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Post subject: Will curiosity kill the cat?
Posted: Sun Feb 02, 2014 4:29 pm
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So I was browsing through several different schematic diagrams and noticed that some amps run a 50uf/70v electrolytic from the center lug of the bias/balance pot to ground and others don't. I noticed this on my '74 bandmaster reverb yet the super reverb, which for all intensive purposes is the exact same diagram minus the output transformer and one coupling cap, doesn't have it. Can someone please enlighten me as to the purpose of this cap?

Thanks in advance.

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Post subject: Re: Will curiosity kill the cat?
Posted: Sun Feb 02, 2014 5:20 pm
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Likely an additional filter for the bias supply to compensate for sloppyass lead dress.

Arjay

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Post subject: Re: Will curiosity kill the cat?
Posted: Sun Feb 02, 2014 6:15 pm
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That is exactly what the cap is, an extra filter cap for the bias power supply. Many later SF model amps used two filter cap --- often placed far apart on the chassis. The lead dressing and wiring layout of these later SF amps are often marginal, at best. This is one reason so many pico-farad snubber caps and local feddback circuits were used. An afterthought and preventative measures to compensate for poor layout.

See the two 80mfd/70VDC filter caps hanging off the bias power line, in this 45Watt Master Volume schemo? Hard to find, as the schematic is drwan in a very funky manner.


http://www.webphix.com/schematic%20heav ... tr_rev.pdf


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Post subject: Re: Will curiosity kill the cat?
Posted: Sun Feb 02, 2014 8:03 pm
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Retro and BMW,

Thanks for the responses.

On another note, is there a difference in using a .01uf or a .001uf cap at the input of one half of the phase inverter? I've noticed both being used on again basically the same circuits but different amps (for instance on the AA1069 BMR and AA763 Super Reverb it has a .001 but on the AA270 super reverb it has a .01). Just curious.....

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Post subject: Re: Will curiosity kill the cat?
Posted: Sun Feb 02, 2014 9:30 pm
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socal,

I can't believe this darn board dumped my latest message... Ok.. attempt number two.

The change from 0.001 mfd to 0.01mfd coupling cap in the phase inverter section (PI) was to increase the overall gain in the amp. This coincides with the change of the plate loading resistors onto the PI's 12AT7, from 82k/100k-ohm to 47k/47k-ohm.

You can see the transition in the AA1069 circuit. Which has the 47k/47k-ohm plate loading resistors, but the original 0.001mfd coupling cap. This must not have been enough gain, as the AA270 soon followed with the 0.01mfd cap change.

But, the price to pay for this gain change was added high frequency instabilties. Which were addressed with the addition of those 2000 pico-farad caps on the 6L6GC's. These route HF to ground. So, what you get is cleaner headroom to a more pronounced crunch with the AA270 over the AB763. But, you lose the relaxed "airiness" and slower onset of OD, with the original BF circuit. And those 2000 pico-farad caps may cause a bit of high end roll-off.

So, it's the gain stage setup and the global & local feedback parameters that define most of the BF tone versus the SF tone. NOT the bias supply topology. Most techs just change the balance/hum pot to a bias pot circuit and call it "blackfacing." And miss the gain stage and PI stage changes, like you noted.

Fender really messed up the Bassman circuit. The SF versions have a lot of local feedback and a bizarre global feedback network. Making the amp way too sterile sounding compared to the Holy Grail tweed Bassman (5F6) tone.


http://www.webphix.com/schematic%20heav ... _schem.pdf


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