JMF Spectra 30-T Bias issues

[01redcobra]

I am having similar issues with the bias voltage being too low as in this post: http://music-electronics-forum.com/t31531/
This amp came in with one blown 6V6 and running very hot. I replaced the tubes and haven't been able to get the bias current down. It's running at close to 100% plate dissipation. In the 30-t there is a 47 ohm resistor that regulates the ac voltage available. I am going to replace it to get the neg voltage up a bit. Right now, the a/c voltage is at 34vac and my bias voltage is at -42v with the trim pot all the way down. My only concern is that the 18v power supply feeds off the same circuit. What is the max voltage that I can allow on that circuit?
There is also a note in the schematic ("bias mod") that says there is a 47K resistor off the wiper leg of the trim pot that this amp doesn't have.
jmf-spectra-120-amplifier-schematic.pdf

[Leo_Gnardo]

Just had a 30-T in with even worse issues - the PT was fried completely. The basic problem with these amps is they tried to run the switching logic, chip supply, and LED's all off the same bias winding = doom for the PT. I found a potential solution but the owner bagged it and we didn't go thru with the repair. Maybe you'll see the "logic". Let the bias winding run the bias, only, as it's designed for. (On the one I was trying to fix, replace the roasted PT with a Princeton PT, re packaged in the original PT's bells.) Add a separate small low voltage transformer to run the channel-switcher & +15 & -15V supplies. They're quite good sounding and very compact amps but just have this Achilles' heel in their design.

[01redcobra]

Leo, Sounds logical to me, but again the owner may balk. Right now I'm seeing 18+ and low -19, if I remember, for the 18v supply. I have the plug off the board for the supply so nothing else might fry. With the lower value resistor on the A/C, I can now get the tube bias in the 20-25ma range. There is a loud hum with the plug for the 18v supply off and I hope that goes away once I hook that back up. But until I have assurance that the voltage won't damage the circuit, it will remain unplugged. In watching the bias voltage, I did notice a lot of variation to the one tube and found a bad .1uf coupling cap which I repaired.

[g1]

That schematic for the bias and low voltage is just wrong. Neither would work as drawn, the bias would be + and there would be no negative low V.
The 15K from the bias pot to ground is where the bias voltage is getting dumped to ground. Increasing the value from 15K would increase the bias voltage. That would probably be preferable to reducing the 47ohm AC feed resistor.
There is very little current flow in the bias circuit (aside from the dump through the 15K), so you should not have to worry about loading down the negative low V supply.

[Mark Black]

Yes the rectifier diodes are drawn reversed - same issue with the 60T and 120T schematics.

[01redcobra]

Thanks for all the input guys, but the question remains, is 18-19v too much at points F and G in the schematic? I really don't want to knock anything out on that board.

[g1]

There are 2 diodes shown at points F and G (across the 470uf caps). I would assume they should be zeners, probably 15 or 16 volt. Check for numbers on them, if they are 18V, then that is probably what was intended.
But +/-j18V seems a bit high for the IC, maybe it has some dropping resistors? Are you measuring the 18 at the IC as well as at points F and G?

[01redcobra]

The 18v is at points F and G. Originally when I checked the amp the voltage was around 18v for both points, Now it is closer to 19. I will check to see if there are any numbers on the diodes.

[01redcobra]

All I can get off the diodes is IN47 the way they are positioned. I'll try and get some voltages at the IC and transistors.

[Jazz P Bass]

All I can get off the diodes is IN47 the way they are positioned. I'll try and get some voltages at the IC and transistors.

1N47 is a zener diode.
There are two numbers at the end that tell you what the zener voltage is.
http://www.cndiodes.com/manage/produ...0762623397.pdf

[01redcobra]

Of course, the last 2 numbers are facing the board. I did check voltage at the transistors and IC on the board and it's hanging around 17.3-17.5

[g1]

Is the IC running on split supply or single rail?
If it is single supply, then 18V would be ok, check pins 4 and 8 of the IC.

[Leo_Gnardo]

Is the IC running on split supply or single rail?

Split supply. With all this razzmatazz running off the bias winding, drawing a lot of current (especially for the inefficient zener regulators.) So much that it unbalances the hi voltage windings. The winding without bias tap contributes most of the current for the high voltage supply. And any problem with the low voltage supplies depresses the bias voltage. Too many not so clever compromises. That's why I recommend a separate transformer.

[01redcobra]

I did notice that the 1K and 820 resistors are 2 watts. I guess I'll bite the bullet and pull the board and replace them to get the voltage down a bit more towards 15v. I just don't want this thing coming back in a week or so with that channel switching and stuff not working. I don't think he wants to do the transformer thing. When I mentioned it he wasn't too excited.

[Jazz P Bass]

You need to find out what the zener diode voltage rating is. (the last two numbers)
If it is an 18 volt diode, then leave the circuit alone.