• E3443.png


    WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.​

Looking for advice on repairing an 845 amp

Hi. I’ve been asked to fix a Voxativ 845 amp. The 845 is driven by a 300B via an interstage transformer.

Around the 845 bias potentiometer there’s a blown 22uF/450V capacitor and a discoloured 5W resistor. In the first picture below, I’ve indicated the blown capacitor. The discoloured resistor is under the red wire below and to the right of the capacitor.

I removed the capacitor and resistor. The capacitor is showing a resistance > 20M. The resistor is open circuit.

A check of the other matching monoblock indicates that the blown resistor is 5.1K. Its matching resistor is not discoloured at all.

I’ve included a picture of the whole amp to give an idea of the complexity of it.

I don’t have the time or desire to try to reverse engineer a schematic for this amp and there’s no way the manufacturer is going to give me the schematic. Shipping it back to Germany from Australia for repair is not a great option.

I’ve researched similar amps and my best guess at the circuit around the pot is shown in the 3rd image. The 5.1K resistor R1 is the badly discoloured and open circuit one. The 22uF capacitor, C1 is the one with the bulging top that's leaked everywhere.

Weird pot value but that’s what I measured. Current should be around 3mA which should be no bother for the 5W resistor. In fact, it makes me wonder why there’s a 5W resistor in there. The pot is tiny (10mm in diameter) so it doesn't have a high power rating.

I could just assume the capacitor went short circuit and replace the resistor and capacitor but I’d rather understand why the components failed in case there is some other problem.

Does the schematic look reasonable and are there any suggestions as to what might have caused the problem please?
 

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Rod Coleman

Member
Paid Member
2004-07-25 6:55 pm
UK
I do not believe a 450V cap blew up with -150V, just by itself. It's possible that the 845 tube failed with a grid-to-filament short. the short may have been transient (if the amp was knocked), or it may fail permanently. A short like this can mean very high voltage appearing on the grid and filament, since the bias vanishes with fil-to-grid shorts.

Fixing the cap in this case may just result in another blowup, and you also take risks with the OT. You can try testing the 845, or better, replace it.

Be sure that the bias voltage is a suitable value, once the R and C are replaced.The IT may have suffered damaged insulation in a blow-up, and should probably be tested too.
 
I would guess grid shorted on the 845 or bias supply went positive and caused the cap to short which in turn took out the resistor. Possible that the cap just died randomly also.

If you have a variac it should be safe enough to replace the cap and resistor and turn things up slowly while monitoring the voltage of the bias supply and the voltage across the cap.

Otherwise best to trace out the rest of the circuit to get a better idea of any other culprits.
 

gideon1990

Member
2009-09-15 4:08 pm
Rod maybe you know more about this; I have heard stories about 845 tubes getting damaged during shipping which may show up after using them for a while, I think it was also associated to some kind of "whispering" noise? Could that be related to this or is this most likely some physical knock/vibration like you already suggested?
 

Diabolical Artificer

Member
Paid Member
2014-05-03 10:34 am
UK
It is indeed! And no, the schematic is confidential. So much for right to repair!
Annoying, but if you look at the underside of the amp, it looks pretty simple and should be easy to draw out. I would think the amp is generic and highly doubt they've done anything complex, it's not Audio Research complex. You'll probably laugh at how simple it is.

It'd be well worth drawing a schematic, it'll give you a far better insight as to how the amp works allowing you to do a better repair, it'll be a service to those techs who follow after you & it will also annoy the numpties at Voxitiv : )

One last thing, one would hope there are some protection circuits in this thing, those valves ain't cheap, not to mention the OPT's. Doesn't look like there is though.

Good luck, Andy.
 
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Postscript

Just to close off this thread, I replaced the blown resistor and capacitor and ramped up the voltage with an autotransformer.

The amp performed perfectly. I've had it in my system for a few weeks and after many power off/on cycles it's still fine.

I hate not being able to find a cause for the original failure and can only put it down to component failure. The amp is being picked up today. Fingers crossed!

I didn't get far trying to reverse engineer a schematic. There's just too much that is hidden from view.

I did test the 845 on my curve tracer and while 1ms HT pulses might not reveal internal shorts, again, the curves were perfect.
 

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