Amp Bias/Tube Questions

[aaronl]

Hey guys, I have a Sovtek Mig 100h I am actually pretty happy with the amp, lots of people have heard it and were impressed as well.

Anyway the question is, the 4 6L6s are named Selectron, never heard of em.

I checked the bias with my amp-head bias meter and they read (from left to right) 1,2,3,4 then the transformer or whatever that big old thing is
32,32,41,38
I am guessing that is way off, the eurotube site says I should be between 36-42, would a new quad make a big diff in sound quality??

I have a Telefunken ECC83/12ax7 in the furtherest left tube slot then GE JAN12AX7WA in the other 2 and i was told the one nearest the tubes was actually the phase inverter, I am guessing these are ok for now.

Anyway, any help ideas insight would be great!

 

[metlking]

Re: Amp Bias/Tube Questions

Are your tubes old...meaning a few years? Your bias readings are not extremely off balance, I have seen much worse! A new set of tubes might make a difference but probably not night and day to the average ear, unless they're just plain worn out! Either the tubes are old and have "drifted" or they really weren't matched all that well to begin with. I've never heard of that brand either..........I'm not familiar with your amp so I don't know which preamp tube is V-1, thats where you should put the Telefunken. It's most likely the one closest to the input....furthest from the powertubes.

 

[aaronl]

Re: Amp Bias/Tube Questions

Are you're tubes old...meaning a few years? Your bias readings are not extremely off balance, I have seen much worse! A new set of tubes might make a difference but probably not night and day to the average ear. Either the tubes are old and have "drifted" or they really weren't matched all that well to begin with. I've never heard of that brand either..........I'm not familiar with your amp so I don't know which preamp tube is V-1, thats where you should put the Telefunken. It's most likely the one closest to the input....furthest from the powertubes.

Then the telefunken is in V1
I really do not know how old the tubes are, they are odd to me at least they have some silver around the bottom but none on top like most I have seen and they have just a little different look in the inside than I am used to.

What should I look at getting if I get new?
TAD, JJ, Sovtek, EH ???

 

[metlking]

Re: Amp Bias/Tube Questions

I've tried and really liked the TAD 6L6GC-STR....I buy from www.dougstubes.com
Contact him, he has a knack for picking out the right tube to get you the sound you're after. Nice guy too! I'd stick with the preamp tubes you've got for now.......as long as they're not microphonic.

 

[missionamps]

Re: Amp Bias/Tube Questions

Hey guys, I have a Sovtek Mig 100h I am actually pretty happy with the amp, lots of people have heard it and were impressed as well.

Anyway the question is, the 4 6L6s are named Selectron, never heard of em.

I checked the bias with my amp-head bias meter and they read (from left to right) 1,2,3,4 then the transformer or whatever that big old thing is
32,32,41,38
I am guessing that is way off, the eurotube site says I should be between 36-42, would a new quad make a big diff in sound quality??

I have a Telefunken ECC83/12ax7 in the furtherest left tube slot then GE JAN12AX7WA in the other 2 and i was told the one nearest the tubes was actually the phase inverter, I am guessing these are ok for now.

Anyway, any help ideas insight would be great!

It's not way off.
Take either the 38ma or the 41ma tube out of it's socket and mate it with one of the 32ma tubes so at least the output transformer has a closer balance of DC power tube current in it while idling.
Here's the bad news... since you don't know the quality of the tubes, you really need to know the B+ voltage on the power tubes while they are idling in the circuit.
Some of those Sovtek amps had high B+ voltage and you need to make sure your power tubes are not idling with too high current vs high plate voltage.
As an example, if any particular classs AB amp was running it's power tubes at 550vdc, then you wouldn't really expect to see any of the power tubes idling at much more them about 32ma each.

 

[aaronl]

Re: Amp Bias/Tube Questions

Hello Bruce, it's Aaron, I called and emailed you a few times!

I guess I need to check what you said, the Eurotube site shows how to do some of this stuff, so I may give it a go. OR I may go to a guy here that said he would look over things for me.

Does this video show how to check what you are talking about??
http://www.eurotubes.com/euro-Generic-Bias.htm

Thanks again guys, BTW Bruce is one of the coolest guys I have ever talked to, when I get enuf together to get a new amp I am going to call Bruce and have him help me work something out, VERY VERY helpfull all for free

It's not way off.
Take either the 38ma or the 41ma tube out of it's socket and mate it with one of the 32ma tubes so at least the output transformer has a closer balance of DC power tube current in it while idling.
Here's the bad news... since you don't know the quality of the tubes, you really need to know the B+ voltage on the power tubes while they are idling in the circuit.
Some of those Sovtek amps had high B+ voltage and you need to make sure your power tubes are not idling with too high current vs high plate voltage.
As an example, if any particular classs AB amp was running it's power tubes at 550vdc, then you wouldn't really expect to see any of the power tubes idling at much more them about 32ma each.

 

[aaronl]

Re: Amp Bias/Tube Questions

Here's the bad news... since you don't know the quality of the tubes, you really need to know the B+ voltage on the power tubes while they are idling in the circuit.
Some of those Sovtek amps had high B+ voltage and you need to make sure your power tubes are not idling with too high current vs high plate voltage.
As an example, if any particular classs AB amp was running it's power tubes at 550vdc, then you wouldn't really expect to see any of the power tubes idling at much more them about 32ma each.

OK I made the tubes line up to:
40,31,31,39 that was after about 5 mins of just on and off standby but of course not playing.

I used that eurotube site and read the pin #3 to get the plate voltage. I used 600 DC V on my multi-meter and ended up with 504 on pin 3.
Then after using his formula I did this
25 / 504 to get .049
then multiplied that by .7 to get to 70% and got .0347 or 35 so i would guess that I am pretty darn close over all to being right since the numbers above average out to 35.25 (the tube readings)

Did I do any of this wrong?

Pin #3 had wires on it and I could only get to 2 of the 4 but they both read at the same number.

Thanks for any ideas, info etc

Note: I was nervous as i could be with the amp apart poking around in there but it was a good feeling to do it right and start to figure this stuff out a little at least (with the help of many others!)

 

[wixedmords]

Re: Amp Bias/Tube Questions

I think you did a good job aaronl.

The equation you are talking about is in my sig.

What I usually do is do the equation based on 50% dissipation and 70% dissipation to find the 2 bias settings. The 70% is what you have already done. From that I turn the pot ever few mA to find out which setting between 50% and 70% dissipation I like the sound of and if there is a tie, I use the lower setting.

 

[aaronl]

Re: Amp Bias/Tube Questions

I think you did a good job aaronl.

The equation you are talking about is in my sig.

What I usually do is do the equation based on 50% dissipation and 70% dissipation to find the 2 bias settings. The 70% is what you have already done. From that I turn the pot ever few mA to find out which setting between 50% and 70% dissipation I like the sound of and if there is a tie, I use the lower setting.

Thanks!!

I am messing around and was wondering if X brand 6L6 will have higher plate voltage than Y brand? Or if a set of 5881s would be different?

I ask because it seems the higher the bias the lower the plate voltage right??

I am trying to learn, and REALLY being very careful and slow.

Should I have used 30watts for the tubes instead of 25??

thanks again!!!