Help Suggestions Trouble Shooting 183 Build

[jlatrace]

Several (maybe 5-6) years ago, with much help from members of this forum, I built a 183. I have been using it regularly since. Yesterday, when playing, sound dropped out, came back & then the 1 amp B+ slow blow B+ fuse failed. This fuse is installed on the center to ground, like a Marshall 100.

I have since run the following tests:
1- Checked power & preamp tubes for shorts using a TV-7 and checked for GM. All passed.
2- Checked value/resistance of all resistors in the power section - all checked out as they should.
3 - Checked for an open diode on the rectifier board & in the bias circuit using a multi-meter - none showed an open short.
4- Put in another 1 Amp slow blow with tubes in. Amp on stand bye, everything is ok. Standby off, a pop and the 1 Amp slow blow B+ fuse failed. Immediately powered down the amp.
5 - Pulled all tubes. Put a 1/2 Amp slow blow fuse in the B+ fuse holder. Turned on the amp - unloaded. Took it off standby, amp was stable. Checked voltages. Readings were high but normal for an unloaded amp.
5. Went back and rechecked the power tubes for shorts including tapping the tubes when checking shorts. No shorts were found.
6. Pulled 1/2 amp and installed 1 amp slow blow fuse in b+ holder.

What tests do you suggest next?
Options include:
1. Swapping power tubes.
2. Swapping preamp tubes.
After that I'm at a loss.
Does this sound like a shorted output transformer winding? How can I test for this - or have I already unknowingly ruled this out?
I am not an engineer nor am I a professional builder. Instead, I'm a self taught (with lots of help) hobby amp builder that has been building tube amps since 2002.

Thanks in advance for yur help.
Les

[erwin_ve]

Hi Les,

I think you can test for Ot: center tap and a tap that is connected to the tubesocket: for a 50w you should read about 50-56 Ohm on both sites. There's always a little off balance, but when shorted you are likely to see on one of the taps a reading of 1 ohm of lower.

Did you check your bias circuit on voltages when in standby? A failure there can put a lot of current going the the tubes and therefore your OT.

I hope your fuse saved your amp, it might Just might be one of the tubes.

[rob@tele]

Check out this nice page about fusing: http://www.valvewizard.co.uk/fuses.html

So I would check the rectifier diodes!

[rob@tele]

In addition, are the power supply caps all ok? No shorts?

[martin manning]

Also check the bias supply caps, which may have been damaged when the CT fuse opened.

[jlatrace]

Awesome help everyone.

One additional thing I noticed. Before I realized the H/T fuse was blown & the center tap was open I apparently overheated, but not to the point it opened up, the 3.9K resistor on the rectifier board. I replaced the 3.9k resistor and it's got me focused on the rectifier diode failure causing bias issues which blow the H/T fuse.

This weekend I will run the diode tests learned from you and others. If not found, I will run other tests and report back the issue and the fix.
Thanks,
Les

[jlatrace]

Found the problem, but only after running every test known to the internet and getting completely frustrated. One Winged C EL34 tube was internally shorted but my tube tester read it as "good". Here is how I found the problem.

Using a 45 watt light bulb as a current limiter, I inserted each tube, one by one, to see if they made the light glow brightly. All preamp tubes and 3 of the power tubes failed to make the light bulb glow. However, one power tube lite up my entire workshop.

Thanks for all of your help.
Les