Ampeg B15 woes.... no sound from guitar

[lstead]

Hi Guys,
i am new to the this thread and so let me say that i have some experience with tube amps.......i am working on an Ampeg B15 Flipover bass amp. there is no guitar sound when it is connected. there is a quiet hum, however. i checked the preamp by using the EXT. Amp socket into another amp and that works fine. So, the problem is the power amp side. I have replaced all the caps except the three in one 40Mfd one and also all the tubes. i have an ESR meter that checks out the three in one capacitor and have gone through all the resistors etc without success. Any bright clever thoughts? Could it be the PT? [please say no] laurie

[pontiacpete]

Isn't the EXT for a speaker extension cab?
Maybe you jhave a blown speaker..
Did you check the speaker?

[Jazz P Bass]

The wirring on the jack may be screwy, also.
In troubleshooting a problem such as this, I look at the power tube grids.
If they are getting a nice large Vac signal & the tubes are known to be good, that leaves the output transformer & speaker circuit as suspect.

[Ted]

If this is the 60/70s B15 with speaker connection XLR, not one of the modern reissues?


The speaker connection is via a 4 pin XLR. Pins 2 & 3 (green wire and white wire) have to be shorted to make the amp work.
This arrangement disconnects one of the cathodes of the phase inverter to prevent the power stage from being driven with no speaker connected. If you undo the two large chrome screws holding the chassis to its tray, there should be a schematic there which will tell you more.

If you do not have this connection made you will get a slight hum and the pre-amp output will still work.

I'm not saying that this is definitely your problem, but the symptoms do seem to fit.

On older B15s the B+ ground connection, rather than the PI cathode goes through this connector. (You can get a significant belt off the unconnected cathode too) So be careful.

[lstead]

If this is the 60/70s B15 with speaker connection XLR, not one of the modern reissues?


The speaker connection is via a 4 pin XLR. Pins 2 & 3 (green wire and white wire) have to be shorted to make the amp work.
This arrangement disconnects one of the cathodes of the phase inverter to prevent the power stage from being driven with no speaker connected. If you undo the two large chrome screws holding the chassis to its tray, there should be a schematic there which will tell you more.

If you do not have this connection made you will get a slight hum and the pre-amp output will still work.

I'm not saying that this is definitely your problem, but the symptoms do seem to fit.


On older B15s the B+ ground connection, rather than the PI cathode goes through this connector. (You can get a significant belt off the unconnected cathode too) So be careful.

No, the amp does not have this style of speaker connection; it is hard wired to the PCB and has an external socket for other speakers. to clarify another poster's response, the socket i was referring to was for the EXTernal Amp, not external speaker .
How do i know if the OT has gone down? the screen voltages are 460 so no problem there,.. THANKS for all you guys helping; i really appreciate it!!!

[lstead]

Actually, all the schematics i have seen show a cathode line to the External socket, but the one i have does not!!!!!!! the schematic on top of the fliptop insides has no cathode connection to the ext. socket!!!! i am baffled.........unless it is an OT failure? i am leaning that way, but it is very costly to replace.

[g1]

For starters, what model and year is printed on the schematic? There are many versions of B-15, is it an N, S, or what?
You mentioned a cathode line to the external socket, you mean ext. speaker jack? or ext.amp? Some of them have the ext.spkr. jack wired in series, so if the jack switch does not close, you get nothing from your speaker. Is the hum you hear coming from the speaker or the chassis?
You said you had 460V at the screens, do you have that at the plates?

[Jazz P Bass]

Ampeg B15 Schematic

'Leaning' towards a faulty output transformer is not the most efficient way of repairing the amp.
OPT's can be tested.
The circuit prior to the OPT can be tested.
From Post #3: "In troubleshooting a problem such as this, I look at the power tube grids.
If they are getting a nice large Vac signal & the tubes are known to be good, that leaves the output transformer & speaker circuit as suspect. "
That still leaves exactly which version of the B15 you are working on.
Here are a few of them.
Study the speaker out & identify which one you have.

[lstead]

'Leaning' towards a faulty output transformer is not the most efficient way of repairing the amp.
OPT's can be tested.
The circuit prior to the OPT can be tested.
From Post #3: "In troubleshooting a problem such as this, I look at the power tube grids.
If they are getting a nice large Vac signal & the tubes are known to be good, that leaves the output transformer & speaker circuit as suspect. "
That still leaves exactly which version of the B15 you are working on.
Here are a few of them.
Study the speaker out & identify which one you have.

SERIAL # 100086. B-15N amplifier. screens on Pin 4 read 490v, screens on pin 5 read -50.00volts. hope this helps.

[Enzo]

So B-15N then.

Screens and grids are fine, you got volts on the plates too?

[Jazz P Bass]

SERIAL # 100086. B-15N amplifier. screens on Pin 4 read 490v, screens on pin 5 read -50.00volts. hope this helps.

O/k. Those are the idle voltages.
Actually what I was alluding to is measuring the grid pins (Pin 5) when a signal was being fed into the amp.
Dynamic voltages.
Measure in Volts ac mode on your meter.

[lstead]

yes , i have 475V on the plates.

[lstead]

it still reads -50.00v no change when a signal is applied?????

[Enzo]

Meter on AC volts for signal readings.

[lstead]

using an AC meter shows 000Vs. nothing at all?