6505+ Bias check and comparison

[::XeS::]

Today I checked the bias in my new 6505+.
Initially I put my Bias probe in the 2 middle tubes (it has only 2 probes), I set the bias and next I put the bias in the external tubes.
The result with the standard bias settings are:

V1 V2 V3 V4

Plate voltage 490 490 495 492

Bias current 14.3 14.5 25.7 22.7

I toke an average Plate voltage of 493 for the formula so:

Ik = (30/493)*0.7*1000 = 42.6 A

The bias current should be 42.6A circa. So my bias current was pretty cold as usual for a 5150.
Actually I've not the bias mode so the bias pot can't go close to the ideal current value so I maxed it and the result now are:

V1 V2 V3 V4

Plate voltage 490 490 495 492

Bias current 22.3 22.8 36.6 33.6

As you can see, the power tubes are totally unmatched and it's not very good...and of course, the current can't reach the ideal value.
Anyway, I recorded 2 samples. One with the standard bias and one with the new bias.

Cold bias: http://dl.getdropbox.com/u/1436721/Sym_5150_Cold.mp3
Actual bias http://dl.getdropbox.com/u/1436721/Sym_5150_Hot.mp3

 

[streetwise]

Today I checked the bias in my new 6505+.
Initially I put my Bias probe in the 2 middle tubes (it has only 2 probes), I set the bias and next I put the bias in the external tubes.
The result with the standard bias settings are:

V1 V2 V3 V4

Plate voltage 490 490 495 492

Bias current 14.3 14.5 25.7 22.7

I toke an average Plate voltage of 493 for the formula so:

Ik = (30/493)*0.7*1000 = 42.6 A

The bias current should be 42.6A circa. So my bias current was pretty cold as usual for a 5150.
Actually I've not the bias mode so the bias pot can't go close to the ideal current value so I maxed it and the result now are:

V1 V2 V3 V4

Plate voltage 490 490 495 492

Bias current 22.3 22.8 36.6 33.6

As you can see, the power tubes are totally unmatched and it's not very good...and of course, the current can't reach the ideal value.
Anyway, I recorded 2 samples. One with the standard bias and one with the new bias.

Cold bias: http://dl.getdropbox.com/u/1436721/Sym_5150_Cold.mp3
Actual bias http://dl.getdropbox.com/u/1436721/Sym_5150_Hot.mp3

WHAT? Bias current can't be >40A, no way And what's that Ik equation?

EDIT: Actually it's the correct equation, but you don't want to put that *1000 in there. You are measuring mA's.

Plate dissipation = plate voltage * plate current (in amperes). With a bias probe you are measuring cathode current though, so your tubes will actually run a bit colder than what your bias meter says.

Your plate dissipation on V4 tube is 16,5 watts (492V * 0,0336A). You could bias it up to 21 watts. That would be 43mA plate current. Your tubes are apparently matched pairs because the other pair has so much bigger values... Get a matched quad when you next replace your tubes.

If you'd like to bias the amp hotter, swap R68 (located on a little piece of PCB on the back side of the amp, it's originally 12K) to a smaller value. I think 8,2k would be ok. This will let more of the bias voltage go to ground, and plate current will increase.

 

[Tachy]

Yes, place an 8 Kohm resistor an d swap the 12K, this is what I do with mine

 

[::XeS::]

Sorry, I mean mA of course.
The equation is Ik = (MPD / Plate voltage) x 0.7 x 1000.

MPD , Tube Max plate dissipation
Plate voltage is 493 (in my case)
0.7 is the 70%
the 1000 is to have the right value you have to use because (30/493)*0.7= 0,0425 * 1000 = 42,5mA
But, anyway, I have a doubt. 6L6's have 25w or 30w of max plate dissipation?

 

[streetwise]

Sorry, I mean mA of course.
The equation is Ik = (MPD / Plate voltage) x 0.7 x 1000.

MPD , Tube Max plate dissipation
Plate voltage is 493 (in my case)
0.7 is the 70%
the 1000 is to have the right value you have to use because (30/493)*0.7= 0,0425 * 1000 = 42,5mA
But, anyway, I have a doubt. 6L6's have 25w or 30w of max plate dissipation?

5881/6L6 is 20 watts, 6L6GC is 30 watts.

 

[::XeS::]

Ok I used the right value. Of course regarding the matched tubes, it's not good to have 2 different couples of matched pair.
Today I'll open the amp to check the resistor...it was already a scheduled mod

 

[GuitarHack]

5881/6L6 is 20 watts, 6L6GC is 30 watts.

Interesting info refarding Sovtek 5881
http://www.harpamps.com/micKtubes/Sovtek-5881-specs.html

I have some Sovtek 5881 in my Krank biased at around 22W at the moment with no Red-plating or other issues. Maybe the Sovtek 5881's are actually rated at 30W.

::XeS:: I had soem ruby 6L6 that were poorly matched in my 6505. They sounded almost identical to JJ6L6's though. Just the poor matching made them harder to bias. One of them did die when biased a tad hot though. The JJ's seem to take a hotter biasing a little better. Haven't killed any of them.

 

[nwright]

Yes, place an 8 Kohm resistor an d swap the 12K, this is what I do with mine

What does this do to the sweep of the bias adjustment knob on the 6505+?

I'd be interested in doing this mod if I knew that the bias adjustment knob still functioned pretty well, like if the coldest setting on the knob AFTER the mod was about where the hottest setting was BEFORE the mod. Make sense?

Did you do any tests to see the change?

 

[::XeS::]

I think the mod allows the trim to extend its range (more sensibility when you turn the trim and bigger range than before).
It's a pretty simple mod in the 6505+ because you have not to work near the caps but there is a little pcb under the trim pot and there you can find the resistor to change.

 

[DevilJaw]

yeah the stock bias range gave me a maximum of 14-15 mA.

 

[DigiV]

what's the sweet spot for the 5150s?

i'm told around 35-42mA

 

[::XeS::]

It depends by the Plate voltage because every amp has its Plate Voltage. You can find 2 5150's with a different plate voltage

 

[DigiV]

i thought they consistently drew around 480?

 

[Wolfeman]

i thought they consistently drew around 480?

Due to competents tolerance and age, things will always slightly differ.....

 

[streetwise]

What does this do to the sweep of the bias adjustment knob on the 6505+?

I'd be interested in doing this mod if I knew that the bias adjustment knob still functioned pretty well, like if the coldest setting on the knob AFTER the mod was about where the hottest setting was BEFORE the mod. Make sense?

Did you do any tests to see the change?

Swapping the resistor will move the bias pot range (both minimum and maximum position) to hotter. If you REALLY need more range, put in a bigger value trim pot in addition to the resistor swap.

Due to competents tolerance and age, things will always slightly differ.....

Yes... But the difference of a few volts doesn't matter. Especially if you're using a bias probe to measure plate current from cathode ...

 

[Wolfeman]

Yes... But the difference of a few volts doesn't matter. Especially if you're using a bias probe to measure plate current from cathode ...

Oh trust me I know, I've built amps

I was just explaining to him why it can differ...

 

[streetwise]

Oh trust me I know, I've built amps

I was just explaining to him why it can differ...

Yeah I see.

Someone asked what's the sweet spot for bias in a 5150: I don't know but you should bias by ear. First make sure you have enough range to adjust bias, say your trimmer goes from like 15mA to 45mA. (Don't use those bias measurement connectors on the back of the head, they tell you nothing about plate dissipation!! Use a bias probe if you're uncertain how to measure plate currents with a DMM). Then have someone play guitar through the amp while you fiddle with the bias trim pot. When you've found the sweet spot for the trim, measure plate dissipation again to see that the tubes aren't running too hot (anything less than 70% will be okay).

 

[::XeS::]

What do you think about these schemes http://www.fjamods.com/5150Models.html ?
For the 5150II, the layout is in order: V6, V1, V2, V5, V3, V4 .....
What does it mean? That the V1 tube is in the 2nd slot? And that the Phase Inverter are in the 1st slot? I can't understand..

 

[TheWinterSnow]

What do you think about these schemes http://www.fjamods.com/5150Models.html ?
For the 5150II, the layout is in order: V6, V1, V2, V5, V3, V4 .....
What does it mean? That the V1 tube is in the 2nd slot? And that the Phase Inverter are in the 1st slot? I can't understand..

the tubes go from right to left in order, the first slot is V6 and they move to the left

V6 First Gain Stage
V1 Second Gain Stage
V2 Third Gain Stage
V5 Tone Stack
V3 FX Loop
V4 Phase Inverter

the phase inverter is the closest tube to the power tubes and first tube from your guitar input comes from is the furthest away from the power tubes (and actually the closest tube to the input jack)

 

[Metaltastic]

Wait, are you fucking kidding me? What could possibly be the logic of doing it that way? (rather than *gasp* going sequentially up in number from left to right, arrggghhh) I'm not doubting you dude, I just hate needlessly confusing things! (or maybe there's some profound wisdom to it that my plebeian brain can't grasp )