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preamp: low volume all of a sudden.

gogofast

gogofast

Addicted Member
been using ampex 402 (i think this is the model) preamp for last few years with no problem. all caps changed out silicon diodes put in. all of a sudden both channel's output is about 1/10 of what it used to be. sound quality is there though.... changed tubes and still same output level. according to the schematic, 280v and 210v should be going into the 12ax7's and one 12au7. but when i tried to measure, somehow readings are all funny..... other than line in, 120v AC, none of the DC readings make sense to me. heater voltage of the 12ax7 is about 7v....due to high line voltage and silicon diode i guess.... will need to adjust the resistors, but right now, i'm puzzled about the low volume.

does it sound like a resistor or two being out of spec? what other reasons could there be? thanks for any help. i can post schematic when i get home.
 
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Sounds like you lost some B+. Depending if its a tube or SS rectifier, check that, and check the resistors in the power supply section. If a cap shorted, it may have taken out a resistor in the power supply, causing low B+.
 
i should be able to measure the B+ with neg to the ground and pos to the pin #6 of that 12au7 (schematic below), right? but i get pretty much nothing. i've replaced the diodes to 1n4007. funny thing is that i was working fine for last 2 years...so something must've drifted away or failed quietly.....

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pin 6 will be high voltage, should be at least 280, probably more like 300 with higher line voltage. If its not in that area, start moving backwards through the resistors until you get to the rectifiers to see what happened.
 
I'm with gadget, also, check the two red high voltage wires at the power transformer. Looks like a real easy circuit.
 
It uses a voltage doubler power supply so the top cap in the schematic is hot both on the negative and positive pins. I don't know what you used to replace the power caps but if the casings weren't insulated you could perhaps be grounding out half your voltage against the chassis. That would make your voltage measurements very weird as well.
 
checked all the diodes and power supply caps and everything seems fine. line voltage is 124v, but the red wires (now faded to white) have no voltage almost!!! green secondary wires are giving about 8v instead of 6.3v so that's understandable, but red wires are supposed to pass 130v according to the schematic, but almost nothing (see pic). after the voltage doubler, obviously nothing..... is the transformer shot?

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I'd say that's your problem. :thmbsp:

Be doubly sure that something else is not loading it down by disconnecting the high voltage wires and measuring them with no load.
 
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cademan said:
I'd say that's your problem. :thmbsp:

Be doubly sure that something else is not loading it down by disconnecting the high voltage wires and measuring them with no load.
Click to expand...

i was thinking it was gonna be an easy fix.......but i a most likely a shot transformer...this sucks.

looking at the schematic, the power transformer secondary has three sections. 2xred, 2xgreen, and 2xblue with green/white and blue/white, since i'm not getting any voltage out of red wires, should i really assume the tranny's dead? or should i check the other two sections' components?

i guess what i'm trying to ask is, are those red wires effected by the other two secondary sections of the transformer?
 
i did disconnect the red secondaries from the circuit and still measure nothing....blue wires do the same thing. :tears:
 
Look for any tube tuner doners. Any tube amp transformer will work if you know how to reduce the voltage to what you want to use.
 
I am sorry now that I threw out the electronics to an Ampex F44. Could have helped you out.
 
cademan said:
Look for any tube tuner doners. Any tube amp transformer will work if you know how to reduce the voltage to what you want to use.
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so i need something like secondaries of 2x 130v, 2x 6.3v, 2x 24v, and what's that white/blue? 126v?
 
You need a 130v transformer that also has 6.3v and 24v windings as well. The white/blue and white/green wires are just the center taps to the 6.3v and 24v windings.

I'm sure this is a redundant question but did you check/replace the fuse and also that every 120v outlet had power? It looks like the tape power outlet should be the definitive one. The schematic shows two contacts in the power switch with one only turning on some convenience outlets while the other is fused and goes to the power tranny.
 
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Scuzzer said:
You need a 130v transformer that also has 6.3v and 24v windings as well. The white/blue and white/green wires are just the center taps to the 6.3v and 24v windings.

I'm sure this is a redundant question but did you check/replace the fuse and also that every 120v outlet had power? It looks like the tape power outlet should be the definitive one. The schematic shows two contacts in the power switch with one only turning on some convenience outlets while the other is fused and goes to the power tranny.
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thanks scuzzer.

the fuse is ok and the unit powers on even the tubes glow. just low volume. all the outlets seem to have power.
 
gogofast said:
thanks scuzzer.

the fuse is ok and the unit powers on even the tubes glow. just low volume. all the outlets seem to have power.
Click to expand...

Pull the transformer, remove the end bells and see if perhaps one of the red wires is broken or damaged. If you need to buy a new transformer you could also look at 120v ones since your power is almost at 125v out of the wall.

edit: actually since you're still getting audio out I wouldn't be tearing it apart just yet. If your B+ is zero volts then you shouldn't be getting any sound out.
 
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gogofast said:
i did disconnect the red secondaries from the circuit and still measure nothing....blue wires do the same thing.
Click to expand...

Did you measure on AC? If both the red and blue wires have no AC then how are you getting sound?

It would also appear that where you're measuring 8.2v in your picture is actually after the rectifiers on the blue wire output. It would be easier to measure the filament dc voltages for V1, V2 and V4 off of pins 4 and 5 on the sockets.
 
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