SinglePower MPX3

[FallenAngel]

Hey guys,

I just got a SinglePower MPX3 to play with and the insides look like a rats nest of messy wiring and crappy looking electrolytics! Luckily, nothing looks unsafe to my untrained eye. After reading just about every article on HV safety and jigging up a 33R 35W resistor to a plastic extender to drain all caps, I looked around, then measured that the transformer output only about 250VAC, 370VDC over the big caps and I think they're rated at 450V. There is barely 250V across the interstage caps (Jensen PIO in this case) and 120V across the HORRIBLE looking little electrolytic output caps - found they these are Panasonic TS-UQ.

Here is the "before" photo, as I will surely be changing things.


Anybody with experience with these or other tube amps is very much welcome to chime in and point out things to change/improve.

Surprises
1) WHOAH! Big discovery - transformer is actually putting out 650VAC! That's 325VAC with center tap to the ground - as in chassis and earth ground on IEC! He uses half-wave rectification to get about 405VDC pre-filtering, drops about 25V through a big ass resistor, then a pair of filter caps.
2) I'm really confused - why parallel a HUGE 220uF film Sonicap with 100uF electrolytic? Doesn't make sense to me at all. If you're putting on a film cap that big, why bother with the electrolytic?
3) Mikhail forgot to solder something Input resistor was attached to wire but not soldered.

Planned upgrades
1) Looks like the 6.3V supply uses 4 1N5408 General Purpose Rectifiers rated for 3A @ 1000V. I've got some nicer considerably nicer MBR360 Schottky rectifiers that are 3A @ 60V. I think this will be the first upgrade.
2) Replacing the 2 Jamicon 4700uF/25V with something decent
3) Getting some decent output caps.

Thanks

[FallenAngel]

First Mod / Findings / Problems

Well, the first thing I did was tossed those 1N5408 rectifiers and threw in the MBR360 diodes... then I realized that 3A really isn't all that much considering the 6SN7 is a 600mA heater tube, 3 of these and you're at 1.8A, but I my amp has 6BL7/6BX7 tubes! Those are 1.5A heaters and with a couple of those plus a 6SN7 driver, that's 3.6A and over the specs of those diodes. That might explain why the originals didn't look overly healthy either. Luckily I have some MUR820's around, those are going in there now.

Noticed that the high voltage rectifiers are also 1N5408, guess he decided to use these everywhere. As the MUR820 is only rated for 200V, I'll keep those in there for for now, but soon that'll change. Yay, now I don't have to worry about running the damn diodes at 120%

Update: I'll be replacing rectifiers with FES16DT for the heater supply and DSEP12-12A for B+.

Ideas on nice caps for heater supply?

[FallenAngel]

Anybody have a clue as to what transformer this is?

[John E Woven]

looks like a hammond 272bx

[Pars]

If you haven't already, you should take a read thru the two sticky threads in the Headphone Amps forum regarding fixing/rebuilding Singlepower amps and getting rid of some of the safety concerns.

[FallenAngel]

Well if it's really 6.3V @ 3A, that's still over spec of the 6BX7 tubes

Perhaps it's a good idea to run 2 different heater supplies. as there's another 5V secondary I can run the two high-powered tubes off the 6.3V and the driver off the 5V.

The arrangement looks there is another diode after the bridge to drop more voltage.

For the 6.3V secondary, I'm thinking this (FES16DT diodes)
6.3V * 1.414 = 8.9 - 1.5V (0.75 *2 forward voltage drop) = 7.4V - 0.75V (another diode drop) = 6.65V looks healthy enough.

For the 5V secondary, I'm thinking this
5.0V * 1.414 = 7.07
This leaves very little for rectifier voltage drop, back to MBR360, at the expected current - 0.6A - 1.5A if I stick a 6BX7 in there, they show between 0.4-0.5V drop. That'll work to get it down to around 6.3V Since I'm expecting a TINY little bit higher voltage output off that 5V as it won't be at full load, we might be safe.

[luvdunhill]

nm. have fun!

[FallenAngel]

John E Woven : Just curious - how can you be sure it's 272BX and not another in the same line?

Luvdunhill : ooh, what did you write?

[diego]

Those filaments will give you a bit higher voltage. I have a similar transformer and with 120v ac input the unloaded voltage is 7.2vac . I don't remember if the voltage in the US is 120 or 115 though. My guess is that with 115vac input you'll get around 6.8 vac. So after rectification you'll get around 8 unloaded so you'll probably be too high. Your idea of using the 5v winding for the input tube would probably work well. The unloaded voltage of that winding is around 5.4 so you'll get around 6 loaded which is pretty good.

You could also use AC to heat the output tubes from the 6.3 winding and take DC for the input tube from the 5v winding. That way the current from the 6.3 winding will be enough.

[John E Woven]

Originally Posted by FallenAngel

John E Woven : Just curious - how can you be sure it's 272BX and not another in the same line?

Luvdunhill : ooh, what did you write?

Just poking around on google and came across this: My Singlepower Supra Experience - Why mine almost blew up like a Hand Grenade (and yours might too)

so it may be a different one.

[FallenAngel]

Well, the problem is that they all look the same, f*ing bastard Mikhail took the label off, I don't even know how badly he f*ed the design!

[bidoux]

Why did you buy a dangerous piece of crap ?

[guzzler]

Having said there's nothing dangerous, just look at the power socket: totally uninsulated and he compounds it by running the mains line all the way across the bottom of the box to the front panel. I would start from scratch on this; it's amazing it even does anything apart from electrocute people's cats...

[skyline889]

Originally Posted by bidoux

Why did you buy a dangerous piece of crap ?

I think some people forget that not all Singlepower amplifiers were crap. The later ones yes, the earlier ones, no.

[rds]

Originally Posted by skyline889

I think some people forget that not all Singlepower amplifiers were crap. The later ones yes, the earlier ones, no.

I want to see internal pictures of these non-crap ones.