Can I use 6550s in my JTM45 clone ?

[pdf64]

60% of 42 = 25
From the perspective of the power supply, that's akin to setting EL34 bias for 100% plate dissipation.

The amp was almost certainly designed / intended for EL34, so I suggest setting the bias so as to achieve a static current draw that would be suitable for them, eg so that the static power tube plate dissipation is ~17 watts.

The 70% guideline was devised in regard of regular tube guitar amps, which traditionally push their power tubes to the max.
6550 in a Marshall can just pootle along with the consequential benefit in their working life, that's likely why Unicord made the mod.
If biased to 70%, the power supply would be subjected to its intended full load on a continuous basis, and the 6550 may barely leave class A.

[gtrplayr1976]

60% of 42 = 25
From the perspective of the power supply, that's akin to setting EL34 bias for 100% plate dissipation.

The amp was almost certainly designed / intended for EL34, so I suggest setting the bias so as to achieve a static current draw that would be suitable for them, eg so that the static power tube plate dissipation is ~17 watts.

The 70% guideline was devised in regard of regular tube guitar amps, which traditionally push their power tubes to the max.
6550 in a Marshall can just pootle along with the consequential benefit in their working life, that's likely why Unicord made the mod.
If biased to 70%, the power supply would be subjected to its intended full load on a continuous basis, and the 6550 may barely leave class A.

I biased according to plate voltage ,rated tube dissipation ,and specific specs on this tube. You're saying I should run it lower ? It sounds great ,and isn't red plating.

[pdf64]

Yes, I think you should run it lower.
The likely reason Unicord modded their Marshalls with monster 6550 (42 watt plate compared to 25) in there was so that they would run cool and unstressed, and so ensure that they would last the warranty period.
That may be seen as equivalent to using a sledgehammer to crack a nut, but there it is.

To idle them at 70% may be seen to act counter to that aim, not that they would likely ever reach 100% under signal though, but rather that stress on other parts of the amp would be raised, ie PT.

What if you took the logic further and fitted KT150 tubes in there, with their 70 watt plates; they would plug straight in, same as 6550 do.
You may be able to see how inadvisable it would be to bias them to a static 70% plate dissipation (49 watt each), as that would likely take it way too far and burn out the PT in short order.

OK their heaters draw more current too, but if we overlook that for the purposes of the exercise, I hope you can see what I'm getting at, ie that power supply capability is a constraint that should be considered in this scenario.

The 70% static dissipation guideline is only really applicable to class AB amps where the total plate dissipation is about the same as the amp's rated power output; that probably still covers 99% of p-p tube guitar amps though.

It's likely to be inappropriate for class A, class B, and the scenario above, class AB in which the total plate dissipation is rather different to the rated power output.

[big_teee]

I agree, you need to run it on the low side, because your Power Transformer HT, is rated at 150ma.
http://www.classictone.net/40-18033.pdf
If you run the 6550s through the calculator below you need over 200ma.
Calculate Tube Amp Power Transformer Current
T

[gtrplayr1976]

Yes, I think you should run it lower.
The likely reason Unicord modded their Marshalls with monster 6550 (42 watt plate compared to 25) in there was so that they would run cool and unstressed, and so ensure that they would last the warranty period.
That may be seen as equivalent to using a sledgehammer to crack a nut, but there it is.

To idle them at 70% may be seen to act counter to that aim, not that they would likely ever reach 100% under signal though, but rather that stress on other parts of the amp would be raised, ie PT.

What if you took the logic further and fitted KT150 tubes in there, with their 70 watt plates; they would plug straight in, same as 6550 do.
You may be able to see how inadvisable it would be to bias them to a static 70% plate dissipation (49 watt each), as that would likely take it way too far and burn out the PT in short order.

OK their heaters draw more current too, but if we overlook that for the purposes of the exercise, I hope you can see what I'm getting at, ie that power supply capability is a constraint that should be considered in this scenario.

The 70% static dissipation guideline is only really applicable to class AB amps where the total plate dissipation is about the same as the amp's rated power output; that probably still covers 99% of p-p tube guitar amps though.

It's likely to be inappropriate for class A, class B, and the scenario above, class AB in which the total plate dissipation is rather different to the rated power output.

So what if I drop it to 38 mA which is 70% of an El34 ?

[gtrplayr1976]

I agree, you need to run it on the low side, because your Power Transformer HT, is rated at 150ma.
http://www.classictone.net/40-18033.pdf
If you run the 6550s through the calculator below you need over 200ma.
Calculate Tube Amp Power Transformer Current
T

That's a handy calculator ! I only see 126.76 calculated. Did I miss something ?

[pdf64]

Yes, that would be a good target / starting point.
Try and get a view on the PT temperature at various static current settings. And the tonal characteristics.
If it runs at a reasonable temperature at higher static currents, and sounds better than it does at eg 38mA, then may as well run it at the higher current.
Whereas if it doesn't sound any better at higher currents than it does at 38mA, then there's no reason to run it more than that.

[big_teee]

The calculator can vary from 180+ to 219ma, depending on options selected.
Be sure to select 345-0-345 for your PT, select 2- 6550s, and 3 preamp tubes, and whatever rectifier type you have.
T

[gtrplayr1976]

The calculator can vary from 180+ to 219ma, depending on options selected.
Be sure to select 345-0-345 for your PT, select 2- 6550s, and 3 preamp tubes.
T

I selected from every option even the OT. This is what it shows.

Selected transformer voltage: 345-0-345

Calculated voltage at first capacitor (B+): 469.2V. Subtract 6V, if using choke.

Calculated filament current (typically the 6.3v secondary): 4.19A

Calculated current: 126.76mA at 8000R load (10% plus factored in). Preamp valves current draw is estimated at typical 12AX7 max dissipation of 1.2W at 330V, i.e. 3.6mA per triode

[pdf64]

Post #1 shows a link for a JTM50, rather than a JTM45.
I think that a JTM50 should have an OT with ~3k5 primary (as per EL34 info suggested value), rather than the 6k6 / 8k of the JTM45.

[gtrplayr1976]

Post #1 shows a link for a JTM50, rather than a JTM45.
I think that a JTM50 should have an OT with ~3k5 primary (as per EL34 info suggested value), rather than the 6k6 / 8k of the JTM45.

That's my fault I used the MV part of that JTM50. The rest is a JTM45. I included specs from all xfmrs I used in here somewhere. Post #6 has the OT specs.

[big_teee]

If you have a 8k OT, then you are correct.
I wasn't specifying that load.
I'm used to 3.6-4.0k OTs.
You get the idea and it is a good tool to see if you're in the ball park.
GL,
T

[pdf64]

OK, an 8k primary will act to limit the power output / peak B+ current draw.
That may give you a little leeway to increase the static current.
But, to reiterate, if thinking of doing that, try to ensure there's a real benefit to tone.

[gtrplayr1976]

If you have a 8k OT, then you are correct.
I wasn't specifying that load.
I'm used to 3.6-4.0k OTs.
You get the idea and it is a good tool to see if you're in the ball park.
GL,
T

It's an excellent tool. Thanks for the link. So I should be ok with these ?

[gtrplayr1976]

OK, an 8k primary will act to limit the power output / peak B+ current draw.
That may give you a little leeway to increase the static current.
But, to reiterate, if thinking of doing that, try to ensure there's a real benefit to tone.

I still may play with a lower current just to see the difference.