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Old 18th September 2019, 06:49 AM   #11
Tesla88 is offline Tesla88  Italy
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I find the pic you provided a bit difficult to read due to "photobucket" logo.

Btw, what kind of preamp tubes ? judging by the heaters wiring i guess ECC83 / ECC81

It seems like all cathodes are at 0V ... are tubes ok?
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Old 18th September 2019, 07:13 AM   #12
jimmy74 is offline jimmy74
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Yes its an Elka bass amp been looking online for a schematic but can't find anything. Yes there is a resistor between the ul taps and the screen

Yes the tubes are all new... I am getting unstable dc readings on most of preamp pin 2 and 7 lugs....jumping around constantly when I put the multimeter on the mv selection and very low readings when I use the V setting. These unstable readings are signed by an asterisk.

V5 is a 12au7 the other 4 are 12ax7's. Tried installing a new output jack and before soldering the wires on again, I tested the common and 8ohm tap secondary and I'm getting 1.3r

Click the image to open in full size.
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Old 18th September 2019, 10:24 AM   #13
Tesla88 is offline Tesla88  Italy
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Click the image to open in full size.

Are preamp tubes "warm" ? Can you see if Heaters are on ?

I don't get how all cathodes are at 0V. It's your meter ok?

Last edited by Tesla88; 18th September 2019 at 10:30 AM.
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Old 18th September 2019, 01:43 PM   #14
kodabmx is offline kodabmx  Canada
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OT question
Quote:
Originally Posted by Tesla88 View Post
I don't get how all cathodes are at 0V. It's your meter ok?
Could it be using grid leak for bias?
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Old 18th September 2019, 01:46 PM   #15
Kay Pirinha is offline Kay Pirinha  Germany
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Appears to be very unusual for bass geetar amplifiers, due to blocking distortion. But only a schematic can tell for sure.
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Old 18th September 2019, 02:56 PM   #16
nigelwright7557 is offline nigelwright7557  United Kingdom
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It will get expensive if you started guessing and replacing random components especially OT's. You need to test the amp stage by stage and see where the signal dies.
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Old 18th September 2019, 06:09 PM   #17
SemperFi is offline SemperFi  Wake Island
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Quote:
Originally Posted by jimmy74 View Post
Yes they are connected to the screens...I have tested for voltage and bias all seem pretty normal around 508v screens, 512v plates, -45v bias and 3.3vac heaters. I'm getting around 100r between the UL windings
3.3VAC heaters? Shouldnt this be 6.3 for EL34s and either 6.3 or 12.6 for preamp tubes?
Or is this 2x3.3VAC measured on each half of a GND (or bias voltage)?
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Old 18th September 2019, 07:12 PM   #18
JMFahey is offline JMFahey  Argentina
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I much suspect he measured each filament pin to ground.

Which is fine.
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Last edited by JMFahey; 18th September 2019 at 07:18 PM.
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Old 18th September 2019, 09:11 PM   #19
6A3sUMMER is offline 6A3sUMMER  United States
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The still unanswered question is about this: Screen 508V Plate 512V

Why is the screen voltage 4V less than the plate voltage?

Swapping plate and screen connections can do that. And that will give a lot less power out.

Either there are resistors from the UL taps to the screens, Or, There are separate Screen UL windings and Plate windings. (that is a very expensive way to build an output transformer). How many guitar amp manufacturers will pay the extra $$$?

Most UL output transformer windings are essentially electrically connected this way: Plate winding all the way to the UL tap, and then continuing on to the Center tap. Another winding from the Center tap, to the UL tap, and then continuing on to the plate winding. That way, there is more B+ voltage drop from the Center tap to the plate tap, than there is from the Center tap to the UL tap. From the Center tap to the UL tap, has Both the screen current plus the plate current. From the UL tap to the plate tap, there is only the plate current. Ohms law. Does that make sense?
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Old 18th September 2019, 09:14 PM   #20
mozz is offline mozz  United States
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Screen/ plate voltages in guitar amps are often very close, i've seen screen higher also. No great power loss if even measurable.
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