Kt120 and class B

[fakeox]

I've got a 1972 Traynor Mark II stuffed full of Mustard caps. It's like the meanest 50w metal face to me. Right now with the KT120s in it, it puts out about 60w in Cathode Bias & a little over 100 in A/B. Would it be better served in Class B? The tranny throws about 560 & the KTs will handle about 800v. Any thoughts anyone? TIA!

[daveweyer]

Wow, KT120s! From what Bob Hovland tells me they are pretty good tubes, nearly up to old American standards.
Go ahead and give class B a try. Once again you may need to beef up the driver a little bit.
And please, give us the benefit of your tests with a clip of how it sounds!
Just love this experimenting.........
Generally speaking, the whole point of the KT120 was a lot of plate dissipation; the purpose being class A amplifiers with more power. Since in class A the tubes dissipate the maximum at idle, it's hard to get high powered class A amplifiers unless you can increase the plate dissipation in the tubes. With KT120s you could conceivably build an amp with 100+ watts of output power with just two tubes, all in class A. You might need a fan though.

[fakeox]

Hi Dave, thanks for the quick reply! The YBA1A Mark II comes with a big fan stock. When it was new it came with Mullard 6CA7s stock. As it puts out 100w in A/B would it be a little more powerful in Class B? I play out as a bassplayer more often than a guitarist so any more juice is a good thing..

[daveweyer]

You could safely get 140 watts in class B. But you will have to have more drive voltage for the output tubes, AND, you will have to have a power supply which can support that much current.
If the amp has a 12AT7 driver, try raising the voltage to its supply up to about 400 volts, so that you can get maybe 230 to 250 volts on the plates of the driver tube, depending on what the voltage on the cathode is. You would be looking to get more swing in that stage.

[fakeox]

Much appreciated Dave!

[fakeox]

Dave I realize SoCal is not a place where Traynors are seen often in the wild but if you have any more tips or thoughts on making this beast all it can be. (50w sounds at 140w)

[daveweyer]

I've got a friend with one of those amps. I may be able to borrow it to get a better idea of its potential.
I've seen number of them around here, pretty much in the ball park with the Marshall, Fender, Hiwatt sort of thing.
If I can check it out thoroughly I might be able to offer more specific information.

[fakeox]

Thanks so much! Mine is the bumper (70s) version. No choke & higher plate voltage than the 60s version. It uses the same PT as its big bro, the Custom Special.

[daveweyer]

Does your amp have about 450 volts on the plates, and originally containing 6CA7 tubes?
If it does, then you will top out at 70 watts, no matter which class you use. The only way to get more from that situation is to use Horizontal deflection output tubes, and they require a much lower screen voltage, about 150 volts. They usually have plate caps too, meaning you have to run a separate wire from the plates to the output transformer.
It's all fun if you are in to soldering and tech sort of stuff. You would most likely also need an output transformer with a lower impedance primary, or else double up your speakers to put more load on the tubes.
Maybe this is too much.

[fakeox]

560v on the plates. Yeah, they had Mullard 6ca7s that tear. Year, sorry

[daveweyer]

Well, if you've got 560 volts, and the regulation is pretty good, you'll get 100 watts no problem. Check the plate voltage at full volume.
The amp schematic of "Big Boy" that Xplorer posted for me had 560 volts on the plates, and it does 180 watts.
The Kt 120 is easily capable of 120 watts and more.

[fakeox]

My favorite cleans are "Wind Cries Mary" & for raunch, 50w Pearly Gates metalface does it for me. The Mark II is kind of big warm & round. Anyone have tips to head towards either sounds?

[fakeox]

Any luck chasing down your friends Mark II Dave? You could make him (& me) very happy..

[daveweyer]

Yeah, he's got it but been real busy with the Christmas riot and all that. Not sure when I'll see him, but he's got no objections to messing with it.

[fakeox]

Cool!