CJ

Fender Musicmaster Bass
« on: March 09, 2019, 06:58:18 PM »
post CBS Silverface 6AQ5 amp from 1973,

unmolested so we left out all the mods people do like bypass the input cap , increase cathode bypass caps, add a DeLuxe tone circuit,  remove the .047 cap from across the mains,

you can do that stuff but souping up a sparrow hawk does not make an eagle,  the above mods add a little but not much, plus the small cathode bypass caps and input cap are there to protect the weak speaker from bass,

this is the only git amp i have seen with an innerstage transformer. we added a long tail to the same amp a while back and it cranked.

the power lytics were fine so we left them.

there are no screen resistors, and only a 1K between B+ and screens, so the screens sit higher than the plates, no big deal, these tubes are cheap, and we can squeeze 8 watts out of a cathode bias so that is cool. the tubes actually still had some life in them so they get to stay.

one thing we noticed is that there was some 'fuzz" on the waveform, this is RF from tube oscillation, so we stuck a couple of .01/1KV caps from plate to cathode and it cured the problem and the amp sounds better.

these can be easily removed as they go into spare holes on the sockets if the customer wants to go back to dead mint.



If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


CJ

Re: Fender Musicmaster Bass
« Reply #1 on: March 09, 2019, 07:02:44 PM »
just an AX and two pwr tubes, OPT has black lead grounded on inside of case,

long bolt protects tubes while working on chassis,

retainers would be a welcome addition,



If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Fender Musicmaster Bass
« Reply #2 on: March 09, 2019, 07:04:25 PM »
took some voltages and iron measurements,

this amp is very quiet, even with the grounded heater circuit,

could be due in part to the innerstage xfmr,
« Last Edit: March 09, 2019, 07:13:26 PM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

volker

Re: Fender Musicmaster Bass
« Reply #3 on: March 09, 2019, 08:01:23 PM »
Nice minimalistic amp.

There are several older Gibson amps that have interstage transformers as phase inverter. Particularly interesting is the version of the EH-150, which runs the 6N6 triodes for the output.

PRR

Re: Fender Musicmaster Bass
« Reply #4 on: March 10, 2019, 01:09:40 AM »
> the screens sit higher than the plates, no big deal

No deal at all. These are pentodes (or beams). The G3 field allows plate to be very far below G2 before any funny-stuff happens. We commonly use them Vp ~~ Vg2 for simplicity, but the "~~" can be many or many-many volts different.

I was looking at IIRC a resistance-loaded 6L6 with G2 at 275V and Vp near 150V. Not a problem. (This was a video 'scope driver so the goal was very wideband voltage swing, not power per se.)


 

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