50W Build - First Startup, Bias Issue

[Cathode Ray]

B+ is 472 V

EH 6CA7 tubes. Bias adjust pot at maximum clockwise rotation.

This amp has bias test points on back panel. One tube reads at 37mV, the other reads 98mV.

Swapped tubes in sockets. Problem stays with socket.

Checked for cold solder joints, reflowed solder on a couple of joints. Same issue.

Removed tubes and took voltage reading at pin 5 both sockets -56.5 VDC.. identical reading on both sockets.

Reinstalled tubes, repeated test. Same result, -56.5 VDC on pin 5 of both sockets.

Re-tensioned pin 5 of trouble socket. Problem remains.

Took continuity reading from tube side of socket to grid leak/bias resistors on turret board. No continuity problem.


I'm stumped here.

[bepone]

is the g2 circuit the same for each tube?
did you connect pin 1 and pin 8 together?

[Littlewyan]

In addition to what bepone is asking, also check the screen grid resistors and measure each side of the Output Transformer winding. They should be very similar.

[pdf64]

Check the resistance of the bias test points. They’re probably supposed to be 1ohm resistors. If one is 2.7ohm then 37mA will drop 98mV.

[Cathode Ray]

is the g2 circuit the same for each tube?
did you connect pin 1 and pin 8 together?

Each of the two output tubes have pins 1 & 8 tied together.

Yes, the g2 circuit is the same for each tube.

[Cathode Ray]

In addition to what bepone is asking, also check the screen grid resistors and measure each side of the Output Transformer winding. They should be very similar.

The OT windings measure 60.8Ω and 60.4Ω to center tap.

Screen grid resistors are both new parts & measure within tolerance (5W 1kΩ).

[Cathode Ray]

Check the resistance of the bias test points. They’re probably supposed to be 1ohm resistors. If one is 2.7ohm then 37mA will drop 98mV.

Each measure 1.1Ω

[bepone]

grid leak resistors present=? 220k and no more (470k, 1M will lead to problems ).

you are reading voltage on 1 ohm resistor inserted from cathode to the ground?

if all is like this and there is no other major fault, this hi current in one side will make red glowing on the problematic tube, in short time.. big discrepancy in output tube currents will also lead to loud hummm from the speaker.

taken all the input parameters from this topic , means oscillation, and layout need to be re-checked.. oscillations can be found with oscilloscope connected to the speaker jack (speaker connected)..

[Cathode Ray]

grid leak resistors present=? 220k and no more (470k, 1M will lead to problems ).

you are reading voltage on 1 ohm resistor inserted from cathode to the ground?

Grid leak resistors measure 221.6KΩ and 221.4KΩ

I'm reading mV at the two bias test points.

if all is like this and there is no other major fault, this hi current in one side will make red glowing on the problematic tube, in short time.. big discrepancy in output tube currents will also lead to loud hummm from the speaker.

I expected the same thing, but both 6CA7 tubes appear to heat/glow normally. The one reading 98mV does not appear to be any brighter or hotter. Have only run the amp 60~90 seconds at a time with power tubes in, just long enough to perform these tests while connected to a 16Ω dummy load. Have not gotten as far as to connect to a speaker cab.

[sluckey]

Check the resistance of the bias test points. They’re probably supposed to be 1ohm resistors. If one is 2.7ohm then 37mA will drop 98mV.

Each measure 1.1Ω

Measure resistance between output tube pin 8 and chassis (one probe on chassis and the other probe directly on the socket pin 8 ). Do this for both tubes. What have you?

[Cathode Ray]

Measure resistance between output tube pin 8 and chassis (one probe on chassis and the other probe directly on the socket pin 8 ). Do this for both tubes. What have you?

1.1Ω on both sockets - pin 8 to chassis.

[pdf64]

Does removing the valve in V3 affect the readings? That will pretty much eliminate the potential for the power amp to oscillate.

What’s the voltage drop reading from the OT CT to each anode? So 2 readings, CT to V4 pin3 and CT to V5 pin3.

[Cathode Ray]

Does removing the valve in V3 affect the readings? That will pretty much eliminate the potential for the power amp to oscillate.

What’s the voltage drop reading from the OT CT to each anode? So 2 readings, CT to V4 pin3 and CT to V5 pin3.

Want to be clear.. this amp has 3 pre-amp tubes, the PI, and two output tubes (6 total).. so when you say V3 you mean the phase inverter tube ?

CT to V5 pin3 and CT to V6 pin3 : 116V

[Littlewyan]

Yes he means the PI valve, that way no signal can get to the power valves.

[Cathode Ray]

Yes he means the PI valve, that way no signal can get to the power valves.

With the phase inverter tube removed, I measure 3.4mV on both output tubes at the bias test points.