Troubleshooting buzzing

[Stevem]

The first rule of thumb when building is that grid wires need to be as short as possible because they act like a antenna.

You should look over your build with a eye towards that.

In fact it’s better to have for example the plate lead feeding a coupling cap to the next gain stage be long and then put that coupling cap right next to the grid of the down stream tube.

That makes for the shortest runs of potential oscillation producing grid wire runs.

Many times in builds there is a long grid wire run from a volume pot to the next gain stage and that can be a big problem!

I had a build a long time ago that was driving me nuts with a problem like yours and I rewired it to get rid of only 2” of unneeded grid wire and that cured the problem.

It may not look pretty but to get this soused out, I would try putting your coupling caps right on the grid pins of the tubes and see what that does for you.

I would also suggest to you to take a look at some photos of old home tube stereo amp chassis like Scott and many others.

These like what your doing are two amps in one and most of them where no bigger then 18 X 14” and yet they did not have oscillation issues because they where layed out right to have those 2 amps working within the same chassis.

[maxkracht]

Your drawing looks fine to me, I'm guessing you can rule out the power supply as the source of your problem and you can rule out interactions between the two amps. So, progress, even if it doesn't seem like it. What's left is wiring errors, layout errors, lead dress, and little mistakes like that. I doubt it is your problem, and I admittedly don't have much experience with torrids in tube amps, but having the power transformer near the inputs could cause some problems. Farthest from the inputs is usually best.

I would trace signal flow from beginning to end, checking each component carefully against the schematic and your layout, marking off each piece as you go. Double check values, and locations. If you post more pictures, or a layout, or annotate which tube is which it would be easier for others to tell what's going on. Is the 1M resistor on the input connected to the cathode instead of ground? That's the only mistake I saw on a first pass.

Move grid wires around with a chopstick to see if there is a quieter spot for them. You can also, carefully, jumper in a small value cap, say 100-500pf from grid to ground and/or plate to ground, systematically on each stage to see if that cuts the oscillation. This could at least give you a better idea of where it is coming from.

[cdemike]

The Fender side lives! Y'all were right about the sloppy lead dress. V1's anode leads were running too close to V1A's grid lead, causing the issue. I ended up replacing the long grid lead with a very short shielded cable and rearranged the anode leads to V1, which has basically resolved the issue as far as I can tell. I also swapped the cathode resistor back to the 330R, which is now sitting at a cool 6V above ground, so I'll need to tweak that, which seems a little odd since the schematics I've seen for similar dual triode-based power amps usually have something more like 820R as a cathode resistor. Since the AC30-XS layout I used doesn't specify voltages, I assumed they're about what I have -- 300V at the OT center taps. I did find another schematic from the Hoffman site (attached) which specifies 400V on the output tube's plates, so maybe I'm being timid about how hot to get the tube.

Anyway, I started trying to get the Vox side working and I'm getting reasonable B+ voltages, but weirdly my filaments aren't getting current on the output tube. All other filaments are getting the specified 6.3V, so I thought at first it must just be bad solder joints. According to my multimeter, though, there's continuity between all every socket's #4-5 pins and every #9 pin. I mentioned before that my big learning project was a Hiwatt build, so I based the filament wiring on a 50W's layout: https://hiwatt.org/Layouts/DR504_v2_Complete.pdf. Is there a reason this approach shouldn't be working? I tried resoldering those leads, but I'm not seeing an improvement. I attached an updated gut shot for reference.

Thank you again to you both for the help so far -- I'm definitely feeling some good momentum after the Fender side came to life.

[R.G.]

Anyway, I started trying to get the Vox side working and I'm getting reasonable B+ voltages, but weirdly my filaments aren't getting current on the output tube. All other filaments are getting the specified 6.3V, so I thought at first it must just be bad solder joints. According to my multimeter, though, there's continuity between all every socket's #4-5 pins and every #9 pin.

(0) pull out the offending tube, arrange your work space so you don't get electrocuted
(1) measure the voltage from pin 4 to pin 5 on the output tube socket; this should be 0 if I read your posts correctly
(2) measure the voltage from pin 4, 5 and 9 to signal ground; this should be 3.15V for each pin
(3) measure the voltage from pin 4 or 5 to pin 9; should be 6.3V

Measure by clipping a short bit of wire to the meter leads so you can insert the wire into the socket holes.

[maxkracht]

I don't have any experience with ecc99s so not sure what the limitations are or usual cathode resistor values at that voltage. Check the datasheet? Make sure you are measuring plate to cathode, not CT to ground for bias measurements.

As RG said, measure the heaters at the tube socket itself, not your solder joint. You can have a bad connection from flux or dirt on the tube pins or socket, a loose socket, or just a bad solder joint. Swap tubes to make sure your tube's heater isn't open for some reason.

Cathode follower on the vox side looks weird to me, but I can't see exactly whats going on, so might just be me...

[cdemike]

Used R.G.'s method and found out I had a loose socket -- thanks so much for the help! Thanks for the extra set of eyes on that, maxkracht, and good catch. I took another look at that section and found the output of the tone stack was feeding into the top of the bass pot. The Fender side had another wiring error with no 56K resistor between the speaker jack and the NFB connection at V2, which was the source of the low volume. Between fixing those, the amp lives and I'm happy to say the amp works as designed with the sides behaving independently and sounding great with good output! Thank again for the help y'all!

[Stevem]

Glad you tuffed it out!
Enjoy !