Heat dissipation issue

[Heavenamps]

Hey there,

So I built a 40 watt amp in the realm of a vox ac30 - i.e. 4 el84's, 2 12ax7s, etc. I was getting around 520 VDC for my HT voltage. This seemed really high to me but the PT I'm using (classictone 40-18060) is rated for 700VAC with a center tap. I ended up using a 50 watt power resistor to drop down to a more reasonable HT voltage. However, because I'm creating such a significant voltage drop, a very high amount of heat is being dissipated which is being absorbed into my aluminum chassis making it hot to the touch. I'm wondering if there is either a good way to insulate the chassis from the heat, or better yet, a way to get a more reasonable HT without having to use a fat power resistor. Any advice is welcome. I feel like the power resistor was likely a noob move, so please feel free to point me in a better direction.

[sluckey]

That PT is not very well suited for a 4 x EL84 amp. Voltage too high and current too low. The 40-18050 would be a much better choice. The 560VAC w/ center tap would not require a big dropping resistor that wastes lots of heat.

[Heavenamps]

Hmm maybe you can help me understand. It looks like the JJ el84's I'm using typically draw 48mA in a class A configuration (according to the data sheet). I've never been clear on how that current draw works in say an AB configuration. Would it be doubled or multiplied by 4 or something else? My transformer says its rated for 125mA so with four el84's and two 12ax7's am I exceeding the current rating? I have no qualms about swapping the PT, I just want to make sure I understand so I don't make the same mistake again in the future. Thanks for your help!

[sluckey]

How did you choose the 40-18060 PT?

[92Volts]

Hmm maybe you can help me understand. It looks like the JJ el84's I'm using typically draw 48mA in a class A configuration (according to the data sheet). I've never been clear on how that current draw works in say an AB configuration. Would it be doubled or multiplied by 4 or something else? My transformer says its rated for 125mA so with four el84's and two 12ax7's am I exceeding the current rating? I have no qualms about swapping the PT, I just want to make sure I understand so I don't make the same mistake again in the future. Thanks for your help!

Other EL84 datasheets give specifications for class AB operation. This one for example: http://www.r-type.org/pdfs/el84.pdf

And in one of these configurations (one of the ultralinear "distributed load" ones) we see a change as large as 2x28ma at idle up to 2x55ma at full power. Note that ALL of these configurations are in the 50ma per-tube ballpark at full power, which means you can use just 1 pair of EL84 if you're limited to 125ma current...

The other issue is no EL84 is rated for anything CLOSE to 500v on the plate.
First off, this means the datasheet examples aren't very informative, we can infer you only want 1 pair of tubes not a quad of tubes, but there's no info provided about the actual voltages you plan to use.
Secondly, that voltage isn't good for the tubes! They might tolerate it but it's high by any standards... even knowing the 300v rating has been exceeded in many amps, I would hesitate to go as high as 500v or even 450v...

The problem with using a resistor to burn off voltage is this difference between idle and full-blast power draw, if you want to get rid of 70v at idle you drop an additional 70v when the power draw doubles at full blast!

The only configuration I can think of which tolerates so much voltage yet doesn't draw excess current would be a pair (not a quad!) of JJ 6V6S tubes which are actually rated at 500v and are otherwise similar to other 6V6s, and in turn similar to EL84s. 6V6 tubes in general have been subjected to more abuse in a wider variety of amps, demonstrating they really can take 400v+ without catastrophic failure.

Of course, you need a higher-impedance transformer to limit the current draw to avoid melting either your power transformer or a single pair of 6V6S, though impedance is always fudged a bit (especially when it comes to speakers and their ratings) so you might get away with the same OT.

[Heavenamps]

I don't know why I was thinking that the two sets of two el84's would draw 96mA total, and then add in 1.2 mA per 12ax7 and I figured I was still under the limit. However I now see that my power tubes are biased with 270 volts at the plate, dissipating 12 watts each so with four tubes I'm pulling nearly 180mA total. Is that the right way to think about it? I'm going to order the proper transformer today.

[92Volts]

In class AB the maximum current can be higher than total tube dissipation limits... because the load (speaker via the output transformer) is also dissipating power.

On a side note, with class AB amps the idle bias current/power should be lower than the tube's limit because the tubes dissipate more power at higher volume.

On the datasheet mentioned in my previous post we can observe typical values for a push-pull pair of tubes with 250V and 300V on the plate:
Plate current at maximum signal: 2x37.5ma and 2x46ma
Screen current at maximum signal: 2x7.5ma and 2x11ma

So depending on your exact plate voltage, it falls somewhere between 45ma per-tube and 57ma per-tube.

[tubeswell]

In class AB the maximum current can be higher than total tube dissipation limits... because the load (speaker via the output transformer) is also dissipating power.

On a side note, with class AB amps the idle bias current/power should be lower than the tube's limit because the tubes dissipate more power at higher volume.

On the datasheet mentioned in my previous post we can observe typical values for a push-pull pair of tubes with 250V and 300V on the plate:
Plate current at maximum signal: 2x37.5ma and 2x46ma
Screen current at maximum signal: 2x7.5ma and 2x11ma

So depending on your exact plate voltage, it falls somewhere between 45ma per-tube and 57ma per-tube.

In Class AB, each output tube can semi-alternately dissipate over the limit when the B-load line kicks in, because each tube also (semi alternately) goes into cutoff for part of the signal cycle. So the average dissipation over time is not exceeded.

[PRR]

700V AC CT is 350V each side, times 1.414 is 490V DC (maybe more no-load).

EL84 is honestly a 300-330V tube, though g-amps do run higher.

This is really a very large difference. As you found, too large to waste-off comfortably.

If I was married to this 700VAC PT, I'd switch to high-volt tubes: 6L6, EL34, etc. A pair could do as much or more than a quad of EL84.

[Garrett335]

That PT is not very well suited for a 4 x EL84 amp. Voltage too high and current too low. The 40-18050 would be a much better choice. The 560VAC w/ center tap would not require a big dropping resistor that wastes lots of heat.

Hey Sluckey!
I just looked up this post as I'm trying to re-design my Marshall 2022 18 watt build running 2 12ax7s and 2 el84.
I want to run a quad pair of el84s to up the output of the new amp.
What do you think about this PT 40-18065  (Classictone) it seems to meet all the requirements of an ac30, and its a little cheaper than the 40-18050.
I should be able to run a quad pair of el84s and even add another 12ax7 stage for a trem, or extra gain stage I think.
Thanks in advance! You guys are awesome.

[sluckey]

I think the 40-18065 would be a good choice.

[Garrett335]

I think the 40-18065 would be a good choice.

Thank you kindly sir :)