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FDP Forum / Fender Amps: Vintage (before 1985) / Reverb and Tremolo on both channels

roadhog96

USA / CT.

Dec 24th, 2015 02:01 PM   Edit   Profile   Print Topic   Search Topic

I've read that there's a better way to do this than the Dan Torres mod something to do with the two channels being out of faze. I found this other mod online. Is there any truth to this with one being better than the other?

Scroll with side arrows to see both layouts

Reverb and Tremolo mod layout

(This message was last edited by roadhog96 at 04:20 PM, Dec 24th, 2015)

guitarcapo

U.S.A.

Dec 25th, 2015 06:45 AM   Edit   Profile   Print Topic   Search Topic

That's the mod I've always used. Pretty simple. both channels are in phase after the mod and you can jumper the two channels as an added bonus.

roadhog96

USA / CT.

Dec 25th, 2015 05:28 PM   Edit   Profile   Print Topic   Search Topic

guitarcapo, which one are you referring to the first one were you relocate the blue wire or the second mod were you remove the .047 cap and add a jumper wire between the 2 100K resistors?

6L6
Contributing Member
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San Francisco, CA

Dec 25th, 2015 10:40 PM   Edit   Profile   Print Topic   Search Topic

Dan was my amp tech for 25 yrs. I had him do his mod on all of my vintage Fender amps and they always worked great.

6

guitarcapo

U.S.A.

Dec 26th, 2015 12:46 PM   Edit   Profile   Print Topic   Search Topic

The second.

amphead4
Contributing Member
********

Cincinnati, USA

Feb 4th, 2016 08:48 AM   Edit   Profile   Print Topic   Search Topic

Sorry to comment so late on this one.

I don't like these methods. If you look at how 2 channels are normally mixed, for example the Bassman AB165 circuit, the signal from each channel flows through a 220k mixing resistor which are tied together to feed the mixer stage before the PI.

I haven't tested this against the other two methods mentioned above but it seems right to me to continue to mix channels through the resistors to minimize one signal modulating the other. This approach can be applied to the guitar amps by using mixer resistors before the reverb driver.

I need to spend the time to draw this.

Steve Dallman
Contributing Member
*********

Merrill, Wisconsin

Age is just a number...mine is big
Feb 4th, 2016 11:29 AM   Edit   Profile   Print Topic   Search Topic

I've done this to dozens of Fenders over the years, and the results have always been great. If there's a drawback, I've yet to see one.

I would also revoice the normal channel so both channels didn't sound identical after the equal weighting. Most customers used an ABY switch after the mod to access each channel or both. They all loved the mods.

I've used two or three slightly different wirings over the years, and they all sounded the same.

amphead4
Contributing Member
********

Cincinnati, USA

Mar 2nd, 2016 06:11 AM   Edit   Profile   Print Topic   Search Topic

One of the ways to do this is to simply tie the 2nd stage plates together and then break the audio path of the normal channel by pulling the coupling cap, as describe by O'Connor in TUT vol. 1 and others on the Internet. Signal from both channels then passes through the coupling cap of the 2nd stage of the vibrato channel.

Effective but the plate load resistors of these stages are now in parallel.

The signal at a plate is created by the plate current change across the plate load resistor. The plates are now tied and consider when signal is routed through a single channel. With two identical plate load resistors in parallel, for the same plate current you would have half the signal available at the coupling cap.

But I suppose you wouldn't have the same plate current as before because the stage now operates on a different curve (the load line shifts).

I haven't had time to test this but it seems this method would shift the operating points of the second stage of both channels. Does anyone have thoughts on this?

TheWayfarer

Raleigh, NC USA

Mar 28th, 2016 05:40 PM   Edit   Profile   Print Topic   Search Topic

Any more thoughts on this? I've tried the method of tying the plates together and while the normal channel sounds good, the vibrato channel seems to suffer a little. Hard to describe it, but it seems off. interested in how to keep the mixing resistors in there and keep the signal path as close to original as possible

amphead4
Contributing Member
********

Cincinnati, USA

Mar 29th, 2016 07:21 AM   Edit   Profile   Print Topic   Search Topic

I haven't had a chance to get back to it.

TheWayfarer

Raleigh, NC USA

Mar 29th, 2016 08:30 AM   Edit   Profile   Print Topic   Search Topic

I'm going to give it a shot by moving both mixing resistors after the coupling caps and see what happens. Read a few positive reports on doing it that way.

FDP Forum / Fender Amps: Vintage (before 1985) / Reverb and Tremolo on both channels




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