Silver face Fender Bassman 10

[hossymandias]

This has no sound output. I checked the resistance between the output transformer primary taps and chassis. I'm getting around 30k ohms for all of them. Is that a short? Bad output transformer? Thanks! PS, this is the 70 watt Bassman 10.

[martin manning]

No, you're measuring the resistance of the power supply filters.

[hossymandias]

Whoops, so how do I test the OT? I was following RG's article, where he says,

"While you're at it, measure the resistance of every lead to the chassis. This reading should be very high, hundreds of K or preferably megohms. A low reading here indicates a short to the transformer core - again, dead transformer."

http://www.geofex.com/ampdbug/outtrans.htm

What am I doing wrong? Thanks for the reply!

[martin manning]

You'll have to disconnect the CT from the power supply, which I'm guessing you did not do based on the reading you got.

[hossymandias]

That makes sense. So I only need to disconnect the B+ lead?

[martin manning]

Just the center tap, yes, but if you are seeing 30K then you don't have a short from the windings to ground.

[Leo_Gnardo]

Lots of places to lose signal. Let's look at a couple easy ones first.

(whispers to self: why do they always assume it's the output transformer??????????????????????????????????????????????????)

First a look at the output department: IIRC there's a fancy switching jack on the extension speaker jack. Dodgy contacts due to corrosion or abuse will stop output signal from getting to speakers. That's the first place I'd go looking for a missing signal.

Two more quick checks before we dig deeper.

Check all your pre tubes, are they all in place, right tubes in the right sockets, no tubes "white" inside, no broken or cracked glass?

Do you have two filaments glowing in each?

Sometimes just pulling out a pre tube & reinserting is enough to break thru a layer of corrosion that's preventing things from working right. On rare occasion you might find a pre tube is missing a pin.

[Stevem]

Do you hear any noise level change taking place when you listen close to a speaker and put the amp into the play mode / out of standby , is there then a low low volume hum level?
If not then in a darkend room while looking at the two output tubes do that same thing, do you see the inside of the tubes ( inside the plate structure) flash up briter, not than your standby switch is shot!

[hossymandias]

okay, okay. I could have just kept quiet, but I think you all deserve to know how right you were (especially Leo_Gnardo). I replaced the output tubes and it worked.

BUT! In my defense the guy who brought it to me swore up and down that he had already checked them and replaced them! So I suppose my lesson is two-fold: always check the basics first, and never make assumptions based on other people's observations.

Thank you all for this wonderful resource!

[xtian]

always check the basics first, and never make assumptions based on other people's observations

Golden words, brotherman.

[Leo_Gnardo]

Deeelighted it worked out for you hoss! I was gettin' kinda worried.

(in Sergeant Friday voice) "just the facts, man, just the facts."

[rfgordon]

You may be way ahead of me on this, but in case you're not, you might want to make a couple of these gadgets to inject and/or tap a signal from different internal points on the amp.

1/4 plug & shielded cable (sacrifice an old instrument cable) at one end.

On the other end, solder an insulated clip lead to the shield and a high voltage rated signal cap (almost any value will do; I used 330 pF/1000V ceramic). Leave one lead long and tape that end to a chopstick with the lead from the cap sticking off the end.

The cap protects whatever you plug the cable into from DC.

So, using this device you can plug it into the headphone jack of a stereo to inject signal into the amp. Likewise, you can plug it into an amp to take signal off.

I have a stereo with both 1/4" headphone and mic jacks on the front, right next to my bench.

In an amp that appears to be dead, feeding it a signal and then tapping all the points along the way will quickly find the piece of the chain that isn't passing signal.

Cheers