can i use 6550's?

[daz]

If you haven't already seen my schematic in the other thread, here it is again below. just wondering...is there any reason i couldn't just swap a pair of old 6550's in there? I have a pair that came out of an early JCM800 that i think are RCA and i recall really liking certain things about thier tone.

http://s24.photobucket.com/albums/c3...urrent=amp.jpg

[tubeswell]

Try them and see how they go for bias. Is ~425VDC (my crude guestimate) enough for the B+?

[daz]

I tried them for a few minutes but didn't have the chassis out. Seemed to sound great. Guess i'll have to check and see how the bias looks, but till then i'm gonna run em a bit cuz they just sound nice ! I love EL34's, but i like the way the 6550's give me a change of pace with thats more 6L6 like wuth the lack of hair on top like the 34's have. A more clean distortion that can be great in it's own way. Wonder how it would sound with one el34 and one 6550?! I recall someone talking about that once. Oh, and i think the plates were at 400v with the el34's last i checked.

[black_labb]

you would probably will have to adjust the bias. el34's tend to have a lower bias voltage than 6550's.

i wouldn't think that a 6550 could be paired with an el34 without some quite asymetrical waveforms. you could possibly have a pair of el34's and a pair of 6550's if you had a different bias level to each of them (or have the el34's fairly cold, which would probably negate the point of the el34's)

Try them and see how they go for bias. Is ~425VDC (my crude guestimate) enough for the B+?

not sure what you mean. are you questioning the ability of a 6550 to run on the lower voltage compared to what they are often run on? i think they would be fine but with lower headroom. i used some 6550's to test my recent amp which runs at ~385V b+ and they sounded fine. i was running them fairly hot though and they seemed to break up sooner than i would have expected.

[tubeswell]

not sure what you mean. are you questioning the ability of a 6550 to run on the lower voltage compared to what they are often run on?

Yeah when I rhetorically asked if that was 'enough', that was what I was meaning. (Sorry for the habitually bad laconic kiwi vocab)

[black_labb]

Yeah when I rhetorically asked if that was 'enough', that was what I was meaning. (Sorry for the habitually bad laconic kiwi vocab)

bloody kiwi's (is wasting time on net at work for kiwi boss )

[tubeswell]

bloody kiwi's (is wasting time on net at work for kiwi boss )

Any further comment here may incriminate me too 8-)

[black_labb]

back on topic, when i used 6550's, they didnt seem to have much as much headroom as el34's. but i was running 4 el34's with one that wasnt conducting, so the extra tube on one side may have changed that. was also done on different days, so i'm not sure wether my judgement is right.

the less headroom could be what you like (you are talking about output tube distortion).
just looked at the amp and noticed it is running cathode bias. the 6550's may be running a bit hot, check the voltage across the cathode resistor on the output tubes and determine the current and thus the idle dissipation (make sure to use the anode to cathode voltage, not anode to ground when calculating)

[MWJB]

What's the PT's 6.3VAC current rating, less than 6A might be pushing it?

[black_labb]

believe its just a single pair, so 3.5A would probably be the lower bound.

[MWJB]

No, 3.5A (about the same as a Fender Deluxe Reverb) would NOT be enough...5A MAY be enough (I have got away with it in the past in my own amps...do it at your own risk), but 6A (e.g. what a 4x6L6 amp would be rated for) would be safer. Don't forget that as well as the 2x6550 (3.2A total) you have 0.9A (0.3A per tube) being consumed by the 3x12AX7 = 4.1A, plus you want some headroom.

The excessive load on the 3.5A PT would also bring down B+ & heater voltages considerably. If the bigger tubes draw significantly more plate & screen current then you need to factor that in against PT B+ secondary current ratings too.

A good rule of thumb is that, if the amp will take the bigger tubes without dropping more than 20v from the B+ (with reasonable plate current draw), or dropping the heaters below 6.2V (with the amp running at correct wall AC voltage), you'll probably get away with it.

[daz]

It's a hammond 272JX with a 8A heather winding so it's fine in that regard. As for the bias i still have to check that tonite. But while i'm not sure about their bias requirements, there was no red plating so i assume that tone aside they should be fine. And tonally at least at low volume they sounded fine.

[defaced]

I think the KT-88s I have run in the -50s range. If you can pull -60v, I'd guess you'll have enough range.

[black_labb]

No, 3.5A (about the same as a Fender Deluxe Reverb) would NOT be enough...5A MAY be enough (I have got away with it in the past in my own amps...do it at your own risk), but 6A (e.g. what a 4x6L6 amp would be rated for) would be safer. Don't forget that as well as the 2x6550 (3.2A total) you have 0.9A (0.3A per tube) being consumed by the 3x12AX7 = 4.1A, plus you want some headroom.

The excessive load on the 3.5A PT would also bring down B+ & heater voltages considerably. If the bigger tubes draw significantly more plate & screen current then you need to factor that in against PT B+ secondary current ratings too.

A good rule of thumb is that, if the amp will take the bigger tubes without dropping more than 20v from the B+ (with reasonable plate current draw), or dropping the heaters below 6.2V (with the amp running at correct wall AC voltage), you'll probably get away with it.

wasnt thinking about the preamp tubes. good point.

i have an amp with fixed non adjustable bias running 4 6550's at 700v. the bias on that was 37v from memory. thats probably pretty hot though, as i replaced the old tubes in it with 6550's. 2 of the tubes in the apparently matched quad redplated, while the other 2 were fine.

the amp in question is cathode bias, so this isn't that helpful.

[daz]

I hate to sound ignorant, but would 6550's require more or less resistance as far as cathode resistors? I can figure it out once i get home tonite by measuring and using webers calculator, but i wanna grab some resistors on the way home in hopes of having several values that may be close enough for now.