heater voltage difference

[kambo]

so i have 3 pairs of 6SN7 wired for 12VDC filtered,
each pair wired serial and parallel to next ones...

i measure 12.6 VDC on far legs of each pair, but when i measure individual tubes,
they all are different....
ie: on one pair, one would measure 5.8VDC and seconds would measure 6.7VDC 
and they all matched tubes in pairs...

5.8 VDC and 6.7 VDC  below/over 5% tolerance for the heater supply.
as far as i know i should keep the heater  voltage within the 5%, and especially avoid under volt.

what should i be looking for ?
or what is the solition, except AC heaters.
thanks

 

[radardoug]

Eventually the side with the higher reading will probably burn out. But it might take a very long time. Series lightbulbs, not a good idea!

 

[mjrippe]

or what is the solition, except AC heaters.
thanks

Run them at 6.3v DC in parallel?

 

[kambo]

Eventually the side with the higher reading will probably burn out. But it might take a very long time. Series lightbulbs, not a good idea!

yeap, one down 

 

[kambo]

Run them at 6.3v DC in parallel?

i guess thats the way to go!

thanks for the bad news guys  8)

 

[ruffrecords]

You do not really have a problem. Tubes are very tolerant of heater supply. They are designed to survive +- 10%. In the old days they used to wire most of the tubes in a TV set in series and connect them straight across the mains via a fat dropper resistor. In my designs I regularly use  pairs of 6922 tubes with heaters wired in series across a 12V supply and I have no problems. However, I do not use NOS tubes. I buy current production.

Cheers

Ian

 

[kambo]

my tubes are NOS, may be i should try new production versions....
one tiode is already started to distort heavily,
i am not an expert,  something else might have caused it, but its one of the high voltage side!
i feel safer switching to 6.3VDC,
i would use AC but its too much of a hassle for me to dial in and calculate, i still have to do many things
before i close the lid on this mixer....

after that, i will work on AC heater.

 

[ruffrecords]

my tubes are NOS, may be i should try new production versions....

I have never had much luck with NOS tubes despite the hype surrounding them. I have tested large numbers of them and found large variations in parameters. I have found current production to be much more consistent.

Cheers

ian

 

[kambo]

i have some 1950' Sylvania pairs, i dont use them on WIP projects, but
i couldnt hold myself and i have tested  a pair on summing amp of my mixer...
they do sound noticeably better then RCA or random NOS 6SN7,
distortion figures are better and more smooth too....  price is pretty high, but i think it worth it!

ordered some Tung-Sol 6SN7GTB new production, hope they sound as good as... we will c

 

[ruffrecords]

i have some 1950' Sylvania pairs, i dont use them on WIP projects, but
i couldnt hold myself and i have tested  a pair on summing amp of my mixer...
they do sound noticeably better then RCA or random NOS 6SN7,
distortion figures are better and more smooth too....  price is pretty high, but i think it worth it!

ordered some Tung-Sol 6SN7GTB new production, hope they sound as good as... we will c

The problem with 'they do sound noticeably better', apart from it being a subjective term, is that it is very dependent on the circuit design. It is quite probable that the average parameters of NOS tubes differ from those of current production (after all, the data sheets only define a 'bogey' tube and production variations of 10% were not uncommon between manufacturers of the same tube). What this means is that, if you use the tube in a simple common cathode circuit, the sound (distortion) depends on the component values and the actual tube. If you just change the tube it is  not surprising the sound changes.  You see lots of articles on the web comparing different makes of the same tube type in a circuit like this and they all fall into the same trap of rating or slating a tube based only on its performance in that circuit with one set of component values.

I don't want to dissuade you from tube rolling because I know a lot of people get a lot of enjoyment from it. Just be aware of its pros and cons.

Cheers

Ian

 

[CJ]

maybe try a couple of 47 ohm 2 watters in series along side the 6SN7's to see if you can coax them into running together, never tried it, can't vouch,

GE, RCA, Sylvania, Westinghouse, they all swapped tubes and rebranded,

"hey dude, just got an order for 1 million 6GH8's for Zenith and only have 500,000 in stock,"
"ok, let;s deal, we need 250,000 6BQ5's and the line went down, we give you the GH8's for the Q5's"

so tube buying has always been kind of a gamble, but you can improve your odds with a little knowledge,

yes we like brand X over here, but do not want to drive prices up, 

here is a list of 6SN7 types with plate resistor and distortion figures along with listening tests,  sorry about the large size, but hey, this is good stuff>