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Post subject: Identifying Silverface circuit
Posted: Fri Oct 11, 2019 8:18 am
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I have a 74 Fender Twin Reverb with Master Volume/Push pull, line out and hum balance. I am trying to identify the correct circuit design number so I can order the right parts... The number is NOT printed on the tube chart, nor anywhere else that I can find. How can I identify which circuit design it is?


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Post subject: Re: Identifying Silverface circuit
Posted: Fri Oct 11, 2019 9:33 am
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The CBS 100 Watt RMS w/ reverb and vibrato dtd 01-82 is the only Master Volume schematic I know of.
While you may most likely have a mix of circuits from the AA270 and the above mentioned schematic.
Pictures would help significantly, as just saying it's a '74 w/ master is very little help in actually identifying the circuit installed in your amp.
What parts are you trying to order?
Unless your trying to mod from one circuit to another, as close to exact replacement of existing parts if your sure there haven't been any modifications is the best route w/o some pictures. Slightly higher value capacitors are fine in most cases.


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Post subject: Re: Identifying Silverface circuit
Posted: Fri Oct 11, 2019 9:44 am
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This circuit was never assigned a specific revision number -- the schematic was simply labeled "100 WATTS RMS AMP W/ REVERB AND VIBRATO".

You'll need to identify the necessary replacement parts by consulting the drawing.

Arjay

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Post subject: Re: Identifying Silverface circuit
Posted: Fri Oct 11, 2019 9:49 am
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sfceric64 wrote:
The CBS 100 Watt RMS w/ reverb and vibrato dtd 01-82 is the only Master Volume schematic I know of.


By 1982 the Twin Reverb had morphed into the 135-watt ultra-linear behemoth (both in terms of output power and weight) and that platform has significant differences from the garden-variety 100-watt variant.

Arjay

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Post subject: Re: Identifying Silverface circuit
Posted: Fri Oct 11, 2019 10:37 am
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The 135W Twin Reverb seems to have appeared in 1977. That was the first time I saw a line out. The numbers on the output transformer should tell you if you have a 100W model or a 135W model.

Image


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Post subject: Re: Identifying Silverface circuit
Posted: Fri Oct 11, 2019 3:44 pm
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Thank you, I would post pics but didn't see a way to attach them here. It's not an ultra-linear. Any links to the proper schematic would be appreciated.

FWIW, I was upgrading caps, 1 switch (bright), 1 pot (volume on chan 2) and wanted to mod the fixed bias.

Perhaps this is the beast? http://www.bustedgear.com/images/schema ... 00W_SF.pdf


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Post subject: Re: Identifying Silverface circuit
Posted: Fri Oct 11, 2019 5:28 pm
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Steviebone wrote:


That's the schematic I referenced but note that it does not have the line-out feature that was added circa 1976.

Arjay

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Post subject: Re: Identifying Silverface circuit
Posted: Fri Oct 11, 2019 9:34 pm
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Your Twin should have all the same iron as the AA270 circuit, my bad mentioning anything '82. That schematic is kinda the catch all of later CBS era models, pre-linear.
Do you mean you want to switch from a balance pot to a bias pot, if so you'll have to adjust that part of the circuit back further to the AB763. Not to difficult, will change the tone some.

If your wanting to blackface the amp & for general reference on the bias mod,
https://robrobinette.com/Silverface_Amp_Mods.htm#Blackface_a_Silverface_Amp


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Post subject: Re: Identifying Silverface circuit
Posted: Sat Oct 12, 2019 12:01 am
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sfceric64 wrote:
Do you mean you want to switch from a balance pot to a bias pot, if so you'll have to adjust that part of the circuit back further to the AB763. Not to difficult, will change the tone some.


Actually, changing out the screen-grid dropping resistors at both halves of the phase inverter to the values found in the AB763 circuit is more responsible for improving the amp's tone than altering the bias-supply control. All the latter really does is make it easier to find the "sweet spot" for the output bottles.

Arjay

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Post subject: Re: Identifying Silverface circuit
Posted: Sat Oct 12, 2019 6:56 pm
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Retroverbial wrote:
Actually, changing out the screen-grid dropping resistors at both halves of the phase inverter to the values found in the AB763 circuit ...


Are you talking about the grid-leak resistors for the control grids of the 6L6's? The screen grids have 470R resistors on every model from AB763 through the 135W UL.


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Post subject: Re: Identifying Silverface circuit
Posted: Sat Oct 12, 2019 7:57 pm
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My bad -- it's the plate resistors at V6. The AB763 is assymetrical (100K and 82K) while the CBS revision uses the matched 47K's. The AB763's lower voltage mismatch produces more ear-pleasing harmonics.

Arjay

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