68 Custom Princeton Reverb tweak!

John N

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Ok ok, we’ve all read lots of advice on these forums - well meaning I’m sure - but it turned out to be useless.
I recently bought a 68 CPR online that I felt was going to be ‘The Fender sound’ in my head that I’ve been wanting as a compliment to my Vox ac15. When it arrived I had an almost immediate letdown. Too much bass, not enough sparkle - the usual 68 CPR complaints.
Googled 68 cpr tweaks and found a posting that suggested replacing 12ax7 in V3 with a 12AY7 as an easy way to revoice the amp. And I’ll be damned it actually worked wonders. No mods. More sparkle, less bass - I can now run the bass up to 5-6! Now all I need is a better Fender voiced speaker.
Am I crazy - could it really be as simple as a lower gain v3 tube?
 

sloppychops

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Interesting. I have the same amp and actually like the darker sound of it, but this looks like a cheap and simple way to experiment with how it sounds. It IS a bit heavy on the bass.
 

John N

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Interesting. I have the same amp and actually like the darker sound of it, but this looks like a cheap and simple way to experiment with how it sounds. It IS a bit heavy on the bass.
For me the perfect Princeton would be a cross between the 68 Custom and the 65 RI. I went with the 68 because I thought I’d rather have more bass than not enough but you can’t dial it out without a mod. I may end up doing the mods but this tube change proves the amp has more to it
 

zhyla

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Exactly how does changing a preamp tube affect the frequency response? I know it's a lower gain tube, so you'll be running the volume knob a bit higher. How does that "tame" the bass response?

I think it's relatively easy to convert the 68 tone stack to 65. Maybe that's what people really want out of those amps.
 

John N

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Exactly how does changing a preamp tube affect the frequency response? I know it's a lower gain tube, so you'll be running the volume knob a bit higher. How does that "tame" the bass response?

I think it's relatively easy to convert the 68 tone stack to 65. Maybe that's what people really want out of those amps.
I can’t answer that question except to say that it did, unquestionably. My understanding is that V3 is part of the reverb recovery gain stage. It’s probably not the ideal solution but I was about to flip the amp. Now I hear the amp has a second chance
 

printer2

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The tube is used for both the reverb recovery and the third stage of gain in the dry signal path. The NFB is introduced in the cathode of this tube. As it has less gain the whole power amp section has less gain. The lower gain should not really change the ratio of the dry to NFB signal. I do not see any reason the balance of the sound should change. That said without trying it I can not say for sure.
 

Tim S

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Ok ok, we’ve all read lots of advice on these forums - well meaning I’m sure - but it turned out to be useless.
Am I crazy - could it really be as simple as a lower gain v3 tube?

Yes, I’d say you’re crazy. Not about the tube, but because you found a lot of the advice here to be useless, but a Google search saved you.
 

Ten Over

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The tube is used for both the reverb recovery and the third stage of gain in the dry signal path. The NFB is introduced in the cathode of this tube. As it has less gain the whole power amp section has less gain. The lower gain should not really change the ratio of the dry to NFB signal. I do not see any reason the balance of the sound should change. That said without trying it I can not say for sure.
Decreasing the open loop gain results is less gain reduction from NFB when all else remains the same.
NFB AA964 Princeton PNG.png

Running with the RCA numbers:
12AY7 gain= (40x100k)/(100k+22.8k)=32.57
Open loop gain=32.57 x 1 x 8.71 x 0.06=17.02
Closed loop gain=17.02/[1+(17.02 x 0.0171)]=13.18
Closed loop gain/open loop gain=13.18/17.02=0.77
0.77=-2.2dB of gain reduction

So -2.2dB with a 12AY7 and -3.8dB with a 12AX7.
 

kuch

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I wish I would have heard this when I had my 68. I finally got rid of it and replaced it with a 65PRRI. I didn't like the "dirty" sound of the 68. I can dirty up the 65 with pedals but couldn't clean up the 68.
I think the stock 10" speaker sounds good on the 65, but I might try something else someday.
 

printer2

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Decreasing the open loop gain results is less gain reduction from NFB when all else remains the same.
View attachment 983102

Running with the RCA numbers:
12AY7 gain= (40x100k)/(100k+22.8k)=32.57
Open loop gain=32.57 x 1 x 8.71 x 0.06=17.02
Closed loop gain=17.02/[1+(17.02 x 0.0171)]=13.18
Closed loop gain/open loop gain=13.18/17.02=0.77
0.77=-2.2dB of gain reduction

So -2.2dB with a 12AY7 and -3.8dB with a 12AX7.
So 1.6 dB, under real controlled conditions it should be audible.
 

Wally

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Using the 12AY7 or a 5751 in V1 might do even more to ‘revoice’ the amp. Changing that tube does not cut the recovery signal for the reverb as does using a 12AY7 in V3. I would change a lot of things before I changed that reverb recovery because I like reverb. Ommv.
Also…and this advice is too late to benefit the OP, if one is looking for the classic BF/SF sound one should not buy a modified amp that is intended NOT to sound exactly like those vintage amps.

but…this could more useless advice to add to the pile that the OP has gotten here, right??. Eeeehaw…
 

hollowman

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I have one of these as well and did the same thing with the 12AY7, now I can run on 5 or 6 volume without the breakup, huge difference. I use this amp now for low volume gigs with the drummer playing "light" (LOL) or using brushes. Sounds real nice with a little slapback and clean OD. Plus it's Surf Green! I can't read circuit schematics, but ears tell me this is more gig worthy with the tube swap
 

Mr Mojo 54

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Ok ok, we’ve all read lots of advice on these forums - well meaning I’m sure - but it turned out to be useless.
I recently bought a 68 CPR online that I felt was going to be ‘The Fender sound’ in my head that I’ve been wanting as a compliment to my Vox ac15. When it arrived I had an almost immediate letdown. Too much bass, not enough sparkle - the usual 68 CPR complaints.
Googled 68 cpr tweaks and found a posting that suggested replacing 12ax7 in V3 with a 12AY7 as an easy way to revoice the amp. And I’ll be damned it actually worked wonders. No mods. More sparkle, less bass - I can now run the bass up to 5-6! Now all I need is a better Fender voiced speaker.
Am I crazy - could it really be as simple as a lower gain v3 tube?

No, you're not crazy at all. It's a perfectly good way to reshape the response. Going down in gain provides a crisper, clearer sound. I've often used lower gain preamp tubes in following stages of multi-stage (12AU7, 12AY7, 5751) for similar purposes. Not only lower gain but they have lower output impedances. If it works, stick with it. It's legitimate. Enjoy!
 




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