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Old 17th October 2012, 03:40 PM   #41
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You posted...


HV1 - 327 - B+
HV2 - 326 - Phase Inverter Stage
HV3 - 286 - Input Stage
V1PA - 65 - Input Plate Voltage
V1PB - 253 - Phase Inverter Plate Voltage



the input voltages 'add-up'. you should have ~60v on the plate.

253 on the PI plate across a 100k is 326-253=73/100k=0.73mA through the PI tube. That's 73v across the cathode and your first stage plate is at 60V dc coupled. What does your cathode voltage read (pin6)? I've never designed a cathodyne but the schematic shows the grid to at 90V and the subsequent current drop putting the cathode at 80V. the opposite of what you have.


On a more serious note, your cathodyne seems to be somewhat slightly off-center biased at 2V, imo, so a ~4Vpp will start to distort. So your gain on the first stage would certainly send in a signal much larger than 4Vpp. The 6SL7 shows a max gain of 70. If you are getting half that with an input of 1vpp you are sending in a signal 7 times greater than the cathodyne can handle. Surely this will cause early onset of distortion and clipping.
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Old 21st October 2012, 08:50 AM   #42
SandyG is offline SandyG  United States
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Ok, made it back for a bit of testing with some interesting results.

Current values for R8 is 150K as changed in a prior test. R7, R8, R9 and R14 are the measured drop across the resistor. Amp was tested with no signal on input and 8 ohm load on at all times.

Test 1 was the baseline that was pretty much as built except for the change of R8 from 270k to150K. This configuration was also tested with a different brand of 6SL7, results were just about the same. Just to make sure no tube issues. Original tube replaced for all test. Output into 8ohms was about 8.5VRMS before clipping

Test 2 was to just replace the 5U4 with a GZ34. Output into 8ohms was about 9.0VRMS before clipping.

Test 3 was with the GZ34 and R16's value dropped from 47K to 4.7K. Clipping before output dropped to 8.75VRMS a drop in output.

Test 4 was back with the 5U4 and R16 at 4.7K Just to see what would happen,
Clipping before output dropped to 8.0VRMS a drop in output

It seems that raising the plate voltage on the input stage causes it to clip earlier.

I need to catch up on some of the comments, just getting some late night time to do the testing.
Code:
Test 1 - 5U4 and R16 47K
HV1 : 326
HV2 : 325
HV3 : 271
Bias :  21
V1PA : 107
VP1B : 213
R7   : Not Measured
R8   : 163 
R9   : 108
R14  : 108

Clipping at 8.5V RMS 8ohms

Test 2 - GZ34 and R16 47K
HV1 : 349
HV2 : 345
HV3 : 289
Bias :  22
V1PA : 116
VP1B : 224
R7   : Not Measured
R8   : 172
R9   : 118
R14  : 118
Clipping at 9V RMS into 8ohms

Test 3 - GZ34 and R16 4.7K
HV1 : 346
HV2 : 343
HV3 : 335
Bias :  22
V1PA : 133
VP1B : 210
R7   : 1.18
R8   : 200
R9   : 132
R14  : 135
Clipping at 8.75V RMS into 8ohms

Test 4 - 5U4 and R16 4.7K
HV1 : 324
HV2 : 321
HV3 : 315
Bias :  21
V1PA : 126
VP1B : 196
R7   : 1.11
R8   : 187
R9   : 123
R14  : 126
Clipping at 8.03V RMS into 8ohms
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Old 21st October 2012, 03:29 PM   #43
rongon is offline rongon  United States
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I think your results reinforce the opinions that the problem is insufficient grid bias on the cathodyne phase splitter. Raising the B+ on the first stage (changing R16 from 47k to 4k7) would increase gain from the first stage 6SL7, making it easier to overload the second stage 6SL7 (cathodyne).

Remember that less than -1V grid bias on a 6SL7 will often cause grid current to flow. I don't know what the effect of grid current would be on the cathodyne stage. I suspect it is not a desirable feature. I'd try to get the grid bias on that cathodyne up to -1.3V.

Just out of curiosity, what is the voltage at pin 8 of the 6V6's?
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Old 21st October 2012, 04:08 PM   #44
rongon is offline rongon  United States
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Part of the problem is that 6SL7, like most high mu tubes, has a high plate resistance, so really wants to work with a high plate voltage. A B+ of only +285 to the first stage is pretty low. More serious (IMO) is that there's only +325 available for the second (cathodyne) stage. You typically want the load resistors to drop a third of the B+, and the tube will drop the last third. In that case, it would be 108V across the tube. A 6SL7 just won't swing many volts with a plate voltage of only 108V.

You could try 6SN7 instead. With an input RMS signal of 1.5V, and the usual gain of about 14 for a 6SN7 common cathode stage, 1.5*14 = 21Vrms. Subtract the slight gain loss through the cathodyne, so 21*0.95 = 19.95. 20 is close enough. To convert back to peak volts, 20*1.414 = 28.2.

So you have 28Vpeak to the 6V6 grids. That will work. It's slow right now, so I think I'm going to scribble some lines on paper and see if I can come up with a 6SN7 based version of this circuit. Should be fun...
--

WAIT... There won't be a lot of gain for negative feedback if you use a 6SN7. 2Vpeak output from your sources might be required. This would be OK if you have a preamp with gain, but not if you have no gain stage before the power amp.

Last edited by rongon; 21st October 2012 at 04:16 PM. Reason: Afterthoughts...
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Old 21st October 2012, 05:20 PM   #45
Jaap is offline Jaap  Netherlands
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From a French magazine
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Old 21st October 2012, 05:21 PM   #46
rongon is offline rongon  United States
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Default Quick 'n dirty...

OK, here's a quick and dirty sketch of the circuit using a 6SN7 instead of a 6SL7. Please forgive any mistakes, I did this in a hurry. The idea was to see if a 6SN7 would even work. I think it will, but not optimally. 6SN7 gets really linear with plate current of 8mA or higher. I couldn't pull more than 5mA per triode with this low a B+, so 5mA it is. Maybe someone else can figure a better way. I hope this is a decent start...

My apologies for the sloppy drawing.
--

I see that I forgot to bypass the cathode bias resistor for the first stage 6SN7. 220uF should work across that 330R.
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Last edited by rongon; 21st October 2012 at 05:25 PM.
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Old 21st October 2012, 05:55 PM   #47
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It seems that raising the plate voltage on the input stage causes it to clip earlier.


I doubt the first stage would clip earlier with More voltage swing possible. In fact, you'd have to be sending in quite a large signal to even get the first stage to clip. See my post just before yours and rongon's post just after. Your signal into the splitter is likely too high and overloading this, second, stage.

A higher B+ or more voltage swing for this stage would reduce this amps weak link.
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Old 21st October 2012, 05:59 PM   #48
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Do you have two spare 100k Rs? If so, gator clip them each parallel to R14 and R9 and do your test again. This should increase both your Plate voltage swing limit as well as the available total swing.
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Old 21st October 2012, 07:00 PM   #49
Jaap is offline Jaap  Netherlands
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Wouldn't it be better to have a CCS on top of the first 6SN7 and a Led under it ?
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Old 21st October 2012, 07:05 PM   #50
rongon is offline rongon  United States
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Quote:
Wouldn't it be better to have a CCS on top of the first 6SN7 and a Led under it ?
Yes. It would.

And I think it might also be better to use an LTP as the phase-splitter/driver, and put a negative supply and CCS in the tail.
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