JJ 6V6S @ 290V / 49mA cathode biased .. 100% plate dissipation. Okay?

[jazbo8]

Yup, can see the schematic now. I am not sure you need the zeners to lower the B+, because you can dial in the bias current with the rheostat anyway. The fact you are seeing the current fluctuate when you play, is proof that you are in Class AB mode, whereas with Class A, the current hardly changes at all. If it still sounds good with the lower bias, then you should measure the resistance between the center pin of the rheostat and ground, and replace it with a power resistor of equal or similar value. You don't want to run the risk of it shorting out, or someone turning it down which could burn up the tubes! Just a safety precaution...

[Gingertube]

I would'nt worry.
The voltage on the anodes may be 290 but the voltage from anode to cathode will be more like 270V. That gives just under 13 Watts for Anode plus Screen dissipation.
Cheers,
Ian

[thehoj]

Any idea what I'm actually measuring over that 1 ohm resistor?.. It seems like exactly what I'd expect in a fixed bias power section using 1 ohm resistors to ground...

[eschertron]

Any idea what I'm actually measuring over that 1 ohm resistor?.. It seems like exactly what I'd expect in a fixed bias power section using 1 ohm resistors to ground...

Yup. The 1R resistors are there for just that purpose. I don't think I've seen them in a cathode-biased rig before, but with variable-cathode bias like you've got, it makes sense in a "all the bells and whistles" way. All you need now is a mV meter permanently attached across one of the 1R resistors, mounted on the faceplate of your amp. Then you can have the bias current displayed as an essential part of your playing style. I'm kidding, of course, but there are folks who do exactly that.

[thehoj]

Okay, slightly off topic question.. Still related to this amp though.. I was playing around with stuff yesterday, and kind of hit a nice sweet spot around 38mA for bias.. The only thing I was noticing after playing around is that I'm just missing a bit of that lower end chunk with that kind of palm muted type stuff. It really sings in general, I just wish I could get a tiny bit more chunk out of it.

I'm running a really small OT, and I was wondering if changing this would potentially open up a bit of that bottom end for me?
I noticed that the hammond site lists the frequency response as 150Hz - 15Khz for the 125D OT I'm running.
I had originally built the amp around 6K6 power tubes, and with the 6V6's in there now I feel like I'm kind of holding them back a bit.

I was looking at this Hammond 1760J which lists frequencey response as 70Hz - 15Khz http://www.hammondmfg.com/pdf/EDB1760J.pdf - I can pick one up for $50.
It's got 4Kohm mains, and then 4, 8, and 16 ohm secondaries. I was thinking with my more like 8K primary impedance, I'd use the 4 ohm secondary as my 8ohm out, and 8ohm secondary as my 16ohm out.

Would this accomplish what I'm talking about? Or am I trying to do too much with a small amp?







On topic edit: I measured the resistance from cathode to ground and it's 200ohms exactly shared for both 6V6's. B+ Measures exactly 280V at the OT, and I measure 40mV exactly over the 1ohm resistor.

[thehoj]

And here I am again with more contradictory information. I was just thinking about it, and the 1 ohm resistor between the cathode and main cathode resistor isn't going to give me an accurate measurement of cathode current because it's not connected to ground.

I adjusted the way I'm calculating current. And this makes a whole heck of a lot more sense to me now.

I've taken all V dropping zeners out of the circuit, I'm sitting right at 300VDC for my B+.
My shared cathode resistor is measuring 180ohms. If I measure the VDrop over that resistance I get 15.3V. That means that I have 15.3/180 = 85mA combined current from both 6V6's flowing from cathode to ground.
If I take my Voltage at the anode of 300V, and subtract the 15.3V measured at the cathode I get 284.7V effectively on each 6V6. That means that my combined plate dissipation is 284.7 * 0.085A = 24.2W.

And that means that each tube is dissipating 12W on the plates, or 85% plate dissipation per tube.

This is the correct way to calculate all of this right?

I apologize for all of the confusing updates in here. I feel like this post is the most accurate so far. Please correct me if I'm wrong.

[jazbo8]

You got it, I often forget to subtract the cathode voltage when doing the Pdiss calculations, thanks to Gingertube for the reminder, so your 12W/tube is fine for the 6V6S.

[jimboyogi]

Also, that 12.2W cathode power is made up of plate power plus screen power. So your plate power (the 14W rating) is actually less than 12.2W.
To measure your screen current, measure the voltage drop across one your screen resistors, I = V/R.
Subtract this screen current from your cathode current (42.5 mA), giving you plate current. Then multiply the plate current by 284.7V to find your plate power.

[thehoj]

Also, that 12.2W cathode power is made up of plate power plus screen power. So your plate power (the 14W rating) is actually less than 12.2W.
To measure your screen current, measure the voltage drop across one your screen resistors, I = V/R.
Subtract this screen current from your cathode current (42.5 mA), giving you plate current. Then multiply the plate current by 284.7V to find your plate power.

Right, I forgot about that.

This is kind of weird.. One screen resistor measures 2.48V dropped over it, the other 1.88V.. So the first one is 2.48/470 = 5mA, the other is 1.88/470 = 4mA.. Is that somewhat normal?

So if I subtract that current, I'm left with 37.5mA for the one, and 38.5mA for the other.. Which equals 10.68W and 10.96W for the other.. Or roughly 75% plate dissipation.. So definitely have some room I could experiment with a bit higher if I wanted to.



EDIT:
For some fun I tried upping this a bit, and something that I have expected happened.
I lowered my cathode resistance to 130ohm, giving me a drop of 13.1V and a total current calculation of 98mA.. But when I measured B+ it was 284V.. The extra current draw brought my main B+ down.

In any case, calculating it all out including accounting for screen current, I'm at 43mA per tube, or 11.7W (84%) Plate dissipation per tube.
I can't play around with the amp right now to compare the effect to the sound with lower B+ / Higher plate diss. vs. Higher B+ / Lower plate diss.

Is it normal that my B+ would drop with increase current draw? Is it because the PT is on the smaller side? It's a Hammond 269JX, which I'm noticing now is rated for 69mA on the HT secondaries.


AHH! I'm starting to feel like maybe I need a bigger OT AND PT for this amp..

[g1]

I still don't get why your 1 ohm cathode resistors are not in agreement with these numbers, or do they agree? Have they changed in value? It doesn't matter that they are not connected to ground, they are still in series with each cathode. So they should actually give you a more accurate reading (in that you can see if they are matched or if one tube is running hotter than the other). Slight difference in bias between the 2 tubes could be why your screen currents are different, although it seems very slight and is probably not significant.

[thehoj]

I still don't get why your 1 ohm cathode resistors are not in agreement with these numbers, or do they agree? Have they changed in value? It doesn't matter that they are not connected to ground, they are still in series with each cathode. So they should actually give you a more accurate reading (in that you can see if they are matched or if one tube is running hotter than the other). Slight difference in bias between the 2 tubes could be why your screen currents are different, although it seems very slight and is probably not significant.

Hm.. good point.. I'll have to double check that.

I'm thinking I should pick up a Hammond 270EX for a little extra B+ and double the current on tap, and go with that Hammond 1760J OT for some extra oomph there too.. I sold my main amp recently, hence the conversion to 6v6 from 6k6.. Trying to get something with more clean headroom and volume for gigging.

[thehoj]

Oh also, just thinking about primary impedance on my OT if I'm effectively running at 271V (284 - 13) on the 6V6's, biased at 43mA (11.7W plate dissipation) per tube.
Should I stick with 8000ohm? Or go 10,800?

I don't know how to tell from any of the datasheets I've looked at.

I see on tdsl for class AB1 PP it shows:

                Va     Vg2   Vg1           Ia      Ig2   Ra             S      Zout          Pout
AB1 P/P 	285 	285 	-19.0 	70.0 	4.0 	70,000 	3.6 	8,000 	14.0 	
AB1 P/P 	250 	250 	-15.0 	70.0 	5.0 	60,000 	3.75 	10,000 	10.0

Two things come up with these numbers for me.
1) I'm between those two Va numbers
2) This shows numbers for negative voltage applied to grid. Is it safe to assume that for the purposes of OT primary impedance that me measuring 13V at my cathode is the same as -13V applied to grid with ground at 0V?