1967 Fender Bassman Question - Power Supply Issues

[keithb7]

The circuit was designed with a current limit in mind. Smaller wires, and components built within this spec. The Bassman amp runs at safe, comfortable 2 amps or less. My home offers 15 amps at the 120V wall plug. Slamming a full 10A for example or 15A into the circuit is going to fry things. Electrons move at the speed of light and can create intense heat quickly in components that restrict its movement. For example a transformer. The heat can melt parts such as the transformer windings. Turning steel windings into melting liquid. Or worse, other parts that may get hot, then combust.

Agreed it was the wrong thing to do. At the time I was pretty confident the transformer was bad and made a poor decision to proceed with 10A.

[Chuck H]

Probably a good idea to check the rectifier as Dude suggests. maybe (probably) the PT is bad, but that doesn't mean it wasn't the rectifier causing the initial symptoms.

[g1]

As well as checking all the rectifier diodes, disconnect all the PT secondary wires.
With only the 2 primary wires connected, power it up on your bulb limiter. If the bulb lights up (more than a faint glow), you can condemn the PT.
If limiter lamp doesn't light up with the secondary all disconnected, check your secondary voltages.

As transformers are the most expensive parts of the amp, we are very hesitant here about suspecting them until all other possibilities are eliminated. I hope you can understand then why the strong reactions about the over-fusing.
Especially with vintage stuff where having original iron can have a big effect on the value.

Error has been noted and acknowledged, I hope we can move on here.
I'm pretty sure everyone who has commented has made some mistake at least as bad (don't make me dig them up).

[Randall]

"I'm pretty sure everyone who has commented has made some mistake at least as bad"

Yes, and have been told so!

[Chuck H]

Aight... I'll play. I have a design that uses a "conjunctive filter". That is, A resistor and capacitor in series across the OT primary. A shunt tuned for impedance and frequency. "Conjunctive" hardly describes it well. "Shunt" is probably better. But I digress...

Anyway, I had no idea that Vp could peak at twice the B+ and so I had used a 600V film cap in the circuit. This was a metalized cap. It kept failing and repairing itself. I could actually hear the process. Not in real time, but in between playing sessions. Once the amp started sounding a bit too far off, and at the time having no idea why, I just said "f#@% !t" and decided to crank and play until something failed just so I would have something to actually fix. After some time the cap pretty much shorted completely, the resistor was starting to "brown" on the color bands and it smelled like a burning plastic bag. That's how I identified the problem and then knew where to search and read here for a better understanding of the circuit I needed to build to replace it. It's fortunate I didn't harm a more expensive part in the effort and the new circuit has caused no trouble (that I know of) in many years on several amps.

But still, the "F#@% !t. I'm gonna break something so I can fix it." method is probably not the most moderate decision I've ever made

[Randall]

"But still, the "F#@% !t. I'm gonna break something so I can fix it." method is probably not the most moderate decision I've ever made"

I dunno, works in psychology sometimes.

[keithb7]

I have been out of town for several days. I am back tonight doing some more troubleshooting.

Removing the secondaries from the PT at the input connection at the rectifier/bias board, the PT draws little to no amps. The light bulb limiter stays very dim. To confirm, with the wires hooked up to bias board again, plenty of amp draw as the light bulb limiter is again bright.

Next I decided to probe for an AC voltage reading at the secondary wires connection to the rectifier board. Everything hooked back up again as normal. Plugged into light bulb limiter, at this point I am getting 11 to 12VAC. Schematic shows 320VAC at this point.
I double checked and measured 120VAC at the mains in.

Using the diode test setting on my DMM, on a few diodes in the amp's SS rectifier section I am getting voltage readings in both directions. I think this is indicative to a bad diode. I have had limited experience testing for problem diodes up to this point. I do however have some new diodes here to compare. A new one nets 0V in one direction and 0.664 or so in the other. Seems to me a diode problem is present.

Does it appear I am on a good course here at this point? Looking at the Bassman schematic, there is no info on the diodes used.For use in this AB165 Bassman circuit which diodes should I be using?

Thanks, Keith

[The Dude]

A good diode should read about .6V drop in one direction and no voltage drop in the other direction (usually OL or something of the sort, depending on meter). If you get the same reading (lower voltage drop) both directions, it's shorted, or something in parallel with the diode is shorted. At that point, you need to unsolder one side of the diode to see if the diode is shorted or something else in parallel with it is. 1N4007 is a commonly used rectifier diode in tube amps.

[keithb7]

Thanks for the confirmation. Any idea on the proper replacement diodes? Thanks.

[The Dude]

1N4007 is a commonly used rectifier diode in tube amps.

^^^^

[g1]

Just a note to make sure the bias cap is has correct polarity (+ end to ground). Put in backwards they will fail and can give symptoms like you are having. It's a simple mistake that is easy to overlook because we are used to seeing (-) end grounded in most cases.

[mac dillard]

Looks like a diode problem but just in case you want to test the PT, take a look at this: https://www.premierguitar.com/articl...ormer-tester-1

[The Dude]

......I do however have some new diodes here to compare......

Which diodes do you have? It may be that they'd work as replacements. 1N4007 is what most use, as I said, but there a plenty of other part numbers that would work there.

[olddawg]

Replace them all btw..

[g1]

Sorry Keith, I think I misread your post. I thought you meant you disconnected the bias winding but it seems you are talking about the HV winding.