Musicman HD-130 Bias Questions

[jackfalbo]

I am working on a 1977 Musicman HD-130. It has the solid state phase inverter.
I have a schematic link:Microsoft Word - covertoc.doc (thetubestore.com).
i cannot get any current more than .1mv on the 6CA7 tubes.
Where on the schematic does the negative Bias voltage come from?
is it on the left where the +16v and -16v (screwdriver slot circle symbol) is located or is it over on the right where the tap from C46 is located?
and which pot adjusts the bias? the right pot on preamp board or the pot on the driver board?
i can upload photos of my amp circuit board.

[Leo_Gnardo]

I am working on a 1977 Musicman HD-130. It has the solid state phase inverter.
I have a schematic link:Microsoft Word - covertoc.doc (thetubestore.com).
i cannot get any current more than .1mv on the 6CA7 tubes.
Where on the schematic does the negative Bias voltage come from?
is it on the left where the +16v and -16v (screwdriver slot circle symbol) is located or is it over on the right where the tap from C46 is located?
and which pot adjusts the bias? the right pot on preamp board or the pot on the driver board?
i can upload photos of my amp circuit board.

These amps used to fox me with their super low idle current. No worries, that's the way it is, though it seems wrong, not what you expect. I would recommend sending a sine wave through the HD-130 and observing its output while driving an appropriate resistive load. See what that looks like on the scope. If it looks like it's making expected power without the waveform showing signs of distress, then it passes the test.

I'll leave it to others to fill in the rest.

[Helmholtz]

i cannot get any current more than .1mv on the 6CA7 tubes.
Where on the schematic does the negative Bias voltage come from?

0.1mV is a voltage, not a current. How do you measure tube current?
Grid voltage is +16V.
As always, bias voltage is grid voltage minus cathode voltage.
So cathode voltage should be around 60V to 70V.

What is the voltage drop across R56 and R61?

What's wrong with the amp?

[jackfalbo]

thanks for replies. hi when i got the amp it had several issues, 2 bad power tubes out of four and one of the 1692 transistors was faulty. i replaced the 2 tubes and 1692 transistor. the amp powers up and when i measure from pin 3 to ground i read say 532 volts. when i turn the trim pot on the driver board. the voltage will drop to 220 but seems like the voltage say 532 volts shifts to the other 2 tubes. i am using a typical bias probe that reads mv and then you assume is the mA reading. the amp is dead quiet no hiss nothing. i do have the reverb tank disconnected, normally that would not matter/ if i pull the tubes i measure 667 vdc on pin 3 to chassis ground. i was not expecting the amp to pass signal. i was hoping to get the output tubes ballpark biased. if i strum a chord, i notice that the dim bulb tester glows brighter but just barely brighter. there is a trimpot on the preamp board that corresponds to pinout B from trimpot wiper on the schematic. nothing(no wire) is hooked to that wiper. i do not know what that trimpot is used for but when i put my meter on the wiper i get from 16 to 37 volts but there is no wire soldered to the tab instead the red wire going up to the driver board is hooked to the 620 ohm resistor just like the schematic.it is confusing because that trimpot has nothing hooked to the wiper unless that pot is used for 12ax7 models and was left like that from the factory. it has me confused

[jackfalbo]

if you look at the top schematic out of the two in that link that i posted. TR-1.(it is 100 ohm) on the driver board is the one that i am adjusting. on the bottom schematic there is a trimpot that is called RV-2 it says 6CA7 Bias out on node B. on my amp there is no wire hooked to that node B wiper. instead it is wired like the top schematic with the bias wire feed going to the + of C46. Being that the transistors are used for the phase inverter. it puzzles me how the output tubes are biased. i keep thinking in my head that there should be a negative voltage wire going to the output tubes for bias.

[Helmholtz]

The lower schematic doesn't apply as it has a tube PI.

TR1 seems to adjust balance.

Please answer my question about R56 and R61 voltage drops.
TR1 should be set for equal R56 and R62 voltage.

[g1]

Forget what you know about grid bias and grid drive with this amp. The grids are both held to fixed +16V and do not get any signal.
It is a cathode drive power amp and the signal is fed to the power tube cathodes.
Bias is set at the cathode and will be a higher +DC voltage as mentioned by Helmholtz in post #3 (60 to 70VDC).
The bias is set by TR1 and is adjusted so there is 25mV across each of R56 and R61 (which are 3.9 ohm resistors). see attachment
When that is done, you will see somewhere between 3 and 6 mA on your bias probe.

Your plate voltages are way out of whack due to the limiter lamp. You can't set bias using it.

[jackfalbo]

hi Helmholtz thank you for the reply. i will perform that test.
but i just plugged a 1/4" cable into my speaker jack while amp was off to check the secondary output transformer winding and i am reading open lead on my meter

[jackfalbo]

i found that one of the series speaker output jacks needed to be bent to complete the normally closed circuit of the output transformer. i am going to continue my testing on the amp. this may explain why i had no sound

[jackfalbo]

thanks for the replies. in low power mode i am reading 350 volts on output tube and the idle bias is zero. when i play a note the bias jumps up to around 20 mA then back to zero. the amp is at least functioning. volume is very low and distorted. i did try putting a 4558 in the socket with no real change. tremolo works. i have a 12" guitar speaker hooked to the amp. it is as loud as a small transistor radio if that but i am happy that it is starting to work. it may have bad bad preamp transistors. i will check the voltage drop on those resistors next Helmholtz. if i try to turn up volume knob on either channel the dim bulb starts glowing brighter. its drawing alot of power with not much volume

[g1]

This is normal dim bulb action. There is no fault that would need you to be using it and it is causing confusing results for your testing.
Also, I believe this is one of the circuits where 4558 will not properly sub for 1458.

Still waiting for DC (mV) at idle with no signal on high power across each of R56 and R61.
see attachment in post #7

[Helmholtz]

I was wrong assuming that TR 1 would adjust balance.
Taking a closer look at the schematic reveals that TR 1 adjusts the base bias and thus the current of both transistors.
The voltage at the wiper should be very low, probably just above 1V.

Transistor and tube(s) are wired in series (it's a cascode arrangement with the transistor dictating tube current), so all cathode current must pass the transistor and its 3.9R emitter resistor.
It's interesting that the instructions make no distinction between 2 or 4 power tube models regarding the voltage drop across the emitter resistor.
This means that the average idle current per tube should be 3.2 mA with 4 power tubes and 6.4 mA with 2 power tubes.

[Tom Phillips]

thanks for the replies....

Unless I missed it, no one has yet mentioned that the output section of this amp is a Class B design. The bias readings are substantially different than the common Class AB output sections commonly used in tube amps. I suggest that you do a little research to discover the unique properties of Class B operation and set up.
Here is a link to get you started http://www.tungsol.com/html/faqs11.html
Cheers,
Tom