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]

Bassman 70

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]

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]

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]

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