Puzzled by cathode bias questions

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j allen shaw
Junior tube assistant
Posts: 36
(4/6/04 6:39 am)
 Puzzled by cathode bias questions
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 I have a puzzling situation with a cathode biased amp as follows:
I have a 1978 Musicmaster Bass Amp that has always been a cookin' "tweedy" sounding little thing.
Here's the schematic.
It started to develop a little hissing, so I changed out the old carbon comps for new, and that solved the hissing problem.
HERE"S MY PROBLEM/QUESTION:
While I was at it, I thought I'd check the bias & see what the plate dissipation was running on the (2) 6V6's. To my surprise, the dissipation is only 4 WATTS!! That's only 28% of max.
I switched out the 220 ohm cathode resistor for a lower value, 50 ohm, and that brought the dissipation up to about 8 watts, or about 57% of max dissipation. I expected the amp to sound better, but it was HARSH & overly distorted sounding. I lowered the R more to 25 ohm, and it got even worse! The dissipation was at about 9.8 watts (70%).
I then put in a 250 ohm, bringing the dissipation to 3.8 watts, and the amp sounds BETTER THAN EVER!!!!!
What am I missing here?? I thought the tubes had to be at about 70% of dissipation to be out of crossover distortion for a sweetened up tone. Or are the principles of cathode biasing vs. fixed biasing different when it comes to optimum dissipation?
(I don't know if it makes any difference, but this amp has a very low plate voltage of 190v on the 6V6's.)
Thanks in advance for any answers to this!!
Jeff
 
Tiny Daddy
I will work on all amps
Posts: 600
(4/6/04 7:19 am)
 Re: Puzzled by cathode bias questions
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 Is the B+ 295V like it shows on the schematic?
 
j allen shaw
Junior tube assistant
Posts: 37
(4/6/04 1:17 pm)
 Re: Puzzled by cathode bias questions
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 No, B+ is 197v, no load, and 190 with the amp on/under load.
The tranny is not stock. It's a Hammond 261G6, 250v CT @130ma. Obviously too small/weak, but the amp sounds great!
Why doesn't it sound like crap, cold, dead?? I have some great amps to compare it with, and it absolutely has a killer tone!
My question is about the low bias/dissipation. Maybe I'm not calculating it correctly. I use Ken's notes as follows:
(changed to match my parameters/situation)
You have a cathode biased pair of 6V6 which share a 250 ohm cathode resistor and have a plate voltage of about 190
Step 1. Measure the plate voltage with respect to ground. 190 V

Step 2. Measure the cathode voltage with repect to ground. 11.2 V

Step 3. Calculate the cathode current.
11.2 V / 250 ohms = 44.8mA total or 22.4mA per tube

Step 4. Calculate the actual plate voltage with repect to the cathode.
190V - 11.2V = 178.8V.

Step 5. Calculate the plate dissipation ( heat being generated by the plate ).
178.8V x 22.4mA = 178.8V x 0.0224 A = 4 Watts.

Step 6. Compare your plate dissipation with the max recommended for the tube ( from the data sheet--14 watts for 6V6 )

4W / 14W= .2857 = 28.57% of max.

You should usually shoot for about 70%.

Am I on the right track, or way off? I just want to try to understand what's going on with this. Everytime I think I have a handle on the bias stuff (or any of this amp building stuff), something like this puts a kink in my way of thinking!

Thanks!! Jeff

 
Lucid Alice
Senior tube assistant
Posts: 189
(4/6/04 2:43 pm)
 Re: Puzzled by cathode bias questions
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 In step 2 your get a cathode voltage of 11.2V. That is a typical voltage for proper tube operation with a typical B+.

The combination of the cathode voltage and the bias is what makes the tube operate in it's optimum range. The reason you are getting such a small power output is tied to the low B+. If you want to run the tube harder you will have to adjust both the cathode resistor and the bias resistors. They are currently scaled for 295V B+.

This is just my 2 cents. YMMV.

 
jimf
Junior tube assistant
Posts: 11
(4/6/04 3:07 pm)
 check the specs for lower plate voltages...
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 I think part of this can be explained by looking at the tube data sheet and the spec for various plate voltages.

www.pmillett.addr.com/tub...V6-GTA.PDF

Your power value of 14W is for a plate voltage of 285V. The power rating for 250V is 10W. The power isn't spec'd for a lower Va, but for class A, one tube, @ 180V the power is 2W. This may be something like 5W with push-pull AB (a guess) based on the other ratings.

Anyway, it's pretty clear that the max power is reduced with a low Va of 200V. If you assume 5W, you're at 80%. If you assume 6W, you're at 66%. It seems reasonable, from the tube data, to make that assumption. And if I'm correct, then you're in the ballpark with the numbers you gave.

I freely admit that I haven't worked out the principles behind the reduced power at reduced Va, but it still looks like a nice simple explanation.

Edited by: jimf at: 4/6/04 3:26 pm
 
Lucid Alice
Senior tube assistant
Posts: 190
(4/6/04 4:42 pm)
 re: cathode bias
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 Nix my reply above. I think jimf is on the track. The 11.2v on the cathode is giving you a good sounding bias for the tube and the current flow is reasonable at 22ma. The power is calculated using the plate voltage and that voltage is low due to a low B+, so the power output is low.

Many amps push the B+ to the limit of the tubes. Then the 70% factor comes into play to avoid going through tubes frequently.

How good does it sound? Does is break up nicely when pushed? If so, a lowered B+ may be a design option for a quieter amp.

 
j allen shaw
Junior tube assistant
Posts: 38
(4/6/04 4:56 pm)
 Re: check the specs for lower plate voltages...
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 OK guys, your theories make some sense to me.
Now let me throw another can of gas on the fire!
I had still not quite found the "sweet spot" on the 5F8-A Twin based amp (using 4 EL84's for power, cathode biased) which was recently completed (if it will EVER be "completed", man I love tinkerin' with these things!)
Anyway, it was still too bassy and flatulent when cranked, even after changing out some of the bypass caps & resistors as had been suggested. I had it biased @ about 70% with 390 volts on the plates (JJ tubes are handling it, so far).
So today, after getting the good results on the Musicmaster amp by lowering the plate dissipation, I tried the same on the Twin.
SAME RESULT!! All the problems went away & the amp sweetened right up after decreasing the dissipation to about 50%!
So-o-o-o, maybe at the ends/extremes of the spectrum of plate voltage (very high or very low for any given tube), the same rules of thumb (70%) don't necessarily apply?
Just thinking, Jeff
 
j allen shaw
Junior tube assistant
Posts: 39
(4/6/04 7:23 pm)
 Re: How does the Musicmaster sound W/ low B+?
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 Lucid A,
The breakup is nice & smooth, and it IS quiet!
The MMB is about 12 watts with just (1) 12AX7 and (2) 6V6's, as the PI is a small transformer (see schematic link above) and 2 little diodes as the rectifier. Very simple setup w/ just Volume & Tone. I have a Weber Signature 12 installed.
It's nice and Fendery clean at low volume, then starts to breakup at around 4-5 on the dial, and is very resposive to pick attack & pickup volume. Dimed it's like a grindin' BF Princeton.
When the dissipation was higher, it got an unpleasant "buzziness" that smoothed out as the ratio was lowered, until I arrived at near perfection at the present whopping 4 watts!
This is making me re-think some of the ideas I have had about power/bias relationships.
Jeff
 
 
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