5E3 running hot!

[Randall]

A '56 Fender 5E3 came in for blowing two fast blow fuses at a gig. I put my meter in place of the fuse and read ~0.7 amps at idle and maybe 0.9 when banging a couple of chords. The 70 year old Tung Sols 6V6GTA that are in it read clean for shorts and pretty strong still on my tester. Same for the rectifier. Then I measure dissipation at idle with a cathode bais probe, and got a whopping 50mA on one tube and 45mA on the other. I think plate voltage was 425v, which puts them at 19 and 21 watts!

The 250 ohm cathode resistor measured 261 ohms in circuit, so I dont think the cathode cap or resistor are bad. One caveat, the PT was replaced years ago, and the guy isn't exactly sure with what, but he has played the amp since then with no issues relating to that.

So I am wondering, why is this amp running so hot? Or to put it in another way, could it be that it has always run this hot, and the vintage tubes have been able to take the beating?

[Enzo]

Did you subtract the cathode voltage from the plate voltage? Dissipation is voltage ACROSS THE TUBE time current. Plate voltage alone won't do.

[Chuck H]

For mains fuses Slo is the way to Go. Really. There are occasions when an amp is being pushed that kickback voltages and old caps trying to keep a charge with heavy bass notes going, etc. may overtax the fuse for nano seconds at a time. A fast blow will see that taxation but a Slo will roll with it. Aside from that...

That plate voltage is way high for a 5e3 with the stock value cathode resistor. Getting into Matchless territory. So, what started the fuse blowing when it didn't before? I dunno, but any wear on the power tubes and filters could have toppled it from the narrow fence rail that the amp was riding on. Rather than go the Matchless route and replace tubes every couple of months you might want to at least try cooling the idle current a bit and see if your guy disagrees with the tone. It can always be put back the way it was.

[pdf64]

What are the actual measured mains, heater and HT Vac ie at the transformer windings; in the 'no tubes in amp' and all tubes in, at idle' conditions?
What rectifier is in there? 5E3 need a real USA made 5Y3.

[mozz]

5e3 drawing about 100watts is not correct. What are the filter caps ages?

[Randall]

"Did you subtract the cathode voltage from the plate voltage? Dissipation is voltage ACROSS THE TUBE time current. Plate voltage alone won't do."

No, I forgot to do that. But, even if I had, I still think it would be running very hot.

"5e3 drawing about 100watts is not correct. What are the filter caps ages?"

100 watts? Do you mean 100 mA? The caps are not new, but they are Sprague Atom replacements.

[eschertron]

"Did you subtract the cathode voltage from the plate voltage? Dissipation is voltage ACROSS THE TUBE time current. Plate voltage alone won't do."

No, I forgot to do that. But, even if I had, I still think it would be running very hot.

"5e3 drawing about 100watts is not correct. What are the filter caps ages?"

100 watts? Do you mean 100 mA? The caps are not new, but they are Sprague Atom replacements.

You posted in #1 that the primary side was drawing 0.9A in peaks. 0.9A x 120V is "about" 100VA, or 100W. I agree that's too much, my 2x6V6 amp draws around .45A max.

0.7A at idle for the amp seems still too much, at 84VA. What's the PT rating?

Also, did you get B+ voltage readings with 6V6 both in and out? What was unloaded voltage? Another indicator of a too-high supply voltage, what was the cathode voltage? I'm guessing 24.7V from your numbers. 10% too high all around.

If the fast blo fuses work for the customer, I guess he's OK. If the amp comes back, suggest a voltage mitigation scheme to get dissipation into reasonable range.

[Randall]

"0.7A at idle for the amp seems still too much, at 84VA. What's the PT rating?

Also, did you get B+ voltage readings with 6V6 both in and out? What was unloaded voltage? Another indicator of a too-high supply voltage, what was the cathode voltage? I'm guessing 24.7V from your numbers. 10% too high all around.

If the fast blo fuses work for the customer, I guess he's OK. If the amp comes back, suggest a voltage mitigation scheme to get dissipation into reasonable range."

The amp is gone and I didn't record my measurements. It did have a vintage 5Y3 in it, and the filaments measured 6.75 volts. He has since played a full gig with a slo-blo in it with no problems. I realize now that forgetting to subtract cathode voltage from plate voltage made it seem like it was running hotter than it actually is, but hot nonetheless. I am going to chalk it up to possibly not a direct replacement PT, with it running hot like that for years and not destroying those old Tung Sols. As for the fuses blowing, fast blo fuses are pretty delicate looking, and I know they do blow from other mechanical reasons, and they did come from the same old 5 pack, so maybe he had two fuses that just wren't up to the task. Time will tell.