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Old 27th June 2011, 07:47 PM   #121
M Gregg is offline M Gregg  United Kingdom
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Post #5 of this thread Tim Mellow OTL link. Half way down.

He has 1M feedback 34K from grid to cap 1uF to wiper of pot.

Wonder if he had the same problem.......Would N1 have an effect under these conditions?


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Old 4th July 2011, 06:10 PM   #122
M Gregg is offline M Gregg  United Kingdom
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Just for interest,

A few ideas of layout.. I'm not slacking.
Still a lot of thought to go into it!


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Old 4th July 2011, 08:46 PM   #123
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Quote:
Originally Posted by M Gregg View Post
Just for interest,

A few ideas of layout.. I'm not slacking.
Still a lot of thought to go into it!


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Old 6th July 2011, 02:04 AM   #124
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I find Alastair's design fascinating and I am wondering whether one could do likewise using say triode connected EL509, 6LQ6 or similar. Thoughts? I know that such tubes are traditionally run pentode mode in Futterman style amps but why could on not use triode mode to simplify things?
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Old 6th July 2011, 09:52 AM   #125
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Quote:
Originally Posted by mashaffer View Post
I find Alastair's design fascinating and I am wondering whether one could do likewise using say triode connected EL509, 6LQ6 or similar. Thoughts? I know that such tubes are traditionally run pentode mode in Futterman style amps but why could on not use triode mode to simplify things?

Well--Yes, I dont see why not....

When I first thought about an OTL years ago, I looked at the 'Sweep-Tube OTL by Allen Kimmel, a Very complicated Circlotron machine....

That one used EL/PL509, and I bought a pile of them in readyness as they were cheap then.
I however, moved over to the 6C33c as it seemed a 'better' choice, and the PL509 remained cold....

Kimmel used a sort of dual drive in his design, Grid 1 driven sorta 'normally' and the Screens driven by MOSFET follower from a 250V supply, from signal on the grid.....

I would think the Autobias idea would work with either layout, std. grid drive or 'super triode-mode', as Kimmel called his idea....
The PL509 I'm thinking would give around half the O/P power per tube in comparison to the 6C33, so you would have to double up on them....

Ive drawn up and tested a more normal input stage with two gain-stages before the splitter, with F.B. into Cathode, I'll put it up later....
--Using a 6SN7 in first/second stages so no need to modify chassis. Only 5 extra parts used.....

Apart from the extra gain, there's no apparent glaring difference in quality...
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Old 6th July 2011, 04:20 PM   #126
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Alastair E View Post

Ive drawn up and tested a more normal input stage with two gain-stages before the splitter, with F.B. into Cathode, I'll put it up later....
--Using a 6SN7 in first/second stages so no need to modify chassis. Only 5 extra parts used.....

Apart from the extra gain, there's no apparent glaring difference in quality...
So no more FB problems?
Sounds good


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Old 6th July 2011, 05:31 PM   #127
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So no more FB problems?
Sounds good


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M. Gregg
No, not at all...

The FB signal goes into the Cathode of first gain-stage, as is 'normal' practice, so the effect of the input or vol-pot has no effect on the FB signal at all...
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Old 6th July 2011, 05:38 PM   #128
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Here you go.....

--Its a complete re-draw, so the part references will be different....
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Old 6th July 2011, 05:58 PM   #129
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by Alastair E View Post
Here you go.....

--Its a complete re-draw, so the part references will be different....
A few questions!

U2 = 12AU7?

I guess this is 6SN7?

R12/13 only has value of 2X100K not total 300K a before?
C5 is now 10uF not 1uF?
Q1 IRF710 not 830?
C4/C3 220n not 470n?

LOL just checking.

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M. Gregg
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Old 6th July 2011, 07:48 PM   #130
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Quote:
Originally Posted by M Gregg View Post
A few questions!

U2 = 12AU7?

I guess this is 6SN7?

R12/13 only has value of 2X100K not total 300K a before?
C5 is now 10uF not 1uF?
Q1 IRF710 not 830?
C4/C3 220n not 470n?

LOL just checking.

Regards
M. Gregg
You're sharp tonight!--Well spotted!

12AU7 was used in the schematic as I havent a 6SN7 'part' in the library, and didnt update its reference....
6SN7 used in real-life though, just check its pinout when you come to wire it up--Not the same as a 12AU7!....

R12/13 are just potential divider, so long as they are equal in this version, no probs, as we are wanting approx half +B for the MOSFET Bias to the first half of 6SN7.
The previous input stage had this voltage at approx 1/3 of +B, to set bias on phase-splitter--That now comes from the second 6SN7 common-cathode stage feeding it.

C5 can be anything around 1-22uF, its not a critical part, just additional smoothing for the MOSFET Bias--You could add a film cap across it too if you like.
--This same MOSFET reference voltage is used for both channels.

Q1, IRF710 has lower Gate-Capacitance than the IRF830, but in the simulations Ive done, it makes very little difference. The breadboard still uses the IRF830 as I havent got the IRF710 yet. Open loop bandwidth is well over 100KHz anyway.
--Works happily with the IRF830, so if you have those, they'll work fine.

C3 and C4, Should be 0.47 or 470N. The 'default' on my Proteus schematic capture is 220N, My error for not correcting...
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