A foray into the world of glowy stuff.

andrew Ivimey · Post by **andrew Ivimey** » Tue Jun 05, 2012 10:15 pm

Didn't Nietzsche say something like ' why use a thousand words when ten words was enough' adn some moribund sub-editor said the picture/words equation thingie.

But then all Friedriche Nietzsche was talking about was the meaning if life ... ek ek ek ek ek ek ek etc.

for valves I like a schematic.

pre65 · Post by **pre65** » Tue Jun 05, 2012 10:25 pm

andrew Ivimey wrote:
for valves I like a schematic.

Me too.

The earliest example known for that phrase is from the text of an instructional talk given by the newspaper editor Arthur Brisbane to the Syracuse Advertising Men's Club, in March 1911:

"Use a picture. It's worth a thousand words."

brig001 · Post by **brig001** » Tue Jun 05, 2012 10:52 pm

Out of interest, how much current do we need to be able to push into the grid for a2 operation? There is significant rounding even when fed directly from the cathode follower, so I imagine we could be talking tens of milliamps - does that sound right?

Anyway, schematic attached and just noticed that R8 can come out.

Brian

Post by **Nick** » Wed Jun 06, 2012 8:12 am

Depends on the valve, but yes, 10ma is entirely possible. Just watch the g1 dissipation levels.

Mike H · Post by **Mike H** » Wed Jun 06, 2012 1:53 pm

Tends to increase exponentially with Voltage, it's not a direct relationship. 10's of mA is quite possible.

E.g. here is the graph for 801A:

Mike H · Post by **Mike H** » Wed Jun 06, 2012 1:54 pm

Oops transparent background

brig001 · Post by **brig001** » Wed Jun 06, 2012 4:51 pm

That makes sense, as the plate voltage drops, the grid takes more current. I think it is worth scoping the anode when we are going into a2. Will have a look later and post back.

Brian.

Paul Barker · Post by **Paul Barker** » Wed Jun 06, 2012 9:43 pm

I can't find confirmation anywhere that it is OK to drive the KT88 in A2.

But I have a set of 6550 triode curves which show you can go +15v. I wouldn't go any further than that.

They don't give the G1 current capability, and they don't show the amount to expect on the characteristic curves. But at least there is a +15v grid curve.

So I am less uneasy now. but I am half expecting to hear a tale of woe any day now.

It was the 6v6 which I was suggesting you drive into A2. For that they give you grid current figures, when the grid is +20v and the anode voltage is the same grid current matches anode current at 40mA.

So we will find the same with the 6550, so if we restrict positive excursion to +15v as indicated by the curves below, when the Anode is also at +15v grid current and anode current will each be 50mA. You can probably extrapolate the rest from the grid current curves for the 6V6 enclosed.

chris661 · Post by **chris661** » Mon Jul 09, 2012 9:54 pm

To get back on with this for a moment...

I'm still trying to get the little output transformer to saturate.
So far, I've been unsuccessful, as the valve previous to the 6550 clips before the output stage gives up. There's about 2k of resistor in series with the output transformer (which itself has a 500ohm primary), and I've been wondering if bypassing those somehow might put the transformer under more stress. Thoughts?

I suspect what's needed is to return to the original class A1 schematic, and actually get a power supply that'll do the 400V DC required: the PSU I'm using sets off around 250v but drops as things warm up.

Perhaps a sturdy mains transformer is in order.

Late edit

Something like this, perhaps?
http://www.ebay.co.uk/itm/BIG-HT-TRANSF ... 51a02c7cc1

Mike H · Post by **Mike H** » Tue Jul 10, 2012 12:30 pm

In what way does it have a 500 Ohms primary? Sounds like DC resistance.

I don't think putting resistors in series will help at all, you'll end up with largely a resistor load, which does not behave like an inductor.

Getting single-ended to clip evenly top and bottom the same is quite awkward actually, it's either valve cut-off first (current falls to zero), or in the other direction, forward grid conduction overloading the driver and clipping it.

From what you're saying could be the latter is happening.

chris661 · Post by **chris661** » Tue Jul 10, 2012 9:36 pm

Mike H wrote:In what way does it have a 500 Ohms primary? Sounds like DC resistance.

The winding itself is 500ohm DC resistance. There's a pair of big resistors (probably 5-8w power rating between them) in series with them to give ~2.5kohm total DC resistance. This was stock with the old valve radio - I decided that would be close enough to the 3kohm primary required for the amplifier to work.

I can see that there's power being lost across those resistors, and wondered if removing them would get me closer to the results I'm after.

Chris

Post by **Nick** » Tue Jul 10, 2012 10:42 pm

I decided that would be close enough to the 3kohm primary required for the amplifier to work.

Thats not how it works Chris. The 3k you are after is the 8R speaker transformed from the secondary to the primary. You want to keep the DC resistance in the anode circuit as low as possible.

chris661 · Post by **chris661** » Tue Jul 10, 2012 11:14 pm

I did wonder how those two bits joined up - thanks for the clarification.

I'll do some thinking tomorrow and decide what the implications are.

Mike H · Post by **Mike H** » Wed Jul 11, 2012 3:48 pm

Just looked again at the original radio circuit, those resistors are HT dropper / filter resistors for the rest of the radio pre output stage, not for the output stage itself, whose OPT gets its HT straight off the power supply (rectifier valve then reservoir C87, 50uF).

No resistors in series!

Non nein niet...

Mike H · Post by **Mike H** » Wed Jul 11, 2012 3:52 pm

Appendix ~ the OPT will have AC reactance which as Nick said is much more than its DC resistance, almost certainly in kilohms.

audio-talk

A foray into the world of glowy stuff.

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