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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Sun 09, 2019 2:55 am 
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John Bartley wrote:


To test your speaker you need to energise the two wires that go into the field coil under the coil dust cover. Then you can send you signal from an audio generator to wires 4,5.




got it and thanks


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Mon 10, 2019 12:46 am 
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Congrats on reading 5.9 ohms on the voice coil! This means that someone who didn't know better bypassed the speaker, maybe trying to correct a fault in the audio, just a guess. 'Why' doesn't really matter, he did a poor job so now you will fix it properly.
That can be considered an 8 ohm speaker, as the meter doesn't give a true reading of speaker impedance.
The field coil has a finite resistance that suggests it is also fine.

The 3 wires come from the amp circuit as shown in the schematic, one from each output tube socket and one from B-. The tubes share the load and the signal swings from one to the other. Get the new filter caps in, all wiring restored to where it was, and see what you got.

Maybe the speaker is raspy from bad alignment of the center. Moisture and wedges can fix that easily in many cases.

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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Mon 10, 2019 12:59 am 
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Location: 13 Critchley Avenue, PO Box 36, Monteith Ont, P0K 1P0
westcoastjohn wrote:
from B-.



One from B+ .... yup, I've done that before too ... what my brain thinks and fingers type aren't even on the same planet ....


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Mon 10, 2019 9:01 pm 
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roger that bro.some pics of the wires put back in place

Image

Image


now on to the caps

I have 3 circles of Caps that I would think is probably to old to stand the strain .would circle 3 be the filter Caps?
and what/why is the resistors/caps piggy back riding these yellow caps in circle 1 and 2 .do you think this is original or do you think maybe and add on ?


Image

Image


looks like the 2 big Caps are 20 MFD at 450 volts

Image

looks like the big blue one is 10. at 450 volts

Image

some pics to show the wiring

Image

Image

question,,,,,seeing how the 2 big Caps at 20 MFD each are wired together with the leads going to the same place could I just get one Cap at 40 MFD to take the place of both of them ?


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Mon 10, 2019 10:24 pm 
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Location: 13 Critchley Avenue, PO Box 36, Monteith Ont, P0K 1P0
blainenbecky wrote:
question,,,,,seeing how the 2 big Caps at 20 MFD each are wired together with the leads going to the same place could I just get one Cap at 40 MFD to take the place of both of them ?


If you refer to to that schematic I posted, you'll see that the first cap following the rectifier tube is 40uf and the second is 10uf. Between them are the wires that go to the speaker field coil (under the cover).

Maybe check your amp's wiring to confirm that?

cheers


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Tue 11, 2019 1:10 am 
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Location: Gainesville, Florida
two 20uF capacitors together in parallel is 40uF. it would be best to have one capacitor at 40uF than two 20uF together. you are going to need to follow the schematic to make the right connections with the new parts.
the other two 10uF with the resistors are cathode bypass.


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Tue 11, 2019 1:21 am 
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thanks guys ,so I guess when they put the 2 20 uf's in they just did not have 1 40uf so they used 2 caps to make 1,right?
I think I now see schematic a little better.its been a while and I have forgot much of what little I knew :lol:
so if I am seeing it right the blue circles in this pic represent the big cape and the red circles represent smaller yellow caps with the piggy back resistors on them ?

Image


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Tue 11, 2019 1:32 am 
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yes and yes


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Tue 11, 2019 1:39 am 
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Location: 13 Critchley Avenue, PO Box 36, Monteith Ont, P0K 1P0
You're doing well. Next .... you might want to learn to search out tube connections.

Do google searches for "6SQ7 Pinout", "6J5 Pinout" and "6V6 Pinout" and then you can see exactly which pins you should be looking at for the correct connections.


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Tue 11, 2019 4:53 pm 
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anybody know of a cap supplier I can get 4 caps (20uf 25v X2,,,,,10uf 450v,,,,,40uf 450v) and not have to have a 25.00
minimal.I tried to get then from this place https://www.justradios.com/cart.html
and it rejected my payment as it was under the minimal ,,,,,eBay doesn't have what I need,,
any ideas ?


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Tue 11, 2019 5:20 pm 
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Location: Gainesville, Florida
edit


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Tue 11, 2019 6:00 pm 
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question ,,,,,I have boxes of old Caps ,,so far I can't find any with the values and voltages I need as I need a 40uf 450v
but I did find a 20uf 450 that test out as 39 uf the cap is plainly out or off its intended value but would it hurt to put it in the amp or would it be to risky IYO

Image


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Tue 11, 2019 7:23 pm 
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Joined: Feb Thu 28, 2019 7:48 pm
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Location: Lawrenceville, Illinois 62439
Personally, I would not waste the solder putting in a 30 year old out of spec capacitor.
If you look for common value caps in 450 volt, you will find 10, 22, 47 mfd. at any supplier or even the bay. I keep several of those around myself. You can always go bigger on voltage, never lower.
I'll let others chime in on if a 47 would be close enough on that radio to sub for the 40.
Dan


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Tue 11, 2019 7:28 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: 13 Critchley Avenue, PO Box 36, Monteith Ont, P0K 1P0
blainenbecky wrote:
anybody know of a cap supplier I can get 4 caps (20uf 25v X2,,,,,10uf 450v,,,,,40uf 450v) and not have to have a 25.00
minimal.I tried to get then from this place https://www.justradios.com/cart.html
and it rejected my payment as it was under the minimal ,,,,,eBay doesn't have what I need,,
any ideas ?


If you plan to continue this hobby, buy about 5 or 10 of each of those values you just listed. You'll use all of them soon enough. Also get a few 20uf @ 450.


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Wed 12, 2019 11:54 am 
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Roger that John ,just ordered a few extra and hopefully they will come in in a few days .till then I will be building a Dim bulb tester for when I power the thing up .


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Wed 12, 2019 12:25 pm 
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while youre at it would be a good idea to get a hand full of replacements for the out of tolerance carbon resistors. I dont know why the drawing shows two resistors in parallel feeding B+ to the 6J5 like that. also there is no bypass capacitor for the output tubes. something you could experiment with using 25uF 50v


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Wed 12, 2019 1:06 pm 
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well I do have a collection of the old type resistors,they are kind of big but this amp has some pretty big ones in it already .

Image

also have a lot of the small ones if I can't find the right value in the big ones

Image

I diden't even think about the resistors.am I right in thinking I can test them in the amp and not have to take them out to get a true reading ?


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Wed 12, 2019 1:26 pm 
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those NOS carbon resistors may be out of tolerance as well. check the with ohm meter. checking resistors in parallel in the amp will not give you singular reading


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Wed 12, 2019 1:59 pm 
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tubeAMP wrote:
I dont know why the drawing shows two resistors in parallel feeding B+ to the 6J5 like that.


over my head bro but here is a closer look

Image

Image


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 Post subject: Re: found an old Gibson Amp can't find model #
PostPosted: Jun Wed 12, 2019 2:01 pm 
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tubeAMP wrote:
those NOS carbon resistors may be out of tolerance as well. check the with ohm meter. checking resistors in parallel in the amp will not give you singular reading




Roger that and thanks


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