What would you do with a CBS 70w Super Reverb?

[pdf64]

Triode connection of pentodes usually means that the screen is linked to the plate, either directly or through a current limiting resistor.
Taking the screen supply from the same node as the plate supply still counts as a pentode configuration, not triode. With a big reservoir cap it might be fine, though stock it might be prone to ghost notes (ripple modulating the signal) at high signal levels.
However it's configured (UL / pentode / triode), those screens will be at the plate dc level around 500V, well over the max 6L6GC spec.
As I understand it, those 70s silverface Fenders were designed around the Sylvania STR387 6L6GC, a special uprated 6L6GC. Paul Rivera said something like 'that tube was so good it should have been given a different designation'. I think that the JJ 6L6GC has a similar rating to the STR387, though the consistancy might not be up to Sylvania's standard!
Hope that helps - Peter.

[lowell]

On the 500v note, aren't the screens at that anyway when connected UL? How is moving them to the B+/CT node going to change the voltage? Just asking cause I don't know.

[Steve Conner]

It's not triode mode. The original poster suggested connecting the screens to the OT center tap, which is the B+ supply, with no signal voltage on it. (ideally, at least.)

It won't change the voltage. The screen voltage and dissipation is the same at idle in UL as in pentode, but under signal conditions, the screens will run hotter in pentode mode. The reason is that in UL the screen current contributes to the power output, whereas in pentode it all goes to heating up the screens.

If this bothers you, just use larger screen resistors than 470 ohms. Everyone has been running EL34s over their maximum screen voltage rating this way for the past 50 years.

[slorinczi]

Back to NFB

I'm a little hazy on where to disconnect the NFB; looking at the schemo below, I would figure that I would interrupt the NFB line from the "line out" jack to the 820ohm resistor etc. I used the now-disconnected ground switch to interrupt the line, but when I throw the switch the amp loses all output. What am I doing wrong here?

I also note that the grn/yellow OT secondary CT meets the 820ohm/.01 junction (this is not called out in the schemo below, is it just a given?). I suspect that's the culprit but I'm still a little dim on CT and their functions....

Thanks,
Seth

[lowell]

That doesn't seem right to me.... maybe someone modded it so the feedback was coming from the other tap (CT) on the secondary. I would disconnect that connection completely... cut and heatshrink it. The NFB you have wired now with the switch should work then.

I would think hooking up the NFB via that other tap on the secondary would simply have a lesser effect than the traditional wiring being that the output from that tap will be far less... someone correct me if I'm wrong cause I'm just surmising.

[slorinczi]

Seems strange to me but again, I'm still a little hazy on all the ramifications of using a CT.

The other thing is, I'm fairly certain that I'm the first person to crack open this amp since it left the factory. Everything is stock, down to the "Fender" branded tubes.

Best,
Seth

[lowell]

Again I think they might have used that CT as a "less" intense NFB loop. The NFB will have more effect if coming from the tap connected to the speaker jack.

My thoughts anyway, not positive.

[slorinczi]

That makes sense to me, but at the moment the amp only works with both CT and "hot" lead wired as NFB. When I disconnect one (the hot lead) why does the output completely die?

[lowell]

i'm suspect of using that ground switch. are you SURE it's ALL lugs no longer connect to ground? Just being thorough.

[slorinczi]

Yes, in fact I switched out to a 2-pole Carling so the amp's owner wouldn't get confused be a middle position....

[joeBefumo]

My new project amp

A few months ago someone gave me a lovely blond 2X12 amp, fitted to accept a Super Reverb chassis. Was going to build from scratch, but stumbled upon a "blackfaced" Super Reverb, which is almost certainly a 70W ultralinear model (haven't picked it up yet, but just judging from the tube complement -- no rectifier tube.

Anyway, I'm not too concerned about that, and once I see what kind of "blackface conversion" was done, I may end up just parting it out, but short of that, have a couple of less extreme ideas...

The higher B+ isn't too much of a concern -- I suspect I could tame that a bit with a swap to a tube rectifier.

I had initially thought I might put a pair of KT66s in there, and maybe turn it into a kind of hybrid Fender/Bluesbreaker with reverb -- but I also have a new pair of Genelex Gold Lion KT88s on hand, so they might be another possibility, though I would prefer less power.

Still trying to find the primary impedance of the OT -- I'm pretty sure the earlier Supers were around 4200 Ohms into 2 Ohms, but since I'll be running it into 2X12 at 4 Ohms, that would effectively put me at around 8K, which would be okay for KT66s at 450V or so.

As for the rest of the circuit, that's going to depend on what I discover inside. If the rest is competently done, I'll probably just leave it. If it's an utter mess, might just let my imagination run a bit wilder.

I guess if there's a question in mind, it's about that OT -- with the UL taps ignored, what's the specs on that trans? Some preliminary investigation seems to suggest that those Supers used a quad of 16-ohm speakers wired for a net of 4-ohms. Is it 4.xk into 4ohms? or maybe 8.x into 4, which would make sense if they were using some older iron and just scaling it for the higher voltages.

Gotta love a mystery!

[potatofarmer]

You should start a new thread instead of bumping an old one.

It's worthwhile to measure the primary impedance and screen taps on the OT; my Twin wasn't too far from a typical Fender OT and the screen taps were barely into distributed load at all, just enough to protect the screens at the ridiculous plate voltage: http://music-electronics-forum.com/t35295/#post330008

[jbefumo]

Other than nebulous 'mojo', I think the biggest practical reason people eschew them is that they are more difficult to retro-convert to BF configuration. You have to install a tube rectifier, replace both transformers, and all the other usual stuff. Overall, prohibitively costly. Also, the chassis, and hence the cabinet, is the same width as a Twin Reverb, so the entire package is pretty unwieldy. Other than that, many of the changes are exactly what many people worship as the 'SRV mods' installed by the late Cesar Diaz, so go figure.

For what it's worth, I picked on up for $356 at a pawn shop -- scarcely looked at it because I planned on using it for parts. When I finally opened it up I discovered it had been impeccably converted to AB763 specs (see attachments). The cabinet was beat to crap, and way bigger than I could easily tote, so I located an unused Mojotone Twin Reverb cabinet for $100, filled it with a pair of Weber alnico 12s (4-ohms each), and now it's the sweetest Fender amp I've ever owned (since 1966).

(What I REALLY wanted to do was install a beloved NOS Altec Lansing 418 and turn it into a Vibroverb, but that would have entailed replacing that Mercury Magnetics output transformer with one offering an 8-ohm tap, and given the workmanship inside, I am/was reluctant to get that deeply into it.)






(Oh yes, the converter had removed the master volume and the Normal Chanel mid control, plugging the holes with these nasty black plastic fillers. I installed my own Post-Phase-Invertor cross-line master volume (for cosmetic reasons), but since I rarely use it, I configured it with a push-pull pot set up so that it's completely out of the circuit by default. Believe it or not, I found a 6.8 K pot in my parts bin, which matches the tail resistor in the original circuit, but while I was at it, I replaced the entire Normal Channel tone stack with one from a 6G5A (pro) tonestack, but with an added mid control - which is one of two I use in all my original designs. I also installed individual bias pots for each output tube, and swapped the 6L6s for Gold Lion KT66s, which I have done with all my Fenders since 1970 or so.)