One output tube in my Marshall 1987X plexi RI?

[No effects]

I can no longer find the thread, but I thought someone said they were running their 50x plexi with only one power tube. I know you can take 2 of the 4 output tubes out of most 100w amps, but running a 50 watter with one tube!? If this can be done, I would really like to know how, what happens to impedence, sound. etc...thanks

 

[Simon_F]

Re: One output tube in my Marshall 1987X plexi RI?

I don't think you can do it - the power tubes work in pairs. In fact, I wouldn't even try it with one of my amps - I suspect you could do some expensive damage. Maybe someone with more amp knowledge can expand on this.....

 

[mrid]

Re: One output tube in my Marshall 1987X plexi RI?

Vhoholic apparently runs his SLP with only one EL-34. Now, according to everything I've read, this should cause problems, since a Marshall is a class A/B push/pull design.

 

[Robert S.]

Re: One output tube in my Marshall 1987X plexi RI?

Pulling a pair of tubes is vastly different from pulling one side of a pair.

Don't do it.

 

[ChrisB]

Re: One output tube in my Marshall 1987X plexi RI?

Try http://www.londonpower.com/faq.htm

 

[msplines]

Re: One output tube in my Marshall 1987X plexi RI?

What interested me in the beginning of the London Power answer was the Fixed-Bias aspect of the tube pulling lark as in :

"Removing tubes from a multi-tube fixed-bias output stage is never a problem"

Wot do typical AB Marshals etc have, is it fixed bias?? i thought my Peavey wiv EL84s was a class A-ish fixed bias, but my Marshall 50 wiv 2 x EL34s was not.
Am I barking up the wrong tree,
Hey! I can bark - I can really bark!!

 

[ChrisB]

Re: One output tube in my Marshall 1987X plexi RI?

I asked Kevin O'Connor @ Londonpower a similar question a couple of days ago about my el84 amp.

He said:

If it is cathode biased with a shared resistor, then you cannot pull a
tube. If it is cathode biased with individual resistors for each tube,
then you can pull tubes. If it is fixed biased, you can pull tubes.

You can look inside and see if pin-3 of the power tubes goes to ground directly or via 1-10 ohm resistors (fixed bias) or if pin-3 goes to a 5-10W 100-180 ohm resistor (cathode bias).
Otherwise, with a voltmeter, you could see if there is negatiove voltage
with respect to ground on pin-2.

If all else fails, contact the manufacturer or have a more knowledgeable
tech friend look inside for you. Any amp can be modified to accommodate
tube pulling and/or other power reduction methods.

The only knowledge I have of tube amps is not to mess about inside them when you don't know what you are doing. So I am going to do what he suggests and get someone more qualified to check for me.

Would love to try his Power Scaling mod but I wouldn't trust anyone I know to fit it into my amp and it is too far away to ship to.