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Advice needed on hot power amp resistor

RichardH

RichardH

Bodging pleb
Finally got to connect my guitar preamp to the Maplins Mosfet power amp module I recently acquired. I'm pretty sure the preamp is built correctly - it's the Rod Elliott one http://sound.westhost.com/project27.htm

The power amp's "big" resistor is getting very warm - too warm. The amp sounds fine at first, then I guess the resistor overheats and the sound craps out.

Any ideas? Ericallan is sending me a copy of the manual, but it would be nice to get this nailed.
 
It may me worth actualy checking the speaker with a meter as the maplin amps state 4 ohms as a minimum.

If you have a spare speaker as a quick check put it in series with the one you are using and see if that lowers the resistor temp.
 
Funnily enough I've just done that. In fact, though it's marked up as 4 ohm, it measures 8.3 ohms, so I can't see that being a prob.
 
Pic here of the module - it's the one bottom right in that pic - R13 it looks like it's labelled.

One thing that's just occurred to me - I'm moved the power transistors off the board and bolted them to the case (built in heatsink). Would that cause a problem - length of wires to the transistors?
 
The board looks like its had a hard life!

Have you moved the transistors and the heat sink that is in the photo or moved them off onto the case itself? Could any grot have got under the transistor and damaged the mica insulation?

How long are the wires?
 
when it read 8.3 ohms it was the leads of the multimeter and the speaker resistance added together. i'd recommend you take the reading of just the leads put end to end then take it off of the speakers resistance, the leads may be 5 ohms which means the speaker is 3.8. get me??!:eek:
 
I see your point but they would have to be rubbish leads to be over 1 ohm and Richard strikes me a someone who would only entertain quality kit, I mean just look at that amp

;)
 
The multimeter leads read zero ohms, and I measured both at the end of the speaker lead and at the drive unit itself - 0.1ohm difference, so I think it's safe to say the speaker measures 8 ohms all right.

That amp cost me 99p, and it came with a Marshall case with transformer and the rest - sold as empty bar the transformer, so this amp module is a bonus, really.

The rear of the case is one big heatsink, with proper plug in locators for the TO3s. I put new silpads on the transistors. Leads to the transistors are around 8" at most.

Pic of amp

The pic above shows how it arrived - the amp module bodged onto the front. The two black circles are the transistor mounting points where I've moved them to - the module itself is now fitted to the case deck on PCB posts.
 
What, Marshalls or the Maplin Mosfet thing?

I think I've fixed it - seems it's just a matter of too much gain on the preamp board - tried plugging the guitar into the other input (designed for active guitars), and the resistor's not even warm, and there's a decent amount of volume too. So I need to drop the gain on the preamp, and I'll be away (probably).
 
The 'big' resistor is one element of the Zobel network of the amplifier's output. It ensures that at high frequencies, the amp has something to bite on. If it gets warm, it show it's doing what it's there for - dumping spurious oscillations from somewhere in the circuit. The circuit looks like the standard Hitachi mosfet offering which was notoriously unstable when even moderately provoked. Look at the small cap under C2 and if 47pF or less, up this to 120-150pF. Do the same with the ceramic which appears to be labelled C6 - (near the mosfet word). Ensure that you're not injecting any HF by looking at your input signal on a 'scope. If so, up the value of the input slugging cap C2 to around 470pF.
 
almichie


If your test leads are 5 ohms they are broken ;)

non of my ones are mor than 0.1 ohms, the same as Richards.

Richard

glad you have got it working OK and an upgrade path from Les !!
 
Originally posted by RichardH
Pic here of the module - it's the one bottom right in that pic - R13 it looks like it's labelled.
Click to expand...

Richard,

My history just flashed by... I use to run a PA for my band through 4 for these and boy did they get hot! You could turn the lights off and watch the resistor glow cherry red! If I remember correctly they went by the nick name of 'biscuit tins' as they and their transformers/caps/rectifiers were mounted in old, metal biscuit tins crudely paint black with a bent piece of sheet alu as a heatsink

They use to hammer out through a pair of horn tweeters and a pair of 15" speakers know as the 'cocktail cabinets' due to their enclosures having striking resemblance to to huge black cabinets.

Ah, student days...:)
 
After all that, I finally found the culprit - a dodgy jack socket! This must have been dumping a load of crud in, as when I tried a different socket, it all went swimmingly...
 
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