5F10 insane tube current

[MonkHouse93]

So I just finished a harvard build with adjustable bias. I fired it up on the bench to make sure it worked and it worked fine: High voltage PT taps read 564VAC, B+ was at 330V, at -27V according to the schematic the voltage across my 1 ohm bias resistor read about .050V, which means 50mA. . . I adjusted the bias to get it down to like 30mA. The amp was extremely quiet too.

Being that I'm a bit of a noob and this is my second build, I have an awful tendency to melt the plastic off my sprague filter caps, so i bought a couple more and replaced the current ones I melted. I didn't have a chance to test it again until just tonight. It sounded overdriven with the volume at about "2", which it shouldn't do that until about full volume, so I started taking some measurements, and the voltage across the 1ohm was .1-- or even .2-- at one point. . . So i started checking the voltages on the new capacitors; the first filter cap had a about 0.8 volts across it, the B+ was reading a 278V and the High taps were 547VAC. I replace the first filter with the melted one that I knew worked. Upon turning it on again, everything was normal for about 5 seconds, quiet and all, then the hum builds up, the speaker pops once, and I'm getting these unusually low B+ and insanely high tube current readings. The only difference is that the filter cap I replaced now retains the 278V across it.

Might it be the power transformer? no other taps have a change in output voltage however. . .

EDIT: taking off the High Taps, it measures 557VAC. I don't know that it's the kinda thing where one can say close enough because it measured 564V with and without the rectifier tube before

[Jazz P Bass]

Pull the output tubes a retake your B+ readings.

Personally, I do not take readings off of the HT Vac taps.
What really matters is what the rectifier is putting out.

[MonkHouse93]

So looking at it again tonight HT VAC says 569, but I guess that's not a big deal then, and the rectifier says 392V, without the 6V6's. I tried a few rectifiers and they all gave me the same thing.

Here's what I thought: Looking at that schematic, If there was less than a volt across the first filter cap I would assume that it's safe to say that there was DC current flowing through it? Electrons fly off the cathodes and to the plates, through the OT 2nd taps to the center tap that connects to the B+, and then through the Rectifier and such. But if that first filter was shorted, then the current will flow directly to ground through there too, providing a less resistant pathway, I suppose, thus the current was huge. And since replacing that cap which maintains a voltage across it, the current is still huge. The rectifier seems to do what it's supposed to be doing, soooo. . . I blew up the power tubes? I'm spit-balling a little bit here

[Enzo]

569vAC from where to where? if you are getting that voltage to ground from anywhere, it is wrong, and i suspect the meter. If you are getting 569vAC from red wire to red wire, I might believe that. In any case, it you get 300-400vDC on the caps then forget the AC.


So the amp was working and then you replaced a bunch of caps and now it doesn;t work. My money is then on something about your work. You miswired one of the new caps, or put it in backwards. Most common error? The bias filter. All those B+ filter caps have the negative end to ground, but the bias supply is negative, so the positive end of that cap goes to ground. get that wrong and your bias is gone, the tubes go nuts and conduct heavily. yes? No?


If you measure close to zero volts on the B+ caps, power off and measuer for shorts to ground.

[MonkHouse93]

569 between the red wires. Maybe I stuck a cap in backwards, but they're all the right direction now. bias filter wasn't touched, and the + side is at ground potential. Rectifier alone shows me 377V now.

Just found a short, my meter didn't catch it at first but after like quadruple checking, where the 220k, 1500, and .1uF cap meet for one of the 6V6 grids is grounded. I can't see why yet but that looks like a task for another day for now. . .

[Tom Phillips]

...where the 220k, 1500, and .1uF cap meet for one of the 6V6 grids is grounded...

Man...All this verbiage is confusing.
Suggestion. If you have the means then mark up the schematic with your readings and post it. That will help the people here help you more effectively. You could just print out the schematic that Jazz P Bass posted and take a photo of the marked up version being sure to point out anything different in your amp.
Cheers,
Tom

[Jazz P Bass]

Just found a short, my meter didn't catch it at first but after like quadruple checking, where the 220k, 1500, and .1uF cap meet for one of the 6V6 grids is grounded.

Well, a grounded grid should surely make a tube red plate.

It probably lowered the bias to the other output tube also.
(orange plate?)

[Leo_Gnardo]

Maybe I stuck a cap in backwards, but they're all the right direction now. bias filter wasn't touched, and the + side is at ground potential. Rectifier alone shows me 377V now.

Good that you found the short in the grid feed, fixing that should clear things up. What worries me is "Maybe I stuck a cap in backwards" - electrolytic caps, once subject to reversed voltage, don't often work well or at all.

NIce to see someone building a Harvard. Sweet amp! Nice "pocket size" and a bit more bark than a Champ.

[MonkHouse93]

I forgot to add a 1ohm at the 6V6 cathodes

Ya know, the tubes aren't red plating
And whatever faulty filter cap I installed, reversed or not, it's not in there anymore

I chose the Harvard because I needed something with a little more power than a champ, albeit my champ build has a 15" speaker and it stands its ground with a full band. I also just wanted to say I have a Fender Harvard for no real reason, and the 5E3 deluxe seems a little cliche Although I had a hard time finding a 5F10 cab and chassis, so I used the 5E3 cab and chassis, so instead of three inputs? Yup, it has FOUR inputs :face palm:

I did a little circuit analysis and found that with the 1 and 2 inputs like on a 5F1, the grid voltage Vg = the guitar's output voltage in #1, and #2 is half that. So the 5F10's three inputs just divides it into thirds, and mine goes by 4ths . . . I didn't have any clever ideas for that.

concerning the short: it's a shielded wire, with the shield going to the cathode side of the 1ohm bias resistor (close enough to ground?), and IF I'm lucky I can just disconnect the the signal lead and shield and restrip it and resolder. Otherwise the whole board has to come out

[Leo_Gnardo]

I forgot to add a 1ohm at the 6V6 cathodes

No biggie, you can do it later or use other methods of determining output tube current. 1) shunt method, which I do, a little tricky and possible to slip & short hi voltage which often pops the meter fuse. But that's me. 2) power off, measure resistance of center tap to each plate. Remember which is which. Then, amp warmed up, measure voltage from CT to each plate. I=V/R and bob's yer uncle.

Ya know, the tubes aren't red plating
And whatever faulty filter cap I installed, reversed or not, it's not in there anymore

I chose the Harvard because I needed something with a little more power than a champ, albeit my champ build has a 15" speaker and it stands its ground with a full band. I also just wanted to say I have a Fender Harvard for no real reason, and the 5E3 deluxe seems a little cliche Although I had a hard time finding a 5F10 cab and chassis, so I used the 5E3 cab and chassis, so instead of three inputs? Yup, it has FOUR inputs :face palm:

I did a little circuit analysis and found that with the 1 and 2 inputs like on a 5F1, the grid voltage Vg = the guitar's output voltage in #1, and #2 is half that. So the 5F10's three inputs just divides it into thirds, and mine goes by 4ths . . . I didn't have any clever ideas for that.

Good progress, and no face palm necessary. Use what you can get reasonably priced and not having to pay a lot extra for custom made. A little bigger box, we expect a bit more oof in the lows, what's not to like.

concerning the short: it's a shielded wire, with the shield going to the cathode side of the 1ohm bias resistor (close enough to ground?), and IF I'm lucky I can just disconnect the the signal lead and shield and restrip it and resolder. Otherwise the whole board has to come out

Let's hope you can avoid having to unass the board. Those under board jumpers & wire runs, I put 'em on top mostly, so I can get at 'em in case of a goof or something goes wrong. Live and learn.

[MonkHouse93]

So it was a simple fix! Just had to move some wires around. Then i was an engineer for a second and determined the plate dissipation to be 69.5% Plates have like 324.5V average (I say average because one had 323 and the other had 326, oops?) @ 30mA per tube. It actually does a lot for only having a volume and a tone. My 5F1 has like 2 sounds, semi driven and driven, this I can get clean, driven, warm, bright. And it's loud enough but not too loud

[Chuck H]

Let's hope you can avoid having to unass the board. Those under board jumpers & wire runs, I put 'em on top mostly, so I can get at 'em in case of a goof or something goes wrong. Live and learn.

A little off topic, but this post made me think of this. Something I do is locate my ground points under the eyelet board for a neat appearance. The lead just wraps from the eyelet to under the board and no spaghetti!!! To do this I need to set up my ground points ahead of time so that after the board is installed I can simply retrieve a lead from the desired ground point and solder it. So I always include extra leads (shrink tubed on the ends) in case I want to add a circuit, change a ground point or an extensive repair requires a new lead. So far this has worked out VERY well.