fizzy and unstable

[DrStrangelove]

I've been working on finishing a slight variation of a Deluxe Reverb. The biggest differences are the tone-stack and independent bias adjustments. The first pic on the following webpage is a nearly up-to-date schematic of it: www.tduryee.com Note: I've since removed the master volume

The general tone is nice, but when I turn it up, even before getting fully into overdrive, there's a fizzy static when I play. I've tried all kinds of things: Snubbing caps around the Anode resistors, putting 68k on all the grids, experimenting with getting more and less gain from different stages, and lots more. Currently I have the reverb side-chain and the tone-stack entirely disconnected/bypassed. I still get the fizz.

A few other hints:
(When everything is connected) If I turn up both the trebble and the presence, the amp produces an extreemly high pitched screech whether playing or not. Also, reguardless of tone controls or volume, turning up the reverb causes uncontrollable feedback/noise.

Anyone have any ideas?

[gofar99]

Hi, I usually stick to hi-fi audio as opposed to guitar equipment. Even though both do many of the same things, there are different goals. My initial thought are, check the power supply filter caps. From your description some type of feedback is taking place. With high gain amps too little or improperly functioning power supply filters can create a path between stages. Without seeing the actual build, it is possible that a component in one stage is close to and is coupling to one in another and causing the problem. Coupling caps are known to do this. In high gain amps, parts placement can become critical. It appears from your description that there is more than one problem. I'm not certain, but is it possible that some of the reverb issues could be phase related. Is it possible to get the tank hooked up wrong? Not in my area of expertise, but I've seen similar problems in transformer coupled stages. The power amp portion seems fine. If the feedback was present at all times I would suspect that the negative feed back was actually positive and the output transformer secondaries needed to be swapped. A quick check on an O-scope would tell. I hope this gives you some new things to check and may lead to a fix.

[ray_moth]

My guess is that your problems could be electro-mechanical, due to the reverb side-chain. The fizzy thing sounds as if something is sticking. I once had a similar problem that I couldn't cure in the amp and it turned out to be a misaligned speaker magnet, The speaker coil was rubbing on the pole-piece. And that high-pitched screech sounds to me awfully like acoustic feedback. Is the speaker near to the side-chain?

[Jeb-D.]

First, make sure it's all wired right.
Secondly, that schematic is littered with 12AX7's.
Lot's of gain + lot's of resistance in the signal path = Lot's of noise.

That is assuming you are refering to background noise. If the sound of your guitar is noise like, then something is def wrong.

[DrStrangelove]

Thanks for the ideas.

I've done some more testing and here's where I'm at:

I ruled out speakers as a culprit by trying two different cabinets with the same result. I forgot to mention in my original post that this is a head, not a combo, and I have the speakers intentionally about 4 feet away to avoid interaction.

I designed the layout to give as much space as possible between caps so I don't think that's it either. Also, for testing purposes I've removed as many unnecessary caps and series resisters as possible (snubbers, cathode bypass, etc.), bypassed the tonestack, bumped the 6v6 grid resisters up to 5.5k, and generally simplified this thing as much as possible.
I updated the schematic here:
www.tduryee.com

I originally built this amp about 2 years ago and it turns out I had covered most of the noise, probably thinking it was just a byproduct of the Baxandahl tonestack in a high-gain amp. There was a snubber on V3a&b and cap paralleling the M.V. to ground. With those removed, the amp is unstable even at "1". If I move any object near the V3b grid wire, there is a definite antennae effect / interference, so my next test will be to replace that with a shielded cable. I suspect there's more to it though.

I can't see it being a schematic design flaw at this point because if it is, the Fender Deluxe would never have worked. It's probably a layout issue, so I'm trying to isolate the main culprit to deal with it.

As far as the reverb tank: Yes, it's possible to hook it up backward but I've been careful not to. I'm guessing the problem with the reverb stems from this other issue though, so I'm bypassing the side-chain to work on one thing at a time.

That said, I'm a little curious about how the mixing circuit works. What is the effect of the 3.3M in parallel with the 10p?
Also, are there impedance matching concerns when coming off V3b anode to the splitter instead of using a cathode follower?

[gofar99]

Hi, Perhaps a long shot, but two possibilities. Is the tube micro-phonic? Can you sub another tube in its place? Second is a cold solder joint at the grid or in the grid of the tube. Check the earlier stage as well.

Just thinking,

Good Listening
Bruce

[DrStrangelove]

I swapped out tubes in all positions without the problem getting much better or worse. The grid solder joint is good, but the other end of that line has me thinking. When I turn the volume knob it sounds like a dirty pot and touching it's case with my needle-nose pliers causes crackle noises. Does it seem reasonable that the pot would be causing problems?

[gofar99]

Hi, You bet. Either it got, cruddy inside, is worn out, damaged or possibly shorted inside to the case. I've had ones right out of the factory be no good. See if perhaps you can hang one along side it and swap the wires. It doesn't have to be a perfect match, just reasonably close. All you want to do is see if the problem goes away. Another possibility is that the case of the pot isn't grounded. I had one amp oscillate badly (at about 100KHZ) because of that.

Bruce

[DrStrangelove]

I swapped out the pot and it didn't change the main problem. It is however slightly less sensitive to touch and doesn't make quite as much noise when turned. My circuit ground is tied to a ground bar rather than running through the chassis. Do you suggest tying the pot-case to the circuit ground or would that add more risk of ground loops?

[kevinkr]

If you have an oscilloscope you might want to check for oscillation. I noted that none of the 12AX7A have grid stopper resistors on them (1K resistor right in series at the grid pin of the tube socket) and this can be a problem particularly if the volume pots are any distance away from the tube they are driving. (Master volume, clean channel gain, dirty channel gain, etc.) Other locations like the phase splitter stage also benefit from this approach.

Note: I've worked on more than a few diy fender clones that had one or more oscillating stages. The above changes fixed the problem - careful lead dress and a compact design in some cases will too, but the grid stopper is pretty bullet proof.

Pot cases if not effectively grounded to the chassis by their bushings should always be grounded.