Question about BIAS Marshall JTM 60

[torcamaniac]

Hi guys,

just biased my JTM 60 with output transformer resistance/voltage method.
I wanted to install 1 or 10 Ohm resistors to the cathode of each El34, but it would've been a mess 'cause of amps layout and printed board.
Anyway, first I measured the resistance of both halves of OT. These where fairly matched. One side 74,2 Ohm and the other 73,9 Ohm.
Then I took the plate voltages of each tube. Also both around 490 V.
I wanted a bias setting of around 68%, so 25W x 0.68= 17 W : 490 V= 0,035 A or 35mA rounded.
So I needed to set the voltage drop on output transformator to 74 Ohm x 35mA= 2.6 V
The only thing I'am not sure about is that while measuring this voltage drop and setting it to around 2.6V it would drop to around 2.2V every 10-15 seconds and back.
Is this right?
Also after this I measured the bias negative grid voltage and it is around -42,7 V ( Marshall techs suggested setting this to -42 V anyway, so I guess that's about right).
The only concern is as I said the changing voltage drop on OT and wanted to ask you if that's normal?
My guess is that this could also be due to capacitors in bias circuit charging/discharging, as these voltage drops are not fast changing back and forth....

[Enzo]

SO monitor the B+ supply, does it dip on the same time schedule? Does the bias voltage dip or increase on this schedule?

[torcamaniac]

Hi Enzo,

thanks for answering.
I'll check the plate voltage tomorrow but I did notice that the bias voltage dipped (pretty slowly) from around -43.5V to -42.7 V, or better said increased because we're talking about negative...

[mozz]

Monitor your input line voltage while you do this if possible. See if the variations are in sync. Most likely you have a bad filter cap somewhere.

[torcamaniac]

Mozz, you probably think one or both of the negative bias filter caps or?
I ordered some anyway and will replace them.
The normal big filter caps are new. Replaced them last year.
Here is the schematic. I colored those I think you meant red.

[Enzo]

We should really be loking for a reason to think those caps are bad before replacing them. Get out your scope or meter, is the bias main supply varying? Is the bias voltage at the pot steady or not?

[torcamaniac]

Enzo, the amp is now around 20 years old, so I wanted to replace them anyway and also all the heater caps.
Don't have a scope unfortunately, but I'll monitor the voltages a bit longer with meter tonight or tomorrow.
Also ordered a second multimeter, so I can monitor voltages for 2 different places at the same time

[Enzo]

I have no problem with replacing old caps, but I try to steer people away from "guessing". Maybe it is this part? Maybe it is that part? I am a firm believer in solving any problems in an amp before wholesale upgrades.

So much of what we can do with a scope, we can also do with a meter, the scope gives a more clear picture, but I can watch a voltage wandering on a meter.

I use good quality Fluke meters in my shop, but I also have some of those ultra cheap (often free with coupon) meters from Harbor Freight. They are perfect for secondary stuff. If you are watching B+ climb up and down, what you care about at that moment is the change, not so much the accuracy. I can clip my free meter to one thing and use my good meter on my point of focus.

[torcamaniac]

I have no problem with replacing old caps, but I try to steer people away from "guessing". Maybe it is this part? Maybe it is that part? I am a firm believer in solving any problems in an amp before wholesale upgrades.

So much of what we can do with a scope, we can also do with a meter, the scope gives a more clear picture, but I can watch a voltage wandering on a meter.

I use good quality Fluke meters in my shop, but I also have some of those ultra cheap (often free with coupon) meters from Harbor Freight. They are perfect for secondary stuff. If you are watching B+ climb up and down, what you care about at that moment is the change, not so much the accuracy. I can clip my free meter to one thing and use my good meter on my point of focus.

You mean how fast it's changing?
If lets say I would monitor the plate voltage and bias voltage parallel with two meters and these dip/increase proportionaly and equally fast, what would that mean?

[Enzo]

That they were probably related. What if you monitor your B+ or whatever it was you first saw was cycling at 10-15 seconds, but the bias voltage was not. That would indicate to me that the bais was not the cause. On the other hand if they both move similarly, then they are related. Maybe causally or maybe just because something else was affecting both. Like transformer voltage, so we also lok at the mains voltage, just as example.

[pdf64]

...I did notice that the bias voltage dipped (pretty slowly) from around -43.5V to -42.7 V, or better said increased because we're talking about negative...

No, we refer to the magnitude of the voltage, so it's dipping when it moves from eg -43 to -42.

It seems a flawed design for the bias supply to be (zener) regulated, whereas the HT is free to follow the mains.
Preferable for both supplies to track the mains, which should tend to stabilise the operating conditions against mains fluctuations.

[torcamaniac]

That they were probably related. What if you monitor your B+ or whatever it was you first saw was cycling at 10-15 seconds, but the bias voltage was not. That would indicate to me that the bais was not the cause. On the other hand if they both move similarly, then they are related. Maybe causally or maybe just because something else was affecting both. Like transformer voltage, so we also lok at the mains voltage, just as example.

Hi Enzo,

I finally found time to check and monitor.
I let the amp warm up nicely. The impendance was 74,5 Ohm on OT. The plate voltage changet to 461 V.
And I monitored the plate voltage and OT voltage drop together and yes the dip/increase is in sync.
To be more exact the voltage drop was increasing/dipping between 2,56V to 2,66V as was the plate voltage between 465V to 460V.
Also checked the negative bias voltage and it was only changing from -39,33 to -39,31 (I guess this one could be ignored).
So this tells me that I'm biased to 36mA at about 66% dissipation.

[torcamaniac]

Well I guess the temperature plays a big role here...
Was checking and setting the bias back and forth.
After a while I checked the impendance again and it was settled and raised at 75,5 Ohm...
So I went for the bias of around 60% . Guess this might be a sweet spot. Checking and setting the bias pot until the plate voltage settled on 461-462V.
So, 25W x 0.60 / 461V = 33mA x 75.5 Ohm = 2,45 V voltage drop. The tubes are well matched. Difference less than 2mA...
I noticed that when biased hotter the hum gets much louder and there is more chanel bleed from gain to clean?
The bias range with pot lets me go to -49.5 V. At settings from -43 to -49 there is almost no hum and no chanel bleed at all... But if I set it this cold tge plate voltage raises and if I calculate the bias current again I would be between 5mA - 19mA???
Weird...

[mozz]

Do you mean resistance when you say impedance? Of course the voltage drop across the output transformer is going to change when the plate voltage changes.

[torcamaniac]

Do you mean resistance when you say impedance? Of course the voltage drop across the output transformer is going to change when the plate voltage changes.

Yes resistance