Re: Ping Prof Turner: Amplifier questions
- From: Patrick Turner <info@xxxxxxxxxxxxxxxxxx>
- Date: Fri, 07 Oct 2005 15:56:44 GMT
flipper wrote:
> On Fri, 07 Oct 2005 11:16:31 GMT, Patrick Turner
> <info@xxxxxxxxxxxxxxxxxx> wrote:
>
> >
> >
> >flipper wrote:
> >
> >> Now that I've got power back I've been able to return to more fun
> >> things like the amplifier project and I've run across some things I'd
> >> like your learned opinion on.
> >>
> >> To recap a bit, the design started out as a simple 12SL7 gain stage
> >> into a conventional self bias 6V6 SE pentode configuration. Then the
> >> idea to make it triode switchable cropped up (necessitating a CF
> >> buffer tube to drive the 6V6) and, currently, I've been thinking to
> >> just do the triode mode.
> >>
> >> Btw, I'm thinking of a zener in the CF plate circuit to keep it under
> >> 300V because it's simply cheaper than a C filtered R dropper and I
> >> wouldn't think B+ ripple should be a big problem on the plate of a CF
> >> driver. Am I wrong?
> >
> >Zeners have their uses, but I try to not use them, mainly
> >because they have a wide noise bandwidth, and so bypassing them with 100uF
> >
> >usually makes sense to control the noise, then all the zener does is
> >control the DC,
> >when the cap has a high impedance.
>
> Ok. Point taken.
>
> >
> >When i do use zeners its usually a string of 5 watt x 75v types,
> >say 4 to shunt reg a voltage of about +300V.
> >You need to have a 350V rated 100 uF cap for the 300V supply point and if
> >you have
> >10mA of current to a tube/s from this point you should have 5mA flow in
> >the zeners,
> >so that they work ok and at a safe temperature without having to
> >use heavy heat sinking.
>
> I understand what you're saying there about a shunt reg.
>
> However, I'm just putting a 51 volt zener in series from the plate to
> B+ so there's a 51 volt drop across it.
Then any movement of the B+ is also delivered to the anode of the CF, plus some
noise.
But not much noise is transfered to the output od the CF because the
Rin to the anode circuit of a CF is very high, and the output voltage is
deterimned by the
grid voltage.
But best practice is to establish a B+ for the CF to be referenced to 0V, not
the main B+,
using shunt regulation, a string of zeners to get 300V, with a resistor down
from te main B+,
and with a 100uF cap to lower noise, and prevent transient voltages from
destroying the zeners; they are only SS devices, and are prone easy failure.
You want to use the worst type of practice.....
I may seem to be splitting hairs, but I'm just telling you my preference.
>
>
> <space snip>
>
> >But if you are to drive an output tube set up for fixed bias from a CF
> >which is direct coupled to the output grid, and also has say 50k taken to
> >a
> >-ve supply, then the CF *must* also have a fixed bias supply to its grid
> >somewhat
> >lower voltage than the actual output tube bias voltage.
> >
> >So you need a -200V supply, and a divider network, then a 470k to the
> >CFR grid, and then you can cap couple the CF grid to any other drive tube
> >you like.
> >
> >Divider values have to be got just right for the correct bias current in
> >the output tube.
>
> Yeah. But I don't follow your description and the -ve supply.
>
> Let me give it another shot.
>
> Working backward
>
> 6V6 plate to OPT. OPT other side to B+ 330V
> screen to plate thru 180Ohm
> cathode to ground
> Grid to top of 12SL7 Rk connecting to 12SL7 cathode
>
> Rk is actually a resistor divider chain
> Bottom of Rk to -50V
> top of Rk to 12SL7 cathode and 6V6 grid
> Middle of Rk, adjustable pot to 12SL7 grid through 680k
>
> 12SL7 cathode to Rk (as just described) and 6V6 grid
> grid to 680k to Rk middle pot which sets grid to -14.7
> Plate to B+ 330V through 51V dropping zener.
>
> 12SL7 grid cap coupled to gain stage.
>
> As the bias goes, the Rk divider sets 12SL7 grid to -14.7V with about
> a 2.6V drop across the tube to cathode Rk which sets the 6V6 grid to
> -12.1 for an idle current of 34ma.
>
> Move the 12SL7 grid up/down with the Rk pot and the 6V6 bias follows
> and idle current is adjusted.
OK, I see what you are doing.
Bu adjusting the cathode bias on the CF, ie, the grid to cathode voltage on the
SL7, the current in the SL7 is varied so the cathode voltage changes so
the output tube is biased for the wanted current by means of adjusting the pot
you have.
Ingenious.
But I would much prefer to keep the pot out of the
main dc current flow of the SL7.
I'd have a separate divider with a pot in the middle between -50V and 0V
and then have a low dc current flow in the pot and then bias the SL7 from
between
0V and -50V.
Rk for the SL7 is say a single 47k or whatever gives about the right current;
I would have a -150V supply so Rk could be as high a value as possible
to provide the CF with a high ohmic load to minimise the work the CF has to do.
A CCS between -50V and the SL7 cathode would be very good.
With 1/2 an SL7, all you'd need is about 1.5mA at idle.
For most of the easy class A action of the SL7 the load in its cathode should
be
the same as what you'd use if you had the load in the anode circuit,
so having only about 35V across a resistor in a cathode circuit isn't doing it
right.
The typical class A load for SL7 is about 150k with say 1mA,
so the drop on the load would be 150V, hence the idea that you should have a
-150V supply.
>
>
>
> >> I may be overly obsessing about fixed bias but as I evaluate
> >> variations in the power supply, like vs line variations, it sure looks
> >> like the bias point shifts all over the place, especially with a
> >> triode. I don't mean the voltage I generate, I mean where the bias
> >> point should be for 34ma through the triode because plate drops as
> >> line voltage does, unless I make a regulated B+.
> >
> >The amount of B+ drop in an SET amp is negligible under musical signal
> >conditions.
> >Even when driven into class A2, the change isn't large.
> >Next time you test, hook up a nusic source and take the levels up until
> >clipping with rock and roll just starts to be visible on the CRO as little
> >flat spots.
> >Monitor the supply dc voltages; I bet they hardly vary much at all.
>
> Yes. It isn't the drop vs signal I was talking about here, it's line
> voltage variation. Mains power to you. I.E. if it's adjusted when
> mains is a nominal 120V then fixed bias seems to be seriously out of
> whack when mains is 105 because B+ is no longer 330 but 288.
Triodes with fixed bias are prone to becoming over biased
when the line volts go high.
The 6V6 has an Ra = about 4k, so if the B+ changes 52 v, then you get an idle
current change
of about 12.5mA, so you should set the 6V6 up for the 330V condition.
When in fact the B+ drops a bit, so to will the negative bias voltage, thus
turning on the tube a bit more
when B+ is low.
Hence there is never a need to regulate the negative fixed bias voltage; one
wants it to wander around
with the B+ voltage.
The changes of bias voltages are inaudible because usually your PS *should*
only very slowly change its voltages even when there is someone doing some arc
welding next door on an old Buick bumper bar.
In most amps I make you can switch them on and off and hear no sign
that any on/off switching is happening.
The mosfet amp I have will run switched off for 20 seconds since the PS caps
are
a pair of 100,000 uF.
Use some real capacitance, you will be so glad you did.
>
>
> <space snip>
>
> >> On another simulation I ran into my first real 'surprise'. I decided
> >> to put the pieces together, in particular the power supply and amp, to
> >> check ripple effects but the surprise came with startup.
> >>
> >> I've got a HV delay and soft start circuit worked out but for the
> >> first simulation cut I just used a plain 'slam it on' SS HV supply and
> >> the original pentode amp (quicker than redrawing the whole thing).
> >> What surprised me is that the 6V6 current spikes to over 150ma for
> >> 250ms, or so, then settles back to the design 34ma, with the culprit
> >> being the gain stage to 6V6 RC coupling time constant. But in all the
> >> reading I've done I've never seen anyone even mention this effect.
> >
> >The current surge during turn ons shouldn't be so great.
> >Something is wrong about how you have your things hooked up because
> >in the countless radios that use a 6V6 and in all the amps and gear I
> >revise or
> >make, such things just don't happen.
>
> The heaters aren't already hot in those I would imagine.
They just all switch on together, and since a 5Y3 is used for a rectifier, the
B+
comes up to its high surge voltage level, since the 5Y3 is a directly heated
diode
so conduction starts within a few seconds.
The high peak currents to get the caps charged up to a high V don't
seem to hurt the cathodes.
>
>
> >From cold with an SS supply, the current always just starts with a slow
> >trickle then over about
> >a 5 second period the current increases to full idle value.
> >
> >In a cathode biased amp the current trickle has to charge the cathode
> >caps.
> >Its always slow from cold, but if the tube is hot, and you turn it off,
> >then back on 2 seconds later, indeed sometimes current is high because the
> >
> >cathode can emit as soon as power is back on, but the cathode voltage has
> >sagged,
> >to the tube is biased for a large current flow.
> >
> >Fixed bias amp should be able to establish the bias voltage faster than
> >any
> >applied B+, and should sag very slowly if the power is turned off.
>
> Yes, well, 'from cold' is the thing. I was trying to take advantage of
> all the wisdom posted in here and put a delayed B+ start to let the
> heaters warm up first but that means it's all primed and ready to
> surge when B+ comes up.
To make a B+ that switches on after 20 seconds and takes a further
20 seconds to slowly rise to full value is not mean achievement,
and really only doable with a series regulator which has a grid control
someplace that
has a slow time constant to allow the slow rise of the output voltage without
any sudden
jerks.
I have never worried about it except in the case of an SE amp with parallel
tubes which
insisted it oscillate .5 of a second after the amp was turned on.
I placed a 470uF cap to bypass the screen supply and fed with a resistor from
the B+
with hot cathodes the anode current is prevented from ever surging, and no
oscillations occured either.
But in this case the amp had cathode feedback in the output stage,
another reason why oscillation is possible.
It's like a Quad II amp cut in half, but with 4 SE output tubes.
Being SE, they can tend to oscillate because the balanced nature of a PP
circuit
tends to prevent it.
> >> Is it that no one takes notice of 50 watts into a 12 watt plate for
> >> such a 'short' duration?
> >
> >if the voltage is average 150V for the 150mA, you only have 22 watts for a
> >short time.
> >The tube is able to have a current swing of up to saturation for a short
> >time.
>
> Looks more like 225 on mine but I imagine that varies with the stage
> RC coupling. Takes a good solid second to get under 50ma. too.
>
> >> Ramping HV up (the soft start circuit) makes matters worse up to a
> >> point because while it reduces the overshoot it lengthens the over
> >> power duration (1 sec ramp). Slow enough, say 7 seconds (0 to 330),
> >> and the 6V6 self bias is under damped and ramps up to 34ma without
> >> overshoot.
> >>
> >> That's with hot heaters, of course.
> >
> >This is the trouble of heating the tubes and then applying the B+.
> >The tubes are able to fully conduct, and when the B+ is turned on,
> >the grids to the output stage may initially be pulled up +ve
> >due to how the driver stage reacts to the B+ turn on.
>
> I think I'll skip the heater delay. Fewer parts, less work, and if it
> strips the cathodes they're 2 buck tubes. Although lovely Sylvanias.
>
> >> It all makes sense it's just that I didn't expect it.
> >
> >I think you need to more carefully measure exactly what
> >are the dynamic voltage movements *all* electrodes, and perhaps take
> >steps to limit any problems.
> >I just turn on the B+ fully using SS diodes and with the heaters.
> >Sometimes an amp will oscilate during turn on or off.
> >Tweets, farts, squeals are not unusual.
>
> This part answers my question because the entire issue goes away if
> the heaters are cold and the tube can't conduct during the B+
> stabilization period.
>
> Thanks for the "Tweets, farts, squeals" tip. If I remember maybe I
> won't piss in my pants from panic the first time I turn it on ;)
>
> >In that case sometimes the best way is to have a very slow rise time for
> >the
> >screen or driver voltage; such unexpected issues nearly
> >always arise with DIY amps.
> >Factory made jobs invariably don't show exactly what troubles lay behind
> >their development.
>
> Right. No one advertises the dirty laundry, especially after it's been
> cleaned and folded ;)
>
> >I had a PV2 Conrad Johnson preamp arrive this week from a guy whose
> >purchases always have led to a trip across town to me for a fix after any
> >purchase he has made on ebay.
> >The CJ was quite unusable.
> >It had been slightly modded internally in an attempt to fix the
> >noise at turn on problems due to the preamp stages including heavy
> >negative feedback loops.
> >An ARC amp also gave huge problems 4 mths ago for the same reasons of
> >instability during turn on.
> >
> >The CJ has a relay which grounds its outputs for some time after turn on,
> >then the relay opens,
> >and the amp is connected to the outside world after the noise action has
> >subsided.
> >The time delay circuit was poorly designed, and wasn't working properly,
> >but after fixing that the amp has become usable and polite.
> >
> >There is a real art, or rather a real science to analysising turn on/off
> >behaviour,
> >and finding solutions that are not damaging to anything.
>
> Interesting that there's little, hell no, mention of these start up
> noise issues in any of the amp articles, DIY or not.
Ben Duncan is the only author to have mentioned start up / shut down horrors
some amps have,
especially solid state; he wrote a great book in 1996 about the history and
development of SS
amps since the days when tubes were being phased out in 1960.
Such problems can destroy speaker drivers.
Most modern SS amps have delayed protection relays between speakers and the
amp.
And they fail to protect so often....
Patrick Turner.
>
>
> >
> >Patrick Turner.
> >
.
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