Traynor YBA-1 issues

[Coreynewdisease]

Hi new to posting here. I have a 1971 Traynor YBA-1 Bass Master 45 watt head. I recently put new power tubes in it which are Svetlana Soft/Red EL34s in replacement to the Groove tube medium EL34s that were in it two weeks ago. The amp sounded amazing and I had no issues with it until last week at rehearsal it blew the main safety fuse that is in the back which is a 3amp glass fuse while I was playing. I got a few from the hardware store today. I put a new one in, turned the amp on and before I even took it off standby it was humming real bad. When I flipped the standby switch it blew the fuse immediately. I tried another fuse it did the same thing. I checked the power tubes and the preamp tubes. All are in fine shape. I dont know what is causing this. Any suggestions? I'm very stumped.

[The Dude]

Schematic available HERE.

Pull out the output tubes, fire it up, and see if the fuse still blows. If it holds a fuse, measure the voltages on the output tube sockets and report back.

[Coreynewdisease]

This is what it's doing. Took a video.

Traynor amp problems - YouTube

[The Dude]

I checked the power tubes and the preamp tubes. All are in fine shape.

Also, how did you check the tubes and have you tried known good ones? You could have just gotten a defective new tube.

[g1]

So why isn't the fuse blowing in the video? I sure hope you haven't used a bigger fuse or tinfoil or anything other than the proper fuse.
In your first post you said it was humming loud even on standby, but in the video the hum is when you go from standby to "play" mode.
Like Dude said, it could just be a bad output tube, remove the EL34's and see if it still blows the fuse.

[Coreynewdisease]

So why isn't the fuse blowing in the video? I sure hope you haven't used a bigger fuse or tinfoil or anything other than the proper fuse.
In your first post you said it was humming loud even on standby, but in the video the hum is when you go from standby to "play" mode.
Like Dude said, it could just be a bad output tube, remove the EL34's and see if it still blows the fuse.

Oh I forgot to mention, I had the wrong type of fuse in there, same amp rating but wrong type at the time of the first post. I went out and got the correct one. It will blow the fuse if I leave it on too long. Sorry about that. Oh it does hum before you take it off standby, its just hard to hear in the video. The amp is usually silent when it is on standby when it is operating correctly.

[g1]

Ok, so will it blow the fuse with the power tubes removed?

[Coreynewdisease]

Took the power tubes out and tried it and yes it is doing the same thing without the power tubes in it.

[The Dude]

I would try pulling the preamp tubes also. B+ for the preamp tubes is after the standby switch, so you could have a shorted preamp tube that causes the fuse to blow when standby is engaged.

Does your amp have the flyback diodes like the last page of g-one's upload (the ones off of the output transformer labled 6RS20SP20...)? There seems to be several versions of this amp. If you can confirm which one you have, it will make things easier.

[Coreynewdisease]

Took the preamp tubes out and fired it up. No change.

[The Dude]

Most likely causes:
Power supply diodes, flyback diode (if you have them), or filter capacitors.

I'd first check all of the diodes 4 or 6 B+ diodes (depending on which build you have), bias supply diode, and flyback diodes if you have them. Measure the resistance from each supply output to ground to see if you have shorted caps on either the B+ or bias supplies. You can also remove the flyback diodes all together to see if that's the problem.