NickK-UK
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- May 20, 2023
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So yesterday was the best type of day - I got a performance related bonus so I have some budget and the significant other has approved expenditure.
I've been mulling this planned design over for several years and with the almost completion of my guitar, my mind is switching back to some form of amplification. I've designed and built hit tube amps (640Vdc!), I have an old 1957 tube based entertainment system I've restored back, plus I have luxury of a scope, sig get and bench supplies.
So what do I want this amp to be able todo in the end (after iterations):
* Able to take a single 7 string 28.625" scale length (actually 29" for low B) drop tunings - allowing both old metal (JCM/Fortin) and new prog metal (abasi) cleaner tones.
* Cope with the Sentient and Pegasus pickup outputs (which are medium hot output) in humbuck and split levels.
* Play at full volume in the following scenarios (ie have an inbuilt speaker load to attenuate for):
* * Connect via DI to be completely silent but allow recording of the sound when using volumes to saturate the output transformer
* * Connect via headphones (mine are 55 ohm AKGs) - pure analogue
* * Creamback G12M-65 - pure analogue
* take upfront pedals well - this should be reasonably easy
* have a pedal effects loop
* have reverb tank
Getting a good power transform is a pain, especially one that supports iterative changes. I have two 117V secondary tranformers I'll use and a 22V transformer - I'll build the power supply with those and then after I'm comfortable switch to a single transformer.
So my starting position was the Robinette jcm800 micro 1W using EF80 output tubes.
A design decisions:
* use the original B+ voltages for the JCM800 preamp section - for this I will have two B+ lines so that the preamp can sit at higher voltages, the down side is less sag up front in the pre based on the power tubes current draw. To maintain the character both B+ lines will have a choke.
* use a solid state rectification - there will be two power supplies. I've looked at EZ81 as a minimum voltage drop but it just seems to increase the power and transformer requirements - especially with additional stage driving etc.
* Use elevated heaters (~75V) with DC rectified 6.3V supplies (the EF80 needs 6.3V anyway) I could do 12V for the preamp section at a later date. This should save the poor cathode follower that is notorious for destroying valves I'll also put a diode over to protect the grid.
* turret board - this allows me to make the stages and then move from the development chassis at a later date. Point to point preferred.
Now for the abasi cleans.. I may look at having a switch over from a 12ax7 for the LTP to something with less gain - two options here 12au7 or a 12bh7a-str which I have. The 12au7 is less linear than the 12bh7 but both will do a fender clarity.
Anyway - that's it so far (I have modelled this in LTSpice to get a finger in the air but that does nothing a soldering iron and a few shorts will teach you).
I will use this thread as my build thread as I progress.
I've been mulling this planned design over for several years and with the almost completion of my guitar, my mind is switching back to some form of amplification. I've designed and built hit tube amps (640Vdc!), I have an old 1957 tube based entertainment system I've restored back, plus I have luxury of a scope, sig get and bench supplies.
So what do I want this amp to be able todo in the end (after iterations):
* Able to take a single 7 string 28.625" scale length (actually 29" for low B) drop tunings - allowing both old metal (JCM/Fortin) and new prog metal (abasi) cleaner tones.
* Cope with the Sentient and Pegasus pickup outputs (which are medium hot output) in humbuck and split levels.
* Play at full volume in the following scenarios (ie have an inbuilt speaker load to attenuate for):
* * Connect via DI to be completely silent but allow recording of the sound when using volumes to saturate the output transformer
* * Connect via headphones (mine are 55 ohm AKGs) - pure analogue
* * Creamback G12M-65 - pure analogue
* take upfront pedals well - this should be reasonably easy
* have a pedal effects loop
* have reverb tank
Getting a good power transform is a pain, especially one that supports iterative changes. I have two 117V secondary tranformers I'll use and a 22V transformer - I'll build the power supply with those and then after I'm comfortable switch to a single transformer.
So my starting position was the Robinette jcm800 micro 1W using EF80 output tubes.
A design decisions:
* use the original B+ voltages for the JCM800 preamp section - for this I will have two B+ lines so that the preamp can sit at higher voltages, the down side is less sag up front in the pre based on the power tubes current draw. To maintain the character both B+ lines will have a choke.
* use a solid state rectification - there will be two power supplies. I've looked at EZ81 as a minimum voltage drop but it just seems to increase the power and transformer requirements - especially with additional stage driving etc.
* Use elevated heaters (~75V) with DC rectified 6.3V supplies (the EF80 needs 6.3V anyway) I could do 12V for the preamp section at a later date. This should save the poor cathode follower that is notorious for destroying valves I'll also put a diode over to protect the grid.
* turret board - this allows me to make the stages and then move from the development chassis at a later date. Point to point preferred.
Now for the abasi cleans.. I may look at having a switch over from a 12ax7 for the LTP to something with less gain - two options here 12au7 or a 12bh7a-str which I have. The 12au7 is less linear than the 12bh7 but both will do a fender clarity.
Anyway - that's it so far (I have modelled this in LTSpice to get a finger in the air but that does nothing a soldering iron and a few shorts will teach you).
I will use this thread as my build thread as I progress.