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Topic: Bias question, Ken, Brad,Donny, Jim ?? |
Lee Jeffriess
From:
Vallejo California
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Posted 11 Nov 2008 10:59 pm |
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I have a Sho Bud christmas tree that's acting a little weird.
About two and a half years ago I had Tim Maag replace all the electrolytics.
He also replaced the bias trim pot with a more quality part, and rebiased it hotter.
I believe the schematic calls for 25mv and he set it too 40.
The amp has probably been out and done six or so gigs, and hasn't seen much action.
To be honest Im not sure when I first noticed the change, but it now sounds like its biased cold.
It sounds more like a Magnatone varsity with a 12'' Jensen.
A far cry from a Xmas tree with an Altec 418B.
I took it in to a repair guy here in town.
He is an old school tube and SS Hi Fi service guy, and at Tim's suggestion we put it on the scope.
Clean as a whistle all the caps are fine nothing weird.
He found a bad connection on one of the boards, and a gunked up volume control thats it.
When I gave him the amp,and the schematic I told him that he may notice that the bias is set differently, and it is intentional.
It still sounds the same, both channels, different speaker.
Is it possible that some how the bias setting has changed?.
Could there be a difference between the bias being set when the electrolytics where new, and now they have broken in, it needs to be reset?.
Lee
Last edited by Lee Jeffriess on 12 Nov 2008 3:50 pm; edited 1 time in total |
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Ken Fox
From:
Nashville GA USA
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Posted 12 Nov 2008 5:51 am
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Bias can and will change over time, especially with new tubes.
If the amp is tested on a proper load bank and displays full power capability with no crossover distortion then you are OK there. Still the bias can be set cold, no crossover distortion and you can still get close to full power by cranking up the signal generator! So the scope is no the final test for bias setting.
Bias setting is not really a random number for 6L6GC or any tubes. Plate voltage must be measured and then the bias calculated for 70 percent of maximum plate power. That is a maximum bias setting and tubes will last longer if biased at a lesser value. If the amp sound OK bias at less than 70 percent, leave it there.
Always recheck the plate voltage and recalculate after setting bias. The higher you set the bias, the lower the plate voltage will be in most tube amps. Reason, they us unregulated power supplies.
If the amp still sounds bad I would strongly suspect speaker damage as the next possibility. |
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J Fletcher
From:
London,Ont,Canada
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Posted 12 Nov 2008 6:54 am
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| Is this a tube amp? My Xmas Tree was solid state...Jerry |
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Lee Jeffriess
From:
Vallejo California
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Posted 12 Nov 2008 9:09 am
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Ken, this amp is solid state.
Lee |
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J Fletcher
From:
London,Ont,Canada
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Posted 12 Nov 2008 9:22 am
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Hi Lee
Does the amp sound like there's something wrong, or broken, or does it just not sound as good as before?
Mine had a problem when I bought it, and was only putting out about 5 watts. Managed to sort it out. Put out around 50 watts, into 8 ohms, when it was working properly...Jerry |
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Lee Jeffriess
From:
Vallejo California
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Posted 12 Nov 2008 9:48 am
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Jerry Yes, it sounds like a little tube amp that is starting to get pushed.
Its not unpleasant,its just not right.
It sounded like this when I first got it, and the bias was down at 7mv.
I think somehow its got colder and im wondering if thats a possibility.
Lee |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 12 Nov 2008 8:26 pm
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| It sounds almost like the power supply has lost overall voltage. Electrolytics don't require a break-in period, but I'd do voltage checks throughout to make sure a new cap hasn't gone south already. |
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Lee Jeffriess
From:
Vallejo California
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Posted 13 Nov 2008 8:51 pm
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It was bias I think?.
Its back to sounding great, I had the service guy look at the bias.
The schematic calls for 20mv, and it was at 24.
So you would think it would sound ok?.
He reset it to 40mv, BINGO.
Its one of those amp X files.
Thank you Ken, Jerry and Donny for your input.
Lee |
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Jim Sliff
From:
Lawndale California, USA
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Posted 13 Nov 2008 8:58 pm
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Yeah, biasing a SS amp is actually FAR more critical - one or two ma can kill the tone. But since I usually avoid working on them I usually don't get involved.
| Quote: |
| Bias setting is not really a random number for 6L6GC or any tubes. Plate voltage must be measured and then the bias calculated for 70 percent of maximum plate power. That is a maximum bias setting and tubes will last longer if biased at a lesser value. If the amp sound OK bias at less than 70 percent, leave it there. |
Regarding Ken's comment on tube amps (I've made the same mistake and jumped in on a SS discussion) is solid for "Safety" but pay CLOSE attention to his last sentence, 'cause that's where the rubber meets the road. THAT is how you get the best tone, but I take it further and try a few ma up, a few down, and try to find the "sweet spot". As long as it's in safe operating range, you're fine - for amateurs it is an excellent idea to buy a Weber Bias Rite with the plate voltage option - you can read that and set the bias without opening up the chassis - much safer. If I recall correctly, Ken and I both use Allesandro Bias meters...or maybe Ken uses a Bias King. I love mine, but I think the Weber is a better deal.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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Lee Jeffriess
From:
Vallejo California
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Posted 13 Nov 2008 9:38 pm
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The funny thing with my situation Jim, was I was already a little warmer than the factory spec.
Though if you would have heard the amp it sounded under biased.
It now is in that sweet spot its very buttery.
I was just over at my friend Gar's house and he has a great sounding 59 bassman head with a couple of celestion bulldogs.
We A/B with his D10 Sho Bud, there wasn't much in it.
When this amp is dialed its hard to tell its SS.
One time this guy came up to the bandstand and said to me you gotta love 6L6s, I told him I was running EL34s (psycho acoustics)
Lee |
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Dave Zirbel
From:
Sebastopol, CA USA
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Posted 13 Nov 2008 11:01 pm
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Lee, if you give my that amp you won't have that problem any more and then your problem will be solved!  |
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Ken Fox
From:
Nashville GA USA
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Posted 14 Nov 2008 6:21 am
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I do have an old Bias King Pro. When I got it the Weber was not available! If I were buying again it would be the Weber.
Well now a transistor amp is far more critical about biasing, if it is adjustable. Just a slight bit too much and you are off to buy a new set of output transistors, been there!!!
If the bias is too hot the transistors will saturate too fast and full power will not be achieved. Also the output transistors will be hotter than usual while idling.
If the amp is not showing crossover distortion on the scope, the voltage rails are at there at full filtered capacity and you are getting full power on a test load your power section is probably just fine.
In an amp that old I would not hesitate to replace all audio coupling caps and electrolytic caps as well. I know you did the power filters but also look at any electrolytic caps used for audio coupling, if applicable! |
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Lee Jeffriess
From:
Vallejo California
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Posted 14 Nov 2008 9:45 am
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Ken, every electrolytic is about 2 years old.
All the voltages speced out in the pre, per the schematic.
In the power amp the rails are measuring 48 volts, and the schematic states 45 volts.
The tech doesn't believe any thing is being taxed.
Lee |
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