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Old 12-23-2017, 09:28 PM
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3:1 interstage transformers?

Hello all,

As we head into yet another long, cold, dreary, dark winter here in Michigan, I've decided I want to try my hand at yet another amplifier build. Everything I've built prior has been RC coupled; I'd like to try something using interstage transformers for a change. It won't measure all that great, but I've heard some pretty decent sounding amplifiers from just prior to and just after WWII that actually sound decent with zero negative feedback.

Since I have a couple of type 5930s lying around (basically a 2A3 shoved into a tubular bottle), I figured I'd give this circuit from September of 1947 a shot:




I was thinking of trying this transformer from Hammond for the two interstage units: https://www.tubesandmore.com/sites/d...es/p-t124b.pdf. It won't break the bank, and it looks to be pretty cheap at $40 to $60 depending on the retailer. Should this work, or does anyone know of something that might better fit the bill?
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Old 12-23-2017, 11:42 PM
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Looks worth a try to me.

It's been a long time since I've analyzed tube circuits, but it looks to me like the tone control works against the plate resistance of the 6C5GT, so if you are planning to sub something else you may have to adjust the value of C-2 to get the planned break frequency.
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Old 12-24-2017, 10:22 AM
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Well the math works out anyway. The 6C5 is more than capable of providing the necessary voltage swing according to the RCA 6C5 datasheet.

Since the output stage of two 2A3s/6B4Gs/6A5Gs is operating in fixed bias at the "standard" datasheet operating points, the grids of the output valves only need about 124 volts peak between them to drive the output stage into clipping. The output stage is class AB1, so the current draw of the grids is negligible.

The output stage requires 124 volts (peak) between the grids. 124 volts on the secondary would require the 6C5 to swing about 42 volts (peak) on the primary, which is well within the abilities of the tube. The 6C5 will be "loafing", so the distortion from the driver stage should be quite low. The distortion of the output stage will predominate, and a total THD for the entire amplifier should be ~3.5% just before clipping. Not bad for pre-war technology with no feedback.

This all assumes I can keep the B+ voltage stiff enough. The power supply simulates well so I'm not terribly worried about B+ regulation.
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Old 12-26-2017, 08:37 PM
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I did another, more accurate, simulation of the circuitry in SPICE. I had forgotten how much I like the back-biasing technique for fixed bias output stages. Some of the safety and convince of cathode bias while diving up only some of the distortion and power output advantages found with a true fixed bias output stage.

I'll definitely use the type 6A5G for lower hum. I'm not spending stupid money for a pair of good 2A3s.

I may substitute the 6C5 for a triode strapped 1620. IIRC, the 6C5 is just an internally strapped 6J7 with the grid brought out to a pin, not the cap.
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