Vox Vetting

soundmasterg · Post by **soundmasterg** » Mon Apr 03, 2006 10:27 am

Peter, those voltages do sound a little high...354v. I haven't checked mine in awhile so I can't recall what the B+ is but I can probably check it sometime later this evening. Mine has the stock cathode resistor value on it. The 6V6, especially JJ's can generally be hit with some high voltages, but the EL84 doesn't like them nearly as much. Did your tech measure the cathode current? Depending on where the tubes are biased they may be able to take those voltages, but I would suggest that returning to stock values would probably be a good idea. If you increase the cathode resistor value then yes, the tubes run a bit cooler, but they also sound colder.

admin · Post by **admin** » Mon Apr 03, 2006 11:31 pm

Thanks Greg. I have passed this information on and appreciate your comments. I'll keep you posted.

soundmasterg · Post by **soundmasterg** » Tue Apr 04, 2006 6:33 pm

Peter, I won't be able to get a voltage check on mine until later this week, but if you still need it I can do it and post here. Let me know.

admin · Post by **admin** » Tue Apr 04, 2006 11:29 pm

Greg: Thanks for this offer, but I consider that it is no longer needed as the plate voltage value is now within acceptable limits, running at about 340VDC. The re-issues seem to run a bit higher than the vintage Vox AC30 amplifiers at 320VDC or so.

soundmasterg · Post by **soundmasterg** » Wed Apr 05, 2006 10:12 am

Ok Peter, I won't bother then. Thanks! The 340v does sound better.

With tube circuits, there is usually a 20% tolerance on many parts and transformers are a little closer tolerance, but there will still be a slight variation due to different loads, tubes, resistors, etc.

I'm working on three Supro Thunderbolts right now from the 60's and all three have different voltages randing from about 370 for the tube rectified one, to 400v for a solid state rectified one, to about 415v for the other solid state one. Its normal, and vintage Fenders will often read 15% or more higher than what their schematics say, partly due to the wall voltage being higher nowadays, and partly because the schematics were low from the beginning.

admin · Post by **admin** » Wed Apr 05, 2006 11:51 am

Thanks for elaborating further Greg. I hadn't even considered the higher wall voltages compared to the 1960s.

soundmasterg · Post by **soundmasterg** » Thu Apr 06, 2006 9:31 pm

It makes a big difference in the internal votlages in the amp. The wall voltages are AC, and inside the amp for the most part it is DC, but it doesn't change linearly, so if the wall votlage is up by 15 v, inside the map it could go up 10 v or 40v, and of course that could really change the sound and the performance of the amp, and could even make it exceed the voltage ratings of parts as in old Sunns.

admin · Post by **admin** » Sun Apr 09, 2006 5:52 am

Greg: My AC30 has returned and fully operational again with a big bonus, at least from my point of view.

A 500pf cap is now on the Normal Channel volume control and for the first time it has a very usable sound. It has the volume of the Brilliant Channel and almost as bright but with very full lows that are not muffled.

soundmasterg · Post by **soundmasterg** » Sun Apr 09, 2006 12:14 pm

That sounds good Peter! It still has less gain than the Top Boost channel, but I can see where that would make it more useable. I just use a randgemaster treble booster when I'm going ot use the normal channel.

admin · Post by **admin** » Sun Apr 09, 2006 1:31 pm

Yes less gain as you say Greg but a different tonality. I tried treble boosters on the Normal channel as well but the result was never this type of clarity.