Schematic for Ampeg BT-25 (obscure solid state head)

[simojo]

I recently picked up an obscure Ampeg head, a BT-25, which is like a solid-state B-25. It's bit smaller physically than a B-25, and it's all-transistor. Component date codes are 1969.

There are three large filter caps inside:
2500uF 90V
2500uF 90V in parallel with a 800uF 160V
There are a bunch of small signal caps as well.

The first 2500uF cap is obviously leaking and needs replacement. All the soldering on the caps looks weird, non-factory. I suspect that some or all of these caps are not stock.

Does anyone have a schematic? Or, the easier question is, does anyone have a stock one and can tell me what the stock values are?

Thanks in advance!

[beans-on-toast]

I don't have a schematic but IIRC, it has the same circuit as the BT-140 and GT-15C. I fear that they are just as hard to locate.

The BT-15 and 18 transistor amps used to have two 2500uF/90V caps and a 0.1uF in parallel and then a 640uF/25V after a dropping resistor. An indication that the 2500uF caps might be correct.

[simojo]

Thanks for the reply.

A dropping resistor between the two caps makes sense. My situation (two big caps in parallel with no dropping resistor) makes no sense. Given the obvious re-soldering, it's likely that someone modded this thing and took a shortcut.

Just curious: You said a 0.1uF cap in parallel with a 2500uF cap. That doesnt sound right. In theory, that's just 2500.1 uF of capacitance. Did I miss something?

Sounds like I should trace out the part of the circuit with the big power caps. I dont know much about SS design, but I imagine the power section is just a lower-voltage analogue to tube design, with multiple sections of capacitance (and resistors between) for improved filtering?

[beans-on-toast]

The three parallel caps add to 5000.1uF. Then there is a dropping resistor, then the 640uF cap.

Don't think of it as a total capacitance but rather their function. The purpose of the 0.1 cap is to catch any high frequency noise that passes through the first two caps.

Here is a link to the BT-15 schematic.

I imagine the power section is just a lower-voltage analogue to tube design, with multiple sections of capacitance (and resistors between) for improved filtering?

exactly.

[joshwilkesbooth]

Have you opened it up yet? Most old Ampegs have the schematic glued to the inside of the head.

[simojo]

Thanks for the schematic. Awesome! I get the cap filtering now. I can probably just remove one of the blown parallel caps for now, just to get it working.

I had no idea what all the blue faceplate-wierdness was. "Electroluminescent Panel" huh? 1969, baby, sounds trippy. I'm not sure it's hooked up, but I'll check for sure.

The cabinet was refinished, so sadly, no schematic inside.

Thanks for all the help!

[KramerBassFan]

I want gut shots!

[karma78]

hello everybody,

Does anybody already have adjust the bias of a BT15 / BT25 head amp ?

Can someone tell me where the 2nd Pot. (r812 15K) is located ?

thank you for your help

Jim - France