Advice for a newbie needed

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Mike H
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#46 Re: Advice for a newbie needed

Post by Mike H »

I can't make it make sense either.

I've also noticed no grid bias resistor for ECC83A ?

Possiblee 1 meg to ground.
 
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#47 Re: Advice for a newbie needed

Post by Ant »

The GE datasheet gives a 200r cathode resistor for ultralinear pp operation with 395v on it, so the op point on that schematic is going to be somewhere around the standard op point on the ge datasheet. Biased for a little more dissipation by the looks of it.
The 220mv may be a typo?
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Mike H
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#48 Re: Advice for a newbie needed

Post by Mike H »

Ant's beat me to it.

In the 6550 data sheet, this most closely compares with: "push-pull, ultra-linear, cathode bias, class A1". Ra-a = 6,500 Ohms. 34W, 1.5% THD.

Yes I also think the "220m?" is most likely a typo for 220mA, which is Ia + Ig2. That's for each valve. :shock:

Data sheet says Ia = 170 mA, Ig2 = 12.5 mA, Rk (R5, R6) = 200R, @ Va 385V.
 
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#49 Re: Advice for a newbie needed

Post by Ant »

Biased pretty hot then, there will be a little less dissipation with 270r in the cathode, but then the anode load is higher at 5k6 rather than the 3k3 or whatever it was on the darasheet. P'raps the discrepancey between the circuit and the datasheet is to keep it at the same op point for the slightly different load?

The mains tx and choke you have up to it shane?
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#50 Re: Advice for a newbie needed

Post by IslandPink »

Mike H wrote: Sun Oct 01, 2017 8:43 pm Yes I also think the "220m?" is most likely a typo for 220mA, which is Ia + Ig2. That's for each valve. :shock:
Doesn't make sense for a max dissipation of 35W ( plate ) or 44W ( total ) ... :D
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#51 Re: Advice for a newbie needed

Post by shane »

Mains txs are rated at 200ma, so max 100ma for each valve. That would put the cathode resistors around 470 to calm things down a bit, I guess?
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#52 Re: Advice for a newbie needed

Post by Ant »

Depends what the curves look like on the datasheet at that current.
I do have a 600ma toroid I can take off the el34 amp and donate if you like, dont use it anymore. its 280 - 0 @ 600ma so will put out give or take 395v with an ss bridge. There will be some losses in the psu, but should be close enough without having to rebias the cct to fit the transformers. Theres a 500ma 10h choke I can donate too, it'd have to be a stereo chassis rather than monos though
Cheers ant

Edit: I also have a 6.3v heater tx that has 3 6 amp windings on it if thats any use
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#53 Re: Advice for a newbie needed

Post by shane »

That’s really generous of you Ant. It’s a bit of a change of direction, so I’ll see what Matt thinks before I commit him to anything!
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#54 Re: Advice for a newbie needed

Post by Ant »

No worries, just drop me a message
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Mike H
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#55 Re: Advice for a newbie needed

Post by Mike H »

IslandPink wrote: Sun Oct 01, 2017 9:47 pm
Mike H wrote: Sun Oct 01, 2017 8:43 pm Yes I also think the "220m?" is most likely a typo for 220mA, which is Ia + Ig2. That's for each valve. :shock:
Doesn't make sense for a max dissipation of 35W ( plate ) or 44W ( total ) ... :D
The 35W is output power to speaker.

Which I suspect is peak.
 
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