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Old 7th January 2004, 10:41 AM   #21
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Quote:
Originally posted by richwalters
...I biassed the whole heater chain to minus 10V. That cured every snippet of hum...
Shouldn't you take the heater =above= cathode potential, i.e. *positive*?
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Old 7th January 2004, 03:48 PM   #22
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Shouldn't you take the heater =above= cathode potential, i.e. *positive*?
I think then you have the problem that electrons are being draw from the cathode - anode stream towards the heater at the cathode, or at least retarded in some way by it's field.

Once they leave the cathode and head towards the anode, I seriously doubt many of them would make it back to the heater, but they may still be influenced by any alternating fields produced by it.

The heater isn't coated with a thermionic layer, so it's quite bad at emitting electrons. It's also quite far away from the anode, so it makes it tricky for electrons to be pulled from it.

Running it at a negative potential, in respect to the cathode, means any stray electrons leaving the heater side of the cathode will be pushed back towards the cathode it's self.

While some electrons will also flow from the heater towards the cathode if the heater is made negative, even thought the heater isn't coated to perform the task very well, this current flow is proportionally less than if the heater was made positive.

I've seen this idea used on a particular guitar amplifier that uses virtually nothing to prevent AC humm from the heaters entering the signal line. Rather than use humm pots, filter caps and such, they just connected the suppressor grid to the heaters by way of resistors formed into voltage dividers, to provide a new reference, dropping the heater voltage to around 50 volts negative, with respect. Apparently it works quite well for the level of humm reduction expected. The amp is a beginners level, hands on model to help people learn about tubes... not an AN.
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Old 8th January 2004, 10:51 AM   #23
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Quote:
Why a 350 in the first place?
I'm not entirely sure of your question. If you mean, why use a design that needs a regulator, it's because I require a DC source for other components in the amp. The way I figured it was, why not make it the heater suppy since it's at about the right voltage anyway.

If I've misunderstood, and you mean why use the 350 regulator in particular, then it's because it was the first that came to mind when I was looking thru the RS catalogue. The K model, in a T03 package, is about five times more expensive than the T0220.

I am genuinely open to any recommendations for another regulator if this is the case.

Quote:
Nevermind, what do you want it to do?
I am worried about heat sinking it. I've checked the manufacturer's sheets, but none of them seem to mention it. When it's running around 1.5 amps it gets seriously hot, far too hot to touch for more than a second or so. I bought a couple of heat sinks and thermal paste, but the heat sink is still very, very hot.

Do you have any rough idea what kind of ratings a heat sink should have to cool these correctly?

I am currently using one with a rating of 17.9C per W.

Many thanks,
John
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Old 8th January 2004, 10:15 PM   #24
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Another Apple for someone to knock-off my head ???......The hum/heater dilemma is an interesting one... Iím not a physics prof but Iíve sort of sussed out whatís going on. Trying out three different vendor 7199 tubes in an identical circuit, they all behaved different in terms of hum performance. The differences were staggering...a Sylvania 7199 in conventional heater supply arrangement gave a hum valve 25dB worse than it's competitors..practically unusable for stage. That knocked the s/n ratio of my amp to -50dB instead of -75dB below 140 Watts.
The AC audio signal performance/THD between all is similiar.. The circuit as per RCA app note; www.triodeel.com/7199.htm.

Now the tricky bit.......One can do a simple experiment to see how the space charge between heater and cathode behaves.... a high u front end tube (pentode) in normal configuration to do this, with a high sens AC voltmeter on the anode o/p which can measure less than mV noise level and/ scope to monitor etc....the phase splitter output is an ideal vantage point but B+ must be low ripple..... Quote......>With input shorted (or via 600ohms), lift the AC 6.3V heater supply off ground, attach 100n cap between ground and heater circuit and then bias heater with minus 10V via high Meg ohm feed res (i.e +10M). Got that ? When the 100n or higher value cap is charged up to this voltage, the residual hum level drastically reduces. With the minus 10V source disconnected,....so long the charge stays in a the cap......which it does for a very long time..; except for leakage in other areas.....Surely this is proof of virtually no space charge flowing between neg biassed heater and cathode; and that the field charge is (just ) static space charge ?
Others better than myself can probably put a finger on this state of metastable field physics further.....

Biassing the heater supply positive volts w.r.t cathode, I needed to apply 50V before I got the same effect with only minus (neg) 10V. > ....The RCA 7199 spec sheet for breakdown voltage for heater/ cathode as plus OR minus 200V max peak volts, is testament to excellent heater to cathode insulation properties. ( I donít trust that figure for non-NOS tubes); Other designs use plus 50V but others also minus....So who wins ?? Unless someone can come up with an explanation, it doesnít seem to matter which way round the heater bias volts is. ..........I do get similiar response from the ECL82 triode sect...but far less reduction..
The bit in the 7199 spec about centre tapped heater; wellí, for non-NOS tubes might make the hum worse or for the better. ...However, the end result is I can use the same tube from different vendors that gives identical hum & noise performances. It may not be the recipe for any high gain tube.........The only way is to lash a simple pentode input high gain circuit up and do the experimenting with same tube type but from different vendors. Mix AC & DC heater supplies with biassed plus or minus volts. . Itís the only way...However, one might not get far with well screened EF86ís but there are always suprises in store....There are deeper sonic issues following on from this.


rich
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Old 8th January 2004, 11:03 PM   #25
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Hi,

Rich,

It shouldn't matter one bit whether you bias the heater + or - with respect to the cathode for as long as the heater to cathode insulation is within the manufacturer's limits.

Mostly we bias positive because:

a) The cathode is at positive potential with respect to the heaters and:

b) A positive voltage is often easily derived from the B+.

Either way I fail to see what difference it would make.

Cheers,
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Old 9th January 2004, 10:46 AM   #26
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Default lm350 getting hot

if you're pulling 1.5A through an lm350 without heatsink (or a small one like you're using) it's quite normal that it gets very hot.
Very possible too hot, and it will shutdown.

What's the voltage drop over the lm350? It is often >3V.
Multiply this voltage by the 1.5A and you've got the wattage that you have to dissipate in heat. (so probably more than 5W!!!)
enough to burn you're fingers....

It doesn't matter if you use lm317, lm350 or something else. That dissipation stays...

Filip.
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Old 9th January 2004, 05:29 PM   #27
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enough to burn you're fingers....


Like I found out this afternoon...

I think I'll mount it to a cup shaped heat sink in the future. That way I can keep my tea warm while I'm at my desk!
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