Old Lafayette valve radio problem

Valves fade away with use, as the emitting surfaces of the cathodes wear out. Old paper capacitors leak. A leaky coupling capacitor will shift the bias on the valve it feeds, which can let a more-than-minimal signal drive the valve to saturation. Another suspect is the power supply, where a dried-out electrolytic capacitor can cause all sorts of symptoms. Carbon resistors can increase in value over time.

Can you find the schematic? It may be glued to the inside of the cabinet. These sets are usually easy to troubleshoot with a DMM if you know what the voltages at the cathode, grid and plate of each valve are supposed to be.

The real problem might be finding parts. I don't know whether those once-common valves are still manufactured anywhere. When the new-old-stock supplies salvaged from bankrupt distributors' warehouses are finally used up, these radios will become pure curiosities. The antique-machinery buffs have it easier. A modern machine shop can make any part of a steam engine. Making a 6SQ7 requires a specialized factory, the last of which probably went to rust and ruin with the collapse of the Soviet Union.

Couldn't this modern machine shop build said specialized factory?

Bob

Could be a lot of things. The usual way to repair these is with a signal tracer or a scope. You need to find the source of the distortion.

In theory. In practice, it would be such a long, capital-intensive bootstrap process that nobody is likely to manufacture the old tubes again. I can imagine an obsessed (and wealthy) hobbyist building a shop that could produce those receiving tubes on a one-off basis, but but it would probably take him years to build the first working samples. Can you still get mica for the element spacers, or would you have to learn to make ceramic wafers?

Oops. I forgot about the one or two companies that supply the more-money-than-sense segment of the audiophile market, but they aren't interested in things like pentagrid converters and remote-cutoff pentodes.

It's far more likely that people will design plugin solid-state substitutes for those tubes, as was done to convert the last generation of vacuum-tube voltmeters to FETVMs.

Ask at rec.antiques.radio-phono, too

Audio Power Pentode. Powers the speaker. Weak tube could cause distortion.

Power Supply Rectifier. Provides High Voltage for tubes. Weak tube could cause low Voltage which causes distortion.

First Audio Amplifier and Detector. Weak tube would likely cause low volume.

Pentode RF/IF Amplifier. Weak tube usually reduces sensitivity.

Oscillator and Mixer. Weak tube usually reduces senstitvity.

If you can find the Model No. on the cabinet or chassis, try to find the schematic and service/parts information in Sam's Fotofacts or Ryder Service Manual. Sometimes, you can find these in the local Public Library. Or there are sources on the WWW.

There are a few sources of old valves on the internet. Also you can find vendors at flea markets and Ham Swap Meets. Probably the easiest way to determine if a tube is worn out is to substitute a known good tube. In the old days, there were tube testers in drug stores and your local radio repair shop. If you have a schematic and know vacuum tube fundamentals, you can make determinations of tube or circuit (Resistors, Capacitors, Etc.) by making voltage and resistance measurements.

The circuits for radios with your tube lineup were all very similar and were referred to as the "All American Five". After repairing a few, most radio service people could visualize the schematic in their head just by looking at the under chassis wiring. They could sometimes diagnose failures by touching grid circuits with a screwdriver or test lead and listening to the audio change.

Be careful in trouble shooting. The power supplies for tube anodes could be from 150 to 300 volts. And capacitors can retain a dangerous charge even with the power switched off.

Good Luck, Ken

If you need tubes, here's a good place to start.

Antique Electronic Supply in Tempe, Arizona

They carry a huge assortment at very reasonable prices. For example, the 6V6GT sells for about $9.00 The 6SK7 runs about $5.00

Living in the Phoenix area, I've done business with them and was more than pleased with their professionalism. They know tubes!!!!

I've never heard of such a thing, and I'm as old as some tubes. ;-) The things that give out are the old caps. Shotgun[1] the caps, and it'll probably work like new. [1] replace every cap in the unit with a new one.

...

Well, maybe you could buy one from these folks:

Cheers! Rich