All things LA2a related
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Paul B
Well-known member
I changed the 22k2w and put in a 6AQ5W. I now have 101V at pin 5 and 132 on pin 6.
Is there a consensus that these voltages are a likely source of my problem? The voltages should be about exactly opposite. It makes sense to my ignorant brain that if pin 3 on the t4b is not getting enough voltage, the EL panel wont get enough juice. or something like that. :?
Is there a consensus that these voltages are a likely source of my problem? The voltages should be about exactly opposite. It makes sense to my ignorant brain that if pin 3 on the t4b is not getting enough voltage, the EL panel wont get enough juice. or something like that. :?
Don't worry about the 6aq5 voltages. They are not that critical.
Sorry if you have done these already, but:
Forget what the meter says. Are you "hearing" any compression? Does the volume go way down when you turn up the GR?
Mess with R3, the useless stereo pot that might be turned the wrong way.
It could be shunting your gr signal to ground.
What is the part number on the LDR's in the T4? You might need to pull them out a bit to find out. They will bend back in so don't sweat it.
You will get far less GR while operating the T4 with the lid off in a bright room. So put the lid on when testing for GR.
Fire up the compressor and turn the GR up all the way. Measure the ac volts on the 0.1 uF plate cap which is hooked to the 6AQ5. You can measure it on either side. The side opposite the tube plate is better, as you will not have the high voltage dc on there, but it dosen't really matter.
Measure it at Pin 3 of the T4 socket also. You need about 100 vac to lite up your life.
If low ac volts on the AQ5 plate output cap, hook ac meter to the cap comming off V3, which is 0.02 on the schematic. This is the 12AX7 that has it's sections running in parallel. Measure the ac volts while the GR knob is cranked up.
Disconnect one side of your VU meter and listen for gain reduction. You can hear it without the meter. We want to eliminate the meter as a trouble spot.
Sorry if you have done these already, but:
Forget what the meter says. Are you "hearing" any compression? Does the volume go way down when you turn up the GR?
Mess with R3, the useless stereo pot that might be turned the wrong way.
It could be shunting your gr signal to ground.
What is the part number on the LDR's in the T4? You might need to pull them out a bit to find out. They will bend back in so don't sweat it.
You will get far less GR while operating the T4 with the lid off in a bright room. So put the lid on when testing for GR.
Fire up the compressor and turn the GR up all the way. Measure the ac volts on the 0.1 uF plate cap which is hooked to the 6AQ5. You can measure it on either side. The side opposite the tube plate is better, as you will not have the high voltage dc on there, but it dosen't really matter.
Measure it at Pin 3 of the T4 socket also. You need about 100 vac to lite up your life.
If low ac volts on the AQ5 plate output cap, hook ac meter to the cap comming off V3, which is 0.02 on the schematic. This is the 12AX7 that has it's sections running in parallel. Measure the ac volts while the GR knob is cranked up.
Disconnect one side of your VU meter and listen for gain reduction. You can hear it without the meter. We want to eliminate the meter as a trouble spot.
Paul B
Well-known member
OK first the good news... IT SQUISHES!! :grin: the music is definitely compressing and it sounds nice, but...
The bad news is that it takes an overbearing input signal to get the thing going, and with PR turned all the way up, it still compresses a max of 7-10db.
I also have a lo level grounding hum...
when you say to measure the cap of the plate of V4, you mean C11 right? the one between V4pin 5 and pin 3 of the t4b) isnt that a .01 uf and not a .1uf??
with PR all way up I am measuring 220VAC before the capacitor (up to 260 with input) and negligible on the pin 3 side. with music on, pin 3 is getting about .3VAC max. hope that tells you something good....
VAC coming off the .02 cap on V3 (C9) I am measuring 215 VAC on the tube side and 0 on the R37 side.
The bad news is that it takes an overbearing input signal to get the thing going, and with PR turned all the way up, it still compresses a max of 7-10db.
I also have a lo level grounding hum...
when you say to measure the cap of the plate of V4, you mean C11 right? the one between V4pin 5 and pin 3 of the t4b) isnt that a .01 uf and not a .1uf??
with PR all way up I am measuring 220VAC before the capacitor (up to 260 with input) and negligible on the pin 3 side. with music on, pin 3 is getting about .3VAC max. hope that tells you something good....
VAC coming off the .02 cap on V3 (C9) I am measuring 215 VAC on the tube side and 0 on the R37 side.
Paul B
Well-known member
Another question, what would you consider a good input signal VAC-wise?? both peak and average.
thanks
Paul
thanks
Paul
Paul B
Well-known member
OK now Im REALLY confused and maybe this is it.....
Schematic in bloo manual shows C11 as .1uf. page 5 of bloo manual does not state value for C11. the instruction for installing C11 on p.5 does not state the value.
Schematic at Sowter, cayacosta, jahnsen and 1968 schematics show C11 as .01 uf
which is it????? :?
Schematic in bloo manual shows C11 as .1uf. page 5 of bloo manual does not state value for C11. the instruction for installing C11 on p.5 does not state the value.
Schematic at Sowter, cayacosta, jahnsen and 1968 schematics show C11 as .01 uf
which is it????? :?
the key is to stay with the revsion of schematic that you prefer to use.
the bloo manual is not based on the same revision as what caycosta used (which is why I dont put his layout in the manual)
I think where people make mistakes is trying to use the bloo manual in conjuction with that layout. This is ok if you identify all the differences but where you end up in trouble is if you dont pay attention to the differences. then you end up with a mix-n-match between the two.
you could put a .01 in there if you want... I dont think this is going to fix your problem..
I really believe the first thing to figure out is what your box is actually doing verses what the meter is showing.
try not to even use the meter for any reference at this point.
are you certain that you have wired your input transformer in correctly? we need to know that you have plenty of signal getting in there first/
-steve
the bloo manual is not based on the same revision as what caycosta used (which is why I dont put his layout in the manual)
I think where people make mistakes is trying to use the bloo manual in conjuction with that layout. This is ok if you identify all the differences but where you end up in trouble is if you dont pay attention to the differences. then you end up with a mix-n-match between the two.
you could put a .01 in there if you want... I dont think this is going to fix your problem..
I really believe the first thing to figure out is what your box is actually doing verses what the meter is showing.
try not to even use the meter for any reference at this point.
are you certain that you have wired your input transformer in correctly? we need to know that you have plenty of signal getting in there first/
-steve
Paul, I think you are measuring DC on those caps. 0.1 or 0.01, don't make no diff.
try it again only with your meter set on ac volts.
Also, compare say, a cd into the LA2 and without the LA2, into a stereo or some other playback amp. Make sure you can get unity gain out of the LA2 without cranking up the Gain.
Go.
try it again only with your meter set on ac volts.
Also, compare say, a cd into the LA2 and without the LA2, into a stereo or some other playback amp. Make sure you can get unity gain out of the LA2 without cranking up the Gain.
Go.
Paul B
Well-known member
Hi Steve / CJ,
Those were actually AC measurements at C11: 220VAC and 100VDC. and 215 VAC at C9 (on the tube side). I will check on the unity gain when I get home.
Steve, the sowter input tranny is wired per sowter schematic and that small cayacosta diagram. I know that they are wired correctly but have no idea if Im getting plenty of signal. But I will check it again. Do you have any reference voltages for the input and output trannies?
I really appreciate you guys hanging in there with me.
Paul
Those were actually AC measurements at C11: 220VAC and 100VDC. and 215 VAC at C9 (on the tube side). I will check on the unity gain when I get home.
Steve, the sowter input tranny is wired per sowter schematic and that small cayacosta diagram. I know that they are wired correctly but have no idea if Im getting plenty of signal. But I will check it again. Do you have any reference voltages for the input and output trannies?
I really appreciate you guys hanging in there with me.
Paul
Unplug the unit and blead the filters.
Then put an ohm meter on the low side of those plate caps on V3 and V4 where your ac voltage is disappearing.
I will make some resistance and AC voltage measuements on my LA2 tonight and get back to ya. am.
cj
Then put an ohm meter on the low side of those plate caps on V3 and V4 where your ac voltage is disappearing.
I will make some resistance and AC voltage measuements on my LA2 tonight and get back to ya. am.
cj
Paul B
Well-known member
Filters bled.
checked resistance at pin3 t4b (that is the low side, right?) and got something over 2000k (beyond the capacity of my DMM)
checked resistance on the other side of C9. (V4) With R37 turned all the way up, resistance = 1086k and as I turn it up the resistance climbs until it goes beyond the capacity of my DMM at 2000k at about a 9 oclock setting.
checked resistance at pin3 t4b (that is the low side, right?) and got something over 2000k (beyond the capacity of my DMM)
checked resistance on the other side of C9. (V4) With R37 turned all the way up, resistance = 1086k and as I turn it up the resistance climbs until it goes beyond the capacity of my DMM at 2000k at about a 9 oclock setting.
Hmmm, I wonder why you only get a good AC reading on one side of those caps.
You are measuring from the chassis ground to the cap, right?
Measure that AC volts coming off the plate of the 6AQ5 with the T4 thing removed. Measure on both sides of the coupling cap, C11.
You are measuring from the chassis ground to the cap, right?
Measure that AC volts coming off the plate of the 6AQ5 with the T4 thing removed. Measure on both sides of the coupling cap, C11.
Paul B
Well-known member
$#!@%^&*! sorry I keep getting interrupted,
I have 222VAC on the tube side of C11. I have zero voltage on the t4b pin3 side of the cap...
I have 222VAC on the tube side of C11. I have zero voltage on the t4b pin3 side of the cap...
AnalogPackrat
Well-known member
Paul,
Not to confuse your session with CJ, but what are your test conditions? Input source, PR setting, meter sw setting, etc? Have you tried CJ's idea of disconnecting one end of the meter (tape up the lead so it doesn't short out o n anything)?
Analog Packrat
Not to confuse your session with CJ, but what are your test conditions? Input source, PR setting, meter sw setting, etc? Have you tried CJ's idea of disconnecting one end of the meter (tape up the lead so it doesn't short out o n anything)?
Analog Packrat
Paul B
Well-known member
I am using a digital recorder as the source and am outputing a signal peaking in the 300mV area into a passive direct box and then into the bloo..
The PR has no effect, either meter or soundwise until about 70, and at 100 it compresses to a max of about 7-10db.
Gainwise, I have parity at a gain setting of 10 :shock: :shock:
Shows ya how much this sucker is putting out!
However, on +4, parity gain barely makes the needle jump - less than a 16th of an inch. To get the meter to read near 0db, I have to turn the gain to 70 and it is a horribly distorted sound.
Like I said tho with the gain at 15 and the PR set between 70-100 I hear a warm compression that tells me when I get this box right its going to be a beautiful thing!
by the way yes I did try it with the VU disconnected and it had no effect on the sound.
The PR has no effect, either meter or soundwise until about 70, and at 100 it compresses to a max of about 7-10db.
Gainwise, I have parity at a gain setting of 10 :shock: :shock:
Shows ya how much this sucker is putting out!
However, on +4, parity gain barely makes the needle jump - less than a 16th of an inch. To get the meter to read near 0db, I have to turn the gain to 70 and it is a horribly distorted sound.
Like I said tho with the gain at 15 and the PR set between 70-100 I hear a warm compression that tells me when I get this box right its going to be a beautiful thing!
by the way yes I did try it with the VU disconnected and it had no effect on the sound.
300mv is pretty low
you need to get around 3.4v p-p or 1.2 volts RMS
it doesnt matter if your getting alot of gain.. if the box isnt seeing much signal its just not going to compress..
why the DI box?
get something that has a +4 level output and send it into the compressor...
the box has plenty of gain... (it COULD be used as a preamp) BUT the threshold of the compressor is based on the amount of signal being sent into it... think of the Gain knob as "make-up" gain... it has no affect on the amount of compression.. its after the compressor.
therefore if you have a weak signal going into the box it might not ever even cross the threshold.. or you might have to turn the PR all the way up to 90 or 100 to even get a little bit of compression..
my feeling is that your compressor is probably working fine.. once you get a nice strong signal going into it.
I just tried 300mv on the AP..
even with 300mv input and your meter switched to +4 you should see it hit 0db around the "40 position on the gain"
if you have to crank it all the way up to 70+ then we need to figure out if its the meter or the gain of the box..
my guess is if the meter is registering rather low (which seems to be the case wether its looking at GR or gain) then its something to do with the metering circuit.
try this.
send a 3.4v p-p signe wave into the box (measure the output of the generator) then switch the compressor meter to +4
put a meter on the output of the compressor and turn up the gain knob until your meter reads 3.4vp-p (this will be unity gain) read the meter on the compressor and tell us what it is showing.
while your doing this make sure the PR knob is turned fully counter clockwise.
if your gain knob is somewhere around 20-35 then your gain structure is looking good.
now put a scope on the output of the compressor and bring up the Gain knob.. as you do this you should see the sine wave getting bigger... there will be a point where you will begin to see some distortion.. (the peaks will begin to square off) take note of where your gain knob is when this happens.. my guess is its going to give you a LOT of gain before this occurs.. now look at the VU meter of the compressor (again while its switched to +4) what does the meter say?if its barely registering then I'd say we have a wiring problem in the meter turret or the rotary switch
you actually might be getting alot of compression but just not hearing it... while the meter isnt showing it (our minds can play tricks on us sometimes)
im typiing this early in the morning so my idiot disclaimer is turnes on (im not liable for any mis-information at this time of day) haha
let us know what you find
-s
you need to get around 3.4v p-p or 1.2 volts RMS
it doesnt matter if your getting alot of gain.. if the box isnt seeing much signal its just not going to compress..
why the DI box?
get something that has a +4 level output and send it into the compressor...
the box has plenty of gain... (it COULD be used as a preamp) BUT the threshold of the compressor is based on the amount of signal being sent into it... think of the Gain knob as "make-up" gain... it has no affect on the amount of compression.. its after the compressor.
therefore if you have a weak signal going into the box it might not ever even cross the threshold.. or you might have to turn the PR all the way up to 90 or 100 to even get a little bit of compression..
my feeling is that your compressor is probably working fine.. once you get a nice strong signal going into it.
I just tried 300mv on the AP..
even with 300mv input and your meter switched to +4 you should see it hit 0db around the "40 position on the gain"
if you have to crank it all the way up to 70+ then we need to figure out if its the meter or the gain of the box..
my guess is if the meter is registering rather low (which seems to be the case wether its looking at GR or gain) then its something to do with the metering circuit.
try this.
send a 3.4v p-p signe wave into the box (measure the output of the generator) then switch the compressor meter to +4
put a meter on the output of the compressor and turn up the gain knob until your meter reads 3.4vp-p (this will be unity gain) read the meter on the compressor and tell us what it is showing.
while your doing this make sure the PR knob is turned fully counter clockwise.
if your gain knob is somewhere around 20-35 then your gain structure is looking good.
now put a scope on the output of the compressor and bring up the Gain knob.. as you do this you should see the sine wave getting bigger... there will be a point where you will begin to see some distortion.. (the peaks will begin to square off) take note of where your gain knob is when this happens.. my guess is its going to give you a LOT of gain before this occurs.. now look at the VU meter of the compressor (again while its switched to +4) what does the meter say?if its barely registering then I'd say we have a wiring problem in the meter turret or the rotary switch
you actually might be getting alot of compression but just not hearing it... while the meter isnt showing it (our minds can play tricks on us sometimes)
im typiing this early in the morning so my idiot disclaimer is turnes on (im not liable for any mis-information at this time of day) haha
let us know what you find
-s
AnalogPackrat
Well-known member
Paul doesn't have a sig gen or a scope, so we have to get creative. Hey, Paul, what digital recorder do you have? Do you have a soundcard for your computer? The reason I ask is that there are some nice test signal wavs at:
http://www.binkster.net/extras.shtml
If you could grab a couple of the sine wave files and play them into your LA2A we'd have a more controlled source. Just a thought.
Analog Packrat
http://www.binkster.net/extras.shtml
If you could grab a couple of the sine wave files and play them into your LA2A we'd have a more controlled source. Just a thought.
Analog Packrat
I measured the ac volts on C11 this morning. As you turn up the GR, the ac volts should gradually increase. At max GR, I had about 60 volts ac on both sides of C11. This makes sense as 60 volts is about 160 volts peak to peak, which is within the limits of the B+ voltage.
220 VAC is Way too high, means someting is goofy with your 6AQ5 circuit. Maybe oscillation is causing that big voltage. I doubt you got your power supply transformer ac on the plate, but check and see just in case.
You might have a bad filter cap, which would explain the hum. Check the ripple voltage on your supply filters, C7a and C7b. Set the meter to ac volts, ground your black lead, put one hand in your pocket for saftey, and get on those caps.
Medium GR only reqiures about 30 vac, I guess audio lites up the EL panel easier than 60 cycles, as you have all those other freqs in there.
In desperation, I usually grab a cup of coffee and sit down with a hi-liter pen and trace out every darn wire and component on a schematic, regardless of how many times I have checked things over. If there is a wiring wrror, this method will find it.
Kind of like this:
(pic on the way)
cj
220 VAC is Way too high, means someting is goofy with your 6AQ5 circuit. Maybe oscillation is causing that big voltage. I doubt you got your power supply transformer ac on the plate, but check and see just in case.
You might have a bad filter cap, which would explain the hum. Check the ripple voltage on your supply filters, C7a and C7b. Set the meter to ac volts, ground your black lead, put one hand in your pocket for saftey, and get on those caps.
Medium GR only reqiures about 30 vac, I guess audio lites up the EL panel easier than 60 cycles, as you have all those other freqs in there.
In desperation, I usually grab a cup of coffee and sit down with a hi-liter pen and trace out every darn wire and component on a schematic, regardless of how many times I have checked things over. If there is a wiring wrror, this method will find it.
Kind of like this:
(pic on the way)
cj
if you are using sowters and have not put the grid stoppers in or in correctly you will get a lot of oscillations on the output.. .which would explain the hi AC reading..
hmm is there some scope software out there thats free that we can have him put in his pc? so he could atleast see if theres oscillations on the output...
hmm is there some scope software out there thats free that we can have him put in his pc? so he could atleast see if theres oscillations on the output...
AnalogPackrat
Well-known member
Good idea. If he's got a soundcard he can try some of the stuff in the meta:
http://www.groupdiy.com/index.php?topic=8387
Maybe we should have a meta on circuit tracing and debugging with minimal tools. Seems to be a fairly common problem.
Analog Packrat
http://www.groupdiy.com/index.php?topic=8387
Maybe we should have a meta on circuit tracing and debugging with minimal tools. Seems to be a fairly common problem.
Analog Packrat
