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Jon Yaeger
Guest
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 Posted:
Sun Nov 21, 2004 11:07 am Post subject:
2A3 Amplifier Question |
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Has anyone built a 2A3 amplifier using a Mu stage driver (as opposed to a
SRPP Loftin-White?)
Any thoughts about what might be an optimum driver? To direct-couple or
not?
TIA,
Jon
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Patrick Turner
Guest
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| Posted:
Sun Nov 21, 2004 11:38 am Post subject:
Re: 2A3 Amplifier Question |
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Jon Yaeger wrote:
| Quote: | Has anyone built a 2A3 amplifier using a Mu stage driver (as opposed to a
SRPP Loftin-White?)
Any thoughts about what might be an optimum driver? To direct-couple or
not?
|
Direct coupling of some type would only give you a benefit of
class AB2, or A2 in the case of SE.
What is wrong with a 6SN7 with B+ = 400v, and
normal RC coupling?
The mu follower will have 2H, and so there is less to cancel the thd of the
2A3,
so the overall 2H will be higher for a mu followeer driver than a normal R
loaded driover.
What about the 6h30pi tube?
See http://www.triodeel.com/6h30.html
It looks like a great driver tube.
Patrick Turner.
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Tom Schlangen
Guest
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| Posted:
Sun Nov 21, 2004 4:35 pm Post subject:
Re: 2A3 Amplifier Question |
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Hi Jon,
| Quote: | Has anyone built a 2A3 amplifier using a Mu stage driver
(as opposed to a SRPP Loftin-White?)
|
Didn't find one browsing through my archive, but lots of
SRPP and mu-follower circuits to drive the 2A3.
There is an Italian design from Mr. Binari available on the
web, whose mu-follower 6SN7 driver looks straightforward.
But considering the gain, this only will allow for no gNFB.
| Quote: | Any thoughts about what might be an optimum driver?
|
As usual, the "optimum" largely depends on personal
preferences.
I for myself wouldn't consider a mu-stage when a "conventional"
design would do nicely. Like for example paralleled halfs of
6SL7, or something.
Tom
--
When in doubt, use brute force.
- Ken Thompson |
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Jon Yaeger
Guest
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| Posted:
Sun Nov 21, 2004 8:05 pm Post subject:
Re: 2A3 Amplifier Question |
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in article 41A037E5.9AC47F39@turneraudio.com.au, Patrick Turner at
info@turneraudio.com.au wrote on 11/21/04 1:38 AM:
| Quote: |
Jon Yaeger wrote:
Has anyone built a 2A3 amplifier using a Mu stage driver (as opposed to a
SRPP Loftin-White?)
Any thoughts about what might be an optimum driver? To direct-couple or
not?
Direct coupling of some type would only give you a benefit of
class AB2, or A2 in the case of SE.
What is wrong with a 6SN7 with B+ = 400v, and
normal RC coupling?
The mu follower will have 2H, and so there is less to cancel the thd of the
2A3,
so the overall 2H will be higher for a mu followeer driver than a normal R
loaded driover.
What about the 6h30pi tube?
See http://www.triodeel.com/6h30.html
It looks like a great driver tube.
Patrick Turner.
TIA,
Jon
|
Patrick,
Thanks for your reply.
Is the apparent "advantage" or eliminating the coupling capacitor outweighed
by disadvantages?
Could you kindly explain a bit more about what you mean by "Direct coupling
of some type would only give you a benefit of class AB2, or A2 in the case
of SE."
TIA
Jon |
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Patrick Turner
Guest
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| Posted:
Mon Nov 22, 2004 4:34 am Post subject:
Re: 2A3 Amplifier Question |
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Jon Yaeger wrote:
| Quote: | in article 41A037E5.9AC47F39@turneraudio.com.au, Patrick Turner at
info@turneraudio.com.au wrote on 11/21/04 1:38 AM:
Jon Yaeger wrote:
Has anyone built a 2A3 amplifier using a Mu stage driver (as opposed to a
SRPP Loftin-White?)
Any thoughts about what might be an optimum driver? To direct-couple or
not?
Direct coupling of some type would only give you a benefit of
class AB2, or A2 in the case of SE.
What is wrong with a 6SN7 with B+ = 400v, and
normal RC coupling?
The mu follower will have 2H, and so there is less to cancel the thd of the
2A3,
so the overall 2H will be higher for a mu followeer driver than a normal R
loaded driover.
What about the 6h30pi tube?
See http://www.triodeel.com/6h30.html
It looks like a great driver tube.
Patrick Turner.
TIA,
Jon
Patrick,
Thanks for your reply.
Is the apparent "advantage" or eliminating the coupling capacitor outweighed
by disadvantages?
|
For class A1 SE, or A1 or AB1 for PP, cap coupling is fine,
and there is no need to be masochistic and build a Loftin White to
be able to direct couple and avoid a cap.
Caps used to be horribly unreliable, so the Loftin White
attempted to address this issue in the 1930s, and avoid using coupling
transformers, which were also pretty awful things then, and darn expensive.
There were no Black Gates or Auricaps available in 1935.
Some folks made their own resistors.
| Quote: |
Could you kindly explain a bit more about what you mean by "Direct coupling
of some type would only give you a benefit of class AB2, or A2 in the case
of SE."
|
The direct coupling allows some grid current drive without the cap charging up
and
displacing the bias point.
A CF can be direct coupled to the output grids to enable grid current drive.
Class A2 means class A, and the 2 indicates grid current.
Class A1 indicates no grid current, just class A.
You cannot have class A2 PP, since in PP class A1 circuits driven into grid
current
one tube will cut off when the other starts to generate grid current.
A pair of 6L6 in a Williamson in triode make about 16 watts.
If you included a cathode follower stage between the balanced driver gain amp
and the output tubes, you could bost the triode power output to 30 watts, class
AB2.
The CF would be direct coupled to the 6L6 grids, and the bias for both CF and
output
tubes is applied at the CF grids via normal 470k from a bias supply.
The CF grids are then driven by caps from the anodes of the balanced amp.
No extra CR couplings are thus introduced to the amp.
You can still get the same blameless first 16 watts at 0.1% thd, but you then
have a reserve for a lot more,
although the grid current and class AB action will increase the thd to say 0.3%.
Patrick Turner.
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Fabio Berutti
Guest
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| Posted:
Mon Nov 22, 2004 2:05 pm Post subject:
Re: 2A3 Amplifier Question |
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My own 2A3 SE (fixed bias, operating point as per RCA data sheets at -43V
grid, 250V anode, 60 mA, anode load 3K instead of 2K5) uses as a driver a
choke-loaded high-gain triode EC86. It is as simple as it can be, and IMHO
it sounds great. The coupling cap is a Jensen paper-in-oil (0.47+0.10uF).
It could be improved using capacitors to shunt the choke's stray inductance
as per the classic GEC scheme (it was developed for PP interstage
transformers, but a SE is just one half of a symmetric PP).
This type of driver only needs 200V B+ while providing an enormous voltage
swing, therefore linearity is very good because only a small part of the
maximum output is used.
Unfortunately, due to the highly non-ideal behaviour of the chokes I used,
flat frequency response is limited to 15K.
(Obviously I'm partial: do not take my words as pure truth when I say MY amp
is good...)
BTW the EC86 can be replaced with 6S45, 417, TFK E8020CC... it is only
cheaper and easier to find.
Ciao
Fabio
"Tom Schlangen" <t.schlangen@ndh.net> ha scritto nel messaggio
news:q5b672-ml3.ln1@julabi.snakes.netcologne.de...
| Quote: | Hi Jon,
Has anyone built a 2A3 amplifier using a Mu stage driver
(as opposed to a SRPP Loftin-White?)
Didn't find one browsing through my archive, but lots of
SRPP and mu-follower circuits to drive the 2A3.
There is an Italian design from Mr. Binari available on the
web, whose mu-follower 6SN7 driver looks straightforward.
But considering the gain, this only will allow for no gNFB.
Any thoughts about what might be an optimum driver?
As usual, the "optimum" largely depends on personal
preferences.
I for myself wouldn't consider a mu-stage when a "conventional"
design would do nicely. Like for example paralleled halfs of
6SL7, or something.
Tom
--
When in doubt, use brute force.
- Ken Thompson |
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