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Old 20th June 2014, 08:42 AM   #21
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Originally Posted by vzaichenko View Post
Yep, you're in the list )

- Right, it would be not so good having less than 60V for the valves (actual anode voltage is even less than that)
- 300V is the maximum value, we are far below
- You're right, filtering caps are not shown for modeling purposes (they increase initial setting-up significantly) - I will add them to the final schematic
So we need 70V+ rails for the front end to supply the valves (more would be better?).

Would it be possible to couple this front-end to an output stage with only one or two pairs of BJT or MOSFET outputs?
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Old 20th June 2014, 09:26 AM   #22
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Quote:
Originally Posted by Ranchu32 View Post
So we need 70V+ rails for the front end to supply the valves (more would be better?).

Would it be possible to couple this front-end to an output stage with only one or two pairs of BJT or MOSFET outputs?
In theory, it is possible to arrange a separate (higher) power supply for the tubes and then arrange a level shift for coupling with the next stage, but it would be too complicated and somewhat pointless - the way I did it is rather simple and effective.

I set relatively low quiescent current for VAS (5.6mA), assuming rather high input impedance of the power section (with 3EF ~ 100 Kohm or so).

Significantly lower impedance will increase the level of distortion. So, in general, 2EF-BJT or MOSFET output stage is possible, but it will require certain increase of the VAS quiescent current to some 9-10mA, increasing heat dissipation requirements for the cascode (Q14, Q15 in post #9). I would use something more powerful there instead of MPSA42/92 in this case - for example KSA1381/3503. More powerful WAS will also help to drive MOSFETs with their high input capacitance.

It is possible to simulate the design you are up to and come up with optimal regime for each stage...
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Old 20th June 2014, 09:54 AM   #23
pergo is offline pergo  Italy
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The problem with tubes in LTP is the startup condition.
What is the output voltage when tubes warm up?
You can easily stuck at rails. You need an output relay with delayed turn-on.
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Old 20th June 2014, 09:58 AM   #24
AJT is offline AJT  Philippines
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the Luxman LV103/105 used the 6cg7 tubes VAS stage, you may want to check it out...
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Old 20th June 2014, 10:08 AM   #25
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Quote:
Originally Posted by pergo View Post
The problem with tubes in LTP is the startup condition.
What is the output voltage when tubes warm up?
You can easily stuck at rails. You need an output relay with delayed turn-on.
That's for sure!
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Old 20th June 2014, 10:10 AM   #26
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Quote:
Originally Posted by AJT View Post
the Luxman LV103/105 used the 6cg7 tubes VAS stage, you may want to check it out...
Thank you - will look at it
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Old 20th June 2014, 11:30 AM   #27
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Originally Posted by vzaichenko View Post
In theory, it is possible to arrange a separate (higher) power supply for the tubes and then arrange a level shift for coupling with the next stage, but it would be too complicated and somewhat pointless - the way I did it is rather simple and effective.

I set relatively low quiescent current for VAS (5.6mA), assuming rather high input impedance of the power section (with 3EF ~ 100 Kohm or so).

Significantly lower impedance will increase the level of distortion. So, in general, 2EF-BJT or MOSFET output stage is possible, but it will require certain increase of the VAS quiescent current to some 9-10mA, increasing heat dissipation requirements for the cascode (Q14, Q15 in post #9). I would use something more powerful there instead of MPSA42/92 in this case - for example KSA1381/3503. More powerful WAS will also help to drive MOSFETs with their high input capacitance.

It is possible to simulate the design you are up to and come up with optimal regime for each stage...
I'm thinking about coupling it to an EF3 output stage with BJT drivers and two pairs of quasi HEXFET outputs. Input impedance to the power section could be very high. Instead of level shifting, it should be possible to add current limiting protections.
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Old 20th June 2014, 11:43 AM   #28
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Quote:
Originally Posted by Ranchu32 View Post
I'm thinking about coupling it to an EF3 output stage with BJT drivers and two pairs of quasi HEXFET outputs. Input impedance to the power section could be very high. Instead of level shifting, it should be possible to add current limiting protections.
In this case it will be fine as is
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Old 20th June 2014, 04:55 PM   #29
Bigun is offline Bigun  Canada
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You can combine a tube LTP with tube VAS if you want to (post 5): Dx hybrid amplifier will be produced soon

Unfortunately, I didn't get around to building it yet.
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Old 20th June 2014, 06:53 PM   #30
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Originally Posted by Bigun View Post
You can combine a tube LTP with tube VAS if you want to (post 5): Dx hybrid amplifier will be produced soon

Unfortunately, I didn't get around to building it yet.
Aaaahhh Interesting... I missed it somehow. Maybe because it started pretty long time ago I like the one in post#52 there. Maybe just needs a CCS at the bottom shoulder... keep it up!
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