converting UL in triode

overdrajv · 23rd December 2007, 09:24 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 · 24th December 2007, 01:40 AM

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 · 24th December 2007, 02:49 AM

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 · 24th December 2007, 03:04 AM

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 · 24th December 2007, 11:42 AM

Quote:

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 · 24th December 2007, 12:01 PM

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 · 24th December 2007, 05:47 PM

Quote:

Originally posted by 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

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

7N7 · 24th December 2007, 07:18 PM

Quote:

Originally posted by Wavebourn

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

Indeed! That certainly doesn't help!

7N7

Wavebourn · 24th December 2007, 07:26 PM

Quote:

Originally posted by 7N7

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 · 24th December 2007, 07:28 PM

Quote:

Originally posted by Wavebourn

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

23rd December 2007, 09:24 AM	#1
overdrajv diyAudio Member Join Date: Dec 2007	converting UL in triode 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?

24th December 2007, 02:49 AM	#3
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ If there's a sucker born every minute, where do the rest of them come from?

24th December 2007, 03:04 AM	#4
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	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. __________________ If I disappear suddenly, that means I finally created a time machine and pushed wrong button that brought me to Stalin's Russia. In any experiment any result is the result. Even if it is negative.

24th December 2007, 12:01 PM	#6
7N7 diyAudio Member Join Date: Jan 2003 Location: 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 __________________ Plug them in and light them up

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24th December 2007, 01:40 AM	#2
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: 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.

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