tester causing bias to rise?

[Clemford]

Originally Posted by M Gregg

R69 is just to stop you adjusting and shorting to Gnd..

The pot TR2 is connected so its track is in series with R69 so the supply does not go through the wiper..ie if the wiper went open then you would still have the track resistance in series with R69..however even if they went open you would still have bias voltage on the tubes..

Regards
M. Gregg

that's a relief, thanks

[M Gregg]

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

[Clemford]

Originally Posted by M Gregg

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

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.

[M Gregg]

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

[Clemford]

Originally Posted by M Gregg

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

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.........

[M Gregg]

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

[Clemford]

Originally Posted by M Gregg

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

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.

[Clemford]

Originally Posted by Clemford

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.

[M Gregg]

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

[Clemford]

Originally Posted by M Gregg

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

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.