Vox AC30-6/TB with Loud Xfmr Mechanical Hum

[nevetslab]

One of the three Vox AC30-6/TB amps that landed on the Repair shelves is now up on the bench. Reported symptom was 'Won't Power On' AC Mains Fuse (T4A) was blown. But why? After replacing it, and having a look inside, I didn't see anything obvious, though I wasn't happy with a prior mounting of a 20uF/500V Axial Electrolytic placed adjacent to the 5AR4 Rectifier tube, with it's ground lead staked to an installed solder lug, some distance away from all the other ground returns to Chassis.

When I brought up the mains via variac and power analyzer, all seemed ok. I had modifed the inventory of our AC30-6/TB amps Standby circuit to interrupt the Cathode connection to the RC cathode load connection with the S/B switch, leaving the HV Anode connections attached to the rectifier tube. So, in Standby Mode, only the output tubes are switched off, the rest are powered up....yielding a MUTE mode as we are accustomed to with Standby.

When I switched into Operate Mode, the power transformer was groaning loudly...with heavy vibration felt on the chassis. AC Mains current was a bit higher than normal...running around 1.32A instead of 1A at Operate mode And, substantial hum heard in the speakers after turning up the volume controls (no surprise). Not much difference there, but, after removing the chassis from the cabinet to have a better look and have access to the instruments, I first swapped out the rectifier tube, just to see if that made any difference. I had to make do with a used 'pull' not having a fresh 5AR4 on hand (tube inventory held captive in the Guitar Dept). Mechanical hum gone, mains current back to 1A

Interested in seeing what's the cause of rattling the power transformer, I installed the Tektronix AM503A current probe plug-in and clipped in the A6302 Current Probe to sense the current flow thru the HV C/T lead to Ground, so I could see both sides of the rectifier tube's current conduction.

I started with the 'bad' 5AR4 tube installed.



It wasn't until I had installed the used 5AR4 tube that I saw there was no conduction of one of the anodes of the 'bad' 5AR4 that was causing all the ruckus with the power transformer. At that point, I grabbed the camera and took some scope images for my files. The man with the keys to the fresh tube stash across the street still hasn't arrived, so after taking comparative images with the other 5AR4 tube, I reached into another of the AC30-6T/B's I brought over for repairs, to see if that rectifier tube was any better. It looked the same as my 'pull'



I did have a number of 5U4GB rectifier tubes on hand, and after placing one of those in (higher voltage drop than th3 5AR4), it yielded equal current flow thru the Anodes, which is what I would have expected. I've never looked close at this before, so I don't have a feel for how well each side of the rectifier tubes balance with regards to current conduction.



I'm guessing the tube that was installed when I got the amp, with the blown T4A mains fuse was a result of the rectifier tube going thru its' pre-death rolls. I do recall another AC30-6/TB that would blow the fuse when switched out of Standby.....prior to revising the Standby Switch circuit. I didn't go as far as identifying which anode wasn't conducting, and disconnect it to see how the amp would behave if strapped for half-wave rectifier operation. Just an odd finding, hearing this with only one-half of the rectifier tube working and the imposed magnetic behavior it causes with the power transformer.

[nevetslab]

Results of new 5AR4 installed

Ah..the man with the keys arrived, so armed with a fresh new 5AR4 Rectifier Tube, I popped it in, still having the test setup on the bench. The results were what I would have expected....equal current conduction thru both anodes, both in the lower-current Standby mode, as well as in the higher-current Operate Mode (just at idle).



None of the 'seasoned' tubes I tried were balanced in the Standby mode, including the 5U4GB, which was balanced in the higher current Operate Mode. As this is uncharted territory for me, I wonder how long the equal current conduction condition lasts in the tube rectifiers before they begin drifting apart. I haven't yet looked at what happens under driven conditions.

Added note:

While the test set-up was still up, I looked at the peak current thru the HV C/T lead with the amp running at 30W output into 16 ohms (22VAC)....peak current increased a little bit...from 560mA peak to 700mA peak, still balanced.

[drewl]

Good that you fixed the poorly designed standby circuit, as that is what kills the rectifier tubes.

I removed it on mine and have a master volume installed there.

These amps seem to have that PT vibration/noise.
Try tightening the trans and try twisting the filament wiring.
This cuts down on some of the hum.

Changing the filter cap values to the original values helps the amp sound a bit better also.

[nevetslab]

Of the nine AC30-6/TB amps we have in our inventory, two of them had so much hum from the heater current's contribution in driving the power transformers into saturation, that I installed a separate filament transformer. Big improvement in the hum problem, but it did raise the plate voltage supply level as a result of lower current load, increasing the power dissipation of the already-too-hot EL84's. I hadn't checked the rectifier tubes back then to see if they had one weak side, that seems to come along with aging and use.

On this amp, looking at the higher peak charging current of the bad rectifier tube, it does two things....increased the load current in the transformer, AND lowers the charging frequency to 60Hz, instead of 120Hz, driving the transformer into saturation, causing the loud hum from the severe vibration. Balanced or near-balanced dual rectifier sections running at 120Hz is a lower charging current demand on the transformer, and keeps it out of the core saturation region. I know R.G. and others would have a far more elegant description and explanation of the phenomena, but that's my simplistic view on what I observed.

This particular PT has a fair amount of rust on the outside edges of the lamination stack....and the core bolts did seem snug....tight enough to not want to test the shear strength of the core bolts. I hadn't re-dressed the heater wiring for tighter twists.

I have one of the two AC30-6/TB amps with the added filament xfmr, and just checked the rectifier tube's conduction. Perfectly balanced, both in Standby and Operate (has the S/B switch mode). I'd have to check my records to see if I had replaced the rectifier tube during the xfmr mod, or thereafter. Induced hum level cranked up seems nominal.