• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Capacitors

Did you watch the World Series?

  • Total voters
    11
dlbraly

dlbraly

New Member
Hello,
I have a few questions about Capacitors and reading the schematic.
I know this is probably on the forum, I've searched just didn't see it.
I have a tube amp (GE 752), and want to replace the original capacitors.
Attached is the schematic with parts list.

1. When I look at the parts list it says (mfd, mmfd, or mmf) Im a little confused exactly which capacitors to buy. I assume it has something to do with the decimal point, but Id hate to buy the wrong ones and stick them in there.
Example:
.005mfd, 600v
47mmfd, 500v
47mmf - Ceramic

2. Should I even replace the ceramic capacitors?
3. Where do you like o buy capacitors?
4. Whats a good brand of capacitor to go with, I see a bunch of different shapes, sizes, and colors.
5. I would like to put a 1/4 input jack in it. where on the schematic should I place it?

Thank You

shout out to tensleep
 

Attachments

Register to hide this ad
dlbraly said:
Hello,
I have a few questions about Capacitors and reading the schematic.
I know this is probably on the forum, I've searched just didn't see it.
I have a tube amp (GE 752), and want to replace the original capacitors.
Attached is the schematic with parts list.

1. When I look at the parts list it says (mfd, mmfd, or mmf) Im a little confused exactly which capacitors to buy. I assume it has something to do with the decimal point, but Id hate to buy the wrong ones and stick them in there.
Example:
.005mfd, 600v
47mmfd, 500v
47mmf - Ceramic

2. Should I even replace the ceramic capacitors?
3. Where do you like o buy capacitors?
4. Whats a good brand of capacitor to go with, I see a bunch of different shapes, sizes, and colors.
5. I would like to put a 1/4 input jack in it. where on the schematic should I place it?

Thank You

shout out to tensleep
Click to expand...

This is NOT an amp, but tabletop AM radio.

mfd means uF
mmfd means pF
ceramic should be replaced with mica or high stability ceramic C0G (NP0) type
you likely need to replace all trimming capacitors with equivalent (watch for voltage rating)

You will need RF signal generator to alight receiver part of this radio.
 
mfd and mf are short for Micro-FARAD (and not the expletive). mmfd is micro micro farad or 1 millionth of a microfarad. pf is pico farad and is the new equivalent of mmfd. uf is the new equivalent of mfd. nf is nanofarad and is in the middle

1uF is same 1,000nF or 1,000,000pF.

There are converters out there that can help you with figuring out what the actual values are. google "capacitor value converter."
Back iun the Early 70's the manufacturers changed the values from a multiple of ten (0.0, 0.5, 1.0) to a more value driven set. As tolerances were all over the place (some as high as +90% and -50%) they standardized on 20% nominal with 10%, 5%, or 1% as higher cost solutions.

So for your examples above.
.005mfd, 600v .0047uf, 630V Use a film cap, and here it depends on whether in the power supply or the signal path as to what you actually get.
47mmfd, 500v 47pf, 600v Film or Mica cap.
47mmf - Ceramic 47pf, unk.

Ceramics very rarely if ever go bad. If you DO replace them you need to learn about the rest of the writing on them as there are different tolerances, voltages, and temperature co-efficients to throw into the equation. Get the wrong one's and you could damage the unit. If it tests good, leave it.

Honestly, the GE is a Radio phonograph. It's not really suitable for Guitar work (if that's what the 1/4" jack is for). Rehab the radio as a practice exercise, then find an amplifier that will allow the substitution of an RCA input for the 1/4" jack.
 
Thanks for all the comments.
yeah, obviously Im new to this hobby (I have a ton of hobbies).
Looking for something to break into the hobby, and this is my first.
So I won't stick a 1/4 guitar jack in it this time. Its a really nice looking console, just needs some love.
 
Last edited:
If you replace the mmf values, probably. On the up side, those are usually not a type that goes bad. I would not advise replacement of those parts or even moving them if you can help it. The stuff in microfarads isn't in the tuned RF portion so it won't have much effect. Do try to get the parts and wires put back as close to their original position as possible though. Some radios are really cranky about that kind of thing.

Also, honestly for AM radios you can align them by ear if you have a couple stations at known frequencies on the dial. The oscillator you just set the dial to the radio station frequency and tune it in. The IF strip you can do by ear, tweaking it for peak signal. Or just don't poke at it if the thing is working and you aren't comfortable.
 
You must log in or register to reply here.
Back
Top Bottom