6V6 in 6L6 amp OT impedance

[Dagan8]

I have a modified Fender "Red Knob" Twin(modded to only use 2 output tubes) which is using a bassman 50W OT with a selector for 2,4,8ohm. The amp also has a "Low power" mode which drops the plate voltage from 470V to about 240V and it also changes the bias voltage as well(increases to less negative).

Since I never use the amp in "High Power" mode I want to try using a pair of 6V6's instead of 6L6's. I don't think the plate voltage and bias voltage will be an issue as long as I always use the amp in "Low Power" with the 6V6's.

Will I have to use the 8ohm selection on the OT with a 4ohm load because of the impedance requirements of the 6V6 vs 6L6's??

[Dagan8]

Sorry for the double post.

I have been looking through several forums on this topic and I am getting mixed answers.

Let me simplify my original question:

My current OT is designed for 2 X 6L6 tubes and it has a primary impedance of 4000. I am reading that the 6V6's like to see a higher impedance of say 6500-8000.

Does this mean I should select the 4 ohm setting on my OT to run an 8ohm load when I use 2 X 6V6?

[ThermionicScott]

You'd either want to double the speaker load or use half of the previous impedance setting. So if you normally use the 4-ohm selection, use the 2-ohm.

- Scott

[Dagan8]

Thank you Scott. That's the way I understood it but I was getting the opposite answer on some other forums which were making me second guess myself.

[Old Tele man]

...FWIW, the dynamic plate resistance of 6V6 (rp ~ 50K) is about twice that of 6L6 (rp ~ 22.5K), so 4x6V6 work fine with normal 2x6L6 OT's.

[Chuck H]

+++ to all answers thus. But I do find that primary Z specs seem to go up as voltage increases. That said there are also a number of amps that run tubes at a lower than spec primary Z to eek a couple more watts out at the expence of the finite distortion level. That AND many amps have an "extension" jack that is in parallel to the internal speaker, effectively altering the primary Z to half it's original spec. Most amps WILL run a 100% mismatch without incident. So... I really think at 270Vp you could just plug and bias a pair of 6V6's with no extra considerations. Maybe try it both ways and do whatever sounds bets. Your safe either way.

[ThermionicScott]

Really? I was under the impression that Z went down with increasing plate/screen voltages. I whipped up this chart for 6L6 (originally just to show that the same primary Z wasn't appropriate for all voltages):



You also can see in the recommended 6V6 operation that the Z goes down from 10K to 8K when the plate/screen voltages are raised from 250V to 285V.

Perhaps not all tubes respond exactly the same way, though.

- Scott

[Old Tele man]

...uh, someone's forgetting about the plate dissiplation limitation. As plate voltage goes UP (moves toward right on the X-axis), the plate current comes DOWN (moves down the leftside on the Y-axis) because of the "limiting" action of the constant power dissipation curve.

...thus, the lower the RL slope, the higher it's resistance (impedance) value.

[soundmasterg]

So OTM, if a 6V6 has roughly twice a 6L6's plate resistance, and you should corespondingly use twice the plate impedance for a pair of them...what should you do for something like a 6973 (73k) or a 6CM6(80k)? Are they really going to need to see a 16k impedance OT or is there a bigger range that is acceptable?

Greg

[Old Tele man]

...selection of operating plate load resistance is not truly an exact science, but rather more "approximation," for instance, the 'rule-of-thumb' for determining a usable-range of loads for tetrode/pentodes (triodes are different):

RL ~ 1/6-to-1/10 of rp.

...thus, for a 6V6, with rp = 50K, that'd be anything between 5K(pp) and 8.3K(pp).

...why the two different numbers? Well, it comes down to which operational characteristics are important?: (A) if you want maximum POWER output, you use the larger 8.3K(pp) OT, but (B) if you want minimum DISTORTION, you use the smaller 5K(pp) OT...and, (C) if you want a compromise, you use something between those two values.

...of course, those numbers assume there's enough plate voltage available to utilize those different OT plate loads.

[ThermionicScott]

OTM, those heuristics sound like the ones you'd use for a single-ended amp, where you will often sacrifice output power and use a higher load to get the most symmetrical output.

My posts were for a push-pull amp using the method I learned from Merlin's site, where you draw the Class B loadline from the B+ voltage and zero current point through the knee of the curve, then multiply that by 4 to get the plate-to-plate load. (And then increase the idle current to get into Class AB operation, of course.) Rest assured I haven't forgotten about the plate dissipation limits.

(It's very possible that I was mistaken/confusing to use "Z" to refer to the load resistance as opposed to the plate resistance. I'll try to better spell out what I mean in the future.)

- Scott