Need help identifying this and its purpose

xtian

Gold Supporting Member
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2,680
Probably a voltage regulator. Tell us what markings are on it.

I can see that it takes B+ from the red wire, but what are the other connections?
 
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2,550
That would be this MOSFET and per the schematic on the same site the photo you posted came from, it's associated with the WATTS control on the front panel. Voltage regulator ICs often use the same kind of case, but AFAIK those aren't for guitar amp B+ voltages.

So, is it being used to achieve a form of power scaling then?

UPDATE: Having looked at the link it looks like more than than that... is the mosfet you linked to situated in some sort of heat sink in the pic?

UPDATE2: Btw, I had no idea where that pic came from. I snapped that 9+ months ago back when I was working on this originally... impressed that you knew where it was from!
 
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1,657
So, is it being used to achieve a form of power scaling then?

UPDATE: Having looked at the link it looks like more than than that... is the mosfet you linked to situated in some sort of heat sink in the pic?

UPDATE2: Btw, I had no idea where that pic came from. I snapped that 9+ months ago back when I was working on this originally... impressed that you knew where it was from!

Yes, it's a heat sink. Pretty typical of transistors.
 

Kyle B

Member
Messages
5,290
So, is it being used to achieve a form of power scaling then?"
Yes, that's exactly what it does. It changes the B+ voltage applied to V1. Lower B+ = lower output from the preamp.

BTW, note the little 'spark' symbol next to it?? That whole heatsink is electrically live when the amp is in operation. Don't touch it ;)

impressed that you knew where it was from!
Google image search. Remarkably good tool. images.google.com
 

PushedGlass

Member
Messages
825
That whole heatsink is electrically live when the amp is in operation. Don't touch it ;)
Yep, the datasheet shows that the heatsink tab is connected to the middle (drain) pin. Always useful to check that with any TO-221 device because sometimes, the expectation is that the tab will be grounded by fastening the tab to the enclosure so as to use it as a heatsink. Other times, that same thing will happen but there's a mica insulator between the device and the heatsink.

Polymoog power supplies had a large heatsink on the instrument's back that had regulators attached to it; I replaced one before but I don't remember if it had the insulator or not.
 

Kyle B

Member
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5,290
Yep, the datasheet shows that the heatsink tab is connected to the middle (drain) pin. Always useful to check that with any TO-221 device because sometimes, the expectation is that the tab will be grounded by fastening the tab to the enclosure so as to use it as a heatsink. Other times, that same thing will happen but there's a mica insulator between the device and the heatsink.

Polymoog power supplies had a large heatsink on the instrument's back that had regulators attached to it; I replaced one before but I don't remember if it had the insulator or not.

I gotta share this story...


When I was a wee young green engineer, about 2 months out of school, I had wired up a bridge rectifier, a MOSFET and some other little goodies to try to drive a 12V lamp with 120VAC without using a transformer (for $$$ reasons). It totally did work, but was too 'noisy' E-mag wise to put it into production.

Anyhow... After I first got it going, my manager - an obnoxious arrogant doofus with a "PhD in Atomic Physics" as he liked reminding us - came over to see my breadboard. The MOSFET wasn't getting excessively hot because I was using it as a switcher, so I didn't bother put a heatsink on it.

The tab on that MOSFET, as mentioned, is electrically live -- In my circuit, it was riding at about -170VDC below earth ground.

Mr. PhD looks the simple circuit and sees the bulb working, is a little amazed. He asks if my circuit is generating heat. I say "Yeah, that MOSFET gets pretty warm, probably will need a heatsink". He then immediately reaches down and pinches that MOSFET with his thumb and forefinger.

LO-fricking-L

So he gets zapped pretty good. And when he looks at his thumb, it's entirely red except for a little white circle that matched the hole in the tab.

I was torn between laughing hysterically and being mortified
 
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2,550
Yes, that's exactly what it does. It changes the B+ voltage applied to V1. Lower B+ = lower output from the preamp.

Actually, now that you helped me find it in the schematic, it appears to be wired to the power tubes and OT vs the preamp V1.

There's another mosfet used in the bias circuit per the schematic. Now i'm curious of the purpose of that one as well.

 

Kyle B

Member
Messages
5,290
Actually, now that you helped me find it in the schematic, it appears to be wired to the power tubes and OT vs the preamp V1.

There's another mosfet used in the bias circuit per the schematic. Now i'm curious of the purpose of that one as well.

You are absolutely right. My bad. That schem was hard to read for me, but once I zoom in now I see it's connected to B+1, not B+3. I was looking at it in normal "browser", so I couldn't really zoom in. I see now how to open the media correctly, and the schem is clear as day.

I should have known better, I did think it odd that the power scaling was on the preamp, not the power amp, but I didn't check closer.

Anyway, verified that it is a power scale feature :)
 

Kyle B

Member
Messages
5,290
Kyle, do you know the purpose of the mosfet in the bias circuit? I've not encountered that before.
OK, found it. It's a crazy setup, yes? There's THREE pots shown in the bias circuit

Yanno, these have been called 'MOSFET's all along here. Now that I can see the schematic, I just realized they're drawn as JFETs. That ain't the same thing :/

The IRF9510 identified in the schem is indeed a power MOSFET in a TO-220 package. Is that what's really on the board????
 

Kyle B

Member
Messages
5,290
The power MOSFET is setup as a higher-power zener diode. It confused me at first because I hadn't seen that setup before - I suspected it was a "Power Zener" but it looked different - then I realized that's gotta be a P-channel device because the bias is a negative voltage circuit. (If they'd drawn it as a MOSFET it would have been clearer LOL)

That MOSFET is basically gonna be a fixed 12V from drain to source in that circuit. Here's a link of how it works (with n-channel devices)

So what you have is VR7 is going to set the maximum negative voltage your bias can hit, doing so by tweaking the voltage divider made up of R55, R66 and itself. In other words, it will adjust the voltage at the MOSFET drain.

VR9 allows the fixed MOSFET 'zener' voltage (12V) to be tweaked some.

So that's what everything is, and how it works.

However, the reason why it's in there escapes me at the moment. This is quite a bit more complex than the typical bias circuit, yes? One would think they had a reason. It's got to be something to do with setting the possible ranges of bias, but why so elaborate? Hmmm...

Which pot is the one you'd actually be adjusting to do a bias-adjust? Is it VR9 or VR10???
 
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2,550
Kyle (and whoever else cares), I'm planning to pull the my VHT out of storage on Monday and start working on it again. I'll try to answer some of your specific questions above then.
 
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6,383
Just lurking....
Could the component in the bias circuit be necessary to attain the proper biasing range when the plate voltage is changed??
 

Kyle B

Member
Messages
5,290
Just lurking....
Could the component in the bias circuit be necessary to attain the proper biasing range when the plate voltage is changed??
Ahhh - Clever!

That'd make sense, but I don't see how it would do that ... The bias circuit isn't connected to the power scaling circuit.
 




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