Rectifier tube blows

[tovaskai]

Hi!
Im quite new to troubleshooting of amps (though know the dangers and how to deload capacitors) and would need some help if I could solve the problem myself (and maybe learn something new!)
I have Yamamoto a08s and the rectifier tube had some electric lightning inside. At that time I didn't know much about these electronics, just thought that the tube needed to be changed. I remember to checked that the ampere meter at the deck showed 0. Every one of the tubes were glowing normally.

After changing the tube the electric lightnings started again. I opened the chassis and saw the diodes blowen (check the drawing). After changing the diodes they soon blowed again - and now it's time to think before doing anything else..

I measured every resistor and they seem to be fine. Also all the capacitors seem to increase their resistance as should when measured with a basic voltage meter. The rectifier tubes filament voltage is about 5.2V and the other pins measure 350V.
I would really appreciate if someone gave me tips what to measure/try to change next (and sorry for my bad English!)

[MrCurwen]

What's the capacitance of the first cap after the rectifier? If it's too large, the PS might be drawing too much current thru the rectifier, thus blowing it. Especially since you seem to be operating at the max rated voltage of the 80.

[Andy5112405]

It could also be a faulty 717A or 45 Valve or Output Transformer.

I would tentatively suggest trying removing the two non-rectifier valves one at a time to see if the arcing stops, however, the unloaded B+ could exceed the ratring of the caps under no load.

The caps seem well specced, so it may be OK.

[Andy5112405]

Is it easy to check that the windings of the mains Transformer are all electrically isolated from each other. If you have a current path from HT to one of the heaters, the heaters will act as anodes. This may explain why the diodes have failed.

[tovaskai]

Originally Posted by Andy5112405

It could also be a faulty 717A or 45 Valve or Output Transformer.

I would tentatively suggest trying removing the two non-rectifier valves one at a time to see if the arcing stops

I removed the 45 tubes one at a time and no arching was seen - but this time the right channel's 717A tube was immediately glowing much much more than the other so I had to pull the power off soon since I had no idea what would happen. I checked that it happens with any 717A in that same channel.
With all the other tubes than the rectifier, both 717A's shine equally. The ampere meter at the deck shows 0 at all times.

[tovaskai]

Originally Posted by MrCurwen

What's the capacitance of the first cap after the rectifier?

Another stupid question from me, but can those caps change their values so that it could be the reason for this prob? The next cap after the rectifier is 5uF (now corrected).

[SY]

Originally Posted by tovaskai

Another stupid question from me, but can those caps change their values so that it could be the reason for this prob? The next cap after the rectifier is 100uF.

WAY too large!!! For this type of tube, that cap should be more like 5-10uF, otherwise death of the tube will occur, sometimes rapidly, sometimes after a bit of use.

[tovaskai]

Originally Posted by SY

WAY too large!!!

Omg, sorry my mistake, the first _after_ the choke is 100uF, before is 5uF.. All the caps are original.

[SY]

Whew!

[M Gregg]

tovaskai,

Which diodes are you refering to?
The heater diodes or the Zeners on the 717a?

Regards
M. Gregg