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Old 1st June 2005, 11:20 AM   #1
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Default GlassFET Design Ideas

Up till now I've been strictly a solid state guy. However, a considerable supply of VT components have come my way. This would include power and impedance matching xfmrs, PS filter chokes, and, of course a nice selction of VTs. Since I've never heard a VT amp, I'd like to give this a go. So far, I've got 807s, which would be the finals in a PP topology. So here are some design ideas I've had:
[list=1][*]Solid state front end, or VTs all the way?[*]Circuit topology: Duplicate the usual solid state topology: LTP -> VAS -> Driver -> Finals? I figure that a 12BY7A would make a good VAS. Or try something more "conventional"?[*]Parallel feedback for each final? Would that help tame down the tetrode "harshness"?[*]Any other ideas?[/list=1]

Thanks in advance.
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Old 1st June 2005, 11:33 AM   #2
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807s are an excellent choice. Be sure to have a look at the STC Application Note for the 807. It is quite extensive.

Unless you're going into class *2, or using screen drive, it is probably best to use valves all the way (plus it looks better - very important )

It shouldn't be necessary to have a four stage amplifier. Three stages should be plenty - input differential pair, drivers and output; or some variation thereof. Getting a huge amount of gain doesn't really help that much to reduce distortion because the amount of global NFB you can apply is limited by the odd stuff the OPT does at the extremes. Applying local feedback from the anode to the grid of each output valve, or from the anode to the grid of the driver of the other output valve may well help tame "tetrode harshness", for the lack of a better term.
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Old 1st June 2005, 01:42 PM   #3
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Depending on how you configure your output stage, you might even be able to get away with something even simpler. If you only need, say, 40V peak of drive, a 6SL7-based LTP will swing that easily. For a 2.5V input sensitivity (roughly 3.7V peak), you'll even have enough gain left to put in 8-9dB of feedback, if you desire.

Interpose cathode followers between LTP and output stage and you've got yourself a pretty nice amp.
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Old 1st June 2005, 02:41 PM   #4
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I agree, I made a bunch of 807 PP pentode config with global NFB and they sound very nice. By now, I'm trying to enhance those results adding ultralinear feedback with separate winding and separate suppressor grid power supply. I aim to less power but better performance.
I think this scheme could be a good start to aproach tubes.
Cheers
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Old 1st June 2005, 03:38 PM   #5
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Default Re: GlassFET Design Ideas

Quote:
Originally posted by Miles Prower
[*]Solid state front end,


Miles,
check this out:
http://www.bonavolta.ch/hobby/fr/audio/pacific.htm

It is a schema that was published by .Jean Hiraga in the French avantgarde magazine "L'Audiophile".
The design was described in the no 43 edition of 1988 with a single FET in the input2SK170BL. the divider was for the cascode 108k/2,6k.
Now the follwing ising to be interesting - it shows the cunningness of Mr Hiraga.
By changing slightly the bias fpor the cascode the content of the harmonics can be tweeked.
The stage gives some 30 V - enough IMO for the 807.

It is very responsive and needs a well laid out circuit. The following is mentioned with care. The earth for the zeners (all 4 - both channels have their own zeners) go to the same and single point. Or there will be
So have fun.
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Old 1st June 2005, 07:38 PM   #6
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For a 2.5V input sensitivity (roughly 3.7V peak), you'll even have enough gain left to put in 8-9dB of feedback, if you desire. Interpose cathode followers between LTP and output stage and you've got yourself a pretty nice amp.

This is pretty much the topology I had in mind. So this design won't require the kind of inverse feedback that solid state needs. So no VAS. That leaves:

LTP -> Drivers (Comon plate) -> 807s (Class AB(1) )

I'm going for an input sensitivity of 1.0V(rms)

Unless you're going into class *2, or using screen drive, it is probably best to use valves all the way (plus it looks better - very important )

So I guess it's best to leave the silicon out. Would it make any difference to use solid state in the power supply (silicon diodes, voltage regulators)?
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Old 1st June 2005, 07:43 PM   #7
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It's an esthetic choice- from a pure performance point of view, solid state is far superior for power supply, current source, and error amp use.
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Old 1st June 2005, 11:07 PM   #8
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> Solid state front end, or VTs all the way?

Are you trying to start a fight?

Sand driving vacuum was tried and widely rejected. When you spend some quality time with tubes, you will see that this is a field for the ultra-geeks, or for cheap gitar amps.

Much "tube sound" IS in the outputs; but the spice is in the preceeding stages. Don't sand-state until you try pure tube.

> Circuit topology: Duplicate the usual solid state topology:

LTP? VAS? Please don't use such foul language around bottles: they hate it.

Tubes (and their power and chassis) are expensive. Tube amps need to be simpler than sand amps.

Tubes give lower gain but also lower distortion than transistors. Like 5%, not 25% open-loop. The sand-head notion of making a LOT of gain to apply a LOT of NFB is misplaced: a zero-NFB tube is very listenable, and as Jason says, VAS-y stuff is pointless when the output transformer gives so much phase shift that even 20dB of negative feedback is difficult. One stage with voltage gain of 70-300 is often all you can use.

LTP practically forces a negative supply rail, very awkward when we only had hollow rectifiers. And it is not necessary, in a speaker amp, to have two high-impedance inputs, nor is the even-order distortion of a non-push-pull feedback comparator any real problem. And a LTP gives half the voltage gain per bottle of a grounded-cathode amp. LTP tube amps exist: most Fender guitar amps use a wacky LPT driver. But this is conciously low-NFB and also a lot about the sound in gross-overdrive, a range HiFi amps (should) never run in.

Given 6L6/807s, build the Standard Amp: hi-Mu voltage amplifier and concertina phase splitter. Old Reliable.

Used in Pentode, 6L6/807 has high output impedance, while modern HiFi speakers expect low source impedance. For this reason, NFB is commonly used. Usually overall, sometimes just back to the driver. Alternatively, you can strap Pentodes as Triodes, which is equivalent to internal NFB and gives low output impedance at the cost of high drive voltage and lower maximum power. 807/6L6 was invented to save us from the hard inefficient work of flogging power out of triodes; however even the 6L6's promoters felt triodes would remain the standard of fidelity. They were probably right: we spent 1938 through 1978 trying to get pentode Watts/Dollar with something like triode sound, and then triodes started sneaking back. OTOH, there is nothing like the pounding sound of a hard-flogged light-NFB pentode amp: perfect for some types of music.
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Old 2nd June 2005, 12:29 AM   #9
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Sue me; I like cathode coupled (LTP) phase splitters. Put a CCS in the tail and apply NFB from the O/P trafo to the non-inverting grid. There's more than 1 way to make up for the lowish gain. Avoiding global NFB (the loop is around only 2 stages) and phase shift oscillation makes sense to me. There are several ways a "high" voltage gain block can be made that's very linear without trotting NFB out.

FWIW, I think FET voltage followers are OK. Otherwise, I have no use for "sand" in the signal path.
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Old 2nd June 2005, 12:51 AM   #10
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Are you trying to start a fight?

No, just trying something new and different.

Tubes give lower gain but also lower distortion than transistors. Like 5%, not 25% open-loop. The sand-head notion of making a LOT of gain to apply a LOT of NFB is misplaced: a zero-NFB tube is very listenable, and as Jason says, VAS-y stuff is pointless when the output transformer gives so much phase shift that even 20dB of negative feedback is difficult. One stage with voltage gain of 70-300 is often all you can use.

What can I say? After all, I am a "sand-head". That's why I figured I'd run this by you before committing to any particular topology. OK, I understand: less inverse feedback; ditch the VAS; hollow state front end.

Given 6L6/807s, build the Standard Amp: hi-Mu voltage amplifier and concertina phase splitter. Old Reliable.

My preliminary research into this pointed out a big problem with concertina type phase splitters: wildly assymmetrical levels of THD between sections. The LTP and cathodyne don't seem to have this problem. That's why I had it narrowed down to one or the other.

OTOH, there is nothing like the pounding sound of a hard-flogged light-NFB pentode amp: perfect for some types of music.

My kind of music anyway.

807s are an excellent choice. Be sure to have a look at the STC Application Note for the 807. It is quite extensive.

TNX audiousername
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