Bias too low 5e5a?

[etneccas]

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?

[alexradium]

you can substitute the 56k with a trimmer 20/25k with a 33k resistor in series,it should work fine,those tubes can sound pretty good at 50% dissipation.
200 mA choke sounds a bit overkill.

[etneccas]

The choke that came with the kit was a mojo 777 Which I was told is undersized and would burn up if I was going with the Fender 5e5a schematic.......so I was advised to use a Hammond T193H 5henry 200ma.
I installed a 47K in place of the 56K and the bias is up around 35ma now
Definitely an improvement but may still be to low
I’ll have to try the potentiometer to see if I can further approach 70% of 6l6wgc current

[etneccas]

What milliamperes would be ideal for TAD 6L6 WGC’s?

[mozz]

https://robrobinette.com/Tube_Bias_Calculator.htm

[etneccas]

👍🏻

[nickb]

The amp sounds anemic.....

With about 30mA of bias per tube that is not the reason for this problem. What exactly is the problem? Low power? Thin (lacking bass)? What speaker are you using/ have you tried a different one? Was it always "anaemic". Or what?

What test equipment do you have?

[Enzo]

And don't confuse low volume and low power. It takes the entire amp circuit to make sound come out the speaker, so it isn't always just the last part - the power amp - that is at issue.

Your amp could be oscillating at RF. It will still pass signal that way, but the sound tends to sound "washed out" and weak.

I agree with nickb, I doubt the bias is your problem.

[etneccas]

Volume is fine and I have plenty of power....
What kind of sound response would a weak bias ((29ma)) give?
Like I said I replaced the 56k resistor with a 47k resistor and noticed an improvement in the response and attack.The notes seem to bloom more......Question.....wouldn’t it make better sense to change the mojotone schematic 6.8k to the fender Schematic 10K first though?......

[Enzo]

The gain monster Peavey 5150 head has a stock idle current of 11ma to 15ma. And it will blow the hair off your head.

Is there a schematic you can post? Never mind, I shuffled through the old Fenders until I found which one was 5E5. It's a Pro.

I am not sure why it would make more sense, the whole thing is just a voltage divider, you change the resistors, you change the division. Raise one, reduce the other, kinda doesn't matter which one.

[nickb]

Volume is fine and I have plenty of power....
What kind of sound response would a weak bias ((29ma)) give?
Like I said I replaced the 56k resistor with a 47k resistor and noticed an improvement in the response and attack.The notes seem to bloom more......Question.....wouldn’t it make better sense to change the mojotone schematic 6.8k to the fender Schematic 10K first though?......

No. The value depends on the tube specifics and the transformer. I would not call 30mA weak by a long way. You did respond that it has enough power but we still don't know what it is you don't like. Please tell us otherwise we're just guessing.

If you the raise the bias current you also increase the gain of the power tubes and of the amp as a while and you seemed to like that. So, why not swap out the 12AY7's for two 12AX7's and get more gain and see if you like it? You could add a capacitor, say 2.2uF 25V, across the 1500 ohm cathode resistor of the first half of the 12AX7 see CapCal

If that does not do what you want, then I suspect you're missing lower frequencies. Nothing in the circuit leaps out at me as throwing away the low end so I suspect the choice of speaker. What are you using?

[The Dude]

I agree with the above. Many Fender amps are biased at 30mV per tube. I doubt bias is your problem. As has been asked, what is the specific problem with the amp?

[christarak]

I agree with the above. Many Fender amps are biased at 30mV per tube. I doubt bias is your problem. As has been asked, what is the specific problem with the amp?

The Dude is right. You can't bias a tube using current alone. You have to monitor the Plate voltage at the same time, which will probably decrease as the idle current increases. Measure Plate voltage, multiply it by your idle current and that will give you your idle power dissipation. Divide that value by the power of your valve (eg 30w). You want to aim for a % of 50-65%.

[pdf64]

The Dude is right. You can't bias a tube using current alone. You have to monitor the Plate voltage at the same time, which will probably decrease as the idle current increases. Measure Plate voltage, multiply it by your idle current and that will give you your idle power dissipation. Divide that value by the power of your valve (eg 30w). You want to aim for a % of 50-65%.

Actually, I think it's completely valid by adjust bias by monitoring plate/cathode current alone; the only exception to that would be amps with super high HT voltage, which isn't the case here.
I don't see any no technical reason why idle cathode current needs to be any higher than 30mA.
Plate dissipation is a limit, not a characteristic or parameter of tube operation.
The only thing that matters about plate dissipation is that the limit isn't exceeded during operation.
A tube's characteristics don't change according to how hot its plate is.
The point of Aiken's 'bias to idle at 70% of the limit' guideline is that guitar amps were generally designed to push their tubes to their limit during operation, so by idling at 70% or less, the limit shouldn't be breached.

Assuming that the original has been cloned competently, note that this amp was designed for 6L6GB with a 23 watt plate; there's no technical reason why 6L6GC should be run any hotter in there than would be reasonable for a 6L6GB.
http://schems.com/schematicheaven.ne...5e5a_schem.pdf

@etneccas when reporting voltages in an amp, the key context is the actual measured mains and heater voltages at that time. If the mains is eg 10% higher than the PT primary was intended for, all PT secondary voltages will be ~10% higher than expected. The heater voltage may be used as a reference point for that assessment to be made.

[christarak]

Actually, I think it's completely valid by adjust bias by monitoring plate/cathode current alone; the only exception to that would be amps with super high HT voltage, which isn't the case here.
I don't see any no technical reason why idle cathode current needs to be any higher than 30mA.
Plate dissipation is a limit, not a characteristic or parameter of tube operation.
The only thing that matters about plate dissipation is that the limit isn't exceeded during operation.
.

Very True, pdf64. 30mA is a good rule of thumb. At Vp=385v that's an idle dissipation of 50%. Even at 450v its under 60%. I must admit that I would not feel comfortable setting an idle power dissipation of 70% despite Aiken's excellent rationale. I don't like the sound when a tube is biased that hot, but that's my personal taste.

For my personal amps, I go for my "Goldilocks setting". I bias with a 100-150mV sine wave and increase the output volume until I see the peaks limiting. I then reduce the bias voltage until I can just see cross-over notching and then increase the bias to about midway. I then check the cathode current (I allow about 5mA for screen) and plate voltage to make sure I am inside my happy place (50-65%). I generally like the sound of this setting, but give myself the liberty of tweaking by ear.