Re: HELP JTM45 Pb
- From: Steve <stephen@xxxxxxx>
- Date: Fri, 20 Mar 2009 20:22:18 +0100
On 2009-03-20 19:59:42 +0100, Lord Valve <detritus@xxxxxxxxxxxxx> said:
Steve wrote:On 2009-03-20 17:13:34 +0100, Lord Valve <detritus@xxxxxxxxxxxxx> said:
I'm a qualified tech, but I have to admit that this time I'm having a
big problem with this 1965 JTM45 I just bought.
I revised it, etc.. But something is wrong:
I'm not sure for 100% but that's probably the cause of your problem:
I suspect the tubes are not handling the higher plate voltage this
particular amp has when compared to the others I have.
the other 3 (circa 1963, 1964, 1965) have a PV in the 395-420V range.
This one has 455V.
Could that be the case. Maybe older NOS tubes can handle the voltage
these particular tubes can't.
Any other explanations? What do you guys think? Are the Shuguang KT66
not robust enough for this amp?
Or is it something else?
The Shuguang KT-66 is excellent. He has no swamp
or screen resistors, and three feet of wire hanging
off his cathodes. His amp oscillates when thus
configured. The fact that it doesn't oscillate with
6L6s is meaningless. I've set many of those amps
up with the Shuguang KT-66s, and they all worked
NOS is one thing and not all are OK.
Then they are ridiculous expensive for the given quality.
Use known good tubes.
JJ can handle high voltages and of the current production EL34's
(example) they can handle the highest voltage (800V)
That spec is valid only when the screen is held to 400V.
JJ E34Ls are no better or worse than anything
else out there, and they will fail when the screens
are abused - as they are in nearly all guitar
amp designs. You want an EL34 that can
handle screen abuse, try the Shuguang EL34B-HT,
which is rated for 600 VDC on both plate *and* screen.
Tough tube. How it sounds, of course, is up to
Now, you want to talk about some suck-ass preamp
tubes, that'd be Shuguang. Yeeccchhh!!!
People's Republic of Obamastan (occupied USA)
How do you explain the fact that all my other JTM45 handle the Shuguang just fine, and this one does not? Is it the plate voltage and the lack of swamp resistors of the original circuit?
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