[John L Stewart]

Just finished servicing a PV JSX. My Grandson Jamie is as happy as a Tomcat in a Creamary. That PV sure is loud! It runs 4X 6CA7 in PPP.

I found it to be OK & simply reset the bias. Along the way did a lot of tests of both the amp & speaker. Quite different than HIFI. But it sure makes Jamie happy.

Did an Impedance Plot for the speaker. I need to get me a white & pink noise source. Would make that kind of measurement easier. Found resonances at 137 & 626 Hz. This the one at 626 Hz is a buzz in the cabinet.

For anyone who is interested-

PV Test Results April 2012

Line at 117 Volts

Standby TP68 500 Volts

Run TP68 479 Volts
TP67 484

TP19 262
TP18 318

TP17 318
TP16 440

Bias Set at –39 Volts
Cathode Current each EL34/6CA7 40 mA

Frequency & Time measurements made using a Pico Technologies ADC216 Virtual Instrument fronted by a Pico Tech M1053 Differential Probe.

Made my own Bias Probe. There is a one ohm resistor inside the octal plug cover barely visible in the photo of the opened assembly. The leads connect to a voltmeter where one millivolt indicates one milliamp of cathode current. Forty (40) milliamps at 484 volts will keep the plate & screen dissipation at a safe level, totaling less than 20 Watts.

At clipping this PV puts 110 Watts into a 16 ohm resistive load at 1000 Hz.
Power measured directly using a MetraHit 29S Precision DMM & Power Meter.

THD measured by HP 334A
THD at 16 Watts was 0.9%
THD at 100 Watts was 2.8%

Cheers to all, John

[John L Stewart]

Over in thread IBM 1953 Phil said-

** How so ??

Impedance plots are best done with a slow sine wave sweep while feeding the
speaker from a high impedance ( 600ohms plus) source.



..... Phil



Not sure how the thread got over there but-

PV Speaker Test Hookup

For the speaker impedance test run I used an HP 200CD (600 ohm) as the audio source. Connected that thru a Hammond 1700 Series OPT so that there was a conversion from 2500 ohms to 16 ohms. Then another 150R straight into the 16 ohm box, much as Phil suggested.

All done with the speaker box outside (open field) in order to avoid reflections. Can’t afford & don’t need an anechoic chamber, long as it don’t rain.

The results were stored with the Pico Technology ADC216 Virtual Instrument set to Peak mode. Very time consuming. I think with a good audio noise source one could set up the equipment & let it run to completion.

Long ago I did quite a few tests with a mike on a stand, altho not calibrated. Seems to me there is a method using three mikes wherein one doesn’t need any of them calibrated. These daze with the digital TE it is possible to do a couple of runs & subtract out a lot of the errors. But I sure didn’t need all that!

"You can lead a horticulture but you can't make her think."

Cheers to all John