Vox Pacemaker mod

Vox and Hiwatt Discussion

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murphyterence
Posts: 20
Joined: Mon Aug 25, 2008 8:43 pm

Vox Pacemaker mod

Post by murphyterence »

I have a couple 1965 tube Vox Pacemakers (California Vox)...twin el84 amp with a nice tone. Does anyone have any experience with these and is there any mods to make these have a little more gain at volume?
Thanks,
T. Murphy
Stevem
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A

Post by Stevem »

Well , gain easy, volume not so easy!
The amp already has bypass caps on all the preamp tube cathodes for added gain so if they are all good ( unlikely if original after all these years!) then the easy way to get more gain is to increase the voltage to the plates of the preamp tubes.
Changing out the 56K 1 watt resistor that feeds the P3 power supply node to about a 40k 2 watt resistor should get you some 30 more volts to the preamp tubes and inturn more gain.
Volume wise a output transformer with a bigger core and rated for some 10 more watts would help relive the way the stock small OT saturates, and a speaker with a greater SPL rating would make the amp louder for sure!
Of course good strong tested tubes will help also in both the gain and volume department as just because the ones you have may be new ,that does not mean they are good to over 85% of what they should be.
On average 40% of the new tubes I test out of 200 come in at 85% or less out of 100.
Then for example if the amps speaker is 8 ohms you could unhook it and run the amp thru 2 16 ohm drivers wired in parallel and then get near twice the volume out of the amp.
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