Mid Seventies Champ - scratchy Vol pot on guitar

[angelodp]

Hi, I hav a mid-seventies Champ that I recently (yesterday), recapped. I pulled the power rail cap can and went with F&T inside the amp. I used 500v 22uf for the first node, then 47uf on the second node to the screen, and then 22uf to the pre-amp. The bypass caps are relatively new. Amp plays great except that I was now getting a scratchy volume pot on my guitar. I have no appreciable DC on the bypass or coupling caps. I do see .6-.9VDC on the board near the Plate resistors. I have cleaned the board with alcohol and shop wipes. I tightened all the jacks and knobs. I put in a new 1M at the input. Dropping R's are new. Voltages are in line. Any guitar produces the same scratchy volume pot on guitar (not amp). The only fix was to install a .1 cap in series with the hi-input. I disconnected the lo-input. Now scratchy guitar vol pot gone. I suppose I can add another .1 cap at lo-input.

Question - is this a bandaid approach? The scratchy vol pot on guitar is quite minor but only appeared after the
new power caps were installed. Should I keep going to find the cause of the scratchy guitar vol pot or simply use the remedy I have? I re-built the new caps a second time to double check the soldering.

I read a thread where Xtian recommended this on a Rocket with VVR.... Funny, my Rocket has VVR but it does not have this issue.

[Stevem]

The first test is to see if the small amount of dc voltage your getting on the input grid is coming from the tube, the socket or the other circuitry that hooks up to the socket.

Try a different 12ax7 in that socket.

If that does not change things then unpower the socket by unhooking the plate load resistor with the tube out of the socket.

If that does the trick then install a new socket.

You should not need caps like you have installed to block dc .

[LOUDthud]

The cause of grid current is usually low plate Voltage. Check the dropping resistors in the B+ string and the plate and cathode resistors. If those checkout, a conductive board is a strong possibility. Move the 68K resistors off the board either on the input jacks or at the tube socket.

What is the big blue cap near the input jacks ?

[Stevem]

He added that cap to block the dc to his guitar.

[sluckey]

The only fix was to install a .1 cap in series with the hi-input. I disconnected the lo-input. Now scratchy guitar vol pot gone. I suppose I can add another .1 cap at lo-input.

This is not a fix! Sure, there's no longer a scratchy guitar pot, but now the V1 grid (pin 2) has no dc resistance path to ground and the tube cannot establish bias. Remove the cap, reconnect the inputs correctly and continue searching for the real problem.

[angelodp]

By taking the grid resistors off of the board, problem solved (.1 cap removed). I have a tacked in setup to test this, with just the hi-input taking the 68k directly to the tube pin. Scratchy pot resolved.This seems to confirm that the DC on the board was getting onto the grid through the 68k"s.

For the permanent fix would you recommend putting the 68k's on the jack and then running a shielded wire back to the grid?

[sluckey]

For the permanent fix would you recommend putting the 68k's on the jack and then running a shielded wire back to the grid?

Yes. Here's a pic that shows how Fender wired those jacks in many of their amps. The shielded conductor connects to the junction of the two 68Ks and the shield ground connects to a jack ground lug.

[angelodp]

Yup, that's how I was proceeding. With the new caps in place this amp has a punch and tone that is marvelous.
PD is at 12.5w so all good I believe. Thanks all.

[angelodp]

Another quick question.

On current setup I have these voltages.

6V6

Plate - 418
Screen - 424
Kdrop - 31.8
K-R - 990

With the screen above the plate by some volts, should I address that with the dropping R? (Dropping R - 10k - 1K)

[sluckey]

K-R - 990

I'll answer with a question. Why are you using a 1K cathode resistor? Stock value is 470Ω.

[angelodp]

I went with the higher Cathode R to bring down the PD.

[Stevem]

I don't know about that 1k cathode resistor, I bet your output wave form is very non-symmetrical. Even in my Vibrochamp running a higher plate voltage due to having a 5AR4 recto, I have never had higher then a 700 ohm resistor in there, and this was with a very strong testing tube!

[angelodp]

I backed off to 666 ohm resistor and now getting 16watts PD?? With the 1k R I was down at 13W. 16watts is high, yes? So what to do? Increase the dropping R slightly or live with the high PD?

I have checked all the components and they are within spec. Amp does sound good...even better with the 666ohm,

[angelodp]

I did some additional research and upped the screen R to 1k from 470R. Now the Plate and Screen voltage are within 1v. My PD is at14.1 100% so I reckon this is ideal.....and it's stone quiet and sounds great. I have a Warehouse 8" incoming to fresh it up and should be the icing on the cake. My grandson has taken up guitar from his cello days and now doing Pink Floyd.... finally a family member that can take all this stuff off my hands at the right moment. I built the Supre Eagle for him some years ago.

[angelodp]

Final outcome