can i use 6550's?

[iknowjohnny]

I asked this before i think, but never got a clear answer. I have a 150MA PT and three 12ax7's, two EL34's and diode rectified. I could use 6550's in my last build which was pretty much the same as this one except it had a 250 MA PT. Is this one's current too low for 6550's? I did try and figure it out myself but wasn't sure of the answer i deducted after looking at the tube data.

[Phil_S]

Go here: http://tubedata.itchurch.org/sheets61.html and look at the tube data sheet for the Tung Sol 6550. Look for typical operating conditions for two tubes. You will see that 150mA doesn't cut it. You need about twice that. IMO, 150mA for a pair of EL34 probably isn't enough either.

The real answer is that it depends on the circuit. There is a difference between "typical" and what you actually do with it, so there is no pat answer. We have no idea what you are doing with it.

You've got to pay attention to the maximum signal load, as well as the zero signal.

Good luck.

[iknowjohnny]

This PT was designed specifically for a 2xEL34 marshall ! How can they sell it is thats true? I guess 6550's are out of the question, but EL34's?!

[Badside]

This PT was designed specifically for a 2xEL34 marshall ! How can they sell it is thats true? I guess 6550's are out of the question, but EL34's?!

Interesting fact, I have a Hammond 290GX in my 50W Marshall clone. It clearly says 213mA on it, yet the new units shipping now say 150mA. They're the exact same size.

I guess it has to do with the way current handling is measured (rectified or not, full wave or half wave, etc)

I can definitely tell you that amp is frickin' loud and doesn't sag much if at all.

As for using 6550s... well technically it's the same B+ and some primary Z, current demand shouldn't increase through the roof. The tube is just a valve, a bigger valve is still limited by the rest of the circuit.

I've put KT77s in mine, you may also consider that, they do something similar.

[Phil_S]

This PT was designed specifically for a 2xEL34 marshall ! How can they sell it is thats true? I guess 6550's are out of the question, but EL34's?!

I'm sure it's fine. What I said was that I didn't know what the circuit was. I guess I wasn't real clear about this. Are you taliking about a JTM45 or a 50W plexi? If your a talking about ~50W at 400V, that's 125mA. What the spec sheet says is that peak could be as much as 240mA. That's why it's so difficult to answer when the circuit isn't known.

A pair of 6550 is going to want more current. Than that.

[iknowjohnny]

It's like a jcm 800 pretty much with fixed bias, 475v on the plates, 100/100uf can for the screens and plates. I think thats all you'd need to know?

http://classictone.net/40-18023.pdf

[iknowjohnny]

By the way, if i need twice that, or 300 MA for 6550's, i shouldn't have been able to use my last PT which wasn't 300 MA or twice my current 150. So how do I figure this out? If I use my other PT in it with 6550's, will i burn it out at stage volumes ? because i never used it woth 6550's to gig with, only at home. i did gig with it with el34's and it was fine.

[Jana]

The RCA tube manual, RC 19, has all the answers. There are reprints available and it is probably scanned and online. For all the tinkering you do, you owe it to yourself to get a copy.

FWIW, the impedance of the OT is going to matter. And, if it was me, I wouldn't use anything less than 250 ma for a pair of EL34's.

[FYL]

This PT was designed specifically for a 2xEL34 marshall !

This PT was cloned from a PT used by Marshall. It's quite poorly dimensioned, heats quite a lot and is quite saggy under load - authentic vintage sound and satisfied bean counters, but not what I call an optimized PT for an EL34 PP.

The 40-18024 100 w version is much better and only a couple of bux more than the 50 w.

http://classictone.net/40-18024.pdf

[Phil_S]

The RCA tube manual, RC 19, has all the answers. There are reprints available and it is probably scanned and online. For all the tinkering you do, you owe it to yourself to get a copy..

All sorts of free stuff here, including the later RC-30, which is, IMO, essential, and the GE similar publication, Essential Characteristics.
http://www.tubebooks.org/tube_data.htm

[Phil_S]

It's like a jcm 800 pretty much with fixed bias, 475v on the plates, 100/100uf can for the screens and plates. I think thats all you'd need to know?
http://classictone.net/40-18023.pdf

Do the math. 50W at 475v is 105mA. That is at idle. It is wise to derate the transformer. I'd go for a derating factor of about 2 (50% efficiency) in that sort of amp. FWIW, I agree with others that the 150mA rating is on the low side. YMMV.

[iknowjohnny]

Then maybe i'll use my hammond out of the old amp or buy a new one. (probably the latter) Do you think it will sound any different at low to medium stage volumes? I never crank any amp, and this one like most marshalls never gets past about 10-11:00 on the master. I don't think it will because my last amp had a 250 MA PT and when i built this it was a clone of it and sounded the same even with a 150MA PT. Thanks all.

[Badside]

As I said, the Hammond 290GX in my 50W built, which is now rated at 150mA by Hammond doesn't get hot at all and doesn't sag.

And people seem to be forgetting that tubes don't "pull current", they just regulate it. Think about your load line, they are the exact same no matter the tube. The only difference is how close to 0V it can go before the grid becomes positive, that's when you're pulling max current. This is the big reason why Triode strapping reduces power.

So putting 6550s in there won't make a drastic difference in current demand. Don't fret it that much.

Didn't Marshall themselves use 6550s for quite a few years? (Or was it 5881)

[Bear]

Didn't Marshall themselves use 6550s for quite a few years? (Or was it 5881)

It was both. Well, sort of. The U.S. importer installed 6550's in the 70's and 80's, apparently for fewer warranty claims. And Marshall went to Russian 5881's in the 90's when EL-34 stock got sketchy--this was for some JCM-900 amps and 9200 power amps, although I think a few JCM-900's still had EL-34's.

[iknowjohnny]

Didn't Marshall themselves use 6550s for quite a few years? (Or was it 5881)

It was both. Well, sort of. The U.S. importer installed 6550's in the 70's and 80's, apparently for fewer warranty claims. And Marshall went to Russian 5881's in the 90's when EL-34 stock got sketchy--this was for some JCM-900 amps and 9200 power amps, although I think a few JCM-900's still had EL-34's.

yeah, they started with el34's but some time after the amp's introduction they switched. I had one of the el34 ones.