Optimum driver valve for an interstage-transformer-driven SE 300B - diyAudio
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Old 30th April 2012, 12:34 PM   #1
needsp is offline needsp  United Kingdom
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Default Optimum driver valve for an interstage-transformer-driven SE 300B

Hi Guys

Iím after suggestions for the best tube to drive an SE 300B- via an interstage transformer. The latter is a 1:1 bifilar wound design, with a primary inductance of 50 H. It was intended for currents in the 15 Ė 20 mA range, although its specified up to 35 mA. Over this range the inductance is pretty fixed.

Other restrictions-

Not a DHT (for now)
Mu ~ 20 (preamp line stage gain is high, so noise a problem with higher gain drivers)
Operating current no higher than 35 mA (~ 20 mA preferable)
Operating bias at least Ė 6 V, preferably Ė 8V, to give an output swing considerably greater than the 300Bís bias voltage (currently about Ė 95 V. Keeps driver await from its cut off region and noticeably increases the out put level at which audible clipping occurs on, e.g. loud piano)
Plate resistance 2k or less (for good bass response)
Plate voltage ~ 180 V (?)
9 pin base if at all possible

Itís quite tricky to come up with suitable types within these criteria. The best sounding compromise Iíve found so far is a triode connected 6P15P- 215 V on the plate, 20 mA, - 6.0 bias. Iím trying a 6N6P at the moment- 210 V plate, 20 mA, 8.0 V bias- but I preferred the 6P15P. For a long time, I used a 6P14P, 230 V plate, 26 mA, -8 V bias. This is a lot below its ideal operating current though, and the 6P15P brought extra clarity (though Iím using that a somewhat low current, to get the bias V up, the plate V against current characteristics are still pretty linear.

For what itís worth, Iíve found that I prefer a zener diode as cathode bias over a resistor + capacitor. I canít directly hear any zener noise, and dynamic impedance at 20 mA of the type I use is only 7 ohms!)

Any thoughts?

Paul N
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Old 30th April 2012, 01:23 PM   #2
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5687 or ECC99

Shoog
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Old 30th April 2012, 02:52 PM   #3
needsp is offline needsp  United Kingdom
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Hi Shoog

5687 is a classic choice for sure, but it’s a bit thin on the ground these days. And the guy who designed the interstage transformer compared it to E182CC years back- and preferred the latter. Not saying I’d find the same, though I did prefer the E182CC as a common cathode connected driver- back in the darks days when I thought this was a good idea! Either way, the E182CC is a rarity too now.

ECC99. I’d forgotten about this. I think I’ve read some underwhelming reports on it in this role, but will definitely give it a go, as it looks an excellent fit on paper.

Any other thoughts?

Paul N
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Old 30th April 2012, 04:03 PM   #4
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A (triode connected) 7788/E810F will satisfy most of your requirements and is hard to beat sonically. Also check Sound Practices Iss.15 pages 11-12-13 for more details about this tube and its use in a transformer coupled driver for a VV52B triode in S.E mode. I don't really like zener diodes used for cathode biasing, I'm allways hearing some soiling of the sound. Zeners are know to be temperature-dependant and noisy components. The proposed mehod using NiCad cells is much better, IMO.
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Old 30th April 2012, 04:15 PM   #5
Bigun is online now Bigun  Canada
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I wouldn't expect much difference between the 6N6P you are trying and an EC99.

The gain is possibly too high without some degeneration, but the 6C45Pi is a nice tube, low resistance and good driver. I've heard an SV811 being driven via an IT by one of these.

If I remember correctly, there is a sweet spot for zener diodes for minimal noise, at around 5.6 to 6.2 V.
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Old 30th April 2012, 05:02 PM   #6
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Hi!

Quote:
Originally Posted by Tubologic View Post
A (triode connected) 7788/E810F will satisfy most of your requirements and is hard to beat sonically.
Have you ever seen the plate curves of a triode connected E810F? Not datasheet curves, real ones. I have examined it with a curve tracer. While good for small signals it gets quite non-linear at the outer regions. I would not use it as a driver in a power amp.

I have used it in LCR phonostages and there it works nicely.

Best regards

Thomas
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Old 30th April 2012, 05:03 PM   #7
needsp is offline needsp  United Kingdom
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Hi Tubologic, Bigun

Possibilities abound if we allow higher mu tubes- with 6C45 and triode connected 7788 both strong contenders. But I didn't, and you cheated! :-)

I was very wary of zener bias until I tried it. I certainly wouldn't use it in a low level stage. I've never tried batteries- many like them, but many loathe them. I use LEDs to bias CCS loaded stages, but would be wary of them here, as several in series would be needed, raising their dynamic impedance

One problem with this topology is we can't lower the stage gain by using an unbypassed cathode resistor- this just raises the anode impedance and thus curtails bass response

Paul N

My long term plan is to try Rod Coleman's shunt cascode driver topology, discussed elsewhere on the site, in my amps. I installed this to brilliant effect in a friend's amps, even though the latter were somewhat compromised in comparison- weedier power supplies, output transformers etc
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Old 30th April 2012, 05:54 PM   #8
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Hi,
I have used E280F as Triode together with LL1600/25mA, Triode curves looks quite good and measure well to.
Best
Karsten
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Old 30th April 2012, 06:42 PM   #9
needsp is offline needsp  United Kingdom
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Hi

Triode connected E280F has a mu of 60- far too high! :-)

Thanks

Paul N
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Old 1st May 2012, 10:02 AM   #10
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Hi,
Mu 60 is correct and LL1660 is app 1:1, makes the E280F drive the 300b as a single stage with an output imp at app 2kohm.

Karsten
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