Fender Pro Reverb: KT88 mod?

[kka]

A customer is interested in putting KT88s in his Pro Reverb. I know these tubes draw more heater current than the PT can support. What would the mods be to accomodate KT88s in this amp?

Any help is appreciated. Thank you.

[Chuck H]

An added filamament transformer or a new PT with capable of more filament current.

[Jazz P Bass]

A customer is interested in putting KT88s in his Pro Reverb.

What does the customer wish to accomplish by putting KT88 tubes in a Pro Reverb

[Steve Conner]

Bigger, tighter low end for those 7-string doom riffs?

[Earl Norton]

Changing to KT 88s won't fix flabby lows. The Pro has an open back cabinet that will tend to cancel anything lower than say 80 Hz. He should instead look at a sealed extention cabinet with a 15 or 18 or even a very good 12. I would personally choose a very good 15. YMMV.

[Steve Conner]

Hey, it was a joke! I was trying to think of the worst thing the Internet could do to a Pro Reverb.

I changed the tubes to KT120s and the speaker to a Pyle car sub, totally nailed the tone off Scrotal Concussion's first album

[kka]

The customer is a steel guitar player and lots of them folks like the KT88s . For now I adviced him to pop in some KT66s as he is not ready to change his PT.

Thanks everyone for your input !

[Chuck H]

As I understand it, the Pro has a small OT compared to other Fenders of the same era and output. So if you really want to make it impressive, an OT upgrade would be the thing to do. 'Course that's still a transformer replacement. But FWIW you DON'T need a different PT for the kt88's. You CAN add a smaller and less expensive 6.3V transformer to run the preamp filaments and just add it to the amp. It's a reversable mod and it won't devalue the amp significantly to have three small holes no one ever sees on the under side of the chassis.

[kka]

Thanks, Chuck. Basically, I need to be sure that if we pop in 2 KT88s in this Pro Reverb, that the PT won't get overloaded. I know that with EL34 Marshalls you can just do that, lower the bias resistor to 100kOhm and bias the KT88 just like an EL34, but with 6L6 based amps some techs advice against it.

[Jazz P Bass]

[QUOTE=kka;277834] Basically, I need to be sure that if we pop in 2 KT88s in this Pro Reverb, that the PT won't get overloaded. QUOTE]

Oh and it will be overloaded.
The KT88 heater current is 1.6 amps per tube.
The 6L6 draws 0.9 amps per tube.

[soundmasterg]

The KT66 has higher filament requirements than a 6L6 also...I believe they are 1.2A per. You might try larger filter caps in the power supply stages and use smaller value dropping resistors to make the preamp voltages higher for the customer. A larger output transformer from a Super Reverb would also help as it would give a lot more headroom. The Pro Reverb is basically a Vibrolux with two 12 inch speakers.

Greg

[Chuck H]

Bigger filters aren't the answer for this one either. The OT is the limiting factor. The Pro might not be my first chouce for steel guitar. Compare the OT in the Super Reverb (top) with the Pro (bottom). The Super has had the baffle replaced, but it is a Super.

[Chuck H]

Is this customer the guy with the Twin Reverb also??? If so I think that adding an extra 6.3V transformer to the Twin and kt88's would make a much more suitable steel guitar amp. Even the Twin by itself is a much more suitable guitar amp with NO mods. Why fuss with trying to turn the Pro into something it's not?

[JoeM]

Bigger filters aren't the answer for this one either. The OT is the limiting factor. The Pro might not be my first chouce for steel guitar. Compare the OT in the Super Reverb (top) with the Pro (bottom). The Super has had the baffle replaced, but it is a Super.

Yep, I'd be concerned that even after all the work it's still not going to yield the desired result.

[kka]

The customer has an old Twin as well. Maybe he should run that one with 1 KT88 on each side for his steel guitar.
If he wants to use the Pro Reverb with KT88s he would have to get a extra filament supply plus a bigger OT. Or he runs KT66s and just replaces the OT for more headroom. I don't think there are any other options for these amps.

Thanks everyone for your input and time.