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Old 16th January 2003, 04:44 AM   #1
mig-ru is offline mig-ru  Russian Federation
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Default first design by me! need feedback

hey, finally decided to start reading, and thought id apply it, so here it is, the most help i need is on the 2A3 cathode resistor, im not sure bout that, thx!
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Old 16th January 2003, 04:56 AM   #2
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Default Resistor values

I think maybe some of the resistors values are not what I would expect. The 6SL7 would work with probably lower than a 120K in the anode and the pot in the cathode of the output tube is probably a typo . I guess it was 100 ohms or 1000 ohms?
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Old 16th January 2003, 07:32 AM   #3
halojoy is offline halojoy  Sweden
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Thumbs up Re: first design by me! need feedback

Quote:
Originally posted by mig-ru
hey, finally decided to start reading, and thought id apply it, so here it is, the most help i need is on the 2A3 cathode resistor, im not sure bout that, thx!
I do not thik we can critise you
by your choice of TUBES

I think you have got just about the best Tubes there!!

Now , what is left is a good circuit,
preferably without transformator
in the signal chain.

So, give this boy a better circuit
PLEASE

I know you all can do it .....

/halo - can't do it
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Old 16th January 2003, 08:00 AM   #4
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Default ...Some quick initial observations..

Hi,

I've just been through looking at how best to drive and bias a 2A3 for a design of my own and I thought I throw down some thoughts on the beast...

1) It's a great little valve and great prices for good examples e.g. Golden Dragon or Sovtek...

2) it has (like all DHTs) quite a lot of capacitance associated with the grid and therefore needs quite a lot of current to prevent the onset of slew rate limiting...

I calculated that I needed +/- 7.5mA of current drive for 3dB headroom at 25kHz. Your driver circuit would not provide that as shown without lots of distortion. I'll look at it later no time now...

3) My favorite operating point for the Sovtek 2A3 is 275Volts Va-c and -48 Volts Vg-c and about 55mA Ia. That would give you a cathode resistor of 870R.

Some people like different operating points and I'm sure we'll see some different recommendations...

Try them all if you can. It's fun!

Ciao

James D.
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Old 16th January 2003, 08:09 AM   #5
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Default Re: Resistor values

Quote:
Originally posted by ashok
the pot in the cathode of the output tube is probably a typo
Actually this is a fairly standard trick for a direct heated triode. You adjust the pot for minimum hum.

dave
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Old 16th January 2003, 08:13 AM   #6
halojoy is offline halojoy  Sweden
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Default Re: Re: Resistor values

Quote:
Originally posted by planet10


Actually this is a fairly standard trick for a direct heated triode. You adjust the pot for minimum hum.

dave
I think that he meant the size of Pot, was a typingerror

Should not be 100 kohm, but 100 ohm

/halo - that is the impression I got, by the comment
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Old 16th January 2003, 11:22 AM   #7
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Default Re-worked design....

I have taken the liberty of re-working the design over lunch to illustrate my advice. As commented above I believe the driver was a little under spec. on current delivery and I have chosen to use an ECC99 driver valve. This valve is designed specifically for driving DHT and, in particular, with 2A3 and 300B in mind. It generates lower distortion under high current delivery conditions than most other alternatives.

How to set it up?

Current demand +/- 7.5mA therefore bias driver for over twice that current for good linear performance i.e. >14mA. ECC99 is specified at 18mA Ia so choose that. Grid bias is -6V therefore cathode resistor is 330R. Anode resistor for Va 150volts at 18mA is 8k1. Stage gain is about 17 times.

2A3 is as per my previous message so the circuit looks like the one below.

Hope this helps, I believe it will sound much cleaner than your circuit. Haven't build it but might do one day....

Ciao James D.
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Old 16th January 2003, 11:30 AM   #8
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Default 2A3

Hi,

On my 2A3 amp I run the same B+ of 330 VDC but with a cathode R of 820 R.

The trimpot for the heaters should indeed be 100R iso 1K.

Cheers,
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Old 16th January 2003, 11:30 AM   #9
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Default James got it wrong...

Just realised I miss read the ECC99 curves and bias should be 4 volts not 6 volts...

ECC99 cathode resistor should be 220R not 330R as shown...Sorry

Shouldn't have rushed it...

James D.
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Old 16th January 2003, 11:45 AM   #10
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James,

MUCH better. I agree that DHTs need a lot of grunt to sound any good, or else they sound all 'tubey' and gutless.

In the last week I've been playing with my PP813, and although an ECC99 should <i>theoretically</i> drive it satisfactorily, adding a power tube as a driver makes a world of difference to how it sounds. I tried EL84's, Sylvania 807 and Svet KT88 (all trioded), and the 807, running at 300V/60mA was incredible. I'll sub a 2A3 in as driver later. [ ECC99 - IT - 807 - IT - 813 all stages differential]

Paul Joppa (amongst others) has made someposts at AA in the past stating that in his experience when a factor of 5x the min theoretical drive current neccessary to slew the outputs is available in the driver, that's when the tubes really come alive and their characteristic signature becomes trivial. The more I experiement the more this seems to bear out, and annoys me because of all the expense it can entail. I've also lost the elegance of a simple two stage design. Oh, well, the sonic benefits are so outstanding it's worth it.
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