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Old 23rd December 2007, 09:24 AM   #1
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Default 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?
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Old 24th December 2007, 01:40 AM   #2
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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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Old 24th December 2007, 02:49 AM   #3
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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.
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Old 24th December 2007, 03:04 AM   #4
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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.
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Old 24th December 2007, 11:42 AM   #5
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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!
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Old 24th December 2007, 12:01 PM   #6
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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.

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Old 24th December 2007, 05:47 PM   #7
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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!
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Old 24th December 2007, 07:18 PM   #8
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Originally posted by Wavebourn


Pins 2 (filament) and 3 (anode) are too close to each other!
Indeed! That certainly doesn't help!

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Old 24th December 2007, 07:26 PM   #9
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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.
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Old 24th December 2007, 07:28 PM   #10
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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).

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