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Post subject: BF Bassman bias supply cap question
Posted: Sat Sep 17, 2011 1:08 pm
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Had my Bassman serviced, including filter replacement. Haven't really used it since getting it back, but the one time I tried it, it seemed to have some hum.

I decided to play around w/ it the other day. The amp was definitely humming, and I got suspicious that the 10uf (150V) cap in the bias supply was too low in value. I clipped in the old 25uf cap and the hum lessened.

I'd simply reinstall the old cap except that the bias supply measures 60V and the original cap is rated for 50V, which jibes w/ all the schematics I've seen. I've measured the various supply voltages in the amp, and they're all correct. The components in the bias supply are correct values, such as the 470 ohm resister in series w/ the diode. The AC voltage from the PT bias supply tap is 50V. I assume this is normal, though I can't find the voltage specified on any schematic.

I could just get a higher voltage cap, but this seems screwy and I'd like to know what's up. 25uf/50V agrees w/ the schematic, but not w/ the actual voltage being produced.

BTW, these measurements are w/ tubes installed.

Thx. -Dave


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Post subject: Re: BF Bassman bias supply cap question
Posted: Sat Sep 17, 2011 2:06 pm
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kk... Jazzmatazz, I presume you have the AA or AB864 circuit Bassman. I usually go higher VDC rating on the Bias Supply cap. 100VDC would be nice. It's ok, on a Bias Supply to increase the mfd a bit, if you can't find a 25mfd/100VDC cap.

33mfd/160VDC rated F&T (AES part # C-ET33-160-FT) is what I use.


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


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Post subject: Re: BF Bassman bias supply cap question
Posted: Sat Sep 17, 2011 2:23 pm
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2002:

It was an AB165. Now it's more like an 864.

Yeah, I know can increase values. And that's what I'll do, since I guess what you're telling me is the voltages I'm seeing on the bias supply are typical. Then the question becomes: did Fender spec that cap at too low a voltage? I know old caps were often very well made, w/ plenty of headroom, but jeez.

Thx for the info, and the AES P/N.

Dave


BMW2002Ti wrote:
kk... Jazzmatazz, I presume you have the AA or AB864 circuit Bassman. I usually go higher VDC rating on the Bias Supply cap. 100VDC would be nice. It's ok, on a Bias Supply to increase the mfd a bit, if you can't find a 25mfd/100VDC cap.

33mfd/160VDC rated F&T (AES part # C-ET33-160-FT) is what I use.


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


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Post subject: Re: BF Bassman bias supply cap question
Posted: Sat Sep 17, 2011 2:35 pm
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J, No Fender spec'ed the caps to the older 110-115VAC outlet voltage. Today's outlet voltage runs higher. Like 118-125VAC. Therefore, the entire circuit is seeing higher voltage.

It's most critical with those Bias Supply and gain/PI/EFX stage bypass caps. Which are usually rated 25VDC from the factory. All should be up'ed to 50VDC (at least).

Coupling caps and Main Power Supply filter caps usually have more than adequate voltage rating to compensate. I would NOT run less than 600 VDC rated coupling caps. Or less than 400VDC rated main power supply filter caps.


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Post subject: Re: BF Bassman bias supply cap question
Posted: Sat Sep 17, 2011 2:49 pm
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BMW2002Ti wrote:
Or less than 400VDC rated main power supply filter caps.


In an ultra-linear silverface, those had better be spec'd to 600vdc lest the amp fail the "smoke test" under sustained full-power conditions.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: BF Bassman bias supply cap question
Posted: Mon Sep 19, 2011 6:42 am
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2002:

I hear you on the AC mains issue. Though on my amp, all the voltages pretty much agree with the schematic, even at 120V line. However, even reducing line voltage to 110 w/ a variac, the bias supply still is at 53-54VDC. FWIW...


BMW2002Ti wrote:
J, No Fender spec'ed the caps to the older 110-115VAC outlet voltage. Today's outlet voltage runs higher. Like 118-125VAC. Therefore, the entire circuit is seeing higher voltage.

It's most critical with those Bias Supply and gain/PI/EFX stage bypass caps. Which are usually rated 25VDC from the factory. All should be up'ed to 50VDC (at least).

Coupling caps and Main Power Supply filter caps usually have more than adequate voltage rating to compensate. I would NOT run less than 600 VDC rated coupling caps. Or less than 400VDC rated main power supply filter caps.


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