heater voltage fix

[Andy Le Blanc]

Assuming that you can use a PT from one design for another can lead to
real issues in the practical build you have to face.

Today's bench quandary is how best to adapt the heater for a different tube compliment.

So...I've gotten around to dressing the AC of my amp, the power cord switch, fuse and heaters.
upon fire up with no load everything checks, the light lights all is good.

the heater ac v no load is 7.5, but with the intended tube compliment its 7.1...

that's too hot, that will promote cathode stripping and shorten tube life, not good.

Looking for a quick fix, the bone pile yields forth a couple high wattage
0.15 r resistors out of the power side of some old SS monster and they
get clipped in series on both legs of the Fil. dress and the heaters get
5.8v ac... that's a bit low, so I pull out one of the 0.15r and get 6.1v

that's not bad, smidge low but that'll promote a longer tube life etc..,

The question is whether or not running unbalanced heaters is a bad thing?
It seems like a good fix, but will it create other issues in the finished amp?

[billyz]

You could also use some diodes instead. 3-5amp rated should be sufficient. Paralled and reversed from each other.

[Jana]

If you use a faux center tap arrangement with a pair of 100 ohm resisters then there won't be a mismatch. You have 6.1 volts that is divided in half by the pair of resistors which is then referenced to ground.

[paulster]

If you use a faux center tap arrangement with a pair of 100 ohm resisters then there won't be a mismatch. You have 6.1 volts that is divided in half by the pair of resistors which is then referenced to ground.

That there is your solution.

The only potential issue with the heaters being unbalanced is the possibility of increased hum, but Jana's solution obviates this issue.

[Phil_S]

Looking for a quick fix, the bone pile yields forth a couple high wattage 0.15 r resistors out of the power side of some old SS monster and they get clipped in series on both legs of the Fil. dress and the heaters get 5.8v ac... that's a bit low, so I pull out one of the 0.15r and get 6.1v that's not bad, smidge low but that'll promote a longer tube life etc..,

I have a feeling, if you parallel both R's on one leg, you'll get much closer to 6.3V. Tack it in and see what happens.

[Andy Le Blanc]

just in from a gig...

I did put the series resistor in before the usual 100r pair
I had to double check this mornings efforts The v was riding high so I
scabbed in the .15 r...

I dont mind going a little low, I seen that referenced in more than one
source as beneficial to tube health

if the tone isn't right I'll parallel, diodes seem like a good idea

I have seen a few schematic with ww pot as a trimmer, dont need to go that far, keep it stupid simple..

I was just glad I noticed, could have been one of those forgotten
assumptions that comes back to bite you on the arse

just might have the power side up and running tomorrow or the next day
its a pentode pre so it will tell me pretty quick if the heaters going to be noisy

[Andy Le Blanc]

got a little bit O build funk going on...

line voltages were different this morning... and maybe a bad solder join

it settled into 6.5 v with .15r on both legs to the heaters, 3. 28 on both legs

I saw the pilot light change brightness, and remeasured. The line ac tends
to run high, so it looks ok, doesn't take much, hair, metal filings...