Download Parallel Gain Stages..

From:
Date:
Subject:
Mark Lavelle ([email protected])
11/24/2001 11:41 PM
parallel gain stages?
As I mentioned in the nearby Spitfire thread, "IMO, the really unusual thing about the Spitfire
design is that V1 uses its two triodes in parallel instead of in series."
I know that the series arrangement gives you more gain 'cause you get to multiply it twice, but
what's the purpose of putting the triodes in parallel? More current? Different impedance (if so,
why?)? More mojo? Help!
From:
Date:
Subject:
kg
11/25/2001 3:59 AM
Re: parallel gain stages?
mark,
paralleling tubes will:
double the gm (transconductance) the measure of how much the plate current changes for a
given change of grid voltage.
halve the rp (dynamic plate impedance), or the measure of how "stiff" the plate is, how well it
can handle lower impedance loads without falling apart. lower noise. less loss of high
frequencies due to stray capacitance.
keep mu the same. this is to be expected, since mu=gm*rp. you have made a composite tube
from two sections.
your electrode capacitances will also add to one another... two plates and two grids means
twice the amount of Cgp.
personally, i like low impedance stages. instead of two parallel 12ax or 12at sections, use one
tube with slightly more authority... look for high gm tubes. gm is your friend --it was known as a
figure of merit for triodes, and for good reason.
for values of other components, halve all the values of Rk, Rp, maxRg, and double the value of
capacitances like cathode bypass c and coupling c.
ken
From:
Date:
Subject:
kyle
11/25/2001 3:59 PM
Re: parallel gain stages?
KG, could you go into more detail about the effects of this? This may be hard to describe, but
what is the "sound" of double the gm? What about how it sounds with 1/2 Rp?
From:
Date:
Subject:
kg
11/25/2001 9:59 PM
Re: parallel gain stages?
personally i find tubes with more than usual values for gm warm, punchy, very touch
responsive. the lower rp usually comes through in a greatly extended high frequency
response... and as a result can somtimes make oscillations break out.
ken
From:
Date:
Subject:
Dave James
11/27/2001 4:27 PM
Re: parallel gain stages?
Hi Mark,
In the op -amp world, two op-amps paralleled can result in a lower noise gain stage. In theory,
both op-amps would be generating random noise (hiss). Since the IC processing is tightly
controlled, the noise characteristic of the two devices is very similar. So, when one device has
a positive noise spike and the other has a negative noise spike, they cancel out...in theory, ok?
In theory. Since "noise" is random, the reduction is not a full factor. So for 2 op-amps in
parallel, a noise reduction of 1.2 or 1.3 or so could be possible.
Did that make sense?
Anyway, I don't know if the designers of tube preamps were thinking about this, or if they were
considering all of KG's info. BUT, it seems to me that there might be some noise-reductionbenifit to paralleling two tube sections.
Later,
DJ
P.S. Hey - might be time for another lunch?
From:
Date:
Subject:
kg
11/27/2001 4:42 PM
Re: parallel gain stages?
dave, you're spot on regarding the noise issue. i should have mentioned that.
ken
From:
Date:
Subject:
Mark Lavelle ([email protected])
11/27/2001 5:13 PM
Re: parallel gain stages?
Thanks to both of you.
Is it generally true that increasing gm decreases noise?
From:
Date:
Subject:
kg
11/27/2001 7:58 PM
Re: parallel gain stages?
yes, that is also true. other circiut parameters being equal, the higher gm tube will PRODUCE
less noise internally. noise is also minimized with a well-bypassed cathode resistor.
though it's not ALWAYS the case, higher gm tubes GENERALLY have lower plate
impedances, which also helps from a noise standpoint .
ken
From:
Date:
Subject:
Mark Lavelle ([email protected])
12/1/2001 1:15 AM
Yo! Dave! Check your email!
P.S. Hey - might be time for another lunch?
Yeah, let's do that lunch thang.
I sent you email, but it seems to have fallen into the bit bucket...
From:
Date:
Subject:
pumpstein ([email protected])
11/27/2001 10:48 PM
Re: parallel gain stages?
Kevin O'Connor's TUT1 covers parallel stages, and iirc, (and I probably don't), the main
attributes, compared to series stages, were:
1) better signal-to-noise ratio
2) less gain
No expert, I - just relaying some info that may help point you in a useful direction.
-P
From:
Shea ([email protected])
Date:
11/29/2001 4:46 PM
Subject:
High gm tubes
KgWhat kinds of tubes with higher gm than the 12AX7 are cool for using in the preamp stage?
6SN7?
I dug up info on the 7788, which sounds interesting. It had a mu in the 50s or 60s, and the gm
was something like 50,000. It's a pentode. There is very little info about these on the 'net. The
spec sheet I was reading (Amperex catalog) didn't list either a plate impedance or a load
impedance, so I guess you'd have to do the math on the mu and the gm to get the ra. Antique
Electronic Supply sells them for around $ 36 each.
Shea
From:
Date:
Subject:
Carl Z ([email protected])
12/1/2001 4:06 AM
Re: High gm tubes
Shea;
If you want to fiddle with an interesting tube check out a 12BZ7. It's a dual triode with each
section looking like a 12AX7 in parallel, mu is still 100 though. I've shagged these into phase
inverter positions in Fenders occasionally and they just kill. Definetly worth looking at as having
some potential
Carl Z
From:
Date:
Subject:
Adam Alpern ([email protected])
12/1/2001 4:58 AM
Re: High gm tubes
Hey, I noticed the 12BZ7 myself just this morning!
I wonder why it hasn't become more popular in place of the 12AX7? At the same plate voltage
it's got twice the transconductance and half the plate resistance of the 12AX7, at the same
mu...certainly looks like a cool tube to play with to me.
-Adam A.
From:
Date:
Subject:
Shea ([email protected])
12/1/2001 3:31 PM
Re: High gm tubes
Great suggestion! Thanks!
From:
Date:
Subject:
MBSetzer ([email protected])
12/1/2001 10:53 PM
Re: High gm tubes
The 12BZ7 & 12BH7 are taller tubes with the same pinout and have higher heater current as
well. I think they were worth listening to.
At one early point I had my SEKT88 with CF drive to the power tube using a 12AX7 that was in
the original layout, then paralleled it with the unused triode and it seemed a little better. Put in a
12AU7 and it was different but seemed like it had potential so the cathode resistors and B+
dropping resistor were more optimized for the 12AU7 and it helped a lot. These lower
impedance triodes are made to handle more current than a 12AX7.
That seemed like pretty low impedance especially in parallel, but the 12BH7 has slightly lower
impedance, and I tried that and it was quite good too. The 12BZ7 has higher gain without much
more impedance, there's not many choices in the 12B*7 catagory. It actually didn't sound as
good tonewise as the 12BH7, maybe there's no benefit to a higher gain tube in a cathode
follower service. In the V1 position the 12BZ7 doesn't give you more gain than a 12AX7 either
(I don't think any traditional dual triode will) but it does give about as much, doesn't bias quite
right but it is interesting to try, I had a couple nice NOS 12BZ7 in with the miscellaneous tubes
but they were both microphonic, one was OK if I didn't play too loud and I left it in for a couple
days. Same thing with the 12BH7 as CF, it was just fine but when I went back to the 12AU7 it
was hard to tell the difference. I tried a few more 12AU7's (love those Hewlett-Packards) and
there was as much difference between them and the 12BH7. After all this I was getting close to
concluding that the performance of the dual triodes was about the highest with the smaller
bottle 12AX7 & 12AU7 tubes at the impedance extremes, and the various well-known choices
in between. Maybe I have to figure that with some amps having more power tubes the 12BH7
might sound different than a 12AU7. But if you want to drive even more power tubes into a little
grid current then you would probably go like KG on to a single triode (or pentode wired that
way) which was lower impedance yet than the parallel dual ones. These tubes were made for a
reason, the 12BH7's are in lots of hifi's but I couldn't even find the 12BZ7 on the data roll for
1962 in my Hickock tube tester. It was in the supplement publication as already obsolete by
1962, there was probably nothing it was intended to do that couldn't then be done better by the
other tubes. I was running almost a gain of 1 with the triodes in parallel so I really did want the
voltage to follow the preamp, it was set to slam the KT88 pretty good with about 69 to 75 volts
of peak-to-peak signal. My bias was hot but conservative at approximately -25Volts so anything
over about 50V peak was overdriving the power tube.
This seemed like a nominal refererence set-up with a real clean signal coming in to the CF
about 75Vp -p, and almost the same non-inverted signal coming from its cathode to the power
tube's grid. You will need a really big coupling cap since the impedance is so low, otherwise
you will lose all the lows, not just the bass :(
I found some 1mf/400V and 3.3mf/350V film caps that were smaller than a matchbox and they
worked well. You could probably use the small 2.2mf electrolytic filter caps but I didn't want to
go there. You shouldn't need more than 250V, depends on your Rk values, mine were all over
the ballpark at one time or another, but you don't want to exceed Vhk of 200V anyway.
Anyway I put in all the tubes, run the amp and check the preamp and CF plate voltage and their
B+ supply. Shut down, take out the KT88 then variac up to the point where the preamp & CF
have about the same voltages as before. The heaters are a little low but no big deal. In comes
the test signal, adjust the knobs so that you know the CF is delivering as clean a drive to the
power tube as you like. You measure it by putting the scope right at the power tube grid pin
when the power tube is not there. I wanted to definitely be sure I was not getting any preamp
distortion but had more than enough amplitude to go beyond the linear range of the power
tube's input into some grid current.
With this rigged, then shut down, put in the KT88 and give it full wall voltage. Stand by with the
fire extinguisher ;)
Your preamp should be giving about the same amplitude to the power tube as before, but when
you try to measure it now, anything into grid current will not be seen due to the rectification
effect of the power tube. No biggie, you know its clean going in.
With more than adequate speakers to make sure there is not enough stress on them to cause
significant speaker distortion, anything unclean witnessed by your ears will be genuine power
tube disortion and no other.
It was clean going in to the power tube but you will probably have lots of gain left on some of
the knobs and the power amp distortion will grow to a point and preamp distortion start showing
up too as you crank it. Returning to reference settings, and changing CF tubes requires no
difference in the knobs to achieve a similar drive voltage to the power tube. The differences in
gain between the different types of dual triodes are not expressed in the CF configuration, so
you can compare apples to apples without having to change any knobs away from the
reference setting when you change tubes.
Hope this helps, it's from memory but the amp played good during this configuration,
Mike