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Post subject: No output from the Normal channel suddenly. Vibrato side OK
Posted: Sat Jul 16, 2016 7:42 pm
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'67 single Showman SF AB673. Recently restored and tested. Everything worked fine and all channels played well and sounded amazing. Played great up until just now after it sat for a month or so and suddenly there's no output from the normal channel (1 or 2). Vibrato side is fine.
Let me clarify. When I say no output I mean when I plug an instrument in, no signal passes through. I can hear the normal "hiss" which gets louder as I increase volume but nothing when plugging in a guitar/bass (other than said hiss).
For now I can play out of the Vibrato side but this obviously needs to get fixed.

New 6L6's installed and biased when restored. All other tubes are original.

Whats the proper way to troubleshoot this?

I should also admit I powered on the amp and let it warm up for about 2 mins before I realized the speaker wasn't plugged in :shock: it was still in standby though so hopefully I avoided a catastrophe. and again Vibrato side is fine.


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Post subject: Re: No output from the Normal channel suddenly. Vibrato side
Posted: Sat Jul 16, 2016 8:01 pm
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dammit, I figured it out.
The #1 pin on the first 7025 tube broke off in the socket. Pulled out of the bottom of the tube just beneath the surface of the glass

Yep, all original.
:x


Last edited by oKors on Sat Jul 16, 2016 8:04 pm, edited 1 time in total.

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Post subject: Re: No output from the Normal channel suddenly. Vibrato side
Posted: Sat Jul 16, 2016 8:03 pm
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Loss of heater voltage or plate voltage to V1 would do it. Check and ascertain that V1 is lit.

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Post subject: Re: No output from the Normal channel suddenly. Vibrato side
Posted: Sat Jul 16, 2016 8:03 pm
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Whoops......disregard.

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Post subject: Re: No output from the Normal channel suddenly. Vibrato side
Posted: Sun Jul 17, 2016 1:51 am
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Look to all wires from these jacks to the tube V1


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Post subject: tube replacement question
Posted: Mon Jul 18, 2016 10:18 am
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Ok,
So its definitely the tube that's bad. I put a known good TAD 7025 in its place and it plays out of the Normal channel again. Can I get a way with replacing just one of the 7025's with a NOS RCA from ebay or someplace reputable? I would attempt to match it best I could; I.E markings/printing on the tube, grey plate, etc.
Or could this create a mismatch problem???

at $80/ tube I cant afford to do all 4 preamp tubes.


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Post subject: Re: No output from the Normal channel suddenly. Vibrato side
Posted: Mon Jul 18, 2016 11:15 am
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There is no need to match pre-amp tubes. Some do, I would say most don't and some mismatch by type(power rating/brand) for subtle tone variations. Buy what you can afford as far as brand or vintage.


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Post subject: Re: No output from the Normal channel suddenly. Vibrato side
Posted: Mon Jul 18, 2016 12:00 pm
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sfceric64 wrote:
There is no need to match pre-amp tubes. Some do, I would say most don't and some mismatch by type(power rating/brand) for subtle tone variations. Buy what you can afford as far as brand or vintage.


Great, thanks.

When I did this restoration I wanted to preserve the original RCA tubes so I shelved them and I got all 4 new preamp tubes (hence the known good TAD I swapped in for testing). But once I got everything powered on and functioning I did some sound/volume testing with the new preamp tubes and then w/ the old RCA ones.
The difference was incredible! An order of magnitude louder and much cleaner sound with the RCA's. Needless to say the original tubes went back in which is why I'm so dead set on trying to find a NOS replacement of the original RCA.


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Post subject: Re: No output from the Normal channel suddenly. Vibrato side
Posted: Mon Jul 18, 2016 6:25 pm
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If you're dead set on a nos rca tube, the only way to go is through a reputable dealer. Ebay is a huge gamble! :wink:

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Post subject: Re: No output from the Normal channel suddenly. Vibrato side
Posted: Mon Jul 18, 2016 8:52 pm
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oKors wrote:
Great, thanks.

When I did this restoration I wanted to preserve the original RCA tubes so I shelved them and I got all 4 new preamp tubes (hence the known good TAD I swapped in for testing). But once I got everything powered on and functioning I did some sound/volume testing with the new preamp tubes and then w/ the old RCA ones.
The difference was incredible! An order of magnitude louder and much cleaner sound with the RCA's. Needless to say the original tubes went back in which is why I'm so dead set on trying to find a NOS replacement of the original RCA.


oKors, you can broaden your tonal search. For vintage Fender tone, the Raytheon 12AX7A is just as good as any RCA 7025 or 12AX7. Both red and yellow (Japan) labels. Sylvania 12AX7 are a bit brighter than RCA, but their 5751 are spot on. Esp the triple mica ones. Same for GE 5-star 5751.

Raytheon black plate 5751 types are considered some of the best ever made. Esp the windmill getter. But, I find the square getter variety just as good.

Key to any of these tubes, is quiet and high testing. Good luck on your search! :)


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Post subject: Re: No output from the Normal channel suddenly. Vibrato side
Posted: Mon Jul 18, 2016 10:16 pm
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BMW2002Ti wrote:
oKors, you can broaden your tonal search. For vintage Fender tone, the Raytheon 12AX7A is just as good as any RCA 7025 or 12AX7. Both red and yellow (Japan) labels. Sylvania 12AX7 are a bit brighter than RCA, but their 5751 are spot on. Esp the triple mica ones. Same for GE 5-star 5751.

Raytheon black plate 5751 types are considered some of the best ever made. Esp the windmill getter. But, I find the square getter variety just as good.

Key to any of these tubes, is quiet and high testing. Good luck on your search! :)


+1 on all counts.

As an aside, you might also consider vintage tubes from Japan. A while back I salvaged a couple of Panasonic-branded (ie: Matsushita) 12AX7's from a derelict Zenith clock radio -- they're some of the nicest-sounding and quietest pre-amp tubes I've found. Most Japanese-made vacuum tubes were built with US tooling, mil-spec materials, and fastidious QC protocols.

Arjay

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