Western Electric 121A, power supply time

First off, thanks again for everyone's advice, history lessons, and interest in general. This forum is awesome.

I don't know how that cap slipped under the radar. NOS parts, what can you do? I thought I checked it, but chances are I checked it's partner twice.

[quote author="CJ"]Did you try bridging C7?[/quote]
Yeah, that's how I found it to be bad. I had another go around poking for more accurate measurements. Then I kinda back tracked and expanded your experiment to include all caps. I had clip leads and all flavors of caps all over the place. I checked in order and of course, 7.2 was the very last one to check.

I actually hooked it up last night and recorded some piano through it. It did surprisingly well. It's almost unusable w/o input attenuation. I had mics internal pads set at -10 dB and R5 just about as low as it would go and I still had an abundance of signal.

As far as padding, I think I possess the right animals for the job. They are Daven "T" network, 500/500 ohm, 1.5 dB/step, taper to infinity pots. Monsters they are.

After reading the Daven attenuator META thread, it sounds like I need attenuation more on the output than the input. I cliped in a couple of the T pads on both sides of the circuit and they certainly make the unit much more managable.

> It's almost unusable w/o input attenuation. I had mics internal pads set at -10 dB

It is a 78dB preamp!

It comes from days when mikes were weak, line-levels were high, and amplifiers were expensive. It tries to do everything too well.

No mike that has a -10 pad ever needs 78dB gain to reach a sane line level.

Try ribbon mikes.

Try the WE suggested change of bypassing the first stage.

You might also want a 6dB or 10dB output pad, but no more. You probably want +18dBm output and it is "only" rated +28dBm output.

I've managed to see it reach roughly 78dB, but if the input signal goes above 1mV p-p, it starts clipping. It's max output is 10V p-p. If I adjust R5 down to where the unit is producing 34dB, I can input 200mV p-p w/o clipping. That's getting down towards the bleeding edge of that pot's range but it isn't a precision turn pot... yet. I initially callibrated it at 40dB but as soon as I hooked up an actual microphone, I knew I had a padding issue.

I take it you're leaning more towards something like a fixed "U" bridge pad on the output somewhere in the neighborhood of 6dB vs. a variable "T" bridge. I'm pretty hell bent on at least using a pair of them on the inputs. I will have just enough space on the front for 4 "T" pads & 2 VU meters. Plus I really want to use them for something.

I was going to try eliminating the 1st stage by making some bypass jumpers out of some old octal tubes, but then I realized I would have to disconnect R3 otherwise I would be putting almost 100VDC on the secondary of the input xfmr. That didn't seem like a smart move.

> max output is 10V p-p.

That's not right. It is rated 600Ω and +28dBm, which is 19.4V RMS, 55V peak-peak. Un-terminated, it might do double that much.

Looks like I smoked the inputs of my lab computer's soundcard that I've been using for testing playback. When I pulled it off the circuit, the output jumped to over 50V p-p. I measured the input of the soundcard at 18 ohms. Kinda brings new meaning to "Creative Sound Blaster". Never fear, that PC is a vintage PII 300 that I keep in the garage just so I don't have to walk back inside to reference something.

Ok so I railed R5, applied 30mV p-p in, and I got 250V p-p out. There's my stinkin 78dB and a hair more. I guess I've proven just how tough this critter is if it'll work with an 18 ohm load.

Here is a revised schematic of the remote power supply. It's the power supply I'm currently using plus a few revisions. Here are my questions:

1. Since the 30K bleed resistor isn't enough load, I wanted to make it so that if the phantom line is disconnected, the power supply shuts down. Note the hot side of the line voltage passes throught pins 7 & 8. Does this seem acceptable?

2. Is this the correct place for the standby switch? I lifted this from a Fender schematic. I've also seen the CT ground switched. I'm not at all a fan of switching grounds.

3. Will the main power indicator light work as shown?

OK

It's almost done. If I can get away from my kitchen remodel long enough to visit my bench, I'll be a happy camper.

Here's the current schematic
http://www.butterylicious.com/WE121A/2/WE121A-Modified8-9.jpg

Does anybody have any suggestions for phantom power on the inputs?

Thanks!
-Richard

Butterylicious said:

http://www.butterylicious.com/WE121A/index.htm
There's the link to the picture gallery.

I finally feel like I have the cajones to tackle this restoration.  I've had the #2 unit (the one with the JAN5693) fired up B4.  I haven't even tested the valves in the other one.  Both units have been slightly modified where in R5 is now a pot and the wiper is connected to the grid of V2 (a little attenuator).  I think each one has had at least 1 canister capacitor replaced.  Obviously none of the tubes are original.

So the questions:


2.  To make a pentode into a triode, do you tie the screen and the supressor to the plate?


Thanks again for all the advice you guys give out.  This site has been a god send for me.  I've learned a ton from just learking about.  I love fixing old equipment & I'm starting to feel like I'm understanding what I'm doing vs. the trial-n-error repair method.

Click to expand...

Typically, some pentodes can be triode configured by connecting the cathode and grid 3 (suppressor) together, and the screen to the plate.