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

[mike_mccue]

I got an old Silverface Super Reverb cleaned up and ready to be maintenanced and put back into service.

Now that it's clean I am able to recognize that it is the unpopular 70watt CBS version of the Super Reverb circuit.

The problem for me... I don't want a 70w Super. I was hoping to get to work on a Silverface to Blackface project... and it's my impression you can't make the 70 watter and the big trannys into a suitable Blackface circuit.

What would you do?

I'm thinking I may just need to get this one service ready and sell/trade it towards an older Silverface Super Reverb.

What else can I do?

best regards,
mike

[jrfrond]

Don't butcher it. Get it back into primo condition to maximize your sale price, sell it and find your SFSR or BFSR that you are looking for.

[TheTinMan]

The "problem" with the transformers is the ultralinear hookup on the OT, correct? The controls on the front panel are certainly all useful - you can keep a master volume circuit, just not that one.

What's the difference in the price you can sell this amp for compared to the cost of a BF or earlier SF Super Reverb? Could you replace the PT and OT and build a new board based on the AB763 circuit for less? My biggest concern would be whether or not a cab from that era would sound as good as a nice BF pine box. Otherwise, you've got a great platform to build an AB763 IMHO.

It's not as if you'd be "butchering" the last Brown Deluxe on the planet.

Chip

[Steve Conner]

I believe these have a higher voltage PT and an ultralinear OT. You can disconnect the ultralinear taps, but you're still left with the problem of the high B+ that it uses to make 70w from a pair of 6L6s. It will force you to bias the tubes at a lower idle current than you'd like.

I'm sure you could make it sound good, lowering the preamp voltages to blackface specs would probably help a lot, but I think it would always sound a little louder, cleaner and colder than a real BFSR, unless you replaced both transformers.

If it was my amp, I'd not try to fight that extra power: I'd blackface the preamp, fit 6550s or KT88s in the power amp, crank the bias as hot as I dared, and let it rip.

We have a CBS Bassman 70 kicking around our rehearsal room. It actually sounds fine for either bass or guitar.

[mike_mccue]

Thanks for the ideas.

I think I'm gonna get this amp into playing shape and look for an earlier Silverface.

I sold my Twins years ago... I enjoy playing a Deluxe Reverb I've owned for about 20 years.

I just don't have any use for 70 watts.

It's all a bonus for me as this Silverface amp literraly was a freebee that got stored away years ago as a basket case. I suspect the reality is that for a while our community did not have a single tube tech that could get stuff working... I personally spent hundreds on repeated repairs only to have the amps die on the next gig. I think that was the experience of the previous owner of this Super Reverb.
I finally started learning a bit about amp maintenance when the Vintage Guitar phenomenom started. I bought and read every book that came out. Things are so much better now. Especially because people like those at this forum are so remarkably generous with info

I'm guessing there's not much wrong with this amp.

Thanks for confirming the the Trannys are way different from the older 40 watt versions.

best regards,
mike

[Rob Mercure]

Hey Mike,

About 10 years ago or so I think we pretty much "cussed and discussed" the UL Fenders to death and the consensus was that the UL circuit wasn't the cause of the linearity and other problems that folks hated but instead small bypass capacitors and such that CBS engineers had added up stream. Some folks reported unhooking the OT screen connections and not having the tonal qualities change that drastically. I've not tried to wade through the archives but I recommend them as a starting place. Or simply analyzing the circuit and removing the "very CBS" features that it has. The amp will still be loud but there are lotsa folks that live with vintage Twins who still manage to get a decent low volume sound.

Rob

[mike_mccue]

Thank you for the heads up... I'll take a look at the archives as well.

best regards,
mike

[Blair]

Originally Posted by Steve Conner

If it was my amp, I'd not try to fight that extra power: I'd blackface the preamp, fit 6550s or KT88s in the power amp, crank the bias as hot as I dared, and let it rip.

We have a CBS Bassman 70 kicking around our rehearsal room. It actually sounds fine for either bass or guitar.

I've actually got a Bassman 70 and wouldn't mind a little more headroom, I've always loved the way KT88's react, so how would I go about doing this.

Cheers in advance.

Blair

[hasserl]

Originally Posted by Rob Mercure

Hey Mike,

About 10 years ago or so I think we pretty much "cussed and discussed" the UL Fenders to death and the consensus was that the UL circuit wasn't the cause of the linearity and other problems that folks hated but instead small bypass capacitors and such that CBS engineers had added up stream. Some folks reported unhooking the OT screen connections and not having the tonal qualities change that drastically. I've not tried to wade through the archives but I recommend them as a starting place. Or simply analyzing the circuit and removing the "very CBS" features that it has. The amp will still be loud but there are lotsa folks that live with vintage Twins who still manage to get a decent low volume sound.

Rob

As I recall one of the main things to do is to disconnect the nfb, this seems to make a big positive difference in the amp. There is nothing inherently wrong with UL, Dr Z seems to use it to good effect. And I've got an old Alamo Paragon Bass amp here with UL operation that sounds stellar for guitar. In any case it won't take any butchering to make a few modifications that may make the amp perfectly fine. 70 watts can come in handy when your bass player decides to crank up his SVT and blow you out.

[Steve Conner]

Originally Posted by Blair

I've actually got a Bassman 70 and wouldn't mind a little more headroom, I've always loved the way KT88's react, so how would I go about doing this.
Blair

Someone else might know better, but I think you could just stick a pair of KT88s in there, and it should work fine after a rebias. The heater current draw is more than with 6L6s, but the power transformer in our Bassman 70 looks pretty meaty, and it might hardly notice another amp or two... Or then again it might burn out, especially when you also overload the B+ by hooking a 2 ohm speaker up to the "4 ohms minimum" outputs and trying for 100 watts output. Your call.

[lowell]

good post... I own a '77 SF Twin Reverb. I converted it to cathode bias, disconnected the ultralinear taps and disconnected the negative feedback. I did this on another of the same amp for someone too and I changed the two inner tubes to 6V6. Ofcourse I rewired the bias much cooler for the 6V6. That amp w/ the 2 6V6 in the middle kicked ass!! It was the great sounding SF Fender I've ever heard. Also, as far as butchering goes all these mods can simply be undone... it's not as if you're drilling the chassis or something permanent like that. Also yeah if there are caps on plates of output tubes I'd ditch those and pursue the "cap across the plate resistor" in the preamp stages to soften that harsh high end that these amps have. Kevin O'Connor's book is great for these mods. If you do the plate resistor bypass cap be sure to put some series resistance in there to minimize capacitive interaction w/ the preamp tubes.

[mike_mccue]

I have to admit, I have a sense of what these later amps sound like... I've played half a dozen twins (extensively) that spanned the range. The circa 1980 version was way different sounding than the older amps.

But I do not, in technical terms understand how a transformer can be "ultralinear" as opposed to not? I guess I previously associated the term with regards to the whole circuit.

Can someone recomend a reference where I can read up about what makes a transformer "ultralinear"?

thanks,
mike

[lowell]

Ultralinear xsformers have 2 xtra taps for the screens of the power tubes and allow for more clean headroom... that's the extent of my knowledge.

[martin manning]

Originally Posted by mike_mccue

Can someone recomend a reference where I can read up about what makes a transformer "ultralinear"?

thanks,
mike

A short explanation is here:

http://www.pentodepress.com/receiving/ultralinear.html

A search on "ultralinear amplifier" yielded a number of good hits.

MPM

[mike_mccue]

Thank you very much.

best regards,
mike

[slorinczi]

I have my bandmate's 70W SFSR on the bench and I'm trying to do some gentle mods. Replaced electrolytics and phase inverter coupling caps (they no longer match, if they ever did), converted to adjustable bias, and did the simple BF phase inverter conversion.

Have I gone too far? I'm really not trying to make this into a different amp, just make it a little friendlier sounding. I'm not going to mess with the UL output, for instance. Thoughts, input, rages, whatever....

Thanks,
Seth

[Blair]

slorinczi
Let us know your thoughts on how the conversion works out. It could be interesting.
Cheers,
Blair

[lowell]

If I were you I'd try disconnecting the UL taps... it's only 2 wires and you can put them back on if you don't like it. Just be sure to put some heatshrink on the ends after removal so they don't short to something. You will then need to add 2 470ohm 2-5watt resistors to the screens and connect the screen supply to them instead of the UL taps.

I also recently saw something very interesting in a schematic... it looked as though it was a great way of reducing high end... which I find these SF amps need. The mod is to add a small value cap (150pf ?) and a resistor or pot across the phase inverter outputs AFTER the coupling caps. The out of phase signals via the cap, which has a high resistance to low frequency thus only passes highs, will cancel eachother out. I've never tried this but will get around to it. Let me know if you do.

[slorinczi]

Interesting...I wasn't planning on going that far but I see the argument. I'm a little confused about the screen grid B+ however; if I understand correctly, I'd have to tap it off somewhere, as it's not normally provided in an ultralinear circuit. (And then run it through 470 ohm dropping resistors).

Below is an excerpt from the schemo. If you have a moment, maybe you can enlighten me as to where the B+ is coming from?

Best,
Seth

[lowell]

Hi,
Oops.. sorry, for some reason I thought you had a 4-output-tube amp. This is even easier now.

Ok so disconnect the wires on pin 6 (I THINK!) - one side of the 470ohm resistors should be soldered there... leave the resistors where they are. Then you will be connecting that side of the resistors to the B+ which is the RED wire in the chassis. The RED B+ wire should be soldered on the eyelet board towards the end closest to the power tubes somewhere. On the schematic it is the point after the standby switch that states 500+ volts... see it? You will want to leave the 470ohm resistors mounted on the tube sockets and just run a new wire from the point on the eyelet board to the socket (pin 6 - I THINK!) where the old screen tap was soldered. Use wire that can handle this voltage/current. 18 gauge is good. Hope this helps, let me know!

[Steve Conner]

I remember a previous thread on the board where someone mentioned that they tried disconnecting the UL taps on a CBS Fender, and they said it made hardly any difference to the tone. Blackfacing the whole preamp would probably be more productive. The CBS amps run high voltages on the preamp tubes, and I heard that lowering them to blackface values mellows the amp out.

[hasserl]

Originally Posted by lowell

Hi,
Oops.. sorry, for some reason I thought you had a 4-output-tube amp. This is even easier now.

Ok so disconnect the wires on pin 6 (I THINK!) - one side of the 470ohm resistors should be soldered there... leave the resistors where they are. Then you will be connecting that side of the resistors to the B+ which is the RED wire in the chassis. The RED B+ wire should be soldered on the eyelet board towards the end closest to the power tubes somewhere. On the schematic it is the point after the standby switch that states 500+ volts... see it? You will want to leave the 470ohm resistors mounted on the tube sockets and just run a new wire from the point on the eyelet board to the socket (pin 6 - I THINK!) where the old screen tap was soldered. Use wire that can handle this voltage/current. 18 gauge is good. Hope this helps, let me know!

Note that connecting the screen grids up like this will cause the tubes to run as triodes. You can try this for shits and giggles, but I wouldn't expect to find it very pleasing. The power will be reduced and the tone will be darker. Not sure what the 500+ volts will do for the screens though, may not be too good for them. My data sheet for 6L6GC's shows a maximum voltage for the plates/screen grids of 450 volts in triode.

[lowell]

No this is not making the output tubes into "simulated" triode mode. That would be if the screen resistors were connected to the plates of the output tubes. Here in this mod we are connecting the screen resistors to the B+ - just like any amp you'd see without an ultra-linear output transformer.

I agree that the preamp would be a good place to alter tone... however as in my previous post I did not touch the preamp in an amp that I converted to cathode-bias and removed UL taps and the amp sounded MUCH better. But as usual all this is subject to the "modder's" opinion and ears.

[slorinczi]

Still unsure as to whether I'm going to try messing with the UL; it seems like the consensus (and my gut feeling) are that the amp just "is what it is."

That said, I'm still trying to figure out the mod. I understand that the screens need B+. Lowell, are you suggesting that the 500V supply feed BOTH the CT of the OT and the screens? That's where I'm hung up.

And while we're on the subject of mods, a tech told me to double up the value of the cap feeding the reverb. However, I'm not sure if me meant the cap feeding the driver or the return. Any thoughts? (Semi-handy diagram below.)

Thanks,
Seth

[lowell]

Yes exactly! The CT and pin 6 side of 470ohm should be one and the same - electronically speaking. Typically there is a choke b/t the CT and the screen supply filter cap but I don't think it's completely necessary.

If you increase the "in" coupling cap then the reverb will affect more low-end because a larger cap allows more lows through. A cap in series w/ the grid with a resistor to ground (as in the schematic) creates a high-pass/low cut filter. Cutoff frequency for the input to the reverb there is 320hertz. I'd try the "in" cap first and take a listen... The "out" cap has a cutoff frequency of 530hertz so the reverb return isn't allowing much below that back into the circuit. If you'd like to know what you're doing frequency-wise use this:

cutoff frequency=1/(6.28(R*C))
(6.28= 2*pi)

In your schematic the "in" cap is 500pf... or .000,000,000,5 farads and R is 1meg, or, 1,000,000 ohms.

Fc=1/(6.28(1,000,000*.000,000,000,5)) = 320hertz

hope this helps.

[slorinczi]

Yes perfectly, thanks much lowell!

[lowell]

Oops I just reread preamp basics in Kevin O'Connor's to be sure I was guiding you in the right direction. You actually also need to take the plate resistor into account when calculating frequency cutoff.

The plate resistor in a typical preamp stage is usually 100k. This value needs to be calculated in parallel w/ the 1meg of the high-pass filter. So 100k in parallel w/ 1meg = 1/(1/100k + 1/1,000,000)=91k.

Fc=1/(2*pi(91,000*.000,000,000,5)=3,500=3.5k hertz.

[hasserl]

Lowell, what you are describing IS triode operation. You're connecting the screen grids to the plate supply, the screens will operate at the same voltage as the plates and the tubes will operate as triodes. In a typical amp there is either a choke or a dropping resistor and a filter cap seperating the plates from the screen grids, this decouples the two stages. In what you're describing there is NO decoupling. It IS triode operation.

slorinczi, don't waste your time. Leave the UL alone. Try disconnecting the nfb as was suggested earlier. It is much easier and will sound much better. Reduce preamp voltage in the power supply as Steve C has suggested.

[lowell]

Ok you're being a little pushy and honestly quite rude here to just discount my suggestions and tell him not to try it. If he wants to try it let him try it. This forum is about people sharing their knowledge not discounting someones opinion, is it not?

Triode mode from what I know is connecting the screen resistor directly to the plate... this is not the same as connecting to the CT or the B+. One half of the output transformer is b/t the plate and screen if screens are tied to B+.

[hasserl]

Sorry about that, but the fact is that is triode operation. You're tieing the plate supply and the screen grids to the same power supply node, with no decoupling between the two.

Don't mean to be pushy or rude, but maybe you're being a little too thin skinned?

Try it, or don't. If you want to experiment, why the hell not? Except the 500 volts exceeds the max ratings for the screen grids. But what the hell, go for it. What's a couple of 6L6's anyway?

[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]

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.