Strange Situation With Modulator

[N0WVA]

I picked up this modulator a while back and some of you guys helped me identify it and the mod iron. Its basically a handbook modulator with a 12ax7, 6c4, and 807's. Well Im in the mood to turn my SG rig into a plate modulated one. I hooked it up and noticed the 6c4's filament is not lighting. Examination under the chassis shows that the pin3 has the appropriate voltage applied and there are wires also feeding the 807s and 12ax7 filaments running off of it. However, pin 4 has no connection, nor does it look like it has EVER been soldered to, and I beleive it should be grounded. This is strange. How did this thing ever work like that? Am I missing something? SOmething about this tells me its not 100% homebrew, like the stenciled writing by the connectors.

[KC4VWU]

Looks like a pretty well built HB, but I'd unsolder those driver iron leads and pop a grommet in there. I've got a pair of 807's worked up HB from the late 50's; I need to get it out and smoke it over. It is strange that pin never got grounded. Wonder if it ever really saw service?

Phil

[W2PFY]

Pin 3 & 4 are the filaments. Check to see if you have 6.3 volts on pin 3. If so, ground pin 4.

Then maybe before hooking it all up to your transmitter, you may want to  load the output of the modulator transformer into an appropriate value resistor rated for the output of the 807's. Perhaps then you'll see what the mod is capable of doing? One thing you might consider before going too far is to see if that driver transformer is good. Let us know how you make out

  

[N2DTS]

Looks very homebrew to me.
I cant imagine putting knobs in strange places if it was not.

It most likely never worked, maybe the person who built it could never figure out why, or never finished it.
Maybe look for other things not done...

Brett

[N0WVA]

Its almost like whoever built it went SK before even trying it out. But they sure did a good job with it. I soldered the floating pin #4 to ground, pulled the 807's and it lit right up! Theres a big rheostat for bias, and I set it for -22 volts. Also on the "output" 8 pin socket, there is another wire besides the voltage in and audio out wires. Looks like they have fixed bias going to a pin. 

I get 650 vdc for plate voltage and around 325 for screen. There is an adjustable slide power resistor for dropping the voltage down between the plate voltage and regulator tubes. I pulled the rectifier tubes. Something strange is going on there. Both of them light up purple and blink,blink,blink, like a turn signal on a car. Its an oa2 and 0b2 wired in series to ground.

What can I use for a load and how do I test this thing before hooking it to the transmitter? High power tube audio is rather new to me and Im not sure how to proceed. Its not like I can use a speaker for a load.

[N2DTS]

The blink blink blink of the regulator tubes sounds like there is to much capacitance after the regulator tubes.
You can put a SMALL cap after regulator tubes, but not much.
You should measure about 25 ma through the smaller regulator tubes, about 35 through the big octal types.

I would just hook it up to a rig and try it.
You can use a power resistor that matches the output impedance and scope it, measure voltage across the resistor to get power, look at the waveform on the scope to see the max power before clipping....

If the mod trans is say 5000 ohms, you use a 5000 ohm resistor, power should be at least 40 watts, that might get quite hot, more is better....

Brett

[N0WVA]

The blink blink blink of the regulator tubes sounds like there is to much capacitance after the regulator tubes.
You can put a SMALL cap after regulator tubes, but not much.
You should measure about 25 ma through the smaller regulator tubes, about 35 through the big octal types.

I would just hook it up to a rig and try it.
You can use a power resistor that matches the output impedance and scope it, measure voltage across the resistor to get power, look at the waveform on the scope to see the max power before clipping....

If the mod trans is say 5000 ohms, you use a 5000 ohm resistor, power should be at least 40 watts, that might get quite hot, more is better....

Brett

Hmm, junkbox has no heavy duty resistors. I guess Ill have to fly by the seat of my pants. Im working on the regulator tubes now.

[N0WVA]

Ok, I figured out why the regulators were snapping. The screens each had a bypass capacitor, and one of them was shorting out some DC to ground. They are even installed backwards. I lifted the ground leads on both and now the regulators work fine. The white one is the one that passes DC. Regulated screen voltage is now sitting at 285vdc.

[KC4VWU]

Don't forget, you're at audio frequencies here, so a kludge of fat wirewounds equivalent to needed specs would be suitable for the load for testing purposes. Get out the scratch pad and a calculator!

Phil

[N4LTA]

Not sure what you mean by the bypass caps after the regulator tubes were put in " with the wrong polarity" - but if he has electrolytic type caps after the VR tubes - you are going to have a relaxation oscillator - the bypass caps should be no larger than .01uF.

Pat
N4LTA

[N0WVA]

Not sure what you mean by the bypass caps after the regulator tubes were put in " with the wrong polarity" - but if he has electrolytic type caps after the VR tubes - you are going to have a relaxation oscillator - the bypass caps should be no larger than .01uF.

Pat
N4LTA

Each of the two capacitors are .0015, so the two together are .003. There is a stripe on one end of them. At first I thought they were polarized, but second thought they arent.


I found an assortment of 10 watt resistors, Im going to build up a dummy load and see what this thing does.

[WA1GFZ]

VR tubes may be pulling the supply below ignition voltage when they conduct current causing a relaxation oscillator. You might need to reduce the series resistor value to keep them lit. Look at the voltage across the series resistor to determine if the current it too low to keep them lit.

[kb2vxa]

Those caps aren't polarized, however the stripe indicates outside foil. Rule of thumb; this lead goes to ground or the low side of the circuit which provides shielding, thus reducing hum and noise pickup.

[KC4VWU]

Or, just bag the VR tubes totally and solid state the reg circuit. Use a low power zener diode to drive a pass transistor like a TIP-50.

Phil