Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fiftynine on February 05, 2011, 02:42:38 pm

Title: 1-Tube Reverb Circuit
Post by: fiftynine on February 05, 2011, 02:42:38 pm

I've stuck it in the AB763 Deluxe Lite and it's beautiful. Just the right voice and plenty of reverb for me. But, it would be nice to have a bit more on tap and also, shorten the decay.

If I disconnect the grounds of both pots (or otherwise send the signal straight through), I get a bit more reverb but not a massive amount. Are we at the limits of what 1 x 12AX7 can do? And it seems the dwell knob is more of a ground shunter than a trail shortener. Shunt more to ground and there's not enough reverb at the other end. Is the only real way to get a tank with a shorter decay?

Even with loads of gain on tap, isn't the decay time a function of the physical damping in the springs not just juggling with the signal strength in and out?

Title: Re: 1-Tube Reverb Circuit
Post by: sluckey on February 05, 2011, 02:57:37 pm

If you want to shorten the decay time, you need a shorter delay time tank. The dwell control used with a beefy reverb drive such as parallel triode 12AT7 or 6K6 can increase the decay time by providing a big enough signal to bounce back and forth on the springs (taking more time) before fading/decaying to a point that you no longer hear it. I don't think a single 12AX7 triode can drive the spring with that much signal though.

Title: Re: 1-Tube Reverb Circuit
Post by: fiftynine on February 06, 2011, 05:29:44 am

You're right as usual, sluckey. Thing is, I was half way there again. I'm learning.

Title: Re: 1-Tube Reverb Circuit
Post by: tubenit on February 06, 2011, 07:05:45 am

There are some modifications listed to increase reverb in the ARCHIVES for one tube reverb here:

http://www.el34world.com/Forum/index.php?topic=7957.0

Next time I try a one tube reverb, I think I'll experiment with this 6BM8 idea.

with respect, Tubenit

Title: Re: 1-Tube Reverb Circuit
Post by: fiftynine on February 07, 2011, 04:23:56 pm

I've tried the tweaks at the link. The 6BM8 idea looks good for the next build.

Lifting the earth on the return pot doesn't gain that much. Maybe 10%. Lifting the earth on the send pot gives another 10% and that's enough. My ears might be playing games on me but I think the reverb looses a bit of brightness and clarity. I can't think why.

Maybe that circuit just needs to see a bit of resistance in and out to sound it's best.

Title: Re: 1-Tube Reverb Circuit
Post by: DummyLoad on February 07, 2011, 05:20:48 pm

12DW7 or as already suggested 6BM8. others to consider, but not currently in production; 6CY7 & 6DR7, 6GW8 (ECL86) ECL83, ECL84, and ECL85. all are 9pin. check specs first though - may have issues in that some tubes can't take 400V+ to the reverb driver.

--ISO

Title: Re: 1-Tube Reverb Circuit
Post by: TIMBO on February 08, 2011, 12:03:36 am

Hi guys, A quick question, I have at the moment my reverb driver connected to node C producing 365v, is this considered to be too high for a 12AT7. If so is it ok to add a dropping resistor as seen in your schem ( would it also need filtering as well) Thanks

Title: Re: 1-Tube Reverb Circuit
Post by: Willabe on February 08, 2011, 12:22:00 am

I got this one wrong not long ago and I should have known it, I had a BF SR for years.    :rolleyes:    Any way the AB763 reverb amps take the verb trans. B+ from the screen B+ node, so 440v dc or so on the 6L6 amps and the 6V6 DR, 410v dc or so.

Look in Dougs schmo library.


          Brad         :smiley:

Title: Re: 1-Tube Reverb Circuit
Post by: TIMBO on February 08, 2011, 01:36:10 am

Thanks Willabe,I get a bit worried when you look at the data sheets and it tells you that the max voltage is 250v. I know that there is room to increase the voltage but i didn't think you could go that much.Thanks :smiley: