65 DRRi Normal channel to 68 Custom DR specs

I am still waiting for someone to post a circuit drawing for the '68. I'm curious what the differences are between the two amps.

 

xjazzy, I have the schematics here. The changes to that amp are minimal, imho....much less 'drastic' than what techs have been doing to Fender amps for years.They changed very little in that amp....the Normal channel has the same gain as in your DRRI, and the tonal differences are not as far into the tweed zone as I would like.
If you have a two channel Fender Reverb guitar amps.....vintage or Reissue...that Normal channel can undergo more effective and noticeable changes if you want to move it toward higher gain and tweed tones.

Jhundt, I'll see if I can email these schematics to you. I can't find a way to save them to my computer and thereby post them here.

 

What are the normal channel higher gain + tweed tone mods?

 

waparker...here are the changes that were made to that Normal channel to make it into the Custom channel.


1) the mid tone cap was changed from a .047mfd to a .022mfd.
2) In the '68 Custom, there is an 18K resistor between the mid cap with the 6.8K resistor that goes to grounds its 6.8K resistor and the bass pot connection. Thsi 18K resistor must bump up the mids???? IF so, then that is the only gain change I see in that channel.

They could have gone farther in bumping up the gain and changing the tones, but that would have demanded a new board because it would require placement points for one more bypass circuit...for the cathode resistor and cap.
When I change a Normal channel in a two-channel Fender amp, I bump up more gain and make a bigger 'tweedification' of the tones in that channel. And, yes...even on the modern PCB amps, that channel can be more radically changed. For Fender to do it in their situation would have required, as I noted, a new PCB.

 

I don't quite follow (2).. They replaced the 6.8k mids resistor with 18k? Like the Allen raw control?

And you are saying since they are using the same PCB, they cannot separate the normal channel from the vibrato's 820 ohm // 25uF (changing it to 1500 ohm?) cathode resistor and run a smaller resistor for normal channel?

I wonder, if I were to sub in some larger resistor for the mid 6.8k resistor, and just ran two contrasting tubes (I have 12BZ7 in V1 and 5751 in V2 currently) if that would make up for the cathodes being joined on the pcb.

 

IF you want to change the channel, change it. Imho, it is best changed by separating those cathodes pins 8 on V1 and V2. Establish a new bypass circuit for pin 8, V1. Maintain 'stock' gain in V2 by installing a 1.5K bias/cathode resistor on pin 8, V2. Change the tone caps to better yield tweed tones. Change the slope ressitor to better yield mids.
I am sure that there are a couple more tricks that someone does, but these changes get me where I am wanting to go with these amps.


These changes are more difficult than when working on handwired vintage amps...but far from impossible in the BF/SF Custom RI's...and the CVR',
6G16 RI.

There are other things to do while one is in there....NFB loop variability, for one.

 

OP:
Basically one cap value change(from .047 to .022) and one extra resisitor(18K) in the Custom channel tone stack; 1.5k ohm NFB resistor instead of 820 ohms; the "Fritz Mod"(one jumper wire and lose one resistor), and a Celestion V-Type speaker.Can't post the schematic (PDF file) here, but it is available from Fender Customer Relations via Email for the asking.

IMO, the speaker has a lot to do with the voice of the '68 CDR. Sort of Greenback-ish, but with a tight lower end and plenty of sparkle. I A/B-ed my Custom DR's Vintage channel and my bone stock '75 SFDR's Vibrato channel thru the CDR's speaker/cab and they are pretty close in tone, with the '75 being slightly cleaner and a little more defined. I'm looking to replace the original speaker in the '75 and I think I'm going with the V-type instead of going thru the usual Weber/Jensen/WGS thing.

 

Schematic sent....

 

I'm not even sure it's worth the mod. I did the reverb vibrato mod for both channels on my '66 DR several years ago and decided I didn't really want it on both channels. Later took it out.

 

Well, jumping the channels works pretty well with the '68 Customs, with their mid-heavy Custom channel. I can plug into the Vintage channel and use the Custom channel as a mid control, as well as other EQ options. That being said, I usually don't bother jumping the channels as I like the Vintage channel just as it is.

 

I modded my Twin Reverb like this: reverb & vibrato (trem) on both channels; .022 mid and bass caps, 47k slope resistor (didn't have a 56k at hand). Pin 8 on V1 and V2 both got their own 1k5 resistor and 25/25 cap for the cathode instead of the shared 820 resistor.

This gives a slightly more mid-rangy tone, less scooped. I like it, just to have a (somewhat) different voice. But it's not dramatic. I want to try changing the mid-pot from 10k to 25k for even more noticeable change. I am also considering raising the 100k resistor in the B+ supply of V1 to lower the anode voltage for a more "brown" sound. Or even adding an extra gain-stage with a FET to steer towards Marshall territory. It's great fun!

I did the 'fritz mod' but still let both channels join thru each their own 220k channel divider resistor, but still the normal channel is considerably louder on the same volume setting (no big deal).

I turned the Master Volume into a variable NFB with a 25k pot in series with the 820 NFB resistor (I have to lower that value, it's pretty much all or nothing now). Less NFB loosens up the amp indeed (no breakup with a Twin though) but also adds a considerable amount of hiss, so I only use this in live situations where the hiss is not noticeable.

All this - except the variable NFB - can of course be done to a '65 RI Deluxe Reverb!

 

LIttle Jay, if you go back in that TR, change that V1 pin 8 cathode resistor to the 820...this pumps the gain a bit more.

 

On a slight sidetrack, where are you guys getting '68 Custom schematics? I can't find one for my Princeton, and Jeff Bielke at Consumer Relations was friendly and polite, but he doesn't have one either.

I'm not sure where else to look if Fender don't have it!

 

Will try that, thanks!

 

That's odd. I got one for my CDR there, though it took a couple days as they first had to get one from Tech Support, or somewhere.