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:

I CANNOT bias the tubes. When my Bias Rite is plugged in, the amp is
motorboating badly!
Which it doen't do when no Bias Rite is plugged in.

I know the Bias Rite works properly as it only has this effect on this
particular amp.

Apart from motorboating, the bias is widely fluctuating from 35mA to
65mA on both tubes.
I changed all parts in the bias circuit and the same thing occurs.
I'm too much involved and need fresh views. I don't see anything anymore.
You know what I mean.

Please, help. Every input will be appreciated.



Check to see if grid swamp resistors have been fitted.

These will be 5.6K, sticking into the air from pin 5.
If they're not there, put 'em in. Ditto for screen
resistors - 1K/5W, in series with the screen voltage
feed to pin 4.

Check to see if your bias probe measures the
cathode current by reading the DC millivolt
drop across a 1-ohm (could be another value)
resistor located INSIDE THE PROBE. If it
*breaks* the cathode connection so that an
external meter can be inserted in series,
that is almost certainly the problem. Older
Marshalls do *not* like having two or three
feet of wire added to the connection between
the cathodes and ground, and will oscillate
like crazy if this is done. Just for shits and
grins, ditch the bias probes and solder in
some 1-ohm resistors in place of the existing
ground wires which run from pin 1 & 8 to the
ground lugs attached to the socket mounting
screws. Read the drop across them with your
meter set to the lowest DC millivolt scale. The
idea here is to leave the ground connection
as SHORT AS POSSIBLE. You can leave the
resistors in place - the tube can't tell 'em from
a piece of wire.

Lord Valve
People's Republic of Obamastan (occupied USA)
BaaaarrrRRRAAAACCCCKKK!!! <Safety!!>


Thank you both.
I have several of these amps. They all have the same original circuit:
2 X 470R screen resistors, 1 X 1K in series with the screen resistors, and no swamp resistors.
But none had this problem.

Oddly enough, it seems the problem comes from the tubes I used.
It is true that I changed the tubes several times around.
However, I always used the same brand and model (Shuguang KT66).
Like on the other original JTM45's I have.

Then, before putting in 5.6K swamp resistors and changing the grid res, I decided to try another set of tubes.
I used NOS GE-6L6GC. The problem is gone. Very weird .... The amp is fine and sounds awesome.

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 ;-)