[CButterworth]

Right now, I have the standard complement of tubes in my Cornet2:

12AU7 - Sylvania
12AX7 - Mullards branded as Amperex and GE (1960's) unmatched

Anyway, when I did tube rolling with the 12AX7's I preferred the "Mullards."

I am thinking of investing a little in more tubes!

I could purchase a single Mullard / Brimar / Telefunken 12AU7, or a pair of cheaper 12AX7's

My Question:

Which tube, the single 12AU7 or the pair of 12AX7's have the largest effect on sound?

What other tube types have people tried and liked?  I may use other tube types, although I do not want to change the gain, or have to swap out any resistors.

Regards,
Charlie

[WGH]

Charlie - Don't forget about the rectifier. I found it can effect the sound as much or more than the 12AU7. Your options then can include trying out a 5AR4/GZ34 (a Mullard?),  indirectly heated NOS 5Y3 variations, and the 5V4GA. I try to keep the heater voltage close to 6.3 volts for optimal sound.

[BobM]

I agree, the rectifier plays a large role. NOS 5Y3 variations generally result in a lower heater voltage. Newer versions are a bit better. The 5V4GA spiked it up a bit more, resulting in better bass response for me.

Enjoy,
Bob

[CButterworth]

I am awaiting a NOS RCA 5V4GA (two actually) to arrive in the mail.  How does the rectifier effect the heater voltages?  Aren't the heaters driven from the filament secondaries of the transformer?

Charlie

[CButterworth]

OK, does R223 act to drop voltage for the heaters? From the schematic, it looks as if the diode bridge should putput something like 8.9VDC  (6.3 x sqrt2).  This is regulated using Q100 and Q101, with voltage being dropped using R223?

I just measured my heaters - 7.36V before R223 and 5.75V after R223.  Now, I have read that a little heater undervoltage extends the lifespan of the tube without much sonic detriment.

Now if I wanted to bring the heater voltage up to 6V.  Then is this correct:

1. I read 7.36V and want 6V, so I need to drop 1.36V.
2. The tube heaters run at 0.3A, which for three tubes = 0.9A total.
3. Voltage drop / heater current = amount of resistance required.
4. 1.36 / 0.9 = 1.51 R 5W resistor.  I expect that 1.5R will be fine as it should give 6.01V.

Is my math correct?


Charlie

[WGH]

Dunno about your math, but there are formulas at the end of this thread
http://www.audiocircle.com/index.php?topic=41430.0

I think you should go for the 6.3 V, tube life be dammed!

[CButterworth]

WGH,

My math seems correct.  I can parallel a 10R to the 1.8R to get 1.41R which brings the voltage on the heaters up to around 6.1V.  I can accept this, especially if my mains voltage fluctuates, which it seems to do.

As for tube life, I am thinking that I want to use the Cornet2 for a long, long time and really like the sound of the old British -made tubes (maybe the Brit in me talking).  Anyhow given their scarcity and cost, if I splash out on good ones, then I don't want to have to replace them ever (maybe that's the Scottish in me!).

Charlie