PT for JCM/JMP?

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

Post Reply
iknowjohnny
Posts: 1070
Joined: Thu Apr 24, 2008 2:10 am
Location: los angeles

PT for JCM/JMP?

Post by iknowjohnny »

When i first built this amp it was slated to be very different amp than it now is and i bought the PT i used because it had a tap for a tube rectifier and the rest of the specs were better than others they had. long story short, the amp is now much more similar to a JCM or JMP style amp and i now have a diode rectifier too. So the PT is 300-0-300, and i'm now wondering if i should shite-can it and get a more suitable PT because i'm seeing that the PT replacement for htis style of amp is 345-0-345. Lately i'm starting to find the tone i'm after is better with considerably more voltage then i get out of the current PT. V1 is only around 200v on each side so as a test i tried tapping from the rail before the PI to raise v1 up. It sounds better as i raise it, and i looked at a JCM voltage chart that was showing at least 100 more volts then i have there. Even the replacement won't yield that downstream, but at least it will be better. I'm not to well versed in this stuff, so any advice is welcome. Should i get a different PT?
breakfastbuddy
Posts: 102
Joined: Wed Aug 02, 2006 7:49 pm
Location: Norway (south)

yes save that one for a trainwreak clone

Post by breakfastbuddy »

the 2204 use that PT voltage . and i want to have it replaced with a PT that gives 350 vac , :)
User avatar
Structo
Posts: 15446
Joined: Wed Oct 17, 2007 1:01 am
Location: Oregon

Re: PT for JCM/JMP?

Post by Structo »

Remember that the voltage will be 1.4 X the secondary when using a full wave SS rectifier.
Tom

Don't let that smoke out!
User avatar
Phil_S
Posts: 6048
Joined: Tue Oct 23, 2007 10:12 pm
Location: Baltimore, MD

Re: PT for JCM/JMP?

Post by Phil_S »

If you want more voltage in the preamp section, reduce the value of the dropping resistors in the B+ ladder. Try 1K 1W resistors and see what happens. Tack them in parallel to the existing ones just to check it out. If you like the result, make it permanent and if not, remove them.
iknowjohnny
Posts: 1070
Joined: Thu Apr 24, 2008 2:10 am
Location: los angeles

Re: PT for JCM/JMP?

Post by iknowjohnny »

Like i said, i HAVE done that. Or at least i tapped off the B+ at the PT screens which is the furthest back i can go without going before the main cap can, and took that directly to V1 where i want higher V. And still voltages are way under a JCM.
Post Reply