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Old 13th March 2013, 01:02 AM   #1
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Default KT120 Bass Amp

I might try some KT120 tubes in a bass guitar amp. So I have several chassis to consider converting; all have pretty big power and output transformers. I'll probably add a dedicated DC heater power supply for the preamp, to offload the stock heater supply, as the tall plates in the KT120 means the heaters take more current than most (besides, it's an excuse to go to DC heaters for the preamp). And of course I'll re-bias and adjust B+.

So I've got to decide which chassis to set aside for getting some KT120. Which would be easiest?

- Fender Super Twin, stock uses 6 6L6GC, 130 watt, 500v B+. Ultralinear output. By the size and weight of the transformers it should be able to make a lot more power than it does. It's in an attractive oversized very tall head cabinet. Looks and works like new, except the mix of Chinese and Russian 6L6GC tubes are less than optimum.
- Peavey Tour VB-2, stock uses 6 EL34, 225 watt. Still being shipped to me. Cabinet smashed in previous shipping; power transformer broke loose and crushed tubes. Seller was profoundly ignorant, so I don't know what I've got here yet; if the printed circuit boards are broken I'll convert it to a turret point-to-point hand-wired amp. Transformers are enormous and I paid about what they're worth.
- Sound City 120, stock uses 6 EL34, large Partridge transformers, about 150 watt. Dirty and old and original, but nearly complete except no tubes, no cabinet, no knobs, no impdance selector. Again, by the size of the transformers it should be able to make a lot more power. The Partidge transformers look more like a 200-watt Marshall than a 100-watt.
- Blackline Ampeg SVT, stock uses 6 6550, 300 watt. I think these run high B+, because it DOES make the power that's expected by the transformer size. Still awaiting its arrival. Unknown condition except has no tubes and left side of cabinet is broken in several places.

The old Ampegs are known for eating tubes, the design's history has gone thru several different tube types, and it has none now, so I guess that's the best one to convert. It needs a lot of work, the wooden cabinet is crushed, so I was planning on making a larger cabinet than stock anyway, moving the preamp chassis farther from the power amp chassis. Plus it's the only one that has fans (though they fan is missing on mine). There's a lot to like in the SVT, as it has its power amp chassis right-side-up with the tubes sticking up, so the chassis isn't an oven. Yet the seperate preamp chassis upside-down makes the knobs conveniently accessible like a Fender. And best of all, it I make it really put out substantial power it will be appreciated. You'd think it would be foolish to alter what's probably the most sought-after classis bass amp; but that all depends on whether your alterations are successful. It's a working-band's type of head, and the KT120 are probably what the SVT has always needed in order to be really reliable.

The KT120 are so large, fat & tall, that the spacing of the tube sockets on the chassis may dictate where they can be used. I have no problems with making bigger cabinets to accomodate their height.

I guess I should wait until the SVT and Peavey chassis arrive, and convert the amp with the biggest transformers.

Sure would be nice to put 4 in my dad's Citation II for his birthday.
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Old 13th March 2013, 01:29 AM   #2
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This post split off from KT120 thread in Tubes / Valves as it is MI related. Should get more attention here.
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Old 13th March 2013, 02:50 AM   #3
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Just FYI the ampeg is worth quite a bit if you can restore it...hate to see it get torn apart if it is salvageable.
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Old 13th March 2013, 05:17 AM   #4
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Yes, it is a classic blackline SVT, badly beat and broken, yet considered one of the best, even among really classic early SVTs. That's why it would be imperative to really do it right, functionally and cosmetically. This one's even got the 2 nice early toggle switches. Highly sought after by real touring pros. Long ago when I was in college two friends bought similar ones new for loud guitar, and they were incredible icons of the era. I'm still deciding exactly what to do with it. But biasing it for 6 KT120's sure wouldn't be degrading it, and it would be completely tasteful and reversible...if the sockets are spaced far enough to fit such a large tube. The opportunity to use a better/taller/deeper head case is now, when I'd be making the new cabinet anyway. That opens the door to a lot of reliability and/or output improvements, room for larger & much taller output tubes, baffled airflow to make the fans much quieter, heat-sink the yet still look very nearly stock. A lot of high-end hi-fi companies are embracing this new tube, and it looks like it's going to be made for quite a while. It's basically a KT88/6550 but with longer/taller plates. Used as a nearly drop-in replacement for the 6550 it's a big improvement in reliability with a bit more power and better bass, but in an optimized circuit a hextet should really be pretty incredible.

The big money comes not from fans, nostalgia, collections, or museums, but from successful touring working pro bass players who buy up every one they run across and deplete the supply. They would want a bone-stock known commodity with absolutely no mods. I would prefer to keep it and make it the best that it could be.

Last edited by cyclecamper; 13th March 2013 at 05:32 AM.
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Old 14th March 2013, 07:07 PM   #5
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I want to get an idea of whether I need to bump B+ to get the most out of KT120s, 'cause that's adding heavy parts. I'm hoping I can just drive a lower-impedance load, as long as the output transformer takes it.

Which load curve do I want to look at? I don't understand the abbreviations for the various voltages...
What are:
Ua V
UG1 V
UG2 V
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