blowing fuses

[jetpack]

Hi everyone.

I just finished a 6A3 g*s*g last night. I'm made a mistake or two and can't get it to work. Upon switching it on, I immediately blow the 1A fuse.

I'm a relative beginner, but I'm certain that most of you guys could take a look at it and see the problem straight away.

Any help would be HUGELY appreciated!

Bill

[20to20]

Edit

[jetpack]

Sorry about the size! They are rescaled on the ftp site now.
Thanks for posting them.
Bill

[20to20]

Edit again

[jetpack]

How about this one which is at 50%?

Thanks again for helping with the post.

[20to20]

How about this one which is at 50%?

Thanks again for helping with the post.

To make your photos show, highlight the link you type and then click on the [Img] button above the editing box.

You can go back up to the first post and edit your original links to work right.

This one is fine.

[TomMcNally]

A 1 amp fuse is too small. The correct fuse would be a 3 amp slow-blow.

[Shannon Parks]

1) I see that New Zealand is 240VAC, so a 1.5A slo blo (half a 3A slo blo) is what you should use. Get a handful of them from you local hardware store as we try to suss this out. You can use a 2A if those aren't handy.
2) Pull your 6A3 tubes and 6SL7. Leave the 6AX5GT (btw, be sure it is exactly a 6AX5GT). Fire it up. Your voltages will be high, but go ahead and measure them. Compare them to page 4 in the manual.
3) Shut it down, wait a minute and put in your 6SL7. Fire it up and write down your voltages.

If you've made it this far, go ahead and post your voltages, just like the format on page 4 in the manual. This will help me eyeball if something is way wrong. Don't worry - we'll figure it out!

Shannon

[jetpack]

Thanks for the help and encouragement.

I've stuck with the 3A as suggested by Tom, as I'm using a 240->120 stepdown transformer in the mains line before the PA774.

Measurements with only the 6AX5GT in place:
V2
p1=0
p2=4
p3=327
p4=0
p5=325
p6=0
p7=4
p8=968 egads!

Adding the 6SL7, I can get no DC measurements on V1.

Any thoughts?
Cheers.
Bill

[Shannon Parks]

I apologize for the snail's pace, but hopefully this weekend I can get faster responses to you.

Disconnect the output connections (at J7-1 & J7-2) and try those voltages again. The unloaded outputs could be screwing with your DC. In particular, what is V2-8 after you disconnect them? At J7? Also give me your AC measurements from:
V2-2 to V2-7
V1-7 to V1-8

Your plate load resistors look suspect to me at R7 & R8. Measure them with the power off with your DMM. Are they really 270k? And not 270? I realize there's a K there, but 5W cement resistors in that range aren't as common.

Shannon

[jetpack]

Certainly no need to apologize about reply speed, especially as you are the one doing me a favor.

R7 and R8 are fine at 265k and 275k.

With the unloaded outputs, V2-8 is still very high. In fact, so high that I'm a little nervous about leaving it turned on and even using my test probe on it. Maybe I've put in wrong values of R17 or R18. Once I've let the caps discharge a bit, I plan on pulling the board and testing those.

Following that, I'll get the AC readings.

Thanks.
Bill

[Ty_Bower]

That last number is seemingly impossible.

Are you reading volts AC with your meter across pins 2 and 7? Four volts AC is quite a bit too low.

Are you reading volts AC with your meter across pin 3 and ground? Or across pin 5 and ground? Your numbers here look about right.

Are you reading volts DC with your meter across pin 8 and ground? This is a full wave center tapped power supply. You shouldn't be able to get more than 1.414 times the voltage from the power transformer. 1.414 times 327 volts AC should give you 462 volts DC. In the actual circuit, it will be even less due to the losses in the rectifier tube, and it will drop further once you load down the supply.

If you truly have >950 volts DC coming off the rectifer, I'd imagine the power supply capacitors will go BANG in very short order. Are you confident that your meter is giving you good readings?

[jetpack]

>>That last number is seemingly impossible.
Great to know. Thanks.

>>Are you reading volts AC with your meter across pins 2 and 7? Four volts AC is quite a bit too low.

Yes. Pins 1 and 2 on both J3 and J4 are also low. Do you think something might be wrong with the power transformer?

>>Are you reading volts AC with your meter across pin 3 and ground? Or across pin 5 and ground? Your numbers here look about right.

I am measuring all voltages across J6, pin 2 which I have grounded to the chassis and then on to the mains ground (green wire in photo).

>>Are you reading volts DC with your meter across pin 8 and ground? This is a full wave center tapped power supply. You shouldn't be able to get more than 1.414 times the voltage from the power transformer. 1.414 times 327 volts AC should give you 462 volts DC. In the actual circuit, it will be even less due to the losses in the rectifier tube, and it will drop further once you load down the supply.

As above, yes, measuring to ground. I guess your point about maximum voltage makes it unimportant at this point to verify the value of R18 (concealed side of board) as I'd previously considered.

>>If you truly have >950 volts DC coming off the rectifer, I'd imagine the power supply capacitors will go BANG in very short order. Are you confident that your meter is giving you good readings?

No, I'm not supremely confident in my meter. I've never had any problems in the past, but it is a cheap meter. I'll borrow a nicer one and give V2 p8 another shot.

Thanks very much for the input.
Bill

[Ty_Bower]

I'd start with the basics by first verifying the AC voltages. Make sure that ALL the tubes are out of the amp.

Measure the AC voltage of the first heater winding. You can check this with your meter probes across J3, terminal 1 and terminal 2. You should have a value within ten percent of 6.3 volts AC. You can double check the voltage of the first heater winding by placing your meter probes across pins 2 and 7 at V2. It should be 6.3 volts AC.

Measure the AC voltage of the second heater winding. Put your meter probes across J4, terminal 1 and 2. Again, it should be approximately 6.3 volts AC. You can double check the voltage of the second heater winding by placing your meter probes across pins 7 and 8 at V1. It should be 6.3 volts AC.

Finally, you should verify the AC voltage of the power transformer's secondary winding. Your meter should be rated Category II, and safe for at least 750 VAC. It will say this somewhere on the front of the meter. Put the meter across J6, terminal 1 and terminal 4. The voltage measured should be within ten percent of 660 volts AC. You should double check this last reading by placing the meter probes across pins 3 and 5 at socket V2. The voltage measured should be the same.

[jetpack]

AC voltages check out fine at:
with all tubes out:
J3 p1-p2; V2 p2-p7: 6.0V
J2 p1-p2; V1 p7-p8: 5.7V
J6 p1-p4; V2 p3-p5: 646V

V2 p8-ground: 434AC
but 0DC on my meter..?

Sorry about the hassles.

Thanks Ty and Shannon.