Asteroid 60 spitting out with gain.

[passfan]

Well I have a head scratcher for sure. I have a newly built asteroid that has a problem I’ve never run into before. On startup all voltages matched the voltage sheet from the plans , and still do. All I can really do is lightly strum a guitar through it at a 1/4 gain setting or less. Any heavy pick attack here causes the guitar to drop out and a spitting noise. Just as quickly back to light strumming and all is well(but not). Happens so quickly you don’t even realize the guitar dropped out at all. Run the gain up to 1/2 volume and any pressure at all on the pick causes a noticeable drop out of audio and a heavy spitting out. All this is on the slow setting. If I got to high it occurs so quickly it’s not worth the effort. With the power tubes out I injected a 1khz 5.4 mv signal and traced it through the pi. I could run it up to 56 volts of drive with no issues. I spent the morning dressing leads and installing shielded wire for V1 thinking oscillation and while I did make the amp sound better my issue is still there. All new parts except for iron. Scratching the hair right off my butt here. I am down to separating the preamp and patching it into another amps loop in an attempt to isolate where it is.

[martin manning]

Did you try disconnecting the NFB?

[passfan]

No I have not. 1st on the list.

[mojotom]

Maybe check for any DC leak on silver mica caps (treble cap and PI cap if any across the PI plates).
Or put new ones in.

[xtian]

After checking NFB, next likely thing is grid-to-ground reference. If a preamp stage's has no proper reference to ground, you can get blatty, gated cutoff ugly noises.

[Stevem]

I don’t understand how much input signal you are driving the first gain stage with, what is 5.4mv?

I test with .150 volts which is basically the output of stock single coil Strat pickups, so are you saying your driving with .154 volts ?
If so it’s it’s written as 154 mv.

[passfan]

The low setting on my signal device is 5.4mv which would be written .0054 using that at the input I was able to trace all the way through the preamp to 56 volts of drive into the output tube grid. My high setting on my signal device is around 200 mv. Not being sure what is going on right now , I opted for the lower setting to run a signal trace with. Keep in mind I’m still in the early stages of troubleshooting this amp. An oscillation is what I feel I have. I don’t want to destroy anything finding it.

[passfan]

It appears lifting the feedback is allowing my notes to ring out and not spit out. I will swap out my plates in the morning and see where this takes me. Definitely needs feedback though sounds like crap without. It has been a while since I built or worked on an amp and my troubleshooting skills got rusted up a bit. I will keep everyone posted as to how it turns out.

[Stevem]

Also 56 volts of audio drive into even a 6L6 type tube no less a EL34 is a lot!

[passfan]

The output tubes were pulled at the time so I’m not sure what effect that would have on it impedance wise.

[martin manning]

That will 56V will be reduced when you hook the FB up correctly.

[roberto]

I don't know if these 56 V are RMS, peak, peak-peak or whatever.
IIRC the schematic (that I don't have with me now) has around 6 dB of NFB, so just consider to have around half that voltage with power tubes in.

[passfan]

So the plates were hooked up correctly to begin with and squealed when I reversed them. I decided to lift the power supply board and found my problem underneath. Using a recovered organ output transformer I installed a tag strip underneath the board to extend the leads for the transformer. I installed the plates on side by side tags and they arced together. With no load everything acted normal and looked good. When I tried to load it down by playing and turning up, the carbon path arced the plates together and dropped the primary right out. I will change the transformer out with a new one and all should be well. Thanks guys for the troubleshooting tips