AlexBerger wrote:

Hello Romy, I just have build the postage used your "End of Live"  scheme and Altec step-up transformers. http://picasaweb.google.com/alex.berger73/Phonostage# I have original EAR. After of 100 hours of working, the new Phonostage  sounds more dynamic, transparent with much better separation. One problem is 100Hz hum. I did hitter supply filter with too weak ripple filtration: C-L-C (13200uF-3mH-6600uF). Now I going to rebuild  hitter supply. Is any difference in sound of semiconductor stabilization filtration, C-L-C-L-C or L-C-L-C filtration.

Alexander, are you sure that it is 100Hz hum but not 120Hz hum? Even I do not like and tend do not use CLC supply for filaments but your hum most likely is NOT due you your PS. The 13200uF-3mH-6600uF is enough to drive the ripples low and kill the hum.

It is VERY difficult to debug any devise “over the phone” in search of hum. Hum is most likely some kind of ground loop and you need to find it yourself. Pay extra attention to the wiring diagram of the old Altec step-up transformers. I have some of them (Altec and EV) and they are in my view need to use extra shielding and they are very difficult to make them to be quit.

What you need to do for sake of simplicity is take a wire of 2 feet long and begin to short different sections of your new pnonocorrector grounds. Soon on later you will find the location that after they were shorted minimize the hum. This will give you an answer where the hum comes from. I would do it separately for the pnonocorrector and then what I find the absolutely silent configuration, only then I would put the transformer to the game.

Also, considering the proximity of the air caps to the filaments I would VERY strongly advise to use twisted pair for filaments. The twisted pair is very singly but it is VERY effective as it is self-shielding. You can detect if the hum come from air caps by disconnecting them. Also use the “disconnecting trick” to see if the problem comes from the filaments. When you turn off the corrector off then it will be still a second or two operational. You will be able to hear how the hum will change in the moment of disconnection. Remember, what you disconnect the B+ still there and the tube is still hot but there is no filament voltage…

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