Bias Marshall JCM 900 model 2501 combo

[pchilson]

Marshall JCM 900 model 2501 1x12 combo

EL34 powertubes
Bias pot only allows to get bias to 21mA
Tubes are pretty well matched about 1mA apart

Some of these amps apparently came with 5881s and some with EL34s
This one has EL34s but I have no way of knowing if it came from the factory with them or not.
I'm seeing reference to "R28" in relation to bias "range".
I see schems with R28 as 47k and some with it at 22k
Is this where I need to go to get the bias range I need?

Also, with my Weber Bias-Rite I read about 519v on the plates. Is this in the range for this amp or high? The caps are only rated for 500v, this makes me wonder.

Thanks for any and all help.

[gomez63]

I'll reference my notes and get back to you...however, how accurate is your meter?

[pchilson]

I'll reference my notes and get back to you...however, how accurate is your meter?

Are you meaning the Weber Bias-Rite?
Are these known to be off? hmmm...

[Jazz P Bass]

Marshall Bias Voltage

What -Vdc do you measure on pin #5 of the EL34's?
At idle, no signal applied.
(I think the last post wanted to know if you have a high quality volt meter)
The plate voltage does seem a tad high.

[pchilson]

What -Vdc do you measure on pin #5 of the EL34's?
At idle, no signal applied.
(I think the last post wanted to know if you have a high quality volt meter)
The plate voltage does seem a tad high.

-45.2 on one
-45.9 on the other
These seem to drop slightly the longer the amp is on. They both started about -46.2 and dropped to the above after about 5 minutes.

Plates are reading 511v tonight, both of them.

My meter is no Fluke but its not a $5 Harbor Freight either. I can't speak to how accurate it is. I don't know.

[gomez63]

Are you meaning the Weber Bias-Rite?
Are these known to be off? hmmm...

It depends on the tolerance of your meter (i.e. quality).

Take 500v with 5% tolerance, the actual voltage could be 525v or 475v.

[pchilson]

It depends on the tolerance of your meter (i.e. quality).

Take 500v with 5% tolerance, the actual voltage could be 525v or 475v.

Ok, I see what you are saying.

Last evening the plates read 511v with my DMM. This was after setting the bias as high as I could get it the other evening.

The other evening the Weber Bias-Rite read 520v on the plates before I changed the bias up.

For the record, the bias readings were in the 8mA range when I first checked. I was able to get it to approx 21mA by use of the adjustment pot hence the reason for the thread.

Thanks

[gomez63]

the fluctuations may be from the wall (ac) or may be from the meter's accuracy. Keep in mind that you should let the amp warm up...your bias readings will run up the range and then slowly backs down...this depends on the amp, in most cases, a two minute time frame works.

My apologies, I forgot to check my notes on the JCM 900. Just looking over the schematic, R28 is the one to adjust from 47k down to 22k...you could go as far as 15k, depending on the tubes.

[pchilson]

the fluctuations may be from the wall (ac) or may be from the meter's accuracy. Keep in mind that you should let the amp warm up...your bias readings will run up the range and then slowly backs down...this depends on the amp, in most cases, a two minute time frame works.

My apologies, I forgot to check my notes on the JCM 900. Just looking over the schematic, R28 is the one to adjust from 47k down to 22k...you could go as far as 15k, depending on the tubes.

Thanks gomez63.
Both times I read the plate voltage the amp had been on and off standby for about 5 minutes warming up. I'm not worried about some variation due to meter difference/inaccuracy or wall flucuation. I am concerned about > 500v on the plates when the filter caps are rated for 500v.

So, changing R28 down to 22k or thereabouts will allow more "range" in the bias pot?

[gomez63]

Thanks gomez63.
Both times I read the plate voltage the amp had been on and off standby for about 5 minutes warming up. I'm not worried about some variation due to meter difference/inaccuracy or wall flucuation. I am concerned about > 500v on the plates when the filter caps are rated for 500v.

So, changing R28 down to 22k or thereabouts will allow more "range" in the bias pot?

Looking over my notes on 900's that I've serviced, I see an average of 485v. 500v may be acceptable...again, depends on the meter accuracy. What to be aware of here is the condition/quality of those caps.

If they are undergoing 500+ volts, it may be time to change them out. Some caps can handle those volts even if rated at 500v max....some just can't. Personally, I've been using the F & T's with great success and do recommend them. The JJ's have done well too.

Dropping R28 down will open the bias range up...depending on the tube...but will work in your case.

[pchilson]

Looking over my notes on 900's that I've serviced, I see an average of 485v. 500v may be acceptable...again, depends on the meter accuracy. What to be aware of here is the condition/quality of those caps.

If they are undergoing 500+ volts, it may be time to change them out. Some caps can handle those volts even if rated at 500v max....some just can't. Personally, I've been using the F & T's with great success and do recommend them. The JJ's have done well too.

Dropping R28 down will open the bias range up...depending on the tube...but will work in your case.

As I indicated, I have no way of knowing any service history on the amp but I can't see evidence of any "work" so I assume all original components other than the tubes.

I'm going to swap out R28 and get the bias corrected and then re-evaluate the amp to see where I want to go from there.

Anybody know what the branding on the tubes that Marshall would have used from the factory? Did they have them branded with their name or no?

These EL34s are branded "Audio Classic" made in China and two of the 12AX7s are something like "ARS"??
I assume these are not what Marshall inserted at the factory...

[pchilson]

Curious, I went in to mess with R28 and it was a 220k resistor and not a 47k as assumed.
A little digging and I find this schematic which is an earlier version pcb and it turns out to match what I have and the "range" resistor is R30 which was 56k. I swapped it for a 47k and now I have the range I need for proper bias.

Funny cause you can actually read that schematic unlike the others I've seen which are blurry and hard to make out values.

Now it would be nice to get the preamp schematic of that version. Anyone?

[pchilson]

Ok. Found the preamp schematic from the same source. It is very clear and readable as is the poweramp schematic.

If anyone needs/wants these versions you should save them, who knows how long they will be up.

Preamp schematic

Poweramp schematic

[jakehop]

500V+ is a lot more than the 400-425V the screen grids of EL34's are rated at. I usually install a voltage regulator if the customer wants it - most customers appreciate the benefit of longer tube life.

Kind regards, Jake

[gomez63]

...you'd be surprised at what an EL34 can handle...I've seen Traynors with 560v on the plates and Marshall 1968's with 495v on the plates...