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Topic: Advice needed: late 60's Blackface amps: 3 prong power, bias |
Marc Jenkins
From:
Victoria, British Columbia, Canada
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Posted 8 Mar 2012 11:03 pm |
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I was just discussing some mods to an early CBS Bassman head with a tech we contract (I work at a music store). I do some tube amp repairs, all else goes to this guy. I had added, at the customer's request, a 3 prong power cable. As per Dave Funk's book, I disconnected the ground switch as well. The tech said this shouldn't be done, as the grounding style (brass plate) may cause some problems due to varying currents and eddies. Whatever that means.
I had also changed the hum balance-style bias adjustment to an actual adjustable bias pot, and he is saying that the hum balance circuit affects only the heaters? I have not heard of this before but I'm not very familiar with all the Fender circuits. When I initially tried to bias the amp (assuming it was a proper bias adjust) I noticed that only one output tubes' bias was being adjusted, and sure enough, the hum balanced at a certain point. Of course, neither tube was biased anywhere near correctly. I then proceeded with the second of what I thought were standard upgrades. The tech said this was wrong to do as well?!
Thoughts? |
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J Fletcher
From:
London,Ont,Canada
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Posted 9 Mar 2012 5:18 am
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Hi Marc
Sounds like there are some misunderstandings there. Later Fenders did have a heater hum control, as well as a bias adjust, but it sounds like the Bassman you are refering to has the output tube hum balance only.
As far as disconnecting the ground switch, I think this is done to remove the cap that used to go from one side of the ac line to the chassis, out of circuit. This might be a UL and/or CSA requirement as I think the old way of doing things creates a shock hazard...Jerry |
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Rich Hlaves
From:
Wildomar, California, USA
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Posted 9 Mar 2012 9:10 am
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Marc,
IMO you did just fine. I have disconnected the polarity switch on all my vintage amps I gig with and added the 3 prong power cord.
The bias balance Fender used is a good idea but when the bias resistor drifts up the amp gets cold. Modern power tubes also draw more current so a bias resistor change is needed if the circuit is left stock.
My favorite way to add the bias adjust is to leave the bias balance in there and add a trim pot for bias adjust. Then you have the best of both worlds.
I think maybe the tech is trying to break your confidense to get more of your tube amp work.
_________________
On man....let the smoke out of another one. |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 9 Mar 2012 1:32 pm
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| Quote: |
| I'm not very familiar with all the Fender circuits |
The tech knows what he is talking about in each instance that you mention and I recommend listening to him before charging ahead. The Fender "hum balance" has nothing to do tube bias at all, as it is simply an adjustable ground tap between the two sides of the heater secondary; in amps without this pot a pair of fixed 100 ohm resitors are used instead. http://www.cornelvis.com/Schematics/faq/humbalance_pros_cons.pdf
The "ground" switch routes alternate sides of the AC mains power through a capacitor to ground, whether or not you can hear a difference in its setting in your current venue, when it is included in the design it has a place in the grand scheme of things whether or not a third grounding pin is present on the AC cord.
Just in general, if you don't know about the circuit you are working on you can do a lot more damage than good while you are "learning" about it. To dismiss advice given by those who are familiar with it is just foolhardy. |
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Marc Jenkins
From:
Victoria, British Columbia, Canada
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Posted 9 Mar 2012 1:46 pm
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Thanks guys, appreciate the input.
Dave, for the record, I didn't say I was unfamiliar with ALL Fender circuits, but that I'm not familiar with all OF them. This circuit originally had the output tube hum balance, not the heater hum balance.
For interest, the circuit was this:
http://ampwares.com/schematics/bassman_ab165.pdf
and I converted the bias circuit, plus removed the extra 220k resistors to the output tube plates, to be as the previous, pre-CBS version:
http://ampwares.com/schematics/bassman_aa165.pdf
As for the ground switch, I made no reference to hum there. Just that I had as following common convention, removed the ground switch. |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 10 Mar 2012 3:02 pm
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Ahhh, understood  |
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