AC on plate - JTM 45 -

Repairs, trouble shooting, modding and so on.

AC on plate - JTM 45 -

Postby zen.state » Fri Nov 30, 2012 10:40 am

Hi

I have a problem I'm trying to figure out with my homemade JTM45 with the Marstran 103 OT.

I had some crackle on the amp at full tilt which lead me to go back and rebias/investigate, as I had changed valves recently...to some Mullards - very un-matched, I'll have to find some matches within several I have. The crackle seems not present with the EH 6CA7's which were replaced.

I have 450 volts DC approx on pins 3 without valves installed...

Powered up with valves I am getting an AC voltage of around 385 Vac - just on one plate/valve base on pin 3, the other has a usual looking DC voltage.
I swap the valves and the same position has the AC, ie the same base and pin...so it's not the valve.

Any idea where to look to fix this one please?

Many thanks

Roy
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Re: AC on plate - JTM 45 -

Postby zen.state » Fri Nov 30, 2012 12:24 pm

Just found

As I turn Volume 1 down the ac voltage disappears. Turn it up and it returns, with a squeal when at full tilt from the positive probe of the meter.

Volume 2 has no effect.....

AHA my Park does this too...

perhaps this is normal...
:)

onwards
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Re: AC on plate - JTM 45 -

Postby dai h. » Fri Nov 30, 2012 5:32 pm

AC voltage on an output tube plate should be signal (this is transformed by the output transformer to lower AC voltage and high current appearing on the speaker side--might be setting off an oscillation by probing--perhaps a C ( = capacitance) across two points?). If you have crackling, maybe try cleaning contacts (say, if the amp hasn't been used for a while, maybe try moving the tubes very gently in their sockets to break up oxides on the pins/socket contact point). If some soldering has been done recently, I would check those joints. BE CAREFUL IN THERE! SERIOUSLY.
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Re: AC on plate - JTM 45 -

Postby zen.state » Fri Nov 30, 2012 7:06 pm

Thanks Dai

It does sound like the probe is setting off an oscillation to me.....when I was measuring primary voltages at full tilt the oscillation or squeal happened as usual, I switched the probe quickly to the other primary/plate pin and the sound was deep and low like the motorboating when the primaries are reversed...this went when the probe was taken away....not loud as I'm using a hotplate


On the crackling:
I tensioned the pins on the sockets and went over a lot of joints
then put the EH 6CA7's back in....it seemed that some valves were problematic and some weren't - can extreme mismatching do this crackling noise I wonder?

I've shortened some wires too...at the input and preamp valve bases

It's sounding good...with the 6ca7's...very smooth, full distortion....nice vibe....on full...

I've just put some NOS XF2's in, that happen to match and I've set the bias using Brian's 50 watt method...there is no crackle now....good result...progress...

will keep an eye on things, these are unused, untested, unselected (in terms of matching), XF2's so I think I'll do a kind of burn in...if that makes sense.....I don't think they've had that.....


thanks again, I appreciate the input....
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Re: AC on plate - JTM 45 -

Postby dai h. » Sat Dec 01, 2012 6:27 am

I think extreme mismatching would be more a source of hum.

With the attenuator, I think you want to be careful since using one can lull you into a sense that the amp isn't working very hard (that is, working very hard to amplify drawing a lot of current through the power transformer and creating high AC voltages (as you witnessed) and current flows through the power tubes and output transformer) even though it might be a low volume acoustically. As I understand, there is some possibility of an attenuator contributing to oscillation, then in combination with maxing the output causing damage when limits of current flow through wire is exceeded (in other words, it melts) (I think there is a general recommendation to check that the amp is stable and not oscillating when hooked up to an attenuator or even just by itself without one). To be clear, I'm not saying attenuators are some sort of absolute danger, but that a bit more care may be required so you don't blow something up inadvertently. (Not that I'm particulaly tech savvy) but I've taken precautions such as trying to ensure the lead dress inside the amp is correct, the wiring outside (input lead and spk. out leads) are correct (not run parallel for long lengths next to each other), and a slight bit of modification to my attenuator (Marshall Power Brake) which reduces the high frequency impedance of the internal load a bit (might sound complicated but it's just the introduction of a resistor across an inductor). Also, something I noticed w/the PB is that Ralle (he of the VH clips) apparently blew his up, so that makes me wonder if the 100W power resistor (the main load the amp output will "see") is adequate (even with the forced air cooling) for use with a 100W amp.
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Re: AC on plate - JTM 45 -

Postby zen.state » Sun Dec 02, 2012 10:21 am

Thank you

I've heard of the old PB breakdown..not so much the THD.

I am hammering the amps a lot more at present and I think putting the jtm 45 vol around 8 seems better than full, for this situation...and at less attenuation I can use the sustain of the overall volume output from the speaker. So that's probably how I'll play it.

The NOS Mullards have now had some hours and seem to have biased OK.

And another pair, unused, seems to have matched fairly well and biased all right on the old jtm100...so that's good ...

Less extreme hammering on the 45, definitely seems better. Bit more respect in a way, for the amp.

Thanks again.
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Re: AC on plate - JTM 45 -

Postby dai h. » Mon Dec 03, 2012 7:19 am

(I remembered this more comprehensive analysis from R.G. Keen, so putting up this link for any future searchers w/a crackling problem) :

http://vintageamps.com/plexiboard/viewt ... =2&t=92141

A small point I would add is if you use old stock or used parts (even wire) make sure the leads, lugs, wire strands, etc. are free and clean of oxidation. If there is significant oxidation solder won't flow and the problem is obvious, but it things are sort of "iffy", then you might manage to make a joint but not a really good one which might be a problem later on. In that vein, I also think it's a good idea to store parts in air tight bags or containers and not keep stuff out in the open for a long time.

Also, I think this ongoing thread over at MEF is a good one discussing failure mechanisms and protection:

http://music-electronics-forum.com/t31452/
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Re: AC on plate - JTM 45 -

Postby zen.state » Mon Dec 03, 2012 10:04 am

Thanks for those excellent links. I'll read those over.

The HT 1k resistor going to Pin 6, I had made up of two 680 ohm Dale 3.75w resistors for some unremembered reason...those are pretty grotty and I couldn't get a shiny joint on those...I've retouched those a few times....but now I've I replaced the two Dale resistors with a 1k 5watt...

the solder I recently bought is cheap and not as good as the previous solder..... and I found this review below on Amazon
so there's a lesson there and I need to get some better solder...chuck this stuff out...

with some lead shortenings/re-jointing changes I think I'm getting there....I feel like mopping the joints and resoldering with better solder...better get onto RS...


Before I slate this product I'd like to make clear that the seller did a good job - the speed of delivery and price are good.
That said, the product is worse than useless. I spend a lot of time soldering. I use a nice heavy duty 45w soldering iron. Up until now I have been using old stockpiled solder... ersin flux multicore usually. This product, whilst still 60/40 lead/tin solder is only comparable in name. The solder only heats locally despite constant heating. It flows poorly. It solidifes to a matt finish irrespective of how long you heat it. Nowhere in any literature does it state the type of flux. I eventually found out that the flux is not in this solder, it's around it. This solder is flux coated solder. It does not have flux cores. This is the reason it is poor quality. If you want a nice shiny joint from good runny solder do not use this product. If you want dull, lumpy solder that is extremely difficult to use then this is the stuff for you. Silverline products also seem to be missing an email address from their webpages. How convenient.
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Re: AC on plate - JTM 45 -

Postby dai h. » Tue Dec 04, 2012 12:39 pm

generally (unless it's say a case of reheaing a joint--which should be done very quickly) I think you want to remove all the old solder when reworking a joint. I like to use "solder-wick" (or sometimes a manual vacuum bulb w/Teflon tip if the amount of solder is relatively larger). With solder-wick (my general advice) :

-make sure it's fresh and not oxidized (just like old component leads or wire, it won't work well if it is)
-choose the right width for the job (if the balance of the soldering iron capacity, and solder joints to desolder is wrong or IOW too much metal to heat to solder melting temperature, things may not go well or at all)
-sometimes tinning the very tip of the wick can help as a "starter point" as well as cutting it to form an acute edge to get into nooks and crannies
-I think it's a good idea to store the wick in air tight plastic or other containers when not being used to help keep it fresh

As far as solder, I think any "regular" 60/40 from a reputable manufacturer/vendor will work fine (nowadays you may have to watch out for substandard stuff probably from China--although I'm sure they make good stuff for shooting down satellites and so on :lol: ). Depending on the work, the gauge (diameter) can matter, but I feel the biggest factor is just basic technique (I'd say just working with it hands on and getting practice is one of the best ways to improve and become more comfortable). Plus general awareness of choosing the right tool (large high-capacity irons with big tips for things like soldering to chassis lugs, big pieces of metal). Possibly, it might be wiser to not use the lower melting point (eutectic) for joints connected to parts that get hot or really hot. General stuff like not drinking 5 cups of coffee and developing shaky hands before you solder and so on. :lol:
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Re: AC on plate - JTM 45 -

Postby zen.state » Tue Dec 04, 2012 8:53 pm

Absolutely, care will be taken

I'm very grateful, you've put a lot of time and info in here...

I got the chem wik braid and ordered RS solder...

fortunately most of the work was done with better solder so just a few things to go over and maybe tidy a few leads further if I find anything....

but it's really getting there now....and I've changed quite a few things....and will continue..but sounding better.....

Thanks again

you deserve a large beer sire 8)
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Re: AC on plate - JTM 45 -

Postby dai h. » Wed Dec 05, 2012 6:13 am

thanks, cheers! (glug glug) :mrgreen:
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