Question about various topologies for tube preamp

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I'm liking this topic a lot , its pulling in a lot of opinions on different angles ,without ending up in a divicive sh1.t slinging match.
For guitars its often awkward to crank your 50 or 100 watt output stage to into Angus Young  levels of feedback/sustain on a small stage or domestic studio set up . I keep a range of amps available, single EL84( modded peavey nano valve) switchable between triode/pentode ,so 1.5 or 5 watt.Next step up is push pull EL84's (Marshall 20watt 1974 head) and also a 2xEL84 with baxandall tone section, split load phase inverter and fender deville o/p transformer , from there your up into 50 watt marshalls then a couple of Hiwatts ,DR103 and DR201. For me there's nothing like the interaction of player,guitar ,pickup, output stage and speaker regenerating into the positive feedback loop , pre amp distortion alone sounds fizzly and lacks dynamics in comparison. Im not a fan of op amp distortion stomp boxes either , so what I used do was set up the 20 watt marshall with a passive ( a box ,two jacks and a pot) footswitchable attenuator on the front end , you set the amp volume so all hell breaks loose then dial in your clean tone with the attenuator ,if needs be and your on a big stage you mic up the 20 watt with a hi-Z mic then plug that into a bigger tube amp with what ever power is required for the size of the venue your in ,then the guitarist has his tone no matter what the ocassion ,he only needs to adjust his stage volume through the final amp to suit the venue. I should add here that although I have a guitar for testing purposes Im not a musician and never played on stage in my life, did plenty of work on front of house though . Anyroad I hope theres some food for thought in that ,might not cut the mustard for speed or death metal heads ,but if its anything from clean with an edge to blues to rock to hard rock its just the ticket .
 
yeah many interesting points from everyone.  I guess I got a little off the original topic, but the thread did reveal some important points for me to remember when designing - pentodes create mostly odd harmonics, and push-pull stages tend to cancel even harmonics.  Definitely  valuable info when I am aiming for higher distortion figures.
 
About pentode/triode, in "linear" mode (not clipping), pentode delivers more harmonic distortion, leading to a more coloured/warm sound IMHO. I can hear it with the Therapy Preamp which has a pentode/triode switch on the 3rd gain stage, as well as on a Vox AC15 guitar amp clone I built on which I implemented this switch for the output stage. When compensating for the gain loss, triode mode still  feels "flat"/"dead" in comparison.
 
ricothetroll said:
About pentode/triode, in "linear" mode (not clipping), pentode delivers more harmonic distortion, leading to a more coloured/warm sound IMHO. I can hear it with the Therapy Preamp which has a pentode/triode switch on the 3rd gain stage, as well as on a Vox AC15 guitar amp clone I built on which I implemented this switch for the output stage. When compensating for the gain loss, triode mode still  feels "flat"/"dead" in comparison.

It is hard to compere triode and pentode sound in that sort of context. For all tubes, the amount of distortion produced depends on the output level. Switching a pentode to triode mode drops the output level so not only does the character of the tube change, but alos the amount of distortion drops due to the lower output level. Usually this is compensated for by extra gain  after the tube but to compare the tubes at similar output levels you really should up the gain before the tube. This probably explains why the triode mode sound flat to you.

Cheers

Ian
 
I think I compensated it on the vox by increasing the input gain as it doesn't have a master volume and I didn't use an attenuator.  I should test it again and record the result to confirm.
BTW the Therapy Preamp is nearly finished as I got rid of the last issues I had with it, so I'll be posting some sounds soon. It's a great circuit to test that triode/pentode difference as you can switch from one mode to the other and compensate with the volume pot before the E81L.
 
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