It’s been a while since I last share some of the experiments. I’ve done a lot with different drivers topologies and pentodes lately, yet not much time to sit down and do a proper write-up.
I have a project in hands which is a small Mono amplifier to drive a Celestion guitar speaker I have in my Marshall clone. I will use this to play my synthesiser gear. Low volume levels needed in my workshop so can do with a nice small Single-Ended Amp. For now, I focused on playing with different driver topologies and settled with the pentode with local feedback as a result of my evolving experiments and the guidance provided by JC Morrison on this topic:
JC Morrison has covered this circuit in detail so I suggest you read his blog post which are fantastic. Therefore I will only add a few notes about the circuit. I want to get a good gain level which can be done without sacrificing too much the input impedance. About 30KΩ-ish would be ok without pushing R2 to be too big. I’m trying to keep cathode bias with diodes to maximise the transconductance of the valve and therefore the open loop gain. The screen voltage stability will be derived from the mu-output of the CCS. In practice will have R6 and R7 with a trimpot in between for fine adjustment.
Why the EF37a? I have a few of them NOS Mullard and have yet a fascination of pre-WWII valves. It sounds and looks great as well!
Here is the nice small modular approach. Both boards are mounted on top of each other. The PCBs have been tested before, so it’s a question of soldering on now:
I hope to report further progress during the Holiday season.
Stay tuned
Hi Ale,
You’ve got my attention…. Have you got a link to the JC Morrison pages that address this design?
Best,
Simon
Morning Ale,
I hope you and your family are staying well.
I’m not sure that it is a significant difference but have you noticed that JC doesn’t use the so-called “mu output” with his CCS loaded pentodes.
Merry Christmas
tim
Hi Tim
All good here, tighter lock-down than before. Doh! Hope you and family are ok?
I find loading on the mu output to have a minor improvement due to the fact that the EF37a hasn’t got the highest of the gm for a pentode. If you load it on the anode, the feedback circuit into the screen grid will lower the open loop gain. With loading in the mu-output you don’t have this, which is marginally better.
I made some progress as have a screen bias PCB I could reuse. I know need to wire the whole thing up. Fun part!
Cheers
Ale