confused by mains transformers HT current specs

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Bieworm
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confused by mains transformers HT current specs

Post by Bieworm »

Hey guys

Sad news. The Dual 50 I built fried the power transformer's HT secondary.
To explain things: I used a JTM45 PT in stead of a JMP50 type. They were the same price, but the specs on the JTM45 were higher, so I thought I'd be better off with that one.
I checked everything else in the amp and all is just fine. There is no direct cause for the winding to start burning.
Specs of the JTM45: HT 345-0-345V @ 200mA / Fil 6.3V CT @ 6A
Specs for the JMP50: HT 345-0-345V @ 150mA / fil 6.3V CT @ 5A

So the JMP50 would be overloaded sooner, no?

Is it because I use the hammond 1750N, which has 3200 Ohm prim, where the JTM45 has 8000 Ohm?

Is there something other different about those power tranformers? There is no further info to be found on them.
290GX.pdf
tt-jtm45-pw-v2020.pdf
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cstenger5
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Re: confused by mains transformers HT current specs

Post by cstenger5 »

Aww man :(
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Re: confused by mains transformers HT current specs

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cstenger5 wrote:
Tue 11/23/21 11:28 am
Aww man :(
No worries. Tube-Town has refunded the transformer. Ordered a JMP50 PT instead. We'll see from there 😉
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geoff 1965
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Re: confused by mains transformers HT current specs

Post by geoff 1965 »

Bieworm did you have the HT centre tap fused? if so you would presume if the fault was in the circuit it would blow the fuse,looks like a duff transformer winding.
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Re: confused by mains transformers HT current specs

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geoff 1965 wrote:
Wed 11/24/21 6:50 am
Bieworm did you have the HT centre tap fused? if so you would presume if the fault was in the circuit it would blow the fuse,looks like a duff transformer winding.
Geoff.. thanks for your reply.
I don't have a fuse on the CT, but on the B+ right after the SS rectifier. It's a 500mA fuse.
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Re: confused by mains transformers HT current specs

Post by geoff 1965 »

would'nt you be better placing the 500mA fuse between the HT centre tap and ground?
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Re: confused by mains transformers HT current specs

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geoff 1965 wrote:
Wed 11/24/21 7:48 am
would'nt you be better placing the 500mA fuse between the HT centre tap and ground?
Not sure... I followed a schematic from Sluckey. I presume he knows his stuff...
You think it's better on the CT? I have to reinstall a transformer, so moving the fuse is easily done I guess...
Marshall_Dual_50.pdf
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geoff 1965
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Re: confused by mains transformers HT current specs

Post by geoff 1965 »

See what you mean,first time i’ve seen the HT fuse in that position.so did that fuse blow when the HT winding fried?
bye the way that looks like a cool amp!
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Re: confused by mains transformers HT current specs

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geoff 1965 wrote:
Wed 11/24/21 9:08 am
See what you mean,first time i’ve seen the HT fuse in that position.so did that fuse blow when the HT winding fried?
bye the way that looks like a cool amp!
No that fuse is still intact. Doesn't that indicate the problem is before the fuse?
Let me explain what I experienced.
The owner called me it was blowing (3A mains) fuses. I received the amp, and not having my light bulb limiter on had I tried another fuse to see it for myself. And it blew indeed...
Then I removed all tubes and put in another fuse. I put the 'on' switch on. That's when it started smoking, even on standby. It didn't blow fuses anymore.. it just smoked.
I disconnected the secondaries and measured resistances..
filaments winding = 0.3 Ohms
HT winding = 38 Ohms
no short on the heaters.. no short on B+ nodes anywhere...
diode test = +/- 500mV on all diodes
filter caps measure fine (preamp filters 250k Ohm, poweramp filters 220k Ohm)
bias elco's 82k Ohm
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geoff 1965
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Re: confused by mains transformers HT current specs

Post by geoff 1965 »

That fuse placement is the same as the Marshall 1987 but what is puzzling me is there is the 50/50uf can cap and the diodes between the winding and fuse.the bias taps off the HT before the diodes so to protect the HT winding if there was a problem would’nt the fuse be better directly on the centre tap to ground?
Tricky one to work out!
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Re: confused by mains transformers HT current specs

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geoff 1965 wrote:
Wed 11/24/21 10:17 am
That fuse placement is the same as the Marshall 1987 but what is puzzling me is there is the 50/50uf can cap and the diodes between the winding and fuse.the bias taps off the HT before the diodes so to protect the HT winding if there was a problem would’nt the fuse be better directly on the centre tap to ground?
Tricky one to work out!
The fuse on the center tap makes more sense if you think about it. Another good solution is to install 2 fuses... one on each side of the HT winding
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Re: confused by mains transformers HT current specs

Post by geoff 1965 »

Bieworm there is a thread over on “EL34 world” concerning the fuse placement,one of the reasons it’s not on the centre tap is because of the bias tap,check it out,still puzzles me how the HT winding fried and the 500mA fuse did’nt blow!
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