Harmony H193

[drfunkphd]

I just got one of these given to me in rough shape. I recapped and 3-prong corded it (the usual...) and got it running, but it seems a bit weak.

Tubes are 12SQ7, 50L6, 35Z5. Yes, there is an isolation tranny or I would have put one in.

I know the 50L6 is rated at 8-10 watts from data sheets I have seen but it seems like it's not putting much out. I'm assuming it has something to do with the plate voltage being low because that tube set up was designed to run on the 117V line voltage.

Any suggestions on how to maximize these tubes? I considered a solid state rectifier instead, but then it screws up my filament voltages, right?

Anyone with some experience in vintage tubes care to offer some insight? Or do I just accept what I have?

[Resonator Guy]

For a 50L6 the power output at maximum voltage might be about 4 watts. The 10 watt figure is plate dissipation. 50L6 tubes are not in the class of 6V6 tubes or anything like that. You may have to accept what you have. They can sound fine, just not very loud.

[loudthud]

First of all you need to verify the amp is working properly. If you don't have the equipment to measure the output power, try a different speaker and/or a fresh 50L6. Check that the cathode resistor has not drifted in value. You could raise the B+ slightly by connecting a 1N4007 across the plate to cathode of the 35Z5.

According to the RCA tube manual, the output is 2.1 watts at 110V B+ with a 2000 ohm primary. The closest Hammond part has a 2500 ohm primary. I looked at trying to mis-match the Hammond part but, the only option is 1250 ohms to a 4 ohm load. You could try one of the Hammond parts, a 125ASE or 125BSE, or try to find another transformer that would work.

[drfunkphd]

I haven't had a chance to measure up all the voltages yet, but the resistors were all changed to fresh new stock, new coupling cap, new e-caps.

I took a second look and realized given the B+, that's probably the expected output. I suspect I can get a little more out of the 12SQ7 as it it grid leak biased, but not to it's full potential in the stock circuit.

Also, came to the realization, looking at old radio schematics, especially the AA5 schematics, that the whole deal was designed around 117V input voltage and then the tubes were designed to add up to 117V so the didn't have to use a transformer.

[drfunkphd]

OK, just an idea, maybe not a great one...

What if i were to use a SS rectifier which should bring the voltage up, replace the 35Z5 with another 50L6 and then I would have about 6-8 watts output. If I split the voltage off the tranny before the full wave rectifier and use that on the heaters, then I can use the rectified power for plates and it should give them another 40 volts or so...

Any thoughts?

[loudthud]

You would have to be careful with a full wave rectifier that you do not exceed the heater to cathode voltage rating of the 12SQ7 which is +/- 90V peak. The rating for the 50L6 is +/- 150V peak. Tubes like the 12SQ7 can pickup hum if the heater to cathode voltage is too high, they usuallly like being on the "ground" side of the heater string when a half wave rectifier is used. Other than that, there is the issue of the optimal output transformer impedance.