Ampeg B-12-N resto

[CJ]

3rd victim i have worked on from a salt water flood due to king tide and low atmospheric pressure at Puget Sound, home of the famous killer whales and man eating giant octopus,

i hope those funky tone chips are going to be alight, (by the pots)

 

[CJ]

gonna take a complete strip down, output transformer is sealed in black tar so hoping for the best, power transformer was an aftermarket splice in so we get rid of that,

 

[CJ]

swapping out the pots on these tone modules was a real pain.

some people wire in discrete components and claim better sound, but i bet the customer wants it stock since this is a rare classic,

 

[CJ]

a can of deoxit and a few days later it is time to rebuild,

old tube sockets were salvaged but had to drill rivets to remove,

 

[CJ]

schematic on mounting plank,

3 input jacks, 470 K plate resistors on stage 1 and 2 are what make this model a bit different.

tubes were dragging 120 ma each! B+ dropped to 346 v, now what?

7 volts on 250 ohm cathode cap, wtf?

that suggests 14 ma for each 6L6 but they are as hot as heck!

well, the new power transformer had a heater CT so i grounded it like any normal idiot.

look close menso, notice the point "K" on the cathodes, runs to the hum cancel pot, i was sinking all the current through the 100 ohm wire wound pot , through the heaters to ground.

clipped the pot center wire and plate volts popped up to 435, and bias to about 32 v, yippee!

even the schematic done got tin roof rusted,

 

[CJ]

had to clip into the output circuit as this amp uses the crazy 4 pin XLR jack ,

lights up like a Christmas tree,

 

[CJ]

replacement power XFMR is like 328 bucks, so i took a guess on this Hammond and it worked out fine, 5 volts off on B+, it has a 115/125 primary tap in case heater voltage runs high, so we used white/blk = 125 tap ,
\
heaters still at 6.6 and 5.3 but that is ok,

Hammond 273BX is good for 201 ma which is better than the stock replacement, and only $125, made in Canada so i am not going to bad mouth Hammond anymore.

cap got shorted out by the salt water so we replaced that also.

 

[CJ]

Tungsol 6SL7 preamp and PI tubes, cleaned up pretty good considering 3 days submerged in the brine, new CTS pots, changed the 1 Meg linear vol pots to log because linear sucks,

the switches come apart if you uncrimp them, so was able to keep all 3 stock

nothing connected to ground switch, cap removed, (3 prong cord used)

 

[GussyLoveridge]

What's going on with those (neon? incandescent?) bulbs? What's that all about? I think maybe I'm seeing one in parallel with the heaters on the schematic but there seems to be two in the chassis?

 

[CJ]

Those light up a plastic panel with Ampeg written on it. There is a slot in the chassis for the panel. They are regular ge #47 lamps.

This thing sounds muddy. I think a 6SN7 in the phase inverter might clear it up a bit. (Stock is 6SL7=12AX7)

 

[soapfoot]

I recently did a full rebuild on a B-15N. PEC couplates were just fine in mine, and got reused (along with the C-D "Greenies"). The rest of the amp was a basket case and almost everything else got replaced (except for the eyelet board, tube sockets, and transformers).

It's sounding good, if a *touch* murky. It's got me interested to try your 6SN7 suggestion; I happen to have one sitting around here so I may give it a bash

 

[CJ]

yeah those cd greenies are rated 400 v, and B+ is 435 but i saw no positive grid volts on the pwr tubes

now i got a fender super sonic with channel switching noise, ugh,

 

[Riley Casey]

Those light up a plastic panel with Ampeg written on it. There is a slot in the chassis for the panel. They are regular ge #47 lamps.

This thing sounds muddy. I think a 6SN7 in the phase inverter might clear it up a bit. (Stock is 6SL7=12AX7)

It was popular back in the day to replace the Ampeg logoed plexi panel with a personalized version, name , initials, band , that sort of thing.

 

[soapfoot]

@CJ can you expand a bit on the 6SN7 sub for the phase inverter? What is it about that tube that may help the bottom end tighten up a touch?

 

[dmp]

I recently did a full rebuild on a B-15N.

What year/era B15n? I have the '70s version with rocker switches

 

[CJ]

1964 on this amp.

6SN7 will cure the early breakup a bit

solid state rects will tighten up the bass

 

[soapfoot]

What year/era B15n? I have the '70s version with rocker switches

It was a mishmash of parts cobbled together, but the chassis and eyelet board were 1964.

Some parts were later