The preamp tests OK--rail voltages are +/- 15v, and with 1KHz sine wave, I can get up to 30v p-p output clean. But the output from the power amp is low.
Opened one up once and sniffed. Belongs to a friend and careful of darkening relations, so suggested they find a pro solid state repair person. They did, and it failed again five or so years later. If forced, I'd put a small single channel class D inside and smile. Poorly engineered silicon inevitably burns over & over.
Best .. Ian
Last edited by didit on Sun Jul 05, 2020 5:54 pm, edited 1 time in total.
nothing burned or smells funny?
if voltages are where they should be the first thing i check is the output transistors with a multimeter: simple diode test tells if the output transistors are bad. if they check good i go up to the driver transistors
last case scenario is checking all component values
i am admittedly a hack but hopefully this was helpful
PRR wrote:
Plotting loadlines is only for the truly desperate, or terminally bored.
Thanks. I made some measurements, as suggested, but before I got too deep, I remembered a 400-watt class D module I had purchased earlier. The module wants 24-0-24 AC input. And...guess what? The Polytone's transformer is 24-0-24!
Guess who's getting a new heart transplant?
I build and repair tube amps. http://amps.monkeymatic.com
I got the power amp module installed and working last night. It fits like a dream, and seems to be working perfectly, though I didn't get to turn it up yet. Photos coming.
I build and repair tube amps. http://amps.monkeymatic.com
xtian wrote: ↑Sun Jul 05, 2020 4:38 pmI got the power amp module installed and working last night. It fits like a dream, and seems to be working perfectly, though I didn't get to turn it up yet. Photos coming.
Do tell! I'm looking forward to your detailed report on this conversion. I detest working on Polytones especially when the power amp is in fault. They seem so twitchy to fix - sometimes I get one working by accident and never do know what I did right.
Also waiting to see what the heck happened to our good neighbor MEF. tboy and his new server seem to be taking a long holiday weekend. It's always disturbing whey they mysteriously go off the air.
The conversion went magically perfectly. Result is quiet at idle, and LOUD when turned up. Very happy with this.
Power amp unit used is XH-M253, a 420w Class D mono power amp, that requires 24-0-24 AC. As I found, the Polytone Mini-Brute III has exactly 24-0-24 AC secondary, so it's a perfect match. Further, the PCB fits snugly inside the chassis.
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I build and repair tube amps. http://amps.monkeymatic.com
What about the two toroidal inductors and terminal block being up against the metal chassis? I'm hoping you found a way to mount it with acceptable clearance.
xtian wrote: ↑Sun Jul 05, 2020 6:13 pmThe conversion went magically perfectly. Result is quiet at idle, and LOUD when turned up. Very happy with this.
Power amp unit used is XH-M253, a 420w Class D mono power amp, that requires 24-0-24 AC. As I found, the Polytone Mini-Brute III has exactly 24-0-24 AC secondary, so it's a perfect match. Further, the PCB fits snugly inside the chassis.
Excellent work xtian! Now I can't imagine the amp really turns out 420W given the current limitations of Polytone's power supply. I'm sure whatever it can deliver, is plenty adequate for the purpose. Curious as to how much power it really develops. Also, where can we buy that amp module?
If it really can go 420W or anywhere near that, I'd keep a fire extinguisher handy for the poor speaker. That's enough to blow the voice coil across the street.
Thanks for the thread Aaron. Super satisfied this working out. Do be careful as it's potentially a speaker burst amp now. Will suggest a similar tweak next time I'm discussing options with my friend with the Polytone. I have never given the amp ample time listening. Been skeptical that except for the size and weight it's poorly compared with the stock Gibson GA-50.
