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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 7:33 pm
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Blues,

Good luck! You know what works, about the board issue? Check voltages. On ALL tubes. Often the one with the shorted lead (shorted to the chassis) effect that stage's voltages. It's how I found that "Torres modded" PR issue of "on and off" reverb. Dang coupling cap lead straight through two breadboards & a 50/50 contact to ground. The voltage on that tube was all funky. And changed with chopstick pressed onto breadboard.


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 7:40 pm
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and a 50uf 50 volt cap too

sorry thats whats in it and on my AB165 bassman schematic so...
Thats what i got

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 7:43 pm
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the board does have a big "warped" area between the screws and it does not press together easily.

Dang !!! Theres no flippin way I want to deal with all them leads and trying to get it all back together :?

I'm a parts swapper guys not a tech !!!

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 7:45 pm
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Blues, 50/50 is fine. Really, the voltage rating is important... but the current load through the bias supply is so low (mA), that any good 'lytic will prolly be fine.

And the noise-injected issue is over-blown. Only hi-fi nuts put full-wave bridges and pi-filters onto bias supplies. Good steady contact with quality issued parts is good for, oh... 40-50 years or so. IME.


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 7:48 pm
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Blues,

Get the bias supply fixed. Then do a careful stage-by-stage voltage check of every point that has reading on the schematic. Tap the board with the chopsticks as you read. And have the negative lead of the DVM clipped firmly to a ground point on the chassis itself (not onto any lead to ground). My bet is you'll find your culprit. Or at least the area to concentrate.


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 7:59 pm
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So just to go back over some ground we have already tread upon, I remove the PI all static goes away, reinstall it comes back, the only other tube that make the static go away is v-2, identical results as the Pi being removed.

We should be looking somewhere in the preamp circuit right ?

Keep in mind this too, the static is fairly intermittent and it goes away for a fair amount of time after a decent warm up, it may then return if you play for say 30 to 50 minutes.

Thanks for being patient with me as I feel my way in the darkness of ignorance
:lol:

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 8:08 pm
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Blues,

Sorry, I was late to this thread and didn't see this. OK... one thing to check (and I'm the biggest missed-check)--- the power supply. Are all the voltages cool on the B+ taps? And the caps look ok? You may have already been down this road... but, has to be cleared before venturing further. That BFSR I'm fixing had one bad 20mfd/500VDC cap under the doghouse. It looked fine, but voltages were not.

Check the resistors, not only on the rail, but the ones splitting the load between the two 70mfd/350VDC caps in the first pi-stage. They are 1 watt, 220k-ohms. Hiding under the second big cap. If you've done this already... then you prolly need to move onto other areas of potential problems.

Bad thing about the Net... sometimes, you just wish you could drive over to someone's house and share the solder fumes & strategy.


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 8:13 pm
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Blues,

I just started to re-read this whole thread and realized we've been down this path. One thing... new caps can sometimes fail on you. I've found that ramping up new caps helps the life, as you have no idea how long they've been sitting on a shelf uncharged.

Maybe, check the voltage taps off those filter caps under the hood. One more time. Before drastic surgery.

Sorry, for the repeated journey.


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 8:24 pm
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No problem I am a debtor to all of you with the knowledge !!!

So keeping it as simple as i can here
Neg terminal on my meter to chassis ground and pos to the + or possitive side of the cap, all plugged in and running (no problem I dont want to fry, i'll be super carefull).

and get my supposed readings to check against from the schematic ?

O.K. so far ?

I wondered about a possible faulty new cap, the reason being that the amp was dead quiet before the cap and resistor job.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 9:50 pm
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Blues, here's the scoop I've gotten from other boards. SOME of the new Sprague Atoms have been noted for early failure. And from one supplier. Now, it doesn't make sense... unless that supplier buys in batches and got a bad batch.

What I try to do is ramp up ALL new power caps (regardless of manufacturer) with a Variac. Use a direct heated rectifier in place of an indirect one (like 5AR4) in those amps (sub with a 5U4GB, for instance)--- so that the caps are "seeing" low voltages.

You prolly won't need to worry about this, as it seems your Bassman has solid-state rectification.

Measure each tap point off the power rail as you ramp up the voltage (start at 60VAC and go up 20VAC increments every 15-30 minutes). The voltages should stay relative. That is is one tap is 430VDC at full voltage and another is 375VDC... then at 80VAC input one might read 200VDC and the other 175VDC---staying about the same difference in voltage from each other. If not... you have a short or a bad cap.

Do that all the way until you hit house voltage (120-125VAC). This seems to be the best way to insure the proper hookup and working power train caps.

As for the other voltages, you may not see them always posted on the schematic, but they may appear on the layout. See the AA165 schemo and layout posted on the Net:

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

But, with known good tubes you need to be within +/- 20% of the stated values. A DVM will read (-) voltages just fine. But, the plate and cathode reading (there are two for each 12AX7 and 12AT7) is a clue to what maybe going on with that section of the amp. Start with either the input stage and work step-by-step to the output tubes. Or go the other way (6L6GC to the 12AX7 input). Don't pull tubes, as this can effect the voltages up or downstream. Make sure the amp has a load (hooked to speaker or 8-10 ohm, 25watt WW resistor). NO INPUT, zero volume on both channels.

A few precautions. Keep your free hand off the chassis. Put in in your pocket, if you tend to slide it over without knowing. Make sure that the DVM had a good long insulated handle (ala Flukes). As you do not want to accidentally slip and touch the probe or chassis while the amp is "hot."

I use a magnifying glass (like those goofy ones you wear) to measure tight spots like 9-pin socket tangs. The green twisted leads are esp dangerous... as they are the heater lines and carry a lot of current. Current is what is dangerous, not voltage, per se. I mean
you will hear the loudest bang (esp with input tubes, as they have highest gain in the amp), if you touch grid to ground-- but it won't do damage. In fact, a lot of techs used that technique to test stages---purposedly tap the grid to ground and listen for the bang!

But, touching the heater line to ground can cause damage. Think of it like touching a tool on your engine to the positive terminal of the battery. Not as many amps, but enough to cause an issue.

Sorry, for the scary stories. But, many are from the school of hard knocks. Something that is more difficult as we get older. :mrgreen:

Good luck! And if the voltage readings has you worried. A good tech could be the best solution. Just a suggestion.


Last edited by BMW2002Ti on Sun Apr 22, 2012 2:30 am, edited 1 time in total.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 21, 2012 11:31 pm
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Beemer has the right advise! Just a note: the recap can provide a higher level of voltage and current than the old caps. So the entire amp is experiencing a change. The components in the higher voltage and current carrying positions, would be the first that I'd consider for a R&R. And yeah, the voltage "pop-test" is a very good method ! Art.......Maybe brother Stratele can help you! He knows his way around these amps!

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 4:02 am
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Hey man, the wave is normal. And as long as the board is not conductive, it's fine. Don't know why you'd worry about the solder stalactites piercing the lower fiber board with an unmoddified amp like yours. But the continuity test is simple. Ah yes the wave, the only problems I've encountered are the components that are on the crests. On 165, the 470K grid leak resistor that runs horizontally, then to ground could have stressed, to the point of breaking the solder joints. Art

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 5:22 am
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The only tech in our area retired after over 50 years in the business, he was fender trained and worked at a local store forever.

he took a few side jobs and I got him to replace caps on that 40's tweed princeton i found a few years back, he tore the grill cloth and lost me some serious value on that pristine little tweed, i have not been back. (not mad just frustrated) well see how good my eyes and hands are if I make it to my 70's 80's etc....

Thats why I've been trying to learn all I can, Just learning the language has been difficult enough :lol:

And so ... i dont have a variable voltage supply (variac)

Are you guys sure none of you wants to see the sights here in Hoosierville ???? :mrgreen:

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1968 Bandmaster with 2X12 cab C-rex speakers
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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 5:49 am
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Maybe this will help the diagnosis.
When I pulled the chassis the last time I put the possitive lead on pin #1 of V-1 and neg to chassis ground, the reading was something like 150 and was falling fast down to single digits.
should the caps bleed off that fast without a resistor ? or is it possible again that we have a bad brand new cap ????

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 6:54 am
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so I pulled off the cap cover and started checking DC voltage (unplugged)
with the power switch on and standby in the play possition, all of the caps had the exact same DC voltage (using the 200m scale) with the exception of the one 20uf 500 volt cap that has the reversed polarity it showed less than half of the stored voltage than the rest.
Is that the nature of the circuit or is it a possible problem ?
after allowing it some time I went back and measured again, all of the caps were gaining voltage, however that last cap (reversed polarity) was still under 50% of the others.
as soon as i put the meter on the circuit it begins to drop voltage rather rapidly.

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VHT Special 6 Ultra combo

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