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Got Bassman going but not loud and distorts easily...

6/14/2004 8:38 PM
Bill Harbour
Got Bassman going but not loud and distorts easily...
Got my Bassman clone together tonight and am getting sound ok but doesn't get loud and distorts with volume about halfway up. Any advice on where to start troubleshooting? I haven't checked any voltages yet except for B+ and bias voltage.  
6/15/2004 10:13 AM
- how's the bias current? sounds like it may be cold.  
- post up some voltage readings, that would help a lot.  
- if you have a signal generator (or a stand in for one) inject a signal into the PI and see what you can get out of the amp. If that's working right, you know you have a pre-amp problem.
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6/15/2004 12:04 PM
Bill Harbour

I will post some voltages as soon as I can get them. One thing I did differently was to use a single 1/4" female speaker jack instead of the 4 RCA plugs. I connected the feedback lead to the positive side of that jack. I don't suppose that could cause my problems could it?  
6/15/2004 8:19 PM
Bill Harbour
I built the 5F6 variant..not the 5F6A.  
Pin 1 & 6 163v & 167v  
Pin 3 2.79v  
1st 12AX7  
Pin 3 1.23v  
Pin 6 330v  
Pin 7 187v  
Pin 8 189v  
Phase Inverter 2nd 12AX7  
Pin 1 379v  
Pin 2 271v  
Pin 6 377v  
Pin 7 273v  
Pin 8 324v  
These all seem too high!  
Pin 3 both tubes 430v  
Bias supply -48v (fixed bias)  
The amp just doesn't seem loud enough for a 50w amp...seems just about as loud as a 22w amp.  
Pin 3 both tubes 430v  
I'll try to get some current readings on the power tubes.
6/15/2004 8:37 PM
ok.. you gots a problem. ;) should not be bad to solve though. IMHO you're 100% correct about the voltages being too high on the PI. You're saying you built the 5F6, not the 5F6-A. How did you ground the cathode on the PI? The cathodes tie together on the socket, then there is the 470-ohm Rk in series with a 10k resistor to ground. Total potential at the cathode should be around 35V (give or take), and the difference between the junction of the 10k and the 470 and the cathodes themselves should only be about 1.5V. If this is off, my bet is your grounding is AFU. Also pin 7 should be run through a .1uF cap to ground so check it's ground. (if you used the orig layout, these are both the same ground point..Hmm.) General rule of thumb here at OEI is when voltages are too high, someone is not drawing proper current which usually means they have a bad ground. This results in very high voltage readings on the various pins. You have 324V on the cathodes.. that's pretty damn high. ;) Check your grounding there.  
Output jack style should not matter as long as you have not reversed the leads or shorted hot to ground etc.  
Did you run the -fb into the tone stack as on the original?  
hope this helps!  
- Joe
6/16/2004 5:26 AM
Bill Harbour

Joe...will check on all this when I get home today. I used the orig layout for sure but I'll have to check the grounding to the PI.  
The -fb is per the orig drawing.  
6/16/2004 10:07 PM
Bill Harbour
Speedracer...looked at grounding but....
I don't think the ground to the PI is the problem. I took it loose and resoldered elswere to ground and the same problem....high voltages everywhere on the PI. I took the PI tube out and checked the voltages and there was no high voltage anywhere except Pins 1 and 6?!?! Go figure? Could I have a bad tube socket? Seems like if it were bad that I'd get high voltages everwhere with the tube removed. I tried different 12AX7's with the same result!  
I'm about to start pulling out my hair!  
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