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Post subject: 67 Dual Showman turns on but low output?
Posted: Thu Nov 29, 2012 5:18 pm
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I have a 67 Dual Showman AB763. Amp turns on, tubes are lit (not all the same brightness) and can barley hear it on 5 and on 10 output is about 1. Any suggestions? I would like to chase down the problem and learn. Any help is appreciated.

Thanks,
Matt


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Thu Nov 29, 2012 6:42 pm
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Be sure the speaker and instrument cables are ok. Sub in another guitar patch cord and see if that helps. And another guitar. Twirl the guitar and amp knobs and see if you can get volume. No luck? You prolly should take the amp to a good tech.

He needs to:

Test the tubes. they maybe tired, or near death. :x

Check the idle bias on the output tubes. And plate voltages to all tubes.

Check speakers for bad connections & voice coils.

There maybe other issues. The entire amp prolly needs a good R&R. Good luck! Keep us posted.


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Thu Nov 29, 2012 6:46 pm
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Cords are good. Speakers are good. Tested everything with my tremolux and all works. Just low volume. What can I test with a volt/ohm meter?


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Thu Nov 29, 2012 6:56 pm
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First, pull tubes, one-by-one. And sub in a known good one. Maybe, it's a weak tube.

You would need to pull the amp. Place in onto a workbench. And check voltages compared to the schematic. Honestly, unless you have done this before and are compfortable working with hundreds of DC voltages (which can kill you) --- I'd look for a good tech.

How much electronics experience do you have? You read a schemo?


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Fri Nov 30, 2012 10:20 pm
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Turns on, but low output. Yeah, had that problem. Not in an amp though. :lol:

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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Sat Dec 01, 2012 3:03 am
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First do what BMW write .

B+ voltages reading is the second test to do after tubes .

If these test are ok , you need a 'scope and audio signal generator to follow the signal ( sound ) and you'll see where it drop.

Could be output transformer.

It is a work for a tech .


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Sat Dec 01, 2012 7:25 am
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Before anything else, try the other speaker jack (the one labeled "speaker"). :idea:

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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Tue Dec 11, 2012 6:00 pm
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Ok, I believe I have it figured out.

1 bad tube (verified in another working amp)

1 bad filter cap, 20MFD 450VDC.
My schematic doesn't show this value. It shows a 20MFD 525VDC. This leads me to believe some has replaced parts incorrectly.

I would like to replace all filter caps. Anyone's help to determine which are typically replaced, electrolyte caps, high voltage, high capacitance, etc. would be appreciated.

Thanks!


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Tue Dec 11, 2012 9:28 pm
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Ok, looking at the scheamtic and layout of the amp:

http://www.webphix.com/schematic%20heav ... _schem.pdf

You have solid-state rectification. Therefore, it"s ok to go up in mfd value on the main filtercaps, within say a 25-50% range, as long as you maintain similar voltage rating on the cap.

I've used 100mfd/450VDC caps for the first section (the most critical) and 22mfd/500VDC caps for the next three sections. All resistors in the power rail should be checked for tolerance.

I usually replace the two 220k-ohm/1 watt resistors between the caps in the first section with good quality metal oxide, 2 watt, 5% tolerance types. These are critical resistors, as their function is to distribute the voltage evenly across the OEM 70mfd/350VDC caps (or 100mfd/450VDC cap, if you go to these replacement types).


http://thetubestore.com/lcr.html


I like low-inductance resistors for the 4700-ohm and 1k-ohm resistors. 1 watt for the 4700-ohm and 2 watts for the 1k-ohm. Wire-wound Mills are the best. They last a long time and keep within tolerance. There is a bit of jazz about how low inductance resistors makes for "faster response" and lower noise. Been using them for awhile. Very stable voltage. And good ceiling on background noise.

If you decide on Mills, 5-watt MRA-5 is fine.

http://www.soniccraft.com/mills_resistors.htm

See if this is a good start on the PS circuit. My experince, is this setup and thorough check of the BIAS POWER SUPPLY circuit are the first must in making your vintage gear reliable.


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Wed Dec 12, 2012 8:22 pm
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BMW2002Ti thanks for your reply.

This amp has been sitting for 25+ years. Would you recommend replacing all the tubes?


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Wed Dec 12, 2012 8:59 pm
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This amp has been sitting for 25+ years. Would you recommend replacing all the tubes?

Nope. As long as they have good vacuum and are strong tubes (prolly are, if never used in 25 years). Try them out, after the 'lytic change. Be sure to replace the lytic in the BIAS POWER SUPPLY. I'd go up in voltage rating and more capacitance here, will help to isolate any noise from this circuit. A 100mfd/100VDC would be fine.

http://www.thetubestore.com/Capacitors/ ... 100uF-100V

Check all resistors in the BIAS SUPPLY for tolerance. And those 5% 220k-ohm ground-to-ground resistors on the 6L6GC grids. All of these parts help determine a steady nominal idle bias. A must for any amp.

Good luck. Keep us posted!

:mrgreen:


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Thu Dec 13, 2012 8:17 pm
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There are 5 caps under the cover ( these are for the PS, correct?)
There are 2 dozen more electrolyte caps on the other side should these be replaced as well?

Again, thanks!


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Thu Dec 13, 2012 8:57 pm
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Matt, the big caps under the hood are the main power supply caps. These should be replaced about once every 15-20 years. Really, depends on usage (or lack or usage). If the amp has been sitting around --- all the cap have lost their "form" from lack of use. And should always be replaced. If the amps has been used constantly over the last 10 years -- the need isn't as acute, unless you are developing a hum or oddball overtones on top of notes. Esp. if you have hum on cold startup, that goes away when the amp warms-up. This is Fate's way of telling you to do a cap job!

Drain the old caps. first!

http://www.justinholton.com/hotrod/caps_p.html

CAUTION! WATCH THE POLARITY of the replacements (if you do it).

The smaller ELETROLYTIC (+/- sides) caps that bypass the cathode-tied resistors on the circuit should prolly be replace when you do a wholesale change on the power supply caps. That way you will know in the future, when all the caps were done.

The non-polar coupling caps only need to be replaced if they LEAK DC voltage (very important) or go way outta spec (for tone, and not as critical for amp life).


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Fri Dec 14, 2012 8:44 pm
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I had checked for broken connection before and found 1. When i was checking voltages tonight I found a second one and a cracked tube 12ax7. Swapped out tubes from my Tremolux and it working and no hum. It sounds great! I still need to order tubes and filter caps. I'll update after new parts are in.

Thanks!


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Post subject: Re: 67 Dual Showman turns on but low output?
Posted: Sat Dec 15, 2012 1:33 am
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Ok, good luck! Mebbe you be ok, for a bit, at least. Though old lytics make me nervous.

:lol:


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