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Post subject: New Caps for a Vintage Twin
Posted: Wed Jan 23, 2013 9:33 am
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My old 72 Twin is starting to distort on the clean channel at high volumes whenever I'm picking on the low E. The amp is loaded with JBL E120s so it shouldnt be the speakers. I was told it probably needs a cap job. What can I expect to pay for this, and what exactly will it improve?

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Post subject: Re: New Caps for a Vintage Twin
Posted: Wed Jan 23, 2013 10:20 am
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It *might* very well need a cap job, but audible symptoms of such a diagnosis usually include a constant hum at any volume level. More likely, the amp is suffering from tired tubes or possibly a drift of the idle-bias setting due to aging components.

Arjay

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Post subject: Re: New Caps for a Vintage Twin
Posted: Thu Jan 24, 2013 3:17 am
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+1 Retroverbial,

Tubes first , with bias on output tubes

Possible drift resistors in Phase Inverter circuit too.


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Post subject: Re: New Caps for a Vintage Twin
Posted: Thu Jan 24, 2013 10:40 am
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These Fenders often have one of the two caps in the PS first stage go dry before the other. Usually, the one that "sees" the B+ first. Happens when caps start to dry out and the 220K-ohm equaliziing resistors start drifting. And sometimes the amp doesn't really make hum --- but, has this oddball harmonics and/or poor bass response.

If all the filter caps in the PS are older than 15 years -- probably best to replace them. As well as checking bias (replace the electrolytic caps in this circuit, at same time as main filter caps). Rebias, as Arjay said, whenever you change any component in the output stage. Not just the 6L6GCs.

It's probably a very good idea to have the tech check the 5% tolerance resistors on the phase inverter and grid-to-ground for th 6L6GCs. As Stratele stated.

Good luck with your amp.


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Post subject: Re: New Caps for a Vintage Twin
Posted: Thu Jan 24, 2013 12:55 pm
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BMW2002Ti ,

If 15 years old it need new caps ?

I totaly desagree, new caps not before 30 years at least. I have lots of electronic stuff bough in '80 = 30 years. And more stuff over 15 years. Don't need caps.


If you don't use amp since 15 year ? Or more ? Maybe caps are dry and is not sure.


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Post subject: Re: New Caps for a Vintage Twin
Posted: Thu Jan 24, 2013 2:46 pm
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Stratele, I should have said who made those caps. There were many poor caps made in the 1990's. The OP's post sounds like first stage failure. And not PI issues.

JMO...

Have a good one!


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Post subject: Re: New Caps for a Vintage Twin
Posted: Thu Jan 24, 2013 2:49 pm
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BMW2002Ti wrote:
There were many poor caps made in the 1990's.


And even worse caps made today!

:shock:

Arjay

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Post subject: Re: New Caps for a Vintage Twin
Posted: Thu Jan 24, 2013 2:59 pm
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BMW2002Ti wrote:
Stratele, I should have said who made those caps. There were many poor caps made in the 1990's. The OP's post sounds like first stage failure. And not PI issues.

JMO...

Have a good one!


If your experience said those caps are very poor an need to be replaced, I bow before you and I take back what I said.

( Is this is a good english ? I ask Google translate )


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Post subject: Re: New Caps for a Vintage Twin
Posted: Thu Jan 24, 2013 3:17 pm
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Stratele, understood. No problem. :D

When tubes starting to be dialed back in production, many corresponding parts (like HV caps) also got dialed back.

In the 1980, Mallory and Sprague (two biggest US suppliers) shifted to SS cap production. There were a TON of generic companies popping up to fill the gap. Many of these companies didn't last as long as their caps. Which was not long at all. Ever hear about the infamous AERO MIM cap cans? I don't know how many leaking or exploded cans I've seen. As their idea of 450VDC rating really = about 300VDC.

In the 1990's --- tube amps were missed, after years of SS amp failures or poor tones. So, companies like New Sensor, Groove Tubes, etc rushed back into the business, as tube amps had their rising form the ashes. Culling whatever new tube productions were still available.

Then, quality HV caps started to appear, again, on the scene. Like Panasonic and SBE.

Have a good one, my French-Canadien friend!


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Post subject: Re: New Caps for a Vintage Twin
Posted: Thu Jan 24, 2013 5:00 pm
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Thank's BMW2002Ti


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Post subject: Re: New Caps for a Vintage Twin
Posted: Fri Jan 25, 2013 5:46 pm
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Retroverbial wrote:
BMW2002Ti wrote:
There were many poor caps made in the 1990's.


And even worse caps made today!

:shock:

Arjay


+1

Art


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