Champ Amp Kit fault diagnosis

WezV

You may have seen my rebuild project in making and modding.   It was going well but a fault has occured.

the amp loses power about 1/3rd of the way round its travel

this is the amp.


the circuit (minus the addition of a master  volume and tone - its the on/gain control that is dropping out)


and the wiring


I would contact the company i got the kit from, but they no longer exist as it was 12 years ago.

I have swapped out the 5Y3GT and 12AX7 with no improvement.  I don;t have a spare 6V6GT to try but if that's the likely culprit i can get one in.

I have done the following tests and some of the readings are higher than they should be.

power on with only the 5y3GT in place.  I get 410v between pin 3 of the 6V6 and ground - about 13% higher than it should be.    With the 6V6 put back in  that drops to 335v - still higher than the instructions tell me it should be


With the 12AX7 added back in i get some more sensible readings on that tube 169v on pin 1, 1.3v on pin 3, 160v on pin6 and 1.3v on pin 8. 
On the 6V6 i get 293v on pin4, 19v on pin 8 and i cant read my writing for pin 3 but its higher than the 320v max the instructions say it should be



Question 1 - whats the likely cause of the volume drop as the gain is tunned up?
Question 2 - are those high voltage readings showing a fault?


This is a bit like painting  by numbers for me

springhead

Your voltages are ok.  320 vs 335 is fine for a simple valve circuit.  Screen is lower than anode, 19V is about right for a single 6V6.

Check your wiring on the tone and volume pots.  Make sure the ground connections are good.  Feels to me like maybe it's parasitic oscillation - up in the 100's of KHz.  With the amp on, and at the point of losing power, CAREFULLY (one hand, not touching the chassis /guitar strings etc. etc. ) move the wires around with something insulated - plastic tweezers, chopstick etc.  in particular that long blue wire from the volume pot onto V1b's grid - pin 7.  that's carrying a higher level inverted version of the input right over the jack and into the first grid.  Could be a culprit. 



  

ICBM

That's a very odd tone/volume control arrangement. In my opinion C5/RT should be connected to the top of the volume control/rotor of the tone control and not to the rotor of the volume/V1B pin 7. (Although this would probably not explain the problem - it will just make the tone control ineffective at low volume settings.)

If C5 is leaking DC even slightly then turning up the volume will result in V1B becoming heavily forward-biased and cutting off.

It could also be caused by crosstalk from V1B to V1A if the ground connections from the cathode resistors (R9 and R10) are not grounded well enough, if it's not layout-related as springhead said. 

It won't be anything to do with the 6V6. If it was a valve fault it would be the 12AX7 for the same reason.

DJH83004

As IC says very unusual tone / pre-amp volume arrangement - so follow his suggestions, you just don't drive the volume pot from the wiper of the tone control. Suspect not your fault at all and not a wiring issue. If you are struggling, you are not a million miles from me (A38) so you could drop it in for me to sort.

WezV

Thanks all.

I will have a look at the control wiring... it was an add on for the kit and not covered in the standard instructions and the extra sheet has them laid out differently. There is a good chance I wired something wrong there

jpfamps

It's worth testing the DC on the pots (as ICBM suggests).

Silvered mica caps can and do leak DC; I don't use them any more. You should have virtually no DC on the top of the pots.

Tone controls look strange too.

The tone control will have more effect as you turn the volume control up.

With the tone turned fully down as you turn the volume control up you will be shunting signal to ground via CT2. The turn over frequency of this will depend on where the volume control is set.

When the tone control is full up, then you will be adding treble via CT. 

I may be tempted to rewire these a bit more conventionally.

Tweed Princeton would be the way to go.