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.

Pluggers comic conjures ‘National Radio News’

For tube fans, today’s Pluggers comic strip conjures memories of the NATIONAL RADIO NEWS magazine cover, Aug-Sept 1945 issue.

The Pluggers comic is in today’s newspaper July 23, 2011, or you can enjoy the comic online at Comic Strip Nation, comic link is HERE.

I mention this comic because many fans of old tube gear will find it interesting, but from my perspective, seeing this cartoon was a one-in-a-million coincidence.  Yesterday, I was rooting through my storage facility, and I found a quantity of these “National Radio News” magazines published by NRI (National Radio Institute).

This exact issue (Aug-Sept 1945) was on the top of the stack of magazines that I hauled home from my storage facility.  And the very next day, the Pluggers comic appeared.  What are the odds of that?


Limitations of cheap DPM’s and DMM’s

©2011, Bob Putnak.  This post examines the performance (directly related to the input impedance) of low-cost meters; specifically, I explore a common multipurpose Colluck PM-128E DPM (digital panel meter) and a bargain-priced Cen-Tech #98025 multimeter.

Limitations in the design of these low-cost meters can severely affect measurement accuracy.  First of all, I prove that the input impedance of the PM-128E is 1-megohm, not the 100-megohms or 10-megohms that is specified by the manufacturer and most vendors that sell this DPM.  Second, I demonstrate that the input impedance of the Cen-Tech #98025 multimeter is also 1-megohm.  The conclusion is that either meter will not accurately measure high-impedance circuits, and both perform poorly at measuring low AC voltages.  They can be suitable for other types of measurements, though.

Explanation from a very old Supreme radio course

First a little background –“Input Impedance” as it pertains to a meter — is the load that the meter places upon the circuit being measured.  Ideally, a perfect meter would have no loading effect, but all meters have some loading effect on the circuit they are measuring.  For example, early analog VOM’s had an input impedance of 1000 ohms per volt, which meant that when the meter was set on the 500v range, the input impedance was 500k ohms.  This input impedance (sometimes called ‘meter sensitivity’) is the exact same as placing a 500k resistor across the circuit. Newer analog VOM’s had an input impedance of 20,000 ohms per volt; therefore using our 500v range as the example, the 20,000 ohms/v meter would only load the circuit at 10-megohms.  VTVM’s (vacuum-tube voltmeters) and TVM’s (transistorized voltmeters) commonly had a fixed loading effect of 11-megohms or 22-megohms, regardless of measurement range.  Most quality modern DMM (digital multimeters) have a fixed input impedance between 10-megohms to 11-megohm.  The higher the input impedance resistance, the more accurate the measurement.  Input impedance is a serious issue when measuring high impedance circuits.

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TubeSound TTM-1 Tube Tester & Matcher

Meet the TubeSound TTM-1:

  • Testing of almost all amplifying tubes (triode, tetrode, pentode, beam power) from antique 4-pin (such as a #10, #45, or #50) through 9-pin novar (such as a 7868).   Socket configuration — 4-pin, 5-pin, 6-pin, 7-pin medium (aka 1625), 7-pin miniature, octal, loctal, 9-pin-miniature, and 9-pin novar.
  • All tests use exact tube operating parameters found in any “Receiving Tube Manual”
  • 5 digital meters (each better than 1% accuracy, as verified with two Fluke DMM’s) continuously monitor the tube operating parameters.  1 meter for each plate voltage, screen voltage, grid voltage.  1 meter for plate current, 1 meter for heater voltage.
  • VR (voltage regulator) tube testing throughout its entire operating range.  VR tube voltage drop continuously monitored, and starting voltage is easily observed
  • Mutual Conductance testing via grid-shift method
  • testing of tube Amplification Factor
  • Plate current matching at any single operating point, or you could plot a set of curves.

Design specifications:

  • regulated plate voltage, variable 0 to 500 VDC (0 to 410 continuous)
  • regulated screen voltage, variable 0 to 500 VDC (0 to 410 continuous)
  • regulated grid/bias voltage, variable 0 to -100 VDC
  • plate current up to 200 ma
  • heater voltage accurate within 0.1v.

My intention was not to replace any vintage tube tester, but instead, to supplement functionality that does not exist in traditional tube testers.  For example, transconductance testing is certainly much easier using the dynamic test of a B&K or Hickok.  Likewise, grid leakage sensitivity is best tested in a Mighty Mite or similar machine.  But none of those machines recreate the static operating parameters that a tube will see in an amplifier, therefore they do not meet the needs of some tube buyers who want their output tubes matched for idle plate current at the operating parameters of a real amp.  Moreover, no standard tube tester will properly test a VR tube and allow you to monitor its performance over its entire operating range.

Photos below show testing of a new Sovtek 5881/6L6WGC using two different receiving tube manual examples from 6L6GC “Typical Operating Conditions, Class A1 Amplifier – Pentode”.  The third photo demonstrates testing a new 0A2 regulator tube.




I have a few cosmetic issues to finish, but otherwise the first model is complete (for now). I have ideas for other features that I may add in the future. The TubeSound TTM-1, in combination with our classic tube testers, covers a wide range of tube analysis that will meet the needs of sophisticated customers.

PR: new hiring of Chief Technical Consultant

April 1, 2011.  TubeSound, a worldwide vendor of audiophile tubes & test equipment and service center, is pleased to announce the appointment of Snickers T. Dogg to the position of Chief Technical Consultant.

“Snickers will be a valuable addition to our team by growing our service center in a valued-added result-driven manner.  He is a strong strategic fit with our core competencies.”

Snickers brings a goal-oriented approach to servicing.  “The endgame is simple — get it done.”  As pioneer of the Spray-and-Pray service technique, he has been proactive in driving down the cost of repairs.  “I once stepped on a can of WD-40, and the rest was history.”  This user-friendly servicing technique has empowered millions of technicians worldwide.

Never satisfied with the status quo, Snickers has leveraged the synergies of spray & service to expand the effectiveness of his Spray-and-Pray methodology.  “If the spray don’t work, you can whack it with the can.”  His outside-of-the-can thinking will allow unparalleled speed-to-repair.  This is a win-win scenario.

“The one thing that impressed us the most was Snicker’s 24/7 customer-service mindset.  His proactive networking creates a strong foundation of trust.”

Snickers also brings to the table a rare ability to find bad transformers without need for any test equipment or powering-on the equipment.  “I must have a nose for it” quips Snickers.

Competition to land Snickers was fierce.  In turning down a position as a jukebox technical consultant with a Pittsburgh-based music distributing company, Snickers explained “I can’t be associated with nothin’ lame.”

Snickers also plays a mean game of “Bullshit Bingo” and feels that he will have many opportunities to play here at TubeSound.  In fact, he is barking “Bingo” right now.


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