8len8
Active member
Anyone know how to adjust the bias on an Iconic? The manufacturer won’t tell me how (yeah, I know about high voltage safety).
glpg80
Well-known member
It’s not about high voltage safety. It’s one of those things that isn’t amp specific…either you have the proper tools to do it along with the skills to follow or you don’t. It falls under if you have to ask mentality.
With all of that said, are you simply asking where the adjustment itself is? If you don’t see it readily available external to the chassis then obviously you’re going to have to open it up.
Are you for sure they come with adjustments available? We are talking about a low budget amp here. It could be that you have to buy a specific idle plate current for a given plate voltage set of tubes, or mod it to bias. All things to consider FWIW.
With all of that said, are you simply asking where the adjustment itself is? If you don’t see it readily available external to the chassis then obviously you’re going to have to open it up.
Are you for sure they come with adjustments available? We are talking about a low budget amp here. It could be that you have to buy a specific idle plate current for a given plate voltage set of tubes, or mod it to bias. All things to consider FWIW.
G
Geo
Well-known member
Give us some gut shots when you have it open.
8len8
Active member
Looks kinda simple. Bias adjust pot, and a resistor labeled “bias”. I’m guessing I could just plug my bias meter in to do the adjustment.
G
Geo
Well-known member
I am glad they made it obvious, but sure do hate those tube sockets mounted to the board. You will be resoldering them eventually.
G
Geo
Well-known member
I have repaired many a Fender amp for that exact reason.
G
Geo
Well-known member
It depends on how much use the amp gets, and how many years you own it. If you use it 10 minutes a month, you will never have a problem.
8len8
Active member
Something doesn’t seem right.
My probe on one of the four 6L6GCs shows 387V and 24 mA.
70% calculations for that tube shows the bias should be 54 mA.
That’s a huge discrepancy. It can’t be biased that cold right now…
My probe on one of the four 6L6GCs shows 387V and 24 mA.
70% calculations for that tube shows the bias should be 54 mA.
That’s a huge discrepancy. It can’t be biased that cold right now…
mnemonic
Well-known member
31%, sounds about right, 5150s are typically biased cold and many manufacturers bias cold from the factory to cut down on failures.
My jet city jca50 was biased at I think 7% dissipation when I got it…
70% isn’t really a rule, colder might sound better.
My jet city jca50 was biased at I think 7% dissipation when I got it…
70% isn’t really a rule, colder might sound better.
311boogieman
Well-known member
Bias to tone not numbers. That's what I do. Not while the probe is in though.
8len8
Active member
This is stupid - the bias pot maxes out at 35 mA. That's nowhere near even 60%. Now I'll probably have to take it to a tech just to get the bias range re-done 
.
.
griff10672
Well-known member
so your plate voltage is 387 ? ??? is that amp rated at 100 watts or 120 ?? it gets tricky when considering the max dissipation of the tube ...
if the amp is rated at 100 watts I would consider the 6L6 as a 25 watt tube in my equation for bias ... if it's 120 ... then I would use 30 watts which is the actual max dissippation of a 6L6GC
so .... 25 divided by 387 x .7 = 45 mA ..... that should be a safe decent setting to go by .....
and you're saying it maxes out at 35 mA ?? Are you using a " matched set of Power Tubes ??
if the amp is rated at 100 watts I would consider the 6L6 as a 25 watt tube in my equation for bias ... if it's 120 ... then I would use 30 watts which is the actual max dissippation of a 6L6GC
so .... 25 divided by 387 x .7 = 45 mA ..... that should be a safe decent setting to go by .....
and you're saying it maxes out at 35 mA ?? Are you using a " matched set of Power Tubes ??
8len8
Active member
It’s rated as 80W. I assumed that was just because of the low plate voltage limiting max tube power. The tubes came with the amp new.
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griff10672
Well-known member
So even if you consider the 6L6 as a 20 watt tube ..... the equation still comes out to 36 mA .... and you're only getting 35 max ???
I don't know enough about the Iconic to know what's going on ..... like someone mentioned earlier ... they may want it Biased that cold from the factory for some reason ...
I don't know enough about the Iconic to know what's going on ..... like someone mentioned earlier ... they may want it Biased that cold from the factory for some reason ...
8len8
Active member
The designer has said it was intentionally biased at 30 mA from the factory so the power section is very clean and you can get a good modern metal “chunk” out of it without a soft low end.
I would have thought it could be set to a more typical value though. Time to get things modded…