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Hi Phil, my thinking is this - there are a lot of very well regarded direct coupled designs that are cathode biassed. I want to build an amp that has pedigree, leaves no doubts as to whether I coulda shoulda done it differently. The dc coupled amp has a delicate balance of operating conditions and the cathode bias provides a robust way to keep things in-check as tubes age, mains voltages vary etc. Yes, there's a cathode bypass capacitor, but fixed bias also has it's capacitors when you look at the signal current loops.
HI Bigun
Thanks;
I have several more 2A3 output transformers maybe i will try Cathode bias. I paid $US 30 for the Sovtek 2A3's so expensive now.
Have plenty of NOS 2A3's
Phil
Thanks;
I have several more 2A3 output transformers maybe i will try Cathode bias. I paid $US 30 for the Sovtek 2A3's so expensive now.
Have plenty of NOS 2A3's
Phil
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Hi Phil, poor old me has 1 tube and 1 OPT 😀
Why don’t you build alongside my effort and see how you like the results
Why don’t you build alongside my effort and see how you like the results
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I was thinking
When it comes time to build this amp, and it will be built (we're only a few years into the project now after all), it's going to run hot. It's the cathode resistor on the 2A3, it's going to burn a good 10W or so depending on the final choice of operating points.
I'd rather not have that heat underneath the chassis, but on top with the other hot bits. I don't want a heatsink up on top. So that leaves the obvious choice of burning it off in time honoured fashion of a hot anode. Placing a tube up on top puts all the heat up on top.
Today I found a schematic by Gordon Rankin for a direct coupled 45 amp in which he used a valve as a CCS in place of the cathode resistor. I've seen this suggestion before but never in the form of a proven-out schematic. Until now. His amplifier has been built by others and much praised. Gordon said it was in his top 5 (according to a post by Gordon on another forum). And that has emboldened me. I'd normally go with the simple approach, just put a resistor in there. But a valve based CCS lets me put the heat somewhere else.
Gordon used a triode wired pentode which has the advantage of lower noise (bear in mind, it's shunted by a capacitor) and no requirement for a separate screen supply. But a pentode is a much better CCS if left as a Pentode so I thought of starting there. See attached. Of course, it may still be better to go with the triode wired pentode option since it's still going to have a higher resistance than the cathode resistor would have had. Not sure which approach is best on paper but Gordon must have given it some thought. Being adjustable could be very handy for messing around - it would allow for some tube rolling between the more robust JJ-2A3-40 and some of the less able tubes, or even substitute a 45.
Getting fancy could mean sizing the screen capacitor (see attached) to slow down the turn-on current draw of the 2A3 since this cap has to charge up through the screen pot.
When it comes time to build this amp, and it will be built (we're only a few years into the project now after all), it's going to run hot. It's the cathode resistor on the 2A3, it's going to burn a good 10W or so depending on the final choice of operating points.
I'd rather not have that heat underneath the chassis, but on top with the other hot bits. I don't want a heatsink up on top. So that leaves the obvious choice of burning it off in time honoured fashion of a hot anode. Placing a tube up on top puts all the heat up on top.
Today I found a schematic by Gordon Rankin for a direct coupled 45 amp in which he used a valve as a CCS in place of the cathode resistor. I've seen this suggestion before but never in the form of a proven-out schematic. Until now. His amplifier has been built by others and much praised. Gordon said it was in his top 5 (according to a post by Gordon on another forum). And that has emboldened me. I'd normally go with the simple approach, just put a resistor in there. But a valve based CCS lets me put the heat somewhere else.
Gordon used a triode wired pentode which has the advantage of lower noise (bear in mind, it's shunted by a capacitor) and no requirement for a separate screen supply. But a pentode is a much better CCS if left as a Pentode so I thought of starting there. See attached. Of course, it may still be better to go with the triode wired pentode option since it's still going to have a higher resistance than the cathode resistor would have had. Not sure which approach is best on paper but Gordon must have given it some thought. Being adjustable could be very handy for messing around - it would allow for some tube rolling between the more robust JJ-2A3-40 and some of the less able tubes, or even substitute a 45.
Getting fancy could mean sizing the screen capacitor (see attached) to slow down the turn-on current draw of the 2A3 since this cap has to charge up through the screen pot.
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A 2A3 and 12ax7 should sound very good. I would spend a little more on the transformer and get a 20w SE transformer making for a very sweet midrange and highs and solid bottom end.
The 6v6 as a current source is an interesting idea that may add flavor as opposed to a resistor bypassed by a 100uf capacitor
The 6v6 as a current source is an interesting idea that may add flavor as opposed to a resistor bypassed by a 100uf capacitor
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Ho stocktrader, thanks for the encouragement with this craziness. I do have already the OPT so that's fixed. It was a gift from my parents specifically for this project. But I have a lot of confidence in Sowter to provide a good transformer, it certainly looks nice with a good heft. It's rated for 15W.
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Hi Chris, those links seem to have been cleaned up, at least I'm not getting weird stuff, and I had a good read of them. Thanks for posting them.
I'm thinking about this project again, it would be good to see it available for the cold months of winter.
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The photo isn’t quite right, it’s a different shade of red, but equally shocking and repulsive 😀
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Hey Bigun,
Good to see you are still going with this project and not shelved it, most honorable.
Yes it my pair of EL34 Baby Huey Monoblocks. I've set up a bit of a test rig in the workshop with a preamp i found here 4S Universal Preamplifier for 12A*7 Tubes
All running dead silent and all ended up red LOL
Good to see you are still going with this project and not shelved it, most honorable.
Yes it my pair of EL34 Baby Huey Monoblocks. I've set up a bit of a test rig in the workshop with a preamp i found here 4S Universal Preamplifier for 12A*7 Tubes
All running dead silent and all ended up red LOL
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Well, they do look pretty as you have them there!
I have another mono chassis in the works to have a go at a push pull amp, who knows, maybe I’ll end up following in your footsteps.
I have another mono chassis in the works to have a go at a push pull amp, who knows, maybe I’ll end up following in your footsteps.
Thanks, we are in level 4 lockdown in NZ at the moment with Delta running wild in Auckland so I've mixed work (remotely) with audio projects. Covers for the transformers are now ready for staining and clear coat ��
What design are you thinking? I can recommend the BH project, these amps sound amazing.
What design are you thinking? I can recommend the BH project, these amps sound amazing.
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I’m afraid that the PP design is already fixed, a single mono block based on a pair of ‘modern’ SV811’s with an LTP splitter/input. It’ll only be suitable for these particular valves but as I had received a pair as a gift I felt it necessary to make use of them. Well, before I run off at the mouth about those plans I think I better get this 2A3 up and running first, with a planned completion this winter (2022) it’ll be nearly a decade in the making!
We’re somewhat stable-ish with the Delta, pushing to get vaccination levels up asap although we've generally done well in my area.
We’re somewhat stable-ish with the Delta, pushing to get vaccination levels up asap although we've generally done well in my area.
Interesting, I will keep an eye out for that one! What a great gift
Think we all have those projects lol. The hard part is the knowledge you gain along the way has a continual change effect on the projects!
Yes we have been very poor on vaccine rates, Im booked for early September for my first jab.....meantime I'm staying home!
Think we all have those projects lol. The hard part is the knowledge you gain along the way has a continual change effect on the projects!
Yes we have been very poor on vaccine rates, Im booked for early September for my first jab.....meantime I'm staying home!
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Now that I have my Kondo-Ashizawa M7 up and running it’s time to revive this thread and finish building my nice little valve amplifier. When I ordered the valves for the preamp I included a JJ 5751 with the idea of it being an option for the input valve of this amp. I’ve also ordered some E180F pentodes which can be triode wired to give another driver option with more drive capability if the 5751 doesn’t satisfy.
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