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Old 22nd Feb 2017, 9:18 pm   #25
G4_Pete
Hexode
 
Join Date: Feb 2014
Location: Southampton, Hampshire, UK.
Posts: 419
Default Re: Another Bush VHF72

If its any help in tracing the lack of volume I have just done the following test on a Bush VHF 71. (UL84 output valve so should be similer)

All with the set Unplugged and Cold.

Signal generator set to 1 Khz into a matching transformer with the speaker winding across the VHF71 speaker. Signal generator set to put 100 mV rms , approx 280 mV P/P across the speaker. All subjective but fairly loud tone from the set under test at this level.

Next shut down the generator and unplug the UL84. Re injected 100 mV rms and measured from the anode pin 7 to the chasis, this measured 2.1 Volts rms / approx 6V P/P.

This does a basic check of the speaker , wiring and transformer all in situ. The only reason for using the external matching transformer was that I did not want to feed my signal generator into a low impedance speaker. If you have a low impedance signal source then fine use that.

Measurements were with a Fluke 179 true rms meter and CRT scope but if you have a sine wave source any DVM meter should do it, scope not required just used it to cross check.

Pete
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