Traynor YBA200 6550 help!

[rockstudio]

Hey everyone. I have an emergency with a Traynor bass amp YBA200-2. Last night a tube blew and popped the cathode resistor and all that stuff. i replaced the resistors, replaced the tubes and turned the amp on to check the voltages and I am really worried.
This amp belongs to a touring band who happened to have today off, but are back on the road tomorrow!
the two High voltage taps on the power tx secondary are 241 VAC.
the rectified voltage to the tubes is 636 VDC! is this normal? The standby switch melted so I assume not. The transformer is a big toroidal.
Any ideas??

[Giulio Maiocco]

Hello,

I will give you my opinion ($0,02 worth), until someone more expert than me, a really easy task, will tell you something.

Is this amp 200W with 4 6550 tubes? If so, 636V could very well be OK, many amps of this power output have this kind of voltages inside, like the Marshall Major, a real new production tube eater because of the UL wiring or the Ampeg SVT. The melted standby switch sounds very strange and allarming, be sure there are no shorts in the B+ path and that some a*****e did not put too big a fuse in the HT.

Before putting new tubes in (I won't recommend you to change only the blown tube, but the whole quartect, unless you have independent trimmers to set the bias), I suggest you check the negative voltage on the output tubes socket(s), a loss of bias could be very dangerous for the tubes life!!

BTW, be very careful around this kind of voltage, I once had a 900+ V shock and I won't repeat the stupid error again!

Cheers

Giulio Maiocco

[Dave Cigna]

Originally posted by rockstudio
the two High voltage taps on the power tx secondary are 241 VAC.
the rectified voltage to the tubes is 636 VDC! is this normal?

If you look closely, you'll probably find that they bridge rectified the entire secondary (482VAC.) I've seen other Traynor amps that did this...

If the standby switch melted then I would check the fuse(s). That shouldn't have happened.

[rockstudio]

Thank you very much.
I forgot to measure the Heater voltage before I posted, it's 73 VAC! If the B+ is ok at 630 VDC then I guess my main concern is the heater supply. This voltage comes right off of the secondarys (2 of them, each supplying two tubes). could this mean a bad power transformer?

[Dave Cigna]

A failure somewhere that caused the standby switch to melt might also do damage to the power transformer, don't you think?

But, I'm trying to image what transformer failure mode would cause the heater supply voltage to go from roughly 6VAC (or 12VAC) to 70VAC while maintaining correct voltage on the HV winding... I can't think of anything. If there were shorted turns anywhere in the transformer if would buzz and draw too much current from the primary, probably blowing the fuse. That's assuming the correct fuse is in place. Did you check it?

I would keep looking; you might be measuring the wrong thing. Maybe what you're looking at is a bias winding.

[rockstudio]

Thank you again!
The fuse is intact and is the correct rating (6.3 amp).
The secondaries that I am measuring are definitely the heaters (pins 2+7) I traced them to the sockets and back. I measured the voltage with and without the taps connected to the PCB and the same thing both times. I disconnected the output tx and same thing.

I wish Yorkville / Traynor tech support had called me back yesterday. I had to send the band back out on the road with a solid state head . Hopefully The company will drop ship a new one to a venue on the way. The amp was one week old!

I think the tube that blew was defective or damaged to begin with. When I started to remove the tube I noticed the getter flashing had dissolved, then I gently pulled out the tube and the glass bulb broke off near the base and slid off like an upside-down cup! I know the tube got red hot the night before, but I have never seen a tube crack before! It must have been damaged somehow.

Thank you everyone!