OT impedance question

[bluemonk]

I built an BF-style amp with a 5k2 ohm OT. I'd like to use it with both 6L6s or 6V6s (transformers are beefed enough and bias can be adjusted).
I believe the OT is in the impedance range for 6L6, not really for 6V6.
With 6V6 should I set OT's output to achieve a reflected impedance of 10k4 ohm?
In other words: is it better to run 6V6 at 5k2 or 10k4 OT impedance?

Thanks in advance

[booj]

testing it with both 6V6's and 6L6's with different loads will tell you best what the circuit likes and you'll get results real fast. Then you can knock around with the formulas. Just my .02

[bluemonk]

Please can you tell me what tests do you mean? My theoretical knowledges are limited but I am ready to learn.
I know that BFDR has 6k6 OT. I was just wondering if 5k2 is too much low impedance for my building (based closely upon BF Princeton Reverb).
Thanks for your attention.

[booj]

Sorry for being vague. I'd put the tubes in the amp you want to try, say starting with the 6V6's. Bias them properly. Hook up a 4 ohm dummy load and scope the output. See if you get a nice clean output with no crossover distortion. Bring it up to the point of clipping. Measure the output voltage. Then use the fourmula Power = voltage x voltage / load resistance to figure your output power.
Do the same thing with an 8 ohm load. Then you'll know what that set-up "likes" (the one with more power)
Do the same with the 6L6's.
Now you'll have real world demonstratable results.
Then you'll know if the tranny will be able to be used woth both 6V6's and 6L6's or not with the given power supply voltage.
I'm not opposed to doing the math, but guess what? After you do the math your going to have to.........(start at beginning)

[bluemonk]

Very clear. Thanks! But I don't own a scope... otherwise I should't need explanations , I think
On the empiric side I've to say that my DIY amp running 6V6 at 5k2 seems to play good. Perhaps should be wise to bias output tubes colder, not to stress them with higher idle dissipation while working mismatched. Just my tought... maybe correct?

[stingray_65]

5k2 is exactly 20% less than 6k6.

operationaly it's in tolerance, so I wouldn't concern myself much with rebaising.

[booj]

after I replied I was thinking "I wonder if he's got a scope?" You could also just measre the output with a voltmeter and put a dependably repeatable signal into the amp. I used to use a 12 volt transformer with a voltage divider and a 50k pot to get 0 - 1 vrms out for a signal source.
jThen run a clean signal out and measure the difference in voltage output between different loads. Should work fine.
But I agree that the 20% difference shouldn't give you any problems.

[Steve Conner]

I think if you had a choice between 5.2k and 10.4k, 5.2k would be better. I tried something similar, and the screen grids in the 6V6s would glow under load at 10k, but at 5k they stayed dark.

In the context of the recent thread we had on OT impedance, the difference between 5.2k and 6.6k is hardly worth worrying about.

[bluemonk]

[Booj] Nice trick to surrogate a scope! I will try.

[Stingray_65] Regular load for a pair of 6V6 is 8k. DR runs them at almost 20% less... a further 15% gave me matter to worry about.

[Steve Conner] Ok, your major experience confirms my impression.

Thank you everybody for your help.
Best regards.