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Old 20th November 2008, 01:25 PM   #11
colinB is offline colinB  United Kingdom
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Hi Ray,

Just a quick update – I used 2x 39k higher wattage resistors in parallel as plate load resistor to give 19.5kR – not 18k as recommend but probably near enough.
ECC82 valve remains paralleled. Measured voltages on ECC82 were then:-

B+ = 380VDC
Voltage After 3k2 resistor = 340VDC
Anode Voltage = 105V
Voltage over cathode R of 210 ohm = 2.6VDC (equates to ~6mA per section of ECC82)

Using oscilloscope + signal generator, a 2V pk/pk (1Khz sine wave) through ECC82 gave 28V pk/pk output so gain is ~14 – just sufficient to drive EL34 to full volume (bias on EL34 is 25VDC).

I then hooked up the EL34 amp to 12B4 pre-amp and Vinyl front end for listening tests. Firstly, gain was now much more controllable with volume knob at 11-12 o’clock (compared with 8-9 o’clock when using paralleled ECC83). No sign of distortion at my normal listening levels which is not particularly loud as I only have a small room. Next tried CD player as front end – again volume knob was at 11 o’clock so no problems with this source. Of course, downside now is that the gain is too low for my cheapo Creative MP3 player as volume setting is at 4-5 o’clock to match volume levels with CD/vinyl source. So, at the moment I’ll continue to use ECC82 as the driver valve.

I’m also considering changing to a 6CG7/6FQ7 when funding allows. Any suggestions for a suitable operating point for non-paralleled valve? Would the following be a good starting point for B+ of 380VDC?

Plate decoupling resistor: 22k
Plate decoupling capacitor: 22uF
Plate load resistor: 22k
Cathode bias resistor: 470 ohm.

AlexG: Thanks for suggestion of 6N1P as driver. This looks like an interesting alternative although I suspect that it will give me too much gain (spec sheet says x30) for my current system. However, out of curiosity, I might just give this a try

Cheers
Colin.
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Old 20th November 2008, 05:31 PM   #12
46davis is offline 46davis  United States
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Why dont you try the musician's trick of subbing a 5751 for the '83. That usually calms things down a bit, and it has a lot better deep bass.
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Old 21st November 2008, 01:47 AM   #13
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Hi Colin,

For a single triode (either 12AU7 or 6CG7/6FQ7, doesn't matter), I would use the following values:

Plate decoupling resistor: 5.6k
Plate decoupling capacitor: 22uF
Plate load resistor: 39k
Cathode bias resistor: 560 ohm.

That should give plate current of about 5.5 mA and plate voltage of about 130v. The higher plate load resistor of 39k should give a little more gain and lower distortion.

As you know, you can adjust the gain to some extent according to whether or not you use a cathode bias decoupling cap.
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Old 21st November 2008, 08:14 AM   #14
colinB is offline colinB  United Kingdom
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Hi Ray,

Thanks for the suggested operating point for 6CG7/6FQ7.

I have one final question: If I change to a 6CG7/6FQ7 driver (in place of ECC82) do you recommend I wire it as a single triode stage or is it better to use it paralleled (as the ECC82 is currently)? I.e. is there a ‘technical’ reason why paralleling the driver tube is better than using the single triode stage or vice versa?

Thanks
Colin.
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Old 21st November 2008, 10:51 AM   #15
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Hi Colin,

AFAIK, parallelling triodes doubles Gm, halves plate impedance and improves signal/noise ratio but leaves mu unchanged. I think there is nothing to be gained in your application by using two parallel triodes of the type you're discussing (12AU7, 6CG7, 6FQ7). Others may be able to add to this.
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Old 21st November 2008, 09:09 PM   #16
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I am very much a believer in the dictum: "If it ain't broke, don't fix it". If the amp sounded good before, and the only problem is excessive gain, then I'd solve the problem with a 6db resistor pad at the input. (Unless, of course, you think you can improve the sonics with a better small signal triode.)

ECC83s (12AX7) aren't real good, but ECC82s (12AU7) are worse. Most of the 12AU7s appear in RF (oscillators, buffers, drivers, frequency multipliers) and digital (astable/monostable vibrators, Schmidt triggers, RS latches) rather than in audio circuits. Make of that what you will. There have been enough complaints over the sonics of the 12AU7 that I wouldn't want to use one as a gain stage. Cathode followers or SRPPs would be OK. Since 12AU7s appear in so many gee-tah amps (where distortion is not a problem, and even desireable) the type is no less expensive than 6FQ7s/6CG7s (the only difference is that the former type doesn't include the electrostatic shield between triode sections that the latter type does include) so I use the 6FQ7/6CG7/6SN7 for cathode follower grid drivers.
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