Can someone explain why you need to re-adjust the balance on a tube amp every time

[ag8908]

Can someone explain why you need to re-adjust the balance on a tube amp every time you change the tube?

I purchased a cheap Little Bear P1 amp from china (which works pretty well actually) and I've been messing around by switching the tube (the tubes cost alas much as the amp lol) but every time I switch a tube I need to readjust these two screws circled in the attached picture in order to get the left-right balance close to right.

Does anyone understand why each tube re-adjusts the left-right balance?

Also, is there a precise method to ensure that you have the right left-right balance? One disturbing thing about this amp is that the left-right balance seems to vary slightly depending on the sound frequency and I'm never sure I have it right. I'll adjust it by ear for one song and then it will seem slightly off on another song.

Overall though I like the tube sound. I'm just not sure why.

 

[funk1980]

Tubes have widely varying specs. Even the triodes in a single envelop can vary. The surrounding circuitry should account for this, but it seems it doesn't.

I assume these are bias pots. In that case, you're not adjusting the balance, but the tube's operating point. The operating point does way more than just set the output level. If you're biasing the tube too hot or cold, the sound level will decrease (first for the frequencies with the biggest voltage swing, i.o.w. the bass frequencies), but also the linearity (distortion) and output impedance among others will be altered. It's best to set the operating point of the tubes the same and asymmetrically attenuate the signal somewhere else in the chain.

Chapter 1.9:
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

 

[ag8908]

Tubes have widely varying specs. Even the triodes in a single envelop can vary. The surrounding circuitry should account for this, but it seems it doesn't.

I assume these are bias pots. In that case, you're not adjusting the balance, but the tube's operating point. The operating point does way more than just set the output level. If you're biasing the tube too hot or cold, the sound level will decrease (first for the frequencies with the biggest voltage swing, i.o.w. the bass frequencies), but also the linearity and output impedance among others will be altered. It's best to set the operating point of the tubes the same and asymmetrically attenuate the signal somewhere else in the chain.

Chapter 1.9:
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

Thanks. Basically there are two screws, and you can turn them counterclockwise or clockwise until they stop (I would guess you can turn them about 270 to 360 degrees total before you can't turn them anymore in either way). Any rule of thumb for setting these two screws, other than by ear?

Edit: I just discovered that if I turn them both all the way down, the volume is lower than if I turn them all the way up.

 

[ag8908]

One more question, is this something you have to do with all tube amplifiers, or do the expensive ones automatically adjust these for you?

Please note I'm driving a pretty new HD800 so I don't want to be ruining it with volts or something.

Thanks in advance.

 

[funk1980]

Thanks. Basically there are two screws, and you can turn them counterclockwise or clockwise until they stop (I would guess you can turn them about 270 to 360 degrees total before you can't turn them anymore in either way). Any rule of thumb for setting these two screws, other than by ear?

Yes, measure the voltage between the cathode and grid of each triode and adjust the screw until you reach the desired voltage. This voltage depends on the tube used, HT, personal taste, etc.

 

[ahankinson]

Hook up a DMM and measure the voltages.

 

[funk1980]

One more question, is this something you have to do with all tube amplifiers, or do the expensive ones automatically adjust these for you?

Depends on a lot of factors. Not just cheap vs. expensive. What type of bias is used, what topology, what stage in the amp, designers choice, etc.

 

[funk1980]

Looking at the amp, it seems to be a low voltage design (24VDC). This region tends to be more non-linear than when it's ran at (the usual) high voltage. This will increase the varying levels you're experiencing. For 'HiFi' amplification, tubes like to be ran at high voltages.

 

[ag8908]

Ok thanks this is all helpful.

I discovered that if I just set both of these dials to the max, then the balance is right although there is a tiny bit of static in the headphones.

So this is good for listening, but I just want to make sure I'm not damaging the HD800s. Is that a possibility?

Any rule of thumb for where I should set the left and right dial (apparently if they're set at the same level the left and right balance is about right) for a 12A7 bulb?

Thanks again. Please answer the damage question. I don't want to damage a $1700 (tax) with an amp I purchased out of curiosity.

 

[ag8908]

Hook up a DMM and measure the voltages.

I might get one later but I don't have one right now. I assume these things aren't expensive.

 

[funk1980]

Dude! With all do respect to your current purchase, but IMO the HD800's deserve better! Those are some awesome cans!

Without any schematic, component values, etc, the 'correct' position of the bias pots is anybody's guess. I doubt you could do any damage to your headphones though, but not knowing the amp, I can never be 100% sure.

 

[ag8908]

Looking at the amp, it seems to be a low voltage design (24VDC). This region tends to be more non-linear than when it's ran at (the usual) high voltage. This will increase the varying levels you're experiencing. For 'HiFi' amplification, tubes like to be ran at high voltages.

Yes the power supply says 24v DC.

Are you saying I should turn both dials all the way up (highest volume) for the best sound? That's the way I have them now.

I really never knew tube amps had all these issues.

 

[ag8908]

Dude! With all do respect to your current purchase, but IMO the HD800's deserve better! Those are some awesome cans!

Without any schematic, component values, etc, the 'correct' position of the bias pots is anybody's guess. I doubt you could do any damage to your headphones though, but not knowing the amp, I can never be 100% sure.

Another website claims you can kill yourself adjusting these bias pots. https://www.tube-town.net/cms/userfiles/media/bias/bias-setup-e.pdf

I can't believe an expensive $1,000 tube amp would have these issues as well.

 

[funk1980]

Hahahaha, I'm not dissing your Little Bear, but it doesn't quite qualify for a proper tube amp.

Concerning the dials, they're usually not designed to be set to the extremes, but rather somewhere around the middle. Between e.g. 10 and 2 o'clock. But again, without a manual, schematics or anything like that, there's really no saying. Measuring some voltages could shed come light.

 

[ag8908]

Hahahaha, I'm not dissing your Little Bear, but it doesn't quite qualify for a proper tube amp.

Concerning the dials, they're usually not designed to be set to the extremes, but rather somewhere around the middle. Between e.g. 10 and 2 o'clock. But again, without a manual, schematics or anything like that, there's really no saying. Measuring some voltages could shed come light.

Yes middle seems to be a good rule of thumb for all life situations. All the way up sounds good though. Let me try it middle.

 

[funk1980]

Another website claims you can kill yourself adjusting these bias pots. https://www.tube-town.net/cms/userfiles/media/bias/bias-setup-e.pdf

I can't believe an expensive $1,000 tube amp would have these issues as well.

'Normal' tube amps use voltages in excess of 250 volts. Even 500 volts are more are not uncommon. These voltages will kill you. 24 volt, not so much. But be careful, the Little Bear might have some boost SMPS, but I'd need the schematic to verify. A DMM will tell you more.

A proper tube amp runs the tubes in an operating region where the effects your experiencing are negligible, and the surrounding circuitry can decrease this even further. But with 24V (I'm assuming), you're so close to the edge of the tube's working region, there's loads of non-linearities and other non-desirable effects for HiFi.

 

[mcandmar]

Are you seriously plugging a pair of HD800's into this piece of ****? ..and i mean that in good humor as i own one and have tweaked it into a decent sounding fun little amp. But seriously i wouldn't plug any headphones of value into it.

This amp is also sold as an Indeed G2, here is a very useful thread on how to set the bias and tweak the amp in general Indeed G2 tweaks and bias adjustment | Rock Grotto

If your going to do anything ditch the stock IRF630 fets to IRF or IRL 510's, the difference is huge, add some resistors across the headphone jack for safety (to your headphones, not to you), and throw a few decent caps into it, the ones supplied are utter rubbish. If you really want to keep it and use it regularly go ahead and replace all the resistors and film caps with known quality items, just about every part is as cheap as they come.

The biggest issue i had with mine was mains hum as the supplied switching power supply was just nasty, when i looked inside the filtering cap and choke for the mains input filter, and the output choke were all missing and jumpered over with wire. That's how much they penny skimped when building this thing

 

[ag8908]

'Normal' tube amps use voltages in excess of 250 volts. Even 500 volts are more are not uncommon. These voltages will kill you. 24 volt, not so much. But be careful, the Little Bear might have some boost SMPS, but I'd need the schematic to verify. A DMM will tell you more.

A proper tube amp runs the tubes in an operating region where the effects your experiencing are negligible, and the surrounding circuitry can decrease this even further. But with 24V (I'm assuming), you're so close to the edge of the tube's working region, there's loads of non-linearities and other non-desirable effects for HiFi.

Oh I see. You're saying this is a consequence of it not having much power.

Well I set them both around the middle and it really didn't sound as good as with both of them turned all the way up (i.e. highest volume) so I turned them back up.

Oh well my HD800 is under warranty.

I'm also trying to figure out why I like the tube sound. Not sure why it sounds more "human" than solid state amps, which seem cold and dry.

 

[ag8908]

Are you seriously plugging a pair of HD800's into this piece of ****? ..and i mean that in good humor as i own one and have tweaked it into a decent sounding fun little amp. But seriously i wouldn't plug any headphones of value into it.

This amp is also sold as an Indeed G2, here is a very useful thread on how to set the bias and tweak the amp in general Indeed G2 tweaks and bias adjustment | Rock Grotto

If your going to do anything ditch the stock IRF630 fets to IRF or IRL 510's, the difference is huge, add some resistors across the headphone jack for safety (to your headphones, not to you), and throw a few decent caps into it, the ones supplied are utter rubbish. If you really want to keep it and use it regularly go ahead and replace all the resistors and film caps with known quality items, just about every part is as cheap as they come.

The biggest issue i had with mine was mains hum as the supplied switching power supply was just nasty, when i looked inside the filtering cap and choke for the mains input filter, and the output choke were all missing and jumpered over with wire. That's how much they penny skimped when building this thing

Yes there's definitely line noise, but that just adds to the old school feel of the music. lol.

Can you think of a way it could damage my HD800s?

 

[ag8908]

mcamdar, I just wanted to see what a tube sounds like, and I like it although I'm not sure why. I'll probably be getting an expensive tube amp, although I want to find one where I don't have to mess with this stuff.

HOLY S MY HD800 JUST CAUGHT FIRE ON MY HEAD, RUNNING AROUND MY APARTMENT. just kididng.

but seriously if there's a way this could damage a headphone I'd like to know. I don't understand the technical stuff you linked at all unfortunately.