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Post subject: Re: Blackface bassman 1967 AB165
Posted: Tue Mar 06, 2012 7:59 pm
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No fuss at all, Bro. Just replace the needed cap. Just speaking for myself, I tend to go overboard with these amps. It's no big deal ! Whatever it takes to keep makin' music, is the bottom line ! You know Blues, playing music with my daughter at church or music fests, is one of the top highlights of my life ! Art

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Tue Mar 06, 2012 8:18 pm
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blues bondsman wrote:
Just how much are you guys gonna fuss if i only change the one with a hole in it ?

$20 a piece for 20uf 600 volt astrons ? Dang I'm not rich ya know.


How much do you think it will cost to replace a vintage Bassman power transformer when one of those twenty-dollar caps you didn't change goes south and drags the entire power supply down with it???

Don't be a sap!

:roll:

Arjay

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Tue Mar 06, 2012 9:13 pm
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Arjay points to a valid risk, especially, with those old Mallory neighbors. But I'm wondering about the choice of the expensive cap. IMO, Sprague or F&T would take care of the job(you should be able to get it at your local music shop). Be sure the amp has a 2amp fuse! Art

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Tue Mar 06, 2012 10:23 pm
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" Completely unnecessary to use the standby switch when draining caps using a resistor. The negative of all caps are to ground already, and the drain resistor connects the positive of the caps to ground through the resistor. The standby switch does nothing when amp is powered off. "

Shimmy, if he clips the draining resistor to the (+) side of the 70mfd/350mfd, before the standby switch. And the switch if OFF. HE will not be draining the other 20mfd/525VDC caps behind the switch. They will be drained through the gain stage tubes to ground, by the 12AX7 or 12AT7 cathodes or the grid-to-ground resistors (could be a long process).

Really, more important--- you won't have a direct-to-ground line for "flyback" in amps with big OPT's. This residual charge can be more than a little voltage is some amps.

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


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Tue Mar 06, 2012 10:49 pm
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The choices are limited in order to get the voltage rating, uF value and size close to the original. Is the bad cap one of the 20s?

Would the 500 volt rated F&T be close enough in this instance? I would rarely, if ever, use a lower voltage rated cap in a power supply. Maybe the 525 volt rating on the schematic for the 20s is overkill? :?:

BMW2002Ti wrote:
...Shimmy, if he clips the draining resistor to the (+) side of the 70mfd/350mfd, before the standby switch. And the switch if OFF. HE will not be draining the other 20mfd/525VDC caps behind the switch. They will be drained through the gain stage tubes to ground, by the 12AX7 or 12AT7 cathodes or the grid-to-ground resistors (could be a long process).

Really, more important--- you won't have a direct-to-ground line for "flyback" in amps with big OPT's. This residual charge can be more than a little voltage is some amps.

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


I see what you mean now. You would clip the resistor to only one cap to drain all caps at the same time.

I use the drain resistor on each cap due to the resistors between the supply caps, and I assumed that's what everyone does, my fault. For example, if you clip onto the 70s, standby switch closed, you still have a about 33K of resistance between the drain resistor and the last cap in the supply (the 8 uF). You could clip onto one of the 20s and split the difference, but still have the other resistors in the discharge path.

Whatever works, always double-check with a meter to be sure that the caps are discharged.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 1:11 am
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Shimmy and Beemer, I'm with you both on the discharge. IMO It's just a matter of the technique and order. But the result must be the same! My procedure is to open the standby, relieve the preamp plates, then the power tube plates, then the power caps, and leave a draining resistor on the power supply. But we all have our own method ! And you make a good point, Beemer, about the OT's memory to produce a voltage. So yeah, I should probably leave the drain resistor on the power tube plates. But like Shimmy said, the meter is the key ! I read that the filter caps can exert a memory charge, that's why I leave it on the power supply. Maybe that info is bogus. So what do you guys think ? Is Blues going for a crap shoot on the single cap R&R ? Art.... Shimmy, the cap appears to be the screen supply for the power tubes. Might be wrong, but that wouldn't be the first time today!

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 6:56 am
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ARGHHHH !!!

So what needs to happen is to replace all the caps and resistors on that board ?

and some wiring under the board too from what I've read.

Yikes !

Here is my concern, I'm not the best solder guy, cost of parts and loosing any of sweetnes the amp is exibiting right now.

I found this
http://bwilliamson.home.mchsi.com/filtercp.htm
Looks like I need a parts list !

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 8:11 am
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I think that is the best bet for reliability and performance. The tone that you have now won't last with weak/bad components anyway. As Retroverbial astutely pointed out, the failure of the small components can cause bigger, more expensive problems, don't skimp on quality here.

To confuse things even more (sorry) :lol:
The article that you linked to brings up another interesting point. I agree about changing the carbon comp resistors to flame-proof metal film resistors. There is much myth about using carbon comp to maintain the "vintage" tone. While I admit that the carbon comps do have different qualities, and they might have a slightly different sound when used in some parts of the circuit (ie, Plate resistors), they also can and do literally catch fire when they fail. No advantage to using carbon comps for screen grid resistors, or power supply resistors, as the voltage drop across them is too small to take advantage of the different qualities of the carbon comps. The carbon comps can also be a source of noise in older amps. In particular, screen grid resistors, and power supply resistors should not be carbon comp, no advantage in using them there. The carbon comps quickly are out of tolerance, as their values can change quickly. I have compared the carbon comps to the metal film, and the only place that I thought there was a difference in sound was with Plate resistors, and even then it was slight at best, just a bit more distortion using the carbon comps. No difference in sound at all using the carbon comps in the power supply and screens.

Plain and simple, the only reason that carbon comps were used back when, was that is all they had, and IMO that has lead to the attributing of mystical tone qualities for the carbon comps. We now have much better/safer/more stable resistors in the metal film.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 8:19 am
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shimmilou wrote:
Plain and simple, the only reason that carbon comps were used back when, was that is all they had, and IMO that has lead to the attributing of mystical tone qualities for the carbon comps. We now have much better/safer/more stable resistors in the metal film.


+1!

Shimmilou knocks another one out of the park.

:wink:

Arjay

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 9:05 am
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sprague atom or F&T ???

The atoms are a bit more in kit form on e-bay $25 plus extra, Worth it ?

I noticed like the guy in the article that they are offering 500 volt in the 20UF caps

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 10:18 am
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Being cautious, I wonder about the voltage rating. It sure seems as if a 500 volt rating would be plenty for the 20s, and 25 less volts might not make that much difference. If others here agree, I would try the F&T as they are excellent quality also.

We might find at least one engineer here that insists on the 525 volt or higher, and I couldn't really disagree with that either. :| :lol:

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 11:46 am
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not sure if the 600's will fit in the pan ?
not gonna button it up without a cover.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 12:30 pm
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Yeah, they're nearly 4 inches long, wasn't sure of the board and cover size. :?:

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 1:13 pm
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Let me start by saying This is a brand new toy and I got a little stage fright so...
Be kind , I recorded a little sound clip of myself noodeling on the 67 Bassman

The post office literaly delivered it today and so I'm sure I don't know what I'm doing.
Its one of those Tascam DR-05 digital pocket recorders.

This is just so you can have a taste of the tone !

http://www.box.com/s/713jui0n7lqyfi5h5bvm

anyone know how to share sound clips without a hassle or worse ?

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Last edited by blues bondsman on Wed Mar 07, 2012 3:49 pm, edited 1 time in total.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Mar 07, 2012 3:32 pm
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Spragues will be more than adequate -- F&T's are merely "gilding the lily".

The only amps that might require caps rated for 600 VDC are the silverface "ultra-linears" from the mid-late '70s. These models (70-watt Pro Reverbs, Super Reverbs, Bassman 70s, and the 135-watt Twin Reverbs, Dual Showman Reverbs, Bassman 135s) ran outrageous plate voltages and require an extra margin of safety.

HTH

Arjay

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