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Old 28th October 2004, 11:17 PM   #1
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Default DHT and resistors in parallel w/ heater

Hi,

I am designing my amp based on DHT (i.e. driver=300B and power=GM70). I have been trying to find maths behind the values of resistors which are connected in parallel (?) with heater/cathode. I guess I know their functio, but what resistance should they be ? Any reference appreciated.

TIA

Yag
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Old 30th October 2004, 07:31 AM   #2
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I posted this so it doesn't look like you're being ignored....

My feeling is that it is not too critical as long as they're well matched and don't load the filament transformer outside its specifications.

Wait for someone more knowledgeable to comment though
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Old 30th October 2004, 08:35 AM   #3
SHiFTY is offline SHiFTY  New Zealand
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Do you mean parallel variable resistors? This is often a hum balance resistor.

If you mean series resistor, this will be the cathode resistor. I would seriously advise studying up a bit on how tube circuits work before embarking on such an ambitious project... Perhaps build a 300B amp first, then a GM70?
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Old 30th October 2004, 09:01 AM   #4
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I'd have to echo SHiFTY's sentiments.....

I think the GM-70 would require an 800V+ power supply capable of significant current output....

Needless to say this is extremely dangerous!

Maybe a 2A3 design first? You could use it to drive the GM-70 later!
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Old 30th October 2004, 10:18 AM   #5
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Guys,

Thank you for your care and warnings. I know HV is dangerous as I worked with Roentgen tubes (something like 70kV - 120 kV) and this is not my first tube amp. If I sounded like a newbie, forgive me. I was burning the midnight oil. But ... I have searched web pages for the criteria of choosing such values for the resistors and found none.

To clarify my question - those are two kinds of resistors:
- in series with a heater which limit the filament current,
- in parallel with a heater which balance hum.

If those values are taken arbitrary - what should they be like ? Any experience ? Do they introduce NFB ?
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Old 30th October 2004, 12:45 PM   #6
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Can you refer to a particular schematic? Very likely the resistors you mention are also part of the biasing network.
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Old 30th October 2004, 06:54 PM   #7
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Quote:
Originally posted by analog_sa
Can you refer to a particular schematic? Very likely the resistors you mention are also part of the biasing network.
The schematics is attached. I am looking for directions on choosing resisitors:
- Rs1 and Rs2
- Rp1 and Rp2

Simply, I don't to re-invent wheel
Attached Images
File Type: jpg res-heater-circ.jpg (7.4 KB, 323 views)
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Old 31st October 2004, 02:13 AM   #8
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i'm not too sure about the function of Rs1 and Rs2.

but for Rp1 and Rp2, they are not required if you are using P1. P1 would be the hum pot. a 100R P1 should be fine. do note wattage has to be sufficient for the current flowing thru.

an alternative is to use just Rp1 and Rp2 without P1. johannes lebong uses 33R for these.

Click the image to open in full size.
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Old 31st October 2004, 06:55 AM   #9
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Hi Yag

Much clearer now. None of these resistors are commonly used. Rs have the effect of reducing the filament voltage, ie if the transformer has higher output than the tube requires. Rp have no particular purpose as far as i can see and may be there to just trim the value of the pot. All these resistors will contribute to the value of Rk and thus have effect on the biasing.
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Old 31st October 2004, 07:34 AM   #10
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This unbypassed resistance in the cathode circuit, I believe, will cause some degenerative feedback.....?

But essentially every amp using DH valves heated by AC uses a similar arrangement so it probably isn't something to worry about
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