Anyone know what i should be measuring as per factory spec?
I measure approx 32mA @ 395 plate voltage with one set of tubes in there.
Cutting max plate dissipation a bit close i think at around 12.6W.
Nothings glowing though at that even when up to clipping.
Boogie Studio .22 Bias
Moderators: pompeiisneaks, Colossal
Re: Boogie Studio .22 Bias
Attached is the manual.
As all mesa boogie amps the .22 cal is a fixed bias circuit. The company expects the consumer to purchase tubes from Mesa for plug and play operation as a form of quality control and continued business.
With that said, if you have modified the bias circuit to a variable fixed bias. you can run the amp with other manufactor tubes.
EL84
Based on 70% Class AB
Plate Voltage of 395 Volts = 21.2mA = ~8 Watts per tube
Based on 90% Class A
PLate Voltage of 395 Volts = 27.3 mA = ~10 Watts per tube
As all mesa boogie amps the .22 cal is a fixed bias circuit. The company expects the consumer to purchase tubes from Mesa for plug and play operation as a form of quality control and continued business.
With that said, if you have modified the bias circuit to a variable fixed bias. you can run the amp with other manufactor tubes.
EL84
Based on 70% Class AB
Plate Voltage of 395 Volts = 21.2mA = ~8 Watts per tube
Based on 90% Class A
PLate Voltage of 395 Volts = 27.3 mA = ~10 Watts per tube
You do not have the required permissions to view the files attached to this post.
My Daughter Build Stone Henge
Re: Boogie Studio .22 Bias
My Daughter Build Stone Henge
Re: Boogie Studio .22 Bias
Hi
Thanks for that, what you say seems more like what i was expecting to see so its definitely a bit higher than it should be.
I am aware of the fixed bias situation with it, i guess i could modify it to allow adjustment but we were trying to get back to factory condition as its already been got at.
I did find that useful website with the bias tables.
I use the method of measuring the voltage across each winding on the output TX and work the current out from there, it does give a very accurate figure per tube. Problem i have is the mains voltage can fluctuate up and down during this so i have to pick my time to measure it.
Its still got an intermittent hum, pops and fizz's so i think im going to have to recap it as well still.
I think its a known issue with these, must be over 25 years old by now.
Thanks for help.
Thanks for that, what you say seems more like what i was expecting to see so its definitely a bit higher than it should be.
I am aware of the fixed bias situation with it, i guess i could modify it to allow adjustment but we were trying to get back to factory condition as its already been got at.
I did find that useful website with the bias tables.
I use the method of measuring the voltage across each winding on the output TX and work the current out from there, it does give a very accurate figure per tube. Problem i have is the mains voltage can fluctuate up and down during this so i have to pick my time to measure it.
Its still got an intermittent hum, pops and fizz's so i think im going to have to recap it as well still.
I think its a known issue with these, must be over 25 years old by now.
Thanks for help.
Re: Boogie Studio .22 Bias
The voltage swing canbe fixed with a constant voltage transformer if it's really an issue.
The amp is probably due for a cap job if it's original parts.
I've had a few of these on the bench with similar problems - it's quicker to clean the pops, sockets, use known Good tubes, tighten the bolts on the iron and touch up the solder points.
The pops and crackles are most likely the resistors in the preamp section, and or the PCB. Clean it up and touch up the solder points it should help with the "noises"
The amp is probably due for a cap job if it's original parts.
I've had a few of these on the bench with similar problems - it's quicker to clean the pops, sockets, use known Good tubes, tighten the bolts on the iron and touch up the solder points.
The pops and crackles are most likely the resistors in the preamp section, and or the PCB. Clean it up and touch up the solder points it should help with the "noises"
My Daughter Build Stone Henge
Re: Boogie Studio .22 Bias
Ive picked up 3 new 33uf 450V caps from RS, pretty reasonable cost.
Do you know a good place to look for ideas for bias mods to make it variable? I was going to have a look at a few Marshall designs to remember how they do it, its been a long time since ive worked on one of those.
Ive got an idea on how i would do it, but better still known good mods that have already been proven with this amp.
Cheers
Do you know a good place to look for ideas for bias mods to make it variable? I was going to have a look at a few Marshall designs to remember how they do it, its been a long time since ive worked on one of those.
Ive got an idea on how i would do it, but better still known good mods that have already been proven with this amp.
Cheers
Re: Boogie Studio .22 Bias
I would locate the bias Resistor.
off the top of my head I want to say it's going to be 22K (don't hold me to this).
Leave it soldered in and bridge it with a potentiometer so if the pot ever fails it'll have a backup. match the pot as close the bias as you can. this will give you basically a zero to double the value of the resistor as you turn the pot. Off the wiper add a 15K resistor to ground as a safety.
Attached is the bias mod - i drew it up for someone working on a V4 it works for most circuits you have to adapt the resistor values for your amp.
off the top of my head I want to say it's going to be 22K (don't hold me to this).
Leave it soldered in and bridge it with a potentiometer so if the pot ever fails it'll have a backup. match the pot as close the bias as you can. this will give you basically a zero to double the value of the resistor as you turn the pot. Off the wiper add a 15K resistor to ground as a safety.
Attached is the bias mod - i drew it up for someone working on a V4 it works for most circuits you have to adapt the resistor values for your amp.
You do not have the required permissions to view the files attached to this post.
My Daughter Build Stone Henge
Re: Boogie Studio .22 Bias
here is the bias mod installed in a Peavey 30
I drilled out a rivet and replaced it with a bolt so I didn't need to drill any holes and the bias pot would be secure.
I drilled out a rivet and replaced it with a bolt so I didn't need to drill any holes and the bias pot would be secure.
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My Daughter Build Stone Henge
Re: Boogie Studio .22 Bias
option for a prefab
http://cgi.ebay.com/Bias-Mod-Mesa-Boogi ... 2748827866
I'll see if i can find the resistor number. If not i'll open up mine this weekend.
http://cgi.ebay.com/Bias-Mod-Mesa-Boogi ... 2748827866
I'll see if i can find the resistor number. If not i'll open up mine this weekend.
My Daughter Build Stone Henge
Re: Boogie Studio .22 Bias
schematic
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My Daughter Build Stone Henge
Re: Boogie Studio .22 Bias
Page 2
Off the Power transformer "D" 33K resistor R111
Off the Power transformer "D" 33K resistor R111
My Daughter Build Stone Henge
Re: Boogie Studio .22 Bias
I believe what ever you do at R111 is going to affect R112 the bias for the PI. A cheap fast fix for the hot running el84s in the boogie is to parallel another 33k with R111. Did not even have to remove the board(pita). It raises the bias voltage just enough to get most el84's to stop red plating on you. You could also parallel a pot across R111 too, but hey it's a boogie, non adjustable.
I just did one of these and it worked get. One of the better sounding Mesa's .
Z
I just did one of these and it worked get. One of the better sounding Mesa's .
Z
Re: Boogie Studio .22 Bias
Hi Guys
Thanks the input, i appreciate your interest.
This amp has become my Sat morning mission, its starting to drag on a bit now...
Anyway i've replaced the 3 x 30uf 450V caps today, its seems to helped the erratic 50hz (UK!) hum that would suddenly cut in then drop out again.
Having tested them all though they all had low ESR and the values seemed ok, maybe playing up when HT is on them though.
I've still got some crackling going on at times i don't think the EQ is working as it should.
Seems ok in rhythm mode but has no real effect when switched to the dirty channel. Graphic EQ switches as should in "auto" or "in" mode.
So is the idea of LDR3 in the EQ circuit to disable it when switched to the dirty channel and you have to use the graphic for EQ as this is what seems to be happening.
Also looked into the bias arrangement.
I tried paralleling another 33K resistor across R111 but its not had any real great effect. I measure around -13V now on the grids but bias current seems about the same measuring the P/D across each side of the O/P TX windings.
Selloutrr with the pot arrangement as in your diagram should the wiper be tied to one end of the track?
So is the idea of the resistor off the wiper to ground to great a potential divider so that's why its not tied to one end of the track?
Looks like the Ebay one just has a resistor in parallel with the pot as there's only 2 wires to connect.
I think i should of replaced the other electrolytics in the bias and low voltage supplies while i was at it. That PCB arrangement is very un-service friendly, ive already had to resolder most of the wires back onto the PCB as they broke off. That was being careful with the board as well...
Amp sounds pretty good though, just need to get to the point where i know everything is as it should be or at least normal for one of these.
Cheers guys
Thanks the input, i appreciate your interest.
This amp has become my Sat morning mission, its starting to drag on a bit now...
Anyway i've replaced the 3 x 30uf 450V caps today, its seems to helped the erratic 50hz (UK!) hum that would suddenly cut in then drop out again.
Having tested them all though they all had low ESR and the values seemed ok, maybe playing up when HT is on them though.
I've still got some crackling going on at times i don't think the EQ is working as it should.
Seems ok in rhythm mode but has no real effect when switched to the dirty channel. Graphic EQ switches as should in "auto" or "in" mode.
So is the idea of LDR3 in the EQ circuit to disable it when switched to the dirty channel and you have to use the graphic for EQ as this is what seems to be happening.
Also looked into the bias arrangement.
I tried paralleling another 33K resistor across R111 but its not had any real great effect. I measure around -13V now on the grids but bias current seems about the same measuring the P/D across each side of the O/P TX windings.
Selloutrr with the pot arrangement as in your diagram should the wiper be tied to one end of the track?
So is the idea of the resistor off the wiper to ground to great a potential divider so that's why its not tied to one end of the track?
Looks like the Ebay one just has a resistor in parallel with the pot as there's only 2 wires to connect.
I think i should of replaced the other electrolytics in the bias and low voltage supplies while i was at it. That PCB arrangement is very un-service friendly, ive already had to resolder most of the wires back onto the PCB as they broke off. That was being careful with the board as well...
Amp sounds pretty good though, just need to get to the point where i know everything is as it should be or at least normal for one of these.
Cheers guys
Re: Boogie Studio .22 Bias
For the .22 cal the bias pot would wire up.
50K Pot (preferably lockable)
Leg 1 - to one side of the 33K resistor
Leg 2 (wiper) - soldered to a 15K - 1/2Watt resistor other end of the resistor to ground.
Leg 3 - to the other end of the 33K resistor
50K Pot (preferably lockable)
Leg 1 - to one side of the 33K resistor
Leg 2 (wiper) - soldered to a 15K - 1/2Watt resistor other end of the resistor to ground.
Leg 3 - to the other end of the 33K resistor
My Daughter Build Stone Henge
Re: Boogie Studio .22 Bias
Thanks for clearing that up, so its basically creating a potential divider if i get the correct idea of it?