#183 voltages

[lionaird]

When wirewound resistors have a big current to swallow they sometimes act like a fuse. Seems that has happened. Question is why was there a big current?

I have no idea. Thanks for all the input. The only thing which is still not working is the LED of the 50/100 switch. Maybe I need a lower voltage LED...

[lionaird]

Not over yet. My choke (Classic Tone 90mA DC, 4Hy) just fried itself (as in started smoking). I have a spare Hammond choke with the same specs as a replacement but obviously need to fix the problem first. Any idea what could have caused that?

[erwin_ve]

All indications are that something is shorting. Screen resistors fried, choke fried. I would put my stakes on the powersection/tubes.

[lionaird]

All indications are that something is shorting. Screen resistors fried, choke fried. I would put my stakes on the powersection/tubes.

So it could be the power tubes? It's a brand new set though...

[erwin_ve]

All indications are that something is shorting. Screen resistors fried, choke fried. I would put my stakes on the powersection/tubes.

So it could be the power tubes? It's a brand new set though...

Maybe,but I would start to measure all parts in the dropping string.(amp off), resistance from all tubesocket pins 3,4 to ground. How is the half power switch wired? Lifting cathodes like #124? Something is shorting, I like to rule out the wiring before the tubes.

[lionaird]

All indications are that something is shorting. Screen resistors fried, choke fried. I would put my stakes on the powersection/tubes.

So it could be the power tubes? It's a brand new set though...

Maybe,but I would start to measure all parts in the dropping string.(amp off), resistance from all tubesocket pins 3,4 to ground. How is the half power switch wired? Lifting cathodes like #124? Something is shorting, I like to rule out the wiring before the tubes.

Thanks, will do. And yes, the half power switch is cathode lifting. Doesn't seem to do a lot at lower volumes anyway so might just take it out completely (considering that the amp was running for a while before I've wired it back in)...

[erwin_ve]

maybe check the bias probe/meter as well, maybe it has a short. If you used it on all 4 tubes it would explain why all 4 screen resistors fried.

[martin manning]

Clearly some unknown "event" damaged the screen resistors, and could have damaged the choke as well. Maybe the half-power LED, too. It will be interesting to hear how this build sounds- the lead dress you have is very different from the the usual D-practice.

[RevD]

Considering the failure rate of new tubes, being new doesn't dismiss them from having a problem.. Might be worth checking them.. Just a thought.

Regards,

Don

[lionaird]

Clearly some unknown "event" damaged the screen resistors, and could have damaged the choke as well. Maybe the half-power LED, too. It will be interesting to hear how this build sounds- the lead dress you have is very different from the the usual D-practice.

Well, I played it for a couple of minutes and thought it sounded great. Very quiet too...

[lionaird]

Well, I have no idea what's going on. I got rid of the 50/100 switch. I put a new choke in. I tried a different set of tubes. I disconnected the feedback wire which goes from the impedance selector to the preamp board. But as soon as I switch the amp out of standby, the choke heats up within seconds...

[da Geezer]

You need to build a light bulb current limiter......you've got a short somewhere.

[img:800:618]http://i23.photobucket.com/albums/b392/ ... imiter.jpg[/img]

[lionaird]

I got a light bulb limiter actually. Used it for the initial startup but really should use it again...

I think the problem are the two 330uF electrolytics. When I switch the amp back into standby after like 2 seconds then there is a hissing noise coming from the caps...

[erwin_ve]

Is the Pt fused on the secondary side? This might save you another choke. And did you measure all parts in the power section for their rated value? Measure the screen resistors too, again!

[martin manning]

I don't see how the 330uF filter caps themselves (either open or shorted) could cause high current flow through the choke. There has to be a wiring error somewhere, I think.