Need help to troubleshoot PTx and rectifier circuit (popping fuses)

[Gtr0]

Hello MEF - this is my first post here.

I am at the very end of a build, an amp design of which I have built several times already so it’s not an entirely new design to me, but for the life of me I can not figure out why I keep blowing the main fuse.

To troubleshoot, I began disconnecting secondaries to try and narrow down which section is causing the fault. After some hours of troubleshooting and re-wiring I have arrived to this fault lying within the rectifier circuit. With all other secondaries disconnected, and even disconnected the power filtering section immediately following the rectifiers, the main fuse is still popping.

Here is where I am at…
Main power cord in to 3.15A fuse (even tried a 4A but no need to go higher I suspect since my other builds of this amp are using a 3.15A), then one lead (L) from the fuse to a thermistor then to the on/off switch, other lead from IEC socket to on/off switch (N). From on/off switch to the transformer primaries.

From the transformer secondaries, both HV leads go through secondary fusing then to the rectifier bridge. This is a standard full wave rectifier bridge (doubler). At this point there is nothing else connected to the power supply.

So, if I leave the secondary fuses in, allowing flow to the rectifiers, the transformer will vibrate and make a short buzzing sound and pop the main fuse. If I pull the secondary fuses, stopping the flow at that point, then the main fuse does not pop and the HV secondaries at the point where the reach the secondary fuses reads 194VAC on each lead.

I have tried re-wiring the section up to this point and I even replaced the original UF5408 diodes with UF4007, just to be thorough. Any resistance readings are within spec according to the datasheet for the power tranny, a Hammond 290HX.

So now I am quite stumped. Can anyone offer up any suggestions or troubleshooting steps? I can only imagine that I have made the biggest boob move but I can't find where.

[Gtr0]

And now, deep in thought I wonder if it is because I grounded the secondary center tap to the chassis. This is a no no with full wave bridge, is it not? I am wondering if I need to connect that CT between the resovior caps.

While I did state that I have built this amp several times, it was with different trannies. And those were set up for solid state full wave in both solid state and tubed (not doubled bridge).

Am I on to it here?

[pdf64]

I suggest that before you do anything else, build and use a light bulb limiter; things will only cope with being slammed into a wall so many times. It would not be good practice to sell an amp with key components that are 'walking wounded'.

By using a FWB with a CT that is connected to 0V, you have created + and - supplies. at the output of the bridge.
If the - bridge output is also connected to 0V, then fault current will flow, as the - output is shorted to 0V.
See http://www.valvewizard.co.uk/bridge.html

[Gtr0]

I suggest that before you do anything else, build and use a light bulb limiter; things will only cope with being slammed into a wall so many times. It would not be good practice to sell an amp with key components that are 'walking wounded'.

By using a FWB with a CT that is connected to 0V, you have created + and - supplies. at the output of the bridge.
If the - bridge output is also connected to 0V, then fault current will flow, as the - output is shorted to 0V.
See The Valve Wizard

Thank you pdf64. I have been sweating a bit over a light bulb limiter since living in Belgium, I have not been successful locating a 100+ incandescent bulb - most here are like "20watts, works like 100watts" - it's a rather new thing for me having moved here from the US where we tend to stick to the archaic.

Here is an image of a bulb I recently purchased in the hopes of an EU friendly LBL, would you happen to know if this is an acceptable bulb for the task?


Thank you for pointing out my bridge errors!!! It has been a while since I wired up a FWB with CT PTx.

[pdf64]

Yes, I think that halogen type bulbs should be fine.
In the UK currently, proper incandescent bulbs can often still be found for sale in small, independent hardware etc / thrift shops.
I think it's fine, better really, to run 2 or 3 40 or 60 watt bulbs in parallel, in order to get the necessary degree of current limiting. Then for the initial trial, switch out all but one for more limiting, and switch them back in as confidence in the build is proven.

[Gtr0]

Thanks! It is now my priority to build one with this bulb.

Also I may be able to come up with some 75 watt incandescents through the friend of a friend of a friend etc... But I am wondering if you could expand a touch on this;

Then for the initial trial, switch out all but one for more limiting, and switch them back in as confidence in the build is proven.

if I place 2 or 3 x 75 watters in parallel, each should have it's own on/off switch? Would I then assume all three together will add up in watts??

Thanks again!

[pdf64]

if I place 2 or 3 x 75 watters in parallel, each should have it's own on/off switch? Would I then assume all three together will add up in watts?

Yes, it would be handy for 2 of the bulbs to be switched, and yes, I think the total wattage should be the sum of those in circuit.

[Enzo]

I would immediately suspect your wiring has the problem.

[Gtr0]

Yes, it would be handy for 2 of the bulbs to be switched, and yes, I think the total wattage should be the sum of those in circuit.

loads of thanks!! Working on that today!!

[mikepukmel]

Can you post some photos and a schematic?