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-   -   EL84 PP output transformer for 6L6 PP class A (http://www.diyaudio.com/forums/tubes-valves/102725-el84-pp-output-transformer-6l6-pp-class.html)

engels 29th May 2007 09:08 PM

EL84 PP output transformer for 6L6 PP class A
 
I've got some old radio transformers which I thought to use for another project, but I'm not sure it won't blow.

The PT is telefunken with 250V secondary (no CT) which should deliver about 290V B+. The total B+ current in the radio should have been around 180mA. It was a PP EL84.

And then I have a PP output transformer from Siemens radio which looks relatively beefy and was used in another EL84 PP.

May I use it for 6L6 PP amplifier? I believe the Siemens amplifier wasn't a 18W but rather around 15W (it wasn't a guitar one) and 6L6 under the above mentioned voltages may deliver around 20W class A with 2% distortion.

Miles Prower 30th May 2007 03:30 AM

Re: EL84 PP output transformer for 6L6 PP class A
 
Quote:

Originally posted by engels
I've got some old radio transformers which I thought to use for another project, but I'm not sure it won't blow.

The PT is telefunken with 250V secondary (no CT) which should deliver about 290V B+. The total B+ current in the radio should have been around 180mA. It was a PP EL84.

And then I have a PP output transformer from Siemens radio which looks relatively beefy and was used in another EL84 PP.

May I use it for 6L6 PP amplifier? I believe the Siemens amplifier wasn't a 18W but rather around 15W (it wasn't a guitar one) and 6L6 under the above mentioned voltages may deliver around 20W class A with 2% distortion.

You might be able to get away with it, however, you can pretty much forget about that 2% distortion from the 6L6s. The EL84 has an Rl= 5.5K (P-2-P) and the 6L6 needs 6.6K (P-2-P). Using the lower Rl will increase distortion. Nor will you be able to take full advantage of the 6L6s since the power of the EL84s is lower.

Finally, "old radio transformers" weren't all that good, so I wouldn't expect good performance there either.

sorenj07 30th May 2007 04:38 AM

Might make a decent guitar amp though, right? :D

engels 30th May 2007 05:31 AM

Re: Re: EL84 PP output transformer for 6L6 PP class A
 
Quote:

Originally posted by Miles Prower

The EL84 has an Rl= 5.5K (P-2-P) and the 6L6 needs 6.6K (P-2-P).

Most datasheets specify 5000 plate to plate load for 6L6 under 300V. Do I miss something?

ray_moth 30th May 2007 08:33 AM

EL84 PP is supposed to have 8k p-p in Class AB1 PP pentode-mode. 6L6 needs 9k p-p for Class A in PP pentode-mode with 360v plate voltage and 270v screen voltage, or 8k p-p for Class A in PP triode-mode with 325v plate & screen voltage.

So, the impedance of your OPTs seems to be in the ball-park. What might be a problem, though, is the primary current demanded by the 6L6s, which may be significantly more than the EL84s and could cause overheating of the OPT primary windings.

Miles Prower 30th May 2007 09:16 AM

Re: Re: Re: EL84 PP output transformer for 6L6 PP class A
 
Quote:

Originally posted by engels
Most datasheets specify 5000 plate to plate load for 6L6 under 300V. Do I miss something?
There are a lot of different specs for running 6L6s. The best one is: Vpp= 360Vdc; Vsgsg= 270Vdc; Rl= 6.6K (P-2-P); Vgk= -22.5 fixed bias. This'll give THD= 1.8%. Other operating points have higher THD. You seemed to be indicating that you did want the lowest THD, and the OPT you mentioned won't get you there.

If you can stand a higher THD, then you can find an operating more in line with the OPts you mentioned, if you can run them at less than max power and they can withstand the current demand.

engels 30th May 2007 10:18 AM

currents
 
If I'm going to take the above mentioned PT with 250V secondary I won't get 360V for the plates. It seems to be more appropriate to aim at 270-280V plates and screens. Duncan Amps page says it's 270-270V with 72.5mA for plates and 8.5mA screens, 125 shared cathode resistor into 5,000 load. The result is 18.5W with 4.0 THD. Everything seems okay except for the currents and that's what I'm worried about.

Of course, less distortion is better but still 4% is much cleaner than 10% with EL84.

SemperFi 30th May 2007 10:35 AM

Go for it! The distortion will be much lower at your normal listening level. (4% just before clipping isn't bad at all).
I have made a bunch of amps with improper transformers and much too low voltages, and of course the tube is not working at full potential, but the results are often surprisingly good.

SemperFi 30th May 2007 10:45 AM

One way to get a little more plate voltage is to sneak in some negative voltage and use fixed bias. You may do so with an AC coupled rectifier to get the negaitve voltage if you are using a fullwave rectifier for the B+. Or you could use a couple (or three) 9volt batteries in series for the bias voltage.


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