Tweet
I'm trying to fix this amp for a friend and I can't seem to find what's causing this. I converted it to a 3 prong power cord and verified that I did it correctly, I grounded the chassis, ran the hot wire to the fuse then switch then transformer, and wired the neutral directly. I removed the wire going to the death cap and removed the cap. Now everything sounds ok, just super quiet. I ran the preamp out into an external amp and sounds fantastic. I checked voltages on all the tube pins, caps, etc. And everything seems ok, if anything the voltages are a bit high. Im going nuts because I just can't seem to find why it sounds great but quiet. I swapped power tubes with a known good pair.
-
-
Does the amp have an extention speaker jack? Check the contacts of the switching part that connects the 8 ohm tap to the speaker when no extention is plugged in.WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
REMEMBER: Everybody knows that smokin' ain't allowed in school ! -
I checked out the jack and it seems to be making a decent enough connection, I also checked the resistance on the output transformer and it read around 1 ohm which is ok. Should I be concerned the the plate voltage is reading close to 500? It's supposed to be around 430. I'm wondering if maybe the 30uf 600v filter cap just pooped out and the power section just isn't getting the current it needs. I also checked the draw on the power tubes and v4 is stable at 36ma and v5 is stable around 31. Those numbers stay about the same with other tubes.Originally posted by loudthud View PostComment
-
How did you measure V4 and V5 current? The high B+ usually indicates that the output tubes are not drawing the normal amount of current. If you are just measuring cathode current, you may have missed an open output transformer winding on one side. The tube with no plate voltage just draws a huge screen current, it looks almost normal at the cathodes. Check for plate voltage on pin 3 of both 6L6s.
The schematic I found (by Joe Piazza) shows a fixed bias output stage. Is that the correct one for this amp?WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
REMEMBER: Everybody knows that smokin' ain't allowed in school !Comment
-
Originally posted by loudthud View Post
the voltage on both tubes shows around 500 volts, and yeah its fixed bias. So you think the transormer might be shot?Comment
-
Since transformers are so expensive, it's a good idea to test them as much as we can before replacing them. One of the most effective ways is to attempt to measure the turns ratio. If we get a good measurement, the transformer is usually good. I would expect the primary impedance to be between 6600 to 4300 ohms. Here are some threads on the subject I found:
Good discussion of transformer resistance checks
http://music-electronics-forum.com/t17775/
RG posts a link to a neon bulb test
http://music-electronics-forum.com/t17678/
Bruce gives a good example and shows the math
http://music-electronics-forum.com/t9119/WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
REMEMBER: Everybody knows that smokin' ain't allowed in school !Comment
-
Its not unusual to see plate voltages getting up towards 500V on B15s. They were designed for a 110V mains and mains voltages have increased to a nominal 120V since then.
Over 500V would be worrying.
Some Blue Chequerboard (65-68) B15s were designated B15NF but have a number of differences from the later black-and-silver models. One of the key differences is that the use a 4-pin XLR to the speaker. 2 of the pins are for the speaker, and the other two make contact when the XLR is plugged into the cabinet. (This is to prevent the O/P stage being driven with no speaker connected.
They close the cathode circuit of the phase inverter allowing it to deliver drive to the output stage. For testing purposes you can solder a jumper onto the main board.
(Thinking back I think I may have seen some early black and silver B15s with this arrangement as well...)Comment
Comment