Check my math? Cathode bias resistor value...

[Gainzilla]

Hi all,

Pretty new at this and wanted to make sure I'm biasing this optimally.

The amp is push-pull, cathode biased 6bm8 thing. I've never worked with these so the suggested values ive seen look a little weird.

Current readings:
239v plate
22vdc across the cathode resistor (400r 10w)

The math:
22/400 = .055
.055/2 = .0275
.0275*(239-22) = 5.9675
5.9675/7 = .8525

So 85%, whereas 7(above) is max plate dissipation in watts.

Does that look right? Furthermore, is 85% ok? If I use the same numbers, but plug in 470 for the other resistor I have, I come up with 72% (which seems more ideal).

Thoughts? Any help would be most appreciated.

Cheers!

[g1]

The math is good. If you put a 470 ohm in the actual circuit, the readings will change, so you will have to recalculate. The 470 should cool the bias a little, so you should end up with less than 72%.

[Gainzilla]

Great, thanks! I'll throw the 470 in later tonight and see what that gets me. What am I shooting for? 70%?

Cheers!

[eschertron]

Great, thanks! I'll throw the 470 in later tonight and see what that gets me. What am I shooting for? 70%?

Cheers!

I know nothing about the tubes you are using, but generally as dissipation goes up (during operation) the cathode current rises and the bias voltage increases, making the bias colder. I don't like the crossover distortion I hear when pushing the tubes, so I bias as close to class A as I dare with cathode bias. 100 percent, 101 percent, what ever it takes!

[Chuck H]

I'm going to back eschertron on this. The figures you're after apply to fixed bias or very high plate Vp circumstances. I might use a 350 ohm resistor. If I experienced excessive crossover distortion I'd try a 22V zener diode clamp to prevent the bias from cooling too far due to voltage rise. Cathode bias is also called "self bias" because it's VERY forgiving. The tube will somewhat adjust itself once it starts conducting. If you bias at 70% with a cathode resistor you are almost certain to be too cold for good sounding power tube clipping.

[Gainzilla]

Thanks guys, I think that gets to the heart of what I'm experiencing. Sounds warm and smooth at lower volumes, gets grainy and glassy when the master is turned up.

Follow up question: what is the trade off here for low volume playing? Is there one, or does it sound good throughout a wide volume range?

Cheers!

[eschertron]

Thanks guys, I think that gets to the heart of what I'm experiencing. Sounds warm and smooth at lower volumes, gets grainy and glassy when the master is turned up.

Follow up question: what is the trade off here for low volume playing? Is there one, or does it sound good throughout a wide volume range?

Cheers!

Sound is in the ears of the beholder; but I'd venture to guess that at low volume playing you won't hear any difference as a result of the bias point being 100% versus 72%. The trade off as I see it is that the tube is running wide open, generating all the heat it can, when you are playing nothing. If there's a down side to that, it will be primarily in mechanical (thermal) stress on the tubes and other amp components. At 7W per tube, that may be of no consequence whatever.

[Chuck H]

Backing eschertron again. But I'll add that some of the harshness you're hearing could be due to voltage spikes that your Hammond OT isn't subduing. The HF response of those 125 series units isn't as wide as the 1600/1700's but it's still higher than we need for guitar. I used a "conjunctive filter" with a relatively small value cap to correct for this and it worked great. I put an amp on the scope and looked at the clipped wave. Using a resistor of 1.4x the OT primary Z (or a close standard value) I tried cap valued until I found the smallest value that took the peak off the spike but didn't round the edge of the waveform. For an 8k OT my values were 10k and 1500pf. You'll barely hear this on clean tones but it'll take the hash off the clipped tones. YMMV.

[Gainzilla]

Ha! Can't believe you remembered which transformer I used! You may be right about the hammond, and it so happens I have a spare .0015 (bought a couple for the presence circuit). I'm running the OT at 5.6k, so something close to 7.8k for the resistor? What wattage rating do they need to be, and how close to 7.8 should I stick? I probably have a couple 2 watters on hand that might get me close.

Cheers!

[Gainzilla]

Great stuff, thanks! I'll see how close I can get it... Luckily I just found a shop near my house that stocks all kinds of components. My personality type does not do well waiting for mail order. Lol

[Chuck H]

That may be problematic. Because you need a resistor and a cap capable of very high voltage. The resistor should be about five watts good to 400V and the capacitor should be rated 2KV. You might get away with a 1KV rated cap. Actual values aren't critical so fudge and listen.

[Gainzilla]

Lame. That means I'll have to place an order and wait. I'll get an assortment so I can dial it in a little. In the meantime, I had originally planned to try a zobel back when this build was planned with el84s... Maybe I'll stick that on for the time being and see what impact it has.

Played this for a bit this morning. What a fun, versatile amp! Nice slightly broken up, chimney cleans, great crunch (nails the benchmark "All right now."), and gets really nice singing, fluid sustain. When I turn it up, though, it reminds me of a Diezel VH4 that I played once and didn't like... Too cold and brittle! I'm hoping getting the bias sorted out and the conjhnctive filter will take care of that. If not, it's still a great little amp for home use and recording!

Thanks again!