What is the normal bias current for different tubes?

[Alan0354]

I am comparing 6L6, JJ 6V6S, Ruby 6V6GT and Yellow Jacket EL84.

The amp is Pro Reverb with one Utah speaker parallel with a 10 ohm resistor to lower the volume a bit. Treble and Bass set at 5.5 and 4 resp. For the clean sound, volume is at slightly over 3. For break up sound, volume at 6. The +B is all at +440V.

1) 6L6GC with bias current at 45mA per tube.

2) JJ 6V6S with bias current at 25mA per tube.

3) Ruby 6V6GT with bias current at 26mA per tube.

4) EL84 with Yellow jacket adaptor for 15W output. It is cathode bias with resistor inside.


Tube comparison - YouTube

[jazbo8]

one Utah speaker parallel with a 10 ohm resistor to lower the volume a bit.

I don't think putting the 10 Ohm in parallel is safe, if anything, you should put it in series with the speaker, so the amplifier is not over-loaded (assuming the speaker is properly matched to the amplifier to begin with...)

[kg]

I like the lowest grid circuit impedance I can get. fwiw.

[Gingertube]

I like the lowest grid circuit impedance I can get. fwiw.

Yes, the biggest and most copied mistake in guitar amp history is inadeqaute control of output tube grid1. It comes as a result of designing down to a price rather than up to a standard. Low grid ciruit impedance gives you less noise and MUCH improved reliability.

In my HiFI Amps I drive output tube grids direct coupled from a mosfet source follower with current source load, bias applied to the mosfet gate. Dead black background (no hiss) androck solid reliability.

Cheers,
Ian

[Alan0354]

I don't think putting the 10 Ohm in parallel is safe, if anything, you should put it in series with the speaker, so the amplifier is not over-loaded (assuming the speaker is properly matched to the amplifier to begin with...)

Why is that? I would think lower impedance is better than higher mismatch. The original setup is 2X8ohm parallel which is 4 ohm. I remove one speaker so I parallel a 10 ohm. Putting in series make it 18 ohm.

For the sake of the OT, lower mismatch put more stress on the tubes. But too high impedance might cause to high a voltage kick at the plate and might over voltage the OT.

[jazbo8]

Why is that? I would think lower impedance is better than higher mismatch. The original setup is 2X8ohm parallel which is 4 ohm. I remove one speaker so I parallel a 10 ohm. Putting in series make it 18 ohm.

For the sake of the OT, lower mismatch put more stress on the tubes. But too high impedance might cause to high a voltage kick at the plate and might over voltage the OT.

I didn't know you took out one of the speakers, so the mismatch isn't too bad in this case. But as you said, a lower mismatch put more stress on the tubes, since more current has to flow through the OPT/tubes. In most case, it is the over-current that kills the tubes, not over-voltage, so it is usually safer to go for a over mismatch (if you had to do it at all).

[Alan0354]

I didn't know you took out one of the speakers, so the mismatch isn't too bad in this case. But as you said, a lower mismatch put more stress on the tubes, since more current flow has to flow through the OPT/tubes. In most case, it is the over-current that kills the tubes, not over-voltage, so it is usually safer to go for a over mismatch (if you had to do it at all).

How about the OT? Over voltage might cause arcing inside the OT. I was more worry about burning the vintage OT than the tube. Those are Mesa tubes already and is quite old, so I take it protecting the OT is more important. What do you think?

[Alan0354]

What do you guys think about the comparison of different tubes? I want to hear some opinions which is better. I figure this is a true comparison of the tube where everything else is constant. Yes the output is mismatched for the 6V6 and EL84. But from my experience, sound don't change much with impedance particular before it breaks up.

I have a switch that I can switch out one speaker to have only one speaker which is 8 ohm load, or switch in the 10 ohm resistor to give it back to close to 4 ohm. I found removing one speaker and leave it at 8 ohm really does not change the sound, just lower the volume a little. I use 10 ohm because I experiment with 4, 8 ohm already. The lower the resistance, the more bass it attenuates, I loss a little fullness with lower resistance. So I choose 10 ohm that affect very little.

I feel this is better comparison than having those tubes in different amps. Of cause you can optimize the tone stack to accommodate the tube, but that defeats the purpose of comparing the sound of the tube. To have true comparison, you have to have everything else the same so you don't run into resistor and capacitor tolerance issue.


BTW, I found having higher bias current does not imply sounding better at all. I actually like the 6L6 bias below 70%. I ended up with 40mA per tube.

[jazbo8]

Alan - Did you try the amp with just the Utah by itself, if so, how did that sound? The increase in the plate voltage swing using a higher load in this case isn't that much based on back of the envelope calculation, so the OT should be ok. By putting a 10 Ohm resistor in parallel with the Utah, you are still very close to the original spec'd 4 Ohm load, the increase in the peak plate current is fairly minor, so my concern about the stress on the tube is moot. Just curious why did you take out the other speaker, was it broken or was the amp too loud or something else?

[Ted]

What do you guys think about the comparison of different tubes? I want to hear some opinions which is better.

EL84s are better in an AC30
Mullard EL34s or RCA 6CA7s are better in a JMP
GEC KT66s are lovely.
NOS American Tubes of the original type are better in old Fenders.
NOS 7591As are nice. Modern ones not so much.
Nearly all small push-pull 6V6 circuits sound good.

There. You have my opinions. Are they of any use?

[Alan0354]

Alan - Did you try the amp with just the Utah by itself, if so, how did that sound? The increase in the plate voltage swing using a higher load in this case isn't that much based on back of the envelope calculation, so the OT should be ok. By putting a 10 Ohm resistor in parallel with the Utah, you are still very close to the original spec'd 4 Ohm load, the increase in the peak plate current is fairly minor, so my concern about the stress on the tube is moot. Just curious why did you take out the other speaker, was it broken or was the amp too loud or something else?

Yes, I tried with one or both Utah speakers. It's sounded the best with two as the sound source is bigger, but the sound quality is the same. I
compare with two difference Celection speakers and the Utah match the Pro Reverb a lot better.

I was referring to what you suggested to put the resistor in series with the single speaker to get 18 ohms when I talked about the over voltage on the OT.

The other speaker is fine, I took it out and put in another combo as speaker cabinet to use as speaker for the amp I am designing. I want as little variable as possible, I got fooled by the speaker of the Marshall JCM900, that's the reason I use the Utah as speaker for the new build. I am trying to use as cheap a speaker as possible. I don't want to start with a speaker that cost over $100!!!