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Discussion Starter #1 (Edited)
I just finished a build on a Fender MusicMaster Bass amp. It's quite simple, but unique in that it used a transformer as a phase inverter rather than a tube.

I fired it up for the first time today, and it makes a loud hum, through the speaker, level of the hum is adjustable with the volume knob. When the volume is fully off, there is no hum. Tone control effects the hum. There is a little amplification on a guitar. The sound is very weak/thin and always well below the volume of the hum.
Hum at full volume is about 85 db.

I traced the schematic twice, everything seems to be in place. Voltages are a little high, but not extreme. Power tubes seem to be making a lot of heat, more than what I'm used to. B+ spec is 295 volts, I'm seeing 323. The first resistor is a 1K and it only drops it to 318 volts. Next resistor is 10K and it only drops the voltage to 294. The next resistor inline drops it to 290. That increased voltage is the only thing I can see wrong. I checked the input jacks, and they are functioning (38K on one, 78K on the other, measured just before the first cap). Only other weird thing is the pilot light doesn't light up. I measure 6.6 volts across the terminals, it's a #47 bulb, but it doesn't light. Heater voltage is good at all the tubes.
Here's the schematic I used
MusicMaster Bass3.jpg
Here's what it looks like
MusicMaster Bass1.jpg

Welcoming any and all help, questions, suggestions, thoughts.
Thank you
 

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Discussion Starter #2 (Edited)
I strayed from the schematic a little, I used a cap can rather than mounting the two 20 mf caps on the board. (see empty spaces at power end of board)

I also modded the tone circuit, following a 5F2 Princeton design. 1 meg pots. Touching almost any power supply wire with a volt meter probe makes a sound from the speaker.
 

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Discussion Starter #3
OK, I did the virtual center tap trick and got rid of most of the hum. But I still have very little amplification. This is a 12 watt amp and it's only about as loud as fender amp on the very lowest setting that makes noise. Sound is still very thin/weak too.
 

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It took me a while to find a strand from the twisted power input wire out loose and touching ground, it was under everything. You’ll find it sooner rather than later I bet. Nice looking build.
 

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Discussion Starter #6 (Edited)
Can you post DC voltages for:
either 6V6 pin8
12AX7 pins 1,3,6, & 8
Thank you sir :)

Both 6V6 pin 8 - 19.47 volts

12AX7 pin 1 - 293 volts
pin 3 - 2.33 volts
pin 6 - 0 volts
pin 8 - 1.67 volts

I think the only reason pin 1 on the 12Ax7 is so high is because one primary lead of the phase inverter transformer is tied in there. The other primary lead is on third step down of the B+
 

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Ok. The 6V6's and the (1/2)12ax7 driving the tx are a little hot but that can be sorted later. (change to larger value cathode resistors)
12AX7 pin6 0volts? Can you recheck? Pin 8 voltage says it is running but you should have around 200V at pin6.
 

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Discussion Starter #8
Ok. The 6V6's and the (1/2)12ax7 driving the tx are a little hot but that can be sorted later. (change to larger value cathode resistors)
12AX7 pin6 0volts? Can you recheck? Pin 8 voltage says it is running but you should have around 200V at pin6.
Zero volts on pin 6 of the AX7. That wire goes to the input jacks, there's no power there. DC or AC.
I better look up some different schematics. There must be an error on the one I'm using.
 

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Discussion Starter #9 (Edited)
wow, that took a whole 15 seconds! Looks like the schematic is wrong. I found a corrected one on EL34 world. Thank you @jb welder

Oops.....hold the celebration. I can't count. Pin 6 is 192 volts, pin 7 is zero. :(
 

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Well, nothing grossly out as far as operating voltages (spec. is often +/-20% for vintage stuff).
So it appears to be a signal issue. Suggest you triple check the tone mod, or totally disconnect the tone circuit and just run a volume for starters.
Can you put a probe (just to act as noise source, other end disconnected) at the 12AX pin2 and bring up the volume? Do you get a loud hum? Does the volume control affect it?
 

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Discussion Starter #11
Well, nothing grossly out as far as operating voltages (spec. is often +/-20% for vintage stuff).
So it appears to be a signal issue. Suggest you triple check the tone mod, or totally disconnect the tone circuit and just run a volume for starters.
Can you put a probe (just to act as noise source, other end disconnected) at the 12AX pin2 and bring up the volume? Do you get a loud hum? Does the volume control affect it?
Voltage probe on pin two of the AX12, makes a loud hum. The hum is off at 1 on the volume pot, increases to maximum at 5 or 6 on the pot, and then decreases until 10.
 

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So better volume than you were getting from the input? So far so good I think.
I would guess if you do the same test with the probe at pin7, the volume level is poor? If so, again the tone circuit is suspect.
If you do get good volume with probe at pin7, the input jack circuitry is suspect.
 

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Also, if you can draw up the modded section of schematic, or post some close up pics of input area, circuit and pots and 12AX7 socket, that would help.
 

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Discussion Starter #14
Eliminating the tone pot from the circuit had no effect on volume. I replaced the volume pot because the one in there was only dropping to 70 ohms minimum. I put in a pot that dropped to 1 ohm minimum. Again no difference in volume.
 

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Discussion Starter #16
So better volume than you were getting from the input? So far so good I think.
I would guess if you do the same test with the probe at pin7, the volume level is poor? If so, again the tone circuit is suspect.
If you do get good volume with probe at pin7, the input jack circuitry is suspect.
Pin 7 probed also makes a hum, not as loud as pin 2 does. Also the quality of the hum on pin 7 is not there. It's not a good solid noise, and it's almost like I can hear a radio station under the hum. There is no interference of any kind on pin 2.
 

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Discussion Starter #18
I tried a new 12AX7 just for shits & giggles, no change. I for sure have a radio station playing when I probe both pin 7 and pin 8.
 
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