Low Voltage but Higher current?

[big mike]

Not getting it.
Was watching a 'lunch with Dan Boul'. (69 superlead today DUDE!)

and he mentioned a lot of the 65 amps are heading that direction.

Anyone know what that means? I'm not sure I follow.


THanks!

[Blue Strat]

Not without context. What was the WHOLE thing he said?

[big mike]

Just a quick little rant...something about how he had a replacement transformer in his 69 Marshall, running at 430 B+ but if you'd heard his previous musings, more 65 amp designs were going to be lower voltage, but use higher current to keep the tubes happy?

Might not be enough info there I guess.

[diagrammatiks]

apparently the 16ohm tap is also the tap to use for true tone.

:shrug:

[big mike]

Yeah didn't make sense to me.
<shrug>

The Marshall sounds killer though. LOL

http://www.ustream.tv/recorded/21104650

[big mike]

Looking through their product line, I'm wondering if he's refering to scaling.

[Blue Strat]

Just a quick little rant...something about how he had a replacement transformer in his 69 Marshall, running at 430 B+ but if you'd heard his previous musings, more 65 amp designs were going to be lower voltage, but use higher current to keep the tubes happy?

Might not be enough info there I guess.

OK, that does make sense. He's lowering the plate voltage but will increase the idle current of the tubes. Same idle power dissipation for the power tubes. Lower plate voltage means earlier breakup, "browner" tone.

[big mike]

Okay that's what I thought. Just trying to wrap my head around how one would accomplish it.

Said the original trans were around 530v or something. Sounds about right on a 69.

[guitarcapo]

I do like the sound of lower plate voltages on the preamp tubes and without a tone stack.

[TweeDLX]

OK, that does make sense. He's lowering the plate voltage but will increase the idle current of the tubes. Same idle power dissipation for the power tubes. Lower plate voltage means earlier breakup, "browner" tone.
Mike, Can you 'splain this to me? Don't you run the risk of sending the tube into cut-off if you lower voltage enough so that the current gets too high? I know that as plate voltage gets lower, it takes more current to hit the 60-70% mark for idle dissipation. Most of my amps run at 400-430VDC on the plates with 6L6GC tubes. I don't follow the 60% rule for most of them, just find a spot between 35mA and whatever 60% would be that sounds good.

['58Bassman]

Ohm's Law- P (Power) = I (Current) x E (Voltage). If P is to remain constant, I must increase when E decreases.

[rmconner80]

Ohm's Law- P (Power) = I (Current) x E (Voltage). If P is to remain constant, I must increase when E decreases.

Exactly. That said, there are practical limits. I.e. on a 25W power tube, assuming you want 70% dissipation, you could run 300VDC plate voltage and 58mA current for around 17.5W... or your could run 450VDC plate voltage and 38mA for 17.5W dissipation.

That said you can't run 1200 volts at 14mA for 17.5W, nor can you run 100 vdc at 175mA.

My own musings with this suggest that tonally you may run into constraints that are within the operating max / min but which just run out of steam in terms of response / feel /tone. I've messed around with higher current / lower B+ and just didn't find it all that great when you go too low on B+. Of course this is all subjective.

[Blue Strat]

Mike, Can you 'splain this to me? Don't you run the risk of sending the tube into cut-off if you lower voltage enough so that the current gets too high? I know that as plate voltage gets lower, it takes more current to hit the 60-70% mark for idle dissipation. Most of my amps run at 400-430VDC on the plates with 6L6GC tubes. I don't follow the 60% rule for most of them, just find a spot between 35mA and whatever 60% would be that sounds good.

Maybe, but anything over 400 (or possibly even 300, 250 or 200) is SO far from that, that it's not an issue here.

If you look at Class AB operation here http://www.drtube.com/datasheets/el34-philips1969.pdf you'll see that they use 375V so it's pretty safe to say that low 400s is well within the ok range.

[Ronsonic]

Actually, pretty simple, but a number of variables so it looks complicated - what I'm typing here is just one aspect.

The preamp and phase inverters in a typical guitar amp have tons of available headroom from the power supply - not that the tubes aren't being overdriven but that they aren't supply limited like in an all triode ultra-fi amp. So that section doesn't care if there's a total of 540 or 460 VDC available on the big rail.

With the power tubes you've got a balance of supply, screen and grid voltages. An amp with 460 on the plates will need a less negative bias voltage than one with 560. Max clean power output should be happening when the PI is swinging peak to peak double the bias voltage. Double 36 is easier to hit than double 42. Might not seem like a lot but the pre and PI stages tend to not change between models the difference happens right on the margin where it makes the most difference.

As for the practical limits, a lot of really great sounding 50W Marshalls run 390-400 on the power tubes and are indeed snarly little beasts.