Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: etneccas on November 05, 2017, 01:17:24 pm
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Bias too low 5e5a?
Recently completed a mojotone kit
I upgraded the circuit board to a Hoffman and installed a bigger 200 ma choke also installed 1 ohm resistors on pin 1,8 to ground on the power tube sockets
I have the stock power (TAD 6l6WGC’s)pre (eh v1 12ay7 v2&3 eh 12ax7’s)and rectifier (eh 5U4G)tubes that came with the kit
My volt reading are high but normal with the rail a419 b416 c335 d280
The heaters are 6.7vac
The pre’s are
V1 12ay7
1. 141
2. 0
3. 2.28
6. 139
7. 0
8. 2.3
V2 12ax7
1. 183
2. 0
3. 1.45
6. 279
7. 183
8. 185
V3 12ax7
1. 214
2. 0
3. 1.79
6. 268
7. 23.7
8. 66.7
V4 TAD 6L6WGC
1. 0
2. H
3. 420
4. 422
5. -45
6. -45
7. H
8. 0
V5 TAD 6L6WGC
Same as V4
V6 EH 5U4G
1. 0
2. 425VDC
3. 0
4. 350VAC
5. 0
6. 350VAC
7. 0
8. 427VDC
The bias reading on each power tube across the 1ohm resistor to ground on pin 1,8
is 29.6 mv(ma)...
The amp sounds anemic.....
The bias resistor is a 56k
If want to raise the bias higher, what value should I shoot for?
Also if I decide to install a pot for adjustment in lieu of the resistor what taper and value would be advisable?
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That 56k resistor needs to be smaller, or the 10k on the other side of the diode (to the left of the 56k) needs to be larger, assuming you built from this schematic: http://el34world.com/charts/Schematics/files/fender/Fender_pro_5e5a_schem.pdf
First off, you can always borrow an adjustable bias circuit from some other amp if you aren't satisfied with this one.
If you wanted to make this basic design adjustable, make that 56k adjustable. Maybe a 20k ohm (linear) pot in series with a 33k ohm resistor. Maximum resistance is real similar to where you're at now (53k total). Minimum resistance is significantly hotter.
Wire one end of the pot to the center (wiper). You're using it as a simple variable resistor, not a voltage divider, so you "could" just connect to the wiper and one end. But this improves safety... if it gets corroded or otherwise bad it will default to the maximum resistance across the whole pot, not infinite resistance due to the lost connection. So bias will default back to (roughly) what you're at now instead of -425v due to an open circuit, in the event that pot fails.
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I used the diagram and schematic that came with the mojotone kit.
They have a 6.8k resistor on the other side of the diode......
They also had conflicting info on the choke location hence why I upgraded the size of the choke and went with the location as per fender....
I was concerned about the value of the pot but It makes good sense what you are describing
29ma for bias is too low?
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Agree with 92Volts - install an adjustable fixed bias circuit and dial in the bias you want.
At 420V on the plates, you want a target bias range around -35V to -50V (aiming at -42V in the middle).
... 29ma for bias is too low?
420V x 0.029A = 12W, which is 53% for a 23W tube (like a real 5881), or 40% for a 6L6GC. Whatever tubes you are running, you want 70% for good sound.
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Why is my bias so low.just wondering if I did something wrong?
Do faulty tubes cause a low bias reading?
What else would cause a low reading?
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You have -45v on pin 5 of the 6L6s. That will make the 6L6s run kinda cold. You need to decrease that negative voltage (Fender schematic shows -32v) to make the tubes run hotter.
One easy way to do this would be to increase the value of that 10K resistor between the PT and the diode. Or you could decrease the size of that 56K resistor. (I'm looking at the Fender schematic) Better yet, follow 92Volts advice and replace the 56K resistor with a pot and series resistor and set the bias however you like.
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Replaced the 56k with a 47k and the bias is up to 35ma definitely an improvement.....
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Leave it running for a couple of hours at 35mA and see how that changes as the unit warms up.
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Will do!......thanks guys👍🏻
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Just ordered some 33k res and 25kl pot from Doug
I also ordered a 10k res to replace the 6.8k that on the mojotone schem/layout
To bring the amp in line with the fender schematic and to get the -45 v on the grid down lower too
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You probably will also want another 80V-100V 10uF cap (i.e. 'Cap#2' in the attached schematic) to decouple the bias pot wiper/tail resistor (depending on how you wire up the pot)
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I was planning on wiring as per 92volts advice
Just using the pot as a variable resistor in series with the new 33k resistor in place of the 56k resistor....
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I think it should sound fine and won't have excessive noise wired like stock with the resistor+pot substituted in for the 56k. Attaching another capacitor straight to the pot may reduce "scratchiness" or other noise while turning the pot, but the capacitor that's already there should do an alright job. Especially while it's stationary, not being adjusted.
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The reason that you would have an extra cap is to decouple any AC that might appear at the output node of the voltage divider, to keep the bias stable. This helps keep extraneous noise to a minimum (seeing as how the bias supply is coupled to the grids)
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Cap#2 is an electrolytic ? What is it’s orientation in your above diagram?
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I don't like that bias circuit. If the pot wiper fails the bias voltage will default to the lowest negative voltage. This may be too hot for the output tubes.
I like the Marshall circuit much better. If the pot fails the bias voltage defaults to the maximum negative voltage which will run the tubes cooler. No possibility of damage.
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🙄 I think I’ll use the pot to fine tune and then replace with a fixed resistance....
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Cap#2 is an electrolytic ? What is it’s orientation in your above diagram?
If you zoom in, you should be able to see the '+' sign. Positive pole to ground.
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I don't like that bias circuit. If the pot wiper fails the bias voltage will default to the lowest negative voltage. This may be too hot for the output tubes...
Depends how hot the 'hot' end of the bias range is set up for.
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Depends how hot the 'hot' end of the bias range is set up for.
Guess so. If the pot goes open though the bias goes to zero. That's smokin' hot! :wink:
Still like that simple Marshall circuit better than any bias circuit I've ever seen.
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You probably will also want another 80V-100V 10uF cap (i.e. 'Cap#2' in the attached schematic) to decouple the bias pot wiper/tail resistor (depending on how you wire up the pot)
So move the grid wire after the pot and keep the 100uf cap#1 before the pot ?
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I’ve seen The grid wire before the pot on some bias diagrams
What’s better?