Vintage tube mixer input impedance

[Levente]

Originally Posted by ms660

I would fit a 47k and take it from there.

If you have the time try and reverse engineer a schematic for your mixer.

Lawrence.

Thanks Lawrence.. my drawing is so crap that I feel really embarrassed about it... i will try tho... I did find some similarities with this kind of design, although I have 3x 12ax7 for input, one PI, 12au7 for output and the 6x4

http://www.preservationsound.com/201...er-circa-1960/

Thanks again for the resistor suggestion.. when I will fire this up with the replaced parts, and cap's will report back here

[Levente]

Originally Posted by ms660

I would fit a 47k and take it from there.

If you have the time try and reverse engineer a schematic for your mixer.

Lawrence.

Hi Lawrence, I hope you are well, i thought I would share what is happening with my mixer so far and ask also your opinion a few things I hope you don't mind...

So the replacement parts arrived, it was a nightmare dismantling and put everything back as so little space given. The burning resistor replaced, the main filtercap ( a huge Aerovox) still the same as I did not get hold of anything similar would fit here. So far all 4 section were within specifications. There was another two section dead filter cap was easier to replace.

I read that these old types of can caps needs reforming, i also read on this and other forums that some suggest just "plug it in and see what happens, if within 30 minutes the cap wont explode its fine"... would love to hear your view on this as I respect and listen to everything you advise here...

well.. after I've put everything back with the new parts, gave a 117V ac for 30 minutes and every approx. 3 minutes did DC (B+)checks... (the cap did not exploded at all, just got very slightly warmer feeling a very gentle vibration touching the can)

Every 2-3minutes during the 30 minutes check the DC voltage increased slightly starting right at the rectifier and following down the line, feeding the 3x 12ax7 and the 12au7 output and the 12ax7 PI

Started at the rectifier going into the 4k resistor
295 VDC, then up to 370,378,380,383 and stayed on this...took at least 20 minutes to get steady

the rest of the 12AX7s were getting 250 at start, then 260,266,267,275,278 and so on... after a good 20 minutes each of them gets 272, 275 and 280 VDC

Nothing was burning, or overheating, or anything. I did not do sound testing yet, attached a few pictures of this mixer while I was testing.

I am wondering how safe I am with that Aerovox filter cap?

Thanks a lot !