how to get more current gain out of this 12BH7 stage?

[makepeace]

helloo there.

what would be the best way to get more current gain out of this 12BH7 reverb drive stage? i'm driving a 2k5 tank.



thanks,

jono

[Chuck H]

There's nothing wrong with that design. You could raise the voltage a little. Max Vp is 300V (and you can probably exceed that by a little too). But you should have plenty of drive for a reverb tank provided:

Your grid drive voltage is sufficient
Your transformer impedances are correct
Your cathode bias is such that the tube is operating in the center of it's range (requires a bench test)

[makepeace]

Thanks Chuck.

I realize there is nothing wrong with that design, it works very well! In fact I have more reverb than I know what to do with. I'd just like the option for a little more y'know?

Will mess around with the cathode voltage and see if I can get the 12AX7 triode before it to drive a bit more.

Thanks again, and cheers!

Jono

[Enzo]

How did you determine you needed to increase the drive into the reverb pan, rather than increasing the gain on the return side? And just what do you mean when you specify "more" reverb? Reverb louder with respect to the dry signal? Longer lasting reverb sound? Those two things are not done by driving harder. Changing the mix resistors in the forst case, and getting a longer delay reverb pan in the second.

[makepeace]

Enzo, from what I've read about designing reverb circuits (and there really isn't that much information available), it is best to drive the tank as hard as possible (obviously within the limits of what produces desirable results which is affected by many factors).

Not sure if you read all of the posts in this thread, but as I say, the reverb that I have now is great - I don't 'need' any more drive. I just would like to try squeeze some more 'depth' out of the circuit (don't know if I've used the right word there, but it definitely is possible, as I noticed as a result of a lowered plate V to the 12BH7's that it had a noticeable effect on the reverb sound). I have an adequate mixing circuit for the wet/dry mix and I also understand that the decay cannot be changed by circuit tweaks, but thanks for reiterating for me.

I'm approaching this systematically, starting with the 'verb tanks direct drive circuit, as from what I've read thats where you really need to drive the tank. Once I'm happy that there is nothing more I can do there, I will move on to the preamp and recovery triodes. I realize that changes to surrounding circuitry will have an impact on that which has supposedly been 'optimized', but I will use an iterative approach.

Regards,
Jono

[tedmich]

check Elliott's reverb drive circuit
Spring Reverb Unit For Guitar

[mooreamps]

There's nothing wrong with that design. You could raise the voltage a little. Max Vp is 300V (and you can probably exceed that by a little too). But you should have plenty of drive for a reverb tank provided:

Your grid drive voltage is sufficient
Your transformer impedances are correct
Your cathode bias is such that the tube is operating in the center of it's range (requires a bench test)

Well, you know me... I'd run the plates at 450V, just to see what it would sound like.... I am also thinking those spec's on the conservative side for voltage.... and yes I understand about the max plate dissipation....

-g

[Chuck H]

Sure. Hit 'em hard. Guitar amps have been doing it for decades. And these are devices that are often run flat out, getting away with over spec voltages.

I don't think more wattage to the tank is an issue though. The circuit, tube and voltages already present should be plenty provided the drive signal is high enough and the tube is biased correctly. And as mentioned, after that there is the recovery and mixing stage that can be altered also.

IIRC one thing I noticed on the original schem (from another post) was a 10uf input cap. Again, IIRC, I did the math at the time and found that value would give you about a 500hz knee. I did this same thing on a reverb amp I built and I think it's a little weak in the bottom end. There are other limitations that may be at fault too, but I do intent to try a 22uf as the input cap just to see how it sounds. This should substantially increase tank drive too since there is more current in the low end. Current drives the transducers that shake the springs, so...

[jmaf]

Adding a 250uF capacitor in parallel with the 390 OHM resistor will add a ton of gain. But like others have mentioned I'm not sure if that's what you need.

[J M Fahey]

Again, IIRC, I did the math at the time and found that value would give you about a 500hz knee

And for good reason.
Too much bass driving springs creates ugly boingy spikes, imposible to remove afterwards.
There are 2 classic ways to deal with that:
1) small input caps at the driver amp. , such as in Twins and even more drastically in Music Man.
2) straight current driving the coils.
Being highly inductive, driving voltage rises linearly with frequency, or to say it in other way, rises at 6dB per octave.
So you end up with a lot of highs, and little bass, even if at first sight you don´t see frequency shaping.

[Chuck H]

Too much bass driving springs creates ugly boingy spikes, imposible to remove afterwards..

Agreed. I remember a Torres mod for BF amps that involved increasing the coupling cap to the reverb driver from 500pf to some larger value I can't remember. I tried it (with poor results) some twenty years ago. You almost couldn't get a clean reverb effect from it. I've built several revrb circuits since then. My favorite so far was a reverb "amp" that filches power from the host amps output to drive the pan. This is the circuit I think has too little bottom end in the actual reverb effect. So I have heard the phenomenon. There is a point of diminishing returns with raising the knee frequency of the driver. Too little bass doesn't sound good either. There is a happy amount that makes for enough bottom end and maybe a pinch of good chaos. Like that more pronounced BOING in good surf tone. Anyway, it's a place to experiment. The smaller cap value can always be put back in.

[mooreamps]

substantially increase tank drive too since there is more current in the low end. Current drives the transducers that shake the springs, so...

Excellent point....

-g

[mooreamps]

There is a happy amount that makes for enough bottom end and maybe a pinch of good chaos. Like that more pronounced BOING in good surf tone. Anyway, it's a place to experiment. The smaller cap value can always be put back in.

....or "switchable" perhaps ???????? you might be on to something.....

[Enzo]

Yes, indeedy, I read ALL THREE posts before my own.


There may be room for more drive or not. It is one thing to say we should drive the pan as hard as we can within its limits, but a separate item to say that we need more than we already have.


When you want "depth" you either want it to last longer - longer decay - which is a mechanical result of the pan design. Or we want it stronger sounding, which means either a higher mix-in percentage or just more level. And of courwse the tone of the reverb is affected by indeed such things as drive, freq band sent to the pan, tone shaping in the recovery, and as mentioned power wsupply voltages.


A fun old trick on Fenders was to take the pan output and plug it into the usually unused "Normal" channel input jack. Now the normal channel becomes the reverb controls. That might suggest some options.

[makepeace]

Hi guys, I've been fiddling around with this again. Realised I haven't checked this thread in a while - thanks for all of the useful posts!

I was just curious about something, Chuck, you mentioned a 10uF input cap, but there isn't one? Here's the schem:



Are you referring to the 50uF cap on the input of the tank?