Power op amp driving valve in A2

[Mike H]

A T-amp is class C output? I wouldn't, could have HF flying around everywhere

[Mike H]

Aaaargh !!! Paul !!! I had just about forgotten about that bloke until you mentioned it again.... Horror !!!

Yes I'd managed to forget it as well!

However a friend of mine aparently wins the hairiest backside contest, I heard tonight when he was passed out naked on the bathroom floor his wife sent for her friend to lift him into bed.

Too much information!

[Mike H]

Off-topic suggestion for Nick ~ as regards use of an op-amp to regulate a supply rail (low impedance negative as well as positive) how about one of these power amp chips like TDA2050? Is wired up same as an op-amp.

[pre65]

A T-amp is class C output? I wouldn't, could have HF flying around everywhere

Class D old chap.

Would do as a first step though, to test the concept without investing to much spondoolicks ?

[Mike H]

Oh right I keep getting it wrong, class D

I know what I meant.

You can try it but I'm not sure meself.

[pre65]

Oh right I keep getting it wrong, class D

I know what I meant.

You can try it but I'm not sure meself.

I might just, as an experimentalisation to keep me amused on cold winters days.

The old T-amp has got to be good for something.

Next up I need a design for a 250v (220-250v variable ideally) regulated PSU to power the 1626 valves. Each valve is 30ma so 60 + ma needed. The transformer will be the one from my original 300b amp so there are a few volts to spare.

[pre65]

I've decided to go with a simple PSU to start with.

I have a Danbury 260v mains transformer and a couple of 10H chokes.

In fact I could use 3 chokes and split the supply after the first choke, that would give each channel it's own choke.

[pre65]

Thinking about this amp, and having discussed it with a friend, I'm confused as how A2 operation will affect the t-amp driving the grid.

I've drawn the circuit as I understand it from P.Millett's article. Is it as simple as having a cap between t-amp and 1626 grid to protect the t-amp from grid current in A2 ?

Or is it more complicated perhaps ?

http://www.pmillett.com/pwrop.htm

PS there are 2 caps shown in the article picture, one seems to be a bypass cap for the cathode resistor.

[Mike H]

Can't put a capacitor in, would just be charged up, and the grid would end up zero or negative, we've been down this road before haven't we?

Output must be DC coupled to grid, and the current will be all in one direction, that's one reason why I wasn't sure a if a T-amp would be suitable.

[pre65]

Output must be DC coupled to grid, and the current will be all in one direction, that's one reason why I wasn't sure a if a T-amp would be suitable.

OK, so would your suggestion of the Maplin amp be any different ?

[Mike H]

I would think so because it's linear not class D. Have no personal experience of class D but am given to understand the output is like a high freq. square wave with variable mark-space ratio, followed by substantial filtering to then get rid of it, leaving just the DC shift resulting from the difference in mark-space ratio. Don't think you can give it complicated one-sided loads, like a diode, which is what a valve grid is like.

[pre65]

Bugger.

[Mike H]

I rather thought so too.

Somone else more up on the theory of how they work might be able to confirm, or not....

[Andrew]

Off-topic suggestion for Nick ~ as regards use of an op-amp to regulate a supply rail (low impedance negative as well as positive)

I did this a while back, with Nick's help I managed to stabilize it, so it didn't continually blow up op amps, was proving a bit expensive up until then.

Andrew

[Nick]

Off-topic suggestion for Nick ~ as regards use of an op-amp to regulate a supply rail (low impedance negative as well as positive)

I did this a while back, with Nick's help I managed to stabilize it, so it didn't continually blow up op amps, was proving a bit expensive up until then.

Andrew

If I remember correctly, we got through a lot of desoldering braid that day