EL34 cathode bias PA

[slideman82]

I wanted to build something with EL34's for guitar use, but don't want 50W, just a bit less, about 30 or 35W. Datasheet says 375V plate voltage and 3,5k load, but I'm looking for a cathode/self bias PP design. Any ideas?

[gingertube]

Go for it - I had, until recently an ex AWA (Australian) PA Amplifier which I rebuilt as a guitar amp. I replaced power supply caps, left the cathode biased EL34 Outputs, replaced the 70 V Line Output Transformer with a spare 50W Marshall Output Tranny I had on the shelf (Raa = 3K4) and rebuilt the preamp section to the web published Trainwreck A0 circuit (with one mod of my own). It gives 35W Output.

So where are we going with this? - I do guitar amp repairs / mods for the local "Guitar Guru". I lent him this amp to try out and now he does'nt want to give it back - uses it for all his gigs. This guy is the best professional guitarist I know and has had tens if not hundreds of amps over his 35 year career - but thinks this cathode biased EL34 35W Amp is amoungst the best he has ever used.

One BIG Caution - when using cathode (automatic) bias DO NOT try to implement a High Voltage Standby Switch. When switching on the HV you get a massive current spike through the tubes until they eastablish the auto bias. If you want a "standby" function do what I did - wire the standby switch as an output tube grid "mute" switch. I started with a HV switched standby and found I was regularly blowing the 500mA HV fuse when switching out of standby. This can't have been doing the tubes much good either.

Let me know if you want me to finish the schematic I have 3/4 drawn and post it (that would take me a day or two).

Cheers,
Ian

[Alain Poitras]

Originally Posted by slideman82

I wanted to build something with EL34's for guitar use, but don't want 50W, just a bit less, about 30 or 35W. Datasheet says 375V plate voltage and 3,5k load, but I'm looking for a cathode/self bias PP design. Any ideas?

Here's an example schematic for a EL34 / 6CA7 guitar amplifier. You can download a free Spice simulator software like SIMetrix and modify parts of the circuit to have it work at the voltages and load you like, if it is possible ...

You can get good Spice models from Duncan Amps but you will have to draw the tubes symbols yourself, it is very easy !

You can first study the circuit by drawing loadlines on their caracteristic curves. In a push-pull circuit, the impedance see by each tube have to be one fourth of the PP impedance, if you use a 3,5K PP transformer, it is 875 ohms for each tube ... Don't go too far from class "A" if you like to have low distortions !

Cheers,

Alain.

[slideman82]

Both are great replies! Alain's one seems to be a bit more difficult... and I'm a bit lazy for doing the maths!

I want to try self bias because it's easier to set, I think, don't have to readjust bias for ever tube change, and could be a warmer sound. I took power and OT from a national made microphone amplifier, seemed to be fixed bias, 30W output, and has no stand by switch.

Power transformer is center-tapped, 300 VAC aprox. each line, and OT has ~ 5K primary impedance... it seems to be I'll have to mess with curves...

[slideman82]

We could use inductors from OT to plates, so current peak won't be so large... just wondering.

[Alain Poitras]

Originally Posted by slideman82

We could use inductors from OT to plates, so current peak won't be so large... just wondering.

No ... Not a good place for an inductor, it oppose itself to fast current change, then the sound ... Just the bias adjust will make the current higher or lower, the supply voltage have nothing to do with that !

But you have to measure the DC resistance of the primary of your output transformer to know what gona be the voltage drop across it.

[Alain Poitras]

Originally Posted by slideman82

Both are great replies! Alain's one seems to be a bit more difficult... and I'm a bit lazy for doing the maths!

I want to try self bias because it's easier to set, I think, don't have to readjust bias for ever tube change, and could be a warmer sound. I took power and OT from a national made microphone amplifier, seemed to be fixed bias, 30W output, and has no stand by switch.

Power transformer is center-tapped, 300 VAC aprox. each line, and OT has ~ 5K primary impedance... it seems to be I'll have to mess with curves...

With a 300-0-300 volts transformer (no load), you can get up to 423 VDC with no load but with a big load, the voltage can drop 15% or 20%, that mean 359 or 338V ... And you need a choke or voltage dropping resistor in the supply to get less hum so it is possible you get only 330V for the supply ... Less the voltage drop in the output transformer primary and the autobias resistor, this can be another 20 or 30 volts gone ...

You will have to make many tests and ajust the autobias resistor for the supply voltage you can get, you have to live with the transformers you got. Some peoples have little problems with a too high supply voltage and others have bigger problems with a too low supply voltage ...

Cheers,

Alain.

[DF96]

I think high voltage standby switches are a bad idea, but if you want to add one then you could reduce the switch-on surge due to slow ramp up of cathode volts by ensuring that the g2 decoupler cap is big enough - bigger perhaps than you might normally use in this position. A pentode can't conduct much while g2 is low, whatever g1 is set to. Provided the cathode voltage can rise more quickly than g2 you should be OK.

[slideman82]

Originally Posted by DF96

I think high voltage standby switches are a bad idea, but if you want to add one then you could reduce the switch-on surge due to slow ramp up of cathode volts by ensuring that the g2 decoupler cap is big enough - bigger perhaps than you might normally use in this position. A pentode can't conduct much while g2 is low, whatever g1 is set to. Provided the cathode voltage can rise more quickly than g2 you should be OK.

Great tip!!!!!!! Also have a choke, 300 ohm, about 3 H.

Last night I checked some EL34 schematics I have, and there are some cathode biased, 470k grid resistor to ground (I think they are from Poland).

This is a project, I planed everything before solder all toghether. And thanks you all for these help!

[costis_n]

Concerning the HV standby switch, if there is a 2-3K or so resistor across it (the amp will not be muted completely in standby), you will be OK for inrush current. I have done it.