6L6 to 6V6

[kkatarn327]

I have this old no-name "frankenstein" 4x6L6 amp that I want to use 6V6s with. The 6L6s are way too boomy and especially at practice volumes are not easily tamed. I dropped in two pairs of 6V6s straight in with no mods and it sounds fine, there wasn't even redplating as long as I could tell so I know that this conversion is realistic. Also the OT is HUGE so that shouldn't be an issue. However I also know that these tubes are running WAY too hot (400 plate voltage @ almost 39mA, that's 110% dissipation!).

Like I said this amp is a 'frankenstein', there's no brand name or serial numbers anywhere, plus it's all 1960s sloppy point-to-point wired so even following traces within the amp is difficult. Hence I can't quite determine if it's fixed or cathode biased. Obviously there's no bias pot of any sort, and there's these huge 300 ohm/25W ceramic resistors + 22uF/50V caps on the cathode (P8) of each power tube pair. There's also a 'half-power' switch (I can't quite figure out how it works, it seems to just disconnect the cathode pairs from each other) which I believe I've read is only possible with cathode biased amps.

First of all, is this in fact a cathode biased amp? Is there any other way to tell? Ideally I would like to have adjustable bias on this amplifier, is this possible without total re-engineering of the output section? Lastly, if adjustable bias is not an option, how to I determine the correct cathode resistor value to bring the plate current within reasonable range for a 6V6 (24.5 mA +/- 2mA)?

Thanks

[octal]

Sounds cathode biased to me. A few amp use combination bias (i.e. a negative voltage on the control grids in addition to cathode resistors) but if you've got 300 ohm resistors there, that's reasonable for a pair of 6L6 in cathode bias.

Remember, when calculating the dissipation of the output tubes, you're measuring the voltage across the tube (plate to cathode) and multiplying that by the current drawn.
There'll be a good 15V across the cathode resistors. (I'm assuming it's one shared 300 ohm resistor for each pair of tubes.)

You need to increase the value of the cathode resistors to reduce the idle current. There's not really any formula per se. Tack a 50 ohm resistor in series with the 300 and see where you end up.

With a cathode biased amp, you don't need to be as obsessive about staying in the safe operating range of the tube, as the bias is automatically reduced as you begin to clip the amp. Get it in the ballpark and don't worry about making it adjustable.

You might also consider changing taps on the output transformer if it has adjustable impedance. 4 6l6s will probably be seeing 2.5- 3.5K load and a pair of 6v6 would like to see double that.

Nathan

[MWJB]

Is that 400vdc on the plates to ground or to the cathode? As Octal says there will be cathode voltage to deduct, I reckon it'll be more like 25vdc+ than 15vdc. So power tubes are running around ~100%...which 6V6s can do in any number of amps. Changing the 300ohms to 330ohms or 390ohms is a possibility, 330ohm would still be fine for 6L6 too.

Some old 6V6 amps ran their tubes in to 4.5K OT primary per tube, so 2.25K for 4x6V6 in push pull? Still considered a match"...it would be useful to determine "OT primary Z" & turns ratio before jumping to any hard & fast conclusions though.