Normal Channel mods for Twin Reverb

[jasjr]

Hello all.

I recently completed a Twin Reverb build from a Mojo kit for a friend of mine.
He is primarily a clean player (very Garcia influenced), and generally plays only the Vibrato channel on his Fender amps. Since this is not a vintage amp, it seems an excellent opportunity to tweak the normal channel a bit and come with a useable alternative for him.

There is LOADS of information available on normal channel mods, but most seem to be geared towards getting more gain or a Marshall-like sound- which is not really what I am looking for here. I have a basic idea of what I would like to do, and would appreciate any feedback or input regarding my thoughts.

Things I have in mind:

1. Reverb on both channels.

2. Change tone stack cap values.
Owner sets his Super Reverb treble to 9-10, mid 3, bass 0-1.
I'm thinking .047(possibly.022)-.022-330pf tone caps- thoughts?

3. A switch for variable NFB in the ground switch position.

4. Lastly- I had bookmarked a link to a site that detailed some of the mods done to Jerry Garcias Twins (thank you Chief Mushroom Cloud), but the content that detailed those changes has been removed and replaced with some very broad and generic statements. If anyone has knowledge of some of those mods I would be greatly appreciative!!

Thanks again to all who contribute to this site.

[pops]

Reverb on both channels is easy, just move the 1st channel wire from it's present location to where the reverb channel pre is. be sure to use the two 220k isolation resistors one for each pre.

.022 caps in the tone, i love it and do it all the time.

can't help on the third part

[rp]

Nice build. Tweed/Marshalizing the normal channel is pretty trite at this point but it's pretty useful too, better than doing reverb in both or tweed + reverb IMO. It's nice to have a dry channel. As it's a clone there really is no point building it w/ the useless normal channel, I wouldn't think twice and just go all the way and Tweed/Marshall it. Here's this if you haven't seen but just google 'fender normal channel mod', check the hoffman forum. Never tried the 220K plate resistor change, seems a bit too much. Let us know what you think.

I wouldn't use those 715/6 caps but that's just me. Too much bright/clear on the reverb amps sounds metalic/clangy. It's good you used CCs and discs. I'm kind of into 225s now, but whatever, it's not like you can hear caps

pdf from http://blueguitar.org/schems.htm

[Roe]

for the normal channel I like a 100ka mid pot, slightly reduced coupling cap values for tighter bass and a switchable bright cap (100pf/5nf)

[Stevem]

How about changing that 12ax7 to a Pentode?

[tictac]

If I ever get another Fender 2 channel reverb amp I'd like to make the Normal channel a JAZZ channel by rewiring it to a Benson preamp.

The Benson was an obscure amp designed with Howard Roberts well known studio guitarist....

I've redrawn one of the preamps below...

You move the 220K mixing resistors from before the P.I. and insert them between the preamp out and the reverb circuit. Then you've got reverb on both channels...

TT

[Firestorm]

Reverb on both channels is easy, just move the 1st channel wire from it's present location to where the reverb channel pre is. be sure to use the two 220k isolation resistors one for each pre.

Even easier is to run a wire connecting the plates of the second triode on each channel, then disconnect the wire to the Normal Channel's 220K. This parallels the plate resistors so the two signals mix across them.

[tictac]

Yes the easy wire both channels is a Torres trick that's been done for decades.

Both Dave Funk and Kevin O'Connor have commented on why this is not a good idea but I can't remember the details of why but it may have something to do with making the channels interactive and changing the tone of both.

Installing the mixing resistors after each preamp was the recommendation from those guy and so is the one I use personally. One thing about this method is you can adjust the individual tone of each preamp by having diffrent coupling capacitors for each preamp out...

TT

[Firestorm]

Actually, this is KOC's trick. The one Dave Funk commented on is if you tie the channels after the caps without mix resistors.

[tictac]

Not sure why we're splitting hairs here.... the one wire reverb trick was in the Torres Newsletter he used to send out back in 1993; long before KOC ever wrote his books....

In any event if you're comfortable tying the plates from two different tubes together that's fine, I've done it, it works... but I do believe there will be some interaction between channels; similar to the interaction between channels of a Tweed deluxe where the volume control of the unused channel effects the tone of the channel in use...

Try playing with the normal channel volume control while plugged into the vibrato channel and see if it has any effect....

TT

[Firestorm]

Not sure why we're splitting hairs here.... the one wire reverb trick was in the Torres Newsletter he used to send out back in 1993; long before KOC ever wrote his books....

In any event if you're comfortable tying the plates from two different tubes together that's fine, I've done it, it works... but I do believe there will be some interaction between channels; similar to the interaction between channels of a Tweed deluxe where the volume control of the unused channel effects the tone of the channel in use...

Try playing with the normal channel volume control while plugged into the vibrato channel and see if it has any effect....

TT

Sorry you think it's splitting hairs, but for the sake of future searchers, the mod Torres published in 1995 is completely different from the one O'Connor published in 1995. I suppose it's possible Dan knew how to do it properly in '93 and forgot two years later, but the mod Funk was inveighing against in 1996 is the one from the Torres book. O'Connor's common anode load idea goes back at least to the thirties and isolates the channels extremely well because the grid/plate impedance is huge.

For the record, I mostly do it your way, too, not only for the coupling cap versatility, but because you can couple different tube types, which does not work so well with the "tweed mixer."

[jasjr]

Thanks everyone for all the replies.

I'm still mulling over which direction to head on this.
The Benson preamp sounds intriguing.

As for the reverb issue- wow! I'm definitely getting an education on that one.
I've done the mod several times, but it was pre-internet in the early 90s. Information was a lot harder to get then, and usually came to me hand drawn or on a crappy xerox copy of a copy! So I don't really remember exactly which way it was done. I do know it worked fine and I don't remember any problems or channel interactions. The thought it may have come from Torres is scaring me a little!

The eventual owner of this amp plays extremely clean, and always with reverb, so it just seemed like a nice option to have reverb on both channels.
I certainly don't want to compromise on the vibrato channel though. I'll have to sort it out and maybe just experiment and see what works(or doesn't).

Thanks again for all the suggestions and information.

[tictac]

OK then, as I further mull over this I think what's happening is I'm getting the Torres mod and KOC's mod mixed up. Both use one piece of wire. What finally clicked in my ever cluttered brain was the Torres article mentioned using a tiny piece of wire, like an inch and a half long. The reason is what he was doing is removing the lead going from the Normal channel out to the 220K P.I. mix resistor and jumpering the Normal and Vibrato channel AFTER the coupling caps and with no mix resistors.

So check me if I'm wrong but I think the three reverb on both channel mods are...
1)Torres mod: jumper both channels after the pre-out coupling caps with no mix resistors.

2) Dave Funk mod: same as Torres except add 220K mix resistors after each coupling cap.

3) KOC mod: remove/ disconnect Normal channel coupling cap; wire the 100K pre-out plate loads together. The vibrato channel pre-out coupling cap becomes the pre-out for both channels.

Does that sound about right? Sorry for the confusion I may have created; hope this sorts it all out...

TT

[Firestorm]

I think that sums it up. The KOC version dosen't require shoehorning 220K resistors on the end of the 3M3, but seems to require both connected triodes to have the same mu. Either way, you mostly wind up with two identical channels, which is of limited use (except you can bridge them), so then you might want to redesign one of the tone stacks for some flavor. In a stock Fender, the affected triodes share a cathode R and bypass cap, so splitting the cathodes and giving them different tones, gains or bias points is versatile. The mod with the moved 220K mix resistors would let you use 12AY7 in V1, so you could have one channel clean and one a little less so.

[drew]

4. Lastly- I had bookmarked a link to a site that detailed some of the mods done to Jerry Garcias Twins (thank you Chief Mushroom Cloud), but the content that detailed those changes has been removed and replaced with some very broad and generic statements. If anyone has knowledge of some of those mods I would be greatly appreciative!!

See the PDF file linked in the first post in this thread: http://www.rukind.com/forum/viewtopic.php?f=419&t=14350