Very low bias readings on 20th Shiva - UPDATE / explanation.

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RockStarNick

RockStarNick

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So I finally got around to taking some plate voltage/bias readings on my new Bogner 20th Anniversary Shiva.

My Shiva's plate voltage is reading 525 right on the nose for each socket. that's what I expected, as everyone says Bogners have relatively high plate voltages.

My TUBES, however, seem to be biased super cold. They're reading between 22 and 24.

Isn't that really cold for a KT88? Does Bogner bias them super cold so that the stock Sovtek's don't explode? :doh:

Would raising the bias fatten up the tone considerably?
 
35 * 600/525 = 35-40mA - would be a reasonable value for them.

But then (with the quality today's tubes) - what is the voltage on PIN 4?
 
OK. I just rebiased my 20th Shiva. Got the KT88's up to around 37-38. I still wanted to be relatively conservative, considering they're Sovteks. Not sure what their plate voltage handling is.

Before I go further, what a freakin' rigamarole. Taking the chassis out, flipping over, balancing it on the transformers, etc. What a pain in the ass. I was spoiled having an amp, previously, that had external bias adjustments.

Anyway...

The amp definitely sounds different with the tubes biased higher. Less harsh on the high end - yet "sizzlier", like you can hear the tubes working more. The bottom end seems fuller as well.

The bias pot has a travel from 7 0'clock to 5 o'clock. It was at noon, from the factory, and the tubes were biased at 23. Now it is at 2:30 or so, and the tubes are at 37-38. (plate voltage is still 525)

I'm still wondering if, judging by the bias pot travel and positions, I could just pop an EL34 in there and bias it correctly. (without having to swap a resistor) Considering that it was at noon to begin with. Hmm...
 
as easy as it is to do external bias, I have no idea why it's not a standard feature on every amp.
 
Jeff Hilligan":3e2kwpke said:
as easy as it is to do external bias, I have no idea why it's not a standard feature on every amp.
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precaution against idiots that don't know what they're doing, fucking things up?
 
Marshall Freak":1anixipt said:
Jeff Hilligan":1anixipt said:
as easy as it is to do external bias, I have no idea why it's not a standard feature on every amp.
Click to expand...


precaution against idiots that don't know what they're doing, fucking things up?
Click to expand...
exactly :lol: :LOL:
 
sovteks are pretty rugged kt88's just not the best sounding or the best handling at really high volume, but they can take what you throw at them. VHT/fryette uses them and they have high plate voltages in some of there amps.
 
Ok, so I just got off the phone with Bogner. Apparently, because of it's design, the Shiva has massive bias-drift depending on the volume of the amp.

If you bias it at idle, after warming up for 5 minutes, it'll read super low (like mine).

If you bias it running it cranked, it will read 10+ HIGHER on the meter.

The tech said that lots of guys (like me) don't know this, crank up the bias, and then blow up their amp when they go to crank it. Not good. I'm probably going to set mine back to factory specs and just play it.
 
Marshall Freak":l6dxcbcp said:
precaution against idiots that don't know what they're doing, fucking things up?
Click to expand...

This is what I've heard from the mouth of more than one amp designer/MFG.
 
RockStarNick":2lks4nw0 said:
Ok, so I just got off the phone with Bogner. Apparently, because of it's design, the Shiva has massive bias-drift depending on the volume of the amp.
Click to expand...
huh? never heard that.

AFAIK Bogner recommends to use only Svet =C= EL34 in it, cause they can handle the huge screen (the reason why I was asking for the volts reading of PIN 4 of it) voltage. Every tube amp shows a bias drift when driven hard . . . from idle to >>100mA/tube, thats pretty normal, but few tubes nowadays can handle screen voltages above 450V and get into trouble when biased >60%. IMO of course.
 
So what would be the best conditions to check bias on this amp?
Clean channel, at idle, moderate volume...then bias on the cold side? Like 60% maybe?
 
When people are talking/writing about "bias" they always mean the idle current.
Any channel, just idle, master set to "0".

60% IMO is fine and a good compromise between good sound and less tube wear.
 
Ok thanks! I just got a 20th Shiva and I've noticed huge bias drift while biasing with the master up.
It just seems to react differently than the typical Marshall circuit that I'm used to.
 
Do you mean master up without playing - or playing thru the amp while reading the current?
 
Thats normal for all tube amps. The current goes up several times compared to idle conditions.
 
RockStarNick":atzpwplk said:
Ok, so I just got off the phone with Bogner. Apparently, because of it's design, the Shiva has massive bias-drift depending on the volume of the amp.

If you bias it at idle, after warming up for 5 minutes, it'll read super low (like mine).

If you bias it running it cranked, it will read 10+ HIGHER on the meter.

The tech said that lots of guys (like me) don't know this, crank up the bias, and then blow up their amp when they go to crank it. Not good. I'm probably going to set mine back to factory specs and just play it.
Click to expand...
that sounds like a reasonable thing to do. FWIW I noticed my EVH was biased a little cold too, my tech jacked it up a bit from the factory 25ma. I actually thought it was too hot and loss some of the definition, so i had him brought it back down. I actually liked the lower bias better tonewise. More bloom and clarity to the notes.
 
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