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Fender Twin Reverb II mods


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5/7/1997 12:46 AM
Mike Schulze
Fender Twin Reverb II mods
Just picked up a Fender Twin Reverb II for $350 and it's very clean. I bought it because I fell in love with the clean channel's tone. But the overdrive bites. ANyone done any mods on this amp? Torres has one...
 
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5/7/1997 1:29 PM
Jim Salman

You got an excellent deal!  
 
As much as you might be enjoying the clean channel's tone, it actually can be improved upon. The bright pull switch circuit is pretty lame - even when it is off, it degrades the signal (I think there's a 820K resister which the signal always passes thru. I strongly suggest changing the brite circuit over to the original blackface/silverface type (but use a larger value bypass cap, maybe 250p instead of 120p - I found that 120p is maybe too subtle on this particular amp). This change should make the clean channel  
noticeably brighter and livelier.  
 
Another change you really should make is to convert the output balance control over to a true  
bias control so you can properly set bias for a matched set of 6L6/5881's.  
 
Another mod you might want to try is to convert the phase inverter circuit over to Marshall specs  
(replace the 820, 6.8K resistors in the common cathode circuit with 470, 10K, replace the 330k grid resistors with 1Meg and place a 47p cap across the plates.) This might also improve the tone somewhat.  
 
Yes,the overdrive channel leaves a lot to be desired. It uses only 3 gain stages and the tone controls come before the distortion, rather than after, causing a muddy response. There is an unused gain stage in one of the 12AX7's; you might want to look into completely changing the circuit to use 4 stages and have the tone controls come at the end. Also, I would consider splitting V1 so  
that each of its two stages are dedicated to separate channels (the stock circuit has the two stages ganged together and shared by both channels), and have the overdrive 1st stage use a lower value bypass cap than 25u. Some terminal strips may need to be added to the chassis in order to implement these mods to this channel.  
 
Now, if only I can locate one of these baby's for myself...
 
5/8/1997 12:07 AM
Mike Schulze

I called Paul Rivera, who designed this amp. Understandably he didn't seem to enthusiastic of mods to this amp, although he told be how to change the mid boost freq from 300 Hz to 500Hz by replacing a 100k resistor with a 47k for a more "marshall" sound. He was not at all enthusiastic about Dan Torre's mods and said he had read lukewarm reviews of them.  
 
Paul told me when they designed the amp they were thinking of a Larry Carlton type of overdrive, and in fact, LC has two of these amps. Rivera's tack seemed to be if you feel the need to do such extensive mods on an amp then you probably just have the wrong amp.  
 
I like the clean channel as is, and I think I will get some LC recordings and try to duplicate the sound so I know what the overdrive is supposed to sound like. I always intended to use stomp boxes with this amp anyhoo - I just bought it 'cuz a 105 watt CLEAN fender tube amp for $350 was a deal. I am trying to convince someone here to sell me a beat up one for $100. I'll restore it and have 210 watts of stereo tube power. POW!!!!!!
 
5/8/1997 3:00 PM
Jeff Lacio Re: Fender/Rivera amps & OD
this is really interesting; is Paul THAT available? i've got a complete set of Rivera-era Fenders (1982-83 models), and i've had a question or two regarding their design -- ESPECIALLY the not-very-good (IMO) overdrive circuits of the Deluxe Rev II, Concert 60, and Twin Rev II. The Super Champ OD is GREAT; and the Princeton Rev II (though similar to the SC) is only fair. Anyway, i find the OD of the DRII, Concert, and TRII to be rather "sterile" and not very smooth or "creamy". (god, now i need a smoke). the OD tends to be like a bad solid-state distortion pedal from the late 60's i once owned (and got rid of). the clean sound is GOOD -- but not as good as a 60's blackface (once again, IMHO). i've often wondered if the optical switching devices that are used in the signal chains of BOTH the clean & overdrive channels are part of the problem. matter-of-fact, i've slighty re-designed the of the DRII to eliminate some of this; the clean got better, and so did the OD -- but still not great, smooth OD.
 
5/8/1997 7:50 PM
Michael Polutta

I have a Concert that I have modded extensively. For starters, the parallel first stage is a total waste of a triode. You only need one triode to drive both channels. Second, I moved the tone stack on the distortion to the end of the line (just before the master). Also, the unused triode (V4a or b, I don't recall) is now used. My distortion channel is 4 stages (counting the first, shared one). The yucky bright switch config is changed to a blackface-style. My preference was for 100pF, which is less, not more, than the standard 120pF. I put a 10pF silver-mica cap across the 3.3M reverb mix resistor (again, like a blackface). Of the 3 triodes dedicated to the distortion channel, only one of them has a bypass cap - and that is switchable using the "mid-boost" pull switch. I have a lot of attenuation between the stages (without it the distortion was so buzzy! Full of "hash" in the tone. Not very musical at all), but I really dig the tone now. I'm probably not done tweaking, but I'll live with it for a while. Might try some different speakers, just to see what difference that will make. I'd be happy to share the dirty details of my circuit. Just e-mail me directly.
 
5/9/1997 12:38 AM
Mike Schulze

Paul was kind enough to spend about 10 minutes answering my stupid questions. He said the overdrive was designed to be used along with some power tube overdrive, so I guess the amp would have to be pretty loud for it to work right. It's definitely not to good at low levels. He said you could set it up so you could control the crunch with your guitar's volume knob. Again, I guess we are talking big stage volume here...  
 
I don't think I will call him again, as nice as he was I think I may have inadvertantly insulted him. He's probably had enough of people telling him how they want to modify his designs. (BTW - he has a photographic memory and was quoting me component values right off the top of his head.) He left Fender after CBS sold the company. Apparently he and his boss were really haking the place up. Paul had a 5 year plan which included the reintroduction of the 59 bassman design (which didn't happen until much later). Apparently his boss had angered someone higher up and got the axe with the new management. Paul went along with him and I've read Paul was somewhat shaken - not working for a while after the incident. So anyway there is some history there. One can only admire Paul for succeeding in such a big way with his own company after all that. I wish I could afford one of his amps...  
 
I'm gonna get the schematics somewhere. Your overdrive mod sounds like just the ticket - more gain and pre-eq. How do you compare the sound of your mods on the clean channel to the original?
 
5/9/1997 12:48 AM
Michael Polutta

<>  
 
While I understand you saying that, that is exactly how he "made his name" in the music biz, modifying Fender amps. Larry Carlton and Jay Graydon being among his clientele. BTW, I e-mailed Mr. Rivera asking for the same advice. His reply was short. He said, "There is no mod advice available. Why don't you try one of my amps?" While I understand that he is in business, I found it odd that I was asking for help in WORKING on an amp, and his response was for me to PURCHASE an amp. Kinda missed my point, I thought.  
 
<>  
They're in Pittman's book.  
 
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I like it much better. I think it sounds sweeter, and it really cooks if you crank the volume. Typical loud Fender blues tone. It can get a little muddy, but that is also just like an old Fender. Louder works better with bridge pickups, IMO. Having split that paralleled tube, I still have an unused triode. Kinda running out of room on the chassis, though.
 

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