eBay yet again!
Up for bid is a mid 1950s Radio shack of boston AF tube mono amplifier. Unit uses 2 6v6s and a 6sl7 for audio . I have had it up and playing for a few hours and it sounds quite good. Now keep in mind this is a 55 year old piece of vacuum tube equipment so it will probably need cap replacement and service. Nice little amp, has phono/mic input as well as one marked radio.
2 x 6V6 gives us a push-pull output, but 6SL7 is only a dual triode. Assuming that one half of this is used as the phase invertor for the power amp, it leaves only one stage of gain for the guitar, which is not very exciting.
May need to add another stage or even go over to 12AX7, although the idea of trying the 6SL7 which seems unheard of in guitar amp designs, is, well... interesting anyway.
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The Fender Deluxe 5C3 schematic below shows a 6V6 pair driven by the 6SC7 phase invertor.
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj1aM0-yvJiCCUSLFM24K_Ley4pRZ4-S2ZWPuY8ZeJJ5XA4nnKe2VNv1FeRH_G7Nm9YYbaYdTDemBKOHz1kgvQuFbd6Uomhg3Kzy2WyZS6rlMmINh4sFmQMwfraV5rbEDI0zgLDeav1KYg/s1600/fender-deluxe.jpg
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj1aM0-yvJiCCUSLFM24K_Ley4pRZ4-S2ZWPuY8ZeJJ5XA4nnKe2VNv1FeRH_G7Nm9YYbaYdTDemBKOHz1kgvQuFbd6Uomhg3Kzy2WyZS6rlMmINh4sFmQMwfraV5rbEDI0zgLDeav1KYg/s1600/fender-deluxe.jpg
I'd like to use the Fender 6G7 Bandmaster bass-treble tone circuit as shown here (see non tremolo channel at bottom left).
http://www.turretboards.com/layouts_schematics/fender/schematics/fender_bandmaster_6g7.gif
This schematic shows an Ampeg Jet amp using the 6SL7 as one channel gain and tremolo.
http://unofficialampeg.com/schematics/j-12-a.gif
This schematic shows an Ampeg Jet amp using the 6SL7 as one channel gain and tremolo.
http://unofficialampeg.com/schematics/j-12-a.gif
Here's an interesting amp with a switch to vary the treble boost in the first gain stage and also bypassing the power amp cathode for a Vox/Fender variation
OK I have just received this unit and it also includes unbeknownst to me a 6SC7. This is a dual triode with the cathodes connected internally so it is suitable for a long-tailed phase invertor application here, quite like the 5C3 Fender Deluxe. I will use the 6SL7 as a switch selectable one-or-two-gain-stage preamp prior to the phase invertor.
ReplyDeleteRather than a single-knob tone control I'd like a two-knob type and perhaps a mid shift switch for laughs.
Also I will have a switch to enable or cut off the negative feedback around the power amp.
Hi Larry,
ReplyDeleteMy name is Jeff and I'm an Electronics teacher and guitar player. I've had what looks like and old Hammond Amp with identical tubes including the 6SC7. I was hoping to learn some more from you about how your third project went. Did you modify the tone control? Did you install the neg feedback option?
Cheers
Jeff