RE: Amp question

["Priest, Jim" <Jim.Priest@xxxxxx>]

Wow ! I'm kinda wishing I'd never asked ;-)

- -----Original Message-----
From: The Bernadettes [mailto:mdestefano@xxxxxxxx]
Sent: 13 March 2003 11:38
To: Priest, Jim
Cc: harp-l@xxxxxxxxxx
Subject: Re: Amp question




"Priest, Jim" wrote:

> Don,
>
> Thanks for the advice. I'll definitely entrust this to a tech. I also found this on Harmony Central:

Be careful about performing mods intended for guitar that you read on Harmony Central, they'll have different requirements...though what is written here is kinda like I would suggest:

>
>
> "A summary of my mods described in the previous two reviews: My recommendations are to change the speaker to a Vintage 30 (or better),

better speaker choices for harp, I'd look at the C12N clone made by Weber with a harp dust cap

> bypass the 68K input resistor and 0.0047 cap, change the 470K input grid resistor to 1 Meg,

Don't bypass the 68K resistor.  Two kinds of inputs to consider, cathode biased or grid leak biased.  For a cathode biased input, dump the .0047 cap, I'd go with the 68K resistors and a 5 meg gridleak resistor if you're using an Astatic mic.  For a grid leak biased input: 68 K resistors, change the cap to a .047mfd, 5 to 10 meg grid leak resistor, cathode direct to ground (you should be familiar with how to implement grid leak biased schemes if doing this one.

> change the 100K plate resistor to 200K,

220K, possibly bypassed with a 1000 to 2000pf ceramic disc cap of a 500 volt rating or better, also up the cathode bypass caps to 25 mfd. or maybe even higher to 100mfd.

> change the 0.01 coupling cap to 0.022,

go to either a .047 mfd. or .1 mfd. cap

> change the 250K volume pot to 1 meg,

yup, but reconfigure the the volume/tone control as is done with a tweed Harvard or Princeton, (check out one of their schematics/layouts to see how it's done.

> and finally change the power supply stacked 20/20/300V cap to 50/50/500V. Now that last one I'm not convinced you need to do. It will give you more bottome end, maybe more than you want.

I'd probably do this.  Don't know if you need to go as high on the microfarad value but would use a higher rated voltage on the filter caps.  The original voltage rating is too low for the kind of voltages you'll find in that amp.  While you're at it, I'd go to a higher voltage rating on the power tube cathode bypass cap and go with a higher wattage rating on the cathode resistor, 2 watts is too low, use a 5 watt resistor.

>
>
> The end result is tone! Grinding 6V6 crunch at a reasonable low volume. Still loud enough to play in a small room with a drummer."
>
> Is this along the same sort of lines you were thinking ? Would you think it would be worth doing ? Any alternative suggestions ?

Read what I suggested.  The Musicmaster is basically a single channel tweed Deluxe (or cathode biased tweed Harvard) with a transformer coupled phase inverter.  Doing these changes will get it closer to a tweed Deluxe/Harvard type amp which BTW are great harp amps.

Good luck, contact me offlist for more info if needed

Don D.

>
>
> Regards
> Jim
>
> -----Original Message-----
> From: The Bernadettes [mailto:mdestefano@xxxxxxxx]
> Sent: 13 March 2003 01:56
> To: Priest, Jim
> Cc: harp-l@xxxxxxxxxx
> Subject: Re: Amp question
>
> "Priest, Jim" wrote:
>
> > Amp wizards,
> >
> > I just acquired a 1970's Fender Musicmaster amp, from the look of it in pretty much original condition: 2 x 6V6GTA and 1 x 12AX7 tubes, Fender 12" speaker, volume and tone controls, on/off switch, two inputs - that's it. Playing through my Astatic JT-30 it sounds OK-ish - plenty of bite though maybe not as much depth as I was expecting (but see below). I'd appreciate some advice on:
> > 1. Is there anything to be gained in replacing any of the basic components: tubes and/or speaker ?
>
> There's some to be gained, especially if the tubes are worn.
>
> >
> > 2. The tone control appears to have no effect, other than switching the volume off completely when I turn it down below 2.
>
> This could be several things, the tone control pot is bad (grounding out) or the tone capacitors in the tone stack circuit or shorting out...or just the design of the circuit.  I have a schematic of a Musicmaster Bass Amp, pretty strange tone stack, don't know why Fender went with that type.  The older tweed style single tone control I think would have worked better and is a proven design.  Also a lot of the component values I
> see would make this amp pretty thin sounding.  I could think of some nice mods to do with this amp, basically convert it to a tweed Princeton type circuit with larger coupling/bypass caps to increase low end response.  Pretty interesting on this amp is the use of a transformer coupled phase inverter, rarely ever see this in a instrument amplifier, usually see this design in tube hi-fi gear...I did this past summer work on a
> Gibson Explorer amp that had this same kind of phase inverter.
>
> > Is this likely to be something I could fix myself, or should I entrust it to an amp tech ? I'm moderately competent with a soldering iron and have worked on hi-fi amps (solid state) in the past, but am wary of the words of warning frequently issued here about the voltages hanging around inside valve amps.
>
> Unless you're hip to reading schematics and know how not to electrocute yourself, this is better left to a good amp tech who's knowledgeable of vintage tube amp designs.  If you need the schematic, lemme know, I have it as a PDF file.
>
> Regards,
> Don D.
>
> >
> >
> > Any advice welcome.
> >
> > Jim
> >
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