Partial, Intermittent Failure of Output Transformer
I am relating the following tale of woe because it is a very unusual failure mode for an output transformer. I had not seen or heard of it before
I was recently working on an obstinate 1969 Vibrolux Reverb with an annoying problem. It presented as a weird, unusual distortion when loud chords were played – kind of a crackling breakup and nasty distortion that was not evident at lower volumes, or on most single notes.
A previous tech had tried to fix the problem, unsuccessfully, by adding huge capacitance to the first filter capacitor (it was 147 uF). The later filter stages had all been increased to 80 uF (the schematic calls for 16 uF). Also, an extra ground had been added between the brass grounding plate and the power transformer ground. The previous tech also bypassed the preamp cathode bypass capacitors with very small value ceramic caps, replaced all electrolytic capacitors, and used shielded wire on the inputs and the volume signal wires. All the filament wiring had been replaced. A three-conductor power cord was installed.
It seems the previous person was trying a lot of things to eliminate the problem, but none of the modifications were successful. I reverted all of the changes back to specification. The problem was still there.
The amp had one weak tube, (V3 12AT7), which was replaced. I couldn’t see how that contributed to the noise, however.
I did all the usual things. The bias was very hot for a VR, so I brought it down to dissipate 35 Watts, the VR’s rating. The scope was not too helpful – the signal was fine all the way through the amp. I started wondering about the (original) output transformer, but it tested fine – no shorts or opens on either the primary or secondary, and no shorts to the core from anywhere.
I did not try R.G. Keen’s Secret Transformer Tester (https://www.premierguitar.com/articles/print/the-super-secret-transformer-tester-1), so I am not sure if that would have detected anything.
The transformer was not rusty or loose, so I discounted eddy currents as contributors to the noise.
On a hunch, I bought a new replacement transformer from Classic Tone (they are out of business, but you can still find some for sale).
I installed the transformer, and the problem was gone.
(I subsequently modified the circuit to Blackface specs. This is one of the Vibrolux Reverb amps with a tube chart that identifies the circuit as AA864, but this circuit doesn’t actually exist – the actual circuit in the amp is the Silverface AB568. It was converted to the Blackface circuit AA964.)
So…
I suspect that the transformer had a weak spot in the windings that worked fine with small signals at low power, but partially failed as demand was increased on the transformer, creating the crackly breakup. The Vibrolux sounded great – bright and chimey, with solid bass and a typical “Fender” midrange.
