Paul Ruby zener diode mod for fixed bias amps?

[stratoblaster80]

Does the *fizz* occur on both the normal and trem channel or just the trem channel? If it is just on the trem channel I'm pretty sure I might know what is going on.

Mike

Hey Mike - I need to hook the normal channel back up to verify this but I think it’s probably a good idea to check it…I have modded the cathode bypass values for V2B (.68uf/2.7k) - do you think I should return them back to stock before testing or will it be okay to test as is?

[stratoblaster80]

Normal channel reconnected - left the bypass values as is for now. Would test but I don’t think my roommates would be much into being sereneded by a cranked Super Reverb at 2 in the morning…

Off work today at 3pm - I’ll report back as soon as I get home!

[stratoblaster80]

Does the *fizz* occur on both the normal and trem channel or just the trem channel? If it is just on the trem channel I'm pretty sure I might know what is going on.

Mike

I just read your thread about your Stunflower build Mike - do you think this might be related to the reverb driver? I have noticed the same-looking waveform when scoping V3 via V4. And it would make sense that since it’s a “fizzle” type of distortion riding the back of the transient that it would be preamp tube-related. Although I do vaguely recall pulling V3 and the fizz still being there but I will have to check that again.

Will hopefully know more by this evening! I just so happen to have a 1M pot so the dwell control would be an easy add.

[stratoblaster80]

Y’all, Mike might be onto something. Spent all morning reading up on issues he (and others) have had with the reverb driver in the AB763 circuit…V3 is the first tube in the preamp to clip and it seems to clip excessively once volume on the vibrato channel increases. This causes grid current at V3 which if I’m looking at the schematic correctly, has a path back to V2Bs plate load (and also to the 220k mix resister of the vibrato channel).

See this thread:

https://el34world.com/Forum/index.php?topic=23084.0

This would explain an oscillation and also perhaps why I’ve found DC from the .1uf output cap of V4 to the mix resistors… I originally thought I had a bad cap, then I thought it was the board conducting but this might be the reason. As soon as I would touch the meter anywhere from the V4 output to the input cap of the PI it would show voltage and then start to drain back to zero. This would also explain why I thought this might be a power section issue.

All of the dots are beginning to connect…

It appears there are multiple ways to skin this cat but it seems the least invasive (not changing the original tone/signal level) is to place a 560pf 1+kv cap across V3’s plate and cathode. Just ordered some 560pf Vishay 562Rs. Will try a 500pf for now (as well as the other mentioned ways (unbypass V3 cathode, insert reverb ‘dwell’ pot, or also saw a couple people adding capacitance from plate to ground).

I am now chomping at the bit to get home from work and see if I can finally slay this dragon!

[romberg]

I cured mine by reducing the signal going into the reverb driver tube. The fizzy overdrive of the reverb tube still happens on mine. But you can't really hear it anymore since it occurs long after both the power tubes and PI have started to clip.

You could do a quick test by pilling the reverb driver tube. And I think my build also cleared up by disconnecting the tank. But I think pulling the tube is safer.

Mike

[stratoblaster80]

Thanks Mike - I was on the right track with your findings.

So I finally had some time to really sit down and chew on it - spent about 4.5 hours this afternoon and evening - lots to go over so hopefully I don’t forget it all, lol…

First and foremost, the reverb driver theory didn’t end up being the pot of gold at the end of the rainbow. I tried just about everything I read that people said had worked for them - all of them had varying effects but none really nipped the problem in the bud. It was more variations in frequency and gain response. With each if I ended up getting to the sweet-spot I like as far as the breakup in concerned, the fizz was still giving me hefty middle fingers.

Here’s a list of everything I tried:

-500pf 1kv cap across V3 plate & cathode pins (noticeable frequency shift but no difference in gain or fizz
-47pf V3 plate to ground (basically same as above)
-10pf V3 plate to ground (no noticeable change in gain or frequency)
-unbypassed V3 cathode and just left the 2.2k resistor (this seemed like it reduced the fizz a slight bit - probably the most effective of the V3 changes I tried)
-47pf V4 plate to ground (frequency shift but no affect on fizz)
-installed a 1meg J taper dwell pot (had a nice effect on getting the reverb under control but no effect on fizz even with pot all the way down)
-68k v3 grid stopper (no effect)
-various combinations of all of the above (again, variances in frequency response and gain - mostly gain reduction, no affect on fizz)

I also pulled V3, fizz still there. V3 back in and just unplugged the reverb send to the tank, no change.

I did get some squeal a couple times on startup when I took it out of standby - it would go for a few seconds and then stop. So oscillation is still on the suspect list.

Reconnected normal channel and plugged in - fizz still there. This appears to isolate the PI and power section/OT with oscillation still being in the running.

Here’s where it got interesting…disconnected one end of 100r negative feedback resistor…at my normal settings (V-7, T-7, M-6, B-3) no fizz but a drastic reduction in gain/breakup. Okay, promising. Let’s dime volume and treble…gain and breakup returned but so did the fizz.

So all in all, out of everything I tried only variations in frequency response and varying affects on gain/breakup in relation to position on volume pot, but in every situation once I returned volume to my “sweet spot” with the breakup I’m after, it’s still fizzalicious.

So I guess now the plan is to go in with the scope and and magnifying glass to see if I can spot any oscillations or clipping irregularities. Outside of that, OT swap is really the only thing left I can think of to try.

If anything I tried lights up the lightbulb for anyone let me know. Otherwise I’m just about all out of straws to grasp at. After OT swap, short of completely gutting and scrapping the old board/repopulating a fresh, new one I think I’ll just be riding my horse off into the fizzy sunset, lol!

Also I uploaded a new video capturing the fizz while playing. It’s especially noticeable (to me anyway) on one chord so forgive the repetitiveness and obvious nod to Stevie.

Thanks again everyone - hoping the thoughts keep rolling in!

Sound clip:

https://drive.google.com/drive/folders/ ... 4sjpE5p72x

[stratoblaster80]

Random thought laying awake and thinking about this…possibility that V3 is coupling to the power tubes via the screens due to them sharing a filter node?

[romberg]

Random thought laying awake and thinking about this…possibility that V3 is coupling to the power tubes via the screens due to them sharing a filter node?

I'm reasonably sure (but not 100%) that the trouble I saw on my build was due to noise being generated at the grid of V3 (probably grid current). This extra noise on the grid will then also be on the grid of V4b since they are more or less connected. Then the noise generated on the overdriving V3 grid gets sent on out through the amp along with the normal signal. The weakness in the design is that the reverb driver is not isolated from the rest of the signal chain. And the reverb is being fed a pretty hot signal no doubt to have good amounts of reverb when played at quiet levels in a store

Mike

[stratoblaster80]

Wanted to update everyone quickly as it’s been a few days…I’ve been moving and my time with the amp has been limited.

I installed the Twin Reverb OT and that made no difference. Also plugged into the norm channel w/ V2, V3 and V4 tubes all pulled and fizz still there.

I ordered a Furman power conditioner and am gonna try that this afternoon after work to see if it’s an issue with the AC in the house. I’ll let everyone know how it goes!

[Stevem]

I would love for that to be your cure but I think the monster at this point is in your amps component board and or the subterranean wiring .

[stratoblaster80]

I would love for that to be your cure but I think the monster at this point is in your amps component board and or the subterranean wiring .

I think you’re right Steve…power conditioner made no change. I have a new set of boards on order and the plan from here is to gut the old board and all the old lead dress and rebuild from scratch. I think it’ll be fun though and will be kind of cool to have it in a state just like it would have been off the factory floor.

The saga continues…