DC heater supply issue

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sonicmojo
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DC heater supply issue

Post by sonicmojo »

Hello,
I recently picked up a Sovtek Mig100 and the heaters on V1 are only getting 1.4VDC with a tube installed but the pins show about 7.1VDC with no tube in it. I suspect a bad regulator chip or the 1000uF supply caps. Which do you think is most likely the culprit to drag down my voltage under load? I'll probably replace them all but could it be anything else?

Amp works fine when I disconnect the DC heater supply and jumper to the nearby V2 AC heater supply.
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JazzGuitarGimp
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Re: DC heater supply issue

Post by JazzGuitarGimp »

I would start by measuring the 360R resistor to make sure it hasn't drifted off value.
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dorrisant
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Re: DC heater supply issue

Post by dorrisant »

Agreed. Then the regulator... then the caps.
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sonicmojo
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Re: DC heater supply issue

Post by sonicmojo »

If it's the one I think it is next to the regulator and accessible from the top, it measured about 190. Do you think that alone could cause it?
After I disconnected that resistor on one end, I was able to get a reading on the regulator pins with my tester while on the board so I think it may be okay. I'll put a trimpot in place of that resistor and try that first since I don't have to flip the board to do anything. Just flying out of town tonight so I'll report back in a week.

Also reading only about 5.5V on the AC heaters so I'll have to debug that also. The schematic shows a ground connection to one side of the AC heater supply which doesn't make much sense to me. I could understand an artificial center tap with resistors to both sides but directly to ground on one side alone????

Thanks!
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dorrisant
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Re: DC heater supply issue

Post by dorrisant »

sonicmojo wrote: Wed Oct 04, 2017 4:01 am Also reading only about 5.5V on the AC heaters so I'll have to debug that also. The schematic shows a ground connection to one side of the AC heater supply which doesn't make much sense to me. I could understand an artificial center tap with resistors to both sides but directly to ground on one side alone????
Lots of older amps grounded one side of the filament winding. Look at some of the blackface and tweed layouts... you'll find it.

190Ω where it should be 360Ω... I would think that resistor is bad. I wonder what happened to it. I would suspect the three-legged device may not be quite right. It is not a regulator, it is just an NPN transistor rated for 1A. Seems to be a bit underated for the job but what do I know? How is it used... 6v or 12v? I have used different ways to accomplish DC filaments for V1, just never seen it like this.
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Re: DC heater supply issue

Post by Stevem »

If those filter caps are leaky they will pull down the AC voltage also!

You might be interested in this way of D.C. Powering V1 that leaves a heat & static failure phone Transistor out of the picture!

Even better with this is to use fast recovery type diodes at a very minor increase in cost.
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JazzGuitarGimp
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Re: DC heater supply issue

Post by JazzGuitarGimp »

I would expect a decrease in the 360R to produce more, not less voltage at the output. Have you measured the voltage at the top of each of the 1000uF caps?
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Ten Over
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Re: DC heater supply issue

Post by Ten Over »

First off, there is a 3.6K resistor from the Base to ground that is not shown on the schematic. The 360r resistor is on turrets so that Sovtek could put the appropriate resistor in there after the amp was up and running. They did the same thing with the bias circuit. It appears as though the quality control was a bit lax on the winding process. A 190r resistor could well be the correct value for this particular amp. It should have the value written on it like "K19".

This is a 6.3VDC supply as opposed to a 12.6VDC supply. It is just an Emitter follower with the Base held at a particular DC voltage. What we need to know is what the Base voltage is and what the Emitter voltage is so that we can get a better guess as to what might be wrong. The Base is the leg closest to the capacitors and the emitter is the leg closest to the edge of the board. At least on all of them that I have seen.
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Re: DC heater supply issue

Post by JazzGuitarGimp »

That makes more sense. Thanks, Ten Over.
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martin manning
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Re: DC heater supply issue

Post by martin manning »

I suspect the resistor trimming is about compensating for transistor variation. I would plan on replacing the diodes, caps, and the transistor, which I'm guessing has a shorted be junction. You could also replace the 3k6 going from base to ground with a 6.8V 1W Zener to get ~6.5V regulated DC out.

edit: Check the current through the 190R resistor and adjust as necessary to get ~10mA.

You should figure out the low main filament voltage first, though, to be sure the PT is ok.
Last edited by martin manning on Fri Oct 06, 2017 10:33 am, edited 1 time in total.
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Re: DC heater supply issue

Post by Ten Over »

martin manning wrote: Thu Oct 05, 2017 4:05 pm . . . You could also replace the 3k6 going from base to ground with a 6.8V 1W Zener to get ~6.5V regulated DC out. . . .
That's a pretty good idea, too bad Sovtek didn't think of it. NPN transistors like that have a 0.6V or so drop from Base to Emitter giving you around 6.2V out, which is even better than your estimate of 6.5V.
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martin manning
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Re: DC heater supply issue

Post by martin manning »

Using a Zener as a voltage reference for a regulated supply such as this is pretty common. The output voltage will be somewhere between 6 and 6.5, depending on the diode and how much current it's passing. This looks like a pretty good solution for DC filament voltage for one 12AX7. The ripple is very low considering there is only 2000 uF of filtering, and for less than a dollar's worth of silicon. It should work fine with 6.3 VAC input and a Schottky FWB. I think it'd be a good idea to increase the transistor's Ice rating to 2A or more to handle the initial surge into a cold filament. BD237STU looks like a good choice.
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didit
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Re: DC heater supply issue

Post by didit »

Hello -

Wondering if a better/easier answer, presuming there's sufficient raw voltage at the transformer secondary, would be to rework it with an off-the-rack three (or four) leg regulator device?

Best .. Ian
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martin manning
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Re: DC heater supply issue

Post by martin manning »

Sure, provided there is enough rectified DC to keep the regulator from dropping out. Maybe it's not important here, but I was thinking no mods to the existing pcb and same package style to mount the transistor as in the original.
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didit
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Re: DC heater supply issue

Post by didit »

Makes sense. Would need to get some images of the PCB to visualize whether it would be easily modified to use an integrated regulator. Also, getting ~6.3V is not quite off-the-rack without resort to using an adjustable, unless some new device has come out since my last search.

BTW - not seeing a 3k6 ohm resistor on the schematic. Is this shown somewhere?

Best .. Ian
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