Musicmaster Switchable Power Question

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Musicmaster Switchable Power Question

Postby mhbgreen on Tue Apr 05, 2016 5:36 pm

Hi everyone, new to the forum. I'm rebuilding an old Musicmaster Bass amp chassis to go into a head cabinet for a 2x12 half stack. I was hoping I could make it switchable for 14 or 30ish watts. I looked at a schematic for the Orange OR15 which is switchable 15 or 30 watts and it looks like the only difference between switch settings is the B+ applied to the power tubes. Since the Musicmaster is also cathode biased like the OR15, I figure I could switch the 6v6 B+ from 290v, like stock, to 420, like a princeton, and accomplish this with a PT that has secondary windings for both voltages like 245-0-245 / 380-0-380. The original PT is 245-0-245.

I'm a little lost figuring if these numbers are right. Should I be able to just switch the B+ like this?

How will this affect the B+ on the preamp tube?

Should I only switch the power tube B+ and make another rectifier to keep the preamp on the low side?

Thanks. I tried to link to schematics below

http://s5.postimg.org/ihltqyohj/Musicma ... Page_1.jpg

http://s5.postimg.org/ssy6jmg6v/OR15_Page_1.jpg
Last edited by mhbgreen on Tue Apr 05, 2016 11:51 pm, edited 1 time in total.
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Re: Musicmaster Switchable Power Question

Postby stratele52 on Tue Apr 05, 2016 10:34 pm

Link do not work.
This amp is so low power with only two 6V6 why do you want to cut power ?
IMO it is not a 30 watts amps, around 20 watts I guess or less.

Musicmaster bass amp

http://el34world.com/charts/Schematics/ ... _schem.pdf
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Re: Musicmaster Switchable Power Question

Postby pdf64 on Wed Apr 06, 2016 10:56 am

I can't see that it's worth getting a new, multi option PT, even if one were available.
One value of cathode bias resistor may not give equally good results for both HT voltage levels.
And 30 watts from cathode biased p-p 6V6 pair seems fanciful/optimistic/a theoretical max; consider that the Princeton is fixed bias and is nowhere near that power output.
But at a fundamental level, yes, increasing the HT voltage will increase the power output; the designer's job is to ensure that the operating conditions are suitable.
Have a play with http://bmamps.com/Tech_tds.html
Click and drag the sliders to change the variables.

If you want to play, why not just switch from cathode to fixed bias; that would change the feel and give a bit more power output.
With the drive to the interstage transformer beefed up (eg 12AU7), it may be possible to achieve AB2 operation in fixed bias, which perhaps would give a good 20 watts, and a different feel again.

But perhaps better to enjoy the amp for what it is and build a new amp, or at least get a more suitable amp as a platform for modification, to achieve you goal.
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Re: Musicmaster Switchable Power Question

Postby mhbgreen on Wed Apr 06, 2016 7:26 pm

Hey guys, thanks for the replies. Those links should work now.

I want to be able to switch it to higher power option. I know it may not be 30 watts, just wanted a high/low output switch. Thanks for the link to that calculator, although I will need more study to really understand it.

I do like that amp just fine the way it is. I picked one up in great condition for $100 with burnt smell, not working. The tube sockets were arcing so I replaced them and all the caps, twisted the heater wires, shielded the input, did the guitar mods and a new speaker.

Pic: http://s5.postimg.org/mob9k1dh3/image.jpg

It sounds so good and the sustain is incredible. My 17 yo son loves it and we found just a chassis to rebuild for him and he would like to have a head and vertical 2x12 cab like a small half stack.

That chassis came with a 240v PT and I can get a replacement 120v from Edcor with the two sets of windings for under $100 probably. This amp will be the 6v6 version so we were thinking we could just switch more power onto the plates like the OR15 if he needs to keep up with a loud drummer or playing outside or what not.

I edited the schematic to show the changes here: http://s5.postimg.org/nzj0si92v/MMBswitch1.jpg
Close up here: http://s5.postimg.org/8fbn1yyyf/MMBswitch2.jpg
Original here: http://s5.postimg.org/oeualiv07/Musicmaster_Sch6_V6.jpg

It seems all too simple. Do you think this would work? Should I just give it a try?
Thanks
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Re: Musicmaster Switchable Power Question

Postby stratele52 on Wed Apr 06, 2016 9:28 pm

6V6 don't like more than 350 DC volts.
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Re: Musicmaster Switchable Power Question

Postby pdf64 on Thu Apr 07, 2016 1:21 pm

Some probs with the schematic -
The reservoir caps noted as 300V on the high HT supply will be subject to > 500Vdc.
The 800V rated bi-phase rectifier diodes on the 360-0-360 winding will be subject to voltages > 1000V.

A 360-0-360 HT winding seems excessive to me, given the application.

Have a read up on how things work http://www.valvewizard.co.uk/bridge.html

The OR15 uses a p-p pair of 12 watt tubes to put out 15 or 7 watts.

You wish to use a p-p pair of 12 watt tubes to put out 15 or 30 watts.

With a HT of ~300V and a p-p pair of 6V6 in cathode bias, the original amp could only have put out about 10 watts.
Have you considered that the output transformer may be unable to accommodate the additional demands placed on it by an increased HT voltage?

Have a read of http://www.valvewizard.co.uk/pp.html to get some background on how to use http://bmamps.com/Tech_tds.html

If the goal is to be louder whilst retaining the characteristics of the amp, I suggest you consider speakers with a higher sensitivity rating; 20 watts through a 97dB/watt speaker will create the same sound pressure level as 10 watts through a 100dB/watt speaker, all else being equal.
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Re: Musicmaster Switchable Power Question

Postby mhbgreen on Tue Apr 26, 2016 3:34 pm

Thanks for the Valve Wizard links. Just what I needed. Looks like I have a lot of studying to do. I guess I will re-build it like the original with a few mods and leave off the dual power option. Thanks for all the help!
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