Romy the Cat wrote:

This is a technical post.

I've made some progress on my assault on the above version of EAR 834. After hard (mostly because of my idiocy)

labouring through the soldering, I've creeped to the level where I can make some static tests.

I must say the biasing of the second stage is a pure mystery for me: +270 on the rail, 330k load, -0.9V

theoretically cuts the tube out according to the curves or I'm completely out after all the solder fumes?

I do get +270-273V on the B+ rail (accross the first 150u fileter cap), I have -0.92V on

the divider (R13 -R14 junction) but tyhe plate is anywhere from 110V to 140V depending on a tube.

For the experiments I use some cheap Chinese

(one I've already sent to hell) and one TAD specimen. The tubes are CCS heated at the ideal 150mA and in fact

they want to stabilize at somewhat lower heater voltages from 10.8V to 11.8V instead of 12.6V, but perhaps

that's a Chinese crap.

Perhaps -0.9V quoted above is a mistake? Theoreticaly the

47k-10k divider gives 1.1V from 6.3V (I have a 12.6V heated so I adjusted the bias to the published 0.9V).

Anybody able to clarify/help? The 1st stage is +/- ok at 100-110V but the bias is rather -0.8V then -1V.

Thanks in advance.

PS I also do get quite some wideband noise at the output (no 100HZ though)...will investigate, perhaps

the VR's (I've put them close to the 2nd & 3rd stages).

Romy the Cat wrote:

Yes, you are right, the voltage divider from 6.3V across 47K and 10K resistors shall give not 0.9V but 1.1V. However, the numbers that are in circuit are not something that I saw in my dreams but the actual numbers that I measured. I might presume that at the time I measured it I did not run the phonostage from my AC stable PP2000 but from wall and the voltage was lower and I did not have 6.3V but perhaps 6.1V or something like this.  All those numbers are perfectly acceptable and with 1V basis the 12AX7 is not near to be closed up. I do not know what curves you looked at but 1V is fine bias for this tube and with 140-150V on plate it is perfectly fine operational point. Thanks for spotting the .2 difference but I think it is in given case negligible.

N-set wrote:

Sorry Romy, I did not want to sound nitpicking. I'm rather trying to understand if my copy works +/- properly.

Romy the Cat wrote:

Anyow, it is not difficult to find out if your copy works +/- properly.  If you have right plate voltage and the tube has the expected gain then the stage works fine. I do not remember now but I believe in closed loop the corrector shall give 55dB or something like this, it is not difficult to calculate. Did you use air caps or cheated and used mica?   The Cat