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 Post subject: Re: Stromberg Carlson AU-35, BIG Amp, Finished with pics
PostPosted: Jun Thu 14, 2012 9:33 pm 
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Location: Marietta/Moultrie GA
BigBandsMan wrote:

Hi, Gordon, I take it that you're planning to replace the power transformer as well as the output transformers...

And why bother with a 6SN7 for a driver? The 6N7 will run rings around it.

:wink: Larry


The original power transformer is already specified for dual 5U4s (that's what it came with), and with some pretty high current levels on the 6L6s. According to my tests on the bench, it wasn't even budging on voltage, pretty much, at 700ma AC draw from the B+ secondary (it was still less than 5% down from open-circuit voltage)! That can easily give me 200ma per channel (peak draw- they'll idle at around 90ma per channel) for the output, once rectified. That power transformer is a monster- it must weigh 40lbs by itself!

By using the 5V3A or 5AR4, I can get another 30-50v of B+ (around 450v, instead of around 400-420v or so stock). With the same cathode current, that's a pretty significant power increase. With 450 on plates and 350 on screens and a 6.6K PP load, a 6L6GC will flat-out pound...

To do that, though, I will be using some pretty high voltage on the driver tube. That's why I'm going with 6SN7GTB. It can handle full rail voltage on the phase inverter, if needed (450v is OK on the GTB)...

Also, unless I misread them badly, the 6SN7 plate curves are a lot more evenly spaced than the 6N7... that's a plus.

In any case, the 6SN7 is equivalent to what was originally specified for the circuit design I'm working with (it was a 6CG7, but that was under slightly lower voltage- about 390v B+). The 6SN7GTB will give me the same response, but "do it louder"... :D

Regards,
Gordon.

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 Post subject: Re: Stromberg Carlson AU-35, BIG Amp, Finished with pics
PostPosted: Jun Fri 15, 2012 12:18 am 
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Sounds OK, if the power transformer will stand the demand.

Curves or no curves, I used a 6N7 driver in BB with a long-tailed pair configuration, and it will give those 7027A output tubes all they can handle. If that old girl ever took you under fire, it might give you a new perspective about the 6N7.

Good luck with your project :wink: !

Larry

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Last edited by BigBandsMan on Jun Fri 15, 2012 1:40 am, edited 1 time in total.

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 Post subject: Re: Stromberg Carlson AU-35, BIG Amp, Finished with pics
PostPosted: Jun Fri 15, 2012 1:16 am 
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OK, Peter, here we go. I washed that schematic out in color palette JPG format, and it will now fit on the screen:

Image

(A): You have two channels up to the grids of V4, the 6SC7 mixer. One channel covers Mike 1 and Mike 3 inputs; the other channel covers Mike 2 and phono inputs. The channels are merged long enough to go through the tone control systems, and then they are split at their master gain controls, R6 and R7. Since they are wired in parallel to accommodate a single feed, R6 and R7 must be separated at the circuit line between the red Y's on the schematic.

We'll call R6 left channel gain, and R7 right channel gain.

(B): To make the 6SC7 a parallel stage instead of a mixer, you must separate its plates (circuit line between the red X's), give each plate its own plate resistor (R24 and R24A, 5600 ohms), and then build two identical channels from the 6SC7 plates to the feed-in terminals of the master gain controls, R6 and R7. Most of the existing parts between V4 and R6/R7 will end up in the left channel, by the looks of the schematic (although the existing treble control, C17 and R8, won't).

Parallel and independent channels will mean ganged bass controls (R5 and R5A, 5 meg) and treble controls (R8 and R8A, 100K), along with a double set of associated parts. Dual concentric-shaft controls may be used if ganged controls can't be found, and in a pinch, you can scale the bass control down to 2 meg, or even 1 meg if you must. The treble control can fall anywhere between 75 and 130 K if it has to.

Hope this helps and good luck with your project :wink: ,

Larry

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 Post subject: Re: Stromberg Carlson AU-35, BIG Amp, Finished with pics
PostPosted: Jun Fri 15, 2012 1:42 am 
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Joined: Nov Sat 27, 2010 6:15 pm
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Sounds like it's nailed. Depending on what's driving it, you might delete the first stage amplification. I've done this in the past to use sockets for Altec input transformers to accept XLR input. Think about it.


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 Post subject: Re: Stromberg Carlson AU-35, BIG Amp, Finished with pics
PostPosted: Jun Fri 15, 2012 3:04 am 
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If tone controls are not needed could simply delete everything before the two master level controls.


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 Post subject: Re: Stromberg Carlson AU-35, BIG Amp, Finished with pics
PostPosted: Jun Fri 15, 2012 3:49 am 
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Aye.


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 Post subject: Re: Stromberg Carlson AU-35, BIG Amp, Finished with pics
PostPosted: Jun Fri 15, 2012 12:10 pm 
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In that case, I would skip the 6SC7 mixer stage, too. With a plate resistor of only 5600 ohms, it isn't likely to be functioning very much above unity gain anyway.

Thus, send its control grid feeds (two 470K resistors) straight into R6 and R7 (after separating R6 and R7).

On the other hand, I wouldn't do it myself, even if it is the quick and easy way out.

:wink: Larry

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