KT88's in a Twin

Brett Lanier · From: Madison, TN

Had to share this...
I pulled the 6L6's out of my twin and put in two kt88's instead (in the outside positions). For pedal steel, the difference in tone was huge, like going from a wimpy speaker to a JBL. I'd recommend this to anyone playing steel through a twin. The lows and low mids are much deeper and cleaner sounding.

It sounded great through the 4ohm eminence that was in there, but I swapped it out for an 8ohm JBL k130 because I had it sitting right there and I believe the impedance goes up to 8 ohms when you use just two tubes.

Tim Marcus · From: San Francisco, CA

do you mean KT66's? Those are much more interchangeable with a 6L6 than a KT88, which demands much higher plate voltage than the twin's power supply offers.
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Brett Lanier · From: Madison, TN

Nope! KT88's! I'm trying it now with 4 of 'em, I have a half power switch on my amp so it's easy go back and forth from 4 and 2. I'm not sure which way is safer, but my twin has the big OT from the late 70's. I've been running it at modest volume levels at home all day keeping an eye on the level of heat coming off the transformers. What do you think could go wrong here Tim? I'm almost willing to find out for the sound I'm getting right now.

Tim Marcus · From: San Francisco, CA

tubes are pretty forgiving - so you can probably interchange without much need to worry

that said, the ideal plate voltage for a KT88 (or 6550 for that matter) is much higher than the ≈405V that goes to the plates of the 6L6's on a Twin Reverb. I think they like to see something along the lines of 500-600VDC (!!)

http://www.drtube.com/tubedata.htm

there are also different impedances and current demands which you may want to at least check into before you bring that thing out to a gig

If you want to try a different tone and keep it totally within spec, you can try a quad of KT66's which use similar plate voltage and have a similar impedance to a 6L6GC

using KT88's for power with a fender style front end was an idea I tried for the Milkman steel amp, however, it would have been super expensive (lots of Iron) and too clean for my personal taste. I like just a little bit of grit in there!
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Brett Lanier · From: Madison, TN

Thanks for the info Tim. That's kind of what I imagined was the case; the tubes are just being under-used from what they are designed for. It sounds great to me though, and there's a little more color to it when using just two. Just wasn't sure if it's putting too much strain on the output transformer, but like I said, it's the big one from a '77 twin so it's pretty rugged.

Brett Lanier · From: Madison, TN

Just found an old thread with a lot of tube swapping info. http://steelguitarforum.com/Forum11/HTML/000431.html

Bob Metzger wrote a few things in there specifically about swapping kt88's into a twin:

A Fender Twin Reverb is not going to sound like a VHT amp but neither is a VHT amp going to sound exactly like a Twin Reverb; they both have their own sound and constituencies. But will using KT-88's or 6650's improve a Twin Reverb for steel guitar use? Absolutely. However, there are some severe technical details that must be addressed. A Twin Reverb cannot support the heater/filament current demands of these aforementioned tubes and YOU WILL DAMAGE YOUR POWER TRANSFORMER (the most expensive single part in the amp) if you drop them in without modification. This is tech work, not hobbyist stuff. The warning is necessary. If you have a Twin and it sounds ok to but want better for steel, consider what has been discussed here then take your Twin to a tech who has successfully performed this tube mod/swap before. Dan, most players know that if were as easy as just changing a few tubes that everyone would be walking around with 1000 watt Champ amps with more lows than the 1929 New York Stock Exchange.
Bob

I know it sounds like I'm really asking for it by running kt88's in the twin, but would I be correct in thinking that its safer on the transformer to use just two tubes?

Tim Marcus · From: San Francisco, CA

well, maybe you can compensate by pulling out a few preamp tubes. You can scrap the one next to the phase inverter - that entire tube is for tremolo which I imagine you do not use much in a steel amp - unless you are missing your right foot Smile

Also, you can scrap V1 and just use the amp as a single channel

that change will not relieve much heater draw, but it will free up a little bit which can't hurt!

or - you can put in a separate 6.3VAC filament transformer for all preamp tubes

or - you can put in a DC power supply and rewire all preamp tube sockets for DC, thus freeing up the entire 6.3 tap of your power transformer for the KT88's

lastly, you can talk to Paul at Mercury and see if he has a marshall power transformer that will fit the Twin chassis. That way you can bump up the voltages to the correct level, and you get the bigger heater supply with a separate winding
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Brett Lanier · From: Madison, TN

So I brought the amp to my tech to have it biased, checked out, and get some harnesses on the tubes so they don't fall out. The plate voltage is at 510, and we brought the bias down from 140 to 121 (seemed like the magic number with my other amp that has kt88's), so that should be a little kinder to the output transformer.

This amp is an ultra-linear twin, so it has a higher plate voltage than say a blackface twin, and bigger transformers I think. The higher voltage compliments the kt88's. Of course I don't really know what I'm talking about, just learning as I go. One thing's for sure is this thing is a clean machine now. Tones of clear low end. If I melt the transformer I'll let you know, but so far so good.

Walter Glockler · From: Northern New Mexico

What specific brand are you using and would that work for a '76 Vibrosonic, too?

Tim Marcus · From: San Francisco, CA

So is the output still ultralinear?
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Brett Lanier · From: Madison, TN

Hey Walt, I'm using JJ kt88's(2). They're enormous.

Tim, I suppose it is but I can't be sure. When Dan worked it over he removed the master volume and also took out some caps or resistors that were on the power tubes. He's gotten pretty good at knowing how to adjust an amp so that it's good for steel. As you know he really understands electronics very well so I just let him make the calls.

Brett Lanier · From: Madison, TN

Walt, I couldn't say with too much confidence that it'll work for your amp because I wouldn't want to be responsible for wrecking your stuff. I'm going to play mine for a while and see how it goes. I'm willing to risk it. Bringing the bias down a bit did help with keeping the output transformer cooler. A vibrasonic is basically the same amp though.

Rick Abbott · From: Indiana, USA

It's probably not a good idea for the Vibrosonic. The ultralinear stuff is different than the other black or silverface stuff. Yeah, there are many things the same, but you can't do all the same mods to the circuits. I might be off on this but I think the Bassman 70 and 135; the Twin 135; and the PA 135 are ultralinear. Could be others?

The idea seems to be that if you have 4-6L6 then 2-KT-88 are ok. Not exactly, but close. So it would follow then that 2-6L6 equal 1 KT-88. Won't work, cause it's not "single ended" like a one output tube champ. The Vibrosonic is a 2 output-tube amp?
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RICK ABBOTT
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Brett Lanier · From: Madison, TN

Rick, perhaps you're thinking of a vibroverb, which has two output tubes. The vibrosonic is like a twin but with a 15".

Rich Gibson · From: Pittsburgh Pa.

Brett I have a 66 showman that was modded to run 2 6550's.I don't know what was done exactly.I bought it that way some years back and it does sound very good for steel.I don't really love it for guitar though.
I just had my tech check it out as I'm thinking about using it again.He measured the output at about 60 watts and the amp wants to see approximately 8 ohms.I asked about going back to 4 6l6's just out of curiosity.He said no big deal. It would need to have the other 2 tube sockets replaced(they were shot)so labor and parts to change back was about 150.00 including the new sockets.
Sorry I don't know anymore specifics but hope that may be informative in some way.

Cartwright Thompson · Posted 3 Mar 2012 7:36 am

Rich, Did you get that amp from me quite a few years ago? Great amp, I don't know why you'd want to go back to 6L6's, unless you're running a four ohm load.
All you need to do to run 6550's in a twin is to beef-up the screen resistors to 5 watt 1Kohm and re bias to about 60 mA per tube plate current per tube. I've had my twin set up like this with a single D-130 and it's great. I'm talking TWO 6550's. Running four is fine too, as long as you keep a fire extinguisher close at hand...

Rich Gibson · From: Pittsburgh Pa.

Hey Cartwright that is indeed your old amp although I bought it from a guy in Chicago(for 300 bucks!) who had said he bought it from you.I believe we talked about the amp at some point.I like that Emmons S10 your selling BTW.
I was just curious about the 6l6's.It had been sitting awhile and the bias had drifted.Once that was rectified it sounds like it's old self again.
So anyway Brett there's the specs.

Cartwright Thompson · Posted 3 Mar 2012 5:40 pm

That's a really great amp. It's actually a '68, I "blackfaced it", me and Larry Rodgers' incredibly accurate faceplates and grills, that is.
Can you still see Louie Catello's signature on the chassis? He was a real character. He had a shop in Berlin, NH that sold guitars, chain saws and Harley parts.

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