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Old 31st December 2008, 01:27 PM   #1
tmhajw is offline tmhajw  United Kingdom
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Default Berman linestage- do I reference heaters or not?!

Happy New Year!
Hi have just built this linestage (76 then cathode followed by 6sn7) only my second ever linestage build...
It sounds fantastic!
However... rustling and popping apparent at 'idle'. Some of the rustling I've found was from a noisey carbon comp anode load resistor.
But.. I'm worried about the heater- cathode voltage of the cathode follower. At idle this is -202v/ -208v on the 6sn7, which is just outside data sheet maximum (200v -ve to cathode). Can't be doing the heater insulation much good!
Sooo, I'm wondering about referencing the (dc) heaters. The 76 data sheet says max 90v + to heater so I could ref the heaters say about 50v above ground. Is this worth it? The 6sn7 would still be -150v heater to cathode...
Or,I'm even toying with just going the whole hog and supplying a 200v referenced heater voltage from a second filament transformer for the cathode followers only.
Or am I just being paranoid? After all, Berman doesnt reference the heaters in his original 'Sound Practices' article..
Advice, please!
Thomas
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Old 31st December 2008, 01:41 PM   #2
kevinkr is offline kevinkr  United States
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You're on the right track, just because Berman didn't and apparently got away with it doesn't mean you shouldn't do as you propose or that in the long term you will be as lucky as he was.

I would probably float the filaments on about 75vdc well decoupled to ground, this should still provide about 20V of margin on the 76 when you consider the cathode bias present, and will ease the stress on the filament/cathode insulation in the 6SN7. A separate transformer is not a bad idea, and probably not overkill, even with 75V on the common filament supply you are in troublesome territory with the 6SN7.

IME you really don't want to go much over 100V differential between the filament and cathode in any 6SN7 I have experience with. (I've lost a few of various brands to this issue.)

The pre-amp may even get a bit quieter with this change..

FWIW my favorite 6SN7 is just about any pre 1980 production Sylvania 6SN7GTA/GTB . Brimars are great too, but are too expensive around here at least.
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Old 1st January 2009, 03:49 PM   #3
tmhajw is offline tmhajw  United Kingdom
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So +75v referenced dc heaters would be ok for the 76 (data sheet max 90v),and better for the cathode followed 6sn7s (heater- cathode voltage now 125v, data sheet max 200v),is this compromise reasonable?!
Why would the 6sn7 put a strain on the one filament transformer, please explain!
Thanks Thomas
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Old 1st January 2009, 07:18 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by tmhajw
So +75v referenced dc heaters would be ok for the 76 (data sheet max 90v),and better for the cathode followed 6sn7s (heater- cathode voltage now 125v, data sheet max 200v),is this compromise reasonable?!
Why would the 6sn7 put a strain on the one filament transformer, please explain!
Thanks Thomas
I think you may have misinterpreted my comments or I was not clear enough, my comments were in regard to the filament/cathode stress in the 6SN7 - using a separate transformer for the 6SN7 would allow you to run the 6SN7 filaments within their insulation design rating and thereby avoid the issue altogether. The transformer(s) would have no issue either way as long as their current rating is adequate for the load.

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Old 1st January 2009, 07:58 PM   #5
tmhajw is offline tmhajw  United Kingdom
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Ahh OK!75v ref would be better than present but 2nd filament transformer and heater voltage well jacked up purely for the 6sn7 would indeed be the way to go!!
I'm a bit new to all this...
I'm surprised Berman didn't reference his- has anyone else built this schematic??!
The 6sn7s I've got hold of to date are a pair of Raytheon wgt and a pair of GE gtb's.. wonder how these fit in to the sound quality heirarchy?
I was wondering about substituting 6j5gt for them- got a late pair of ITT labelled ones- wonder what the sound quality of these is like?!
Thanks for your help
Thomas
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Old 1st January 2009, 08:06 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by tmhajw
Ahh OK!75v ref would be better than present but 2nd filament transformer and heater voltage well jacked up purely for the 6sn7 would indeed be the way to go!!
I'm a bit new to all this...
I'm surprised Berman didn't reference his- has anyone else built this schematic??!
The 6sn7s I've got hold of to date are a pair of Raytheon wgt and a pair of GE gtb's.. wonder how these fit in to the sound quality heirarchy?
I was wondering about substituting 6j5gt for them- got a late pair of ITT labelled ones- wonder what the sound quality of these is like?!
Thanks for your help
Thomas
It's more than a little topology dependent but as these are used as cathode followers I would not expect the differences to be very significant. 6J5 are equivalent and would work just fine..
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Old 2nd January 2009, 07:32 PM   #7
tmhajw is offline tmhajw  United Kingdom
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ummm Berman rubbished most 6sn7's in his Sound Practices article as 'second rate'...(!) Think I'll try my cheapo 6j5's but the pin outs are different...
Out of interest does the audio signal go anywhere near the voltage divider/heater circuit for referenced heaters?, and could that be why Berman didn't reference his?
Thanks
Thomas
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Old 2nd January 2009, 07:56 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by tmhajw
ummm Berman rubbished most 6sn7's in his Sound Practices article as 'second rate'...(!) Think I'll try my cheapo 6j5's but the pin outs are different...
Out of interest does the audio signal go anywhere near the voltage divider/heater circuit for referenced heaters?, and could that be why Berman didn't reference his?
Thanks
Thomas
No, not really and having floating filaments is also likely to create audible problems like hum in the output in many instances. I think it was either an oversight or ignorance of good engineering practice.

I liked Sound Practices (it would have been nice if I had gotten most of the issues I actually paid for, but that is a story for another time) but I would take many of the opinions expressed there with a grain of salt. Everyone has an axe to grind over something. (I have met more than a few brands of 6SN7 I don't like either, but I can say just about the same about every type I use with a few exceptions - some brands are just much better than others, and probably cost a bit more in their heyday as well.)

I think the 6J5 approach is a good idea, but good ones are a bit rarer and pairs can be much more expensive than the comparable quality 6SN7.
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Old 2nd January 2009, 10:42 PM   #9
tmhajw is offline tmhajw  United Kingdom
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I shall re wire one channel of the pre to take a 6j5 & compare vs 6sn7 over the weekend! May reference heaters up to +70vdc or so & see if 'rustle and pop' goes at the same time...
Thomas
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Old 17th January 2009, 11:45 PM   #10
tmhajw is offline tmhajw  United Kingdom
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Just to update-
Firstly I referenced the heaters up to +70v, so the heaters in the cathode follower stage were about -130v to the cathode. The rustle and pop remained unchanged.
I then went the whole hog, put in a second heater supply for the cathode follower, referenced to about 205v, so heaters now a few volts + to cathode; - what a difference- much quieter, no more pops! Definitely the way to go!
I tried a cheapie itt labelled 6j5 on one channel- seemed a lot more 'brightly lit' sound compared with the 6sn7, too much, in fact, so I took it out...
6j5s seem to be more expensive than 6sn7s... any advice on a 'good' brand?!
Cheers
Thomas
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