6l6 push pull output question(s)

[Bas van Dijk]

Hi All,

I'm restoring a 1956 rockola Comet120.

You can find the amp schematics and more info here:

Amplifier | Rock-Ola 1438 Comet Fireball

I'm new to tube amps but I read a lot and I think I understand the basics.

Anyway, the amp is completely rebuild, replaced all tubes, resistors and caps.
I reconnected it but there are 3 problems:

1) The output power is lower than I remember is. At full volume the volume is 'ok' but not loud.
2) There is a distinct hum at 100 Hz (2xmains frequency here), independent on volume.
3) the music sounds slightly distorted

So I've been trying to figure out what is wrong.

The output stage (see schematic) consists of 3 6l6 tubes (common Cathode) in push pull mode. The center tap of the output transformer sits at the high voltage output of the rectifier.

I've measured the impedance of the ouput coil (pin 3 on the 6l6 to pin 8 on the 5u4) for both tubed, and the voltage across it when there is no input.

First of all the resistance on the coil is 144 ohm for tube 1 and 173 ohm for tube 2, I was surprised about this difference.
Voltage read 6.8 and 8.3 respectively.

this leads to anode currents of 47.2 and 47.9 mA in both tubes (surprisingly close considering the different loads..??)

The common cathode potential is 21.4 and the anodes are at 410 V

So the tube plate dissipation is about 18W.

Now, this is clearly above the limit for the 6l6 (12W max). So this can explain the distortion, and the hum? (50Hz being distorted to 100Hz??).

My questions are these:

1) Since the Cathode and Anode voltage are very close to the specification in the schematics, why is the tube running so hot? Can it be that the output transformer is to low in impedance (see also question 2)
2) Is the difference between the two output transformer impedances normal? Should I worry about it? If so what should I do/check?
3) I can of course rebias the tube by changing the cathode resistor, but this will (I guess??) make the output even softer?

My guess is that there is something fishy about the output transformer, what do you think?

BW<
Bas

 

[rayma]

There is a distinct hum at 100 Hz (2xmains frequency here), independent on volume.

The hum is 100Hz because the 50Hz mains is rectified to 100Hz for the DC supply voltage.
Since only a small electrolytic filters the hum, it's not surprising. Probably a 10R, 10W resistor
and another electrolytic before the output transformer center tap will lower the hum enough.

 

[Bas van Dijk]

Thanks Rayma, what value should you make the Cap? The same as the other's (20u)?

And you mean a R between the 300V point in the schematic and the center tap , followed by a cap to ground correct?

 

[sser2]

Are you using 6L6 or EL34 tubes?

 

[Bas van Dijk]

6L6

 

[Bas van Dijk]

In the meantime I found this thread

Output transformer DC resistance..

explaining why the difference in resistance is normal.
and or course because the tubes are matched the currents are equal so that part makes sense.

So that answers question 2, but I still don't understand why the tubes are so hot. The Anode voltage is on the high side of course...

 

[sser2]

Is speaker original field coil? If you are using a different speaker, you need to determine correct speaker impedance by measuring output transformer turns ratio.

Re. hum, pull out phase inverter tube. If hum goes away, it is likely from ground loops. If it doesn't, it is likely from power supply.

Check inverter voltages. If they are more than 20% off, try different 6SN7 tube in this position.

 

[Bas van Dijk]

Hi SSER2

Thanks. The speaker is original.

Without the phase inverter the hum is gone. How do I check/fix ground loops?

Do you mean to check the AC voltages from the inverter? i.e. the input voltages to the control grids of the 6l6?

 

[rayma]

Thanks Rayma, what value should you make the Cap? The same as the other's (20u)?
And you mean a R between the 300V point in the schematic and the center tap ,
followed by a cap to ground correct?

Rectifier - 20uF - 10R - 30uF - output transformer center tap. That's a Pi filter.

The first cap is actually a little undersized, it could be 30uF also.

The hum (at least in part) is due to the small 20uF input capacitor,
that also has no further filtering before the output stage.

 

[Bas van Dijk]

Thanks, any thoughts on the biasing?

 

[rayma]

Thanks, any thoughts on the biasing?

The stock biasing circuit should be fine, with a fresh capacitor.

 

[Bas van Dijk]

Thanks Rayma, but why does it sound so bad? I expect the hum is unrelated to the distortion, also my measurement show that the tube is running above it's maximum rating? I'm confused.

 

[rayma]

I expect the hum is unrelated to the distortion, also my measurement show that the tube is running above it's maximum rating?

Right, the hum is not related to the distortion. If you want to reduce the plate current and dissipation,
increase the value of the cathode resistor. The cathode bypass capacitor can remain the same.
Try a combined plate current of 60mA instead of 100mA.

 

[Bas van Dijk]

Thanks.

BTW I just realized that the tubes are 6L6GC, rated at 30W, so the 18W should not be the problem.

 

[rayma]

I just realized that the tubes are 6L6GC, rated at 30W, so the 18W should not be the problem.

Yes, the amp is working in close-to-data book operation. Check your tubes, if they are ok, check the voltages for
the input tubes in circuit. Maybe a resistor is way off. If not, try replacing the two 0.1uF coupling capacitors.

 

[sser2]

If hum is gone w/o phase inverter tube, then power supply is fine and you don't need to do anything about it. 6L6 bias and idle current are OK.

Next step in your troubleshooting is to measure all voltages on both 6SN7 tubes.

 

[Bas van Dijk]

All voltage on the input tubes are close to normal. The only voltages that are off are the plate voltages on the output tubes.

Maybe the speaker is broken? I'll also check my input signal.

Thanks

 

[Alllensoncanon]

Also check the Screen voltage (pin 4) of 6L6 is around 300V per design.

 

[Bas van Dijk]

pin 4 is at 310 V
Pin 3 at 410 (higher than in the schematic)
Pin 8 at 22V

 

[Alllensoncanon]

These seems normal. Both 6L6 have similar voltages?