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Post by mauro on Nov 28, 2017 22:59:48 GMT
Hi Again,
I'm really getting my value from this site this week! I've been pretty quiet for a while and now 3 question inside of a week.
Love it!
I've recently rebuilt a few amps, not all Wurlitzers but I've noticed that some start loud and then decrease in volume at the start and others start soft and quickly increase in volume in the first 5 seconds or so. The last amp I did was a Model 530 and I remember Rob (from NYC) suggesting to change the value of the 1uf and 2 uf caps to the same value but I can't find the thread.
I think my computer skills are diminishing as I age or perhaps my computer is getting old? I'll go with that.
Anyway if anyone remembers this modification please refresh my memory.
Thanks again, Mauro
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Post by robnyc on Nov 29, 2017 5:45:24 GMT
Mauro, what I described back then would not address the problem of records starting loud (overshoot -those mods were part of a larger set of mods to improve the AGC action.
In your case you have to determine if the squelch voltage is reaching it maximum (or at all) before the mute/squelch contacts open.
What model amp are you asking about-?
Rob
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Post by mauro on Nov 29, 2017 11:22:38 GMT
I'm asking about a model 530 amp from a W1900.
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Post by mauro on Dec 1, 2017 21:05:47 GMT
Let me approach this another way,
What would happen if I changed the value of C30 1mf and C31 2mf to be the same value?
Either both 1 or both 2 mf?
Would it decrease the time it takes the ampliier to make the volume correction.
Failing that, will removing the 12AX7 disable this circuit?
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Post by robnyc on Dec 2, 2017 2:57:57 GMT
Mauro, lowering the capacitance will shorten the attack time, but also quicken the release time. With the existing circuit I would not go below a .5 mfd on C30 and a 1 mfd on C31. In addition to making the circuit too active and noticeable, a too-short time constant causes thumps in the audio as the DC on the reactance (the 1/2 12AX7)tube changes too rapidly.
The real issue here is why there is overshoot at the beginning of loud records. In theory the original squelch voltage should be slightly higher than the highest level record the system will encounter. Given that record cutting tech allowed much louder disks in the years after that amp was designed, it is possibly that the squelch voltage in inadequate.
Inadequate squelch may also be the result of circuit failure. The schematic does not give an accurate reading of what a full gain reduction would mean in terms of voltage on pin 2 (grid of the 12AU7 section 2). In general the grid vdc on these circuits should not exceed the cathode n either squelch or highest conduction 9loud record).
measuring is tricky though. Even the 10 megohm input of a meter will load the circuit and pull down the control voltage htus making the sound louder and confusing things. In general if your meter is fast enough you can take quick readings that are "close enough for rock'n roll".
select a slot w/no record. Remove the mute-squelch plug. Select the empty slot and after about a minute take a measurement of the voltage at C31. W/mute plug removed the squelch voltage will build up as far as it can go. Put mute plug back in , select a loud record and once playing take a few readings and compare those with the squelch reading.
Check also the value of R24 and R21.
Removing the 12AX7 will result in major distortion.
I've spent the day putting Christmas music in two locations and need some sleep. I'll check back later.
RobNYC
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Post by mauro on Dec 8, 2017 23:11:37 GMT
Mauro, lowering the capacitance will shorten the attack time, but also quicken the release time. With the existing circuit I would not go below a .5 mfd on C30 and a 1 mfd on C31. In addition to making the circuit too active and noticeable, a too-short time constant causes thumps in the audio as the DC on the reactance (the 1/2 12AX7)tube changes too rapidly. The real issue here is why there is overshoot at the beginning of loud records. In theory the original squelch voltage should be slightly higher than the highest level record the system will encounter. Given that record cutting tech allowed much louder disks in the years after that amp was designed, it is possibly that the squelch voltage in inadequate. Inadequate squelch may also be the result of circuit failure. The schematic does not give an accurate reading of what a full gain reduction would mean in terms of voltage on pin 2 (grid of the 12AU7 section 2). In general the grid vdc on these circuits should not exceed the cathode n either squelch or highest conduction 9loud record). measuring is tricky though. Even the 10 megohm input of a meter will load the circuit and pull down the control voltage htus making the sound louder and confusing things. In general if your meter is fast enough you can take quick readings that are "close enough for rock'n roll". select a slot w/no record. Remove the mute-squelch plug. Select the empty slot and after about a minute take a measurement of the voltage at C31. W/mute plug removed the squelch voltage will build up as far as it can go. Put mute plug back in , select a loud record and once playing take a few readings and compare those with the squelch reading. Check also the value of R24 and R21. Removing the 12AX7 will result in major distortion. I've spent the day putting Christmas music in two locations and need some sleep. I'll check back later. RobNYC Rob, thanks for your explanation, I think I may have the solution now that you explained it. Ok, don't throw hard object at me when you read this, I'm bench testing the amp with a 1700 mech so I don't have the mute plug in, is this my problem? Yikes, duck! |
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Post by Ron Rich on Dec 9, 2017 0:48:56 GMT
yep--duck, duck, duck !!!
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Post by robnyc on Dec 9, 2017 2:43:29 GMT
Rob, thanks for your explanation, I think I may have the solution now that you explained it. Ok, don't throw hard object at me when you read this, I'm bench testing the amp with a 1700 mech so I don't have the mute plug in, is this my problem? Yikes, duck! Mauro, yes, the squelch voltage is applied by the mute/squelch plug but, where two different way of applying it. The older design on some amps used the closure of a set of contacts to ground out the squelch. On others it was applied by a contact closure. With no mute plug in-place and no input, and amp in play mode check the AGC time constant for any significant voltage. No voltage means that squelch is applied with closure so you will get overshoot w/out the plug. And no, I'm not throwing anything your way. This just points out the shortcomings of trying to troubleshoot by remote...and of me assuming too much. RobNYC |
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Post by mauro on Dec 10, 2017 0:17:44 GMT
Perhaps so, but this was very educational, I learned alot working through this.
Thank you professor.
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