Single Ended EL34 Amplifier + rectifiers

[mastermanx2001]

I want to change the valve rectifiers to silicon rectifiers in my single ended EL34 valve amplifier.

It is of course a vintage type amplifier, though of new construction.

What rectifiers are recommended, how should they be connected, are there any problems that can be foreseen?

The idea is to reduce the temperature (it does get very hot), power consumption and perhaps get more output from it. I feel, but am not sure, that the valve rectification is limiting the output of the amp.

Any suggestions?

[thermionic]

Hi.

I would use a couple of 1N4007 1 amp diodes & use a 10 ohm current limiting resistor between the junction of the 2 cathodes and the main HT rail to protect the diodes when the HT resevoir capacitor charges.

Cheers. SimonT.

[Tractorfan]

I'm no expert but, personally, I wouldn't do it. For these reasons:
1, At switch on the solid state rectifiers will give full HT before the valves have warmed up with possible overvoltage on the smoothing caps.
2, If the valve rectifiers are supplying sufficient HT current and the correct voltage to drive the amp properly, you probably won't increase the output by any worthwhile amount.
3, Valve amps run hot anyway so the bit extra from the rectifiers won't make much difference. It might be worth increasing the ventilation.
4, Erm... maybe other forum members can think of a fourth
These are just my own thoughts but other members may think otherwise.
Hope this is useful to you.
Cheers de Pete

[Herald1360]

I doubt that the valve rectifiers are limiting the output, the biggest voltage drop in the circuit is probably the 330ohm resistor in the PSU. Since the amp is class A the average current draw is constant and the huge (by vintage standards) HT smoothing capacitors will happily handle the audio frequency variations in HT current. If you want more output and the EL34's will stand it, why not use the higher voltage taps on the transformer?

As it is, the operating conditions for the EL34 bear little resemblance to the Mullard data sheet suggestions, so being charitable one has to assume that they are the result of some meaningful experimentation by the designer of this particular amplifier. In particular, the screen grid tapped off the output transformer bears more resemblance to half of an ultra-linear push pull output than a traditional class A single ended setup.

You will of course reduce the power dissipation a bit (by at least the heater power anyway) by going solid state, but it would be worth also putting some suitable 10n or so capacitors across the diodes to suppress any RF hash which they will assuredly generate.

Have fun!

Chris

[flyingtech55]

More output? Feed the screen of the EL34 from the HT rail. Increase the value of the anode resistor of EL86 type valve to about 220K or so and adjust the HT to the stage accordingly. Check the cathode bias of the EL34 is correct according to the vale data.

HTH

Tim R

[Sideband]

I wouldn't bother with silicon reccy's either. I think the weak point is the ultra-linear connection of the output valves. This will give reduced output (it effectively introduces negative feedback within the output valves) but does give reduced distortion. Probably the original designer tried to reduce 3rd harmonic distortion in the output valves by using ultra-linear connection.

Try Flyingtecs idea. I've never seen ultra-linear connection with single-ended output valves before. It was common with quality push-pull power amps (see Mullard 5-20)


Rich.

[PJL]

There is gain and there is output power. It's class A so the maximum output power is obtained by running the valve as hot as you dare, and matching the output transformer so the voltage swing is as wide as possible over a linear part of the characteristics allowing headroom for the peaks in the music.

Because the design has no feedback from the output, distortion will increase with output power as the voltage swing extends into the non-linear areas. The ultra-linear design will be an attempt to reduce this distortion and although it could be described as feedback, it is designed to straighten the curvature in the characteristics.

Quad and Leak both use some negative feedback to acheive low distortion.

That's the limit of my knowledge...Peter

[mastermanx2001]

Wow, very good answers and some things I had not thought about. The more I read, the more I realise how very little I know.

Just for interest here is a picture of the amp sitting on top a a leak Stereo 70.

Thanks to all, I hope to see even more replies.