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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 1:16 am
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Blues, Arjay has the right advise! After trying his recommendation,and there is still a problem, the microphonics and crackle could be addressed, by a component R&R. I'd go with quality 100K- 1watt plate load resistors. Capacitors do become micro, over time. The tone caps are the least desirable to be changed. Bypass and coupling(decoupling) caps are reasonable candidates. BTW That feed cap for the PI works very hard, might be worth consideration. And, like Beemer suggests, that bias supply must have a rock solid voltage supply. IMO Take it slow, don't R&R anything that's not too noisy. Microphonics are desirable to a certain extent. Believe me, I've totally rehabbed amps with choice components and tubes, only to find it's quiet but clinical sounding. The character is not always located in the perfect spec parts! JMHO, Art

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 4:38 am
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Yes guys I did retension the sockets per your advise a while back.

Used non lubricating contact cleaner too.

Like I said I have ordered the two pack of genuine Fender tube sockets from the guys at Music Guru and some 470K resistors to replace the one i can only get like 85K from on my meter, strange that every other resisiter that has gone way off its original mark have gone the other way and added resistance.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 5:51 am
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I just went back and did the elimination proceedure as per Webers Tube Amp Talk book.

What i found suprised me, I started with the PI and removed a tube, it wasn't untill I got to v-2 that all of the crackle went completely away, so I wiggled V-2 in the socket as perscribed and it didn't seam to make any difference, as soon as you remove v-2 the amp is quiet as it could be.

I would have to say that somewhere in V-2's circuit is the culprit.

The microphonics are still in v-1 but that is about a 1% concern compared to the crackle in v-2

Time to pull out the schematic and follow the yellow brick road :lol:

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 6:46 am
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I pulled the chassis again this morning

The two 470K resistors indirectly in line with V-2 read 545K ohms & 185K ohms
Then there is the 25uf 25V Cap, any way to test it ?

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 10:47 am
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That's well beyond the 20% tolerance spec.

Break out the iron and solder......

Arjay

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 12:16 pm
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All I could find localy are the tan radio shack resistors.

Metal oxides online

????

will the radio shack stuff be bad for the tone or safety of this amp ???

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 2:46 pm
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In the 165, both channels run through V2. So pulling that tube disables the entire pre amp. I'd still go with the plate load resistors. The 470K resistors should be good quality. One is a local negative feedback circuit the other is a grid leak resistor. Usually R&R that 25-25 with two seperate Sprague caps. Yes you can test the old cap, but it should be removed for the test(depending on the meter). Those ceramic disc caps can also become very resonant(micro). Keep at it! Art

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 3:26 pm
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seams like I'm chasing the wind here folks !
I used the Radio Shack cheapies just to test it out and see if it made any difference (the 470K) and it didn't.
It was quiet before the cap job, remember I was so hesitant to open it up.

I'm starting to regret messing with it.

I'll put the metal film 470K resistors in it when they get here and most likely stop there.
all I'm doing is making the amp less desirable every time I pull old parts, not like you can replace the old carbon comps, they crumble at the slightest touch with any tools.

BLAHHHHHHHHHHH ~!!!!!!!!!

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 3:55 pm
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So
I turned down the volume of my guitar and left the amp all the way on and at volume #2
I let it stay on for about an hour (a little burn in time).
First thing i noticed is that again the hum balance is off and it needs adjusting again.
so........ Maybe just maybe that little cap under the rectifier diodes is the main trouble ????
And if so, just exactly what should I replace it with ?

Sorry frustration is taking over !!!

On top of that the idiot who sold me the speakers for my cab taped two boxes together using two strips of tape, I know you'll never guess but only one box and one speaker arrived (I hate E-Bay) !!! and the one that did arrive had zero packing material in the box.
so its been one of those weekends !

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 4:52 pm
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Blues,

That cap a 50 mfd/70 VDC rated one? I use two F&T 33mfd/160 vdc in parallel. Or a single 33mfd/160 VDC if it was a 20 or 30 mfd/70 VDC cap. These really work well and fit nicely.

Did you clean the bias pot? They are kinda notorious for getting pitted after years of not being tweaked. Heat is a common malady. When the bias is stone steady, then starts creeping around with use, suspect dirty pot. And dirty, changing contact with the 6L6GC sockets.

Good luck! You know, I find putting the thing away and walking away from the bench is often the best solution to a nagging problem (esp works well with nosy sig others--- :lol: ).

Just leave a love note with all you have done and what is still the problem taped to the chassis. Go play golf. Or take a nice long drive somewhere cool.

I had to learn this the hard way. I had this VT-22 on a bench and it's schemo and layout wasn't CLOSE to the printed papers on the amp. I was basically gutting the entire beast to "find" the problem. And getting nowhere. After a week of not looking at the thing, I went back and found the socket on the PI tube (12AU7) was hinky. New socket and a Hytron 5418A tube... the guy still gigging with it. Boy those 7027A are GREAT tubes.


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 6:04 pm
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This is the Cap

Image

I could change nearly every resistor on this amp, they all seem to be way out of wack !
funny thing is the one 470K I changed out, the new one had the same reading as the old and the old one was like 477K so.........
something about the placement in the circuit changed the reading,

Chop stick testing hasn't done a thing, the crackle does not want to show off its location, glad this isnt my job ! frustrating enough for my own stuff, imagine having someone calling every day "is it done yet? " I think I would flip out !!!

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 6:45 pm
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it looks like the schematic says 50uf 50 volts

I found 50mfd but no voltage reading on the cap

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 7:09 pm
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Yeah that's norm. I'd go with a 50-100vdc. Art

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 7:11 pm
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Blues,

It's a 50 mfd/70 VDC per schematic cap. I use the 33mfd/160 VDC F&T in parallel (=66mfd total capacitance). Works well.

The amp tech I knew that got me started actually had a contract with WB. He'd have to drop everything in the middle of the night, if a contracted band's amps started going bad. They fly him to the concerts and all that. But, he'd have to fix the thing on the "fly." Literally. I'm glad to have a day job. Even if it means working graveyards. :lol:

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 7:24 pm
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I just ordered
one 10K bias pot
three 470ohm metal oxides (bias board
two replacement NOS 470K C/C resistors
and a few tube shields to cover the naked ones.

I just pray I wont have to ever pull the main board, so far I've been able to wiggle out old resistors with help from some heat, it hasn't made me real happy with the resulting look of the board due to the fact that seating in the new leads is a real pain in the Tail feathers !!!!! the bend soooooo easily.

All the while I'm doing my very best to not get bent myself, and when your a Cherokee, Irish, German, brother that aint easy !!!!!!! :mrgreen:

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Last edited by blues bondsman on Sat Apr 21, 2012 7:45 pm, edited 1 time in total.

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