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More power from a Frank head. [message #20489] Wed, 16 April 2014 06:43 Go to next message
stevem is currently offline  stevem
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Well my ship came in so to speak.
I found this Frank PA head and its owner 5 years ago but he would nit sell it even though due to the condition of his hands he could hardly play anymore.
Well to make a long story short I kept in touch with him and was able to buy it from him last Friday.
I get it home and play it and it sounds ok, but is lacking some volume and for sure punch.
I then hook her up to my test rig and she is only putting out 63 watts rms and the bottom of the sign wave is clipping off very early!
I check the output from the amps PI transformer and its even so I head over to the output transistors and check the six resistors that make up its bias circuit and sure enough they have drifted off value.
One of the 5 watt emitter resistors read .700 ohms down from its needed 1 ohm.
One of the two 2 watt 820 ohm resistors had gone up to 906 and the other 860 ohms respectively.
The two 8.2 ohm 1/2 watt resistors where both up at over 11 ohms.

Replacing all of these got the amp back up to its 100 watt rms power output and made me notice one fact that I never had before, that being that these amps when set up right idle twice as hot as the A or B serise heads and only when puched really hard to they show any sign of crossover distortion.
What people like about the tone of these amps is the fact that I now see that these amps output stage put out twice the amount of class A power as the latter heads do.
This makes for a far warmer sound from these amps even discounting the tone shaping of the amps PI transformer.
Anyway I fixer here up and sat and player it with a big smile on my face for over an hour!!

[Updated on: Wed, 16 April 2014 06:48]

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Re: More power from a Frank head. [message #20490 is a reply to message #20489] Wed, 16 April 2014 13:36 Go to previous messageGo to next message
carlc is currently offline  carlc
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Good deal!

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Re: More power from a Frank head. [message #21036 is a reply to message #20489] Thu, 31 July 2014 22:47 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Hey Steve...
Hope you are still in Frank Head Heaven!

I haven't posted here for a while, but this week I decided to spend some bench time on my trusty K200a5 head (which I use for bass).

I have a similar question about early clipping on this head...a little like what you found in your Frank.

The positive going side is clipping a little early I notice. Into my 4 ohm test load at about 17 volts RMS. The negative side seems pretty good out to about 20 volts @ 4 ohms.

The clipping is quite symmetrical at 8 ohms. I measure about 70 watts into 8 ohms...but not much more than 75 watts at clipping into 4 ohms due to that positive side going flat early. The clipping is smooth and flat. Nothing peaky or ugly in any way.

I've replaced the filter caps (which were very tired) with new 15,000uf @ 75V caps ...no real change. Sure did get rid of the AC ripple showing up at full power on the output though!

I replaced the bridge rectifier with a new 25 amp...no change.

I replaced all 4 output transistors with some very well matched MJ15015's (a heavy-duty version of the 2N3055). No change in the clipping, but the half-power distortion is now down to about 0.15%! Less cross-over distortion I guess. We use these transistors as modulators in AM broadcast transmitters BTW with great results.

Three of the 1 ohm, 5 watt resistors are reading 1.1, and the other is 1.3 ohms.

I've looked at the input to the power amp (monitor jack), and there's no clipping at all there...and loads of head-room.

I'm thinking perhaps some of the bias resistors might have drifted off, but I can't tell exactly which ones might be the most likely to cause problems.?.

Any ideas?
Dave O.

Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >

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Re: More power from a Frank head. [message #21037 is a reply to message #21036] Thu, 31 July 2014 22:51 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Oh, and I should add that the power supply rails are quite well matched. Within about 0.1 volts at clipping.

Thanks,
Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21040 is a reply to message #20489] Fri, 01 August 2014 01:58 Go to previous messageGo to next message
chicagobill
Messages: 2005
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Do you know how well matched the driver transistors are?

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Re: More power from a Frank head. [message #21042 is a reply to message #20489] Fri, 01 August 2014 08:27 Go to previous messageGo to next message
stevem is currently offline  stevem
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Yeah Bill, that was going to be my question to ask, also how matched are the bias resistors on those driver transistors, are they pulling the same current?

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Re: More power from a Frank head. [message #21047 is a reply to message #21042] Sat, 02 August 2014 11:00 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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The drivers are sort-of like this...

The pair farthest from the power transformer are original Kustom numbered RCAs. 38735 or 36.?. Very hard to make out the last number on them however.

The pair closest to the transformer are (I believe) NTE129 (perhaps the matched pair 128/129?) Can't really see the numbers because of the slip-on heat sinks, which are VERY tight. To avoid damage to the circuit board I will likely need to unsolder them before trying to remove the head-sinks.

I've been running through all the resistors on the power amp board, and I've found a LOT of them that are way off...like some of the old carbon comps are near 200% of the rated value. I'll be replacing them shortly.

Do you think I should just shot-gun the drivers too? I'm completely fine with doing that. (much easier while I've got the board pulled out changing resistors) If so, what are your suggested types and sources these days? Is the NTE 128/129 matched pair a good way to go?

Can you guys tell me which way the NPN and PNP pairs go in? Without board layout (and nothing on the board), it's hard for me to see which driver is supposed to go where. I don't think the amp would be too happy with PNP's in the NPN holes. Smile

Thanks VERY much for the feedback!
Dave O.

Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >

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Re: More power from a Frank head. [message #21048 is a reply to message #20489] Sat, 02 August 2014 11:03 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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So the short answer is, I don't think they are matched-up at all.
Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21049 is a reply to message #20489] Sat, 02 August 2014 11:51 Go to previous messageGo to next message
chicagobill
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Well that could be a big part of your problem. I have no problems with using NTE parts other than the fact that they are really overpriced and that they really don't match the original parts. I would suggest replacing all four drivers with the same parts numbers and not mixing the new with the old.

I know that I've posted the part numbers that I use for replacements somewhere here, but I will look them up and repost here later.

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Re: More power from a Frank head. [message #21050 is a reply to message #20489] Sun, 03 August 2014 12:43 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Thanks Bill!

I agree, the NTE stuff is certainly over-priced...but sometimes these days it's about all I can find.

From a couple of years ago, in a reply from you and Bart (which started with suggestions for filter cap upgrades), there were links to Mouser for a 2N3440, and 2N5415. They don't seem to stock them any more. Those cross to NTE 396/397, but 350 volt transistors might be over-kill anywhere in these amps.

At Vintage Kustom I also found a transistor cross reference done in the past by Bill.
It suggests the NTE128/129 as replacements for the 38736/37...which I can get matched pairs for...although that might not make much sense since in this application, a pair of matched pairs would be needed.

Thanks Guys!
Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21051 is a reply to message #20489] Sun, 03 August 2014 12:53 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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As it turns out, I had some matched pair NTE 128/129 (the NTE129MCP) in my parts stock. I'll change-out the drivers in this K200 later today with those and see where that gets me.

While I have the board out, I'll also change the way off-value carbon-comp resistors.

I'll report back here after that's done and let you know if the asymmetrical clipping / reduced power issue is solved.

Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21052 is a reply to message #20489] Sun, 03 August 2014 23:22 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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After an afternoon of soldering on the PC 702 output board in this K200A5, here's where I am...

All 4 of the driver transistors were changed for NTE 128/129 matched pairs (which I checked for Hfe before installing, and they were all within less than 5% of each other).

I found that someone in the past had changed out Q707 with an incorrect cross-ref ECG transistor. It was a PNP, but was under-rated. I changed that to an NTE 129, which should be closer to being correct.

I measured all the resistors, and changed all I could find that were more than 10% off tolerance. This wound up being more than half the resistors on the board. One 100 ohm was reading 250 ohms! That was the worst of it, but a lot were 20-30% off.

I changed the two 80uf caps with new 100uf, low ESR versions (Panasonic FM).

After all this, the positive side of the waveform is still clipping early. I gained perhaps a couple of watts in output at 3% distortion (on-set of clipping), which yields 17.5 volts RMS into my 4 ohm load...75 watts.

The negative side of the wave still doesn't get into clipping till about 20 volts RMS.

The input to the power amplifier is still very clean, and free of clipping, with plenty of headroom.

I suppose the only place left to look is at the earlier stages of the power amplifier board. I do not have any modern, generic transistor cross reference for those however. Those are all original Kustom/RCA transistors.

Any additional suggestions would be greatly appreciated...but I'm getting pretty close to giving up on ever again getting 100 watts out of this amp.

Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21053 is a reply to message #20489] Mon, 04 August 2014 06:41 Go to previous messageGo to next message
stevem is currently offline  stevem
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I agree!
At the point you are at now you need to look for early clipping of the other stages that are class A and may be going into saturation or cut off early.

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Re: More power from a Frank head. [message #21054 is a reply to message #20489] Mon, 04 August 2014 12:44 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Thanks Steve!

With the feedback loop running through this power amp, it's a little hard to just look back at the early stages and tell where the clipping is coming from. I had wondered if it was OK to power it up without the output transistors in their sockets.?. That would open the feedback, but might cause other problems too, so to avoid smoke, I didn't do it.

It's pretty easy just to replace the pre-driver transistors...Q701-Q707.
(although I just changed Q707) That is, if I knew for sure what they were (see below).

I don't think this PC702 board has the Q700 on it. I wish I could find a schematic and parts lay-out for it. It would stop a lot of guess-work I'm having to do.

If anyone has suggestions for the replacement transistors for the early stages of a PC702, I'd appreciate hearing it.

The Kustom/RCA part numbers look like 38734, and 38735.

On a schematic I found at this site it looks like 38734 is 2N4249...which crosses to an NTE159 PNP. That's easy. I even have some in stock.

I have no idea what a 38735 might be. It appears to be in the position where Q703 might be...but I'm using a parts lay-out from the PC704 boards, since I don't really have a schematic or parts lay-out for the PC702.

At least I know a lot of what's not causing the early clipping now. Smile At some point, we will stumble across the real cause.

Thanks!
Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21055 is a reply to message #20489] Mon, 04 August 2014 13:26 Go to previous messageGo to next message
chicagobill
Messages: 2005
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Darn! I was hoping that the replacement of the off value power amp parts would fix the problem.

There is a PC702 schematic in the technical section here. Transistor Q703 K#38735 is the predriver, the RCA generic number is RCA40408. The differential pair at the input Q701 and Q702 K#38734 are RCA40406.

I did look up the drivers that I have been using (2N3440 and 2N5415) and I did check with Mouser and they are only stocking the 2N3440 now. I guess that I'll have to find a new set of replacements once mine have been used up. I just liked the fact that I could remount them on the original heatsinks. I started using these as replacements, because I had them stocked to fix Gibson Lab Series amps. The Lab amps use them as drivers as well.

Well back to your problem, Q705 and Q709 are the protection circuit. They monitor the voltage across the emitter/ballast resistors. I suppose that if one of the transistors is turning on too early it would decrease the drive to that side of the power amp. Check Q705, Q709, CR704 and CR705 if you haven't already done so. Or I guess that you could just pull the diodes and see if the asymmetrical clipping goes away.

I would not recommend running the amp without the outputs. Even though this design is very similar to a thousand other power amps, it just seems that the drivers will fail if you try and do this. Maybe it was those original RCA40409 and 40410 drivers that couldn't handle it, but I'm gun shy from experience. In any case I doubt that the feedback loop would only effect the the positive side of the signal.

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Re: More power from a Frank head. [message #21056 is a reply to message #20489] Mon, 04 August 2014 15:34 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Ya, I was hopeful too Bill. But my hope remains as long as you guys are willing to put up with my questions! Smile

I totally missed the schematic for the PC702 in the schematics section of this great site. Got it now. Things make much more sense now that I'm looking at the right schematic. Now I have concerns about the parts lay-out (see below).

I also missed that the 36735 is really a 40408. That shows up on a list I have that you (or some other Bill here) did long ago. I saved that one back in 2007.

The drivers were changed yesterday with new NTE 128/129s...at least I think that's what I changed. Now that I look at the PC702 schematic, I could have changed something else. I was working with the parts lay-out for the PC704, which was about the closest thing I could find. That PC704 lay-out doesn't even show a Q710 driver. Surely the heat-sinked transistors on the board almost directly above the output transistors are the drivers...right.?. Those are what I changed.

You might be on to something with the protection transistors/diodes!
I'm also wondering which diodes are actually CR704 & CR705. There are 5 diodes on this board (including the one on the chassis), and they are 3 different types in appearance. Three are clearly in series, so those must be the CR701,702,703 (heat-sinked to the chassis). I'm very worried about breaking the leads on that one after pulling this board so many times BTW.

The schematic calls for FD111 for CR704,705. I'm not sure what those look like.?. At least one of them (or what I think is them) has clearly been changed at some point in it's life. One is a little glass thing that looks like a 1N914 germanium. The other is a plastic thing that looks more like a 1N4001. One of these is down near the output transistors, the other is up near the output tab and fuse.

Am I seeing this right? Are those the protection circuit diodes (CR704/705)? I just wanna be sure I'm pulling the right ones for this test.

Thanks for the advice on not running it without the output transistors. I was nervous about doing that...but seriously considered it.

Thanks again,
Dave O.

Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >

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Re: More power from a Frank head. [message #21057 is a reply to message #20489] Mon, 04 August 2014 15:39 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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BTW, I also just checked for voltage drop across the skinny red and green wiring between the filter caps, and the output transistors. There really wasn't any. About 0.05 volts of drop from the filters to the transistors. I don't think that's enough to matter much...but that sure is skinny wire!

Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21058 is a reply to message #20489] Mon, 04 August 2014 15:40 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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That 0.05 volts was at full output BTW, well into the clipping.
Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21059 is a reply to message #20489] Mon, 04 August 2014 15:48 Go to previous messageGo to next message
chicagobill
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I'm nowhere near my K200A head, so I can't direct you to the specific part locations, but you are correct, the three diode in series including the one off the board are the bias string diodes CR701-703. Leave the 1N3754 diode un-hooked from the heat sink clip to avoid some of the motion induced stress on the leads. Steve suggests dabbing silicon RTV where the leads come out of the body to help secure the leads, I just avoid moving them as best I can.

Most of the FD111 or 1N3193 diodes that I've seen on these boards are white plastic with a black stripe and no part number. If one has been replaced, that would point a suspicious finger as a possible cause of your problem.

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Re: More power from a Frank head. [message #21060 is a reply to message #20489] Mon, 04 August 2014 15:49 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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I just checked those diodes. In-circuit, both just check as normal diodes on my Fluke 115. I changed one of the transistors yesterday...the one up next to the output tab. I think that's one of the protection transistors.?.

The other protection transistor could still be the source of the clipping though. It looks like an original Kustom.

I have to go meet a tower crew shortly, so I won't have time to do the "pull the diodes" test today. Maybe tomorrow.

Thanks again for this lead...you and Steve are a God-send!
Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21061 is a reply to message #20489] Tue, 05 August 2014 06:36 Go to previous messageGo to next message
stevem is currently offline  stevem
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Your welcome! let's keep pluggin away as I am sure we can track it down!
It's like a quest now right?

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Re: More power from a Frank head. [message #21062 is a reply to message #20489] Tue, 05 August 2014 10:33 Go to previous messageGo to next message
stevem is currently offline  stevem
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One thing that just came to mind here guys is that any if the electrolytic caps in the circuit that could effect gain and also have drifted up in mfd value may make for the gain issue /. Off set clipping going on!

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Re: More power from a Frank head. [message #21063 is a reply to message #20489] Tue, 05 August 2014 10:59 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Yea...Chicago Bill saves the day (and my K200)!!!

Your idea about the protection circuits firing early on the positive side of the waveform was absolutely correct!

I lifted one side of diode CR704, and now the positive and negative sides clip symmetrically at 4 ohms. 3% distortion (which is what most folks define as clipping) is reached now at a VERY respectable 21 volts RMS.

That calculates to 110.25 watts into 4 ohms! My K200 is once again a BASS BEAST!!! Smile

I'm also measuring even less THD now up near clipping into 4 ohms. It stays down around 0.2% till the clipping begins, and 0.13% at half power. That's about half of what I was seeing into 4 ohms. This means that protection circuit was even kicking in early before I was seeing the actual clipping.

Note to self (and everyone else): The transistor and diode combination that's down near the output transistors (Q705/CR704 on the PC702, and Q706/CR3 on the more common PC703 boards) affects the positive going side of the wave. I assume the set up by the output tab affect the negative going side of the wave...but I've had no problem there, so I can't confirm that for sure.

Now I need to replace that transistor with an NTE123, and the diode, and make sure we are still at 100+ watts with the protection restored.

I should have known it was something like this. Whenever I run into a road-block servicing a solid state amp, it's usually got to do with protection circuits. Until Bill make it clear where how the protection circuit works in the K200, I was in the dark without a flashlight.

Thanks SO very much!!!
Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21064 is a reply to message #20489] Tue, 05 August 2014 11:07 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Ya know, if some guitar player wanted to put a "half-power" switch on the back of his K200, it would be pretty easy just to install a switch to re-bias those protection transistors to kick-in at 50 watts.

For that matter, you could even make it clip asymmetrically, so that it generated more even order harmonics at the on-set of clipping.

That would (I guess) basically make it a more robust K100, and very simply.

Of course, for bass, we like all the clean, UNclipped power we can get!

Just a thought,
Dave O.

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Re: More power from a Frank head. [message #21065 is a reply to message #21062] Tue, 05 August 2014 11:33 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Thanks for the thought Steve...and the advice!

The first place I usually start with any old electronics is with replacing all the electrolytic caps. So, that had already been done on this CR702 output board...and most of the rest of this K200.

It's a really good thought though. Newer caps have a LOT less ESR than the old ones used back in 1970. I can imagine a situation where replacing old, tired aluminum or tantalum electrolytic caps could actually cause a protection circuit to fire early.

In the K200, it looks like the protection transistors monitor the current through the 1 ohm, 5 watt resistors...or the voltage at the emitter of the output transistors.?. Although, it looks (to me) like it might only monitor that current on one transistor of the output pair. But that was probably enough. It might also just be monitoring the voltage on the output rail. Or maybe, it's all of the above.?. I'm more of a tube guy than a transistor guy, so I defer to the superior knowledge of you and Bill.

I'm not seeing any caps in the protection circuit of the PC702 or PC703, but I could be missing something.

Thanks again,
Dave O.

Dave 0 Wall-O-Bass
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Re: More power from a Frank head. [message #21066 is a reply to message #20489] Tue, 05 August 2014 12:47 Go to previous messageGo to next message
chicagobill
Messages: 2005
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Congrats! I'm so glad to hear you found the problem.

The protection circuits do monitor all four outputs. If you look at the schematic, each of the output transistor emitters is connected to the base of the protection transistor through a 1K resistor.

I'm wondering if the problem is the transistor or the diode? My gut reaction is that it is the diode that has gone bad. We know that the transistor is working because it's clipping the signal, but I think that the forward conduction voltage of the diode sets the turn on point of the circuit.

Earlier on you had posted that one of the diodes in the protection circuits had been replaced, which one was it CR704 or CR705. I'm thinking that the two diodes should be reasonably well matched as far as conduction voltage is concerned to help keep the clipping symmetrical.

Anyway, let us know how it turns out.

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Re: More power from a Frank head. [message #21067 is a reply to message #20489] Tue, 05 August 2014 15:33 Go to previous messageGo to next message
stevem is currently offline  stevem
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Hey glad the baby's fixed now and congrats on hanging in there!
My comment on the caps was in regards to the preamp section and if one stage with more gain but not clipping yet would push the next stage into asymmetric clipping .
I,am a bit more of a tube guy also and I always try to keep in mind with SS output sections how messed up they can be with just minor resistance changes on some of the small values they use in comparison to a tube circuit !

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Re: More power from a Frank head. [message #21068 is a reply to message #20489] Wed, 06 August 2014 08:40 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Well actually, you guys found the problems. I just soldered and lifted leads. Smile

Bill... I'm working with tower guys again today doing antenna work. Yesterday was 12 hours of work on a much bigger amplifier...like 20kw. Later this week I should have time to change both the resistor and the diodes. CR704 is the one I lifted to restore normal operation. I'll just change that, and the Q705 transistor. I have an NTE123 in stock, and that looks like what it crosses to.

The diode CR705 (associated with Q705) is the one that has surely been changed. I'll make sure the diodes match before putting the board back in place. As I look at the specs for those diodes, they don't seem to be anything special. Perhaps a fast turn-on time is good for them. I may put in something faster than a generic 4001.

I'll report back when I get that done.

Thanks again guys!
Dave O.

Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >

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Re: More power from a Frank head. [message #21069 is a reply to message #20489] Wed, 06 August 2014 08:41 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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That should have read... "both the transistor, and the diodes"

Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >

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Re: More power from a Frank head. [message #21070 is a reply to message #20489] Thu, 07 August 2014 06:30 Go to previous messageGo to next message
stevem is currently offline  stevem
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I am not sure if a fast recovery type diode would help or hurt in this application ?

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Re: More power from a Frank head. [message #21071 is a reply to message #20489] Thu, 07 August 2014 12:49 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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I'm not sure either Steve, but I used them just find out. 1N4948 "fast recovery" diodes is what I used...mostly because that's what I had (from Mouser). They seem to be working just fine, and not causing any trouble in this circuit. I've used them in the past to get rid of rectifier switching noise in power supplies.

The original diode on the positive side protection set CR704/Q705 (the one that was clipping early) was an original FD111. Since I replaced that, and the original Kustom transistor it's associated with (Q705 on this PC702 board) we will never know which was to blame.

I'll type up a summery of what's been done on this K200a5, and where it's at now, and post that next. To say this amp is improved would be an understatement! This has been quite the learning experience!

Dave O.

Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >

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Re: More power from a Frank head. [message #21072 is a reply to message #20489] Thu, 07 August 2014 13:40 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Ok, This should be (almost) the final update on the quest to improve my K200a5 head. I use this amp for bass BTW, into a T&R Kustom 2-15 enclosure loaded with JBL E140-8 drivers (for 4 ohms). I may post a final update after I actually make some music through it.

The journey began when I started to notice the amp was distorting early, and just generally not sounding "right". After you play through an amp for many years, you just kinda know when things aren't right.

Once on the bench, I noticed this K200 was only making about 70-75 watts at the point of fairly heavy clipping on the positive side of the wave-form. There was also quite a bit of 120hz hum superimposed on the clipped wave-form. Clearly the original filter caps were tired, and due for replacement...but there was MUCH more.

What's been done so far (with bench measurements):

New 15,000uf 75 volt filter caps installed (had to mount them horizontally due to their height)
(this completely cured the 120hz hum on the clipped wave-form)

New MJ15015 output transistors were installed to replace the '75 date coded Westinghouse 2N3055's
(these are beefed-up versions of the 2N3055 used for industrial applications. They also have very tight matching)

New driver transistors installed. I used NTE129MCP matched pairs, which is an NTE128/129 pair.

Nearly all the resistors on the PC702 output board were replaced with new carbon film versions.

All electrolytic caps on the PC702 board were replaced with new low ESR/high temp types (Panasonic FM series).

Protection circuit diodes and transistors were replaced. (this was found to be the source of the early positive side clipping) I used "fast recovery" 1N3938 diodes. A 2N4401 NPN was used for Q705. An NTE129 PNP was used for Q709.

To reduce super-sonic noise pick-up, I rolled the high frequency response off above 20khz by adding a 330pf cap across the monitor output (to ground) from the power amp board. This made very little change at 20khz, but brought down the response at 100khz by about 10db. There's little point in having 100khz frequency response in a guitar amp IMHO.

A new, low impedance grounding buss was added from the common point of the filter caps, to the power amp and regulator boards.

I had previously changed most of the transistors on the pre-amp boards, and also on the summing/reverb driver board with newer, lower noise transistors. Mostly 2N4401 NPN, and 2N4402 PNP. Some resistors were also changed on these boards.

Here are the final bench measurement results...
(all measurements done with tone controls set as close to flat response as possible...Bass at 12:00, Treble at 9:30)
All measurements done at 4 ohms into my 800 watt dummy load.

The 3% distortion point (which is defined by most folks as the point of significant clipping) now happens at 21 volts RMS. This equals 110.25 watts! The clipping is quite symmetrical now.

The amp can produce 110 watts from 30hz to 20khz at clipping. The frequency response is fairly flat over this range too...+/- 1.5db.

Below clipping, the distortion is mostly below 0.2%.

At lower power (5 watts) the distortion is an astonishingly low 0.04%! I attribute this to the superior matching of the driver and power output transistors, which yields very low cross-over distortion.

Keep in mind, all these measurements have been done from the instrument inputs on the front of the amp.

Frankly, this is WAY beyond the performance I was expecting to achieve out of a 1970 solid state guitar/PA amp head! I hope this feedback can aid others along the way. I'm not sure how (or where) to post it in a place where others would be more likely to see it.?.

Many thanks to Chicago Bill, and Stevem for the advice along this journey!

Dave O.

Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >

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Re: More power from a Frank head. [message #21073 is a reply to message #20489] Thu, 07 August 2014 13:45 Go to previous messageGo to next message
daveobergoenner@gmail.com is currently offline  daveobergoenner@gmail.com
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Oh ya, I forgot to mention that a new 25 amp bridge rectifier was also installed.
Dave O.

Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >

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Re: More power from a Frank head. [message #21172 is a reply to message #20489] Fri, 15 August 2014 09:28 Go to previous message
stevem is currently offline  stevem
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Thanks for the summery on that repair Dave, if you get the time retyping it in the sites repair section would be a great help!
Also if you still got her opended up you might want to add a series inductor and a Zoble network to the output stage.
This is a very important mod to put in place in the first series Frank heads with no protection circuit , as that 5 amp fuse is just way to slow, in fact many times I have put in a 3 amp fuse for a faster response to a issue and just dealed with it blowing if I played too loud!

[Updated on: Fri, 15 August 2014 09:35]

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