SF Pro Reverb Kinda Bright Sounding...

[mbratch]

Hey all, I have a SF Pro Reverb w/AB668 circuit. The one with 150 ohm resistors on the power tube cathodes...

Anyway, the the amp sounds full and fairly noiseless, but I find the vibrato channel to be fairly bright. Well, almost excessively, surprisingly bright. With any guitar, including single coils, I have the bright switch off and the treble set to about 5 or so just to keep it tame. Currently, the speakers are one Jensen C12N reissue, and one vintage Oxford (12L6). My experience with the C12N RI is that it tends to be kind of bright anyway, and it is louder than the Oxford. So maybe that's it (?). But just wondering what to expect out of this particular amp.

Thanks.

[slider313]

Do you have another 2x12 cabinet to cross it over to? Also,the 12ax7 in V2 and V4 can play a part.

[kevkyle39]

I have a silvertone 1484 with the 2x12 cab that I replaced the blown speakers with the C12N ri and they are really bright/harsh compared to the originals. Anyways I also have a 66 pro reverb that tried into the silvertone 2x12 and that did not sound good at all. The pro reverb sounds great with the original oxfords however. I was thinking about replacing the oxfords with the new jensens but after I heard them there is no way that is happening. I think I would try a different speaker in place of the jensen...alot of people like some of the eminence (cannibis rex, redwhiteblue) or try something from ted weber.

[mbratch]

Thanks kevkyle39. I have a 12F150 in a DR I may move over to it and try for now. IIRC, my DR had the C12N in it and I didn't like it for the same reason, but the 12F150 is smoother.

I'm actually awaiting my other Oxford to show up in the mail. It's being reconed. But I have an outdoor gig this weekend and want to use this amp if I can.

[PRNDL]

Hey all, I have a SF Pro Reverb w/AB668 circuit. The one with 150 ohm resistors on the power tube cathodes...

Anyway, the the amp sounds full and fairly noiseless, but I find the vibrato channel to be fairly bright. Well, almost excessively, surprisingly bright. With any guitar, including single coils, I have the bright switch off and the treble set to about 5 or so just to keep it tame. Currently, the speakers are one Jensen C12N reissue, and one vintage Oxford (12L6). My experience with the C12N RI is that it tends to be kind of bright anyway, and it is louder than the Oxford. So maybe that's it (?). But just wondering what to expect out of this particular amp.

It's actually fairly common to set the treble that low ... the Magic Six is 6-3-2.
A bright speaker will also affect that.

I'd check the bias, since those 150 ohm resistors make it a combination of fixed and cathode bias. It has a balance pot that is used to equalize the current in both tubes.
In my experience, these amps are biased a little cold, and are bright.

[mbratch]

It's actually fairly common to set the treble that low ... the Magic Six is 6-3-2.
A bright speaker will also affect that.

I'd check the bias, since those 150 ohm resistors make it a combination of fixed and cathode bias. It has a balance pot that is used to equalize the current in both tubes.
In my experience, these amps are biased a little cold, and are bright.Thanks PRNDL. What surprised me was how bright it was with the bright switch in the off position. It's way over the top with the bright switch on. My BFDR, with the bright cap in, has treble set to about 7 for a crisp, but not painfully bright sound.

I'll check the bias. The AB668 actually has a pseudo-balance bias adjustment: it changes the bias on one of the power tubes rather than both. So one's fixed, the other adjusts.

[mbratch]

I'd check the bias, since those 150 ohm resistors make it a combination of fixed and cathode bias.
The "Soul of Tone" book I believe describes these particular changes Fender made as "sonically inappropriate".

[RedMan]

All Fender amps of that era with reverb are bright. The bright cap across the 3.3M resistor has a lot to do with it and of course the aforementioned Jensens. I like to swap the ceramic caps out for silver micas to lessen the harshness of the bright. Setting the treble on 5 isn't really trying to tame the treble IMO either.

[Blue Strat]

All Fender amps of that era with reverb are bright. The bright cap across the 3.3M resistor has a lot to do with it and of course the aforementioned Jensens. I like to swap the ceramic caps out for silver micas to lessen the harshness of the bright. Setting the treble on 5 isn't really trying to tame the treble IMO either.

Every Fender reverb amp I can think of ALWAYS used that cap across the 3.3M resitor. That's not a bright cap, it restores loss of highs in the part of the circuit.

[mbratch]

All Fender amps of that era with reverb are bright. The bright cap across the 3.3M resistor has a lot to do with it and of course the aforementioned Jensens. I like to swap the ceramic caps out for silver micas to lessen the harshness of the bright. Setting the treble on 5 isn't really trying to tame the treble IMO either.
I was comparing the treble setting to what I normally do on my BFDR. I have my BFDR treble set on about 7, with the bright cap installed, and the tone is fine. It's bright but not ridiculous. The Pro Reverb has the bright switch OFF and treble set of 5 before it starts to get reasonable. Any lower setting on the Treble and it starts reducing too much upper midrange as well and doesn't sound good.

[Tinman]

I have '73 Pro Reverb that's been blackfaced. I never engage the bright switch, and I keep the treble at 5 and the bass a hair over 3. I don't think thats an unusual setting.

[RedMan]

Every Fender reverb amp I can think of ALWAYS used that cap across the 3.3M resitor. That's not a bright cap, it restores loss of highs in the part of the circuit.

Isn't that what all bright caps do?

[jay42]

Isn't that what all bright caps do?Yes and no...Mike is right. A bright cap is on a volume control and is used to gate more highs when the volume is low. As the volume is turned up, the cap has less and less effect. The 3M3||10pf is always there. Some prefer silver mica there vs. ceramic. It's a legitimate area to tweak, but the OP probably would do well to have some blackfacing done, beam blockers installed, and maybe try some 5751's or 12ay7's up front.

[RedMan]

Yes and no...Mike is right. A bright cap is on a volume control and is used to gate more highs when the volume is low. As the volume is turned up, the cap has less and less effect. The 3M3||10pf is always there. Some prefer silver mica there vs. ceramic. It's a legitimate area to tweak, but the OP probably would do well to have some blackfacing done, beam blockers installed, and maybe try some 5751's or 12ay7's up front.

Well....if you think about it a volume control is really nothing more than a variable resistor. The reason you lose highs on a 1M pot when it's down low is because the signal is going thru almost a meg of resistance before it's Miller time. 3.3M is over 3X that so you lose even more highs across it but it's the same principle. Bypassing the highs across it with a cap is a valiant effort but what happens is the highs become more prominent, even more so in the case of a 3.3M. It's one of the main reasons the second channel sounds brighter than the first one in a BF/SF era Fender amp with reverb. I'm not saying the cure is to do something with the cap at all, just trying to help someone understand why this happens. If you don't need reverb the first channel always sounds less bright on a Fender of this era. I guess it was my roundabout way of trying to say that. Dammit Jim I'm a doctor, not an english teacher!

[Blue Strat]

Isn't that what all bright caps do?

Maybe, but the cap in question is not THE bright cap;)

[RedMan]

True, but it's the only bright cap across a 3.3M resistor that I know of in those amps.

[Blue Strat]

True, but it's the only bright cap across a 3.3M resistor that I know of in those amps.

Semantics:)

It's actually a "compensation cap"...to compensate for lost highs in that voltage divider circuit:D

[RedMan]

Same as the one across the pot.

[WaltC]

well.... on the twin I'm working on now (SF Master Vol w/ all new solen couplers and a BF PI and output stage, not done by me) won't go to zero volume if the 10pf cap across the 3.3M resistor is removed, the volume stays at about 2 or so with the pot set to zero if the cap is disconnected (as it was when I got the amp up on the bench). With the cap re-connected the volume pot works as you'd expect.

the amp is on the bench because the tone on the tremelo channel is harsh, slightly buzzy and breaks up way too soon. the normal channel is much better but shows the same symptoms but at a much less obnoxious level.

back to the original subject (my search will continue of course <G>), I didn't notice any appreciable boost to the "highs" in the amp when I re-connected the 10pf cap. But... on this amp the bright switch doesn't do much on either channel, so maybe it's naturally too bright as is....hmmmm... we'll see (I hope <G>)

[Blue Strat]

Same as the one across the pot.

...which is known as the "bright cap". :D

[RedMan]

Well you say tomato I say tomato, it makes the same sauce either way.

[Blue Strat]

One thing of note is that the "bright cap" (the one on the volume pot) is often removed to make an amp more usable. I've never heard of that being done with the one across the 3Meg resistor. Removing that one just makes Fenders sound muffled.

[RedMan]

I've never heard of removing them either.

[jay42]

hmm...removing the volume pot's bright cap on a JCM800 2203/2204 is a hugely popular first stab at becoming a modding guru. I've heard of people increasing the 10pF to 20pF, or decreasing the 3M3 to 2M2 on reverb Fenders.

[RedMan]

Hey I just tried removing the (whatever) cap across the 3.3M in my DR and I really like it! It's less shrill especially when cranking it up a bit and still has plenty of treble bite. Not dull at all. I have tried different value caps and resistors there many times before but I never really tried taking it out completely. Me like!

[RedMan]

hmm...removing the volume pot's bright cap on a JCM800 2203/2204 is a hugely popular first stab at becoming a modding guru. I've heard of people increasing the 10pF to 20pF, or decreasing the 3M3 to 2M2 on reverb Fenders.

That and removing the bright cap on the bright channel of 4 hole Marshalls across the mixer resistor.

[WaltC]

Put a 15pf cap in the one I was working on and replaced three leaky Solen caps w/ 150Ms and the Twin is back up and operating as it should. Still noticed the "can't go to zero" volume problem with the 3.3m bypass cap (10pf, 15pf on mine...<g>) disconnected though.

FWIW

[jay42]

...replaced three leaky Solen capsoh?!?!
Those are too expensive to crap out.

[slider313]

http://photobucket.com/albums/b376/slider313/pro2.jpghttp://photobucket.com/albums/b376/slider313/pro.jpgI just put a pair of Jensen Neos in my '66 Pro Reverb.Real nice warm,clean tone with smooth top end.I really thought I was going to hate these speakers,even though all the reviews I read were positive.This amp had a nice warm tone to begin with and that's with two Oxfords,so these made a good amp great.