Blackfacing a 135W ultralinear Twin Reverb?

[AllenBradley]

Does anyone out there have any experience/pointers for blackfacing a 135W ultralinear Twin Reverb? I was thinking about converting the PI & Driver to AB763, and just wanted to be sure this is safe in these models. I have heard that this is not a good idea. Any other pointers for "helping out" an ultralinear Fender would be appreciated as well! Thanks!

[JoeM]

I dont know how successful BF'ing these work out. They're such clean sounding amps. I owned a UL (70W) SFSR one of these, very clean and just seemed about impossible to get to break up. I think there's some other threads on these with suggestions, try a search on the forums.

[Gtr_tech]

Not worth the trouble if you ask me. I'd be a pretty big project. You'd have to add a screen supply since you'd not be using the output xfmr's screen taps.....just for starters. Those are loud clean amps and there's ppl out there who want that. Steel players for example...and country pickers too. May be great for stage monitoring of keyboards as well. Hell...you could even use it for a bass amp if you plugged the output into a bass cab. Sell it to someone who likes it for what it is and use the cash to get something you want.

[Chuck H]

+++ on Gtr_tech's post.

If you really want to get a more guitar friendly amp out of THAT amp I'd say forget BFing it. That'd be like trying to make a moped out of tractor parts. In other words, they're two very different amps. So much so that it doesn't make sense to change either into the other for any reason. But you could mod and revoice that amp to be more guitar friendly...

If it were my project I would start by reducing voltages. A couple of big zeners would do it. I wouldn't bother with changing the UL design, UL amps overdrive just fine (ask DR.Z). Change any bias circuit goofiness so you have a normal adjustable bias circuit, trim down the plate volts and re-bias the amp. Once the amp is able to be overdriven without ripping your head off you can start revoicing as needed. Don't expect a BF out of that amp, but perhaps something else equally good.

Chuck

[pdf64]

If you've got new production tubes in there, I would strongly advise against overdriving the power amp. For the sake of the tubes, as well as your hearing. Due to the very high B+, UL configuration and low supply impedance, the screen grid dissipation quickly increases to massive levels when pushed past clipping - I've measured over 20 watts. That seems to render modern tubes useless after a few hours.
I'd guess that those amps were designed around the Sylvania STR387, which I think was a 7581A spec, rather than 6L6GC.
Those are super expensive, but to reduce the B+ means big zeners or a new power transformer. Post #3 is the best response.

[JoeM]

...
I'd guess that those amps were designed around the Sylvania STR387, which I think was a 7581A spec, rather than 6L6GC.
Those are super expensive, but to reduce the B+ means big zeners or a new power transformer. Post #3 is the best response.

The Sylvanias are what these came with , at least mine did. IIRC the UL SF I had ran about 510V!

...Post #3 is the best response

Yeah, not worth the attempt, they are what they are.

[AllenBradley]

Thanks guys!
BTW, this is not my amp, but a friend's. He heard a '75 Twin that I had blackfaced & blueprinted and freaked out because his never sounded so good! I already recommended the sell it / buy another scenario!
Now I'm just trying to learn more about these ultralinear Fenders since the opportunity has come up.
Again, anyone who has successfully tweaked these amps, please share! Any problems experienced would be appreciated as well.
Does anyone have any modern tube recommendations for these UL's that run at such insanely high voltages?

[pdf64]

Advise that you increase the screen grid resistor value. You will lose a little output power but protect the tubes. 1k minimum, 2k2 5 watts is better.

[Ronsonic]

Thanks guys!
BTW, this is not my amp, but a friend's. He heard a '75 Twin that I had blackfaced & blueprinted and freaked out because his never sounded so good! I already recommended the sell it / buy another scenario!
Now I'm just trying to learn more about these ultralinear Fenders since the opportunity has come up.
Again, anyone who has successfully tweaked these amps, please share! Any problems experienced would be appreciated as well.
Does anyone have any modern tube recommendations for these UL's that run at such insanely high voltages?

Giving these the BF treatment is simply unrewarding. These are great amps for the guys they are, uh, great amps for The best "tweak" is to get it running right. My customers who play Jazz or do really big Country gigs love them. Sell it to one of those guys and use the proceeds to buy one of the very many under-priced SF Twins that are out on the market. Mostly those don't even need the BF treatment, just some good power tubes and speakers.

Tubes for the UL amp isn't hard. The crappy Sovtek 5881 loves it some high voltages and actually comes alive with them - very rugged tube. The =C= Svets are good and the JJs work at a low price.

[AllenBradley]

Thanks Ronsonic! I've heard the same thing about the Sovtek 5881s before, so I'll have to give 'em a whirl.

[Gtr_tech]

Please note these Sovtek "5881"s are not a real 5881, so don't go cramming old stock 5881s in your UL twin/super/etc. They are more of an industrial 6L6 variant intended for utilitarian use in servo drivers and such non audio applications. Explains why they are very durable, but somewhat toneless. I use them in my HV pwr supplies as pass regs.

Sovtek 5881:
http://www.voodooamps.com/home/Porta...5881%20WXT.jpg

Real 5881:
http://www.qsl.net/dl7avf/roerbild/ts5881.jpeg

Real 5881s are smaller than the sovtek, just barely bigger than a 6V6. They won't take the voltage/current that the sovwreck will. Think of a 5881 as a downgraded 6L6 type.

[imaradiostar]

The circuit isn't so far from an older twin in many respects...many of the things that are inherently "Fender" are still there like the mid-scooped Fender tone stack and the 3M3/10pF reverb blend/treble boost circuit. Don't expect pretty phase inverter or power tube distortion thought!

It'd probably be a fabulous rhodes amp!

I think increasing the screen grid resistor value and wattage was a great idea to help reliability. It would also be a good idea to move the standby switch in front of the filter caps so it won't have to switch DC- use a DPDT switch off of the bridge rectifier since the circuit uses stacked capacitors.

The "ultralinear" aspect of this amp is unconventional. I won't say poorly engineered because that's subjective, though that is how I feel. I have one of these amps and I measured the ultralinear taps at 10% or something like that- not really ultralinear in the traditional sense. I'd be tempted to make a series pass mosfet vreg on a little terminal strip and supply the screens with 400 or so volts instead. This would make the output tubes happier and they'd require less grid bias.

jamie

[isaac]

I have one of these, and blackfacing everything on it besides the ultralinear sounds awesome, and its REALLY easy. Many people say that its hard to blackface, but besides the UL its so easy, but I guess if I had a normal one it would be even easier, although it sounds SOO nice now. I imagine it wouldn't be very hard to install a choke and make it do non UL, but I'm not really sure how that would turn out. I'm still not sure why some people say you have to replace the power transformer, the voltages are a little high for this particular purpose but you can drop the voltage down using several different ways. But i havent tried it. but i gues i "half"-blackfaced it and it sounds really good. but now has a warmer, sweeter voicing, and maybe its just the way these amps react at low volumes but it almost sounds a little compressed which is nice with my tele doing twang. ;-) This amp is great and I think people should know that, because for SSOOOO long people have almost trashed them.

[bob p]

What you've got there is a World Class clean amp -- the type of amp that some people would absolutely love to own: pedal steel players, keyboard players, bass players, and guitar players who shape their tone with stompboxes and want to own one single amp that can play any sized gig. I agree with the recommendation to move the amp to someone who will love it as it is, and look for something that's more suited to your end goals. If you already know what you want, it makes sense to start off with an amp that's already close to being there and start modding it instead. That's more efficient use of your time. Just two cents. YMMV.

[olddawg]

Does he own a boat?