API

Bias and voltage problem with Marshall JTM45 - New finds!
« on: October 18, 2008, 04:00:48 PM »
Hi.

I am working on a reissue Marshall JTM45 and have a problem.
When I got the amp the power tubes were burnt and also one of the input jacks had slightly melted.
Not sure what had caused this.

Anyway, I changed the filter caps, retubed it and checked voltages which all seems ok.
The amp plays and sounds good.
But, there is some oscillation going on, it almost sounds like a tremolo when played.
Also you can see the power tubes glows blue in pulses!
The Bias voltage goes up and down about + - 4 volts in the same pulses.
What could cause this??
I am running JJ KT66 and vintage Siemens and Brimar preamp tubes.
The same thing happened with the old preamp tubes in, but I did not check with other power tubes.

Any help appriciated.

Regards,
Dennis


mrphotodude

Bias and voltage problem with Marshall JTM45 - New finds!
« Reply #1 on: October 18, 2008, 04:14:56 PM »
that is odd.
That is exactly how a tremelo works out!
If the old tubes were burnt then they were either long overdue for a change, or had a pretty high B+ on them. If a jack was melted then obviously there was a heat issue or fire.

Have you checked all the resistors? check your grid leak resistors and your screen resistors closely. Also make sure none of the cathode resistors are shorting.
Oscillations can be caused by signal wires running too close to voltage wires(really keep them away from heater wires!), but that shouldn't give you too much of a trem sound.

Go to
http://vintageamps.com/plexiboard/index.php  and get an account and ask there.
those guys could probally point you in a better direction then i can.
I would just say check everything you can with a meter!
Even pull one end of the diodes out and check them.
another day...another dalek

gemini86

Bias and voltage problem with Marshall JTM45 - New finds!
« Reply #2 on: October 18, 2008, 05:13:43 PM »
first off, make sure all is well within the amp before powering it up with the tubes in. the blue arcing will make short work of that new kt66..
- Rodney

"...you better call Kenny Loggins, 'cause you're in the danger zone."

PRR

Bias and voltage problem with Marshall JTM45 - New finds!
« Reply #3 on: October 19, 2008, 01:35:18 AM »
> Oscillations can be caused by signal wires running too close to....

That gives high frequency squeal. It is possible that a hypersonic oscillation comes and goes, but...

> really keep them away from heater wires!

That gives hum.

> I changed the filter caps

Filter caps are what keep power stage signal current from leaking down the B+ chain to sensitive input stages. And since caps suck highs the best, when they get weak it is usually a subsonic oscillation. So are you sure of your cap-job?

> Have you checked all the resistors?

This too. Designers can skimp B+ filtering on push-pull stages because rail wobble cancels. The first motorboat problem I ever met turned out to be two "47K 5%" resistor which had drifted to ~~70K and ~~120K. The unbalance was enough (and in the right phase) to let power stage B+ wobble into the push-pull driver which amplified it around and around and around....

VacuumVoodoo

Bias and voltage problem with Marshall JTM45 - New finds!
« Reply #4 on: October 19, 2008, 03:51:02 AM »
Is this oscillation/tremolo effect in any way affected by the Presence control?
If yes, then connections of output transformer primary to power tubes' anodes are reversed. Swap them and check again.
Alex Niemand
_____________________________________
Life's a party but you get invited only once...
Tubewonder amps
"L

API

Bias and voltage problem with Marshall JTM45 - New finds!
« Reply #5 on: October 19, 2008, 05:58:35 AM »
Hi everyone.

Thanks for the great input!!
I will check everything pointed out and report back later today.

Dennis

API

Bias and voltage problem with Marshall JTM45 - New finds!
« Reply #6 on: October 19, 2008, 04:17:58 PM »
Hi again.

Ok, first thing I found out today, and yes, please call me dumb, is that I had put a pair of KT88´s in the amp!
I gross JJ tubes and this pair had ended up in the KT66 box and I did not notice that since the look identical on the outside.
Anyway, this was not the fault and I do not think that was the root of the problem, or?

Actually the pulsating noise and audible tremolo effect was gone but there is still something strange going on inside the amp.

First, I have a hard time to bias the amp properly.
I wanted to set the bias at 40 mA idle but I can not do that with the stock value resistors.
I could do 20 mA and the amp works this way but sounds a bit cold and hard.
The current is stable though with the KT66´s in and when the amp is on the bench.
The strange thing though is that when I powered it up there is a blue glow to the power tubes, nothing strange in itself i believe, but when I plug in a cable to the input jack and short the other end of the cable the KT66´s start to "swing" and the blue shine is alternating beteween the tubes!
First slowly then stronger and faster.
There is no audible effect of this though.
At this point I turn off the amp.

Does this ring a bell with anyone?
Why does the oscillation (if it is that) start first when I plug something in?
The melted input jack may have had something to do with this before I believe and maybe this is why the amp broke in the first place.

Tomorrow I will go back and check resistors to see if any have drifted.

Dennis

API

Bias and voltage problem with Marshall JTM45 - New finds!
« Reply #7 on: November 11, 2008, 09:02:23 AM »
So, I have finally had some time to do some tests.
I have checked all resistors and they are all good, so that seems not to be the problem.

What I do have foud out though is that I seem to have low voltages in the amp.
I found a voltage chart for the JTM45 on the Metroupolis page and my voltages are about 30-50 volts low everywhere in the whole amp.
I did some tests with and without the output tubes in the amp.
Voltages going into the rectifier are ok at 320v AC, coming out of the recitfier without the output tubes are 440volts DC which is good I think (I tried several rectifiers all with the same result) but when I plug in the tubes (JJ KT66´s) the voltage coming out of the rectifier drops to 350 volts, this voltage should be around 400 volts according to the voltage chart I have.
I checked the output transformer and that seems ok measuring 75ohm for each side of the center tap.
The inductor measured 100 ohm across it (could not measure the inductance).

I changed the output tubes for a pair of new JJ EL34 and these were giving a slightly higher voltage at 365 volts coming out of the rectifier.

I assume that it is because I have this low voltage in the amp that I can not get the amp biased correctly.

What could be the problem??
Is it normal for the voltage to drop this much with the output tubes plugged in or is it something else down the line that drag down the voltage??

Any thoughts appriciated

Dennis


 

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