question about power transformer temp.

Glass Tornado · Post by **Glass Tornado** » Fri Feb 09, 2007 4:43 pm

so i've just recently finished a build that is very similiar to the 66 jtm 45/100 layout on BillyBatz fileshare from a w*ber kit. i have replaced all the components on the board with carbon comp resistors, bc and f&t caps from george. i've already went through the first power transformer in this kit. when i first played it really loud for an extended amount of time, to my best judgement it got WAY hotter than it was supposed to and opened the primary winding. after asking around about this, high end parasitic oscillation was the overall concensus as to what might have made the transformer get that hot. so i got a replacement transformer, in the time it took to get it, like i said i replaced all the components on the board and rewired it all. changed grounding schemes. it's a much quieter amp now in the lines of signal noise.

here's my question.
how hot is TOO hot for a power transformer to get. i practiced again with the amp for the first time the other night. it didn't die, but it got pretty hot. i've never played or known anyone that has had a good marshall 100 watt, so i really have no point of reference. any suggestions that might keep me from buying another transformer?

thank you

jake unruh

novosibir · Post by **novosibir** » Fri Feb 09, 2007 5:30 pm

Glass Tornado wrote:how hot is TOO hot for a power transformer to get. i practiced again with the amp for the first time the other night. it didn't die, but it got pretty hot.

As long as you can't fry an egg on the PT's top, it isn't too hot!

In fact PT's especially from the older amps with the lower grade steel in it may become so hot, that you already cant't touch it - but it still isn't in danger. If you have a replik PT in your amp, it's the same.

But when you'd smell something strange like burned paper or so or when smoke is seen from around the PT, then better switch the amp off and give it a check.

Really not easy to say with words, what's too hot for a PT

Larry

Glass Tornado · Post by **Glass Tornado** » Sat Feb 10, 2007 2:12 pm

yeah, i've kinda noticed that there is only so far you can explain as far as temp, but what you just said helped alot. we played another show with it last night, and when i was done i could lay my hand on it and it wasn't to hot to not leave on there. so i think i'm ok. i do appreciate your help. thank you so much.

jake

Blast · Post by **Blast** » Wed Feb 21, 2007 11:13 pm

My '70 100w SL has a replacement PT that someone installed for me in the early '80's due to failure of the original. It's been running fine with no problems and I haven't a clue where the replacement PT came from (was most likely used) nor what brand it is. So after twenty years one day I was running it without the back on it and felt the PT-

it was so UNBELIEVEABLY hot that I really don't know why it hadn't gone toes up yet. After reading Jake's post maybe there IS a problem in the circuit that caused the original to fail that just hasn't gotten to the replacement yet.

I tucked a cheap DMM's thermometer probe into the top of its case to check the temp. Going strictly off memory I'm now thinking I was seeing 250-300 degree readings (if anyone asks I would go to the trouble of checking it again to confirm). I now run a 4" fan on it during normal operation which might be keeping the temp down into the upper one hundreds (you can put your hand on it now). I'd like to think I'm making at least a little difference in its longevity.

Mr G · Post by **Mr G** » Sat Feb 24, 2007 2:45 pm

I had the same problem of severe excess heat when i first installed my new dagnall repro PT....

1) first check for tube current draw (you may have a shorted tube) = excess current draw...replace defective tube(s) accordingly

2) Try adding a single 5k 7W screen resistor (the value changes depending on the ohmidge of the PT...based on their ability to balance out parasitic oscilation...i think) on every tube base - instead of just the one of each pair (typical traditional 1959 marshall schematic)...this is a precautionary measure that doesent affect the tone..helps to save your pt/tubes if you have a tube failure..

The weird thing is, my PT finally (after a week) just ~cooked~ itself better:shock: ...and now runs totally cool...with great tone, and over 120W clean power before clipping...Im told that (sometimes) new tx's have a minor short inside them, the heat could potentially fuse the shorted wires together...thus curing itself...

let us know how you progress...dont give up on that PT yet!

Mr G · Post by **Mr G** » Tue Feb 27, 2007 10:30 am

Type-O in above reply...try adding 5K 1W screen grid resistors....or play around with a similar value that cancels out enough Parasitic Oscillation

ScottW · Post by **ScottW** » Tue Feb 27, 2007 11:35 am

Mr G wrote:Type-O in above reply...try adding 5K 1W screen grid resistors....or play around with a similar value that cancels out enough Parasitic Oscillation

I use 1K 5W resistors...

Mr G · Post by **Mr G** » Tue Feb 27, 2007 1:41 pm

Double type-0!:roll: ....indeed 1k 5W work with me....

Blast · Post by **Blast** » Tue Feb 27, 2007 5:00 pm

So you're saying to start by adding 1k 5W (in the red circles, see diagram)? And if that makes little or no improvement, try 1.5k or 2k in all four places?

ScottW · Post by **ScottW** » Tue Feb 27, 2007 5:41 pm

Yep, what you'd do is disconnect the wire to pin 4 of each power tube. Put a 1K5W resistor between pins 4 and 6 on each power tube. For each tube, reconnect the wire (that used to go to pin 4) to pin 6. That's it, now you have properly installed screen grid resistors.

Blast · Post by **Blast** » Tue Feb 27, 2007 5:54 pm

Cool

That just went closer to the top of my "to do" list!

Glass Tornado · Post by **Glass Tornado** » Wed Feb 28, 2007 12:34 pm

so after about a month of playing it about every other day pretty brutally, i seem to be staying afloat. I still have this hinting suspicion that i'm not completely out of the woods just yet though.

i have grid resistors on mine. so i guess that's good.

say i have two tubes, when checking bias that are at like 32 or 33 mA and the other two are at about 43 and 45 mA. that's probably a bit too "unmatched" or is that not going to be causing me an issue. i probably have it biased where the two at 43 and 45 are biased too hot?

would this be causing weird issues with the PT. (as i'm asking this question, i feel like the answer is yes before i ask, so go easy on me

)

i also still get some wierd oscillation stuff going on after i get to about 5 or 6 volume-wise...but putting a 220k resistor on pin 2 of v3 seemed to help bit. i have 5.6k swamper resistors on all four tubes as well.

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question about power transformer temp.

question about power transformer temp.

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