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Tube Technology Unisis: should I replace the mains transformer and how ?

david winter

david winter

Classical music lover.
Hi,

Some time ago a client brought me a Unisis for repair (version with phono preamp included).
Basically one of the big fat electrolytics pissed in the amp and probably shorted or at least generated excessive current draw.
I had to clean the board and even the tube sockets (once removed) because the electrolyte was resistive.
Now that everything is back in place, new fresh caps are installed, new russian equivalents of 7189 installed and biased at 28mA (as per original inspection label inside the amp), I tried the amplifier and voila: the mains transformer becomes warm within 20 minutes or so. It's hot after an hour and then really hot. Not as much as to burn, but definitely too hot.
The client told me that this was not the case before, so I'm concerned about the possibility that this transformer shorts soon or later.
As the client wants to sell his amplifier, I wondered how I could replace the transformer. I tried to contact Tube Technology but no answers.
I have no specs for this transformer as well.
Any suggestion appreciated.


Thanks,

David
 
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Pull the output tubes , and if the , transformer stays cool to the touch its ok , most tube amps power transformer run 140 F + (60 C) when driven . Measure the temp and just don't say it's hot , some run as hot as 160 F (71.1C)
 
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david winter said:
Hi,

Some time ago a client brought me a Unisis for repair (version with phono preamp included).
Basically one of the big fat electrolytics pissed in the amp and probably shorted or at least generated excessive current draw.
I had to clean the board and even the tube sockets (once removed) because the electrolyte was resistive.
Now that everything is back in place, new fresh caps are installed, new russian equivalents of 7189 installed and biased at 28mA (as per original inspection label inside the amp), I tried the amplifier and voila: the mains transformer becomes warm within 20 minutes or so. It's hot after an hour and then really hot. Not as much as to burn, but definitely too hot.
The client told me that this was not the case before, so I'm concerned about the possibility that this transformer shorts soon or later.
As the client wants to sell his amplifier, I wondered how I could replace the transformer. I tried to contact Tube Technology but no answers.
I have no specs for this transformer as well.
Any suggestion appreciated.


Thanks,

David
Click to expand...

This is a classical case of "dead product" where the vendor has disappeared / or ignoring support demands.
In your case, the transformer _might_ be damaged ( it might also be the case that it's still working but the amp draws more current now, maybe it's correctly biased )

Your options a few, all of them leaving the amp in status "homemeade unsupported" as
you cannot replace with original parts.

There is a few supported el-84 amps today, one of them is dynaco st-35 clones. They sell
a main transformer that just might be suitable. Try dynakitparts.com trideelectronics and
tubed4hifi.com , they all sell this amp and has parts for it.

The above is a clear warning to buy anything from http://www.tubetechnology.co.uk/ until they have resolved their support tasks. And some googling seems to indicate that you are not the first person to have trouble getting support from them.
 
I'll try to test without the output tubes, however I'm not sure if the replacement transformer you're talking about will be suitable. This amp uses eight EL84 tubes with 350V at the plate (from memory after summer holidays).
The original inspection sticker shows an individual bias of 28mA on each tube. I used a quad bias tester to make sure that all tubes were correctly biased.

I think the last option is to have the transformer rewound, but again, I'm not sure of the original voltages. Best would be that somebody here can test the voltages from his mains transformers with wires disconnected (they are not soldered except the heaters winding).


Thanks,

David
 
david winter said:
I'll try to test without the output tubes, however I'm not sure if the replacement transformer you're talking about will be suitable. This amp uses eight EL84 tubes with 350V at the plate (from memory after summer holidays).
The original inspection sticker shows an individual bias of 28mA on each tube. I used a quad bias tester to make sure that all tubes were correctly biased.

I think the last option is to have the transformer rewound, but again, I'm not sure of the original voltages. Best would be that somebody here can test the voltages from his mains transformers with wires disconnected (they are not soldered except the heaters winding).


Thanks,

David
Click to expand...
David,
I was not aware of the topology, but as you say a st-35 transformer won't do here.
So it's even worse then i initially assumed.

I feel sorry for You and your customer. Hopefully someone might hint about a replacement transformer.
Du you have schematics for this amp? Could you post it here ?
 
You might look through the Hammond site, they do make tube power transformers. Essentially total up your heater loads and voltages if other than 6 volts. If the rectifier is a 5 volt type, you need a winding for that, and for the B voltage, a 325 volt AC will produce the 350 DC when filtered, it is just a matter of knowing what the dissipation total is for all the tubes, and going with a transformer that is above that. off the cuff I would estimate the B at 325-0-325 At about 300 to 325 milliamps depending on the number of tubes in the chassis. The 7189's have a maximum plate current rating of 65ma@400VDC, and this is measured between the plate and cathode. The EL84/6BQ5 is 50@300VDC

So even if you are not using the exact replacement from that company, you can make a decent substitution that is more reliable.

I would think they would ship worldwide, otherwise there are other ways of getting it. At a minimum, you have have a starting point for finding something local to France or EU.
 
All you GUY'S are getting ahead of your self , we don't know for sure that the transformer is bad , it may be something as simple as a leaky diode or bad rectifier tube .Too hot touch is different for every one is different , I can hold things that others will drop from being too hot . Lets get some temperature and let him check some things out .
 
There is no tube rectifier, it's all made of indivitual diodes for the output power and diode bridges for the rest (pre-amplifier and probably some heaters).

I forgot one essential problem: after cleaning the main board and changing the two large electrolytics, I noticed some arcing from under the mains transformer with a very very small and light hair of smoke.
After cleaning a few spots of electrolyte which I found to be resistive, the problem disappeared but the heat problem was still there. The client reported the heating trouble but used the amp until the electrolytic capacitor leaked, so I'm pretty sure that the mains transformer has been overloaded. The resistive shorts generated by the electrolyte must have aggravated the problem.

So I'm not sure if the transformer is still ok or not and whether the voltages measured on the disconnected secondaries would make sense.

I'll measure that tomorrow and let you know. The other problem of course is to find a transformer with the same dimensions as it's on the visible part (not covered). There is a silver lid on it as well (see on the google pictures).

The tubes powered by the mains transformer are:
- 8x EL84M (or 7189 or russian equivalent)
- 2x ECC81

If there is one single heater secondary, this brings the total current to 8x0.76 + 2x0.3 = 6.68Amps. Should I consiver 8A ?

The preamplifier stages run from another smaller core transformer attached to the rear.


Thanks,

David
 
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peterh said:
This is a classical case of "dead product" where the vendor has disappeared / or ignoring support demands.
In your case, the transformer _might_ be damaged ( it might also be the case that it's still working but the amp draws more current now, maybe it's correctly biased )

Your options a few, all of them leaving the amp in status "homemeade unsupported" as
you cannot replace with original parts.

There is a few supported el-84 amps today, one of them is dynaco st-35 clones. They sell
a main transformer that just might be suitable. Try dynakitparts.com trideelectronics and
tubed4hifi.com , they all sell this amp and has parts for it.

The above is a clear warning to buy anything from http://www.tubetechnology.co.uk/ until they have resolved their support tasks. And some googling seems to indicate that you are not the first person to have trouble getting support from them.
Click to expand...

I dunno. I have a pair of the Genesis 100w mono's and they are outstanding. If you get a good deal on some used gear I say its worth a chance.
But you are right, it will be unsupported as the original designer has left due to a death in the family. However, a local tech looked at my amps and sees nothing that cannot be serviced. But obviously transformers are a different thing.
 
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This is where a megger comes in handy (AKA insulation tester). I run all used TXs up to 1000v then measure the insulation resistance. If it's not infinity my trust goes way down. I might add that I bake them first to remove any moisture that could be trapped in the windings.
 
battradio said:
All you GUY'S are getting ahead of your self , we don't know for sure that the transformer is bad , it may be something as simple as a leaky diode or bad rectifier tube .Too hot touch is different for every one is different , I can hold things that others will drop from being too hot . Lets get some temperature and let him check some things out .
Click to expand...

While in part true- it is also true that once over heated- you cannot "unheat" the transformer. (I would never trust a transformer high voltage to be safe even after attempting to infuse it with varnish. An OPT maybe.) I fully expected with the at least 8 time lag between Paris and most of the US that the condition of the transformer would be fully known by the time David Winter logged back on, besides from his initial description, he gave the impression that the unit had been operated for a fair amount of time even after the heating issue was noticed. HiPot testing might reveal something. Similar yet different to a Megger.

End bells can be swapped. Since it is the power transformer, it is only 1, so even if a different height, it will not look entirely out of place. Finding one that can work on a 250 VAC mains of the EU, that migth be more difficult, but not impossible. There should be a listing of EU corporations that produce transformers, it should not be extremely lengthy.
 
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