6B4G schematic - opinions please!!

andyjevans · 20th June 2009, 10:17 AM

I've just built this amp - schematic enclosed. Problem I'm having is distortion on both channels. voltages seem correct - except output 6b4gs are not perfectly matched - filaments range from 5 to 6.5 volts. Bought new in russia!

Power supply is on a separate chassis, and is nice enough - cap>choke>cap with last cap being polypropylene. 80uF cathode bypass on 6B4Gs also polypropylene. One PSU for outputs, another for input and driver.

What can be causing this high distortion - sounds horrible!

thanks for any suggestions on the circuit.

andy evans

andyjevans · 20th June 2009, 10:31 AM

Sorry - schematic isn't loading. How do I get it to load off my hard disc?

andy

45 · 20th June 2009, 10:36 AM

Could you be more precise about filaments supply?
45

andyjevans · 20th June 2009, 10:43 AM

Filament supply to 6b4gs is 9v secondary, Schottky bridge, 22,000uF cap and LM1084 as current source. This kind supply has always worked for me before.

andy

STILL can't see how to upload schematic. Grrrrr Says "attach file" and "choose file" but it just doesn't work.

45 · 20th June 2009, 11:00 AM

Usually, for DHT's, you can have slightly different filament currents from one tube to another(for a given voltage).
IMHO, the same filament voltage is a must for a good sound.
Have you checked the anode currents for each tube?

45

SY · 20th June 2009, 11:23 AM

Andy, what's the file type?

45 · 20th June 2009, 11:27 AM

In your place I would use a capacity multiplier after the 22000 uF cap ( maybe you should decrease this value).
It is rather simple (one darlington transistor, two resistors, two 1N4001 diodes).
You will get a noiseless supply and a smooth transient when switching on.

45

andyjevans · 20th June 2009, 01:09 PM

File is a jpg. 96k Here's another go!

45 · 20th June 2009, 01:38 PM

Overall distortion could be due to (the order is not relevant):
1) Excessive distortion from the driver. It could be unbalanced and/or 27K a bit low for the max swing you need
2) Transformer primary load (with auto-bias I would suggest higher load than typical 5K)
3) Substantial unbalance of the output stage (it can also be just a consequence of different filament operative conditions)

5V rather than 6.3V for the filament voltage will alter considerably the sound.
Sometimes a lower voltage (say 5.7V instead of 6.3V) results in a better sound. Such a difference is more than one could expect....

45

SY · 20th June 2009, 03:25 PM

Is the distortion at all levels or just on peaks or louder stuff? Constant or cyclical?

20th June 2009, 10:17 AM	#1
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	6B4G schematic - opinions please!! I've just built this amp - schematic enclosed. Problem I'm having is distortion on both channels. voltages seem correct - except output 6b4gs are not perfectly matched - filaments range from 5 to 6.5 volts. Bought new in russia! Power supply is on a separate chassis, and is nice enough - cap>choke>cap with last cap being polypropylene. 80uF cathode bypass on 6B4Gs also polypropylene. One PSU for outputs, another for input and driver. What can be causing this high distortion - sounds horrible! thanks for any suggestions on the circuit. andy evans

20th June 2009, 11:23 AM	#6
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Andy, what's the file type? __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

20th June 2009, 03:25 PM	#10
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Is the distortion at all levels or just on peaks or louder stuff? Constant or cyclical? __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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20th June 2009, 10:31 AM	#2
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	Sorry - schematic isn't loading. How do I get it to load off my hard disc? andy

20th June 2009, 10:36 AM	#3
45 diyAudio Member Join Date: Dec 2008 Location: United Kingdom	Could you be more precise about filaments supply? 45

20th June 2009, 10:43 AM	#4
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	Filament supply to 6b4gs is 9v secondary, Schottky bridge, 22,000uF cap and LM1084 as current source. This kind supply has always worked for me before. andy STILL can't see how to upload schematic. Grrrrr Says "attach file" and "choose file" but it just doesn't work.

20th June 2009, 11:00 AM	#5
45 diyAudio Member Join Date: Dec 2008 Location: United Kingdom	Usually, for DHT's, you can have slightly different filament currents from one tube to another(for a given voltage). IMHO, the same filament voltage is a must for a good sound. Have you checked the anode currents for each tube? 45

20th June 2009, 11:27 AM	#7
45 diyAudio Member Join Date: Dec 2008 Location: United Kingdom	In your place I would use a capacity multiplier after the 22000 uF cap ( maybe you should decrease this value). It is rather simple (one darlington transistor, two resistors, two 1N4001 diodes). You will get a noiseless supply and a smooth transient when switching on. 45

20th June 2009, 01:38 PM	#9
45 diyAudio Member Join Date: Dec 2008 Location: United Kingdom	Overall distortion could be due to (the order is not relevant): 1) Excessive distortion from the driver. It could be unbalanced and/or 27K a bit low for the max swing you need 2) Transformer primary load (with auto-bias I would suggest higher load than typical 5K) 3) Substantial unbalance of the output stage (it can also be just a consequence of different filament operative conditions) 5V rather than 6.3V for the filament voltage will alter considerably the sound. Sometimes a lower voltage (say 5.7V instead of 6.3V) results in a better sound. Such a difference is more than one could expect.... 45

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