Crackle/sparks in output transformer of tube amp - Page 1
Forum section: electronics / repair

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 12, 2021, 08:11:22 pm
content: So, I've recently recapped an old tube amplifier, reconnected everything, connected the tubes and slowly increased the voltage on my variac. Amp seems to work ok, takes input signal and gives output signal. However, when I get close to 120V input (amp is rated for 117V), there comes crackling from the output transformer, as if there are shorts inside. I immediately turn down off the power, but a tiny burn mark has appeared on the outside paper. Internal voltages seem ok; main power transformer works fine.Question is, could I have blown it somehow? Or was the transformer already damaged? Is there anything I can do to repair it? I have no idea how to find a replacement, apparently it should have a primary impedance of 9400ohm. Secondary is 4, 8 and 16 ohm.Any help would be greatly appreciated, I've put a lot of time into this machine. :O

author: Gyro
author_rank: Super Contributor
timestamp: March 12, 2021, 08:48:51 pm
content: Silly question, but have you got a load on the speaker output? Valve amplifiers hate open circuit outputs, this can easily damage an output transformer.Edit: re-reading, it sounds as if you have.

author: TimFox
author_rank: Super Contributor
timestamp: March 12, 2021, 08:59:43 pm
content: I don’t see a mention of load connected. Specifically, if you do not load a tube amplifier secondary, with substantial drive the instantaneous plate-to-ground voltage may become very high.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 12, 2021, 09:05:09 pm
content: Yes, I did put an 8 ohm load on it, always do. Could it be that the previous owner, or someone who found the amp tried turning it on without a load? How much would be needed before a transformer fail?And again, is it repairable, or simple to replace?

author: Gyro
author_rank: Super Contributor
timestamp: March 12, 2021, 09:16:30 pm
content: Could be.If it's crackling then it sounds more like insulation breakdown rather than shorts - although one will very quickly cause the other.If you have a suspect burn on the surface then it may indicate that the problem is on the outer winding. Personally, I would go in and investigate. If you have access to a megger, then you could check primary to secondary, and to frame, insulation. The Megohms range on Dmm might show something, but I doubt it.If electrical test don't show anything, I would remove the outer paper and see what you find - it may be repairable with modern insulation materials. There doesn't seem much to lose at this stage.I'm assuming that the transformer isn't damp from long storage.P.S. It is probably possible to source a modern replacement but not cheaply. I don't know what the local purchasing conditions are like in Norway.

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 12, 2021, 09:21:31 pm
content: Simple to replace in the mechanical sense, hifi tube output transformers are still made, not cheap though. Repair depends greatly on your free time and skill level fighting magnet wire. Pictures are always helpful, as is the model number of the amp in question.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 12, 2021, 09:22:50 pm
content: Thanks, I guess I will have to try. I have a portable LCR meter that goes up to 200Mohm, will that be enough? Would an insulation fault be easily visible, or is there anything in particular I should look out for?

author: Gyro
author_rank: Super Contributor
timestamp: March 12, 2021, 09:32:35 pm
content: A 200M range might be enough - if the insulation has carbonised. The L function will be helpful for detecting shorted turns.If you're sure that the tiny burn mark is new, then that is where to focus. If it is insulation breakdown between layers of insulation, then that should be visible under that spot. It might be possible to repair that by partially unwinding the outer layer and adding something like Kapton or Mylar as a patch.It's very hard to say without a close-up photo of whatever damage you find.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 12, 2021, 09:33:21 pm
content: Amp is a Bogen DB10. I've attached a picture showing the burn mark. This amp has an unknown history. Input RCA jacks are heavily corroded, but otherwise there is no indication of damp storage, on the contrary it seems well cared for.

author: Gyro
author_rank: Super Contributor
timestamp: March 12, 2021, 09:36:30 pm
content: Is the burn mark the dark spot on the inside, below the '9' in T292?If it is, then it looks as if the paper insulation had failed at the end of the winding layers and is arcing across from the primary to the laminations. there doesn't look to be any gap between the end of the insulation and the laminated core (judging by the squashed paper at the other side).The only thing I can think of is to introduce some Polyurethane varnish into the hole, ensuring that you avoid air bubbles and allowing to dry very thoroughly. With luck it might be that only the last turn of the layer was involved and there are no shorted turns.At the very least, it indicates that the insulation is on the verge of breakdown generally. Even if you manage to repair this weak spot, you will need to reduce its supply voltage. Giving the transformer some heat, might help the insulation properties (and dry the Polyurethane).

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 14, 2021, 09:24:22 am
content: Thank you very much for invaluable feedback! More questions..Quote from: Gyro on March 12, 2021, 09:36:30 pmWouldn't that be like firing in the blind? Could I peel off the outer layer and have a look? I'm asking, because I don't know how they made these transformers, and wondering if opening it up might worsen the issue.I do have data both for input impedance and DC resistance, so I guess it would be simple to test for shorted turns.Quote from: Gyro on March 12, 2021, 09:36:30 pmHeat: Like, giving it a blow dry? Or baking it in the oven? What temperatures, and how would it improve the insulation (what can I assume the insulation is made of in a transformer like this)?Voltage: Could I just slap on another power resistor, or would that potentially degrade sound?Sorry for all those questions - I'm trying to read up on everything I can find, but surprisingly not many seem to bother repairing transformers

author: Gyro
author_rank: Super Contributor
timestamp: March 14, 2021, 07:34:37 pm
content: You're welcome.Yes, it is rather 'firing blind', but options are limited I'm afraid. The construction of the windings will be similar to this photo...The layers of winding are separated by layers of thin transformer paper, primary and secondary interleaved too. Once wound, strips of transformer paper are wrapped around the windings at right-angle to insulate the ends. Your arcing seems to be from the end of a winding layer to the transformer core, where the insulation has broken down. There is no way to get to the very inner layers (where the breakdown seems to be) without unwinding the whole transformer. Hence my 'firing blind' suggestion.While writing this reply, I suddenly realised that there is something in your favour. The output stage is single ended, not push-pull. That means that the magnetic core laminations must have an air gap (to prevent magnetic saturation from DC current) rather than interleaved (as a normal mains or push-pull output transformer).Normally the 'E' and 'I' shaped laminations are interleaved and varnished, making it almost impossible to separate them (without ruining the windings anyway). On a single-ended transformer, all of the 'E' laminations are stacked together and all of the 'I' laminations are stacked together. Between them is an 'air gap', in practice probably just a single sheet of transformer paper as a spacer.This means that if you carefully remove the outer clamp, it ought to be possible to separate the 'E' and 'I' lamination stacks. The paper spacer is probably varnished and will split, but if you mark the orientation of the laminations so that the go back the same way and don't clean off the paper residue, then the gap should still be about the same when you re-assemble.The gap spacing is critical, too narrow and the core will saturate with the DC standing current through the output tube, causing distortion and reducing bass response. Too wide and the inductance will be reduced - also reducing bass response and increasing dissipation in the output tube.If you are lucky, it might be possible to slide the winding bobbin (probably card) off the E laminations. If you are even more lucky, the breakdown may be at the air gap end. Either way, any insulation that you remove must be substituted. Ordinary paper is too unreliable for use. You will need to replace it with yellow Mylar or Kapton tape - not ordinary PVC, it is too soft and punctures.The question is whether the heat of the arcing has broken down the wire enamel and caused shorted turns, also whether it has arced through the paper separating the primary and secondary layers. There is no way to replace this without unwinding the transformer.So yes, the air-gapped transformer does give you more opportunity to investigate and replace the outer insulation than I originally thought, but there is still a risk that there is too much internal damage. My soak some Polyurethane varnish in to fill air spaces is probably still probably needed, but it may give you the opportunity to replace the outer insulation tape. Cutting away the outer tape without being able to replace it will only make the insulation worse (hence my original suggestion).Regarding warming and drying the transformer insulation. At a different time of year, I would suggest a putting it outside on a dry day with nice hot sunshine! A blow dry with a hair dryer or heat gun will probably be too short a duration to fully heat the full depth of the winding. Baking or putting it on a hot surface for several hours is the sort of thing that is needed. Don't exceed 80 - 100'C though.I found (and lost again!) a reference to Bogen manufacturing - they wound their own transformers and used rather good quality transformer lamination metal, which apparently made their sound quality better than many of their competitors. Yes, there are still specialist companies that will rewind transformers (at a cost), audio transformers tend to be more expensive due to the interleaved primary and secondary windings and several taps (rather than side-by-side split bobbin). They would be able to work it out by careful notes during disassembly, the figures you found would also help.Regarding dropping the supply voltage by a few volts, the most efficient way is to use a small low voltage transformer. Connect the primary and secondary in series (ensuring correct polarity) apply mains across the combined primary and secondary, and take the output across the primary only - you are effectively creating a fixed tap variac (or autotransformer). The transformer can be very small (just a few VA) - the secondary current rating only needs to be a little bigger than the supply current of the amplifier it will drop the voltage by a little less than the secondary voltage rating. Doing it this way will reduce stress on the amplifier mains transformer (you don't want that saturating and overheating) and ensure that the filament voltages are correct too. I though Norway was 230/240V mains though.So yes, you have a bit of a choice to make - whether to try to reinforce the existing damage by injecting (must be polyurethane) varnish, or to dismantle, replace the outer insulation, and have more ability to see more clearly. Personally I would risk the latter.I hope the above 'essay' helps.EDIT: Damn, on further searching, I think the DB10 is push-pull. I can't think what I was looking at now, sorry.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 19, 2021, 10:24:19 pm
content: Gyro: Thank you!! Your answers, as always, are very in-depth and invaluable!So, if I understand this correctly: a transformer for a push-pull amp shouldn't be pulled apart? Which, in my case, means polyurethane varnish would be my best option?Speaking of varnish.. I've tried researching this, and in my language (Norwegian), there's no difference as far as I can tell between lacquer and varnish. However, there seems to be a lot of different polyurethane varnishes/lacquers.. air drying, non-air drying, 2 component, 1 component etc.. Is there anything in particular I should look out for? Is viscosity a thing?

author: Gyro
author_rank: Super Contributor
timestamp: March 20, 2021, 02:20:19 pm
content: Again you're very welcome.Yes, after single-ended essay, I foolishly realized that it was a push -pull output stage with interleaved laminations, leaving you little choice.The distinction between lacquer an varnish is a tenuous one - sometimes lacquer is associated with being a finer finish but I've seen the two used interchangeably. Lacquers are maybe lower viscosity. The sort of Polyurethane I was referring to was the simple single part smelly air drying one - the sort that you would brush straight from the can onto wood for protection. Use a clear gloss one.Warm the varnish (put the can in hot water, it is flammable!) and transformer to maybe around 40'C to reduce the viscosity and allow it to soak into the windings as much as possible. Apply slowly to the hole with a fine artist's brush, cotton swab, cocktail stick, or similar to avoid air bubbles and keep filling slowly until the hole is full - with luck the level may go down to indicate that it is flowing into the windings so you can add more.Leave the transformer in a warm place to fully set - the smell fading is a good indication. Don't be tempted to apply voltage until it has.If the fix doesn't work, at least it won't affect the ability for somebody to fully dismantle the laminations and re-wind the transformer later.