OT question

[grtamp]

Hi , what happen if I use 4x6L6 with an Plexi OT 1700ohm primary?

tubes can fail or consume before?

Thanks

[Stevem]

First off those tubes will need about 10 to 12 more volts of bias voltage on there pin 5 or they will burn up in short order, and once biased right they will not last as long due to being run into a lower impedance , but you could set the impedance selector and change that!

[ampgeek]

A 4 X 6L6 output stage will be looking for a ~2000 ohm primary impedance. The Fender Twin Reverb would make a good initial "model" for study and consideration.

I wouldn't anticipate any significant excess wear on the tubes running into a 1700 ohm primary. The rule of thumb is that you can run a ~2X imbalance without any significant tube/OT damage issues.

However, as Steve implies, the tubes should always be biased within their maximum allowable plate dissipation specification (~75% of stated max for a typical push-pull output stage) to avoid excess wear and tear, redplating, etc..etc..

Cheers,
Dave O.

[martin manning]

The answer depends upon the plate and screen voltages you will be running. I think you will be ok up to ~450V plate voltage with the screen at or below that. The required bias voltage will be in the usual range expected for 6L6's at the plate and screen voltages you have; the OT primary impedance won't change that unless it's way off.

[Leo_Gnardo]

What stevem and martin said ^^^ plus remember an OT can be thought of as a 'gearbox" between output tubes and speaker. There's a good discussion going on recently at our friends MEF. Have a look here, and particularly at posts 14 and 18.

http://music-electronics-forum.com/t37620/

Someone mentioned, I forget if it was here or there, that he reckoned 5500 was an ideal primary load for a pair of 6L6. Let's go with that for the moment. Then 2750 would be best for a 6L6 quad. Your proposed transformer choice isn't going to cause anything to melt down, but the power transfer from tubes to speakers won't be ideal. Upping the load your OT's output tap by running say 8 ohms speakers on the 4 ohm tap puts you at 4000 ohms, still a ways from ideal in the other direction.

Transformers intended for use with 4 x EL34 or 6550 or KT88 usually rated 1900 ohms primary or thereabouts. At least that's the rating on some of the popular Hammonds.

FWIW I've converted lots of Fender amps to EL34, surprise, extra power to the speakers. Properly biased, the tubes can last a long time, in some cases over 10 years in amps that are used a couple times a week for gigs. Some may consider that heresy but the customers are satisfied. Nothing's melted down yet - so far so good.

[grtamp]

a friend of mine has a Sovtek MIG100 with 4x6L6 and the OT seems to me like a plexy/800 type , and never had problem , therefore the thing has me curious!!

[Firestorm]

Fender used 4K2 on a lot of their 2x6L6 models at typical Fender voltages (460ish).A quad would then be 2K1, so you aren't really all that far off.

[Leo_Gnardo]

Fender used 4K2 on a lot of their 2x6L6 models at typical Fender voltages (460ish).A quad would then be 2K1, so you aren't really all that far off.

Given that, I'd say go ahead. It's not a matter of "get all the power you can, points off if you're not top of the power curve." Do it sound good? If yes, you hit the target.

No wonder the power figure rises when EL34 are used in Fenders.

[Reeltarded]

It works fine!