DC bias on heater winding ?

Question

Hi,

positive DC biasing the heater winding of a tube amp is occasionally used t o eliminate H-K hum in the pre-amp - a clever alternative to using a well f iltered DC supply for the same job as done in many guitar amps.

However, it means the heater circuit is no longer firmly grounded and could acquire a high voltage under certain fault conditions.

Marshall in their infinitesimal wisdom have done this on their latest model s

- the JVM 400 series. The 6.3V heater winding is floated to about +70V DC u sing a voltage divider off the HT.

Now, output tubes like the EL34s used are prone to failures that send their cathode to a high voltage or short pin 3 to pin 2 ( anode to heater arc ov er). When that happens, the heater winding rises to meet the B+ supply. Non e of the other tubes will like that - max rated H-K voltage is about 100V.

Maybe the idea is *too* clever.

.... Phil

Jeff Urban · Accepted Answer

If the cathode is positive I can see why to put DC slightly higher on the f ilament. It will prevent any conduction I would think. However +70 is up th ere. What cathodes are at that high a positive voltage ? All you need is an equal or slightly higher positive voltage.

I guess maybe for a common plate stage. (cathode follower) Perhaps a triode single phase splitter to drive push pull output tubes.

As far as getting rid of hum, you have to rectify that 70 volts, so why not just rectify the damn filament voltage in the first place ?

You never know what kind of drugs these engineers are on.

I remember an RCA TV like a CTC211 or something, projection. They had a str ing of like seven transistors just to turn on the convergence subsystem. Wh y ? And it wasn't like there were a bunch of protection lines going to it, just a voltage and that's it. It was also not in the feedback loop.

I think they just want s*it to be as hard to fix as possible.

John-Del · Answer

filament. It will prevent any conduction I would think. However +70 is up there. What cathodes are at that high a positive voltage ? All you need is an equal or slightly higher positive voltage.

de single phase splitter to drive push pull output tubes.

ot just rectify the damn filament voltage in the first place ?

tring of like seven transistors just to turn on the convergence subsystem. Why ? And it wasn't like there were a bunch of protection lines going to it , just a voltage and that's it. It was also not in the feedback loop.

I think it's time to go back to A batteries..

Phil Allison · Answer

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** Marshalls all have cathode follower in the pre-amp where the the cathode is about 140V above ground.

** Yep - about 90V above ground there too.

** Derived from the HT so just two Rs and one small C.

DC heater volts are used on many guitar amps and some hi-fi ones as well.

.... Phil

Jeff Urban · Answer

I am aware of that. If they use a single triode for a splitter the cathode voltage should be what, a third ? In a hifi amp, if they got a full blown W illiamson and I mean with the cathode taps they are going to need a hell of alot of voltage on it. I am not sure the old ECC83 an do it. Most full Wil liamsons with the cathode taps have another stage, sometimes with a bootstr ap to get the drive they need. But that type of amp is rare. Mac made one o r two, my Uncle built a pair of monoblocks using Chicago BO-15 transformers . The BO-14s do not have cathode taps. Those were actually popular transfor mers back then, but the BO-15 was really hard to get.

Tubes, I tell you what, those amps my Uncle built sounded better than any t ube amps I ever heard till then or since.

Phil Allison · Answer

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e voltage should be what, a third ? In a hifi amp, if they got a full blown Williamson and I mean with the cathode taps they are going to need a hell of alot of voltage on it. I am not sure the old ECC83 an do it. Most full W illiamsons with the cathode taps have another stage, sometimes with a boots trap to get the drive they need. But that type of amp is rare.

** I have rarely seen a total red herring chased down a rabbit hole so thor oughly before.

Quite something.

...... Phil

Jeff Urban · Answer

Glad you like it.

DC bias on heater winding ?

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