Is there any advantage in using preamp tubes in output stages ?? - diyAudio
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Old 6th January 2005, 11:29 PM   #1
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Default Is there any advantage in using preamp tubes in output stages ??

Hey guys,

I often come across several schemes using many input triodes in output stages...

Is there any real advantage of using preamp tubes (such as multiple 6SN7 or even E88CC !!) in output stages?

I know that they wouldn't give out much power, but do they simply sound better or something.... ??

And just an addition (frankly, to save some space... ),
Any opinion on the "6AS7 OTL" scheme on Sch3matic's web??

It uses fairly low voltage on its 6as7 output stage and I think I can get a transformer for that quite easily (I might already have a few in somewhere, saved for my hi-power SS amps......).

The real trouble I always have to face everytime when I plan to build a HV OTL is that, they consume so much power and I would get soooooo much trouble to find a large HV tranny. They tend to be quite expensive too!! (and even worse thing is the shipping fee that I have to pay for'em )

Thanks,
James
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Old 7th January 2005, 09:02 AM   #2
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I don't really know why people would use input triodes as an output stage, though it clearly is feasible if you parallel enough of them.

It might be because they tend to be able to provide higher gain, or maybe they're more linear than the output valves on hand.... Or perhaps just because they have a few hundred sitting in a box and looking for something to do with them?

On the other question, Tim's (Sch3mat1c) 6AS7 OTL... 60V on the plates allows you 250mA of current before going into positive grid bias, which would impose catastrophic loading on the driver. You've probably seen Bruce Rozenbilt's 6AS7 OTL around somewhere (btw, that driver/output arrangement provides higher output impedance than Tim's arrangement), which uses higher (150V) idle plate voltage. The advantage of using higher plate voltages, is that it allows you more current before going into that area (and thus more power). This is important because OTLs are invariably current limited.

The circuit looks sound (pardon the pun), though I do wonder how good the regulation on that voltage doubler power supply is...
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Old 7th January 2005, 12:28 PM   #3
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That circuit was intended to keep the tubes within limits. It just goes to show you how dangerously extreme an average OTL is.

If you want more power (than the 20W? it indicates), you can up the voltage.

A doubler will work quite fine, if the resistance is low and capacitance high. I personally have used an 18V transformer (okay, wound with 8AWG wire, so there's no shortage of conductivity here ) doubled to 40V 5A as measured, which is pretty close to unloaded voltage (48V or so). So good regulation. It was with around 10mF total (4700uF top and bottom). So 1000 to 5000uF (total) would be alright in the OTL.

As for massively parallel amplifiers, I think the only reason anyone would ever think about, let alone design and build one, is to be weird and try to waste a few tubes. Hey, I should know. And coincidentially, I do have a glut of 12AU7s...

I doubt it has much to do with sound, although I suspect 6DJ8 would work nicely. Depsite the lower plate disspation I bet you can get 1-2W out of a single tube. I seem to recall hearing Tim De P did a 12AX7 amp, that would sound great judging by the curves - but in class 1 (i.e., no grid current), it'll take about 50. There goes an OPT's cost in the *sockets* alone!

Tim
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Old 7th January 2005, 05:13 PM   #4
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James,
One drawback of using several tubes paralleled is the multiplication of the interelectrode capacitance, that shortens your frequency range at the high end. This is still complicated by the Miller effect.
You need a very low driver impedance ( such as an interstage transformer) but then, unless you pay a fortune, IS trannies don't have a wide range.
The present trend is to amplify beyond the arbitrarily set range of 20 Hz - 20 kHz. Supersonic frequencies contribute to keep the sound envelope close to the original, even if you don't hear them.
In my designs I prefer to use a strong powerful valve, driven by a wide frequency range driver tube.
The problem then is the OPT.
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Old 7th January 2005, 06:27 PM   #5
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Quote:
Originally posted by ari polisois
James,
One drawback of using several tubes paralleled is the multiplication of the interelectrode capacitance, that shortens your frequency range at the high end. This is still complicated by the Miller effect.
Which can be somewhat overcome by heavy NFB, but that introduces its own grief.
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Old 7th January 2005, 07:37 PM   #6
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...Which is why local NFB in that location is preferred. Some day I'm going to pull the IST from Hept'AU7 and replace it with a 12AU7 dual CF, switching the 6SN7 to a preamp and cathodyne splitter.

Personally I'd never use NFB to correct anything but OPT response. Trying to compensate one or two stages in addition to that blob of lumped-constant capacitance and inductance is just begging for trouble.

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Old 7th January 2005, 11:30 PM   #7
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Quote:
Originally posted by Sch3mat1c
Personally I'd never use NFB to correct anything but OPT response.
Here, here

(I was converted to no-NFB about the same time as mu-stages )
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Old 8th January 2005, 05:17 AM   #8
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Quote:
Originally posted by ari polisois

You need a very low driver impedance ( such as an interstage transformer) but then, unless you pay a fortune, IS trannies don't have a wide range.
Or you could use a cathode follower. It isn't too hard to get one with Zo<100ohm.

The non-existent gain of the cathode follower isn't as much of a problem because the output stage would have a relatively high gain in itself.
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Old 8th January 2005, 01:08 PM   #9
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In terms of tube matching, preamp triodes may be easier than double triodes. I'm just building a PP amp with two pairs of 12b4 per monoblok. And running into driver problems. Right now I have a 6SN7 diff pair driving them. Problem is I only have one socket space for a driver valve - could be octal, noval, or what. I don't know how to solve this without getting an interstage - any ideas? What I can hear of the 12b4 sounds very detailed - there's a good sound in there somewhere. I'm running them at 300v B+, 40v bias, 20mA, 2.2K cathode resistor into a 6.6K OPT. Solutions appreciated. Andy
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