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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 10:57 am
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Strange as it may sound changing just the .1 caps to .047 gave a little more head room , however the tubes are now flashing harder again and it added back some of the noise.

new .047's will most likely be needed here

Maybe i sould go forward with the 100K's down to 47K ???

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 11:02 am
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The picture shows the caps that I returned to .047

The resistors are the ones we talked about reverting back to 47K

Image

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 11:04 am
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Chris,

You can try the 47k-ohm resistors -- but this will INCREASE the plate voltage. And run the 12AT7 harder.

Do you have new Mallory-150, 0.022mfd/600VDC caps to try in place of the old 0.047 mfd ones? Just to see if you can get the amp to a better OD point.

BTW.. yes to both questions. Try the cap changes, first. Then, go for resistor change. You need to see how the PI reacts to one BEFORE the other.

Soldering is very critical in this stage. As well as socket contact. Just to keep in mind.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 12:44 pm
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And so while checking my solder work i noticed the wires from the 220K resistors and to the pin #1 of the 6L6GC's looked burnt, I replaced them with some 22 gauge cloth wire and popped the amp back on.
The 6L6's red plated instantly (fast shut down) !!!
doesn't make any sense ! The voltage at pin #1 was -42.7 dc volts
What did I do ?????

Image

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1968 Bandmaster with 2X12 cab C-rex speakers
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Last edited by blues bondsman on Sat Feb 16, 2013 12:55 pm, edited 1 time in total.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 12:53 pm
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Chris,

Ok... something is hinky. And it maybe the sockets. Try cleaning them first. Insure tight contact.

1.) Turn the bias pot full CCW (from the top of the chassis --- tranny side)

2.) Replace 6L6GC. Turn amp on. Check for red plating. If none, adjust your bias to get about 18 watts of dissipation per tube. Check, again, for red plating.

3.) If 6L6GC red plates --- try pulling that 12AT7 and see if the same thing happens.

4.) Are the ground straps from the 6L6GC cathode (pin #8) to ground secure? Double check them.

Good luck. Keep us posted.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 12:58 pm
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cranked all the way down and the lowest it would go was in the 40's ma

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 4:24 pm
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Used deoxit without lube and cleaned the sockets
adjusted the springs in the socket (again)
Swapped out the 220K resistors, one was 260K the other 250K

How could changing a wire from a burnt up looking thin nothing to a good piece of wire cause all this ????

Its gotta be the socket ??? cracked somewhere maybe ???
only the one tube is red plating now and it does it in a hurry, so I watch the meter as I'm testing.
Its not the tube, any tube I put in there does it so........

I really do enjoy this ! :roll:
It will be great when its over and it sounds like Angles singing 8)

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 5:53 pm
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Chris,

1.) What electrolytic cap did you put into the bias power supply? ONE 100mfd/100VDC Sprague Atom? Or did you go to TWO 50mfd/50VDC caps in parallel?

Is that negative voltage STEADY, at the two 220k-ohm points?

2.) With everything else the same, there should be no way the 6L6GC would red plate by going to a smaller mfd rated coupling cap off the 12AT7 (going from 0.1mfd to 0.047mfd). This could indicate that there is a poor connection(s) somewhere in the circuit or the socket.

Every time you manipulate the board, the faulty connection(s) rears its ugly head. With your issue of noise and static, it could very well be a ground point.

Try one thing. Pull all tubes. Clip the DVM leads securely onto the junction of those two 220k-ohm resistors going to the grid of the 6L6GC. And the black lead to a GOOD clean grounding point. Now, get a WOODEN chopstick and probe the components and the board. See if the voltage jumps around. If so, try to locate the bad portion of the circuit, which is causing the voltage to jump.

If the voltage jumps around --- inspect the board. It may have a poor solder or damage from a carelessly placed wire, which has pierced the two boards and is causing an intermittent short.


3.) With amp off. CLEAN the top surface of the socket where the 6L6GC plug into the sockets with rubbing alcohol. You may have a carbon track between the pins, from a shorting event. Caused by a bad 6L6GC.

4.) How steady are those plate voltage readings? Esp the one on the 12AT7 and the 6L6GC? As Art said, you may need rectifier diodes. Or at least touch-up the solder points. You need to use a heat sink on them while soldering. I like a light alligator clip (no wire attached), on the body of the diode, as I solder it.

5.) There are cases when the unseen wiring between the boards of the main circuit and the boards under the doghouse maybe cracked or bad connection. This can really be perplexing, as you change out EVERYTHING & the amp still is not stable.

6.) After all the checks--- let's pull ALL tubes and only bias the two 6L6GCs. You have to get them to a steady idle bias with no red plating &no other tube inserted (the 12AT7 and 12AX7s). If you cannot --- there is one more resistor to check. That 1k-ohm resistor just before the diode of the bias power supply (on the separate board, top left corner). I believe you went to 470-ohm, 1 watt metal oxide. Right?

Check the resistance on a meter. It may have drifted upward or not have good soldering points. You may need to really go over that POWER SECTION from power supply caps through to the 6L6GC with a magnifying glass. These sections MUST be rock steady or you'll never have a reliable amp. But rather, a tube eater or fuse blower or both.

Check in after all the above yaddy-yaddy is checked. You need to VERY carefully check ALL of the circuit, at this point. Section-by-section.

Sorry about that, but a systemically review of the amp is needed. Or you run a risk of going crazy, trying to find the culprit.


Last edited by BMW2002Ti on Sat Feb 16, 2013 6:01 pm, edited 1 time in total.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 5:53 pm
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edit: double post erased. So Sorry...

:mrgreen:


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 16, 2013 8:53 pm
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Thanks !

Yes I have followed up on all counts.

The next step is that I will pull the elements from the 6L6 V-5 that is doing the funky stuff and remove the socket for a detailed inspection, if I find fault of any kind, I'll replace both 8 pin sockets with a good replacement (best I can find) !

After all of the checking, touching up etc.... the only unknown right now seems to be the sockets, don't happen to have any right now and Monday is a holliday so I guess we'll let this rest a bit eh ?

I will let ya know if I find an obvious fault in the socket.

Thanks for keeping watch !!!!!!

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 12:31 am
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Chris,

Please read my email about socket dimensions (punch hole size and screw-to-screw distance). Fender went to cheaper, smaller octal sockets, in the mid 1970's.

These are the best you can find. If they fit your chassis. (Esp the brown micanol mil spec or ceramic kind):


http://www.ebay.com/sch/i.html?_trksid= ... &_from=R40


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 8:02 am
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Well I removed the offender and can't see any evidence of a crack, the pins are all pretty loose in the holes.
It is the dreaded 1" mounting hole and so I'm limited to what I can use without opening the hole and I'm not interested in grinding on the chassis.
AES has a ceramic socket with a 1" mount.

One pin had a lot of solder built up on it that I couldn't see from its mounted possition, It is possible that that large drip made contact with the chassis too.

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2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 9:24 am
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Chris,

Good morning! Do you have a solder wick? Suck all the solder off the offending socket. Check all wiring to the pins. Esp the wiring of the resistors on the sockets. See if the resistors are causing stress on the tangs of the sockets.

If the tangs look loose and the resistors seem to stress an overly worn socket tangs --- may need new sockets. Try to solder the resistors so that they don't pull the tangs too much. If that make sense.

What I do is put the resistor onto the socket tangs with leads coming out of the tang holes. So that they are not pulling the tangs, too much. Then, put a touch of solder on both tangs. Cut the excess leads. Use the least amount of solder as possible.

This works best with the sockets off the chassis. Which at this point prolly be best with new sockets.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 9:39 am
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I'm thinking all that excess solder
Have been the problem

Are those ceramic sockets going to
Be ok ?
I may reinstall the offending socket
After I clean up the mess just to see
If it gets better
Then put in new ones next week

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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: Sun Feb 17, 2013 9:55 am
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Chris, who makes those ceramic sockets? Some ceramic and porcelain Chinese ones are pretty marginal, at best.

I'm not a big fan of "cinch" type sockets. But, these Amphenol 1-inch type will be a LOT better than most new Chinese made stuff available, these days.

http://www.ebay.com/itm/8-Pin-Ceramic-A ... 2ec5d35434

Here's a pretty nice NOS Beltron-style black octal socket (about 1/2 way down the page). Non-cinch type.

http://www.vacuumtubesinc.com/Products/ ... ckets.aspx

Believe me, GOOD NOS sockets are harder to find than NOS tubes. I always keep a look out for mil spec ones on EBay. Worth their weigh in gold. If you ever had to use some of the new junk on the market, you'll know what I mean.


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