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Old 30th September 2012, 02:19 AM   #1
SandyG is offline SandyG  United States
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Default New 6V6 Build - Suggestions Needed

Back again for some help on a 6V6 Project. I Mashed together a few different schematics and came up with the EZ10. Trying to make a pair of small mono-blocks for my office and figured I would try an all octal 6V6 based amp.

The circuit is a cross between the Silvertone and the Dynaco 6V6 using ultra-linear.

Initial testing looks awesome, except for early clipping early at about 5watts. Power transformer and output transformer are about 2x the required size so pretty confident it's in the input stage.

Waveforms (sine and square) before it gets into clipping are beautiful across the audio range, so off to a pretty good start!

I need some help in getting the right operating points for the each of the sections for the 6SL7 and getting the power supply in order to set that up.

Other then trial and error what are good places to start (current and voltage) for this type of amplifier? And how to apply the math so I can learn a bit.

My Initial voltage measurement seems like the plate voltage on the input stage is way too low (maybe not the inverter stage).

From the test points on the board, with no signal, connected to load 8ohm, and rectifier tube as a 5U4GB, and 270ohm cathode resistor -

Code:
HV1 - 327  - B+ 
HV2 - 326  - Phase Inverter Stage
HV3 - 286  - Input Stage
V1PA - 65  - Input Plate Voltage 
V1PB - 253 - Phase Inverter Plate Voltage
Bias Voltage across Cathode resistor (270ohm) 21v, and current measured at each 6V6 Tube at about 37ma (might be a tad high)

So the big question is what are good operating points for the 6SL7 and how do I figure it all out the math for the resistor (is it just simple voltage drop once I find the operating point???)

As always thanks for any help folks have to offer!

Sandy
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Old 30th September 2012, 01:34 PM   #2
DF96 is offline DF96  England
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Google for valve/tube data sheets. Then Google for valve/tube load lines.
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Old 30th September 2012, 02:03 PM   #3
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Hey Sandy,
I just built a 6V6 version of Dynaco's "Low Power High Quality Amplifier." and I ran into the same problems. I'm an amateur so my advice should be taken with a grain of salt.
The first thing I noticed is your schematic specs a GZ34 rectifier and you're using a 5U4GB. I think this might be a mistake. The GZ34 has a voltage drop of ten volts. The 5U4GB has a drop of 50 volts. This may explain why your voltages are low across the board.
The second thing is the feedback circuit. Most of these older designs drew feedback from a 16ohm tap. An 8ohm output has considerably less voltage than a 16ohm. So I would replace that 10k feedback resistor with a 10k pot and then tune the circuit while listening to it. The first time I did this I was amazed at how much the sound varies with feedback amount. Too little feedback and the amp will be distorted and tinny. Too much and it will sound flat and dull.

My two cents, Kevin
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Old 30th September 2012, 02:34 PM   #4
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C6 shorts out the feedback. Put 100Ohms between C6/R5 and GND and connect the feedback there. Be sure to have negative feedback otherwise happy tinnitus.
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Old 30th September 2012, 03:28 PM   #5
roline is offline roline  United States
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If you are willing to try a 6SN7......
I'm more of a fan of 6SN7 tubes running with a little more current. ( 2.5ma min)
R8 to 47k 1watt or 33k 1 watt to run at about 3.5ma
R7 1200 to 1500ohms to bias cathode to about 90v to drive the cathodyne. 93 volts across the cathode resister, 93 volts across the tube, 93 volts across the anode resister.
C6, leave out
R9, R14 to 33k 1watt that will run the cathodyne at 2.7ma.,to 27k 1watt that will run the cathodyne at about 3.5ma
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Old 30th September 2012, 04:31 PM   #6
SandyG is offline SandyG  United States
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Quote:
Originally Posted by DF96 View Post
Google for valve/tube data sheets. Then Google for valve/tube load lines.
I have the data sheet and will look at some of the google results. Hopefully it will start to make sense!

Sandy
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Old 30th September 2012, 04:36 PM   #7
SandyG is offline SandyG  United States
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Quote:
Originally Posted by mr2racer View Post
Hey Sandy,
I just built a 6V6 version of Dynaco's "Low Power High Quality Amplifier." and I ran into the same problems. I'm an amateur so my advice should be taken with a grain of salt.
The first thing I noticed is your schematic specs a GZ34 rectifier and you're using a 5U4GB. I think this might be a mistake. The GZ34 has a voltage drop of ten volts. The 5U4GB has a drop of 50 volts. This may explain why your voltages are low across the board.
The second thing is the feedback circuit. Most of these older designs drew feedback from a 16ohm tap. An 8ohm output has considerably less voltage than a 16ohm. So I would replace that 10k feedback resistor with a 10k pot and then tune the circuit while listening to it. The first time I did this I was amazed at how much the sound varies with feedback amount. Too little feedback and the amp will be distorted and tinny. Too much and it will sound flat and dull.

My two cents, Kevin
Kevin - Thanks for the tips, the GZ34 / 5U4 swap was to keep the B+ voltage under 350, with the GZ34 it was a bit higher then the tubes recommended voltage. I have some JJ 6V6's that would be fine with it, but for starters I figured I would keep it closer to stock 6V6 specs while I figure things out with a good baseline of testing. The 10k pot is likely a good idea for testing as well.

Sandy
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Old 30th September 2012, 04:38 PM   #8
SandyG is offline SandyG  United States
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Quote:
Originally Posted by the_manta View Post
C6 shorts out the feedback. Put 100Ohms between C6/R5 and GND and connect the feedback there. Be sure to have negative feedback otherwise happy tinnitus.
Yep, forgot to put a optional resistor in that part of the circuit. Currently C6 is OPEN and not used, but put it in since I had some extra room on the board and might want to experiment with it. For now will be left open, but will update the schematic to add it in.

Sandy
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Old 30th September 2012, 04:42 PM   #9
SandyG is offline SandyG  United States
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Quote:
Originally Posted by roline View Post
If you are willing to try a 6SN7......
I'm more of a fan of 6SN7 tubes running with a little more current. ( 2.5ma min)
R8 to 47k 1watt or 33k 1 watt to run at about 3.5ma
R7 1200 to 1500ohms to bias cathode to about 90v to drive the cathodyne. 93 volts across the cathode resister, 93 volts across the tube, 93 volts across the anode resister.
C6, leave out
R9, R14 to 33k 1watt that will run the cathodyne at 2.7ma.,to 27k 1watt that will run the cathodyne at about 3.5ma
I like the 6SN7's too, have a bunch. The question on this will the 6SN7 have enough gain vs. the 6SL7? I will have to look at the data sheet and see if I can figure it out but my guess is that it will be lower then the 6SL7?

Learning as I go

Sandy
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Old 30th September 2012, 05:46 PM   #10
roline is offline roline  United States
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6SL7 is much higher gain especially with a 270K in the anode. Personnel preference to lower gain stages, lower output impedances, good stable drive stages to the next stage and minimal GNFB needed to keep it clean at full swing. You have both to play with, so have fun! From you earlier post you have extra current to play with if you use the GZ34 with less of a drop.... so you can run the first couple stages hotter to bring the B+ down a little.
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Last edited by roline; 30th September 2012 at 05:49 PM.
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