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Unexpectedly good EL84 amp
Unexpectedly good EL84 amp
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Old 27th March 2011, 12:11 AM   #21
trobbins is offline trobbins  Australia
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A poor mans way to alleviate the power limiting characteristic of the cathode biased/bypassed output is to shunt a zener across the RC network (eg. 15V) - this constrains the stage from getting too cold biased (rising cross-over distortion) at higher power levels and rolls in to fixed bias mode.

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Old 27th March 2011, 12:16 AM   #22
cjkpkg is offline cjkpkg  United States
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Essentially what I did...plus replaced the 340 ohm resistor on the front section of the 6n1p with a red LED. Sounds Fantastic. Iron is all Edcor with broskie PS-4 PSU using a 5ar4 rectifier.

Click the image to open in full size.
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Old 27th March 2011, 12:33 AM   #23
woodturner-fran is offline woodturner-fran  Ireland
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Thank you for the schematics - I'll have a go at that - removing the cathode bypass is first up as shoog says, then the ccs. I'll try the led too - I get about 2v across the 340r right now so it's a straight forward swap...
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Old 27th March 2011, 01:56 AM   #24
trobbins is offline trobbins  Australia
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If you wanted to double-protect cjkpkg's output stage then you could add say a 30V zener across the 317 to 0V, and put wiper open-circuit protect resistors on the pot.
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Old 27th March 2011, 07:03 AM   #25
Ian444 is offline Ian444  Australia
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I find the coupling method between the 6N1P stages to be interesting, I have not seen it in other schematics. Does anyone know of the pro's and cons of this method of coupling versus DC coupling, or versus having the split load inverter self-biased?
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Old 28th March 2011, 07:23 AM   #26
Wavebourn is offline Wavebourn  United States
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Unexpectedly good EL84 amp
Quote:
Originally Posted by Ian444 View Post
I find the coupling method between the 6N1P stages to be interesting, I have not seen it in other schematics. Does anyone know of the pro's and cons of this method of coupling versus DC coupling, or versus having the split load inverter self-biased?
Have not seen schematics of my Pyramid amps?
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Old 28th March 2011, 08:09 AM   #27
Ian444 is offline Ian444  Australia
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Yes I have, but do not remember seeing this coupling method. Pro's and cons are?

One advantage I can see is the ability to easily set DC operating conditions at idle.

Last edited by Ian444; 28th March 2011 at 08:12 AM.
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Old 28th March 2011, 05:45 PM   #28
Wavebourn is offline Wavebourn  United States
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Unexpectedly good EL84 amp
Cons are higher number of parts needed (R6, C5, R8 in Pyramid-VII-M).

Concertina has high input resistance and low input capacitance, so I even omitted that shunting capacitor: anyway I had a Zobel compensation network there, so resistance of the voltage divider and input capacitance of Concertina formed one more Zobel in parallel with the existing one.

Main advantages are: flexibility in choosing of working point, and higher stability on low end. Coupling all tubes in the amp directly (well, except output tubes) I stabilized working point of all of them at once feeding screen grid of the 1'st tube from tail of the LTP driver.

Here is one more time my Pyramid attached, for the reference.
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File Type: jpg Pyramid-VII-M-Driver.jpg (180.8 KB, 2219 views)
File Type: gif Pyramid-VII-M-OS.gif (21.2 KB, 2182 views)
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Old 28th March 2011, 11:32 PM   #29
Ian444 is offline Ian444  Australia
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Thanks for that. I have the Pyramid 8 schem and couldn't see it, because the cap was gone Still, its not a common coupling technique, but is definitely worth remembering. With direct coupling, with no divider or cap, it is difficult to try different tubes due to the changing DC conditions.

If one put a trimmer resistor in place of the lower resistor of the voltage divider in the Tubecad circuit, one could easily tweak the DC conditions. Probably no point in tube-rolling, the 6N1P certainly sounds very good to me, I run it in 2 out of 3 small PP amps I have.
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Old 28th March 2011, 11:51 PM   #30
Wavebourn is offline Wavebourn  United States
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Unexpectedly good EL84 amp
Quote:
Originally Posted by Ian444 View Post
Thanks for that. I have the Pyramid 8 schem and couldn't see it, because the cap was gone Still, its not a common coupling technique, but is definitely worth remembering. With direct coupling, with no divider or cap, it is difficult to try different tubes due to the changing DC conditions.
Yeah, and I got lots of criticism from people who think by patterns.

Quote:
If one put a trimmer resistor in place of the lower resistor of the voltage divider in the Tubecad circuit, one could easily tweak the DC conditions. Probably no point in tube-rolling, the 6N1P certainly sounds very good to me, I run it in 2 out of 3 small PP amps I have.
Good idea, I would not leave that pot in place under a voltage, in sensitive AC position, being afraid of noises.
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