Amp biasing question -- Bogner EL34 Shiva...

[Red_Label]

*** WARNING -- LONG-WINDED POST*** I apologise in advance for a long-winded post. Gotta get all the info out there...

Okay, so a week ago I got my newer model, mint condition Shiva EL-34 Reverb head. It was a "sick puppy" out of the gate with zero clean headroom. So I ordered a matched quad (so I'd have a spare new pair) of new Winged-C Svetlana tubes. And also ordered a complete set of 7 new Chinese 12AX7B preamp tubes, as well as an AmpHead brand dual bias tester so I could bias the new power tubes.

I received the EL34s last Friday and thought I'd pop them in there over the weekend even without biasing. INSTANT fix for my new sick puppy. Sounded great after that. Tonight I got the preamp tubes and the bias tester. So I put in the preamp tubes and warmed the amp up. After it was at full temp I checked the tube levels and found that one was reading 31ma and the other about 24. This was a bigger spread than I wanted, so I did some swapping with the other two new tubes and ended-up pairing the hotter two for now. After warming back up fully I got a final and consistent spread of 4ma (28ma and 32ma). That was as close as I was gonna get with this quad. A little disappointing, as I'd hoped for closer to 2 or less. But since I've never tested tubes or biased before, maybe my expectations were a bit much.

Anyways... now this thing is a RAGING BEAST. Before swapping the tubes out to decrease the 7ma spread (24 and 31ma), I had the amp purring like a kitten and was nailing the old plexi tones ala Brother Cane "Seeds" album. I've always loved that smoldering, fat tone that Damon Johnson gets. But now... this thing is just a pissed-off raging beast ala modded JCM800. The cleans went from sparkly and slightly spanky, to VERY spanky. I still get decent headroom on those, but it's definitely closer to "hairy" now. I'm torn, because I can tell that the current setting is how it's "supposed" to be -- because I keep reading superlatives like "when biased right the amp comes alive" etc. I can tell that this thing has come alive indeed! But thus far I seem less able to dial the beast back to that smoldering old dark plexi tone. It wants to rage and it's got more presence on both channels.

Bogner's factory bias is set at 29 - 31ma. Being that I'm at 28 and 32, I guess you could say that I'm at their specs. I dunno. Maybe I should dial it back to maybe 26 and 30? Just thinking out loud here and hoping that other Shiva owners, or just other owners of great tube amps with such mood swings will chime in with your opinions and experiences. Thanks for reading and sorry again for going-on so long.

BTW... no matter what... I LOVE this schizophrenic amp! I think that my search for THE amp for me is over. The only question is do I want it to be a sultry brunette or a fiery red head. I can't believe that people say you can't do metal with this thing. When I kick in my OD pedal and dial back the mids it's definitely knocking on metal's door!

 

[That90'sGuy]

Re: Amp biasing question -- Bogner EL34 Shiva...

It's been a while since I checked the bias on mine, but I don't seem to remember them biased that far apart. Nevertheless, you can never go wrong setting it by ear (provided you know the relative range on the trimpots where they should be). Find the right balance for you! I know that when I've had it set too cold the overdrive really suffered, so I ended up having to set it just a little hot.

 

[Red_Label]

Re: Amp biasing question -- Bogner EL34 Shiva...

It's been a while since I checked the bias on mine, but I don't seem to remember them biased that far apart. Nevertheless, you can never go wrong setting it by ear (provided you know the relative range on the trimpots where they should be). Find the right balance for you! I know that when I've had it set too cold the overdrive really suffered, so I ended up having to set it just a little hot.

,

Well... it seems that the quad I got isn't that close. But the tutorial I got with the AmpHead tester said that less than 7ma would be acceptable as a "matched" set. I dunno though... seems like a lot to me. The other two tubes in my quad were colder, but I think they were closer to each other. I figured I'd use the hotter ones first. That way, if I'm on a gig and the hot ones go bad, I can replace them on-site with the new cold ones and they'll be fine at the gig. Whereas, if I used the cold ones first and biased the amp up for those, if I threw the hotter ones in at the gig they'd be biased way too hot. That's my logic anyways.

Yeah... I'm thinking that I should probably back-off 1-2ma to save some tube life and maybe get little closer to that smoldering tone I had for a bit. Hopefully that would be a good comprimise between that dark old school tone and the brighter raging tones that I was bouncing between. Definitely sounds like I need to bias by ear since both extremes seem to be within safe range.

 

[ehdwuld]

Re: Amp biasing question -- Bogner EL34 Shiva...

My Carvin Belair was of a similar vein

nice pleasent softspoken
I retubed her with some Rubys ( EL84s but .... )
biased with the "Carvin Method" across the standby switch

factory set at 70 ma
for the Sovtek
the rubys were much hotter rolled the pot back from noon to bout 10
still kinda high at 79

shes got a personality now

I'm diggin it

 

[glassman]

Re: Amp biasing question -- Bogner EL34 Shiva...

I wouldn't worry for a second about a 4mA spread...you'll never be able to hear a sonic difference between 4mA and a "perfect" match.

By the way, reducing bias current will make the amp a tad brighter and really won't improve tube life appreciably...being centered on 30mA idle current is a pretty good place to be. If you like the tone, I'd stick with it.

You may consider putting some of your old preamp tubes back in to tone down the gain of the amp.

 

[Red_Label]

Re: Amp biasing question -- Bogner EL34 Shiva...

I wouldn't worry for a second about a 4mA spread...you'll never be able to hear a sonic difference between 4mA and a "perfect" match.

By the way, reducing bias current will make the amp a tad brighter and really won't improve tube life appreciably...being centered on 30mA idle current is a pretty good place to be. If you like the tone, I'd stick with it.

You may consider putting some of your old preamp tubes back in to tone down the gain of the amp.

.

This morning I ended-up going back in there and playing with the bias again. This time I watched the meter, but used my ears to set it where I felt it should be. Ended-up with the higher of the two tubes at 31ma (which is the high side of what Bogner sets them at). The lower one is sitting at 27.4ma. That seemed to be the most pleasing spot to me. Right on the edge of being a monster, but controllable.

Reminds me of a two-stroke dirt bike that's just sitting on the lower edge of the powerband. Whereas, as 32ma it was deep in the powerband and pulling my arms out of their sockets! Or like a "pet" tiger. LOL. I much prefer the feeling that it's right there on the edge of the leash, than the feeling that it's completely off the leash and eating innocent bystanders! ;-)

And yeah... I think you may be right about the complete new set of preamp tubes being part of the brightness. But I was happy with the overall tone with the current bias, so I left all the new stuff in there.

BTW... your avatar drives me nuts. Every time I see it I instantly think of a hot chick wearing a thong bikini bottom. Always have to look harder and remind myself it's just a lamp.

 

[Gearjoneser]

Re: Amp biasing question -- Bogner EL34 Shiva...

29mA sounds low for EL-34's. If there's any room on the bias pot to heat them up more, get them up around 34-35.

On my XTC, Bogner told me that factory bias is 35ma, and that's probably where it is now, since I haven't checked it since they set it a few months ago. But before that, I was running my JJ EL-34's at 38mA and I think the amp may have sounded slightly better.

In general, I've gotten the best tone with 6L6's at 32mA and EL-34's around 38mA. I can't say what the voltages were, because my bias tool is a mA tool.

 

[Red_Label]

Re: Amp biasing question -- Bogner EL34 Shiva...

29mA sounds low for EL-34's. If there's any room on the bias pot to heat them up more, get them up around 34-35.

On my XTC, Bogner told me that factory bias is 35ma, and that's probably where it is now, since I haven't checked it since they set it a few months ago. But before that, I was running my JJ EL-34's at 38mA and I think the amp may have sounded slightly better.

In general, I've gotten the best tone with 6L6's at 32mA and EL-34's around 38mA. I can't say what the voltages were, because my bias tool is a mA tool.

Hmmm... wow. The tech at Bogner who responded to my email said that they set the Shiva at the factory at 29-31mA. I'm gonna reply to him and make sure that I understood that right.

BTW... there is room left on the pot to heat them up more. Seems like I could definitely get them past 34 or 35 with what's left to go on the dial.

 

[Fresh_Start]

Re: Amp biasing question -- Bogner EL34 Shiva...

Guys, at the risk of stating the obvious, there is no magic current (milliamps) for a specific tube type. Plate dissipation (watts) is current * voltage (drop from plate to cathode). The "right" current reading depends on plate voltage. "Right" with 300 VDC on the plates is WRONG with 500 VDC on the plates. IOW 29ma may be exactly right if his Bogner runs a significantly higher plate voltage.

FWIW starting with the recommendation of the Bogner tech makes sense to me. He knows what the plate voltage is supposed to be and what operating class sounds best for that amp (Class A, hot Class AB, etc.). For true Class A (not Vox), your target plate dissipation should be 100% of max (maybe minus a touch). Usually, only single-ended amps are true Class A. For Class AB like most amps, the target could be anywhere from 60% to 80% although 70% is a typical starting point.

It's not an exact science, so if it sounds good to you and the tubes aren't glowing red (or blue) then go for it! IOW if you're in the ballpark in terms of a reasonable percentage of rated dissipation or whatever and you like the tone, go play guitar.

GJ - another difference could be how you're measuring the plate current. If your amp has test points, you're actually measuring plate and screen grid current. An EL-34's screen grid is rated for up to 8 watts dissipation while the plate is rated at 25 watts...

NEVERMIND the AmpHead bias tester measures cathode current too. Actually, it measures cathode voltage. "35 millivolts = 35 milliamps" and the 1-ohm 1% resistors give it away. There is nothing wrong with that approach - it's how everyone sets up bias test points. However, it is helpful to know that you have an extra cushion in that your measurement includes the screen grid current in addition to the plate current.

If you are compulsive (like me), you get screen grid current by measuring the voltage drop across the screen grid resistor and then dividing that voltage by the value (resistance) of the screen grid resistor. Subtract the resulting screen grid current from the cathode current gives you the actual plate current alone. The good news is that you only have to do the measurement & calculation of screen grid current once for a given amp unless you change something pretty major.

Note: There are bias testers which actually measure plate current directly. Doug Hoffman's "kit" is one example.

Sorry for the techno-rant. Just hope it helped more than it hurt.

Chip