5F4 Head Needs Debugging Help

[Rocketrob]

Well, after a few months of getting things together for this build the results are rather underwhelming. 5F4 head with my own chassis design. Hammond 276X PT, and 159Q choke, with a Allen Amps TO35MT OT.The amp went together pretty well; everything fitting where it is supposed to go. This is only my fourth one so I'm still trying to feel my way around.

Then I fired it up...and a few problems that I can't seem to fix right off. The first and most serious problem is the Presence circuit; I had to disconnect it to get the amp to operate somewhat correctly. I couldn't get accurate readings from v2 and v3 and the amp would start a low frequency feedback when I tried to do this with the Presence connected. Also I noticed that the bias was being affected by the position of the Presence control. That doesn't seem right.

The other problem is that the amp sounds a little stiff and not as loud as it should be. Right now the power tubes are biased at around 32mA. I want to bias them a bit warmer, but I have to change the resistor.

Here are some numbers from the last test session ( with Presence disconnected).

AC line in: 118v
Recto(5u4GB) pins 2&8: 4.9v, pins 4&6: 711v
1st filter cap: 427v
2nd filter cap: 421v
3rd filter cap: 344v
4th filter cap: 289v
Heaters: 6.4v
V1(12ay7) pin1: 146v, pin2: 0v, pin3: 2.3v, pin6: 146v, pin7: 0v, pin8: 2.3v
V2(12ax7) pin1: 157v, pin2: 0v, pin3: 1.4v, pin6: 289v, pin7: 157v, pin8: 170v
V3(12ax7) pin1: 211v, pin2: 0v, pin3: 1.8v, pin6: 274v, pin7: 3.7v, pin8: 65.6v
V4(6L6GC) pin3: 420v, pin4: 421v, pin5: -39.9v
V5(6L6GC) pin3: 420v, pin4: 421v, pin5: -39.7v

I have checked my wiring several times on both sides of the board, but can't find my mistake(s) any insight would be greatly appreciated. Thanks, RD.

[Rocketrob]

Here are a few pics of the guts.

[Bruce / Mission Amps]

Here are a few pics of the guts.

Sounds like NFB induced oscillations.
I could be lead dress of the presence control and NFB circuit.
Try flipping your two output tranny wires (blue and brown is standard) around on lugs 3 of the power tubes.
Probably the reason your bias is weird is because the amp is oscillating due to NFB ... which the presence control is part of.
Most likely is that you're measuring static idle current and AC power output current at the same time but close to a frequency your speaker isn't producing much sound at or your ears don't hear... but power is power and most all these tubes we use can easily run in IF frequencies or MHz.

[Rocketrob]

Thanks for the suggestions Bruce. I didn't have time to try them today. Hopefully, tomorrow. If you look at the first picture, the orange wire comming off the output jacks is the feed for the NFB circuit. Maybe wrapping it around the common and ground of the jacks is not a good idea? I'll switch the OT leads at the power tubes too.

[Rocketrob]

Thanks, Bruce!

Once again you are right on it. I changed the routing of the NFB circuit and reversed the OT primary leads; now the amp is working pretty well. I think some tone shaping sessions are in order to get the sound the way I want it. Right now it's a little too trebley and a bit harsh. I changed the coupling cap on the bright channel to a .047 to see if that would give a little more body; it got a little louder and more dynamic, but not exactly what I am looking for. I'll keep trying 'till it's right for me; just like the other amps I've built. At least I know the amp is working correcly and is stable. Stay tuned....RD