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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 8:41 am
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1/2 watt, 10% tolerance, carbon comp. These are dark brown, squared ends.

The metal film are 2% tolerance, blue color, rounded ends.

Don't use the carbon film (tan/light-brown colored), they are just as bad, if not worse than the carbon comp. Both the carbon comp and carbon film, can and do literally catch on fire. :o

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 9:34 am
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blues bondsman wrote:
its very small ! 1/2 watt ???


If you check the notations on the schematic you'll see that all resistors are 1/2 watt unless otherwise specified on the drawing itself.

HTH

Arjay

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 10:20 am
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I did see that, well now I do :roll:

I picked up some 1/2 watt carbon comps, they lasted 45 years the first go round.

I found most of them well within 10% two are so far out it isn't funny.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 1:02 pm
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well that didn't make a lot of difference, although I'm sure it needed to be done.
So I started checking some of the other carbon comps and they are all over the board.
I had only checked the 100K's before.
Looks like i could spend a bunch of time getting the resistors up to snuff on this board and maybe still not get rid of the slight crackle. :(

I've never seen a tube circuit act so much like a microphone and it does not matter what tube is in the socket, could it be the socket itself ????

P.S. the switch on the bass side "deep" goes POP !!! when you flip it up, like a cap discharging, is that normal ?

Thanks

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 1:38 pm
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It's possible that the bakelite insulator in one or more of the tube sockets may be the culprit. Inspect them all very carefully (both top and bottom) for any tell-tale signs of age- or thermal-related issues. The older this material is, the more brittle and prone to cracks or flaking it becomes.

HTH

Arjay

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 3:34 pm
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OK. My experience with CC resistors is the made in Orient crap is crap. Tonal, tolerance, length of life. You can still get Allen-Bradleys, which I would recommend in places, like signal line, where heat isn't a big issue (like right under the power tubes).

I've had really good success with Mills in the PS rail & cathode resistor position. Dales where heat and tolerance is an issue.

http://www.hificollective.co.uk/compone ... stors.html


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 5:59 pm
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the whole microphonic v-3 is whats puzzling me the most
I have installed a brand new Old Stock RCA, a known to be 90% 5751 and a tung-sol reissue in the socket and yet I'm betting you could here a voice through it if you dared get close enough.

Isn't that kinda wierd ?

I have a few more resistors that are supposed to be 470K and they are 80K so no doubt I'll get back to the resistor swap (which I am not happy at all with).

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 6:06 pm
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Sorry, late to this thread. But, did you "chopstick-test" the coupling caps, too? I've had some get noisy. SBE OD's seem more notorious for this. It's why a lot of tech gunk the crap outta them, after installing, with silicone sealer.

What I do is place the cap's body slightly above the board and put a dab of sealer between the cap and board. It's all you need. Don't gunk the crap outta all the caps. Worst looking mess by any professed "tech." IMHO.


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 6:20 pm
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Glad to have ya as usual !

yes I did the chopstick test to every componant on the board.

Nothing was made to buzz or crackle on demand, i even went back to the filter caps and cleaned up any and all solder joints.

Again all I can figure is a bad tube socket or more way out of range resistors.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 6:36 pm
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Socket sounds like a candidate. I know it's a bit more of a beetch to replace those (esp 9-pin minis), but you may need to go there.

BTW... I'd had resistor and caps get noisy on me, after they heat up a bit. Let the amp idle for a good 10 minutes, then try the chopstick test. For the socket what I do is grab the "tit" of the 12AX7 and gently rock it while the amp is on. I use Mechanix gloves with hot tubes--- they really work and you don't lose dexterity. You'll hear the microphony or static sometimes this way. Change tube, same results? New socket needed.

Good luck!


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 6:50 pm
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YUK !

But I found some real deal Fender replacements at the guru store and hey their close by too.

I hate to keep replacing so much on this vintage amp ! but I want it to play, not a museum piece to look at and so here we go again.

Now i still have not replaced that funky look'n cap on the bias board, possible added hum ? however hum isn't the problem, its crackle.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 7:18 pm
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Maybe just check the negative volts coming off the bias supply. Then, tap the component of the supply. You may want to pull your 6L6GC's.

The volts should be rock steady. If not... first pull the separate bias board off the chassis and check underneath for excess globs of solder. I've seen several post-tech jobs that had enough solder to poke through to the chassis.

Good luck!


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 7:31 pm
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pretty sure its original
Black with red on one end and yellow on the other (if my memory serves me)

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Fri Apr 20, 2012 7:53 pm
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Just for fun I read this entire thread (daughter is at homeschool prom, wifes watching a scary movie) and its been a very cool learning experience.

I ordered two 9 pin sockets and will no doubt change one connection at a time when i replace V-3.

I'm pretty sure thats were my main sourse of noise is from, thats my hypothesis and I'm stick'n to it, at least untill I install the new parts.

I feel very connected to this amp now, kind of like building your first Hot Rod 8)

My hope is that the next guy to want to tune up an old Bassman finds this and gets the answers he's looking for.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 12:51 am
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If you have not already done so, try cleaning and re-tensioning the contact pins on all of the sockets, especially those for the 9-pin subminiatures. Your crackle issue may be nothing more than the product of some errant corrosion or a loose cathode pin.

Arjay

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