Blocking Distortion or mis-biased tube???

[ChrisM]

http://www.youtube.com/watch?v=Z3yD0JxO ... e=youtu.be

I've been troubleshooting this build off and on for months. Can't decide if the sound in the YouTube clip is blocking distortion or a mis-biased triode. What do yo think?

100W ODS HRM, tele on bridge pickup and volume set low.

[cbass]

I'm not sure what your problem is but i've had a bad filter cap give that wierd out of tune note thing.

[ChrisM]

I am using the precision power supply Dumble's have.

The only odd thing I noticed about it is the filter cap stacks (caps in series) well the top one (or the one supply the HV feed) has a HV on the caps outer top casing. It is like this for all the caps. Odd...

[ampgeek]

The HV on the outer cap skin can happen when the negative lead of the cap is common with the skin and caps are in series.

I saw that on my first Ampeg V4 re-cap job when I accidently bought cap cans configured that way.

Here is what the V4 topology looks like.

Dave O.

[ChrisM]

Oh good to know, thanks!

[rp]

The HV on the outer cap skin can happen when the negative lead of the cap is common with the skin and caps are in series. I saw that on my first Ampeg V4 re-cap job when I accidently bought cap cans configured that way. Here is what the V4 topology looks like. Dave O.

TAG tip of the week. Thanks!

[ampgeek]

My pleasure guys!
Hopefully it helps in resolving the observed issue.
Cheers,
Dave O.

[rp]

So not all cap cans are connected to the negative? I always thought grounding the can was some sort of shielding & safety issue, but now I see it can be a hazard too. I never checked but you can buy caps that the case can be either grounded or not? What's the best way to go here?

[ampgeek]

Yes. You can buy them both ways.

I prefer the "un-connected" version of the cap cans.

With those, you can implement a refined grounding scheme (e.g., not having to use the chassis ground plane as the capacitor negative) independant of the capacitor(s) mounting location.

Really good technical information regarding grounding and power supply topics can be found here:

http://www.valvewizard.co.uk/Grounding.html

http://www.aikenamps.com/StarGround.html

Cheers,
Dave O.

[ChrisM]

Any ideas on this terrible sound?

[LeftyStrat]

It almost sounds like ring modulation or an octave effect. Which could mean something is going on that's only affecting half the waveform. I'd perhaps look for something odd around the PI.

If you have a scope or a signal tracer, you should be able to identify where the distortion is occurring. You can make a simple audio probe:

http://www.diystompboxes.com/pedals/debug.html

[Reeltarded]

Wow. Is that annoying!? It actually sounds like once you fix the trouble that one will be really good!!!

Do you have an effects loop? Have you tried isolating the trouble from there in both directions? It almost sounds as simple as a single funky ground point or crosstalk beteen two stages.

I am not so familiar with that amp. Let me go build one. I am bound to have that problem at least once!

[pdf64]

A stage is being biased into cut off. What is the static dissipation of the power tubes. Do all the cathode and plate voltages look ok?
Pete

[Firestorm]

A stage is being biased into cut off. What is the static dissipation of the power tubes. Do all the cathode and plate voltages look ok?
Pete

That might be a good call. But I don't think output tubes are the issue (unless there is really bad phantom coupling to a grid. Sounds PI or preamp. You have cathode followers in there?
And, not to offend, but are all the filter caps oriented and soldered correctly. That's a complex (but excellent) power supply.

[Structo]

Wow, that's a cool sitar sound.

One time I had the preamp cathode resistors on the wrong sides of the tubes.
In other words, for the 220K plate side I had the 2K2 and the 150K side had the 3K3.
Made weird sounds.

What size couplers to the power tubes and what size grid stoppers on the power tubes?