Please help amend 6AS7PP circuit

[Shoog]

I just had a discussion with Brian Beck about that cap value. You may be interested to read the whole of the "vacuum State dpa300b" thread, if you haven't been following it. Brian recons that Triodeguy is doing something wierd with resonances and that has dictated his choice of cap. Since you are using a different transformer that cap value is definately wrong. Brian agreed with me that he couldn't see any reason why this cap value would be different than for a regular cathode bypass. In that case I would suggest something in the range of 500uf to 2000uf. I am going to up mine from 1000uf to 2000uf. This is because in my setup the two cathode cap are in series and give an effective bypass of only 1000uf. Hope that helps.

One more general question - is there any preference between 6080's and 6AS7's? Or the Russian 6N5S or 6N13S?

I am currently using the russian 6H13's. Can't say as they sounded much different to the 6080's I tried. The only real difference I can think of is that the Russians look the nicest. I think the driver has the most influence over the sound of the amp. I might just try some NOS Mullard 6080's in the hope that they have that Mullard magic - but I doubt it.

Shoog

[Yvesm]

Originally posted by sorenj07

Hi, I'm also interested in the 6AS7, but driven by a CCS LTP 12AT7 with your 400V, with a regular grounded-cathode 12AT7 input stage. All the gain anyone could need, I hope. My question is this: With your power supply, aren't you unbalancing the power transformer by only using half a winding to feed the 150mA to each 6AS7 pair? Would a transformer with completely separate 200V windings work better? Maybe I'm just reading the schematic wrong...
. . .

Just look at the power supply as being two full waves rectifiers on a 400V CT transformer, one positive, the other negative.

You obtain two rectified voltages : +200 and -200V relative to the center tap.

Now, remove the ground from the center tap and move it to the -200 output.
The result is now 0, +200V (from the formerly grounded center tap), +400V.
The transformer is not abused in any way.

(All numbers rounded !)

Yves.

[Klimon]

Brian recons that Triodeguy is doing something wierd with resonances and that has dictated his choice of cap.

Yes, there's a paragraph about this in the audioxpress article. I'm waiting for a reply from triodeguy; naturally I'ld prefer lower value cap (all the more because I've got many of those in stock). The huge difference in capacitance between what he uses and what you use is intriguing..

Simon

[cotdt]

Klimon, unrelated to this thread but how did you triode-strap the EL84 into a triode? I use a resistor, but I heard you got a special trick. Care to explain it?

[Klimon]

Hi Cotdt,

I remember your mail but forgot to answer it, sorry for that.

You can find the schematic here: http://www.t-linespeakers.org/tubes/triode-trick.html

the original text on joelist by Bill Perkins connects the 10ohms resistor to plate, not to the screen (and so did I) but this probably doesn't matter

Eddy Vaughn uses a similar trick in his Carina and gave some advice on how to improve the schematic by Bill Perkins, see his wise words on: http://www.hawthorneaudio.com/forum/...hlight=perkins

http://www.hawthorneaudio.com/forum/...er=asc&start=0

As my initial impressions throughout the web indicate: I'm very enthousiastic about the tweak (in spite of the fact that very few triode-strappers seem to have implemented it and there's much scepticism) . FWIW I use el84 (philips = mullard = valvo) with ecc40 as driver (previously used e80cc, both are improved versions of ecc82 and 5965 if you will) and added small amount of anode to anode feedback (RH84 style) using a 300k resistor. This does wonders for making the bass tighter and faster without any apparent drawbacks.

Chiao,

Simon

[Shoog]

I just added in my extra 1000uf per cathode (o' the joys of diy) and first impressions are that the bass has filled out again.

I really don't think that their will be a cheap and easy way of avoiding those big caps.

Shoog

[Klimon]

Interpreting the part of the article where triodeguy describes the ac-current balance circuit (= the capacitor), the formula must be easy. Variables are:

L (primary inductance opt)
C
Rplate
fhz (resonant frequency of the circuit)

The author says the resonant circuit formed by L and C causes a (in his case calculated) peak at 5hz "that extends upwards into the frequency region where the reactance of the capacitor would otherwise interact with the cathode to cause an LF rolloff"

I think the simple formula for calculating the resonance frequency of a given circuit is all that it takes to find the capacitor value. I'll aim for 5hz as triodeguy does. When thinking about it, it's the same principle as a parallel zobel filter. Just fill in the values for L, C and R in a online zobel calculator and pronto! (off to the cellar to measure the primary inductance of my opts)

Simon

[Klimon]

EDIT: read notch, not zobel

The notch calculators I can find do not allow input of L, C and R so it'll have to go the manual way.

Shoog,
You're probably right that a much higher value cap is necessary. The big difference between those toroids and iron-cored opts is the primary inductance; 100H for the toroids triodeguy used, 8H for Hammond 1645 and 5.5 / 6.7 for the old opts I plan to use.

Simon

[sorenj07]

Originally posted by Shoog
I really don't think that their will be a cheap and easy way of avoiding those big caps.

Fixed bias maybe? Assuming that the 6AS7's don't complain and drift, etc.

[sorenj07]

I just realized that this amp is a perfect candidate for those big industrial control transformers. I have one that buzzes a tiny bit when plugged in, but it's 550VA and has selectable 115V "secondaries" and 230V "primaries" Basically you can end up with two ~230V windings at around 1.2 amps. That's an extreme example but a smaller one would also work well. You'd need an auxiliary filament transformer but with the greedy 6AS7's you'd probably need one anyway