marshall 2204 el34 to 6550

[GCKelloch]

Right. You can tell the GE 6CA7 beam tetrode by the how the glass sticks out over the base. I think I might have had those in my HiWatt. The Sylvania weren't as well made as time went on. The GE are very expensive and increasingly rare. A big gamble if you don't know you will like the sweet meaty sound. The JJ are so much cheaper. from what I've heard, I'd say the JJ 6CA7 are "harder" sounding, but some have reported they sound more open after warming up for a while. Eurotubes claims the JJ KT77 were actually reverse-engineered from a set of GEC KT77 they gave the JJ company.

I have quads of the GE 6CA7 and Sylvania 6CA7, they both sound good. I think I like the Sylvania more though.

The JJ KT77 is okay, but I really like the new production Genelex KT77.

In both cases, would you say you prefer the softer sounding tube?

 

[harddriver]

If you bias 6550s in a Marshall to 65% as though they were 35w, you'd actually be at 91% for the OT spec and 25w tubes. So while the 6550s can handle it just fine, you are stressing the OT. I've always been told regardless of the tubes you sub in, you need to bias them according to what the amp was designed for, and not for the tubes themselves. Because if you use 35w tubes, your amp isn't suddenly putting out 70 watts. It's still a 50-watt amp unless you change the transformer.

I understand what you are saying and it probably isn't bad advice being we know the OT weren't designed for them. I have not read that recommendation in any of my reference amp books but I can see the logic to it.

If you bias a 6550 as a 25 watt tube then you will end up with a colder bias(less current draw thru the tube) less chance of a runaway bias condition and stressing of the output transformer.
Typical bias range in a Marshall amp:
100 watt amp EL34 -36 to -45
100 watt amp 6550 -44 to -55
50 watt amp El34 -34 to -40
50 watt amp 6550 -38 to -48

 

[GCKelloch]

My understanding is to figure the bias % from the tube specs and the amp's plate voltage. Using the JJ 6550 specs (as I understand them
600/35 = 17.14
If your 2204 is indeed 400Vp:
400/17 = 24W
For 70% at 400Vp:
24/400 = 0.06 x 0.7 = 0.042 = 42mA.

The plate voltage may drop a bit when increasing the bias current, but it shouldn't more than ~10% at 70% bias current. Eurotubes makes a few devices to measure that. I'm not an expert, but I don't see how that would stress a 2204 OT. Granted a JJ KT77 (at 18W) or 6CA7 (at 20W) would be a bit safer, but I doubt it would matter as long as the impedance load is OK.

AFAIK, increasing the bias current doesn't increase the tube power output. It just lowers headroom and adds even harmonics. Don't quote me, but I think you could run a 6ooV/35W rated tube at 100% bias current at 400V without danger, no? My understanding is the tube would then only generate even harmonics. Maybe ask your amp tech about that. Hendrix couldn't do that with his JTM100's because they were probably running 550Vp+. EVH apparently ran his Sylvania 6CA7's at full bias current, but they burnt out frequently because they were 500V/25W tubes and he loaded the amps pretty high with an attenuator, and dropping the AC supply with a Variac doesn't necessarily help the situation for a few reasons.

Incidentally, I believe the Shuguang 6550B is 42W, but the standard Shuguang 6550A may just be just fine, especially at ~400Vp. I've read good reports from at least one person who ordered them direct from China.

 

[FourT6and2]

You can't get 70 watts of power from a 50-watt power supply. Original 6550s were 35w. The 6550A is 42 watts. So it's not like the PT and OT in a 50w Marshall were designed to run 82w at idle. And bias is just idle current. If you bias up using a 35w+ max dissipation and you crank the amp... bias is just the idle setting. Current will spike when you crank the amp and play. If your idle is up in the 40s or higher and you play cranked, you'll run into a problem eventually. That's why I've always been told to bias 'em as though they were 25 watts, becasue that's whawt the amp was designed for. But ya you can get away with higher. Just depends on a bunch of things, as well as luck.

I'm not sure what original Marshalls were biased to at the factory for 6550s. Just something to think about is all.

 

[GCKelloch]

My bad. I confused the TAD 6550A-STR with the original Shuguang 6550A, which I recall reading as 600V/35W. The 680V/42W TAD would then put out 25W at 400Vp. I really don't know enough about biasing to say whether it could handle 100% or even 70% bias current at 400V, but the desired % can be calculated from those figures.

 

[GCKelloch]

It should also be noted how the lack of grid alignment in EL34's contributes to runaway current. Beam tetrodes have aligned grids, so there isn't the kind of electron build up on the screen grid when the tube is driven hard.

My guess is some EL34 types (maybe JJ) have aligned grids, which may be partially why some guitar players do not to like them? In any case, I'd think the beam tetrode 6550A-STR could easily be biased to 70% or a bit more without issue in a 2204 with 150k grid leak resistors, and the 440V G2 limit allows plenty of room in a 400Vp Marshall. Treating them as 25W tubes doesn't really tell the whole story though. Assuming 400Vp in the 2204, biasing up to 70% might yield ~375Vp. 375Vp would normally dissipate 23W for a 6550A-STR if biased to 50%. I assume dissipation would then be as high as ~34W (for only half of the cycles) at 70% bias. I guess that would equal ~30W? You may want to bias a bit higher or lower depending on taste, but I'd think it would be safer than EL34's. I believe the recommended primary OT Z is the same as for EL34, so that shouldn't be a consideration.

Here's an informative thread about the JJ vs GL KT77 if you decide to go that route:
https://www.thegearpage.net/board/index ... 77.557752/

Update 1: It turns out the Shuguang 6550A-98 are 680V/42W tubes with a 440V G2 limit. My guess is they are the same as the TAD for 1/2 the price direct from China. Note the differences from the old standard Shuguang 6550 in the Chinese datasheet on this page:
https://www.ebay.com/itm/2pcs-Shuguang- ... 2248911703

Matched sets won't be tested like the TAD's. Maybe get two sets in case you get a seriously microphonic one?

You could try some Eurotube 'Tube Damper Rings' in that case:
https://www.eurotubes.com/store/pc/view ... Category=8

They're the only one's I'd consider buying, and only $6 each.

Check out this great 6550 comparison video: https://www.youtube.com/watch?time_cont ... gKkAJJwPWQ
We both liked the TAD's best.