Pentode experiments

tobyk

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Would be interested to know if any of you gents have experimented with pentodes, ie put them in a later preamp stage, and/or tried different component values for screen compressions, etc..
 

VintageSG

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Not so much experimented, but I did build the Madamp G3 Blues, which has an interesting pentode/triode front end.
The 'gain' control has the effect of varying compression as well as overall 'gain'
It's a lovely amp!
 

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tobyk

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Exactly what I was looking for, thanks!
So how does it work, turn clockwise and you get more gain and compression at the same time? Think Merlin has something like that in his book.
Not so much experimented, but I did build the Madamp G3 Blues, which has an interesting pentode/triode front end.
The 'gain' control has the effect of varying compression as well as overall 'gain'
It's a lovely amp!
 

VintageSG

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Clockwise gives more gain, less compression. Used in conjunction with a clean-ish boost pedal and/or the guitar volume control, some great sounds can be had. Keep the compression up by keeping 'gain' set quite low, and a Tele bridge pickup sparkles in a most agreeable way. The gain side of it isn't for metal-heads, but for sustain heaven, for bluesy leads ( and gronked out fuzz-garage ) it's a nice setup.
 

tobyk

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Clockwise gives more gain, less compression. Used in conjunction with a clean-ish boost pedal and/or the guitar volume control, some great sounds can be had. Keep the compression up by keeping 'gain' set quite low, and a Tele bridge pickup sparkles in a most agreeable way. The gain side of it isn't for metal-heads, but for sustain heaven, for bluesy leads ( and gronked out fuzz-garage ) it's a nice setup.
Sounds great, gotta try it – very easy mod too!
 

VintageSG

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One thing to watch for is the use of a type 'C' pot. A normal 'A' pot'll work, but may cause head scratching, being bass-ackwards :)
 

tobyk

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One thing to watch for is the use of a type 'C' pot. A normal 'A' pot'll work, but may cause head scratching, being bass-ackwards :)
I was just going to ask about that, so a reverse audio pot it will be!
 

tobyk

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Clockwise gives more gain, less compression. Used in conjunction with a clean-ish boost pedal and/or the guitar volume control, some great sounds can be had. Keep the compression up by keeping 'gain' set quite low, and a Tele bridge pickup sparkles in a most agreeable way. The gain side of it isn't for metal-heads, but for sustain heaven, for bluesy leads ( and gronked out fuzz-garage ) it's a nice setup.
Thinking about it, if I put the pentode after two gain stages, there would be no need for a boost pedal I guess!
 
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printer2

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I messed around with a pentode but in the first position. Rather than copying other's values I had a pot for each of the resistors, plate cathode and screen. I spun the knobs until I found a combination that sounded best. I found there was a sweet spot that sounded best to me. I only tried it with one tube and did not get around to checking others of the same type or different pentodes.
 

tobyk

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On a preamp pentode with say 100K anode resistor, what will dropping the screen resistor from say 1M to 220K do to tone? Will it compress more?
Will the result be similar to instead adding a 500K resistor after the screen bypass cap?
 

2L man

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On a preamp pentode with say 100K anode resistor, what will dropping the screen resistor from say 1M to 220K do to tone? Will it compress more?
Will the result be similar to instead adding a 500K resistor after the screen bypass cap?
It will increase screen voltage and it lessen control grid signal ability to adjust anode current. Operating point moves and I think whole EF86 rebiasing is needed. I have used "default" values when I have used EF86 few times. Tube datasheet usually list max values!

I think you are ready to begin using this links loadline calculator :)

 




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