Plexi VS jcm800 power amp section

[fossilshark]

Hi all! time for my yearly noob question!

So I want to do a JMP but with an extra gain stage, I am pricing out components and really looking hard at the JCM800 2204 and 1987 schematic.

My question is: what is the sonic/practical difference between the 1987 and 2204 power section? Obviously the 2204 has 5.6K "control grid" resistors and 1.5K "screen grid" resistors while the 1987 does not. Were those additions to the circuit purely for stability reasons or does it have to do more with the HT voltage of the two amps?

I am going to be running some old ClassicTone power/OT transformers I bought back when they were in business and if I remember correctly they were sold as JCM800/Plexi replacements.

Any advice greatly appreciated!

[tubeswell]

My question is: what is the sonic/practical difference between the 1987 and 2204 power section? Obviously the 2204 has 5.6K "control grid" resistors and 1.5K "screen grid" resistors while the 1987 does not. Were those additions to the circuit purely for stability reasons or does it have to do more with the HT voltage of the two amps?

Cork sniffy difference IMO. Maybe get a more satisfying answer in one of the Marshall amp forums

[pdf64]

For any given production era, my understanding is that the power amp / supplies of 2204 and 1987 models are pretty much identical.
Screen grid resistors are 1k 5W.
Other than perhaps UK Mullards, it would be very injurious to their operating life to run any EL34 without them.
Also it would be a bad idea to copy the JMP era hot switching standby arrangement, in which the bias supply gets switched along with the HT.

[fossilshark]

Ok awesome, that cleared it up for me. Thanks!

[glass54]

Totally agree with pdf64!!
Consider it an opportunity to build a reliable Bias supply with your ClassicTone Mains Tx.
Also, in the late 90's and onwards, I found that some of the Marshall amps had Heater Volts well in excess of 7V AC. So I put in series resistors (balanced low value resistors, in each Heater leg to adjust for around 6.4V AC). This helped with problem amps that got very hot and ATE EL34s .
Regards
Mirek

By the way, did you have a look at this? (Its a good guide  )
https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf

[fossilshark]

Totally agree with pdf64!!
Consider it an opportunity to build a reliable Bias supply with your ClassicTone Mains Tx.
Also, in the late 90's and onwards, I found that some of the Marshall amps had Heater Volts well in excess of 7V AC. So I put in series resistors (balanced low value resistors, in each Heater leg to adjust for around 6.4V AC). This helped with problem amps that got very hot and ATE EL34s .
Regards
Mirek

By the way, did you have a look at this? (Its a good guide  )
https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf

wow ya sometimes i forgot just how much knowledge is on this site. I will definitely be looking at this as I finish my design. Thanks!