Metropoulos Forum

As I understand it the 70s Marshalls ran on a lower HT voltage, roughly 380V. What wattage would these amps put out? I have a JTM50 I built which runs on 460V with a GZ34 and it easily puts out 50W clean and about 90W full blast. Obviously as you can imagine its far too loud so I was wondering what a low HT Marshall would be like.

If they ran 380v then they would run 410+ in todays voltage. But I think they ran more like 400 then.
Cant answer your question but in reality any el34 amp is going to be too loud anyways for wattage to really matter

Its more to keep my attenuator cool to be honest. It runs a bit hot with my current Marshall. Any drop in wattage will help.

But I agree it will always be too loud!

What attenuator?

Its one I built, basically a copy of an Airbrake but I modified it so it doesn't mismatch the impedance quite so much. After using it on the Marshall it can be too hot to touch.

about 35w clean with 380v on the plates, at least theoretically

410v on the plates gives ca 40w clean theoretically. 400v gives ca 38w

Thanks Roe thats really helpful.

I really want to build this amp but at the moment the parts required are coming to £660. Might wait a bit to see if the Exchange rate improves as the transformers are coming from the US. I like to use Classictone. They have a PT with multiple secondaries.

If I do build it then I will put in a 3 way Standby Switch so I can go between 380V and 460V. Just to make it a bit more interesting . I think the 380V setting will be better as it should make the amp a bit more.....chewy? As well as obviously lowering the output a tad.

For low voltages, reduce the screen resistors and the 8k2/10k power resistor between the power amp and pre amp

Would it not make it a bit softer and more compressed if I left the 1K screen resistors and the 8k2/10k resistors in?

I noticed they only used one 10K resistor between the PI and Screens in JCM800s, which makes sense if they kept them on 380V but they bumped them up to 460V later . I bet V2 blew quite often!

Basically the JMP plexi circuit had the 2x 8k2 and 2x 10k. Then it switched to the 4x 10k around 72" that would drop the preamp volts a bit more too. Later, 76", most likely for the lower volt transformers they only used 3x 10ks (4 x 10ks would have really low volts) to compensate the preamp volts for the already lower volts to the whole amp.

It would be nice for someone who had most of the models or doubles to take volts (with the same bias/tubes installed for accuracy) to make a JMP volt chart.

I looked at some of the amps on amparchives.com and the 72 amps use 2 x 10K droppers between screens and PI. I don't think it was until the very late 70s that they used one 10K dropper. Thats going by schematics I've seen though.

Yes you are correct. I was referencing the whole B+ dropping resistors.

I did find a post from someone on another forum where someone had been inside various Marshalls and had measured voltages.

#1: '69 marshall #1992 (US) - 493V
#2: '72 marshall #1986 (HOME) - 409V
#3: '74 marshall #1987 #1 (HOME) - 358V
#4: '74 marshall #1987 #2 (US) - 343V (set-up as a #2204)
#5: '78 marshall #2204 (HOME) - 360V
#6: '74 hiwatt #sa212 (50w) - 530V

I didn't know that some Marshalls were as low as that until recently, I thought 380V was the lowest! Shame they didn't measure their mains voltage as well and tell us what primary tap they used.