Question about Pentode reverb drive circuit

[nickb]

This has been a great thread showing examples of expected design results, simulated results, and measured results.
You all have been really helpful. I think Helm is right, time to give it a listen. I just received a 6U8A and Im just waiting for my buddy to bring ovr the tank.
Anyone see any issues with the recovery stage? Its not a high gain triode, but I was able to get a gain pretty close to mu.

My $0.02 worth..

I suggest you roll off the bottom end below 100Hz or so pretty quickly to attenuate hum. Making the cathode bypass cap 2.2uF will help a little. Add a 3.3nf cap in series to the grid and a grid leak of 470k roll to off below 100Hz.

You may want to tame the HF output from the tank with a resistor in parallel with the output coil above 2KHz or so with a 5.1k resistor.

Honestly it's very much a guess as I really don't have a good handle on the frequency response of the tank. I think it would be tough to measure. All to your own taste, naturally in any case.

I think you'll see about 30dB of gain on that stage.

PS: I forgot to mention that a cap in parallel with output coil will give you a resonance that you may or may not want.

[66 Kicks]

You may want to tame the HF output from the tank with a resistor in parallel with the output coil above 2KHz or so with a 5.1k resistor.

PS: I forgot to mention that a cap in parallel with output coil will give you a resonance that you may or may not want.

Hey! You're the only one, other than me, that I have ever seen say that the parallel capacitor adds treble (at least I think that's what you said). Everybody always thinks that it will cut the highs like it does in other circuits and they won't listen to me because I have low post counts around the forums and because I am usually a kook. I got the output of an Accutronics tank to jump up 20.6dB at 5.3KHz by adding a 2.2nF capacitor in parallel. Of course there's not much material at that frequency, but it also pulls the output up at frequencies on both sides of 5.3KHz.

A 10K resistor in parallel with the coil dulls the sound, so I suspect that 5.1K would be unacceptable. If you want to use a resonance capacitor, you need to use a resistor that is greater than 100K because less than that reduces the resonant peak. 5.1K will eliminate the resonant peak altogether.

[Helmholtz]

Hey! You're the only one, other than me, that I have ever seen say that the parallel capacitor adds treble (at least I think that's what you said). Everybody always thinks that it will cut the highs like it does in other circuits and they won't listen to me because I have low post counts around the forums and because I am usually a kook. I got the output of an Accutronics tank to jump up 20.6dB at 5.3KHz by adding a 2.2nF capacitor in parallel. Of course there's not much material at that frequency, but it also pulls the output up at frequencies on both sides of 5.3KHz.

A 10K resistor in parallel with the coil dulls the sound, so I suspect that 5.1K would be unacceptable. If you want to use a resonance capacitor, you need to use a resistor that is greater than 100K because less than that reduces the resonant peak. 5.1K will eliminate the resonant peak altogether.

A capacitor across the receiver coil produces a parallel resonance and thus a tuned emphasis at the resonance frequency. Parallel resistance damps the peak. I know at least one book that describes this method.

[66 Kicks]

A capacitor across the receiver coil produces a parallel resonance and thus a tuned emphasis at the resonance frequency. Parallel resistance damps the peak. I know at least one book that describes this method.

I don't have that book. I had to figure it out myself back in the 80's.

[Helmholtz]

I don't have that book. I had to figure it out myself back in the 80's.

So you were right all the time .
BTW, my Fender Super Champ from '82 employs this nice trick.

[SoulFetish]

My $0.02 worth..

I suggest you roll off the bottom end below 100Hz or so pretty quickly to attenuate hum. Making the cathode bypass cap 2.2uF will help a little.

Well, using (≈)1µF cathode bypass gives me a half boost frequency of around 144Hz and -3dB at roughly 120Hz. This might be a better option if we keep with Accutronics recommendations. Plus this gives us the added benefits of reducing power supply ripply and any 60Hz picked up.

Add a 3.3nf cap in series to the grid and a grid leak of 470k roll to off below 100Hz. You may want to tame the HF output from the tank with a resistor in parallel with the output coil above 2KHz or so with a 5.1k resistor.

My thought was to include any R/C filtering in the proceeding/mixing stage so I can keep added resistor noise to a minimum. Although, with a series grid resistance of 5k1, tube noise might be the dominant noise. I haven't figured for it.
Oh wait, I'm sorry, scratch that. You were talking about adding the 5k1 in parallel with the output coil. My B.

I did want to double check something though, would any added grid leak resistance (ie 470k) essentially be in parallel with the 2k5 impedance of the output coil?

I think you'll see about 30dB of gain on that stage.

That's what I got too. Well, depending on the AC load, between +30dB to 31dB.

[nickb]

Well, using (≈)1µF cathode bypass gives me a half boost frequency of around 144Hz and -3dB at roughly 120Hz. This might be a better option if we keep with Accutronics recommendations. Plus this gives us the added benefits of reducing power supply ripply and any 60Hz picked up.


My thought was to include any R/C filtering in the proceeding/mixing stage so I can keep added resistor noise to a minimum. Although, with a series grid resistance of 5k1, tube noise might be the dominant noise. I haven't figured for it.
Oh wait, I'm sorry, scratch that. You were talking about adding the 5k1 in parallel with the output coil. My B.

I did want to double check something though, would any added grid leak resistance (ie 470k) essentially be in parallel with the 2k5 impedance of the output coil?



That's what I got too. Well, depending on the AC load, between +30dB to 31dB.

I was just throwing out ideas and my initial choices for you to experiment with. I chose 2.2uF for essentially the same reasons as you chose 1uF. I was coming up with something like 1.6uF and so 2.2 was the next higher readily available preferred value. Suck it and see.

I don't think the position of the filtering makes too much difference other than hum suppression has to be after and I think HF roll off is better after ( to suppress interference.


The 5.1k ( or whatever) in in parallel with output coil as is the 470K. Neither of these will contribute much noise.

The 470K in parallel with the coil will make no difference. The 5.1k does, but that is why it's there. Here the is frequency response for that arrangement. Take this with a large pinch of salt as I didn't include the driver or tank responses.

After a little thought I think the way to measure the tank response would be the same way that you do speakers i.e take the averages in bands. Could be done with a PC and a bit of software. I must add that to my growing todo list (which has a stand-alone reverb in it, all solid state by the way, I have the PCB here but no time...)

(I'm having a hard time with adding pictures as the preview during edit seems to have broken so I hope this works)

[Helmholtz]

Sorry, but meanwhile I am getting confused.

Isn't the main reason for using high impedance/current drive to achieve a more linear tank response? In his book Zollner shows the frequency responses for current drive vs. the typical Fender "impedance matching" drive. The current drive is far superior with respect to frequency linearity and response bandwidth. The transformer drive circuit instead produces a mid peaked response.

Now when I see all these proposed cut-off/shaping measures, I suspect that this benefit gets lost, resulting in an overall frequency response not much different from the typical transformer drive circuit.

[SoulFetish]

Sorry, but meanwhile I am getting confused.

Isn't the main reason for using high impedance/current drive to achieve a more linear tank response? In his book Zollner shows the frequency responses for current drive vs. the typical Fender "impedance matching" drive. The current drive is far superior with respect to frequency linearity and response bandwidth. The transformer drive circuit instead produces a mid peaked response.

Now when I see all these proposed cut-off/shaping measures, I suspect that this benefit gets lost, resulting in an overall frequency response not much different from the typical transformer drive circuit.

Perhaps. But I, lile yourself, like the idea of a nice even response curve across the the spectrum.
I do think this circuit would benefit from trimming the highs and lows, which is where most of the discussion is centered around (IMO).

[SoulFetish]

. ...Suck it and see.

Uhhhhhh... do I have to?
(I feel like I need to go tell an adult )

[SoulFetish]

What am I doing wrong here?:
After fixing my friends Silvertone 1474 and getting it ready, I installed this reverb circuit. But I'm getting strange results. First, Here is a schematic of the original amp and a schematic with 6U8A driver/recovery amp. -



So, here's what's going on...
With the reverb tank connected, I was able to get and control the reverb level. The "reverb" part sounds really nice with a nice mix and decay. But I noticed that there is a serious lack of output from the amp. At first, I thought I had wired the speakers out of phase by accident (It's a stereo amp with 2 output transformers - each going to a designated speaker and sharing a common). But, turns out, that wasn't the issue. Interestingly, with the reverb control set to minimum, there is no reverb signal but the output is still very weak. If I disconnect the reverb return rca cable, leaving the "send" rca cable connected, I still get the same symptom. Only when I pull both rca cables (disconnecting the tank), the amp performs normally the way you would expect it to with full output!
Can you guys see something I missed? I mean, it either sounds really attenuated or as if I'm getting phase cancellation or something. I would almost think that I might have to find another stage location to mix the reverb signal back in, but I'm still getting the problem with no return signal present. I'm kind of stumped here. Any thoughts?

[Enzo]

Have you scoped it all to see if the amp is making RF oscillation? That often results in a weak signal.

Any chance one of your cables is wired in reverse? A hot/shield reversal?

SO both cables have to be pulled to make it stop. What if you leave the cables but pull the new tube?

Both B+ A and B in your addition are the same node as the input stages. Your high gain reverb drive shares B+ with the input stages and that can couple the stages causing instability, which brings me back to RF.

MY working theory is you need to make an isolated B+ node for this.

[nickb]

Did you accidentally miss-wire the tank to the the hot side of the 1uF cap?

[SoulFetish]

Any chance one of your cables is wired in reverse? A hot/shield reversal?

Almost certainly no, but I should try some other cables in case the cable is the issue

SO both cables have to be pulled to make it stop. What if you leave the cables but pull the new tube?

I'll try that and report back.

Both B+ A and B in your addition are the same node as the input stages. Your high gain reverb drive shares B+ with the input stages and that can couple the stages causing instability, which brings me back to RF.

MY working theory is you need to make an isolated B+ node for this.

See, I was wondering if I was going to be able to get away with that at the very beginning. there are space/layout constraints and I was trying to keep my footprint small with these changes. I doubled the two preamp coupling caps as an added measure. But ultimately I was prepared tap off the B+ and run a designated dropping resistor and decoupling filter cap for the 6U8 tube. I'll try the quick steps first, but I think you may be right.

[SoulFetish]

Did you accidentally miss-wire the tank to the the hot side of the 1uF cap?

definitely not, the amp is basically wired point to point. So the 1µF cap is soldered directly to pin 6 (plate). Then to a 3 terminal tag strip right next to the input/output rca jacks.