Re: HELP JTM45 Pb
- From: Lord Valve <detritus@xxxxxxxxxxxxx>
- Date: Sat, 21 Mar 2009 08:22:12 -0600
Steve wrote:
On 2009-03-20 20:51:07 +0100, Lord Valve <detritus@xxxxxxxxxxxxx> said:
Steve wrote:
On 2009-03-20 19:59:42 +0100, Lord Valve <detritus@xxxxxxxxxxxxx> said:
Zilog wrote:
Steve wrote:
On 2009-03-20 17:13:34 +0100, Lord Valve <detritus@xxxxxxxxxxxxx> said:
Steve wrote:
Hi all,
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:
Snip....
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?
Thanks ;-)
Chinese tubes...
Nonsense.
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
just fine.
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
you.
Now, you want to talk about some suck-ass preamp
tubes, that'd be Shuguang. Yeeccchhh!!!
Lord Valve
People's Republic of Obamastan (occupied USA)
BaaaarrrrRRRAAAAACCCCKKK!! <Safety!!>
One
Big
Ass
Mistake,
America!
How do you explain the fact that all my other JTM45 handle the Shuguang
just fine, and this one does not?
I don't explain it, I merely note that it is occurring.
Same here...
I'd need test equipment to explain it.
Of course. I was just asking because I thought you had already seen
this particular behavior. ;-)
Is it the plate voltage and the lack
of swamp resistors of the original circuit?
If it has no swamp resistors and it's oscillating...you tell me.
Well, I can't explain it either. I can only suspect, as I said, the
higher plate voltage in this amp.
My other JTM45 are dead mint originals and, of course, have no swamp resistors.
And with te same tubes, they are not oscillating at all.
I'd really like you, or someone else for that matter, to give a clue as
to what it could be besides the higher plate voltage.
I'd like to understand.
All my JTM45's have the same tubes: NOS Mullard's ECC83, NOS Mullard
GZ34, Shuguang KT66 ( which I bought from you and are perfectly matched
BTW )
Thank you.
Steve
Comes under the heading of Shit Happens. ;-)
Put some swamp resistors in and see if it goes away.
Lord Valve
People's Republic of Obamastan (occupied USA)
BaaaarrrrRRRAAAACCCCKKK!! <Safety!!>
One
Big
Ass
Mistake,
America!
.
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