On-Board Reverb in Parallel

[Bloomfield]

Hi, it's been a while since I've done much with amps, but I've been getting the urge lately to build something new and use up some of the parts that have been hanging around for a while. I was thinking of using an EF86 as the first pre-amp stage, likely followed by a cathode follower-driven tone stack and on into a typical 2 x 6V6 power amp. I've built more-or-less similar before, but this time I'd like to add reverb. Seeming as I'd like the EF86 preamp to be high gain and capable of getting quite dirty, whereas the reverb would ideally be much cleaner, my thought is to use a circuit very similar to the 6G15 in parallel with the 'dry' preamp, and then mix them back together just ahead of the phase inverter. Looking at the 6G15 schematic I see that Fender simply split the wet and dry at the input jack. Should I be able to do similar here? I'd like the input to go straight to the EF86, so avoiding a buffer stage here would be my preference.

Any thoughts?

Many thanks for all of the knowledge shared on this site.

Andy

[Malcolm Irving]

I don’t think there would be a problem splitting the signal to go to two parallel ‘first’ pre-amp stages. You could have a shared grid-leak resistor (on the input jack) followed by two grid stoppers to the grids of the two ‘first’ stages. The input impedances at the tube grids are very high, so there will be no signal loading problem.

Because the shared grid-leak carries current from two tubes, you would need to check its value according to the tube data sheets. For example, if each tube has a maximum specified grid-leak of 2M, then it would be alright to share a 1M.

[Enzo]

On the other hand, it seems odd to me hearing a distorted guitar signal with an undistorted reverb behind it. generally the idea behind reverb is to add an echo effect to whatever sound the guitar amp makes. On the other hand if you like the sound then it is good for you. Isn't important if I like it or not.

[Bloomfield]

Thanks for the replies Ian and Enzo. I figured that it should work with the right grid leak and stopper resistors. It may indeed sound weird, maybe in a bad way, but if so it's easy enough to swap things into a more normal arrangement. I'll give it a shot and see.

Incidentally, any idea why the 6G15 omits the usual 68k grid stop, at least on the schematic I'm looking at?

Andy

[Justin Thomas]

Check the Fender Vibro-King - seems to be a similar idea to what you want to do. Especially the first edition with the EL84 reverb driver. Note: that is a full-out tbree-knob Fender Reverb Unit incorporated into the amp, not the usual BF/SF one-knobber...

Justin

[Bloomfield]

Thanks Justin, that's somewhat along the lines of what I was thinking, using the three-knob reverb circuit, although they still have it stuck on the front end. Looking at it again though, my EF86 would just go in place of V1B and its tone stack would be ahead of the mixer on the dry side. The reverb recovery will probably need another gain stage ahead of the mixer too to bring it up to a compatible level. I'll have to draw it up soon and make sure that things are properly in phase with one another but I'm too tired for that at the moment. Thanks for the suggestion.

Andy

[Chuck H]

Just to add my $.02...

I agree with Enzo. A reverb incorporated into a "system" like a complete amp circuit should reverberate the FINAL sound of AT LEAST the preamp. That said, you would need a mixer, secondary recovery stage. That is how virtually every reverb incorporated into an integral amp circuit has been done. That adds a tube stage beyond the preamp and tank recovery. In other words, you should really make sure you have an extra triode in the amp to do this in case you find that you want to. The drawback to incorporating the reverb early in the preamp stage (ala 6g15 or Vibro King) is that you really (and absolutely) only have good reverb for the cleanest tones. That's fine if you want to sound like Dick Dale, but cumbersome if you want to drive the amp for some dirt. I don't know your intended use of the finished amp so I'm just offering this as something to consider.

[Bloomfield]

Hi Chuck, I know what you guys are saying; it may sound really weird, maybe even really bad, but I'm just trying to get a sound I'm imagining in my head which is not the conventional reverb sound. As I said I realise I'll need another stage ahead of the mixer. I won't have time to sort that out for a while, but I have lots of space (and power supply) to work with so adding another tube isn't a big deal. I just want to see if it does what I want; if not I'll reconfigure it into something more conventional.

Thanks,

Andy

[Enzo]

Absolutely, try anything you like. If the results are pleasing, then it is good.

[Bloomfield]

Thanks for the encouragement; I think I'll go strip the last experiment out of my old Traynor Mark III chassis and get ready to start piecing this together before I build it up in its own new chassis. It'll likely be Christmas by the time I get anything working, but I'll let you know how it goes.

Andy

[Bloomfield]

Success! More or less. As a last bit of tinkering for the year I just got the circuit up and running in prototype form. A bit hard to tell with all the noise, but it seems like with a little tweaking and proper lead dress and shielding, it should do what I want. The pentode (dry) channel is a good bit louder than the wet but I think I can get them fairly even without having to add another gain stage in the reverb recovery side. Anyhow, enough of that for now, it's a new year in five hours and I should get ready for the festivities.

Happy New Year everyone, and thanks for all I've learned on here.

Andy