Massaging the Cathode Bias of a Cathode Biased Amp

syscokid

syscokid

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My Allen Amp Chihuahua, that I built from a kit, is a single-ended cathode biased amp that can operate with a 6V6 at 6 watts or a 6L6/EL34 at 10 watts. An 8 ohm speaker is always used. There’s a tube selector switch that does a couple things in order to operate the different tubes safely. When the switch is set to 6L6, the tube’s cathode biased resistance is 335 ohms and the OT’s 8 ohm output impedance is connected to the speaker jack. When the switch is set to 6V6, the cathode bias resistance is 500 ohms and the OT’s 4 ohm output impedance is connected to the speaker jack to still operate with an 8 ohm speaker.

A couple days ago I decided to check the bias of the amp with a 6V6 installed, and I read 35 mA’s which is 93% of the tube’s dissipation. Definitely on the hot side! Installed a 6L6, flipped the switch to 6L6, and got a current reading of 59 mA’s which is 65% of the tube’s dissipation. This is very cold according to the online tube bias calculator. By altering the bias resistance of 335 ohms down to 200 ohms, the 6L6 now has 76 mA’s of bias current for 84% of dissipation… A cool, but not super cold bias.

Since I’m favoring the amp’s tones with the 6L6’s warmer bias, I feel like making this a more permanent alteration. What I don’t understand is, did the designer of the Chihuahua circuit, Mr David Allen, do this on purpose? Was it to compensate for the use of a EL34 instead of a 6L6? Other safety concerns?

Tube Bias calculator showing the 6L6 with a 200 ohm bias resistor:
Chihauhua with 6L6 Tube Bias Calculator.jpeg

Changing the 1K/3W resistor at the tube selector switch to a 330 ohm/3W resistor, will make the 6L6 position produce a bias resistance of 200 ohms:
Chihuahua Schematic CATHODE BIAS MOD V8-24-19.jpeg

Maybe I should try a different 6L6 first, just in case the one that’s in the amp is a little wonky!

What y’all think?
 
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On cathode bias you can safely go 100% at idle, but I usually go a bit lighter so I would think the 93% would be ideal. The 6L6 may have been biased a bit cold to get the reverb to behave better to the Allen's ear? Did you notice any tone difference by raising the 6L6 up to 84%?
 
T-Rex said:
The 6L6 may have been biased a bit cold to get the reverb to behave better to the Allen's ear? Did you notice any tone difference by raising the 6L6 up to 84%?
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Not a huge difference, but still noticeable. Clean tones are little meatier and richer. Overdrive tones are sweeter, and some natural feedback is easier to obtain. In fact, with the warmer bias of the 6L6, the amp's response felt a little more intuitive between my fingers and what's coming out of the speaker. I also need to go back and pay more attention to the response of the reverb between the differences in the bias. The amp has a beautiful sounding spring reverb with good range.
 
CoyotesGator said:
Maybe add a pot?

A good use for tubes that otherwise don't play well with others.
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That's a good idea. But adding a pot ain't going to happen on this amp. Just want to verify that warming up the bias of the 6L6 tube up to nearly the same levels of the 6V6 tube, isn't going to cause something catastrophic. I was already assuming that there won't be a problem except for shortening the life of the 6L6 by a small bit. But relying on assumptions is like gambling with questionable odds.... :whistle:
 
syscokid said:
That's a good idea. But adding a pot ain't going to happen on this amp. Just want to verify that warming up the bias of the 6L6 tube up to nearly the same levels of the 6V6 tube, isn't going to cause something catastrophic. I was already assuming that there won't be a problem except for shortening the life of the 6L6 by a small bit. But relying on assumptions is like gambling with questionable odds.... :whistle:
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Nah. You're good as long as the PT is ok. Watch for VD and you should be OK.
 
Another one of my chapters of mindless tinkering is closing. Parts arrived much earlier than expected. A 330 ohm/3 watt resistor is the only part I really needed, but I ordered three packages of other power resistor values just to keep the aesthetics of the amp’s components more uniformed. Also ordered a new JJ 6L6GC and a new JJ 6V6S (double mica) for the amp. If I were to try a EL34 (25 watt) in this amp, I’m guessing it would bias up a bit warmer than the 6L6 (30 watt) but still remain under max dissipation.

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Another interesting note that happened during this recent endeavor: When I first set up the amp for bench testing, I saw bias readings of 125% of dissipation while the 6V6 was still installed. When I went to swap out the 6V6 for a 6L6, I noticed that the tube selector switch was set for 6L6 operation! As soon as I set the switch to 6V6, the bias went down to 93%. Big f’n difference, huh? I don’t know if I inadvertently hit the switch while I was trying to remove the chassis and place it on the chassis cradle? Or maybe I accidentally hit the switch a long time ago! I tested the 6V6 tube, and the tester showed that the tube is slightly on the worn-out side… :BH:

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On the other hand, I very rarely use the 6L6 at 10-watt option at home. Too loud when cranking without an attenuator. A 6V6 at 6 watts is still loud, but not as intense.
IMG_4895.JPG

Killer sounding Blackface style low powered amp. If anybody is considering a kit build with the preceding specs, it would be hard to beat this Allen Amp product.
 
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