OT Wired Wrong - Noobie Mistake

[steveneddy]

Man - built a new amp - but it never was loud enough in my opinion.

Took it to gigs - rehearsals - tried various 16 ohm cabs - to no avail.

What was wrong?

It was built with a Hammond 1750Q OT for like a JTM45 - with a choke like a Bassman.

I finally got around to really looking at the amp this morning wanting to finally put this to rest.

Looked at every solder joint. Resoldered a few.

Checked EVERY resistor - even removed a few just to ensure they measured correctly out of the circuit.

Looked at all caps.

Checked the voltages so many times it's not funny.

Everything looked good - what was the issue?

So - schematic out - build noted spread all over the table - DVOM on - check EVERY wire - looking - looking.....

Wait a minute - look again...

The OT was wired to the 4-8-16 ohm selector switch incorrectly - it was wired 16-8-4!

When I had the switch in the 16 ohm position according to the markings on the rear panel it was accessing the 4 ohm tap on the OT.

I couldn't believe it.

This is a testament to the Hammond OT and the Sovtek 6L6WXT+ power tubes - the HD components used throughout - and dumb luck.

A simple test by moving the switch to the 4 ohm position made the amp come ALIVE!

I feel so stupid. I haven't made a mistake like that in many moons.

But - it happens to the best of us - and it happens to the worst of us - like me - :^)

[martin manning]

Confession is good for the soul. I agree, tough tubes! Miss-matching the other way is what will take out the OT, due to fly-back voltage.

[Stevem]

Yup, having a lower impedance tap driving a higher load can be a sure invite to a winding flash over arc dependant on the Plate voltage.
Lucky you plate voltage is under 480 and the Hammond OT is overbuilt!
The first time many moon's ago that I replaced a Marshall Major OT I did not make note of the color code and when I 2 weeks latter installed the new OT I used the hook up for that color code that non Marshall amps use for that color code I regards to hook up.
Lucky for me I rechecked before I powered the amp up and found my mistake, or that would have been 350 bucks for another new OT out of my pocket!

[xtian]

I ran into trouble when trying to drive a 16 ohm cab with a 2 ohm Bassman OT in my Marshall 1987 build. A plate in one EL34 arced to its heater, burning out the 100R heater balance resistors. Valuable lesson learned cheaply!

[RWood]

Does running 16 ohm cab from 2ohm tap burn out OT?
I thought it was the other way around.
I ran 16 ohm cab from 4 ohm bassman for several years practicing (though never at gig). The only ill effect I noticed was loss of volume. Maybe I just got lucky.

[tubeswell]

Running 'a notch' above or below the desired load resistance most likely won't cause the thing to fail. But running way-too-high-a-load will cause problems (as will running no load) - both can cause voltage spikes in the OT that will puncture through the winding insulation.

However if running within a notch above or below, then think about this - running a slightly higher load resistance (and getting a flatter load line) is better in the long term than running a slightly lower load resistance (and getting a steeper load line) that puts the plate into running over the maximum rated dissipation more than 50% of the time. Having said that, you can compensate for a lower load resistance with colder bias.