[david62]

I have been more than happy with my Clarinet.I have recently noticed that the frontmost 12au7 tube does not glow quite as bright as the rear 12au7.I have also had to adjust the balance control because one channel is louder than the other with the balance control centered.The situation was the same with a different set of 12au7 tubes.It sounds fine with the balance adjusted,but it bothers me that the channels are not the same output.What should I check first to find out what the problem is?Thanks,
Dave

[poty]

I'm not sure about glow brightness (you can still check the heaters voltage on both tubes anyway - between pins 4 and 9 of each tube should be something like 6.2V), but as soon as both channels share both tubes I doubt that the visual appearance somehow led to the described behavior.
I'd check the voltages on C300 for both channels (should be near 280V) and on the cathodes of V301 (pin 3 to ground, pin 8 to ground) - around 140V. Please be very cautious, because there are potentially lethal voltages here. If the voltages are identical and good - it should be something in the input circuits or output circuits (RCA, input switch, balance and volume regulators, C302, C303) - bad joint or contacts, pots and something like this.

[david62]

Thanks much for the reply and your help.I will have to find some time to put the Clarinet on the bench and measure the voltages that you listed above.I certainly will take care with the high voltage.I will report back.Thanks again.
Dave

[hagtech]

All tubes have differing amounts of glow.  Some brands have way more than others.  As for channel balance, swap your interconnects and see if the shift goes the other way.  Also, you can double check resistor values (visually). 

jh

[david62]

The solder joints look good,but I am going to re-heat the right channel connections,which is the weak channel.Is there a diagram published that tells what the tube pin numbers are from the bottom side of the PC board?The PC board is attached to the top plate of the case and it would strain the RCA wire connections if I set the board free.Thanks,
Dave

[poty]

If we look from the bottom: the numbering is clockwise beginning from the gap.

[david62]

Thanks.

[david62]

 Here is the situation. Measuring from pin 8 to ground on the 12au7 closest to the front panel,I only get .04 volts,and the same .04 volts on pin 3.On the other 12au7 I get 132 volts on pin 8 and 128 volts on pin 3. My heater voltages are 5.92 volts on both tubes.I tried changing the rectifier tube and got the same measurements on the front tube of .04 v on pins 8 and 3.I switched the 12au7's with no change.Clearly there is a problem.I'm surprised that it has worked as well as it has with one channel weaker! I would appreciate any advice to diagnose the problem.The voltages on the C 300R is 263v and C300L is 261v.Thanks.
Dave

[poty]

First of all, the situation is absolutly normal. Half of the "front" tube serves as "bottom triode" for the right channel, the other half - for the left channel. The same for the "back" tube, the only difference - the tube serves "top triodes".
I guess the 0.04V is a mistake. I believe the readings mean 4V. If my guess is right - then all other voltages seem OK! Please check my guess before proceeding further!
As far as all parts are soldered to the PCB the further procedure can be a guess and try. Another option includes some soldering which at the point is unnecessary. Could you use a short wire and connect it between R303s (one end to one channel, another - to the second; the best leads are those connected to the pin 2 and 7 of the front tube)? Please do this after switching the device off. You can curl the wire around the leads or use an alligator. Check that the wire does not touch anything else! Then turn on the Clarinet and listen to the volume of both channels. If they are the same - the problem are in the input side, if not - on the output.

[david62]

Poty,

   Thank you very much for your help.I will go and measure the voltages again.Idon't quite understand your instructions for using a jumper wire.If the voltage is 4v,then I will try to re-heat the solder joints ont the weak rt.channel outputs that connect a silver jumper wire to the output RCAs since that is the waek channel and the joints look a little dull to me.
Dave

[david62]

Potty,

    You are right.The voltages on pins 3 and 8 are 4 volts on the 12au7 near the front panel.I went over all of the solder joints between jumper wires connecting the PC board and the panel mounted RCAs.The access is very difficult.I took the side panels off of the Lansing chasis and used a braded solder wick to remove most of the existing solder before re-flowing the joints.I had always suspected those connections since the access is so tight and it was difficult to get in there with a soldering iron.If I had it to do over I would have used the stock RCAs specified.I will clean up the flux and try it tomorrow.
Dave

[poty]

Hello Dave,
Could you post some photos of insides? It could be not only bad solder joints, but volume or balance pot, input capacitors and so on...
The main idea of my test with wire - connect the grids of the bottom triodes (V300). If something is wrong on the input side - the output will be the same for both channels. If on the output - the equal signals on the grids won't help. So you can just solder a short wire between the pins 2 and 7 of V300.
Vladislav

[david62]

Vladislav,

       I will try to take some pictures later today.I will try the jumper wire that you suggested if it still has a weak channel after re-heating the input and output connections.Could I spray the volume and balance pot with contact cleaner?It is only a few years old,so I'm not sure that the pots would be dirty.I appreciate your help.
Dave

[david62]

All seems to be well now!I put the panels back on the chasis and reconnected everything.The balance of the channels was restored.I attribute the difficult access of making the solder connections between the PC board and the panel mounted RCAs.I did the best that could at the time of the original build,but it wasn't 100% right.A head lamp and loupes helped me examine the solder joints and re-do a few of them.Thanks for all of the good advice which had me learn a few things.
Dave

[poty]

Great news! Enjoy the Clarinet!

[hagtech]

Thanks Poty - I was off to Kauai for a couple of days...

jh

[david62]

With the channel balance working properly now,my Clarinet sounds fantastic once again.
Dave