Checking current

[The Captain]

Ive noticed my PT getting very hot, so tried a current check with a MM but the amp made a horrid loud ERRR noise before standby flipped (closed), as if it coincided with caps charging up.. I had my MM set to 200m for Amps, red probe on standby switch, black probe on 6v6 pin 3, all tubes and speaker in too (is this right?). I want to check this in case its wrong, if anyone can help- thanks, Captain.

[stokes]

With the speaker connected and the meter on your plate,you will get noise.Not sure where you have the standby wired,I usually put it on my PT's CT to ground.Just to be clear,the red lead goes to your OT's CT or first filter cap or pin 8 on the rectifier and the black lead to the plate (pin 3,6v6).If you have a shorting jack on the output jack to the speaker,just pull the speaker jack out to avoid any noise when checking the current,if not you will have to short the output.I have to think that if it were the power tubes current causing the PT to heat up,the tubes would be red plating.After you check the current on the 6V6's and are sure they are within safe limits,pull all your tubes and check the volts at each filter cap,with no tubes the volts at the rectifier output (pin 8) and all the filter caps should be the same,with no load (tubes) there will be no voltage drop across any of the dropping resistors in the B+ rail.If you see a voltage drop anywhere in the rail with no tubes,you have a problem with a leaky cap.If you remember when we last did the test with the current limiter,I wasnt 100% convinced there was no problem,the way you described the lights brightness levels.A leaky cap wont always cause the fuse to blow,but the increase in current will cause the PT to slowly overheat and can eventually burn it out,which was what I thought with your first PT problem.Check the volts as I described at each of the 3 filter caps and pin 8 of the rectifier and post your findings,we'll get to the bottom of this.

[The Captain]

Great help as always Mr. Stokes- for some reason Im nervous about this current check, so I have to double check..

I've a non-shorting spkr output (2 lug) so Ive pulled spkr jack out & wire-linked output lugs for the check, so thats now shorted? I've tubes in and flipping standby(closed) after AC switch on. Im looking to get 20-28mA with MM set to 200m on its A section: Once Im sure this is right I can post up voltages of all caps with and without tubes in. cheers Capt.

[stokes]

Its always best to use a "shorting" jack on the speaker out,in case you forget to plug the speaker in.20-28ma's is a safe range for the 6V6's.No need to check the volts on the B+ rail with the tubes in.What we are looking for with this test is to see if there is any current being drawn in your power supply.With no tubes in the amp there should be no current being drawn,so you shouldnt see more than a couple of volts dropped from one cap to the next,the volts at each cap and the rectifiers output (pin 8) should all be the same,or within 2-3 volts at most.So if you see,lets say,a 20v drop at one of the caps that would indicate that the cap is leaking current and causing the PT to overheat.

[The Captain]

Assuming Im checking current with all tubes in-? after AC switched on, and waiting for a bit I read 88.5. I shut it down before flipping standby(closed) as I guess summat is not as it should be/ I have a prob? (Black probe was on 6v6 pin 3/ red probe pin 8 5y3gt, reading with MM set to A, spkr unplugged, 2 lugs joined of spkr output -configed as a shorting socket would be ie tip to switch to gnd all linked).

[stokes]

If I am reading this right,you are seeing 88.5ma's with the amp in standby?If that is the case then I have to assume you have the switch wired so you get dc to the first filter before you turn the standby switch to the play mode.If this is the case,you must check the current with the amp in play mode.If the current is still that high in play mode,then we have to adjust the cathode resistor to lower it.Dont be concerned about flipping that standby switch to play,as long as you have no signal going thru the amp,it'll be okay.Yes,you have to have the tubes in the amp to check the current.

[The Captain]

I get 88.5 with just AC switch on- I didn't dare flip standby to play as I thought there was a major prob. So thats AC switched on, and standby switch...

I really hate this standby terminology: Standby On means effectively 'off', or 'not flipped'? All I can do is hope my 'Open' and 'Closed' (as in switch contacts) is understood, being the clearest way I can describe its position; I can never be sure of anyone's On or Off, or 'Amp in Standby' either. 'Play' I am taking as my closed/ ie a singing, ringing 5e3. If not I am considering opiates. Play is clear- thanks Mr. Stokes.

So I will check current again, this time with standby to play/ closed, and report back.
-Am I keeping tubes in for this current check?

[Tom Phillips]

Assuming Im checking current with all tubes in-? after AC switched on, and waiting for a bit I read 88.5. I shut it down before flipping standby(closed) as I guess summat is not as it should be/ I have a prob? (Black probe was on 6v6 pin 3/ red probe pin 8 5y3gt, reading with MM set to A, spkr unplugged, 2 lugs joined of spkr output -configed as a shorting socket would be ie tip to switch to gnd all linked).

Captain,
Do you realize that with your current meter connected as described above, you are providing a path for current to flow around the standby switch even if the switch is in “Standby mode” (Open contacts) Then the current can flow from pin 3 of one 6V6 to the output transformer secondary. From there it can flow through the OT secondary and out the other side and out the center tap to effectively try to power the whole amp. Think of the current meter as a low resistance wire, draw it that way on the schematic and I think you will see what I mean.
This can be a cause for confusing results

Regards,
Tom

[stokes]

When I say "amp in standby"I mean you get no sound,you say open.Standby on means you get sound,or as you say,closed.The tubes must be in to check the current,you are checking the current the tube is drawing.Is my assumption of your standby being wired so you get dc voltage at the first cap when the switch is "open"?

[The Captain]

So Standby On is closed/ play? well Im a dutchman... not to worry, I'll use my terms just to be as clear as poss. Cheers Tom, now Ive redone checks your post makes sense.

Anyway, current checks redone: at 1st 6v6 (nxt to recto) pin 3= 40.3 mA. And at other 6v6= 43.3 mA. Both switches flipped/ play.

[The Captain]

I can't understand this..

V at pin 8 recto (no tubes now)= 0.26V.
And at 1st filter cap (25uf/ 250r 5w)= 0V.
And at adjacent 16uf sprague= 0.11V.

I checked each 3 times. Both switches to play/ closed. No spkr in as before(?). MM set to DC, all connections sound- WTF?!

[stokes]

It sounds like you have no rectifier tube in there.The rectifier should be in the amp or you will get no dc volts.I guess i should have made that clear when I said "with no tubes".You have to leave the rectifier tube in.

[Tom Phillips]

So Standby On is closed/ play? well Im a dutchman... not to worry, I'll use my terms just to be as clear as poss. Cheers Tom, now Ive redone checks your post makes sense.

Standby terminology is a little confusing the way people use the terms differently. I prefer to just say the amp is in “standby” mode or “operate mode.” Anyway…on to the other issues.

Anyway, current checks redone: at 1st 6v6 (nxt to recto) pin 3= 40.3 mA. And at other 6v6= 43.3 mA. Both switches flipped/ play.

OK. That’s making more sense now. So the amp seems to be under biased therefore drawing too much bias current. You have not posted your plate and cathode voltage so we need to sort that out in order to calculate the plate dissipation. The current is higher than you will want but should not hurt anything during the time it’s on for temporary tests.

I can't understand this.. V at pin 8 recto (no tubes now)= 0.26V. And at 1st filter cap (25uf/ 250r 5w)= 0V. And at adjacent 16uf sprague= 0.11V.I checked each 3 times. Both switches to play/ closed. No spkr in as before(?). MM set to DC, all connections sound- WTF?!

You wouldn’t have measured the above currents if the voltage was not present.
Either something just failed like the rectifier tube as Stokes mentioned or it’s a measurement problem.
Checkthese things:
1) Are your meter leads still plugged in the the current inputs? If so, you need to plug back into the voltage measuring inputs. Note: If you try to measure the DV voltage with the meter configured for current measurement you could blow a fuse in the meter. This would make the current measurement section stop working.
2) Check where your meter ground lead is connected when you make the voltage measurements. It should be on chassis ground.
3) Check another known DC voltage (like in another working amp) to verify that the meter is working OK.

Regards,
Tom

[The Captain]

Ah Ok. So with just recto tube in, and spkr plugged back in (?)..

1st 16uf (layout left of three)= 523V
2nd 16uf= 518V
3rd 16uf...... the 1A amp fuse blew/ pilot light lit briefly-? this on AC switch flipped, standby switch not applied yet (open).. wtf.

[stokes]

If you are seeing 523v at the main and 518v at the screen with the standby open then we have a problem.The idea of the standby switch is to keep dc from going beyond the rectifier till the tubes have a chance to warm up.Are you saying the fuse blew before you had a chance to measure the voltage at the 3rd cap?Or are you shutting down after each measurement?Where exactly is the standby switch wired into the circuit.When I install a standby switch,I connect it between the PT CT and ground,this keeps any dc volts out of the circuit altogether.Where is your standby switch?