Well,
Looking at the Peavey...Just for interest the amp has flyback (Back EMF) protection diodes..eg cr200/cr201 cr202.cr203..not important just interesting..
OK so the bias is via fixed resistors R107/R108...connected to Pin 2 of J211 connector..this feeds the grids via r229 and r234/206 and 211
Bias is also connected via pin 2 of connector J201 for the other channel. No adjustment that I can see..Unless someone else knows any different..
So it would seem that peavey don't regard this bias as adjustable it is one supply connected to to many tubes and no seperate adjustment is provided.
Regards
M. Gregg
Looking at the Peavey...Just for interest the amp has flyback (Back EMF) protection diodes..eg cr200/cr201 cr202.cr203..not important just interesting..
OK so the bias is via fixed resistors R107/R108...connected to Pin 2 of J211 connector..this feeds the grids via r229 and r234/206 and 211
Bias is also connected via pin 2 of connector J201 for the other channel. No adjustment that I can see..Unless someone else knows any different..
So it would seem that peavey don't regard this bias as adjustable it is one supply connected to to many tubes and no seperate adjustment is provided.
Regards
M. Gregg
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I read a suggestion somewhere to split and replace the 47K resistor with maybe a 27k pot and a 20 k resistor in series for adjustable bias. that way the pot wouldn't be able to zero bias with the 20k resistor in series as a safety resistor.
The mod as you describe,
Would only allow a reduction in bias supply and increase in idle current.
If you use a 30K resistor and 27K pot then you could increase or reduce bias supply and so increase or reduce idle current. The pot track would have to be connected as a standard resistor and wiper connected to the one end. Same as HK did it. I would probably change R107 same value to a 2 Watt ...kiwame are small sized.
You could now split J214/J209 and measure current for four tubes at idle or split J210/213/215/216 and measure for two tubes on each side..However the adjustment you have would only change "all tube bias" at the same time.
It depends how far you want to go..by creating another new R107 and R108 c110 and taking each one to pin 2 J211 and J201 you could bias the two sets of 4 tubes separately and so have more control..even with this you can only set bias for each set of four OP tubes.
I would probably remove the 47K and leave the resistor between the caps to give a filter then create two resistor /pot networks after the last bias cap for each channel..as long as you can adjust above the same negative value you have now for all the tubes..again make sure its fail safe.
Again remember you are measuring B+ current.
Regards
M. Gregg
Would only allow a reduction in bias supply and increase in idle current.
If you use a 30K resistor and 27K pot then you could increase or reduce bias supply and so increase or reduce idle current. The pot track would have to be connected as a standard resistor and wiper connected to the one end. Same as HK did it. I would probably change R107 same value to a 2 Watt ...kiwame are small sized.
You could now split J214/J209 and measure current for four tubes at idle or split J210/213/215/216 and measure for two tubes on each side..However the adjustment you have would only change "all tube bias" at the same time.
It depends how far you want to go..by creating another new R107 and R108 c110 and taking each one to pin 2 J211 and J201 you could bias the two sets of 4 tubes separately and so have more control..even with this you can only set bias for each set of four OP tubes.
I would probably remove the 47K and leave the resistor between the caps to give a filter then create two resistor /pot networks after the last bias cap for each channel..as long as you can adjust above the same negative value you have now for all the tubes..again make sure its fail safe.
Again remember you are measuring B+ current.
Regards
M. Gregg
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Thanks M. for looking over the schematics and taking the time to figure it out. I'm ok with adjusting the current for all 4 tubes at the same time. I'm only concerned about keeping the bias at a safe level, not individual tweaking. I'm not sure what values in trimmer pots are available but so long as they both add up to 57K I should be ok with a good range of adjustment in either direction? And splitting the total value with a fixed resistor will keep the bias from zeroing out? And the current value's of the 4 tubes I measure at J214/209 should be divided by 4, so I should expect to see 80ma if I want each at 20ma? Or at J210/213/215/216 should be divided by 2 and I should expect to see 40ma if I want them at 20ma each?
I think when I used my bias tool on the individual tube it read around 35ma initially which I assume would be way too high and the reason why I get so hot playing in front of the amp since the fan blows across the tubes from the back to the vents in the front where I sit. It's nice in the winter but in the summer.........
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Looking at the schematic,
Only just noticed the bias switching..
It seems that you might as well just adjust all the tubes togeather, because there is a relay called parallel A/B that switches the inputs signal to the grids on the bottom power section on the drawing...The problem is it also switches the bias off that section so its dependant on the bias from the top set of tubes, so all the work setting each channel is pointless. So just set all eight togeather..and measure each set of four to get a balance....not the best of situations...
The trouble with setting each set of 4 with a different resistor pot network is when you switch over the one set of tubes could be biased to high...because they are now on the bias setting for the top set of tubes.
So lets hope you don't get an open contact on the relay... so now it makes sense that they just do all of them togeather..
Regards
M. Gregg
Only just noticed the bias switching..
It seems that you might as well just adjust all the tubes togeather, because there is a relay called parallel A/B that switches the inputs signal to the grids on the bottom power section on the drawing...The problem is it also switches the bias off that section so its dependant on the bias from the top set of tubes, so all the work setting each channel is pointless. So just set all eight togeather..and measure each set of four to get a balance....not the best of situations...
The trouble with setting each set of 4 with a different resistor pot network is when you switch over the one set of tubes could be biased to high...because they are now on the bias setting for the top set of tubes.
So lets hope you don't get an open contact on the relay... so now it makes sense that they just do all of them togeather..
Regards
M. Gregg
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what switch is activating the relay and why wouldn't they just supply all 8 tubes without a relay . Does it have something to do with the individual standby switches.
There's another section , I/O of the schematics missing. The relay ground goes to a stereo/mono switch and I think the relay is n/c in the non grounded position . In other words, it's energized only when the switch is set to mono. I don't know why the schematic shows bias off at channel B. If I understand correctly, if the relay goes open, it would always have bias to channel 1 regardless of the relay but what would happen if the relay opened while channel 2 was cranked in stereo, and if the contacts are normally closed for that channel, what would the odds be that it opens. I would think that closed contacts are as good as hard wired. It would be nice to post the other schematic so you could have a look.
The relay,
Looking at the schematic you show..remember the bias is never off..just changes over (with the signal) to the other channel bias (which should be the same value)
The function of the relay is to disconnect the grids for the bottom power OP section from the signal, and connect them to the top OP tubes grids so the signal would be common to both channels (as you say Mono)..however the bias for the bottom section on the drawing comes from the (grid side of) coupling caps via the grid resistors through the relay. This means that as the relay changes over the bottom power tubes loose bias for a split second as the relay changes over to the top tube signal and bias.(This is not a major problem as long as its very fast)
Now you have to remember..that any relay contact can go high resistance and that the voltage across it and current drawn have an effect on the reliability of connection..so you could argue that the bias voltage will be the important factor to ensure this contact resistance is over come...
You would have to be very unlucky for the contacts to not be in contact with either the top or bottom tube bias (ie broken or corroded)
A faulty relay coil would not damage the tubes because it would only mean the contacts did not change over so bias would still be on for all tubes..
Now the down side ..if for some reason the contact went open circuit or high resistance due to oxidisation then the bottom set of tubes could be without bias control..being an engineer I know this does happen I have seen it. Normaly when the contact breaks off after many operations.
Why they decided to switch the bias I don't know..except that with no current flow across a signal contact you do get a lot of problems (if the relay is not designed for low level signal switching "Gold contacts etc")..and the bias supply would overcome this resistance..
Its easy to pull circuits apart, however it must work OK or they would modify it...Its probably better if I don't say because once the seed is planted you start to doubt the system..I guess its been OK so far so there is no reason to doubt it now..
Regards
M. Gregg
Looking at the schematic you show..remember the bias is never off..just changes over (with the signal) to the other channel bias (which should be the same value)
The function of the relay is to disconnect the grids for the bottom power OP section from the signal, and connect them to the top OP tubes grids so the signal would be common to both channels (as you say Mono)..however the bias for the bottom section on the drawing comes from the (grid side of) coupling caps via the grid resistors through the relay. This means that as the relay changes over the bottom power tubes loose bias for a split second as the relay changes over to the top tube signal and bias.(This is not a major problem as long as its very fast)
Now you have to remember..that any relay contact can go high resistance and that the voltage across it and current drawn have an effect on the reliability of connection..so you could argue that the bias voltage will be the important factor to ensure this contact resistance is over come...
You would have to be very unlucky for the contacts to not be in contact with either the top or bottom tube bias (ie broken or corroded)
A faulty relay coil would not damage the tubes because it would only mean the contacts did not change over so bias would still be on for all tubes..
Now the down side ..if for some reason the contact went open circuit or high resistance due to oxidisation then the bottom set of tubes could be without bias control..being an engineer I know this does happen I have seen it. Normaly when the contact breaks off after many operations.
Why they decided to switch the bias I don't know..except that with no current flow across a signal contact you do get a lot of problems (if the relay is not designed for low level signal switching "Gold contacts etc")..and the bias supply would overcome this resistance..
Its easy to pull circuits apart, however it must work OK or they would modify it...Its probably better if I don't say because once the seed is planted you start to doubt the system..I guess its been OK so far so there is no reason to doubt it now..
Regards
M. Gregg
Luckily these Peavey's are known to be workhorses so hopefully I'll never have a relay problem. Since it's for a mono/stereo operation, and the use a slide switch anyway ,I don't see the need for the relay other than faster guaranteed switching in their eyes.
Just to clarify, when I measure the two tubes on each side of the primary, should I adjust it to 40ma since it's reading across both tubes.
The rule of thumb,
I guess you already know this...
Always measure what you have now..then if anything goes wrong you have a reference to work to..I would not increase idle current higher than you already have the amp running at now... (you should follow any information from the manufacturer..)
However if all is within normal (existing) values I would set to same as before 17-20mA per tube so on each side its 34-40mA remember that one tube could be carrying more than the other. Remember to listen to your amp after you set bias...reducing the bias current will have an effect at higher volumes.
Always watch for red plateing on any tube, this shows a problem..(over current). Also remember that the primary of your OP Tx can be damaged with over current..so its at your own risk..And again its at B+ voltage..
Just for interest link:
http://www.r-type.org/pdfs/el84.pdf
Regards
M. Gregg
I guess you already know this...
Always measure what you have now..then if anything goes wrong you have a reference to work to..I would not increase idle current higher than you already have the amp running at now... (you should follow any information from the manufacturer..)
However if all is within normal (existing) values I would set to same as before 17-20mA per tube so on each side its 34-40mA remember that one tube could be carrying more than the other. Remember to listen to your amp after you set bias...reducing the bias current will have an effect at higher volumes.
Always watch for red plateing on any tube, this shows a problem..(over current). Also remember that the primary of your OP Tx can be damaged with over current..so its at your own risk..And again its at B+ voltage..
Just for interest link:
http://www.r-type.org/pdfs/el84.pdf
Regards
M. Gregg
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I think it's way to high now if the bias tool is somewhat accurate at initial cold reading. It was reading around 35ma on the individual tube. That's why I am concerned and want to lower it. The tubes weren't redplating but the amp really runs hot as far as the trannys go. The tubes are JJ's. I've seen a radical difference in bias current between different brand tubes. TAD's seem to run high. The stock tubes were Sovtek 6n14n's. A lot of people complain about how hot these get when rack mounted. It's probably why it comes with jumbo rubber feet for non rack mounting. I'll have a better idea what the bias is at the primary. I figure around 17 to 20ma for 65 to 70% of 12 watts @ 400v.
I have a 6L6 amp that runs high with new TAD's in it and that has fixed bias too and the manufacturer say's not to worry about it. I can't understand why they don't just put a simple adjustable circuit in these amps. Even though they all say to use the same spec tubes in them, they can still vary. The 6L6 amp uses zener diodes to fix the grid voltage so at least that's an easy change. And the bias tool stays nice and stable on the 6L6's. At least I'm getting educated with these other amps now as far as the more negative the bias goes the higher the current is (I think) so I can pick the right value zener later on. That's a new post down the road.
Please rethink..electrons are negative...bias is negative..
Fun Link:
What is the Bias Point
Regards
M. Gregg
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