DC heaters

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Andy Le Blanc
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Joined: Sat Dec 22, 2007 1:16 am
Location: central Maine

DC heaters

Post by Andy Le Blanc »

Looking for a quick stupid simple DC solution for one tube.
I'm noodling with a 6sl7 zero bias stage,.. grounded cathode.
Great tone but very prone to hum.
I can get away with running the entire pre series at 12 v, less current, but a solution that takes less space
than a voltage double'r and filter would be nice.
lazymaryamps
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martin manning
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Re: DC heaters

Post by martin manning »

Did you try elevating the string? Two resistors and a relatively small cap for that. Otherwise, you can do a FWB like below. !000u cap works fine for one 12AX7, you might want to go larger.
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Andy Le Blanc
Posts: 2582
Joined: Sat Dec 22, 2007 1:16 am
Location: central Maine

Re: DC heaters

Post by Andy Le Blanc »

Yes, but its been remarkably stubborn. Trying to use old tubes
In as old of a way as possible. Its just the first half of the pre tube.
If dc heaters dont help ill be looking very closely at layout.
Very very finicky. Im useing 6sl7 6j5.. Amp is nearly silent
Other wise.

Im assuming the junction of the 100r is to ground?

Best
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martin manning
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Location: 39°06' N 84°30' W

Re: DC heaters

Post by martin manning »

Andy Le Blanc wrote: Sat Jun 03, 2017 9:00 pmIm assuming the junction of the 100r is to ground?
Yes, correct.
bal704
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Joined: Tue Sep 20, 2011 10:45 pm
Location: USA

Re: DC heaters

Post by bal704 »

Got room for a small 12vdc power supply (like a meanwell type)? Add a simple voltage divider with a couple of resistors to get you 6V.
Andy Le Blanc
Posts: 2582
Joined: Sat Dec 22, 2007 1:16 am
Location: central Maine

Re: DC heaters

Post by Andy Le Blanc »

Oky...

Reworked the pre towards recommend layout considerations.
Attention to wire placement and high impedance stray capacitances....
For example, one of the grids of a 6sl7 is adjacent to the heater on the socket.
Use the other one.... Much quieter and better perfoming amp.

Now I can very clearly hear the 60 cycle heater hum.

I want the zero bias first gain stage for the 5% harmonic content.

But jeez... The simplest way to go is to cathode bias the rig on
Pre amp heaters as a series string. Like an old dano..

But then you have to contend with a controlled heater warm up period.
Grampy's old black and white TV.. seems to take forever.. But it works.

I'll series the heaters the lower the ma. requirement and try a voltage
doubler before replacement of the 6sl7 and 6j5 with 12sl7 amd 12j5.
lazymaryamps
Andy Le Blanc
Posts: 2582
Joined: Sat Dec 22, 2007 1:16 am
Location: central Maine

Re: DC heaters

Post by Andy Le Blanc »

Un-expected resolution...

The problem was heater hum. I like old octal base NOS tube types.
But these come with a strong likelihood of heater cathode leakage.

So the focus became the heater... Floating the 6.3 fil., hum dinger,
And DC heater supply... Etc...

Nothing worked, which was a surprise... Even dc supplies resulted
in even worse switching noise... Not even brute force large cap values.

The resolution was feedback. Nothing crazy, less than typical
fender ratio... I try to avoid feed back but in this case it was the solution
lazymaryamps
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