Bias problem in JVM410H

Pete Farrington

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I have also never looked at transient/instantaneous peak voltage/current during high demand playing situations.
Why not?
When you’ve next got an amp on the bench with 1ohm cathode resistors, it’s quicker to do it than to write the above.
Even more illuminating is to scope the waveform, eg blue trace is the VAC across an 8ohm dummy load at the onset of clipping of a Marshall type amp, red trace is the voltage across the 1ohm cathode resistor,

the low power rating for these should be enough but in reality it aint so
It’s fine until the valve shorts.

even 1W resistors burn up sooner or later
No, they’re fine until the valve shorts.
If there’s a suitably rated fast acting HT fuse in place, then 1/2W should be fine.
If there’s a diode in parallel, then their max dissipation can’t ever get to 1W, so even without a HT fuse, 1W with a diode will be fine when a valve shorts. And in that scenario, that arrangement will dissipate less heat than 5W 1ohm resistors.
As per the Super Reverb RI, see my previous post.

Flipping a switch like the STANDBY allows inrush
That inrush current flows around the HT winding, rectifier and caps, not via the valve cathodes.
 
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Malima

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From reading these comments and looking again to the schematics, it looks to me that R26 and R27 are there for the only purpose of reading the current value flowing to GND in idle, hence the BIAS setting. By selecting 1R, the designer gives us a very simple way of reading it by I(bias) = V(R26) / 1.

It doesn't seem to me that R26 or R27 are there for any kind of protection so, using an higher wattage it shouldn't be an issue. Maybe a fuse will blow in the HT when a valve shorts, but IMHO that's why they're for.
 

Spanngitter

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Do whatevery u want, cases where people do reveal themselves as resistent to all suggestions do go to my ignore list...I'll prefer to use my valuable time on other topics where real help is asked for...
I'm outta here...
 

Malima

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Do whatevery u want, cases where people do reveal themselves as resistent to all suggestions do go to my ignore list...I'll prefer to use my valuable time on other topics where real help is asked for...
I'm outta here...
yeah, suggestions like "or do you troubleshoot russian style by replacing fuses with going higher in rating until the root cause identifies itself with the BTSS* Signal ?" or "your statement does clear show a lack in understanding of electrical circuit" I can live without.

I also prefer to use my valuable time sharing knowledge and not trolling others.

See you
 

_dazzmac

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Thanks for posting this. I’ve had the same problem with the bias resistor burning only FS6 blew As well.
 

alpha al

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I'm guessing the amp has been overfused at some point. It should blow a .5A or 1A HT fuse before blowing a 1 Ohm cathode resistor.
 

Malima

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I'm guessing the amp has been overfused at some point. It should blow a .5A or 1A HT fuse before blowing a 1 Ohm cathode resistor.

We all discussed a lot why it blew and if it is supposed to act as a fuse or not.

But, in the end, these resistors will end up burning. So, if you get strange noises from your amp, and BIAS values gone wild, check theses Rs.
 


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