70s Marshalls Low HT Wattage

[RockinRocket]

Hmm that's very interesting. I was under the impression it was basically late 70s that had low volts. Bias can change volts by 20 or so. Were they using a variac at 110v for to simulate the 70s line power? It seems odd to me all the volts were low on mk2's... It's a tough topic with out doing apples to apples comparisons as I've struggled to find any good accurate info on this

[Littlewyan]

They didn't say unfortunately. I heard that the 70s Marshalls were all on pretty low voltages, even the Master Volume Amps. It wasn't until the JCM800 line came out that they went back up to 460VDCish. Thats what I've heard anyway.

However at the same time I heard that it was a crapshoot with regards to what you'd get. The Transformers varied a lot I believe.

[Tazin]

I believe it's around 1972 that the 50w amps started to get the lower (apprx. 390vdc) H.T. voltage transformer.

[RockinRocket]

Its a bit confusing. When was it 390V? Back in the 70s when the wall put out 110v or modern 120v? My 72 puts out 430 or so at 120 wall. But like I said previously it can be anywhere between 420 and 440 depending on 50% to 70% bias. Id imagine it be 400v at 110v.

The transformers weren't consistent back then were told and its believable. But what spec were Marshall aiming for? Probably 400v back then that would make 430v with todays USA voltage. Thats basically the voltage all transformer clones are putting out.

Now does it matter what country your amp was designed for too?

Also I think its excepted talk that Marshall JMP 50s got gainer and more aggressive through out the decade. But HOW could that happen if low volt tranformers started around the PCB era. Think about it. The B+ dropper line had more volts in the preamp during the 68-71 era amps. The preamp volts got lower for the 72' - 76'ish with 4 -10ks. Now when the 2204 amps came about in 76' we start to see only 3 -10ks. That would raise the voltage to the preamp making the amp not as gainy or aggressive as the 72-76' amps. I believe to raise the preamp voltages that would have been crazy low otherwise.

My guess, the earlier amps with low volts were just one offs oddballs that didn't meet Marshalls specs of 400v then/ 430n at modern volts

[Tazin]

From what I remember the PT's with the lower H.T. (B+) voltage started sometime in 1972. Then sometime near the end of the 70's the H.T. (B+) voltage went up again.

[Tazin]

In 1970 AC voltage in the USA was between 115Vac through 120Vac depending upon where you lived in the country. In the northeast the voltage was 120Vac, and was considered 'Type 3' when it came to design specifications.