Framus Cobra Head

[dpm309]

I just received a Framus Cobra Head a friend that just stopped working.  The main fuse was blown so after replacing it, it powered up but when I flipped the standby switch on, the fuse blew again.  Am I looking at a blown PT?  I have not pulled the chassis yet so I am trying to get some ideas on what could be the problem.  Any suggestions?

Thanks,

Dan

[PRR]

> Am I looking at a blown PT?

Unlikely.

Also costly and time-consuming.

Pull the chassis, check the $1 and $10 stuff first. Rectifiers, main filter cap.

[RicharD]

This is a classic example of where a lamp limiter is your best friend. 

Connect the lamp limiter.  Fire up the amp.  The lamp glows bright.  Something is shorted but now you're not blowing fuses.

Pull output tubes.  If the lamp went dim, check the output tubes.  If they're OK, start getting depressed and check the output transformer.  If lamp is still bright, something is still shorted but you've narrowed it down to the power supply.

Disconnect the wire between the rectifier and filter caps.  If lamp goes dim, it's a bad filter cap.  Replace them all.  If the lamp is still bright, then it's either the PT or the rectifier. 

Disconnect the rectifier from the PT.  If the lamp goes dim, replace the rectifier.  If the lamp is still bright, bum out cuz it's looking like a bad PT.

Divide and conquer.  Always be careful.

[dpm309]

Thanks for the suggestion of looking at the inexpensive stuff first as it turns out the problem was a cracked and shorted out diode (see diagram - D7).  I was told that both D6 and D7 are there for transient voltages.  This diode, BY509, is rated at 11.5kV working reverse voltage and 4mA average forward current and is not manfactured anymore.  I have an e-mail into Framus asking if these diodes are neccessary (amps have been built without these for years) and second where I can find one or a suitable replacement.  I will pass on what I find but I was wondering if anyone has had experienced with these type of diodes?

Thanks,

Dan

[FYL]

You may still find BY509's in Europe - Conrad DE lists them for €2.90 a pop. More recent models such as the BY8410 (10kV, 5mA) and BY8412 (12kV, 5mA) are cheaper and easier to source.

Or as you don't really need HV diodes with a 400V B+, you may use a string of three 1KV diodes for app. 3KV - basic 1N4007's are OK.

 

[dpm309]

I have been scouring the Internet for these transient suppression diodes (BY8412 or BY509) and can't find anyone that will sell only a couple.  I found a couple of companies that have the BY8412 but they want a minimum $400 order.  I don't really have enough room to put 3 IN4007 in series as I am dealing with a PCB.  Are these diodes really necessary?  I have read that Peavey and MusicMan use transient suppression diodes and they seem to be the cause of these amps failing occasionally.  Would it hurt this amp to run it without them?  This is Uli Jon Roth's amp (of The Scorpions fame) and he is planning on taking on tour to New Zealand in August so I have some time to track these parts down if absolutely necessary.  Let me know what you think.

Thanks,

Dan