Info Needed: Silverstone Model 18801 Stereo (Tubes)

You don't say how many other tubes there are. There may be two parallel strings of tubes. Do any of the tubes go out when you [pull one of the 50 EH5's?

H. R. Hofmann

Tim: This is a typical (of the era) hot chassis design that wired the heaters in series to add up to 120 VAC.... if there were not enough tubes to add up to the line voltage then a suitable high wattage resistor was placed in series. The only isolation available was with the speaker output transformer that isolated the speakers, speaker jacks and terminals. Everything else was "hot", the input jacks, the phono cartridge wiring and phono cartridge (probably a low quality ceramic or crystal) and the metal shafts of the chassis mounted switches and controls.... be very careful. Are one of the channels working? If the tubes are lit, you should get some kind of hum as long as you have rectified DC getting to the plates of the 50EH5 tubes..... the red & blue wires on the speaker ouutput transformer are from the Primary windings.... usually the red wire goes to the B+... and the blue wire goes to the plate of the output tube. You should have approximately 140 to 160 V DC at these wires, assuming that the selenium or tube or whatever rectifier is working and that there are no open resisitors, shorted capacitors (old electrolytics are immediately suspect and if bad would cause tremedous hum) or other faults causing the lack of B+. This is not a complicated circuit and you should be able to figure this out without a schematic.... many times a high out phono cartridge could be connected directly to the grids (through a resistor) of the 50EH5 and they could produce ample output without a preamp tube.

50EH5 pin 1 Cathode & G3, usually connected to ground through a resistor ... 150 ohm approx. pins 2 & 5 G1 signal input pins 3 & 4 heater (50V) pin 6 G2, usually connected to B+ through a dropping resistor, should measure at least 100 VDC pin 7 Plate

Good luck and be careful with this hot chassis design. electricitym . . .

Hi!

You might wish to check the speaker system and its wiring. Sometimes the voice coil in a speaker opens up, either because of abuse or normal wear and tear.

If the speakers in this set are a conventional permanent magnet type, you can disconnect the leads from them and use either an ohmmeter on the resistance range OR a nine volt battery across the contacts of each speaker. Clicking sounds or static from the speaker when you connect the meter or battery indicate that it is good. Don't leave the meter or battery connected for very long, or you may open the voice coil.

This is a so-called "line connected" or "series string" set. The tubes are wired in series so their heater voltages will all add up to roughly 110/120 volts. If the number of tubes and their voltages don't add up to line voltage, then a large resistor may be used to drop the voltage to the point where it is needed.

You might look into replacing this resistor (if your set has one) with something more modern, such as a diode. This will make the set run cooler and more efficiently.

Be VERY careful when working on this chassis under power. Better yet-- don't have the set on or plugged in when you're working on it. There is absolutely no isolation from the power line and you could really hurt or kill yourself by doing the wrong thing. If you want to test the set while it is apart, plug it in, make sure the (hopefully nonconductive) knobs are on, conduct your tests and then unplug the set when you are done.

Give this tube a visual inspection. You may be able to see the "getter' inside it. This should normally be a silver color. If it has turned milky white (or *maybe* red), the tube's vacuum is gone.

Maybe. It depends upon how the unit is designed. Chances are most of the tubes are good. You will find the most problems with supporting components, such as capacitors and resistors. The capacitors may be left alone if they work, but for reliability (and possibly safety) reasons you should replace them if it all possible. You should also give thought to reworking the power system if it turns out that the radio does in fact use a dropping resistor.

For more information, you might also try posting to: rec.antiques.radio+phono

William

Several of the previous posters ignored the fact that the original poster stated that when one of the tubes was removed, the other filaments stayed lit This is not quite as simple as they believe.

H. R. Hofmann

Well I do see a huge wirewound resistor bolted to the chassis near the ac input, so that is probably is limiting the current to the heaters. There is no sound from either channel. I tried my little signal injector into pin 5 of the tubes, but got no output at all. If I inject the tone into the 12ax7, I get a very faint tone out of the speakers.

I will check into the dc supply though, as it would take out both channels if it was absent. I have found an old schematic from a Fleetwood 50eh5 that looks similar to what I see in the chassis.

I guess I did not mention that I have the specs for the tube, so I know the voltage range that should be present. My concern was that someone had removed the transformer and hard wired the set to AC. The AC Interlock was defeated, so I was a little suspicious that something else may have been modified too.

This particular set seems to have a lot of extra components compared to most other circuits of type I have found.

Thanks for the help,

- Tim -

Ac interlock? I've never seen a valve radio or amp with an interlock of any sort.

If putting signal on the op tube grids isnt giving you anything, it should be simple to go round those op tubes and see if the voltages are ok or not, and if not where not. And if they are, you know the fault is somewhere between tubes and speakers.

There are a few problems with neutral chassis stuff (I'm presuming this is single rectifier giving N chassis, not full wave rec giving a live chassis as was popular in the 80s) .

1. Plug it into the wrong socket and everything plugged into it will be live.
2. Ditto if you get an electrical fault of some types
3. When handling the stuff plugged in, eg tape deck, cd etc, youre connected direct to the mains, which isnt best practice.
4. Anything earthed plugged in connects N to E, and in some cases this can melt wiring.

TBH I'd forget about giving it to someone. Interesting item maybe, but is it really worth the risk.

NT

Yup it had one of those interlocks that were common on tv sets of the same era. It was the kind that separated the ac power when you removed the back. We all used to have cheater cords to work on the bench with the back off.

I will be doing a more complete diagnosis of the tubes now that I have a bit more info and know the dangers of the mission....

- Tim -

In your first reply post you mentioned that there could be 2 parallel heater strings connected to the AC line, which could be true.... that fact was not ignored but may be irrelevant..... even if one of the tubes are bad, some kind of hum or noise should be heard out of the other channel. The best bet is probably a B+ problem or open speakers??

Is it a "SIlverstone" or "Silvertone" The latter was the name Sears Roebuck used for many of its products.

H. R. Hofmann

What is/are the total list of tubes. We need to see what the line-up is, then we can start giving you much better advice.

H. R. Hofmann

I have to put this project on hold for a bit. I have many, many things on the go right now, and the person I was fixing it up for is not interested in a tubed stereo system.

I did check the 50EH5 tubes voltages and they are way off, so I guess the problem is somewhere in the power supply side. The chassis is a jumble of wires and most of 'em are brown, so it makes it difficult to trace the lines. I will fix it up though, just to add it to my list of done deeds.

Oh, the 50EH5s *are* wired in series, with the rest of the tube filaments off another line. When I pulled one out, the other one went dark, but the rest of the tubes stayed lit.

- Tim -

Was this US practice? I ask as I've never seen anything like that here (UK)

NT

Well I'm in Canada, and it was very common until the 80's

- Tim -