EVJ head - bacon frying sound

[ronh]

I am looking for help in fixing my Epiphone Valve Junior head. It has developed a sound like bacon frying. (no smell). The sound is there right after the tubes get to operating point. The level of the sound is un-affected by the volume control. The level of the sound doesn't change that much, it goes away and then comes back. The amp is hum free and very little hiss. What I have done so far, changed tubes, cleaned the input/output jacks. Cleaned the tube sockets, cleaned the ground point and tighten. Changed the 2 plate resistors. Measured the remaining resistors with the tubes out. Cleaned the board with iso-alcohol to remove old flux. Re-soldered funny looking joints.
Measure for shorts from board to ground. Measure the dc voltages with the tubes in and dummy load connected. V2 plate voltage is 7 volts higher than V1 plate voltage. B+ to plate resistors is the same. V1-Re is 1.5K and V2 - Re is 1.2K. Both Re have a cap across. V1 -Re voltage is 1.1 and V2 -Re voltage is 1.0. I am suspecting power supply caps. Could this sound be caused by a cap breaking down? Am I missing something? Searched the "net and found nothing really helpful. Thanks.

[Helmholtz]

Schematic?

[Jon Snell]

It is unlikely to be a smoothing capacitor. If the sound goes when you remove the 12AX7, try a new valve. If it doesn't, it is probably the EL84.

[ronh]

Thanks for the replies. I swapped the tubes with other ones. Also Re are the cathode resistors. (summer heat) I found a schematic. Changes are R7 is removed, R8 is 1.5K and R9 is 1.2K.

[Enzo]

Re as in emitter? Cathode is k on shorthand, so Rk would be cathode resistor. You have two triodes with different value cathode resistors, so it makes sense the plate voltages would differ.

Voltages being a little off won't usually cause noise. A noisy resistor can still measure dead on, and a resistor can be way off value and still not be noisy.

AMp makes noise. Isolate the problem. Pull the 12AX7, still noise or did it stop? Still noise? SHort across R5. Not to chassis, but directly across the part. Noise?

[ronh]

Thanks for the reply Enzo. Will do as suggested and report back. Divide and conquer.

[The Dude]

It could also be a leaky coupling cap. The volume control should ground the grid of V1B. Is the noise still there with the volume all the way down? Also try grounding the grid of V2. Is the noise still there? If you have a scope, you could also trace out the signal path and find the point at which the noise starts.

[Jon Snell]

This schematic is probably easier to read. I didn't post the power supply as it is very simple. Separate the 12AX7 from the EL84 to fault find without confusion. If the sizzle has gone with no 12AX7 then look around the second half/driver side of the 12AX7. If it is still there with no 12AX7, it is only one of five components, simplifying the fault finding process.

[ronh]

Thanks for all the replies. I followed Enzo's suggestions and it turned out to be the power tube grid resistor. R5 in the posted schematic. Changed it out, powered up with the power tube in only, no noise. Installed the AX7, and no noise. The amp is very quiet now, so it seems to me. The bad resistor measures good with my Fluke meter. Strange defect. I learned something new today, thanks to everyone, so its a great day.

[Helmholtz]

Is the input grid leak resistor (R1) really only 68k?

[nhbassguitar]

Is the input grid leak resistor (R1) really only 68k?

I saw that, too. That would present a ridiculously low impedance to a passive magnetic pickup. Significant signal loss as well as flabby bass and extreme loss of highs. IMO R1 should be at least an order of magnitude higher value and its right leg moved to the left side of R2. And R2's right leg soldered directly to the socket pin. (Well, at least that's how I build, fwiw...) I'm guessing error in the schematic.

[Helmholtz]

The input wiring resembles the low sensitivity inputs of vintage Fenders and Marshalls. Also R6 and R7 just waste gain.

[nhbassguitar]

The input wiring resembles the low sensitivity inputs of vintage Fenders and Marshalls. Also R6 and R7 just waste gain.

R6 & R7: Probably intentional. I can think of more than one reason this would be done. 1) Amp too loud for the speaker. 2) Amp too loud period. 3) Stability problems at high volume settings. 4) Reducing gain *significantly* at V1A with different values for Rp and Rk wasn't an option since they're already close to the low end of gain recommended in the mfr's application notes (page 647 of RC30 for example). 5) Had thousands of AX7 tubes already in stock, engineers fought for AT7s or AY7s but the bean counters said, "Nope, you guys gotta use those up those AX7s." 6) They liked the tone and/or reliability of the AX7-WA tube and were willing to do something unconventional to make it work for this particular amp.

Just guessing, of course.