Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bsmif on April 02, 2012, 06:46:56 pm
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I am trying to bring back to life a dusty old Magnatone M10a suitcase amp.
SLuckey's article has been invaluable.
http://home.comcast.net/~seluckey/amps/magnatone/magnatone.htm (http://home.comcast.net/~seluckey/amps/magnatone/magnatone.htm)
I got it at an auction and it didn't look like it had been used in a while so I immediately put in a IEC socket and replaced the power supply capacitors as well as the rectifier diodes. I started powering it up on the Variac with no tubes ( dont want to fry the 7189a's!) and all seemed to be going well but once I got to 40V I noticed the power transformer getting very hot. COuld it be doing that because there is no load? ANy other things to check before I order a new PT? What Replacement PT would work 300V..right? if it comes to that?
272DX
600V C.T. @ 144ma.
272FX
600V C.T. @ 172ma.
272HX
600V C.T. @ 230ma.
272JX
600V C.T. @ 287ma.
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Search this forum for "light bulb limiter". Build and use it with this amp during initial power-up.
There is probably a short somewhere, which will light the lamp up brightly. You will need to go through the amp attempting to find and eliminate the short circuit before proceeding. You don't want to spend $$$ on a transformer that wasn't needed, or to pop it in an amp with a short somewhere else in the chassis, which would just damage the new transformer.
Also make sure you didn't make a small wiring error or solder blob with your newly changed items.
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COuld it be doing that because there is no load?
With no load it should stay nice and cold. It's the load (or short) that warms 'em up, as far I know. (which ain't far)
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Here's a link to the lamp limiter, thanks to sluckey. :icon_biggrin:
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf (http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf)
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Before you replace the tranny, if the light bulb limiter shows a short, the short could be somewhere other than the tranny.
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Divide and conquer. Just unplug the connector between the PS chassis and main chassis. Put a jumper between pins 5 and 9 and fire up the power supply, checking for B+ on the cap can. If still getting hot, disconnect the diodes and try again. If still hot, disconnect the bias diode and try again.
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Yeah something is weird in the PS. I had screwed up and connected the"middle" of a pair of the diodes to the same tab as the orange lead (negative bias). Now I fixed that but I'm getting VERY high B+ voltages from the Red leads from the PT. For instance if (thru the Variac) I apply 15v in I get 82v (!!) on the Red leads...even when its only on "Standby" Havent had time to take it apart and look for shorts
Also getting very high voltages ( @ 15v in I am reading 100+v!) at the #1 connection which connects to the 600v "death cap" Can I safely just disconnect that cap? Or will that require rewiring the Power switch too?
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Yeah something is weird in the PS. I had screwed up and connected the"middle" of a pair of the diodes to the same tab as the orange lead (negative bias). Now I fixed that but I'm getting VERY high B+ voltages from the Red leads from the PT. For instance if (thru the Variac) I apply 15v in I get 82v (!!) on the Red leads...even when its only on "Standby" Havent had time to take it apart and look for shorts
Also getting very high voltages ( @ 15v in I am reading 100+v!) at the #1 connection which connects to the 600v "death cap" Can I safely just disconnect that cap? Or will that require rewiring the Power switch too?
When it comes to the death cap, leaving the switch in is up to you. Primary thing is the death cap is removed from the circuit completely. 2 OR 3 prong cord. (If 2-prong update the cord for safety reasons before you're finished, which can be a good time to get the switch removed if you choose to do so.) The power switch itself shouldn't need to be touched unless rewiring from the power source.
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I'm getting VERY high B+ voltages from the Red leads from the PT. For instance if (thru the Variac) I apply 15v in I get 82v (!!) on the Red leads
Ain't no B+ on the PT red leads. That's AC. And there's nothing wrong with getting 82v on the red leads with the variac set for 15v. 15:82 ratio is the same as 120:656. 656v between red leads with 120v on the primary is very normal.
Also getting very high voltages ( @ 15v in I am reading 100+v!) at the #1 connection which connects to the 600v "death cap" Can I safely just disconnect that cap? Or will that require rewiring the Power switch too?
Forget about the voltage on that cap. Just remove the cap. But, look at the schematic and read the little yellow notes. You'll notice that there is another 'death' cap shown on the switch. Remove it too.
And since you boogered the bias tap you'll need to verify that you have about -17vdc on pins 1,2 of the output tubes.
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Hey THings might not be so bad! At 117V I'm getting 423 on the output tubes @ 7 and 8 and -15v on #2 (#1 isn't connected to anything). This is without any tubes in it so I imagine with a load the voltages will drop. Is the discrepency on the bias tap a problem? Would it make sense to lower the B+ on the power tubes a bit to preserve them?
Thanks sluckey. Your site on the m10a has been so helpful.
The leads of the rotary switch (that combine power and hum cancelling) are a little harder to figure out than a pair of DPDT's (and one end of the switch Death cap is encased in rubber insulation). Im a little unsure how to proceed. Can I just cut the death caps out of the circuit without any further changes?
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Hey THings might not be so bad! At 117V I'm getting 423 on the output tubes @ 7 and 8 and -15v on #2 (#1 isn't connected to anything). This is without any tubes in it so I imagine with a load the voltages will drop. Is the discrepency on the bias tap a problem? Would it make sense to lower the B+ on the power tubes a bit to preserve them?
I'd say your voltages are all very good. No need to do anything.
The leads of the rotary switch (that combine power and hum cancelling) are a little harder to figure out than a pair of DPDT's (and one end of the switch Death cap is encased in rubber insulation). Im a little unsure how to proceed. Can I just cut the death caps out of the circuit without any further changes?
Yes, just snip them out, leaving no dangling cap leads to get tangled with something. However, if you have properly installed a 3-prong power cord, the 'death' cap is now just safely working as a line filter. I took my caps out, but it wasn't because I thought they were unsafe. Section S1A of my power switch that provided the standby function was bad. (The amp would not make any sound and that's why I got it so cheap.) So, I removed the 'anti hum' caps to free up a switch section to be used for the standby function.
I think you're ready to try it with tubes. Have fun.
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Great! I will leave them be for the time being and warm it up slowly with tubes. I'll check the voltages from your M10a pdf to mak sure the other voltages are in the ballpark.
I didnt want to monkey with the Death caps if they arent hurting anything so I'll leave them be for now.
Thanks for the help! What a great way to mark Jim Marshall's passing.
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Thanks for the tips! Its Alive!. A tad noisy but I think I might have to clean out the pots and sockets a bit. Love the crazy vibrato
Not bad for a $75 find!
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All Right it lives! :happy1:
Guard those varisistors in the vibe circuit, their hard to come by and probably could be sold for $75.00 just for 2.
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Between the Varisistors, the 7189a's, the vintage Jensen speaker, and the old Mullard and Telefunken 12Ax7's it would cost a mint to build from scratch!
;-)
http://www.flickr.com/photos/bsmif/6905553648/in/photostream (http://www.flickr.com/photos/bsmif/6905553648/in/photostream)