Corroded battery on gain selection switch, old tubed voltage amplifier

Yesterday I picked up a couple Technology Instruments Corp. Type 500 Wide Band Decade Amplifiers.



Before bringing up on a variac I like to give the guts a quick visual.



Which is when I saw this:



Which looks to be a pair of type 625 mercury batteries, hanging off the back wafer of the 10-100-1000 gain switch. I can't find a schematic, and outside of some EEG amplifiers which presumably used such small batteries as a reference to calibrate I've not come across this sort of thing.

Does anyone have any insight as to why there would be batteries in here? Calibration of the phase meters mentioned in the literature blurb? There's no calibrate mode selectable on front panel.

I'm attracted by the notion of having a couple 20-40-60 dBv preamps on hand, so I think my current course of action is to oust the selinium rectifier, clip the battery and see what happens. I'm just curious if there's a precedent for retaining this sort of part..

crochambeau said:

Does anyone have any insight as to why there would be batteries in here?

Click to expand...

Tube bias maybe.

http://www3.telus.net/radiomuseum/projects/RCA97BK/beforeBiasCell800.jpg

moamps said:

crochambeau said:

Does anyone have any insight as to why there would be batteries in here?

Click to expand...

Tube bias maybe.

http://www3.telus.net/radiomuseum/projects/RCA97BK/beforeBiasCell800.jpg

Click to expand...

Aha! That makes sense, thanks. I guess it may play a bigger part in response than I had hoped..

The bias-batteries may or may not be Mercury. In low-price gear they were a special low-leakage zinc(?).

Read the voltage. If a NO-current bias, and if the chemistry was VERY clean, they may still be good.

If not, mount a AAA-cell holder nearby but where leakage won't do much harm. (Nearby, because with 2mc response layout and wire-length matters.)

I assumed mercury due to the old 1.35 volt 625 batteries in 1960s camera light meters (the number 625 is also present). Given the corrosive mess, I removed this one, and from what I can see from the other one it may get removed.

Though, now that I've typed that, I guess measuring the voltage won't hurt. The chemical reaction doesn't extend too far into the switch.

I like the battery carrier idea. My bandwidth interest is pretty much limited to audio range, do you think longer leads would invite instability?

Ideally the leads would be long enough to allow changing the battery without complete disassembly, I'm not keen on the idea of cutting any more holes in the unit:



That said, being able to adjust bias externally and run it into non-linear response might be fun too.

> do you think longer leads would invite instability?

Without knowing details: yes.

It's probably an input stage bias. Upset the output stage bias for dirt.

Lovely case!

if is mercury, be careful not to reverse the leads as they can explode in some situations, like being across a pwr supply as a backup battery,

when they do explode they can throw mercury around the room,