PACO SA-50 Tube Amp Rebuild

I find it exciting to discover a nice audiophile amp from a nondescript brand.  In this article, I chronicle my rebuilding of a PACO preamp-amplifier model SA-50.

PACO is the Precision Apparatus Company, best known for building quality test equipment such as tube testers, signal generators, and oscilloscopes.  The “PACO” branded gear (as opposed to gear that used the full name of “Precision Apparatus”)  was sold as a build-it-yourself kit.  During this rebuild and subsequent troubleshooting, I found two connections that were never soldered, and one loose solder joint.  Those issues are extremely common to find when servicing vintage kit-built gear.

I do not have any paperwork or background information about this model.  I did find a schematic for a model SA-40, which appears to be largely the same circuitry.

The SA-50 is a stereo preamp-amp with these design provisions: Read the rest of this entry »

…more Phase Linear (round 2)

My neighbor was thrilled to find someone local who could repair has Phase Linear audiophile gear.  One of his friends was also a Phase fan, and his gear had been sitting for a longtime in disrepair.  So now I had 3 more Phase Linear units to repair: a Phase Linear 2000 preamp, Phase Linear 200 power amp, and a Phase Linear 400 power amp.

All three units were very dirty (cigarette smoke yellowing) and dusty.  After cleaning, each looks excellent.  It was obvious that all three units had not been in use for many years.

Here is a summary of each repair:

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Phase Linear …on the bench!

Here is something that I don’t see everyday… Phase Linear audiophile gear! In fact, this was my first opportunity to use, repair, and evaluate any Phase Linear gear.

While I only use tube audio gear, it is hard to argue with the exceptional quality of a premium audiophile solid-state amplifier such as this Phase Linear 700 Series Two. It makes me want to reconsider tube audio …at least for a day or two!

I agreed to tackle this Phase Linear gear for my neighbor, even though I do not typically service solid state gear. He had originally purchased this pair brand new back in 1979 — a Phase Linear 700 Series Two amplifier, and complimentary Phase Linear 4000 Series Two autocorrelation preamplifier. Both units had defects. My good buddy Donnie and I spent most of entire day working on this gear.

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Transformers can blow in the blink of an eye

Recently my longtime trusted Eico 1030 regulated high voltage DC power supply succumbed to a catastrophic failure — a shorted transformer winding.  The power transformer has a separate winding that supplies screen voltage to the 6L6 pass tubes.  This is a half-wave circuit that was rectified by one original 1N4006 silicon diode.  Unfortunately, this old diode shorted and took the transformer winding with it, thereby ruining the transformer.

I use my HV power supplies on a daily basis for a number of tasks such as component testing (high voltage diodes), capacitor reforming, insulation testing, mocking up tube circuits, and many other purposes.  Fortunately, I kept a number of “backup” HV power supplies in case such a failure occurred.  (Sidenote: testing HV diodes on a DMM is a futile task.  The applied voltage is much too low to indicate whether the diode is shorted.  Many diodes will pass a DMM diode test but are, in fact, shorted when operated at the high voltages seen in the circuit.)

This transformer failure caused me to consider how unprotected most transformers are in vintage gear.  In many cases, a shorted cheap component (such as diode or filter capacitor) can ruin a very expensive or sentimental piece of equipment.  Typically, the only circuit protection that you see on most vintage gear is a line fuse, which does a poor job of protecting the transformer’s secondary windings, especially if the fuse is up-rated to accommodate inrush current at startup.

In rebuilding my backup Eico 1030, I took several steps that may help to protect it’s transformer.  I say “may” because fuses & circuit protection often do not blow on a consistent basis, so these steps should help but are not guaranteed.

I installed an ICL to limit inrush current at power-on.  I replaced the rectifier tubes with fresh silicon ultra-fast diodes because I wanted to reduce the heat inside the cabinet, promote stability, and reduce transformer load (2.26A saved by eliminating the rectifier tube heaters).

One of the cheapest circuit insurance you can buy is to place diodes in series.  If one diode shorts, you still have another one to continue proper operation of the circuit.  Likewise, series diodes sum their voltage ratings, so the circuit is more robust anyway.  For the price of a quality ultrafast diode from a reputable supplier — approx 20-cents each for a Fairchild UF4007 (1A, 1000v) — this protection is a bargain.  In each 5AR4 plate circuit, I used two UF4007 in series, or a total of four diodes in this circuit.  The bias supply (6X4) is a half-wave circuit, so I used two in series.  Likewise, I used two UF4007 in series in the screen supply, which was the winding that failed in my old power supply.

In addition to stacking the diodes, I installed fuses for each secondary winding. First, I fused the center-tap of the HV 440-0-440 winding with a 200ma fuse (200ma-250ma fuse should be reasonable in this circuit).  I fused the screen winding with a 62ma slo-blo fuse, which was the smallest that I had available.   I fused both 6.3vac front panel windings with 3A slo-blo fuses, which will accommodate the occasional overload and still remain within design parameters.

I have experimented with using PTC’s (resettable fuses) in each circuit, but PTC’s have three problems that make them less suitable for this type of circuit protection : (1) they do not trip with any operating current certainty (ex: a 150ma PTC may trip at 200ma or 600ma, you just have no way of knowing),  (2) they have a base resistance that may affect circuit operation, and the resistance increases as the PTC heats and approaches the upper range of its hold current design, (3) they trip slowly because the trip is based upon the PTC heating up.  Due to these characteristics, it would seem as though PTC’s would not have much practical use in protecting audio transformers.  In this power supply, though, I did leave a 40ma PTC in the screen supply, which helped to mitigate inrush current and possibly added some transformer protection.

Finally,  I decided to add protection for the current meter, which would always slam backwards if a connected load (such as a capacitor) discharged when the voltage switch was toggled off.  I replaced the SPST switch with a DPDT switch.  The first section of the DPDT switch replaced the original in the same manner.  I added a protection diode in series with the current meter to prevent current from reversing thru the current meter.  The second section of the DPDT switch is wired to dump the (+) terminal to ground through a 500 ohm high wattage wirewound resistor, which assures that any load attached to the power supply (such as a capacitor that was being reformed) is safely discharged as soon as the voltage switch is toggled off.

rare Rockola jukebox amp hum problem

Rockola 1426 jukebox "O" amp

I wanted to write about a unique problem in a Rockola model “O” jukebox amplifier (from a Rockola 1426 jukebox, vintage 1946).  The amp is a very primitive design: class A, 4 tube operation consisting of 5U4 rectifier, push-pull 6L6’s, and 6J5 preamp tube.  An interstage transformer drives the P-P 6L6’s instead of a more modern phase inverter tube circuit.

The amp was recapped but had a hum problem.  The hum was not a single frequency, such as the common 60-hz or 120-hz hum that you would expect in a typical amp.  The hum was a combination of 60hz with a strong 3rd harmonic of 180-hz.

The problem turned out to be that the interstage transformer was inductively coupling hum from the power transformer.  This problem would have existed in this amp from the day that it left the factory.  Often it is necessary to carefully position and orient interstage transformers so that they will not inductively couple hum into the amp.  In this case, remounting the interstage transformer at a 45-degree angle from original mount, as shown in photo, “magically” eliminated the hum.  Trial-and-error (rotating the interstage transformer) is the only way to find what mounting orientation will cancel the hum, and each case would be unique.  In this instance, the hum cancelled best at the orientation that you see in the photo.

I wonder whether other Rockola model “O” amps were shipped with this problem?

The amp also had somewhat slightly reduced power output, which was tracked down to a resistor/capacitor combo from grid to ground on each 6L6 tube.  Scope analysis showed that these components added no benefit but did reduce the power output a little more than preferred, therefore the parts were removed.  Finally, the amp had poor reproduction of treble notes, and this was tracked down to a plate-to-plate capacitor on the 6L6’s.  This cap was probably intended to prevent oscillation in some circumstances.  Removing this cap dramatically increased the frequency response of the amp and without any oscillation.

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