Tested the dual 807 amp

[N4LTA]

The amp tuned up but was a little disappointing but very clean and parasitic free.

The grid tank Q was way too high - impossible to peak with the variable it was so sharp. I guess I'll try to damp it with a parallel resistor - anyone got an idea where to start - I'm thinking 5-10K or so at 2 watts carbon.

The power supply choke buzzed so bad it rattled the chassis at full load. That one will go back to Hammond. It buzzed pretty bad at near no load.

The power transformer is rated at 400-0-400 at 230 Ma.  I should get 200 Ma out with little drop but it is squatting more than I like - down to 350 volt at 150 mA.  Didn't measure the drop in the choke - so I need to do that.

Got about 36 watts measured on the Bird and on the scope with about  52 watt input  - close to 70% efficiency  - not bad but not great - but a very clean sign wave with no sign of parasitics or instability. No color on plates or hot smell - so the 807s are loafing.

I may put in a 200 Ma choke rather than trhe 150 one that I have in there now. I think they are close enough in size to not show the holes.

Just glad it is clean and stable.

Pat
N4LTA

[KE6DF]

If you are using FWCT with a 400-0-400 volt transformer and a choke input filter, then about 350 volts out is what you would expect, isn't it?

I just tried this on the duncan amps simulator and I got almost exactly 350 volts out.

If you are using cap input, then more like 450-500 VDC under load would be expected.

Dave

[N4LTA]

The Hammond charts show a little more than that but not much. I was expecting about 370 and I get that fairly steady up to 100 mA and then it squats like the transformer or choke is bogging down. The transformer should deliver 200 Ma.

I need to do some more measuring and see what is happening. I am not really unhappy with 36 watts as I made this to test some alternate modulation methods and wanted an amp that is very stable and not working hard.

First thing I need to do is de-Q the grid circuit. It is so high that breathing on it changes it.

Thanks for putting the circuit in the Duncan Amps sim as it makes me feel a am not that bad off. It probably takes accout of diode drop, transformer drop and choke drop.

Pat
N4LTA

[KE6DF]

Thanks for putting the circuit in the Duncan Amps sim as it makes me feel a am not that bad off. It probably takes accout of diode drop, transformer drop and choke drop.

In the simulator, you can plug in the DCR of the transformer and the choke.

I didn't have that info so I just took a swag at it, but you can put the real numbers in and see what you get.

Dave

[N4LTA]

Changed out the choke and that sure made it a different amp - Totally silent compared to sounding like a rattle trap with the other choke. Got about 39 watts out with the 200 Ma choke and still might get a bit more out with de-Q ing the grid as it is just impossible to tune now. I'll try that tomarrow.

Now to start the modulator. Plan to use a Hammond 125GSE  with a 30H modified Heising choke and Steve's 3 diode limiter with a keep alive power supply. I'll drive it with a solid state 100 watt amp so I'll have plenty of audio drive.

What symptoms would I have if I have the grid tank turns ratio to high? As I add drive power, I reach a point where the output power drops rather than increases and I don't think the tubes are fully driven into saturation.

Pat
N4LTA