Hello All,
after completing my gu46 amp I am about to embark on a new (and life threatening) adventure. My boss asked for a nice, medium powered (<30W) amplifier with a consistent "aesthetic" factor to it.
Well, I came up with a gu48 (833A) amplifier.
I have everything I need except the opts.
These are the specs:
gu48 @ 1500v and 100ma + bias current / 5k load
driver: 6sn7 input feeding 307A in triode mode CCS loaded with a mosfet follower (fet to be determined in due course)
The power transformer is a universal 1000VA unit I designed some time ago which features multiple secondaries and generous power ratings on all rails.
I have a big issue with OPTS. I used the 1628SEA units from Hammond loading the gu46 tubes of my other amps @ 1500v and 100mA idle current. The result is good in terms of sound quality and power output but with the gu48 I would like to investigate other options.
So.... are there alternatives to the hammond 1628SEA opts with greater idle current capability and in the same price range?
Any and all suggestions concerning the project are welcome of course.
Cheers,
after completing my gu46 amp I am about to embark on a new (and life threatening) adventure. My boss asked for a nice, medium powered (<30W) amplifier with a consistent "aesthetic" factor to it.
Well, I came up with a gu48 (833A) amplifier.
I have everything I need except the opts.
These are the specs:
gu48 @ 1500v and 100ma + bias current / 5k load
driver: 6sn7 input feeding 307A in triode mode CCS loaded with a mosfet follower (fet to be determined in due course)
The power transformer is a universal 1000VA unit I designed some time ago which features multiple secondaries and generous power ratings on all rails.
I have a big issue with OPTS. I used the 1628SEA units from Hammond loading the gu46 tubes of my other amps @ 1500v and 100mA idle current. The result is good in terms of sound quality and power output but with the gu48 I would like to investigate other options.
So.... are there alternatives to the hammond 1628SEA opts with greater idle current capability and in the same price range?
Any and all suggestions concerning the project are welcome of course.
Cheers,
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Why three stage? Why not two stage direct coupled, using a gyrator/mu-follower load on the driver tube to push the output tube into A2?
You would need to drop the voltage and change the OPT of course, and you may want to tailor the biasing scheme to your own tastes. Try it on LTSpice and see how it looks to you.
You would need to drop the voltage and change the OPT of course, and you may want to tailor the biasing scheme to your own tastes. Try it on LTSpice and see how it looks to you.
Attachments
That's interesting. I was thinking D3A direct coupled to 4E27 for myself. What kind of shunt reg that could easily do 160mA did you use?
@Magz, I've read all about the Midlife Crisis! Great project.
The requirement in my case are, however, a bit different and are unfortunately limited buy the iron I can source (i.e. low voltage low current SE opts).
In addition I will supply (cost containment) the gu48 with switching supplies hence I cannot elevate the filament above ground potential.
A2 operation would in any case be achieved through a mosfet follower such as a CCS loaded irf device, ixys or supertex.
Also the second big requirement is that the amplifier must sound mellow (personal taste) which I think can be achieved by manipulating the OP of both the 6sn7 and the 307A.
Your schematic is beautiful and I suspect very good soundwise but I am pretty much set on keeping things simple, cheap as can be (obviously not that cheap given the use of gu48).
I am wondering if the GU48 can effectively work with "only" 100mA of bias current.
Thanks!
The requirement in my case are, however, a bit different and are unfortunately limited buy the iron I can source (i.e. low voltage low current SE opts).
In addition I will supply (cost containment) the gu48 with switching supplies hence I cannot elevate the filament above ground potential.
A2 operation would in any case be achieved through a mosfet follower such as a CCS loaded irf device, ixys or supertex.
Also the second big requirement is that the amplifier must sound mellow (personal taste) which I think can be achieved by manipulating the OP of both the 6sn7 and the 307A.
Your schematic is beautiful and I suspect very good soundwise but I am pretty much set on keeping things simple, cheap as can be (obviously not that cheap given the use of gu48).
I am wondering if the GU48 can effectively work with "only" 100mA of bias current.
Thanks!
I used a K&K Audio HV adjustable shunt regulator kit. I replaced the stock pass MOSFET with a beefy TO-247 IRFP450 which I have bolted to the chassis side with a heatsink on the outside of the chassis. It dissipates 40W at idle - so far it's held up fine for more than a year now.
Kevin of K&K (great guy to work with) also supplied me with a better quality voltage reference for the regulator; I found that using the stock reference circuit the bias fluctuated up and down quite a bit, but with the upgraded reference it's rock steady where I set it. On startup the output current is ~130-135mA, and after warming up for 15 minutes or so it gets to 160mA. The regulator adjustment is accessible from the top of the unit so it's a simple matter to dial in the proper current using the front panel meter. Since it's a constant voltage source there's no need to bypass it with a cap. The sound is dynamic, clear and immediate.
The regulator on the other end (V7) isn't really a regulator - I ended up using a simple cascoded IXYS 10M45S CCS feeding a resistor to ground to set the bias voltage for the MOSFET. Works great.
Gotcha. I knew you'd need to modify things but perhaps the idea of the driver tube with a mu-follower/gyrator load can be applicable. You can plug a lot of different drivers in there, s.a. D3a or other higher gain tubes if you need more gain. One thing's for sure, though, the sound of my amp is not "mellow", it's very clean and dynamic, with just enough SET warmth for my taste - YMMV.
BTW, I have to give credit to Michael Koster and his Meteor design (posted here a few years ago) for inspiration. I just took it to another power level and used very different power supplies.
The idea is excellent and I also like clear transparent amps (my gu46, gm100 and gu81m amps are that way for a very good reason) but the "client" wants warm, tubey and mellow so I figure I have to deliver the goods as ordered. 🙂
I am just hesitant because of the bias current on the 833A which, at 100mA/1500v seems lowish..
I am just hesitant because of the bias current on the 833A which, at 100mA/1500v seems lowish..
Hello all
Which is best MOSFET driver for RCA 833A.
anyone provide me the suitable mosfet driver to drive rca 833a.
Which is best MOSFET driver for RCA 833A.
anyone provide me the suitable mosfet driver to drive rca 833a.
Hello
do you have the schematics about gu81 , gm70, gm100, gu81 tubeampas?
im locking for some ideas,, because i would try such a projekt
best regards brudertuck
Maybe usefull: I think that there is something wrong with the curves for the 307A in triode mode on the "Bartola Valves" site. See:
https://www.diyaudio.com/community/threads/dht-headphone-amp.370331/#post-6987112
I commented on this but the discussion stalled 9 days ago. See:
http://www.bartola.co.uk/valves/2013/03/16/307a-dht-in-triode-and-schade-feedback/
These are curves for the 307A in triode mode that I once found on the internet (they probably come from the site of Pete Millett but Pete writes he also once found them on the internet). These curves better match the Western Electric datasheet for the 307A (see attachment).
https://www.diyaudio.com/community/threads/dht-headphone-amp.370331/#post-6987112
I commented on this but the discussion stalled 9 days ago. See:
http://www.bartola.co.uk/valves/2013/03/16/307a-dht-in-triode-and-schade-feedback/
These are curves for the 307A in triode mode that I once found on the internet (they probably come from the site of Pete Millett but Pete writes he also once found them on the internet). These curves better match the Western Electric datasheet for the 307A (see attachment).