1967 Marshall Super PA KT66 screen voltage question

[Charlie Wilson]

Hello, customer brought a nice 1967 Marshall Super PA to me to go through before he sells it. He told me it wasn't very loud and didn't sound that good. This amp should sound loud and great but I found that he had EL34s in an amp that uses KT66s and... the power transformer has no 110v tap. He was running 120v into the 220v tap. Since NOS GEC KT66 are going for about $1400 for a quad and they are the only tubes that I would feel comfortable with 560v+ on the plates and screens, I decided to have Chris Merren make me a Drake 1202-43 power transformer rather than have him get a step up transformer. Merren's transformer has the 560v secondaries as well as 500v and 460v.
My plan was to use the 500v taps so I could put some new production KT66 in the amp. Everything was great until I realized that the new production TAD KT66 has a limit of 500v on the plates but a 400v limit on the screens. So now the question; Has anyone used new production KT66 in a similar Marshall amp and how much wiggle room do I have with the screen voltage?
CW

[Matthews Guitars]

Your answer is....the Shuguang KT-66s are great tubes but you have that high plate voltage to deal with. So...get a good variac and run the amp at 90 volts and that'll both keep the plate voltages within the 500 volt limit and sound great.

Given the PT choice you've made, I'd really recommend that you use the 460 volt tap. Many legendary tones were crafted using the lower 460 volt B+ supply.

But the bias circuit has to be reworked and fixed resistor values have to change when switching from EL34s to KT66s or vice versa. I'd give you the new values to use if I was certain of them.

I've changed over my own '69 Superlead to use KT66s but I don't remember the values I ended up with for the bias circuit. I THINK I dropped the 220K to about 150K to put the bias into proper range.

[Roe]

changing the PT will reduce the value of the amp greatly. the plate voltage isn't the problem as much the screens voltage. With a normal 3h 110 ohms choke, the screens will draw too much current. But if you have the Radiospares 20h choke, with a 690 dc resistance, the screens should be fine, given screen resistors.

For high voltage, the best new tubes are chinese ones, sold by TAD and others.

The Russian gold lion tubes work at around 515v or 535v with the RS choke. the chinese kt66s handle even higher voltages. I recommend 500-540v plate voltage to get the right tone and response.

[Roe]

check if it is the 122-84 or 1202-119 output transformer, since both were used in 1967. the latter has a 1k7 primary and came with el34s.

[Charlie Wilson]

Thanks guys. Yeah this amp has a 100 ohm Drake choke so the measured screen voltage is the same as the plate voltage. I have the TAD KT66 but I believe that they are now made in Russia and the spec sheet says 400v for the screens. I am just wondering how literally to take that.
CW

[bepone]

new tubes should be avoided to run over 480V due to flashing and arcing inside, use lowest possible voltage

[Roe]

500V at the plate should be no problem with a 4k primary (given the -84 OT, not the -119 or -132!). The real problem is the screens. but you could always add more screen resistors. A shared, flying resistor between the screens filter cap and the tube socket individual resistors works well

[Stevem]

I agree fully with Matt!

Don’t butcher a original amp, get a Veriac!

One with enough amperage is likely cheaper then a new PT anyway!

[Roe]

yes, best option is to keep the amp original, using a variac (at roughly 220v) into the 245v primary

[Charlie Wilson]

Hmmm, let me rephrase my original question. Will TAD KT66 take higher than the spec sheet 400v? As far as variacs and butchering vintage amps. This amp is to be sold and I feel better selling an amp to a buyer that can plug the amp into the wall than having to get a step up or variac and hope they know what they are doing. Obviously the original transformer is included with the amp and is easily put back in if they choose. Again, I repeat, NOS GEC KT66 are $1400+ if you can find them so that is not an option.
CW

[martin manning]

CW, do you know what the OT primary impedance is?

[Charlie Wilson]

Martin, it has the Drake 1202-84 with a 4k primary. The amp had el34s in it when it was brought to me and sounded horrible.
CW

[martin manning]

So it's a 50W, then. I would keep the plate and screen voltage at 450 or a bit lower.

[Charlie Wilson]

Martin, 100w Marshall Super PA.
CW

[martin manning]

4k is 2x too high. I would expect problems with the screens overheating. Or maybe I should say it's a poor combination of plate and screen voltage and load.
edit: after looking up the spec I guess that was what they did, but it looks pretty bad, with the load line way below the knee.