RE: Amp question

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

Don,

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

"A summary of my mods described in the previous two reviews: My recommendations are to change the speaker to a Vintage 30 (or better), bypass the 68K input resistor and 0.0047 cap, change the 470K input grid resistor to 1 Meg, change the 100K plate resistor to 200K, change the 0.01 coupling cap to 0.022, change the 250K volume pot to 1 meg, 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.


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 ?

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
>
> --
> Harp-l is sponsored by SPAH.
> Hosted by ValuePricehosting.com, www.valuepricehosting.com