Can I use 6550s in my JTM45 clone ?

[dai h.]

I think that the grid leak limiting value may become critical if the tube is being operated close to its limiting plate dissipation; the higher temperatures resulting in increased control grid secondary emission, which may result in thermal runaway if that grid current, passing through the grid leak resistor, causes the control grid to become more positive than intended.
Hence the grid leak limit is lower in fixed bias than cathode bias, which would tend to self regulate in that scenario.

In a Marshall, 6550 shouldn't get near their plate dissipation limit.

Just to note that their static dissipation shouldn't be anywhere near 70%, but rather should take the intended load / likely capabilities of the power supply into account, eg similar static current draw to EL34.

Thanks. That makes sense from what I've read and seen. Marshall (or their American distributor) apparently biased the 6550s as EL34s (i.e. as 25W plate dissipation tubes not 42W tubes--that the PT and OT were unchanged also seem to point to this).

[gtrplayr1976]

Thanks. That makes sense from what I've read and seen. Marshall (or their American distributor) apparently biased the 6550s as EL34s (i.e. as 25W plate dissipation tubes not 42W tubes--that the PT and OT were unchanged also seem to point to this).

I biased as a 35W.

[big_teee]

So what tubes have you had in it, and how do you think each compares and sounds?
T

[gtrplayr1976]

So what tubes have you had in it, and how do you think each compares and sounds?
T

I started with some Sovtek 6l6s when I first powered it up. I didn't get to play them much before I decided to add a MV ,but they sounded clean for the most part. Had to turn it up some to get crunch. The first MV I tried was the simple one between the treble pot. I tried 6V6Ss ,and 5881s with it ,but didn't like them. Sounded too fuzzy. I changed the MV to PPIMV ,and tried the tubes again ,but still sounded fuzzy. I decided to try the 6550s ,and there was a huge difference. Bigger sounding ,and distortion was much smoother not fuzzy. I have some good matched 6l6s I had thought about trying also ,but these 6550s sound so good I may not want to.

[dai h.]

I biased as a 35W.

sounds risky (at least from the amp's perspective)--to be a bit more specific--drawing a lot more current from the B+ winding if it's not designed for it would create excess heat and nudge things towards where Marshall apparently didn't want to go (biasing as 25W tube and not 42W--or to put it another way, using an overrated tube and not running it hard (i.e. the same way mA wise as the original tubes)).

I also wonder how much influence the (presumably) lower B+ (with the hotter bias) is doing for the sound.

[gtrplayr1976]

sounds risky (at least from the amp's perspective)--to be a bit more specific--drawing a lot more current from the B+ winding if it's not designed for it would create excess heat and nudge things towards where Marshall apparently didn't want to go (biasing as 25W tube and not 42W--or to put it another way, using an overrated tube and not running it hard (i.e. the same way mA wise as the original tubes)).

I also wonder how much influence the (presumably) lower B+ (with the hotter bias) is doing for the sound.

Well it says 150 mA on the PT ,and I think we figured we were drawing 126 +/- 2 .

[dai h.]

well it's not something I've worked out but it just seems a lot more to ask from the power supply as compared to say, adding an extra 12AX7 (if similarly biased as the other ones, maybe 1mA x2, plus extra 300mA from heater winding). And I'd wonder about the difference when signal is applied and output goes up drawing more current. Anyway, just being cautious!

[gtrplayr1976]

well it's not something I've worked out but it just seems a lot more to ask from the power supply as compared to say, adding an extra 12AX7 (if similarly biased as the other ones, maybe 1mA x2, plus extra 300mA from heater winding). And I'd wonder about the difference when signal is applied and output goes up drawing more current. Anyway, just being cautious!

I'm going to bias it more as an EL34. I biased as a 35W plate dissipation instead of 42 originally ,and at about 60% of that ,but I will be experimenting with lower settings. Check the link on post #9 big_teee shared. It's a handy calculator.

[big_teee]

If your checking bias with a 1 ohm resistor?
I would think 40-45mv or less would be ok?

T

[gtrplayr1976]

If your checking bias with a 1 ohm resistor?
I would think 40-45mv or less would be ok?

T

I have a Weber bias probe ,so I set it that way.

[pdf64]

Check the link on post #9 big_teee shared. It's a handy calculator.

I think the calculator assumes regular class AB operation (though I've not analysed it).
If the bias was adjusted much hotter, for class A, the current draw could be higher than the calculated value

[gtrplayr1976]

I think the calculator assumes regular class AB operation (though I've not analysed it).
If the bias was adjusted much hotter, for class A, the current draw could be higher than the calculated value

This is an AB amp though right ? Push/pull ?