My un-named console which up until yesterday was a great performer has just started to play up. Yesterday upon turning it on it made a loud mains buzz noise, and wouldn't get a station, I turned on my Kriesler plum pudding on top of it and it was working fine (to test whether it was atmospheric interference), but still an angry buzz and no music....I swapped out the 80 rectifier with a NOS one and the buzz was gone but instead had a sht sht sht sht sht sht at about two sht's/ second, still no music, so I swapped the 42 valve with a NOS Philco one and then had music, but only at half volume and with a faint sht sht sht sht in the background and I'm not getting all of the stations and good sensitivity it used to have, when I turn it up to three quarters It is nice and clear and loud, but gets to a point where it cuts out leaving a loud sht sht sht sht, but that's nearly full volume and if I turn it down the music comes back. It used to be really sensitive and listenable at a fraction of a turn (about one eighth) volume and really sensitive. Tonight it was doing sht sht sht sht again, I wiggled valves but to no avail, no reception, then after dinner turned it back on and got reception but at half volume to get any sound and not sensitive can only get 2 or 3 stations and the sht sht sht is in the background? I recapped this one about a year ago and it's been great ever since up until now so I'm guessing either a resistor gone high or a valve has gone punk? I can't get a schematic for it as it's unbranded, but has an 80, 42, 75, 57 and mystery valve? Any Ideas where I should start? Very loud buzz if I touch the pickup, so the amp stage must be ok...

Jamie ,
Your having lots of problems with noise lately....
They say things come in waves or runs of 3

Dirty switch contacts. Did you clean the valve sockets?.

That is "motorboating" and don't bet on the amp being OK. That's instability, or feedback. One cause is where there is feedback from the secondary & the primary / secondary gets reversed. Unlikely in this combo.

Something seems odd. 42 & 75 are 6.3V heaters the missing link may be 6A7.

But that begs the question of what the hell is a 57 doing in there, unless it also has a 2A7 as well both of these have 2.5V heaters. That would be bizarre?

Methinks 6D6 should be there: Not 57. Check the heater & filament voltages & wiring. This does not add up.

Marc

Maybe one of the main electros dropped its bundle?

I only say that because one day I turned on a 11-7 and after about 10 minutes the music suddenly turned into a loud buzz. Replacing the first filter cap fixed it.

Yes it might be a 6D6, it was a while ago I checked. After dinner when I turned it on the set was dead as a door nail, my supposedly NOS rectifier cold as a corpse, I pulled out the chassis and found HV from the power transformer, plucked out the 80 and found no continuity across the filaments! Put the old one back and it was good, so I checked resistors, one 15k was 16.9, so I replaced three 500k ones were around 680k, so put in 470k's as I don't have any 500k's, one 100k one was 120k, so replaced, then I noticed the transformer input has 3 taps one measured 590 ohms, the second with the active wired to it measured 650 ohms and the last one 700 ohms, so I'm thinking it was wired to the 220v tap. When I got it I replaced the cord simply wiring it where the old one had been. I've put it back together and it seems to be working ok, very slight sht sht sht, almost imperceptible. I'll just have to see how it goes, It worked well for the last year, maybe the rectifier was cooked by the extra voltage. The NOS one was supposedly NOS NIB, but had smudgy dust on it which aroused my suspicions when I first got it, I'm guessing the old 42 was cooked along with the resistors due to excessive voltage? Hopefully all is ok now, I'll just have to see, the remaining resistors check ok, there's a few wire wounds whose values I don't know.

The first filter cap is ok, well I assume as I got a boot from it! lol, but any more shenanigans and I'll swap them both out with new ones.

Well it's holding up well now, one 500k resistor was actually reading 920k, so somebody in years gone by has most likely moved the transformer input to the 220v tap to juice it up, rather than replace the resistor. The sht sht has gone now when I disconnected the antenna from the lounge room where my van Ruyten, is connected, which works wonderfully but interferes with the other radios when it's antenna is connected to the long wire with all of the others!

I would still check heater voltages & if the 57 is getting 6.3V on its heater it not going to last long. Using 450V caps with filament rectifiers, like #80 I consider, asking for trouble. Over voltage with them is bound to accelerate failure.

If you have the sht sht sht still in the background, that may be the other electro being strained. I suggest you change it and see.

Ok I will, it seems to have subsided now. It's not a 57, I think I made a boo boo there, it's a 6D6 methinks

It is a 6D6 actually, just had it to bits and checked and the mystery valve is a 6C6, could just read it up to a light after breathing on the glass. I've swapped out both 450V 8μF caps and replaced with 10μF. One question, how high can I go with smoothing caps on an 80 valve, lots of consoles only have 8μF caps, which seems unusually low considering later ones had 16μF and 25μF? I also put a 1μF polyester across the 10μF after the field coil for extra smoothing, it seems good now.

According to the miniwatt manual, a 5Y3 (80) should have 10μF maximum as the first filter. The second one can be larger if you wish, since the choke / resistor protects the rectifier from the effects.

Regarding 'Motorboating', what Marcc said is correct.

Motorboating is a low frequency oscillation caused by positive feedback in the audio stage. THE AUDIO STAGE IS OSCILLATING

This often happens when the Power Supply impedance has gone too high. If there are any old codgers out there, think of the early transistor radios----some models would often motorboat when the battery was flat.

Why? Because the battery's impedance became too high.

So, your motorboating is trying to tell you something. Two possibilities spring to mind;

a) The power supply is unable to filter the positive feedback signal affecting the audio output valve

b) Incorrect component/s causing excessive power drain

Thank you all, well if it is motorboating, It'll have to just motorboat till it's little hearts content as the output valve is brand new, the caps and resistors are new except the ones in spec. in any case it is barely perceptible and I'm thinking it's most likely atmospheric interference, it's just a very faint sh sh sh sh which you can only hear in gaps between songs when the announcer isn't talking. The 80 valve I'm using isn't new but it works fine, my supposedly new one went oc the day after I put it in, so I don't have another new one yet. In any case I had it on all day today and it performed well, it's sensitivity has returned and It's working fine.