New 5E3 build power tube anomaly

[jyoung]

New to forum - I posted this over at the Gear Page and thought I would see if you guys have any additional wisdom for me. I got very helpful comments. I am hoping that some one has experienced a similar situation:

New 5E3 build from gathered parts. Power tubes will not heat up from time to time, especially after sitting for a while. Check other tubes same behavior. Not blowing fuses, no smoke, no funny noises. Amp will also randomly stop - and power tubes will shut down. Temp fix is to pull one power tube while amp is powered up and then plug it right back in. After this all is well.

When power tubes are working amp sounds great. I have checked all wiring paths and connections. I have read it that it might oscillations outside of hearing range? I am really scratching my head on this one.

Thanks
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#2 Today, 12:49 PM
WaltC
Gold Supporting Member Join Date: Feb 2006
Location: Northern CA (Sacramento area)
Posts: 179

I'd first recheck *all* the filament connections and re-solder all the connections, paying particularly close attention to the ones at the pilot light and the power tubes (since in these amps the filaments are often wired in parallel from the PT to the light to the power tubes then on to the preamp tubes.

Being very careful of the hazards of course!
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#3 Today, 12:52 PM
arwhite
New Member Join Date: Jan 2008
Posts: 12

Are you losing heater voltage when the output tubes shut down? Maybe it's your power transformer or the connections from it to the tubes...


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#4 Today, 01:52 PM
TungstenAmp
Gold Supporting Member Join Date: Jan 2006
Location: Central Florida
Posts: 97

One more vote for filament connections. Double check pins 2 and 7 on the power tube that you have to pull out to make it work.

I've seen vintage Fender amps where the wires were in the hole with no solder. Worked fine for 40 years until the wires were bumped. Scratched my head for a few minutes on that one.
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#5 Today, 02:43 PM
Roundtone
New Member Join Date: Jul 2008
Location: Webb City, MO
Posts: 3

Thanks for all the ideas- I will check all you mentioned over the holiday and post a follow-up.

Adam - I did find one such connection in my second circuit search exactly as you described. (which was self-inflicted - nobodys fault but mine).

Also both 6V6GTs go down together - rectifier stays alive, as well as pre amp and inverter. I had been using Sylvanias (6V6) and just got a set of GEs hoping it was just a tube glitch - but to no avail.

Thanks again
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#6 Today, 03:34 PM
TungstenAmp
Gold Supporting Member Join Date: Jan 2006
Location: Central Florida
Posts: 97

Gotcha. From the OP, I thought your preamp tubes were also losing their filaments.

Are you using quality octal sockets? If they are the old style, perhaps check to make sure they are properly tensioned and grabbing the pins securely.
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#7 Today, 03:36 PM
donnyjaguar
Senior Member Join Date: Nov 2005
Location: Canada
Posts: 1,129

If the 6V6 are wired in series then check for a receiving pin that's not seated properly in the tube socket. If they are in parallel, it can only be a bum connection. If you're using solid wire for the filaments, check for a break inside the insulation.
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[tubeswell]

When the amp dies, can you see the heaters going off? (i.e.; are the heater filaments still glowing or are they dark?)

In the stock 5E3 the heaters are wired in parallel, so if one heater went off, the others would still work, which, if it is the heaters going off, makes me guess it could possibly be:

1) the heater winding on the PT intermittently shorting

2) (if the heater winding has a CT) the heater winding ground lifting, through not being soldered properly perhaps

3) (if you use 100R resistors from each heater wire to ground) one (or possibly, but less likely, both) resistors drifting through heat and either shorting or grounding the heater directly, thus cancelling it.

4) a loose solder joint/bad contact in the heater circuit set off by vibration

My only other thought is elsewhere in the power supply (although perhaps this is less likely than a heater fault), e.g.;

1) loose solder joint bad contact/dodgy socket pin in the B+/power supply

2) rectifier playing up and not delivering the B+, through bad tube/diodes

3) faulty HT winding (CT ground lifting??) (hard to see how it could be this if unplugging and replugging a tube brings it back on tho')

4) some intermittent fault probably related to heat and supply resistors drifting, or filter caps shorting

[jyoung]

Thanks for scratching the old noggin for me. I have work to do and will let you know what I find.

Thanks again.

[jyoung]

Update - sorry it took so long to let you know what I found --

NO solder on one heater pin of power tube 2 (closest to rectifier). I am such a goof. During the search I was able to move some wires around to decrease what little hum I have, so I felt a little better after that. I really appreciate all of you taking the time to respond.

I am building a Tweed Super Amp (5F4) at the moment, so I will probably be back with questions. Bob Sickler made the cabinet, and let me tell you it is a work of art. If you know Bob you know what I mean. Finished the tweed application yesterday and started the chassis.

Anybody out there have experience troubleshooting a Silvertone 1434 Medalist? It is next for a going over on the bench.

Thanks again