DCR on Gibson GA8T Discoverer power Xfromer sec.

[JW]

Here's the schematic https://elektrotanya.com/gibson_ga-8t_schematic.pdf/download.html

This concerns pins 2 and 8 on the 5Y3, which are the rectifier tube's heaters. What kind of DCR should I be getting between these secondaries (with no tube in the socket)

The amp is blowing fuses. I'm getting like 0.5 ohms or something. Almost full continuity. Is that normal. or not normal?

I don't really want to power this thing up again with the current transformer if that is the issue.

 

[Khron]

How about disconnecting all the secondaries, and see if you have something in the region of 6-7V on that secondary? You'd expect the voltage to be a bit higher (than the nominal 5v) if unloaded.

I see the datasheets specs the filament as 5V @ 2A - you wouldn't expect much DCR in the associated transformer winding anyway. Otherwise that would just cook the transformer, wouldn't it? :

For what it's worth, one quick measurement of a transformer at hand (a potted PCB-mount 1.5VA unit with a 9V secondary) shows a winding DCR of 13 ohms or so, but that's for a 160mA-ish rated current (if not less).

 

[Tubetec]

If you pull the 5Y3 out of its socket and try powering it up again you should be able to determine if the issue is either an interwinding short in the power transformer or something further down the line like filter caps, output transformer or tube interelectrode short . With the rectifier tube removed your signal tube heaters should still light up . You could always measure your rectifier heater supply volts while the tube is out , bear in mind high ac voltage will still be present on the pins of that socket .

 

[Tubetec]

That 0.22uf cap off the primary could be shorted , Id fit the amp with a proper modern three wire flex, plug and make a good solid ground connection before doing anything else , you wont need the 0.22uf cap once thats done.

 

[JW]

Thanks for the replies.

I've tried powering it on both with no rectifier and ONLY the rectifier. It doesn't blow fuses with no rectifier, and with only the rectifier it does. I checked the 5y3 tube on another amp and it seems to work fine. Also tried another 5y3 in this amp and it still blows fuses.

I didn't mention that when I powered it on with no rectifier (even though it didn't blow the fuse) there was still a sound of what you might call sizzling at the 5y3 socket. I've replaced the socket thinking maybe there was a short I wasn't seeing.

Then I also saw very charred cloth on the secondary wire that goes to pin 8. And indeed this is where the sizzling sound is coming from.

Okay, I just need more fuses and I'll get some Voltage measurements hopefully before the cooking kills something. But first I'll check that cap. . .

 

[Tubetec]

Sizzling and peculiar smells are a good indication of a problem with tube amps
Check and replace the wires to the rectifier heater if the insulation has crumbled.

 

[Khron]

Might i suggest cobbling together a so-called "bulb tester"? Ie. an old-school 60-100W tungsten filament bulb, in line with the mains. If the bulb lights up bright, something's drawing too much current; at idle you would / should only expect a low glow.

Saves on fuses, y'know... :

 

[JW]

Hi folks,

Just wanted to bump this one with some new info. I tested the AC heater leads for the 5y3 and they are 5.7V unloaded. So, I think I'm in the right range there. Which is good news I think because I believe the power transformer is okay.

So, with no tubes in, I still get sizzling and the amp starts to cook the 4 watt 250 ohm resistor that goes from the 5y3 heater to the output transformer. This junction at the output transformer center tap also connects to a 20uF cap and 470ohm 1 watt resistor. When I remove this part of the circuit (20uF and 470ohm resistor) there is no more cooking of the 4 watt 250 ohm resistor.

Would this indicate a bad 20uF cap perhaps? The brand that is in there is a replacement already so I was betting it was okay. But. . . .

 

[Khron]

Wouldn't be the first shorted capacitor I've seen. I'm gonna guess it's not one of the very few reliable Japanese brands?

 

[JW]

Dang. Replaced both the 20uF and 10uF right by this sizzling 250ohm resistor and still it wants to burn the 250ohm. The 470ohm specs out. The 22K specs out. I must have a short somewhere.

The 22nF cap on the power transformer primary is fine (I assume) at 31nF.

 

[Walrus]

Shorted primary to secondary on the output transformer?

 

[radardoug]

You are just shotgunning parts without testing. Do you have an ohmeter? Use it without the power turned on. Lift connections so you can verify high resistances. Tip, the ohmeter will take time to charge the caps on high resistance scales, give it time to do that.

 

[JW]

Now I've disconnected the 470ohm resistor on the positive side of the 20uF cap. I've also disconnected the output transformer. So, the only thing in the circuit is the 250ohm resistor and the 20uF cap. With no cap there's no cooking, but as soon as I connect the cap, it starts cooking the 250ohm resistor.

Am I on the right track here, or is disconnecting the rest of the circuit causing same symptoms?

 

[radardoug]

Well, you have resistor going to the cap and nothing else? Then its the cap. Have you tried just measuring the cap for ohms? Also, if you put your voltmeter across the cap and switch it on, how many volts there?

 

[JW]

It's a brand new cap. A replacement of a replacement. I could try another but I'm guessing that ain't it. I'll get voltage readings after the cap when I get home, but am away at the moment.

Anyway, thanks for the help.

So, if it is not the cap, then it's the power transformer right?

By the way, I don't think it matters, but the 250 ohm 4 watt resistor is in actuality two 5 watt resistors in parallel. 560R and 470R. But as I understand it, it doesn't cut the wattage in half by having two parallel resistors right? Just thought I'd mention that . . .

 

[radardoug]

You need to apply elementary faultfinding techniques. If you only have a resistor feeding a cap to ground, and the resistor smokes, then its the cap OR a short across the cap. Break the problem into sections that you can understand, and move through it logically.
1/ Remove rectifier valve. Power up and measure on rectifier plates. Voltage OK? (AC)
2/ connect first filter section and put in rectifier. Switch on and measure. Smoke? Theres your problem.
How the resistor is composed is not an issue.