Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: munkeyboy on January 03, 2018, 12:15:40 pm

Title: Diagnosing low volume (old magnecorder)
Post by: munkeyboy on January 03, 2018, 12:15:40 pm

So I have an old magnecorder, but unfortunately not any of the documented ones (can't find any schematics or specs).  It is the CM6K, which was an part of a reel to reel combo PT6-A portable.  The PT6 has some docs a schemas out there, but nothing for this particular CM6K model.  This has 6V6x2, 12AX7x2 and GY3.  I'm guessing the output should be around 10 to 20 watts.   However, it seems to be quieter than I expected.  No way I'm upsetting the neighbors with this one.    Even at max volume, there is hardly any breakup.   I get way more volume and breakup out of my 5watt valve jr.

It could be that the circuit was setup this way on purpose, which is fine.   But I'd love to understand what I should be looking for.  Any pointers on where to start?

Title: Re: Diagnosing low volume (old magnecorder)
Post by: shooter on January 03, 2018, 12:45:31 pm

Quote

Any pointers on where to start?

spend some time tracing the 6v6 circuit n sketch up a partial schematic, determine If it's fixed or cathode biased, calculate the power with that info then you have a pretty good idea if it was engineered low watts.

full set of DC volts measurements on the tubes
 more circuit sketching from input to output of V1a

Go slow, know "what you think each action should be", verify, resolve, walla

Title: Re: Diagnosing low volume (old magnecorder)
Post by: munkeyboy on January 03, 2018, 01:00:27 pm

Cool, I'll start with the bias section.   Well first I think l'll find a digestible book on tube amp circuits... Or just rewatch uncle Doug's vids 12 more times.  I still get lost when talking about biasing.  This will be a good learning exercise tho.

Title: Re: Diagnosing low volume (old magnecorder)
Post by: sluckey on January 03, 2018, 01:04:46 pm

It could be that you simply need a guitar friendly preamp to achieve the volume it's probably capable of doing.

Title: Re: Diagnosing low volume (old magnecorder)
Post by: munkeyboy on January 03, 2018, 05:26:24 pm

probably.  Need to recap and possibly rewire anyways.   Cap can just started smoking, bubbling and leaking some foul smelling 65 year old fluid.   Maybe rewire/configure for a tweed.   The cleans did sound good tho, just a little dark and not a whole lot of volume or dirt.   It's a thin chassis so it's all gonna be point to point and will need to use a cap can. 

Title: Re: Diagnosing low volume (old magnecorder)
Post by: munkeyboy on January 03, 2018, 05:57:30 pm

Pics in case anyone's interested.