Benefit connecting 6L6 in triode mode
Georges Vasileiadis
I have a pair of Fisher monoblocks with EL37s. Will be any sonic
benefit if I will connect them in triode mode ( grid 2 to plate)
instead of the std pentode mode ?
By the why should I put a 100Ohm resistor in series for the connection ?
thank you
george
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*--------------------------------------*
| George Vasileiadis |
| SLAC, Stanford Univ. |
| email : vasi...@slac.stanford.edu | |______________________________________|
| Tel : 1-650-926-4463 |
*--------------------------------------*
Tim
> benefit if I will connect them in triode mode ( grid 2 to plate)
> instead of the std pentode mode ?
Dunno anything about the amp, but usually triode mode has lower distortion
and power. In my 50C5 SE amp, I prefer it. I think it also sounds better
at higher volume (even though power output is lower).
> By the why should I put a 100Ohm resistor in series for the connection ?
All the data sheets I've seen say direct connection, but I've always heard
to use a resistor. Go figure.
Tim
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The 100 OHM resistors are stopper resistors to discourage oscillation, or so
I have been told. It will not hurt to use them.
Porter B
BT
Tim <tmor...@charter.net> wrote in message
news:tn11kb4...@corp.supernews.com...
Jonathan R. Knight
transformers in the event of a short. It's a good idea to include them on UL
amps, which also have a low-impedance DC path through the output transformer
to the power supply. I've replaced them in a good number of amps that have
had this problem, e.g. Citation II, Marantz 9's, etc.
Jonathan Knight
Jonathan R. Knight
that when you connect the power tubes as triodes, you will remove the isolation
of the Miller capacitance (grid-to-plate) which the screen grid provides. The
capacitive load seen by the driver stage will be about two orders of magnitude
larger as a result. This means that, short of driving the grids with cathode
followers (which the Fishers don't), the bandwidth of the driver stage will
be significantly reduced. Again, it depends on the design, but in some cases,
this can mean drastically reduced phase margin. In an extreme case, this can
mean a tendency to oscillate. In a less extreme case, it will just mean
transient ringing and edgy, tiring sound.
Still, it's so easy to make a temporary change, it doesn't hurt to try. If
you have access to a square wave generator and an oscilloscope, you can
look at square wave performance before and after the change. If you see a
large increase in ringing on a 1kHz square wave, then you know you need to
go back and make changes to the driver stage. Otherwise you're not making
a valid comparison of the two modes of operation.
Jonathan Knight
Fred Nachbaur
I think it has to do more with "just being sure" than any
other consideration. Much like grid-stoppers at control
grids; they're not always needed, or even desirable, but
people like to put them in just in case something goes wrong.
Putting a grid-stopper in a screen grid seems to me
counter-productive, since there is a varying screen current
drawn which translates to a varying screen voltage (relative
to plate)... if this is too great, there goes true triode
mode. Whereas on control grids, generally the only thing
that happens with too large a grid stopper (unless you're
running class AB2 or B2) is that high frequencies can be
undesirably attenuated.
Fred N
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Wiz
I'm guessing that you have SA-300 amps. These are very good amps... far better
than many classic amps that are highly sought-after. They are likely to become
valuable in the future, when their great performance becomes known. Seriously
consider keeping any modifications reversible.
I'd suggest that you replace every coupling capacitor in there. Most will be
leaking by now and Fisher didn't use the best ones originally :-)
As for triode mode, I never connected mine in triode (wish I still had them; I
would :-) but I've done ti to lots of others and always found a great
improvement.
I definitely would use 100R between the screen grid and the plate to control
oscillations... otherwise, you might exceed screen max. dissipation.
Also, you say 6L6, but mention EL34s. I expect that the amp will sound better
with EL34s if you have good ones available.
The SA300 is one of the quietest amps I've ever used. You may be able to lift
the FB loop as well. I have found that most Fisher amps will work without FB
and require no other mod, except maybe a choke in the PSU. Yours may get by
without the choke unless you have very efficient speakers. When I lifted the FB
loop on my current Fisher X100-B, I had to add a choke (96dB speakers) to stop
the hum. 6 Hy was sufficient.
I think you will be pleased with quasi-triode operation as long as you have
sufficient power for your speakers.
Cheers/Carron
"LABOR SVGIT"
Patrick Turner
If your amps are now using an 8 ohm output setting, and there is a 4
ohm setting available, you could switch to triode (with the 100 ohm resistors)
and connect your 8 ohm speakers to the 4 ohm outlet,
the resulting loading for the triodes will be double the present
loading and be "more better" for them.
Power output will halve, but if all you do is use a couple of watts, there
may be some benefit sonically.
Patrick Turner.
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"Patrick Turner" <in...@turneraudio.com.au> wrote in message
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Steve O'Neill
Wiz wrote in message <20010808072219...@ng-fu1.aol.com>...
>George,
>
>I'm guessing that you have SA-300 amps. These are very good amps... far
better
>than many classic amps that are highly sought-after. They are likely to
become
>valuable in the future, when their great performance becomes known.
Seriously
>consider keeping any modifications reversible.
You've probably guessed wrong: Fisher made three monoblocs (boy do I dislike
that word) that used PP EL37s. There was the 70AZ, the 80AZ and the 100.
It seems that they employed the same basic circuit with cosmetic and circuit
feature variations. All employed split loading using a conventional plate
winding and a less conventional cathode winding. As a result, the plate
load is quite low. The output transformer was probably optimized for
pentode operation. Assuming this to be the case, I would be very wary of
converting to triode operation. In my mind this would be akin to converting
a pentode unity coupled Mac amp to triode. BTW the EL37 is a true pentode
and not a beam tetrode like the 6L6 although in these amps they're
interchangeable.
Steve
Jonathan R. Knight
> HI:
>
> Wiz wrote in message <20010808072219...@ng-fu1.aol.com>...
> >George,
> >
> >I'm guessing that you have SA-300 amps. These are very good amps... far
> better
> >than many classic amps that are highly sought-after. They are likely to
> become
> >valuable in the future, when their great performance becomes known.
> Seriously
> >consider keeping any modifications reversible.
>
> You've probably guessed wrong: Fisher made three monoblocs (boy do I dislike
> that word) that used PP EL37s. There was the 70AZ, the 80AZ and the 100.
I'd have to think you're right about this. They're funky-looking amps, by the
way.
> It seems that they employed the same basic circuit with cosmetic and circuit
> feature variations. All employed split loading using a conventional plate
> winding and a less conventional cathode winding. As a result, the plate
> load is quite low. The output transformer was probably optimized for
> pentode operation. Assuming this to be the case, I would be very wary of
> converting to triode operation. In my mind this would be akin to converting
> a pentode unity coupled Mac amp to triode.
I have plans of doing exactly this to my MC-30's one day, in hopes that they'll
sound a little more like my MI-200's (which blow the circuit breaker in my
house, overheat the living room in the summer and use rare, expensive output
tubes). I do expect it to be a big project, requiring careful selection of
tube type and modification to the drive circuitry. But I think it could be
promising if done right.
> BTW the EL37 is a true pentode
> and not a beam tetrode like the 6L6 although in these amps they're
> interchangeable.
>
> Steve
Jonathan