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Ampeg B25 layout?

Voodoo Guitar

Voodoo Guitar

Guest
Oct 12, 2016
29
22
#1
Hey all, i'm restoring an original B25 and getting lost in the convoluted circuit board... mainly, I can't find a few of the components! The 10k 5w & 47k near the 40uF filter caps don't appear to exist on this amp. Also, the "death cap" was actually mounted on the main board, which I found odd... and it was cracked open, to boot. Removed that and replaced all the filter electrolytics, wanted to check the resistors as well, but can't do that if they're not there...
 
#2
You can see the 10K/5W to the far right here. I replaced the polarity cap with a safety cap (the large blue one) but you can just remove it and disable the switch.

pukG32u.jpg


I don't think I have pictures that will allow me to trace down the connection on the 47K. The cap connects to the PCB at the lower side in this image (original layout before I started the recap here), the traces go across the board from there so you'll have to follow them to the right spot

zxB5Qkm.jpg
 
#6
coreyfyfe said:
The original wirewounds were the tubular kind. You can find those if you want to get a similar style but the cement block ones are a bit more common to find these days.
Click to expand...

Looks like mine are still good, barely any drift, so i'll leave em. the 250 ohm, looks like that was used as a failsafe in case the amp was turned on without a speaker, or just using the preamp put jack?
 
#8
The circuit board looks to be in good shape. I find that they generally need a good cleaning. Re-soldering all the joints helps but it has to be done very carefully. Not too much heat, especially on the shielded wire and other wire connections. Those fragile wires can be brittle, often the lightest gauge wires need to be cut back to fresh metal.
 
#11
Voodoo Guitar said:
I may just remove it entirely, as I think I'll redo the output connections (remove original 4-pin XLR, add 16/8 switch).
Click to expand...

I rewired mine to separate 8 and 16 ohm jacks, a switch works too. The 16 ohm tap is also the connection for the negative feedback circuit and I think on mine the 250 ohm resistor was wired to the same point. Shouldn't be an issue to remove but unless there's a problem with it I'd just leave it.
 
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#12
The resistor serves to protect the OPT and power tubes in the event that you turn on the amp without the speaker cab connected. It will work long enough for you to realize that something boneheaded was done. Inexpensive insurance.

A number of amps employ this feature. It doesn't affect the impedance of the speaker load. Calculate a parallel 250 ohms and 8 or 16 ohms. It yields about 8 or 16 ohms.
 
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#14
coreyfyfe said:
I rewired mine to separate 8 and 16 ohm jacks, a switch works too. The 16 ohm tap is also the connection for the negative feedback circuit and I think on mine the 250 ohm resistor was wired to the same point. Shouldn't be an issue to remove but unless there's a problem with it I'd just leave it.
Click to expand...

Just wired up a switch with one speaker jack, since that's what I had handy... question re: negative feedback and dummy load, i should be able to move them both to the + lead on the speaker jack at this point with no issues, rather than both coming from the 16ohm tap directly, eh? Ideally i'd want a smaller resistor, say 33k, on the 8ohm NFB, but i'm not sure it would make a huge difference in sound.
 
#15
I just left mine on the 16 ohm jack, with the switch I'd leave them connected to the 16 ohm winding at the switch. If you have separate connections for the feedback and dummy resistor (I don't think this is the case in these amps?) you could wire the dummy load to a switching contact on the jack.
 
#17
Thanks for all the tips guys! I stuck with the NFB off the 16ohm tap and left the dummy load resistor alone. Powering up, all the voltages look good after a recap with 6L6s for now.

I'm afraid the 7199 PI tube is shot... first it wasn't lighting up at all, then after a reseating it worked fine for a bit but began making some serious noise after 5 mins or so (along with a faint burning smell, no smoke). With preamp tubes pulled, still made noise after a few chopstick taps, then wouldn't light again next power up. Heater voltages are fine throughout. Thoughts? Planning to try a 6U8A w/Tube Depot converter socket.
 
#18
Voodoo Guitar said:
Thanks for all the tips guys! I stuck with the NFB off the 16ohm tap and left the dummy load resistor alone. Powering up, all the voltages look good after a recap with 6L6s for now.

I'm afraid the 7199 PI tube is shot... first it wasn't lighting up at all, then after a reseating it worked fine for a bit but began making some serious noise after 5 mins or so (along with a faint burning smell, no smoke). With preamp tubes pulled, still made noise after a few chopstick taps, then wouldn't light again next power up. Heater voltages are fine throughout. Thoughts? Planning to try a 6U8A w/Tube Depot converter socket.
Click to expand...

It could be the tube but I would check the tube socket carefully. If the tube is not sliding in and out with some normal friction, the socket contacts should be retensioned. With the tube removed, carefully pinch the tube pin contacts closed with a small jewelers sctredriver or a needle. Inspect the contacts to ensure that they are clean with not brown or black depositits, no oxidation. If they are dirty use deoxit. Next I would reflow the socket solder joints. Clean the entire area carefully, sometimes a small strans of wire or a solder bridge can be under the socket, above the board. I find that a pipe cleaner soaked in isopropyl alcohol (99% from a pharmacy) is good for cleaning under sockets.

If you go with a 7199 adapter, check that it will fit in the amp. Sometimes the tube extension can be a problem height wise. It should be alright in this case. Also check out this tube adapter: Invalid Link Removed and tube as a substitute. I haven't tried this substitution.
 
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#19
Thanks for the great info!!

Just tried all of that... cleaned socket contacts with deoxit, tightened with screwdriver, reflowed solder on all pins... seemed to help with the inconsistent filament heating, but the noise still starts up after a few moments of playing. Damn shame, since it sounds great for those first few moments.
 
#20
Voodoo Guitar said:
Thanks for the great info!!

Just tried all of that... cleaned socket contacts with deoxit, tightened with screwdriver, reflowed solder on all pins... seemed to help with the inconsistent filament heating, but the noise still starts up after a few moments of playing. Damn shame, since it sounds great for those first few moments.
Click to expand...

You've taken all the reasonable steps. Sounds like the tube is heating and something is going intermittent. Noise is a sign of a bad tube.

The 7199 tubes cost so much these days. Often around $60.
 

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