4x 6AG7 Amplifier

Discussion in '"Boat Anchor" & Classic Equipment' started by NW2K, Feb 11, 2019.

1. W1BRHam MemberQRZ Page

thanks! I was sure someone did it!

2. W9BRDHam MemberQRZ Page

Yes, and, if so, 837s were a stupid option. They're power pentodes, not beam power tubes, essentially 12.6-V 802s/RK-25s, with a much higher plate Z (and much lower max plate I and plate E ratings) than 6L6s/807s/1625s.

Last edited: Apr 19, 2020
3. W9BRDHam MemberQRZ Page

In class C you can run 75 W input per tube at max ICAS ratings (750 V at 100 mA plate) for the 807/1625. So you could safely run 200 W input with a trio -- in class C. In AB1 or AB2, they'd be less efficient and dissipate more power per input W, so you might need four to handle 200 W input within ICAS ratings in linear service.

N2EY likes this.
4. W1BRHam MemberQRZ Page

I was talking about output Z.

5. N2EYHam MemberQRZ Page

Yes! But, slight correction - it says combined plate impedance of 75 ohms and feeds RG-11 through a DC isolating circuit and presumably a low-pass filter.

The power supply uses 6 TV power transformers with 400 mA secondaries, and the filter capacitor has a total of 900 uF. (Must have a heck of a soft-start circuit too.....). The 1625s are operated Class AB1, grid-driven.

73 de Jim, N2EY

6. W1BRHam MemberQRZ Page

So, a few more toooobes

KM1H likes this.
7. W9BRDHam MemberQRZ Page

Yes, I paid most attention to the pic. Close enough.

N2EY likes this.
8. N2EYHam MemberQRZ Page

Let's do the math!

If 30 tubes in parallel gives 75 ohms output Z, then 45 in parallel would give 50 ohms output Z (all else being equal).

45 1625s at 25 cents each is only \$11.25.

Of course the plate current will be rather high....plus the cost of all the sockets, and all the wiring, and all the bypass capacitors.....

9. W1BRHam MemberQRZ Page

Filament transformer, cathode choke.... but what a wonder to behold.

WB4IUY likes this.
10. N2EYHam MemberQRZ Page

Don't need a cathode choke; the design was grid-driven AB1.

The heater transformer.....let's do some more math.....

The 1625 has a 12.6 volt 0.45 amp heater. Thirty in parallel would require a 12.6 volt 13.5 amp transformer. Or, wire them in series-parallel, and use a 25.2 volt 6.75 amp transformer. Or split the string to use what's available. (Since the PSU used six TV power transformers, heater power might not even be an issue.)

Or - no transformer at all. Ten 1625 heaters in series could run directly from the 120 volt AC line - properly fused, of course. Three strings of ten each, and there you go.

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