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Post subject: Vibro-Champ PT replacement
Posted: Thu Jun 16, 2011 5:49 pm
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Never had to replace one of these. Mercury Mag PT comes with a center tap for its 6.3VAC heater lines. I can remove the one stock green heater line soldered to ground. Then solder the new CT to this point. Twsiting the two new green heater lines to each tube (removing their OEM heater leads to ground)---this should reduce some of the hum. Right?

Stock Vibro-Champ single line heater to tubes:

http://www.ampwares.com/schematics/cham ... _aa764.pdf



Converting the lines to this (PR AA964 style):

http://www.ampwares.com/schematics/princeton_aa964.pdf


Is there any point in added a 100-ohm resistor to each end of heater line (usually by the lamp), if the 6.3 VAC heater taps already has a grounded CT?

Such as in this PR (AA1164):

http://www.ampwares.com/schematics/prin ... aa1164.pdf

Thanks!


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Post subject: Re: Vibro-Champ PT replacement
Posted: Thu Jun 16, 2011 10:55 pm
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To be honest, I'm not sure that any of these machinations will perform their intended purpose (to reduce the heater-induced hum). That tranny tap uses un-rectified AC -- how does inserting a 100-ohm at either end of the line accomplish anything?

Just wondering......

Arjay

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Post subject: Re: Vibro-Champ PT replacement
Posted: Thu Jun 16, 2011 11:38 pm
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Arjay,

I think they act as a pseudo-CT for those PT's that only have the two 6.3VAC taps & no center tap. Like the Princeton B1270 circuit:

http://www.ampwares.com/schematics/PRIN ... AB1270.pdf


Valve Wizard gives a pretty good run-through about how to reduce heater line hum:

http://www.freewebs.com/valvewizard/heater.html


But, for the life of me (maybe it's those crazy CBS bean counters, again)---- I cannot figure why you'd put 100-ohm resistors at the tail end of a heater line that HAS a CT.

http://www.ampwares.com/schematics/prin ... aa1164.pdf


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Post subject: Re: Vibro-Champ PT replacement
Posted: Thu Jun 16, 2011 11:55 pm
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BMW2002Ti wrote:
I think they act as a pseudo-CT for those PT's that only have the two 6.3VAC taps & no center tap.


Okay, got it now.

Still not sure if it will work though.

Give it a try and see.

My '68 Bronco idles pretty quietly but I'm not sure if the PT is center-tapped or not. I'll have to check next time I have the chassis out, which will prolly be soon since it needs a new on/off switch. I'll also have a look-see to determine if there are extra resistors at the ends of the heater-tap line.

Arjay

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Post subject: Re: Vibro-Champ PT replacement
Posted: Fri Jun 17, 2011 12:38 am
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Arjay,

I think you'll find that your Bronco has one green heater line soldered right to the chassis from the PT. And only one green line going to each tube. Each tube's other heater pin is actually grounded to the chassis. So the dang chassis is carrying 6.3VAC (or let's say +/-3.15VAC) on the chassis, at all times. The chassis is acting as the other "green" line used in the more expensive push-pull amp.

To use the OEM PT and have two twisted heater lines to each tube, you have to pull the one green line from the PT, off ground. PLUS ALL the grounded heater pins from the other three tubes and the lamp (rectifier has its own 5 VAC heater lines). Then, put in a separate, new green line from the PT. Wrapped around the original one---to each tube and the lamp. Connect two green lines to one 6V6GT and two 12AX7.

The original PT has no center tap. So, to help with the hum... a 100-ohm resistor will need to be wired (usually from each lamp connection) to ground. It's a LOT of work.

I've never done this on a stock Champ or Vibro Champ. Hardly seems worth it, as single-ended amps usually always have some level of hum. (no push-pull topology to cancel noise).

But, since I have this new Mercury PT with the CT on the heater line.... I tell you how it works out. The amp is coming into the shop next week.


Bronco schematic:

http://www.ampwares.com/schematics/bronco_ab764.pdf


What I'm thinking of doing (like this PR schemo):

http://www.ampwares.com/schematics/princeton_aa964.pdf


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Post subject: Re: Vibro-Champ PT replacement
Posted: Fri Jun 17, 2011 5:20 am
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I did not read all the answers, but this is waht I know;

Vibro Champ is a single end amp = only one output tube. These kind of amp are always noisy ( hum ) .

6.3 volts center tap is usuful for hum cancellation. If your transformer did not have it, like Fender original. Two 100 ohms 1/2 watt , resistor do the same job as a center tap.


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