converting UL in triode

[overdrajv]

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]

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.

 

[SY]

Ray, in triode mode, Mullard's spec sheet indicates a 600V max for plate and g2.

od: Yes, a 100R screen stopper seems to be a good idea. And you'll certainly have restricted treble, but for this application, that may be fine.

 

[Wavebourn]

It already looks like a Fender's guitar amp's output stage. However, Fender preferred to connect the cap from the right grid to the earth instead of to the GNFB, for more musical overdrive.

 

[ray_moth]

Ray, in triode mode, Mullard's spec sheet indicates a 600V max for plate and g2.

So it is! Furthermore, I see that the max g2 even in pentode mode is 500v with signal and 800v with no signal. I dunno where I got 420v from but I eat my words!

 

[7N7]

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

 

[Wavebourn]

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!

 

[7N7]

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

Indeed! That certainly doesn't help!

7N7

 

[Wavebourn]

Indeed! That certainly doesn't help!

7N7

Neither ceramic sockets... That's why I use GU-50 tubes in amps for more than 50W output.

 

[7N7]

Neither ceramic sockets... That's why I use GU-50 tubes in amps for more than 50W output.

Well I have to submit that ceramic is better than cheap plastics (which is what I used to find in the guitar amplifiers).

7N7

 

[overdrajv]

Thanks for the response!


Anyway I found on www.aikenamps.com under data sheet, that g2 should not be above 425V.. and I would like to run it class A for even less power, so how can I drop B+ ?

 

[ray_moth]

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.