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escaped smoke

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Folks, My Chinese kit EL34 amp has died. The smoke escaped from the mains transformer and the fuse literally exploded. The primary is now a dead short. I asked the original supplier about a replacement transformer and got an unhelpful response because of Chinese New Year apparently!

The transformer has 3 primary taps at 220V, 230V, & 240V

Secondary Taps
320V-0-320V 0.18A
3.15V-0-3.15V 2A
6.3A 3A
5V 3A

It has a part number BD-200W-9650.

Any suggestions where I could source a replacement that is not going to cost me a custom winding fee? Alternatively could I replace with a couple of transformers to supply all the tappings? Currently all are in use, the balanced heater feed to the first stage valves the unbalanced to the EL34 and the 5V to the valve rectifier. The amp does sound surprisingly good when it works, very clean and quiet with enough power to drive decent speakers.

Thanks in advance for any help with this!

AVO111:)
 
Nigel,

I will be doing. The thing blew up while playing and sounding OK, no odd balance, distortion, odd noises or what ever I will be going through the circuit bit by bit when I get a new transformer to put in, I don't want to rip it apart just in case it is a while before i get a new tranny and I forget what I had linked to what!


Actually I might put fuses on the secondaries this time particularly heaters!

Cheers!
 
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Don't fuse heaters. The fuse will drop the filament voltage.
And what kind of fuse will you use, a slow blow, or a fast blow?
Filament currents are very large at turn-on, and much lower when the filament is warm.

If the filament shorts (I never saw one do that, and don't remember hearing anybody report that), when the fuse blows, the tube will go cold. Do that to the power tubes, and the B+ will go up.
B+ might now be more than the voltage rating of the electrolytic filter cap, and it will stay that way, not just be a short surge (caps have continuous voltage ratings, and some have a higher surge voltage rating).

B+ Surges normally occur when the amp is first turned on before the tubes warm up.
This is particularly true when the output tubes have indirectly heated cathodes (i.e. EL34).
Solid state rectifiers cause almost immediate B+; Direct heated filament rectifiers take a little longer (i.e. 5Y3).
Indirect heated cathode rectifiers (i.e. 5AR4) have the least surge effect, because when the rectifier cathode heats up, the other tubes cathodes are almost heated up too.

I agree with nigelwright7557
Check the rectifier tube, and the B+ filter caps.

Also check the output transformer primary for shorts to ground.
If there is a grid bias supply, check the diode(s) and filter caps, and that the pot wiper(s) are connected (read with an Ohmmeter from wiper to the pot ends.
 
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Agreed. Also check to see what the wall voltage is and what it was set to. When theres a failure in the amp it would usually kill the secondary, not the primary. Perhaps it was set to 220V and running from 240V+?

The Hammond 376X is close enough. You would just parallel the two heater strings in theory.
 
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Thanks folks!

Problem 1 is the Hammond 376 transformer costs about twice what the amp cost in the first place, at least it does in the UK! It also doesn't have a 5A winding for the rectifier though I guess that can be a separate transformer.

I am really looking for a cheaper solution than this if one can be found.

Cheers AVO III
 
While shipping magnetics long distances can be "dicey" business, it's hard to better AnTek in the price dept.

AnTek's AS-2T300 has more than enough stones for this job. Connect the HT windings in parallel and bridge rectify with 4X pieces of this part or a flat pack equivalent. Schottky's are dead quiet and the need to replace is most unlikely.

Use resistors to create a pseudo CT on 1 of the 6.3 VAC windings.

BTW, Chinese 5U4 "equivalents" are guano. :mad: A crappy rectifier could have wrecked the power trafo.
 
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Since the Hammond is locally available I would just get one and get it over with. It should fit with a minimum of modification.

The Antec Eli suggests is a good option, but I imagine the shipping to the UK will reduce the difference in cost considerably. If you do go that route make sure you have sufficient room and purchase a cover.

I expect Sowter is more expensive than the Hammond, but I would check with them too.

The economics are skewed, if it sounded decent to you you can regard it as having more utility value to you than what you paid for it.

OR

Just put the money towards a new and better amp. Maybe build one of Tubelab's amp projects.
 
The 5U4 rating is 75 Ohms effective resistance in each plate (Turns Ratio x Primary DCR) + (DCR secondary to CT).

i.e., 300Vct to 220V, turns ratio = 300/220 = 1.36. If the primary DCR is 30 Ohms, that part of the effective resistance to the plate is 30 x 1.36 = 40.8 Ohms. If the CT to one side of the secondary is 35 Ohms, the resistance to the plate is 40.8 + 35 = 75.8 Ohms.

If you have more than 40uF in the first capacitor after the 5U4, you need more than 75 Ohms effective in each plate. Otherwise, the rectifier may go bad or even short.
 
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The 5U4 rating is 75 Ohms effective resistance in each plate (Turns Ratio x Primary DCR) + (DCR secondary to CT).

i.e., 300Vct to 220V, turns ratio = 300/220 = 1.36. If the primary DCR is 30 Ohms, that part of the effective resistance to the plate is 30 x 1.36 = 40.8 Ohms. If the CT to one side of the secondary is 35 Ohms, the resistance to the plate is 40.8 + 35 = 75.8 Ohms.

If you have more than 40uF in the first capacitor after the 5U4, you need more than 75 Ohms effective in each plate. Otherwise, the rectifier may go bad or even short.

Quite true. My loathing of Russian and Chinese ST bottle 5U4 "equivalents" stems from the many times I've helped people with power on arcing. The 1st filter cap. was within documented limits and switching to a U.S. made 5U4GB ended the "fireworks".
 
I priced a custom transformer with your specs (toroidal though) from audiophonics. It comes to 95€ (200VA, 7 windings total, two windings above 100V and electrostatic shield). Not cheap but cheaper than Hammond.

If you used a separate transformer for the 5V (15VA transformers can be found very cheap), you could step down to a 150VA/6 windings transformer which only cost 72€ (66€ without the shield).

I've had decent experience with these. They're a bit slow to ship if you miss the time when they order a batch.

edit: I assumed that the specs you gave were for AC current rating. Some manufacturers, such as Hammond, rate the current of the HV winding for DC.
 
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