• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

converting UL in triode

overdrajv

Member
2007-12-19 11:56 am
Hi people!

I have a simms-watts PA 100 head http://www.drtube.com/schematics/simms/100w-amp.gif , that im trying to convert in a guitar amp. I already turned off two tubes (OT still sees impedances), this softened the sound a bit..

I wonder if it is posible to convert the power amp in a triode mode. The plate voltage is around 500 V, I think this is too high. And should i conect the screens with 100 Ohm resistors too the plates?

Any suggestions?
 

ray_moth

diyAudio Moderator Emeritus
2004-01-27 8:55 am
Jakarta
Yes, 500v is much too high for triode operation, especially since that amp uses grid bias, so there is no voltage lost across cathode resistors. In fact, 500v is really too high for ultralinear operation too. EL34 screens are rated at 420v max. What you need to do is drop B+ to about 420v.

As you say, you can just connect the screens with 100 Ohm resistors to the plates and leave the UL taps on the OPT unconnected (and safely insulated). The OPT should still be a reasonable match for triode operation. Just watch the bias when you switch it on - set the two bias pots to maximum negative voltage (minimum current) and adjust from there.
 

7N7

Member
2003-01-18 10:43 pm
England
Yes, I think that 800V is pushing it a bit for octal bases.

Certainly in my guitar amp days I had enough trouble with arc-overs on valve bases in guitar amplifiers, most of which were running at around 440V.

I suppose a clean ceramic or (wishful thinking) PTFE base would be OK, but very often the interiors of guitar amplifiers left something to be desired in respect of cleanliness! And the plastic octal bases were probably not ideal.

7N7
 
7N7 said:
Yes, I think that 800V is pushing it a bit for octal bases.

Certainly in my guitar amp days I had enough trouble with arc-overs on valve bases in guitar amplifiers, most of which were running at around 440V.

I suppose a clean ceramic or (wishful thinking) PTFE base would be OK, but very often the interiors of guitar amplifiers left something to be desired in respect of cleanliness! And the plastic octal bases were probably not ideal.

7N7

Pins 2 (filament) and 3 (anode) are too close to each other!
 

ray_moth

diyAudio Moderator Emeritus
2004-01-27 8:55 am
Jakarta
Ah, that's where I got the idea of 420v from. Thanks!

If you replace that 25 ohm 10w resistor with a 5H 600mA choke, you should be OK, That will give you about 320v which is fine for Class A. You'll need a much higher quiescent current - around 90mA per tube - and that may require some changes to your bias network.