Cornet2 Right channel low sound

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mucool

  • Jr. Member
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Cornet2 Right channel low sound
« on: 11 Jun 2014, 04:40 am »
Hi there,
Just finished building the Cornet2. Thanks Jim for the awesome piece of electronics. Everything works okay, but not perfect.
Unfortunately, the right channel is low, not by much, but noticeably lower to shift the balance to left side.
I swapped the 12AX7's and it doesn't solve the problem, but alters the levels making right channel worse off.
These are my voltage readings -
B+ - 366 (should be 365)
R211L - 338 and 311 (should be 330 and 300)
R211R - 339 and 314 (should be 330 and 300)
R219L - 175 (should be 155)
R219R - 193 (should be 155)
V200 Pin 1,6 Left - 185 and right - 203 (should be 150 each)
V201 Pin 1,6 Left 168 and right 185 (should be 150)

If I swap the tubes, the values become:
B+ - 364 (should be 365)
R211L - 337 and 310 (should be 330 and 300)
R211R - 338 and 313 (should be 330 and 300)
R219L - 190 (should be 155)
R219R - 209 (should be 155)
V200 Pin 1,6 Left - 168 and right - 187 (should be 150 each)
V201 Pin 1,6 Left 185 and right 203 (should be 150)

As you can see, Pin1,6 voltages also get swapped when changing the tube. Also the R219 voltage gets increased by 15 in both channels by swapping.

To me it seems like a tube problem, but I don't know jack about such complicated circuits.

There is also a hum/buzz (dont know the difference) in the sound. Both channels. Have properly grounded everything.
Please help!!

poty

  • Full Member
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Re: Cornet2 Right channel low sound
« Reply #1 on: 15 Jun 2014, 12:12 pm »
The readings
R211L - ... and 311 ...
R211R - ... and 314 ...
...
V200 Pin 1,6 Left - 185 and right - 203 (should be 150 each)
V201 Pin 1,6 Left 168 and right 185 (should be 150)

If I swap the tubes, the values become:
...
R211L - ... and 310 ...
R211R - ... and 313 ...
...
V200 Pin 1,6 Left - 168 and right - 187 (should be 150 each)
V201 Pin 1,6 Left 185 and right 203 (should be 150)
means the V200 and V201 draw too little current. It could be bad tubes (both), wrong tubes (not 12AX7) or mistakes in the R203, R210 values. As soon as the resistors are not paralleled with someting in the circuit you can easily measure their resistanse right on the PCB (Cornet 2 must be switched off!!! and the plug is pulled out of the mains!!!). If the values of the resistors are OK, then you should put new tubes in both positions.

mucool

  • Jr. Member
  • Posts: 8
Re: Cornet2 Right channel low sound
« Reply #2 on: 16 Jun 2014, 02:58 pm »
Thanks Poty.
So after a lot of correspondence with Mr. Hagerman, I was able to do these:

Got a new pair of matched AX7's.
Drop the resistance for R223.
This brought up the heater voltage to 5.7 from 4.5 and also all the other voltages are now very close to the specs, albeit a tad bit lower.
More importantly it also solved the problem of low right channel sound. Now both are balanced. So major problem is fixed.

Now as I understand that 5.7 is still not the ideal voltage and may be my Cornet 2 is not performing to its potential.
But the problem I am having right now is that if I keep dropping the R223 to bump up the H+, the other voltages drop even lower. I am not too sure how to go about it.
Thanks.


















poty

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Re: Cornet2 Right channel low sound
« Reply #3 on: 16 Jun 2014, 04:17 pm »
If there was no mistakes in the resistor values, it seems very strange that the heater voltage was so low. What is your mains voltage and how the power transformer primaries are wired?
Could you measure the real voltage in mains?
It's normal that changed load of one sort affects other power lines as far as the voltages are not regulated in this design.

mucool

  • Jr. Member
  • Posts: 8
Re: Cornet2 Right channel low sound
« Reply #4 on: 16 Jun 2014, 05:13 pm »
Mains voltage is 120V. I measured and it came around 119V on my dmm. I have wired it according to that. I rechecked all resistor values one by one tediously and they are good.
One thing I noticed was that the voltage before the diodes rectifier bridge is 3.26 in each green wire and becomes 6.52 after the rectification. Is that low?
I mean if the voltage before R223 is only 6.52, then dropping it to 6.3 seems like it'd have to use a very low resistance. I read somewhere that someone was getting 7.5 or so from that.
Thanks,

mucool

  • Jr. Member
  • Posts: 8
Re: Cornet2 Right channel low sound
« Reply #5 on: 16 Jun 2014, 05:27 pm »
Hey Poty,

So I also checked for the voltages before the rectification bridge (green terminals coming from the tranny) and they are 6.3 V. I checked the tranny spec sheet and that is indeed the case.

So now I am wondering, adding a resistor is obviously going to drop the voltage lower than 6.3. Do you think at this point I should be bypassing the resistor or at least keep adding more in parallel?
Thanks!

Here are the tranny specs http://www.tubesandmore.com/sites/default/files/associated_files/p-t370bx.pdf




poty

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Re: Cornet2 Right channel low sound
« Reply #6 on: 16 Jun 2014, 07:51 pm »
The 6.3V AC is RMS voltage, sort to say if it is applied to a resistor the dissipated power will be the same as the DC voltage of the same value. It's max (amplitude) value is the RMS multiplied to square root of 2. So, after the rectification and filtering there may be higher voltage. So, this is not the case. You have very high losses somewhere. I cannot now say exactly where, but if you measured 6.3VAC at the entrance of the diode bridge your primary wiring should be OK.
Could you check the diodes and the capacitors in the heaters circuit? Is the value of the capacitors OK? It should be around 7V and probably higher before the resistor.

mucool

  • Jr. Member
  • Posts: 8
Re: Cornet2 Right channel low sound
« Reply #7 on: 17 Jun 2014, 03:21 pm »
on a slightly different note, I am having trouble removing the crimped wires from the terminals on the PCB. They are too tight. If I try hard, the pink rubber insulation of the crimp come out.
I need to detach the PCB to check for solder errors and also in future planning to upgrade with auricaps.
Thanks!