Carvin Bellair 212

[Tubetec]

Just had this amp back for the second time in six months for  repair .
Twice now its blown in4007 ht rectifier diodes .
Theres two diodes in series with each leg of a centre tapped secondary to make the Ht.
The guy that uses it is young and Im wondering if he's not using the standby switch and thats whats causing the diodes to blow .
Ive swapped out the power tubes now as well as replacing the diodes ,just in case there was some kind of intermittant short in one of the power tubes.

Its a few years old now the amp ,but looks like it hasnt got that much road use on it ,when it works it works great and sounds good then for no apparent reason it just dies ,the blown diodes taking out the mains fuse .
If anyone has seen similar trouble from one of these amps Id be greatfull for any advice .
Thanks again,

 

[PRR]

Some folks have not memorized all Carvin schematics. Show the guts of this one.

"212" suggests a big amp which *may* be on the edge of what 1N400x can manage, current-wise, but Carvin is fading and I don't see that model on their site.

Standby is to shut-up while being instantly ready. It should not be essential to life.

 

[Tubetec]

Not a massive beast , 2x12 speakers and a quad of EL84's on output ,5x 12ax7 on pre-amp duty , although not quite on Boogie standards in terms of build ,its still very good .
I agree , even in the hands of a greenhorn who goes straight to full ht volts from cold start it shouldnt die like that ,
anyway she's running away happily for a full 24 hours before it goes back to the customer , If it comes back again I'll up the ante diode wise ,and maybe despence with the fusible resistors and fit a proper ht fuse . Im going to wave the fee for replacing the rectifier diodes this time and just charge for the new output tubes ,fingers crossed it'll turn out good ,
Thanks again PRR,

 

[CJ]

sounds like the output transformer arcing out,  measure the DCR of both primaries from the CT.

crank it up full power on a dummy load at 1 K Hz to 10 k Hz and see what happens,

Carvin is no more, and not too helpful with schematics, there are a few archived on the website that use a quad of EL 84's. 

http://carvinimages.com/schematics/77501revD.pdf

"In October 2017, Carvin Audio announced that the factory will be closing its doors after over 70 years"

 

[Tubetec]

Hi Cj,
thanks for the tip , Ive seen intermittant arcing in transformers before ,but I think this fault hasnt occured under actual driven conditions of the amp and transformer ,either way before the customer comes to collect it tomorrow I'll check the winding resistances and drive it balls to the wall and see if anything untoward happens .
I did find the actual schematic of this amp on the previous occassion , Ill have another look tomorrow and see if I can find it again,
Real shame to see long standing manufacturers of sound gear having to shut the doors ,must be a real heartbreaker for the employees and their families .

 

[Gus]

I looked at the schematic CJ linked. Maybe the Bellair has a similar power supply and output stage

Did the diode(s) fail open or shorted. If shorted I would check the power supply electrolytics.

Does the amp use a cap at the spot of C48 in the linked schematic?
Is it a metallized film?
If so can you check the cap to measure if the value marked on the cap is much higher than the measured value?

What electrolytic caps (value(s) and brand)are used for the first stage of the power supply?

Do you have a current clamp? Can you check the current waveform after the diodes and before the first power supply caps?

 

[Tubetec]

Hi,thanks for the input Gus ,
Just got off the phone with the dad of the guy who owns the amp , apparently the first time the issue cropped up the band was playing open air on generator power , the second time it failed someone knocked it over at rehearsals,the lads were having a few beers and things got messy by the sounds of it  . The diodes blew short on both ocassions . Anyway for now I have replaced a pair of diodes on one leg of the mains transformer ,and put in a fresh set of Sovtek EL84's.Its sitting there powered up now for about twelve hours now ,and everything seems fine with it . The bellair does look almost identical to the amp in the schematic Cj posted , it doesnt have the twin fuses on the Ht windings though, theres a couple of low ohm resistors there instead .
The old tubes were a bit rattly and microphonic ,so its possible flashover or arching occured within the tubes , Its also possible a big transient from the reverb tank cooked the diodes when the unit fell over . Not sure about the brand of power supply electrolytics in there ,it uses axial mounted 22uf's ,all the seals are good and theres no sign of any heat ,which often shows up as distortion of the shrink wrap on the caps .I'll do a couple of saturation tests later before the guy calls round to pick it up .
Thank you all kindly for your input.

 

[Gus]

I would change all of the HV power supply diodes if one side shorted the other might be damaged.

I posted about the small cap because metallized film can "heal" burn away the metallisation at the section that fails, an indication this might have happened is a lower than marked with tolerance taken into account measured value.

Diode are fairly robust I would not be sure the two events are the only cause of this issue.

IMO you really need to check every part in the power supply section. Remove parts from the amp if needed and measure them Caps leakage at rated voltage resistor values, look at the solder connection around the power tube sockets, etc.  this includes testing the transformers as in CJ's post.

 

[CJ]

those generators can chew amps, what other amps were online? 

 

[Tubetec]

Yeah I believe someone plugged out a a mains board with loads of stuff running off it ,it obviously takes time for the generator to adjust voltage and current ,so most likely the amp saw a big spike in mains voltage ,and the diodes fizzled out .
anyway gave it a good looking over and a long time on test over night ,all seems good again so its back with the owner now  .
Many thanks for all of your helpfull suggestions and tips on this .

 

[mjrippe]

those generators can chew amps, what other amps were online? 

Cheap generators and inverters can also put out something closer to a 50/60Hz *square* wave than a sine wave.

 

[Tubetec]

Ugh 50/60 hz ,bristling with harmonics ,doesnt do much for sound.
Even regular mains supplies arent as quiet as they once were ,poor quality switchmodes can spew dirt back into the wireing .
Had another strange experience with a switchmode and digital switching amplifier the other day also,
Sometime back I made a speaker /amp out of an old bakelite radio case , I fitted new stereo speakers ,and a switching amp pulled from a set of Fostex pm.03d's ,and its switchmode psu . The idea was just to use it with a modern dab radio as an external speaker using the headphone out . Location of the wireing connecting the two sets and psu's  made the difference between it working or becoming some kind of ultrasonic osscillator . its one of the downsides of smps's ,sometimes when you interconnect different pieces of gear with different supplies very strange things can happen . Another time I was recording a band ,the bass player had an ultramodern genz benz switching amp ,its was all working fine until I hooked the xlr di output of the genz benz to my my recorder, the bass amp simply refused to work in that set up . swapped over to an active di box and everything worked great again , didnt stop the bass player throwing a hissy fit that his brand spanking new Genz benz wouldnt work though , dont blame him really the ghost in the machine type symptoms are a bit beyond comprehension of non technically minded people .Id be interested to hear of any other strange experiences people have had with these technologies.