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Old 8th April 2013, 10:21 PM   #1
surfstu is offline surfstu  United Kingdom
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Default Transformer bubbling

I've built an ecl86 amp based on the babyhuey design. The amp has been running extremley hot after half hour. The area around the valves are untouchable without burn pains - as is the power transformer and also the components which are all soldered close to the valve pins.

I seem to remember the power transformer leaking wax before I salvaged it from an old heathkit amp.

I switched it on today and as soon as the HT came up (gz34) - I started to hear an awful squeal through the speakers followed by a bubbling sound from the amp.

Is this my transformer?

Could this have been made worse by using a bleed resistor that is too small (33K)?

cheers Stuart
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Old 8th April 2013, 10:47 PM   #2
12E1 is offline 12E1  United Kingdom
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Did the amplifier ever work, or is this a new build? If the latter then this could be the transformer, but equally it could be a build error. The squeal could be some form of instability, and that could be pulling excess current. Since the power transformer is previously used, did you test it before wiring it into the circuit? It's a case of power off, check and double check everything - all components, their values, the wiring. Then I would remove all valves, apply power, and check voltages. Are they normal, or suspect? If voltages are normal and if you have valve rectification, I would power up with only the rectifiers installed and check voltages again. If in doubt post a schematic, pictures of your build, details of the measurements taken, and so on. We can only make some guesses from the information so far. Something is clearly wrong, but there are too many possibilities.

Old transformers can have "leaked" wax or tar in the past - it does not necessarily mean a problem. It can just be due to age. 33K is on the low end for a bleed resistor, but would not draw so much current as to cause problems in most cases, although it might get very hot of course.

Last edited by 12E1; 8th April 2013 at 10:50 PM.
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Old 9th April 2013, 06:15 AM   #3
surfstu is offline surfstu  United Kingdom
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Thankyou for the reply... yes - the amplifier worked but It is also a new build. It has been running for a month on and off (an hour every other day) but I think it has always suffered the heat issue.

The beeld resistor has always been extremley hot, so have the output caps as they are soldered directly to pin 9 of the ecl86.

I remember that the old heathkit developed a fault that caused the amp to bubble and wax dripped from the power transformer onto the chassis. I thought the transformer was dead until someone from this forum persuaded me that it may not be. So when I built the new amp I tested the transformer unloaded and it seemed to work. Although I only tested it for a few seconds to see if I had secondary/heater currents etc.

I have enclosed a copy of heathkit schematic that all the transfomers were pulled from.

Have aslo attached new schematic that I used to build this amp.

The PSU is CRC using 50uf + 100uf and a dropper of approx 330R.

I notice the heathkit schematic does not use a bleed resistor, however it powers a preamp section that I don't use. And considering it also uses 4 ecl86 in PP - I always thought it would be able to handle the power.
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Old 9th April 2013, 08:54 AM   #4
12E1 is offline 12E1  United Kingdom
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I don't think its a power limitation in the transformer. As you say, the original design has more valves and has similar operating currents. If the transformer has in the past suffered some internal damage, then it should be possible to establish what.

Another thing to check - are you able (very carefully) to measure the two AC voltages on the HT leads relative to the centre tap? Try this with and without the rectifier valve fitted. They should both pretty much the same, but can be quite a bit higher without the rectifier. If they are much different, that can point to an internal short in the HT windings.

Also, can you check the heater voltages for both channels? Heaters my glow, but again there could be a partial short resulting in an unexpected difference.
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Old 9th April 2013, 09:05 AM   #5
Matt BH is offline Matt BH  United Kingdom
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As 12E1 says, I think it is oscillating. Can you post a pic of your build? Have you got the UL taps on the output transformer round the right way? I guess you have not swapped the driver LTP anode connections as they are ECL86.

Cheers Matt.
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Old 9th April 2013, 11:05 AM   #6
DF96 is offline DF96  England
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Polypropylene caps are large and ECL86 triodes have high anode impedance, so you have to be careful not to create an HF oscillator from stray capacitive feedback. Both signal phases are present so it doesn't take much for some positive feedback to appear.
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Old 9th April 2013, 11:16 AM   #7
12E1 is offline 12E1  United Kingdom
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@surfstu...

As mentioned before, if you are able to post a photo or two showing the wiring detail, it may help with checking and analysis.
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Old 9th April 2013, 11:34 AM   #8
MelB is offline MelB  Canada
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Does the transformer heat up with no tubes in the amp? Try it just without the rectifier tube too.
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Old 9th April 2013, 11:41 AM   #9
AndrewT is offline AndrewT  Scotland
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re post8,
It was a clever and very practical idea to use sockets to fit the majority of tubes/valves.

Why do we use so few sockets for transistors?
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Last edited by AndrewT; 9th April 2013 at 11:46 AM.
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Old 9th April 2013, 12:16 PM   #10
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I thought transistors were suppose to last forever? Why socket it if so. Provided of course that DIYer's like me learn how to keep the magic smoke from leaking out.
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