High voltage in a Blackface Bandmaster

[guitarjesus]

Years ago I was a dumb kid, couldn't afford a Marshall and I let con-man Dan Torres take my BFBM and replace the Ajax caps with Orange Drops, put a JTM 50 watt output transformer and EL34s in it. Yes, I was dumb, BUT I was 15.

NOW, I'm putting my amp back to stock and slowly getting parts back.

Poking around in the amp with a volt meter some of the measurements are way off. On the vibrato tube the schematic is calling for 440v going into the 220k plate resistor and 280v coming out of the plate. I'm getting 435 after the plate resistor, and the resistor is testing good at 227k, this is at pin 1.  I have no voltage on the cathode at all.  The voltage at pin 6 is 390v.  If I pull the tube, the voltage doesn't change, so there's no current draw it would seem in that tube socket (Ive tried other good tubes). I've pulled the first two preamp tubes and the voltage shoots up to the same 425-435 range when not plugged in, but drops to the 270-290v range when the tubes are in the socket.

What should I be looking for to remedy this? An extra 150v going to a 12ax7 can't be good.

[sluckey]

The Vibrato/tremolo is switched off. Your voltages are correct for switched off mode. Plug in your footswitch and turn the trem on. If you don't have a footswitch just ground the tip of the VIB footswitch jack. You can use a gator clip test lead or even make a dummy RCA phono plug with the tip connected to the shell.

[guitarjesus]

The Vibrato/tremolo is switched off. Your voltages are correct for switched off mode. Plug in your footswitch and turn the trem on. If you don't have a footswitch just ground the tip of the VIB footswitch jack. You can use a gator clip test lead or even make a dummy RCA phono plug with the tip connected to the shell.

That did the trick.  You sir, are a gentleman and a scholar!  Now to find the source of scratching and popping, find a 125a6a transformer and some 60's molded blue capacitors.

[Ed_Chambley]

The Vibrato/tremolo is switched off. Your voltages are correct for switched off mode. Plug in your footswitch and turn the trem on. If you don't have a footswitch just ground the tip of the VIB footswitch jack. You can use a gator clip test lead or even make a dummy RCA phono plug with the tip connected to the shell.

That did the trick.  You sir, are a gentleman and a scholar!  Now to find the source of scratching and popping, find a 125a6a transformer and some 60's molded blue capacitors.

The blue molded caps are a nice addition, but are not a big value killer.  I would replace the orange drops in a vintage amp to axial caps.  Solens are usually embraced fairly well to collector value.  Another option is SoZo blue, which lots of people like because they are almost the same color but since they are called SOZO, everything you play will sound like Jimmy Page!

[guitarjesus]

The Vibrato/tremolo is switched off. Your voltages are correct for switched off mode. Plug in your footswitch and turn the trem on. If you don't have a footswitch just ground the tip of the VIB footswitch jack. You can use a gator clip test lead or even make a dummy RCA phono plug with the tip connected to the shell.

That did the trick.  You sir, are a gentleman and a scholar!  Now to find the source of scratching and popping, find a 125a6a transformer and some 60's molded blue capacitors.

The blue molded caps are a nice addition, but are not a big value killer.  I would replace the orange drops in a vintage amp to axial caps.  Solens are usually embraced fairly well to collector value.  Another option is SoZo blue, which lots of people like because they are almost the same color but since they are called SOZO, everything you play will sound like Jimmy Page!

I'm just wanting to get the amp back to where it was.  The Sozo caps are only an investment of $40 so I didn't care about the name per se, but at least in Marshall circles they have a good reputation.  The biggest thing is going back to an original era OT and 6L6 tubes, the JMP 50 OT and EL34s ruined what was a good amp.

[Ed_Chambley]

The Vibrato/tremolo is switched off. Your voltages are correct for switched off mode. Plug in your footswitch and turn the trem on. If you don't have a footswitch just ground the tip of the VIB footswitch jack. You can use a gator clip test lead or even make a dummy RCA phono plug with the tip connected to the shell.

That did the trick.  You sir, are a gentleman and a scholar!  Now to find the source of scratching and popping, find a 125a6a transformer and some 60's molded blue capacitors.

The blue molded caps are a nice addition, but are not a big value killer.  I would replace the orange drops in a vintage amp to axial caps.  Solens are usually embraced fairly well to collector value.  Another option is SoZo blue, which lots of people like because they are almost the same color but since they are called SOZO, everything you play will sound like Jimmy Page!

I'm just wanting to get the amp back to where it was.  The Sozo caps are only an investment of $40 so I didn't care about the name per se, but at least in Marshall circles they have a good reputation.  The biggest thing is going back to an original era OT and 6L6 tubes, the JMP 50 OT and EL34s ruined what was a good amp.

I would bet ole Dan Torres got a nice chunk of change from those old parts.
No worries, hindsight is 20 20. I sold a jtm45 100 watt and a Marshall 8, 10 cab when I was 19 for $250 to have the money to wine and dine this gal. I did not get laid, spent my money and lost an amp.


What can you do but move on.

[guitarjesus]

Found the scratching, banging noises when I barely touch the volume.



Luckily I have a backup from my Marshall to use.