Wrong power terminal voltages in non-functioning S.E.X. amp from 2007 [resolved]

landaunp · 1829

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Offline landaunp

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Scroll to the bottom for just the bones of the problem. For those that want some story....


In the beginning:
Hello all! Back around 2005 I caught the audiophile bug. Around 2007 I bought and built my sex amp and it loved it! It faithfully powered some of my favorite headphones like my much loved AKG K340's.
Then:
Fast forward a couple of years and my S.E.X. amp stopped working. No sound in either channel. I always meant to fix my beloved S.E.X amp, but between more rigorous college classes, and other interests, music and headphones took a backseat and I unfortunately never got around to it. I still used my less demanding (read: lower impedance) headphones, but I stopped listening as closely as I used it. Listening to music became a backdrop--a soundtrack to my life, rather than something I took time to enjoy just by itself. And it stayed that way except for the occasional listen here and there until...
Now:
Fast forward SEVEN YEARS until today. I graduated college plus some, and I landed a job in the industry I studied for. I was digging out things I had stored at my parents house when I found my old K340's and my S.E.X amp! I want to get these up and running again, but after all these years, I'm still not sure what's wrong with the amp.



Here's the problem...
The technical details:
I measured all the resistances and they match up with builders manual I found. So did the DC voltages. The AC voltages DID NOT. Specifically, the Power Transformer and the Plate Choke do not come even close to what they should measure.

T1 measures at 120VAC and T2 measures 0VAC, but that's about it. The rest are as follows:
T4=7VAC
T5=7VAC
T6=426VAC
T7=415VAC
T9=435VAC
T10=428VAC
The plate choke measurements are similarly high.

Any help with how to fix my poor, neglected amplifier would be greatly appreciated! I miss listening to music with it, and now that I find myself in a more calm place in my life, I finally feel like I have the time to. Thank for reading if you got this far.

-Nick
« Last Edit: January 19, 2016, 07:09:21 AM by Caucasian Blackplate »



Offline Doc B.

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You're sure all the DC voltages are correct? Are you also sure you measured the AC voltage as described in the manual?  If DCV are correct the AC voltages pretty much have to be correct. This is often an issue of not setting the meter to AC volts or not measuring across the proper terminals.

If the DC voltages are correct it means the amp is properly drawing current and your issue is probably in the signal path wiring, i.e., input jacks shorted by solder, headphone jack improperly connected, parafeed caps miswired, something along those lines.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline Paul Birkeland

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The plate choke measurements are DC (recheck the manual, they follow the other AC measurements, but specify DC).

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline landaunp

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So I "fixed" my amp. As it turns out, the only thing wrong were the tubes I had. As soon as I swapped them out with a pair of RCA tubes, they problems went away and it sounds amazing again! Thanks for your replies. I'm glad it's working wonderfully again. Next thing to do is replace the wood housing with something a little nicer.