New find, old amp.

[davidwpack]

Hey guys. I happened to be in the right place in the right time and scored these 2 nonworking Ampeg B25s for free. The top one appears to be quite clean but the bottom one has some issues. These were kept in storage for a while and the pots are frozen solid. They're all 1 M but I can't seem to find exactly these pots anywhere. I'm considering just getting regular pots with solder lugs and just running some heavier type buss wire to the pcb. Any suggestions about this? The can cap and giant electric sausage caps are crusty and need to be replaced but I think the pots might be the only issue of getting these back to working condition. Anyways here's a couple of pics. If anyone has some suggestions it would be really cool. Thanks!....Dave

[John]

That's how I'd do it. First I'd take the pots apart though, see if I couldn't free them up.

[davidwpack]

It looks like someone has tried that with 3 of them. Parts are missing from a couple housing etc. I think the shaft fell out of 2 of them. Another issue was that it took a 7199 tube in the preamp which is all but defunct but I found that someone makes an adapter to accept a 6U8. This is a very interesting amp.

[2deaf]

6GH8 is a typical replacement.  Adapters are available.

[davidwpack]

Thanks. I'll check into that.

[pompeiisneaks]

You could also just use regular pots and wire jumpers from them to the board, possibly easier than trying to find exact replacements.

Edit: and I'm very jealous, great find!

~Phil

[davidwpack]

Thanks. I'll probably start working on it this coming week. I'll post any progress I make.

[davidwpack]

Update: new caps installed. Getting ready to power up with tubes installed. Looking at the schematic it looks like the polarity circuit is in parallel with power in so I simply clipped it out. When turning the power switch on,  in standby, I'm getting about 180-200VDC on pins 3&4 of the 7027A. Flipping the standby switch, the voltage comes up the rest of the way in the normal range. Weird. Standby switch is halfway failing? Anyways, back to more voltage readings...Dave

[PRR]

> Looking at the schematic

Got a copy to link-to to show us?

The half-voltage strongly suggests the power supply is mis-wired. For more fun, Ampeg often put the SB switch in the negative leg. If you (or a previous tech) has not kept it straight, it can act very funny. (Better what you have, than the time I mis-wired my VT40 to make 1,150V......)

[davidwpack]

Success! This amp sounds phenomenal. Yeah this is pretty much it. The power supply looks identical. http://https://www.google.com/search?q=ampeg+b25+b+schematic&oq=ampeg+b25+b+schem&aqs=chrome.1.69i57j33.26600j0j4&client=ms-android-uscellular-us&sourceid=chrome-mobile&ie=UTF-8#imgrc=c2i-E2ZCS7zhhM:

[davidwpack]

Don't Know why link doesn't work. It's an Ampeg B25-B.

[PRR]

> Don't Know why link doesn't work.

#1 - there are both http and https on the front of the URLs. That's no good.

#2 - you linked to a Google search page, not any specific result on that page

(#3 - the URLs have stray crap about your web device and internet carrier, which probably does no harm, but no good.)

I agree that cell-phones are terrible web-tools. Can you find it on a real computer and capture the specific link?

I have seen several B25 plans that may not all be the same. Here is a B25-B; is this authentic?

[pompeiisneaks]

I edited the link to make it work, removed the http://https// and changed it to just https:// and it loaded, The first result was:
http://www.kbapps.com/audio/schematics/tubeamps/ampeg/images/b25.gif

Is that it?

[davidwpack]

Yep. That's it. Thanks for fixing the link! The only difference I see is that there's a second pilot light on mine. When you take it off standby it activates the second pilot light.

[davidwpack]

Btw, can you make out the symbol above the diode in the bias supply? It looks the same as the schematic attached to the amp but I can't make it out.

[shooter]

can you make out the symbol above the diode in the bias supply

F4

[davidwpack]

Yeah, that's what it looked like to me too.

[tubeswell]

You could also just use regular pots and wire jumpers from them to the board, possibly easier than trying to find exact replacements.

Edit: and I'm very jealous, great find!

~Phil

And probably less prone to causing damage than trying to screw the original style PCB-mount pots back into the chassis.

[sluckey]

I believe F4 is the diode part number. Use 1N4007 today.

[davidwpack]

It does look like a 1n4007. It was giving weird readings too till I figured the standby switch is wonky. Turns out it's fine. Overall I replaced a couple caps, can cap, tube socket, 2 fuses, 5W resistor, 3 pots, and tubes. Not too bad. It sounds great. Still haven't played bass through it yet though.

[Joel]

Those style pots are used pretty regularly in the stompbox world.

http://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=12_44&products_id=1381

Its a 16mm pot, not 25mm like int he pic, but the legs are of the style you're after.

[davidwpack]

Thanks! I'll check into that for the second  Ampeg I'm getting ready to start on. I kinda like the normal pots and just running the buss wire though.