ugly distortion sound when notes fading

[mat]

My friend has bought a champ style guitar amp made out of an old tube-radio. It has one EL84 for output tube and two 12ax7's other being for the tremolo. The amp has invented a problem with ugly distortion sound when notes fading. This happends after the amp has been about 20 minutes on. Changing the V1 tube removes the problem but after a few days the symptom comes back.

I sprayed cleaner to the tube socket today and I'm going to measure the voltages tomorrow and check the circuit for cold joints and maybe check for any leaking caps.

The transformers are original, maybe 50-60 years old but they look to be in good condition in outside at least.

Any advice for where the problem might lie ?

[dartanion]

If the amp is that old, I would wager that all the electrolytics are probably bad or on their way out.

I would also measure all the components on the board and check against the schematic to ensure components haven't drifted wildly.

[mat]

If the amp is that old, I would wager that all the electrolytics are probably bad or on their way out.

I would also measure all the components on the board and check against the schematic to ensure components haven't drifted wildly.

The amp is made out of an old radio. The components are all new (but two pots and the diode bridge). Only the transformers are old. No schem for this one.

[ampgeek]

Does the problem come back instantly if you put one of the previously used tubes back in?

Posting up the voltages on all of the tubes, even without a schematic, would be a very valuable troubleshooting tool.

Cheers,
Dave O.

[rfgordon]

It may be blocking distortion on one or more gain stages. Try putting grid stoppers on the preamp stages. Values can range from 33k to 470k. I've had this problem in an amp, and grid stoppers fixed it.

[mat]

Does the problem come back instantly if you put one of the previously used tubes back in?

That is something I dont know, I'll ask my friend.

I'll write down the voltages and take few pictures and post them.

Are grid stoppers a resistors that are in series with the signal coming to the grid Something that TWexpress lacks in V1 ? I remember seeing one on V1a on my friends amp.

[mat]

Does the problem come back instantly if you put one of the previously used tubes back in?

I asked and He hasn't tried it that way.

I fired the amp and measured these voltages to ground:

V1a 148V,0.99V
V1b 137V,1.06V

I presume the V2 being the tremolo tube.

V2a 113V,1.28V
V2b 243V,116V

V3(EL84) nc,-0.02V,6.98V,3.3V,3.3V,nc,277V,nc,243V

The heaters measure 6.66V

I tried the amp with its own speaker and as soon as I connected the guitar in it it gave ugly sound when notes fading. I chopsticked the innards and when tapping the 1M to ground on the input jack or the 68k grid resistor it gave crackling noise.

I turned the amp off and connected it to another speaker cab and it worked just wonderfully. I did not have chance to play it for 20 minutes or more but I will try to do that tomorrow.

Btw. The amp sounds superb ! Very warm and round sound + the tremolo is really good.

I'll swap new 1M and 68K resistors in it and see if that helps getting rid of the crackling when chopsticking those spots.

Any further suggestions ?

Here are few links to the pictures of the build:

http://www.pichotel.com/pic/16028B31e/158354.jpg
http://www.pichotel.com/pic/16028B31e/158355.jpg
http://www.pichotel.com/pic/16028B31e/158356.jpg
http://www.pichotel.com/pic/16028B31e/158357.jpg
http://www.pichotel.com/pic/16028B31e/158358.jpg
http://www.pichotel.com/pic/16028B31e/158359.jpg
http://www.pichotel.com/pic/16028B31e/158360.jpg
http://www.pichotel.com/pic/16028B31e/158361.jpg

[Leka]

Interesting build!

What are the buttons function in the amp?

[mat]

Interesting build!

What are the buttons function in the amp?

tremolo speed

[mat]

Interesting build!

What are the buttons function in the amp?

tremolo speed

For now the amp sounds great and functions properly after swapping the speaker. I swapped also the 1M and 68K to fresh ones but still get crackling noise when chopsticking them.

How does the voltages seem ?

[ampgeek]

Hey Mat,
Based on the observation that a V1 change eliminated the problem, I was guessing that some problem was stripping the cathode(s). Hence, the request to determine if the tube was being permanently damaged by subing a used one back in.

Your V1 voltages (and subsequent observations) indicate that this is unlikely. You are running in the 1.2ma and 1.0 ma range which is very reasonable.

Dave O.

[mat]

Hey Mat,
Based on the observation that a V1 change eliminated the problem, I was guessing that some problem was stripping the cathode(s). Hence, the request to determine if the tube was being permanently damaged by subing a used one back in.

Your V1 voltages (and subsequent observations) indicate that this is unlikely. You are running in the 1.2ma and 1.0 ma range which is very reasonable.

Dave O.

Ok, good to know, thanks Dave O.

Btw. I'm still amazed that single EL84 can give such a full and fat sound