I plugged the speaker in on my DuMont

I put the chassis back in and plugged the speaker in but left off the HV for now and got nothing, nada. There was the slightest hiss for a few seconds, but even that went away. I tried tuning in a station, both FM and TV and nothing. The tuning eye didn't budge either.

So I'm thinking that maybe I have a bad rectifier tube (5U4) and I'm not really getting any juice going through, just the filaments. When I measured the new electrolytics after running the chassis by itself for about a half an hour, there was no charge at all on any of them, which I thought odd too.

I don't have a tube tester, but I know someone that does that keeps short hours in his shop. The other thought I had was substituting, just to give it a try, a 5AR4 that runs my Dynaco. They're not completely compatible, but they've been known to work. Or trying it the other way, putting the 5U4 in my Dynaco. It is mentioned as a short term substitute in the manual, but nobody on the Dynaco board recommends running one for very long. Is either of these options a good idea? I suppose it could be a bad tube elsewhere in the audio section as well.

Any other thoughts on what could be wrong? I checked continuity on everything as I went along replacing caps. I brought the power up slowly when I was done with a variac. No smoke. I suppose I could check voltages, I have the Sams folder with the proper voltages at the check points. I hate to poke around in there with the power going. Resistance checks were mostly on target, or at least ballpark, except where modifications were made that aren't completely documented by Sams.

[blue_lateral]

As I remember, (going completely from memory, so take this with a grain of salt) the 5ar4 is similar in load capability to the 5u4. The 5ar4 was an improved tube for Hi-Fi, having two major differences. (1). The 5ar4 had cathodes instead of filaments, so it warmed up slower. the idea was that it is bad to have the plates hot on the tubes before the filaments come up. (2) The 5ar4 had less voltage drop, so you could in theory design a power supply with less sag, and possibly an amp with less distortion. If you substitute a 5ar4 for a 5u4 the plate voltage will be higher, probably too high. SInce you're running it on a variac, though, and you have schematic voltages, I'd just hang a meter on the highest b+ tap and use the variac to make sure it doesnt get out of hand.

If you go the other way, and put a 5u4 in where a 5ar4 belongs, you'll just have low plate voltage, more sag, and you wont have the benefit of the plate voltage delay on startup.

I like your idea of checking the DC voltages. Almost any bad part will screw them up (so some of them will be horribly wrong). I'll bet you find the problem this way.

John

[Chad Hauris]

I wouldn't recommend substituting 5U4 in a circuit designed for 5AR4 as the filament current draw of 5U4 is 3 amps vs. 1.9 amps for 5AR4...this may overheat the power transformer.

I believe that 5AR4, even though it's smaller, has the same or higher plate current ratings as 5U4 so it should be able to replace it...however the tube resistance is less as Blue lateral says so the output voltage will be higher.

[Bill Cahill]

Did you replace all electrolytics, and caps? Check b+ resistors? Is your 5U4 lighting? What color? If flashing, you have a short in B+. Same with plates turning red, or white. Check audio out. tube, When you ran set without speaker tube got very un-happy. Could be you dammaged it. Filters shouldn't keep b+ after set was turned off after warming up. Bill

I replaced all the caps. All the tubes look OK. I didn't check the resistors.

when i blew a 5ar4 in my dynaco, all the heaters would light, but the tube didnt pass any current, same as what i see now.

Unfortunately I've broken my wrist in a cycling accident and badly bruised the other one and I won't be looking at the tv again for a while. I can barely type.

good to know the forum is back up and running. i thought it was some spyware on my end.