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Investigating Kustom 2 Valve Problem [message #19047]

Wed, 07 August 2013 10:27

javelin
Messages: 6
Registered: August 2013
Location: vancouver canada

Junior Member

Hi I am a newbie to Kustom Amplifiers but am very impressed with the quality and common sense layout I saw when I opened up my newly aquired Studio 2 Valve Head.The problem is there is a lack of volume ( it plays at a volume I would consider 1 or 2 even at maxiumum ) and there is a raspy dull sounding distortion sound , when increasing the master volume to 10 the sound actually gets a little lower..... I have put in new tubes to test but there is no difference, I have checked the 5w resistors and almost all I can access and they all have been within 10% The large 100uf caps look pristine however all I have done to check is to see that they all are testing over 25K however my understanding is that this is a primitive sort of test.... the pre-amp works fine thru a different amp so I am hoping to just replace the 4 main 100uf caps and the two smaller 50V ?uf caps to start with.... any words of advice or previous experience would be appreciated, This head looks old school tough I can not wait to hear what kind of unique sound it will deliver..... Thanks for listening Tim javelin53@gmail.com

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Re: Investigating Kustom 2 Valve Problem [message #19049 is a reply to message #19047]

Wed, 07 August 2013 11:39

Kochens
Messages: 80
Registered: April 2009
Location: Denmark

Member

Hello javelin
This amp sounds great when it is working. Channel 1. is awesome, the best on this amp.It has a clean dark sound, and does not get bad with the volume on 2/3.
I had a problem some years ago with it. The molex conections were pure, but I do not think this is your problem, as the pre-amp works with other power amps.

[Updated on: Wed, 07 August 2013 11:41]

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Re: Investigating Kustom 2 Valve Problem [message #19050 is a reply to message #19047]

Wed, 07 August 2013 12:09

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

I'm not sure that repairing an amp by randomly replacing parts is a very smart approach. The likelihood that you fix the problem is probably just as good as the odds that you damage something else in the process.

You have narrowed down the problem to the power amp, so the first thing to do is to start checking voltage levels. What sort of technical experience do you have? This is a tube amp and will have voltages inside that can physically harm you. Do you feel comfortable in doing this sort of work? If not then get someone who is, involved.

And remember that even though the amp is turned off and unplugged, the main filter caps can hold a charge for a while.

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Re: Investigating Kustom 2 Valve Problem [message #19055 is a reply to message #19047]

Thu, 08 August 2013 07:30

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

If the amp has any tube sockets that are mounted right on a circuit board, you need to inspect those and resolder if needed, any rings around the solder point means a bad connection from expansion and contraction due to heat cycles.
You can check for DC voltage on pins 3 and 4 of each output tube.This will be very high voltage, like over 400 volts so be very carefull and only keep you one hand that is holding the hot probe near/in the amp, make your ground for the meter thru a clip lead.
You need to also check that pin 8 is at ground on each output tube by means of a continuity check.
If these test pan out I would have to say that you very likly have a bad output transformer that has a shorted primary winding.

If you play thru the amp in a dark room and can see into the slot`s/holes in main plate struture of the output tubes you will see the inside glow briter when playing, this means the output tubes are being drivin by the phase inverter section, but can not inturn drive the primary of the output transfomer due to the short in the winding.
Another way to test this is to set your meter for AC volts and hook it up across pin 5 of each output tube, not pin 5 to ground, but from 5 to 5.
This should show at least 8 volts of signal drive to the outputts when playing, which is more than enough for some good volume from the amps output stage if all is well.
Its most important to be carefull working in a amp with these voltages inside!!
A easy way to discharge the power filters is to turn off the main AC power switch and leave the standby switch on, this will drain the power supply within 3 minutes and make it safe to dive into the amp if the AC cord is unplugged.
Keep us informed on your progress.

[Updated on: Thu, 08 August 2013 07:35]

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Re: Investigating Kustom 2 Valve Problem [message #19141 is a reply to message #19055]

Thu, 15 August 2013 00:41

javelin
Messages: 6
Registered: August 2013
Location: vancouver canada

Junior Member

Thanks all for the advice Part of the problem is I have this curiosity that keeps me coming back to a problem when I know i should be calling in professional help... So here is where i now wallow in grey matter.... I have new tubes installed with no change I have 2 470 Ohm 5W resistors changed with no change I have 4 new 100uf/450V capacitors changed with no change , I have changed all other capacitors with no change.... I have used another pre-amp connected to the amp in with no change I have used the existing pre-amp in another amp with proper working conditions and effects, I have checked all the footswitch and aux inputs with no change....,. I still have low volume and a mild distorsion at all levels ... no real clipping or squealing but very little difference in volume on the master between 2 and 10 The tube voltages read as follows Across 2and7 6.3 VAC Pin 3 to ground 475VDC across pin 4 and 3 437 VDC Pin 5 to ground -47VDC across pins 8&3 473VDC...... any words of wisdom would be greatly appreciated ....... Thanks.... Javelin

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Re: Investigating Kustom 2 Valve Problem [message #19142 is a reply to message #19047]

Thu, 15 August 2013 01:50

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

All of the voltages that you have posted seem about right. At least nothing there stands out a being really wrong.

Have you checked the speaker jacks and wiring? How about the speakers themselves? Did you do any testing of the output transformer?

If the speaker jacks are shorted or if you use the extension speaker jack instead of the main one, the amp will distort and lose power.

I assume that the speaker that you are using with this head works correctly with other amp heads.

The output transformer can have a shorted turn that can cause this symptom. There is a simple transformer tester that you can make to test this this theory. RG Keen's website has a schematic for it.

Edit: I looked at the voltages posted again and one thing looks odd, you say that you have "across pin 4 and 3 473 VDC". Is this correct or is this the reading from pin 4 to ground?

[Updated on: Thu, 15 August 2013 13:35]

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Re: Investigating Kustom 2 Valve Problem [message #19150 is a reply to message #19047]

Fri, 16 August 2013 09:09

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

One thing to mention here is that I have built Keen`s Transfomer winding tester and it is not 100% infalable, as I have checked it on many Transformers and two known bad Output transfomers still tested good.
The best way to test a output transfomer is to apply steady 10 volts of AC across the primary, read the secondary voltage and then figure the turns ratio thing out.

Javelin, I also need to ask why, with all the test you have done, you have not done my simple test I posted before of checking the audio drive signal on pin 5 of each output tube?
In 60 seconds you can tell if you have the drive signal and if so then you have a bad output transformer, real easy!!

[Updated on: Fri, 16 August 2013 09:15]

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Re: Investigating Kustom 2 Valve Problem [message #19151 is a reply to message #19150]

Fri, 16 August 2013 09:27

javelin
Messages: 6
Registered: August 2013
Location: vancouver canada

Junior Member

Thanks I am going to do that prior to anything else as soon as I can get back to the amp.I think I overwelmed myself with downloaded articles from the web and forgot your critical step......Thanks again Javelin

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Re: Investigating Kustom 2 Valve Problem [message #19157 is a reply to message #19151]

Fri, 16 August 2013 15:07

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Pleat was nice enough to send me the schematic for your amp (Thanks Pleat!), and in reviewing it I see two more things to check.

As I had mentioned before, there is an extension speaker jack on the amp. This jack has a switch built into it that switches between the 8 ohm and 4 ohm windings of the output transformer. If this switch contact becomes oxidized, it will cut off the sound to the speaker or reduce the volume. Check both of the speaker jacks to see that they are clean and making good contacts.

There also is an fet transistor at the front of the power amp that is used to mute the power amp at power up and power down. If it goes bad, the signal going into the power amp will be shunted to ground. Check this fet and the voltage that turns it off. It is marked as Q3, 2N5484.

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Re: Investigating Kustom 2 Valve Problem [message #19161 is a reply to message #19157]

Fri, 16 August 2013 16:26

javelin
Messages: 6
Registered: August 2013
Location: vancouver canada

Junior Member

Thanks Again Chicagobill
I swear I am going to name and rebadge this amp after you when i get it working properly.Great suggestions to follow up, I have no regret in replacing the componets to date as given the age of the amp these items would be almost routine maintainance,I am however learning a great deal about needing to be more patient and do more testing prior to replacing parts all willy-nilly. I will be back in a few days to post my latest findings.....Javelin

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