mickysama

hello guys
I am new to the forum.

recently I brought my Marshall JCM2000 TSL 100 for retube. I provided 4 Svetlana EL34 for my technician, and after a few days, he called me and said one of them is faulty, but he doesn't have Svetlana so we put a Electro Hamornix EL34 instead.

so my question is would mixing poweramp tube cause any probem to my amp? I know some people mix preamp tube with different brands.

The reason I ask is because when i was playing it yesterday, at clean channel with volume full, the voulme suddenly droped about 20-30%, so I checked the head and saw the 1st and 2nd EL34 is bright red (the 2nd tube is E. Harmonix), and the other 2 tubes are only about 65-80% as bright. this is not usual right?


can someone please give me suggestion on what to do? should I change the electro harmonix to Svetlana? or I just need to bias the tubes again?

any help will be appreciated
cheers

Marshall Amps

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Wilder Amplification

If by "glowing bright red" you're referring to the outer plates glowing red, the amp needs to go back to the tech.

I personally don't condone mixing brands of valves like that, but if they're all closely matched in terms of idle plate current draw as well as transconductance then yes it's OK to do. But unless there's something electrically wrong with one or more of those valves, I'm thinking you may have a problem with the bias supply circuit.

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Jon Wilder

Wilder Amplification - The official home of the 80s cock of the walk "Schwank"

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mickysama

first of all thanks Wilder Amplification.
Yes I am referring to the outer plates glowing red, but it only happend when I turn the volume all the way up. when I noticed this problem I turn the volum done a bit, and it doesn't glow as bright anymore.

at lower volume the 2nd EL34 (Electro harmonix) is still slightly brighter than oters, is that usual?

sorry I have no background understanding of electronics, how can I tell if "all tubes are closely matched in terms of idle plate current draw as well as transconductance."

thanks

Wilder Amplification

If it's doing it at high volume, more than likely either the amp is biased too hot or you've got your speaker cabinet hooked up wrong/plugged into the wrong jack. What kind of speaker cabinet are you running?

As far as being able to tell if the valves are matched, this would require a valve tester, which hopefully your tech should have. As far as the EH valve "glowing slightly brighter than the others", are you referring to the orange glow of the heater/cathode? Or are you referring to the florescent blue glow that exists inside of the valve?

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Jon Wilder

Wilder Amplification - The official home of the 80s cock of the walk "Schwank"

http://www.wilderamps.com

http://www.facebook.com/wilderamps

mickysama

I use a jcm2000 TSL 12"x2 speaker and both connect to 16 ohm.
the cab looks like this one http://guitars.musiciansfriend.com/p...net?sku=482789
but it has a sticker says 16 ohm only.

yes, I mean the orange glow of the heater/cathode.

he told me the EH EL34 matchs the Svetlana so I am assuming he had done the test before.

thanks again for reply

Wilder Amplification

Ah OK. The EH cathode is more than likely made of different materials that glow differently.

I would definitely have the bias re-checked in that thing. Shouldn't be redplating at any volume.

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Jon Wilder

Wilder Amplification - The official home of the 80s cock of the walk "Schwank"

http://www.wilderamps.com

http://www.facebook.com/wilderamps

gemather

Dear new member,
ever mix in a pair.
Configurate pair "A" the same pair "B".
Prefer same brand.
If the circuit is not continous, ask a technician,
he can do his measuring.
I mixed same brand, short socket with whole socket.

mickysama

thank you very much, so in conclusion, as long as they are EL34, mixing power amp tube is ok, providing they are biased correctly, is that right?

Wilder Amplification

Provided they're balanced and matched as well as biased. As long as they match to each other within a certain tolerance, they'll pull the same amount of current at a given grid voltage. The amp could care less about the brand.

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Jon Wilder

Wilder Amplification - The official home of the 80s cock of the walk "Schwank"

http://www.wilderamps.com

http://www.facebook.com/wilderamps

Jesstaa

It's a bit different, power tubes are all working with one another, on a single bias circuit, preamp tubes are all individual tubes.

Wilder has explained the rest well. But just remember, if the plates start glowing red, it isn't so good.

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mickysama

thanks very much to you all for help, I will get my tech to check the bias again.

cheers

p.s. Hey Jesstaa I am from brisbane, so cool to see aussie here

Jesstaa

Ohh there's plenty of us around here.
Keep your eyes out.

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