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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Feb 06, 2013 12:52 am
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Just to clarify, The amp should have(2) leads running from the 220K resistors( possibly under board) to pin 1 of each power tube. These are seperate. The connection to pin 1 is at the opposite end of the apex of the two 220K resistors that are connected to the bias pot. On the socket there should be a 1500ohm resistor going straight to pin 5. This is the usual Fender route..... The screen supply comes from a 20uf filter cap, directly from the power filter board. This would usually go to pin 6 of the closest power tube and then jumpered to pin 6 of the adjacent power tube. From pin 6 to pin 4(on each power tube socket) would be the 470ohm 1watt resistor(or 2 watt). Is this what you've got going on, Chris? Art......And yeah, Beemer is spot-on about the lead length!!!

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Feb 06, 2013 7:34 am
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I'll follow up later today

It's state audit day for me, Yippe !!!!!

A state employee who has never done my job telling me how to do my job.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Feb 06, 2013 11:10 am
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I went back over everything I had done and retouched every solder joint and now we are back to where we left of before replacing the broken 470ohm resistor

Quiet operation , excellent sound and that little annoying zzzbbb sound that is oh so faint, but when you hear it, the bias reading drops 2ma for a second or two then rises back. = some form of instability :x

I am going to first try replacing every one of those 32 gauge ???? wires on the board with 22 gauge cloth covered and shortening the leads that need to be shortened.

Note to self, order some red cloth covered wire :mrgreen:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Feb 06, 2013 12:04 pm
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" Quiet operation , excellent sound and that little annoying zzzbbb sound that is oh so faint, but when you hear it, the bias reading drops 2ma for a second or two then rises back. = some form of instability "

Instability or oscillation. Do both 6L6GC drop a couple of mA when you hear the "bbzzz?" Maybe lead dressing and/or grounding issue.

How is the purple glow issue, now?

Getting close...


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Feb 06, 2013 12:16 pm
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much better ! (blue glow)

I keep changing wires as we go, I'm thinking after finding that "blown up" resistor on the power tube that I need to pull some legs and check the rest of the board.
That 470 ohm resistor was 26 ohms when i found it.

Also another curious thing, plugging into the bass channel, the volume pot has the most abrupt Nothing to loud ! I've ever seen and it causes buzzing and tube flash.
Have yet to touch anything in that circuit so......
Could an issue over there be causing some of the issues ?
Can pulling V-1 remove enough of the circuit to solve it ?

Thanks
Next I'll post a picture of the overall circuit so far

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Feb 06, 2013 12:30 pm
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OK I took two semi close up shots of the circuit
You'll notice that I messed up when I showed you the burned wafer cap earlier
see picture two for the correct (circled in red) burned cap.

Image

Image

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Feb 06, 2013 4:28 pm
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Hey that fancy Brimar 12AT7 wasnt exactly balanced
Now there is a sweet & tested Telefunken and it smoothed it out a bunch !

So here is a recording
First bit is my favorite setting volume 1.5 and my route 66 comp only

second is the amp alone volume 3 ish

Third waving the neck near the amp (you'll get it)

and last is volume at around five Strat into amp again

The recorder was a bit too close so forgive the signal getting a little wild

The only thing I still see is flashing at the very top of V-5 at higher volume.

https://www.box.com/s/redlxwh9y1isa9w3csc0

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Feb 06, 2013 5:08 pm
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Chris,

It sounds pretty good. How many hours are on your 6L6GCs? What brand and vintage are they? That f;ashing you are seeing could be gas contaminates still burning off. Does the amp hold a consistant bias after use?

The noise you are getting right after switching on the STANDBY sounds like bad switch. You have a spare to try?

Maybe a good idea to replace that 0.01 mfd disc cap with a good 0.01 mfd/600 VDC Sprague SBE 716P or Mallory-150. this cap is 100% in the signal path. So, if it's hinky, you are going to hear it.

I believe from the schematic, you can pull V1 and V2 (BASS INSTRUMENT) --- plug your amp into the NORMAL channel and play. May want to try this, as the fried disc cap is in the gain stage ot the BASS channel.

Good lucK!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Feb 07, 2013 6:30 am
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The .01 caps are on my order list.

I also have a couple of new 1M pots in my in my parts stash and looks like the bass channels volume pot is shot, Deoxit didn't help this time.

If I believed in any of the local "techs" I would have taken it to them, however I've taken in gear locally from craigs list that these "experts" have "fixed" and had to undo a lot of their very poor work.

But after all the rabbit trails this thing has taken me through, I believe by golly we have it ! or at least on the way to becoming a sweet vintage fender !
Image
Looks good with the Bandmaster too 8)

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Feb 07, 2013 6:44 am
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Chris,

Nice tandem. And look at ALL the great tech info you learned! You'll soon be the "AC568 to BF" wizard of the Midwest.

:mrgreen:


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Feb 07, 2013 7:06 am
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Please don't let the word out !!!!

I had a guy at the local Mom & Pop send me a 1967 bassman drip edge thats blowing fuses.
So far I found again mixed old and new filter caps, a very bad 12AT7, 5881's (old sovtek fender branded) that are way out of spec and rust.

He is a very good friend and so I reluctantly agreed to take a look :(

Must be plum out of my mind !!!!

This one has the nice blue tubular caps and all cloth wiring and matching 1967 codes, untouched bias adjustment (tube matching) ARGHHHHHH !!!!

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Feb 07, 2013 1:46 pm
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Chris, from a quick look at the photos this friend's amp, which you email'ed to me --- looks like an AB165.

See the difference in the layout of those resistors off the balance pot (compared to the AA165 or AA864)? And the lack of 7-watt sandbox resistors hanging off the 6L6GCs.

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


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Feb 07, 2013 1:53 pm
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Yes I noticed that first thing.

Also the bright nearly white light emited from the tubes and a loud HONK out of my poor speakers.

I do believe we have a short somewhere, no evidence of burnt stuff.

Funny how I am getting to know these circuits like an old friend or enemy :shock:

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Feb 07, 2013 6:33 pm
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Chris, Just outta curiosity, does the chassis on your amp have a plate over a pre-punched socket hole to the left of the 6L6GCs? (as you look at the back of the back, in the cab).

If that sudden popping, fizzing distortion keeps on being a problem --- even with a new standby switch (SBS) or after you clean & rebuild the old one --- you could think about going to a nice indirect-heated rectifier (like a 5AR4) or a tough direct-heated one with a nice voltage sag (like an NOS GE or RCA 5U4GB). And remove those silcon-diode rectifier.

Just a wild thought. But, not only do these tube rectified amp sound better for guitar use. Esp Blues. But, also tend to cause less sudden votlage in-rush, when amp is cold or when you flip the SBS.

See cover plate in photo, right by "death cap":

Image


EDIT: translated into English. I hope. See what happens after the third straight graveyard shift? Only four more to go, in the next 5 days! WOO-HOO! :lol: :( :lol: :?:


Last edited by BMW2002Ti on Fri Feb 08, 2013 7:17 am, edited 3 times in total.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Feb 07, 2013 6:56 pm
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No such luck

I have a few GZ34's & 5U4GB rectifiers too

and 5Y3 and.............

That would be so cool and another excellent bit of info !!!!

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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