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 Post subject: Re: Silvertone 1448
PostPosted: Oct Wed 17, 2012 12:56 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4866
Location: Gainesville, Florida
no reason to use top of the line in this low output amp. Jensen MOD6-15 reviews are very good. similar to the 8" speaker used in the 1449 - 1457 Silvertone amp-in-case.
this original speaker in my unit has issues. nothing visible or obvious mechanical but sounds blown. reproduces noise not music. for any collector out there I have saved all the original worn components. they can put them all back-in for that lousy sound.
8ohm 25W that I am using until my new speaker arrives sounds really good. reproduces music. brings out a lot of the amp and guitars good characteristics. the design is for 4ohm. thats what I am going with


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 Post subject: Re: Silvertone 1448
PostPosted: Oct Wed 17, 2012 4:39 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4265
Location: Boston, MA USA
tubeAMP wrote:
P = I E
I = E/R

And P = E^2 / R, and P = I^2 * R. Any more?

-David


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 Post subject: Re: Silvertone 1448
PostPosted: Oct Wed 17, 2012 5:52 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4866
Location: Gainesville, Florida
P = (E/R) E


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 Post subject: Re: Silvertone 1448
PostPosted: Oct Wed 17, 2012 7:19 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4866
Location: Gainesville, Florida
R4 drifted to 2.7M having effect on 12AU6 screen. using all I had 1.9M is working well raising screen lowering plate some. reducing R3 would lower its voltage drop across it as well and lowering voltage amplification. considering that the 12AU6 AC signal is over 4vac and 50C5 bias is a little over 8vdc I would go with a larger bias resistor if anything. there is some clipping at high volume that may improve with a slightly larger bias resistor. smaller bias resistor increasing clipping. FedX tracking says that my speaker is waiting on the porch for me when I get home. more fun


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 Post subject: Re: Silvertone 1448
PostPosted: Oct Thu 18, 2012 12:16 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4866
Location: Gainesville, Florida
someone recommended Tube Depot. on the porch in two days from TN to FL. packaged very well. I am recommending Tube Depot for speakers at least.
the Jensen MOD6-15 speaker construction is marvelous. foam surround ceramic magnet beautiful all around. smells like a new pair of Nikes. 21 bucks plus 5 shipping. I am pleased with this purchase so far. this weekend will put it to the test. 8ohm v/s 4ohm


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 Post subject: Re: Silvertone 1448
PostPosted: Oct Sat 20, 2012 11:19 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4866
Location: Gainesville, Florida
end of the trail. 4ohm speaker installed. seems a bit tighter. a little bit better attack. not screaming volume but good for what it is. has all that Blues tone now. definitely leave it original design. sounds good enough for what it is


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Sun 11, 2019 10:15 pm 
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Joined: Aug Sun 11, 2019 10:02 pm
Posts: 10
Hey there...I'ts many many years later from your original post.

But, I'm dealing with the same guitar/amp combo and I'd really like to ask about the output transformer when replacing the speaker with the Jensen Mon6-15. Did you retain the trans that was attached to the original speaker basket?

Best Regards


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 12:11 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4866
Location: Gainesville, Florida
gawd that was so long ago I had to look at it. the output transformer is original. seems I managed to mount it on one of the replacement speakers mounting screws near the speaker terminals as the transformer leads are short


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 3:24 pm 
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Joined: Aug Sun 11, 2019 10:02 pm
Posts: 10
Great! Wow. Thanks for checking and responding so quickly. I really appreciate it!

I bought a 1448 without the amp in the case back in the 80s. I bought this one and found the schematic so I could build a 2nd one from scratch and have two working setups. I'm very new to amps and psu.

Once it arrived, I thought the speaker was bad since the amp sounded like death...crackling, in and out with horrible noise. I thought the speaker was blown. However, I just tested it with a 9v battery and it seems to work ok as it makes a pop sound. Now, I don't know what is going on with it. I bought this from somebody who claims it worked fine and an amp tech replaced caps etc...

I'm going to open it up and see what's what. I'm planning to add a 1:1 isolation Xfmr as my daughter will be practicing with one of these gtr/amp setups.

I don't really understand the T1 heater Xfmr.

But, now I know I need to retain the Xfmr on the speaker! Thanks, again.


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 3:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4866
Location: Gainesville, Florida
the filament transformer is for the 12AU6 pre amp. maybe for noise reduction.

if you depress the speaker cone gently is there any rubbing sound?

this amp has no power transformer like an AC/DC radio. the high tension mains power one side is connected to the chassis.

http://schematicheaven.net/bargainbin/s ... e_1448.pdf


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 4:39 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20207
Location: Warner Robins, GA
Suggest wiring in an isolation transformer which will isolate the circuit from the AC line.


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 7:40 pm 
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Joined: Aug Sun 11, 2019 10:02 pm
Posts: 10
thank you for the info.

Yes, it makes a scraping sound when I evenly and gently depress it. So, I believe it is toast.


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 7:44 pm 
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Joined: Aug Sun 11, 2019 10:02 pm
Posts: 10
Yes, I plan to wire in an isolation Transformer. I was looking at this one:
https://www.mouser.com/ProductDetail/Tr ... q5ob3xQ%3D


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 9:05 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20207
Location: Warner Robins, GA
For that isolation transformer you will either need to wire in a 12uF film cap between the secondary and amp due to the transformer putting out 126Vac for a 120Vac input under lighter loads.

Or you can just put a resistor in the heater string to drop 6Vac which will provide a slightly higher B+ and a little more audio power.

Since you are using the isolation transformer perhaps the transformer used for the single tube heater can be removed and that tube put in series to eliminate the need for a dropping resistor.


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 9:26 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4866
Location: Gainesville, Florida
yep the speaker is shot. time for a new one. save the original for nostalgia 8)

the filament transformer and the 100ohm resister are dropping about 40vac. you might have to leave T1 in there for a ballast with the 12AU6 in the string.

I always thought that was a dumb place for a volume control. its like a second guitar control in a row. it should be where R6 is and a 68K grid stopper where R1 is now


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 9:58 pm 
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Posts: 20207
Location: Warner Robins, GA
Oh ok so basically the 100 ohm resistor will need to be increased to account for the extra 6 volts AC.

That said the film cap cannot be used due to the transformer for the 12AU6.


Last edited by Tube Radio on Aug Mon 12, 2019 10:06 pm, edited 2 times in total.

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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 10:01 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4866
Location: Gainesville, Florida
I cant believe that I wrote all that stuff on pages 1 and 2 :lol:


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Mon 12, 2019 10:07 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20207
Location: Warner Robins, GA
Looks like the transformer is dropping 17Vac with the 100 ohm resistor dropping 15 volts.

Will need a 160 ohm 6 watt resistor.

If not available a 150 ohm 6 watt resistor in series with a 10 ohm 1 watt resistor will work fine.


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Tue 13, 2019 3:34 am 
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Joined: Aug Sun 11, 2019 10:02 pm
Posts: 10
update on the speaker: I used some alligator test clips to temporarily hook up an 8", 4 ohm Eminence speaker I had laying around. The amp works!!

It sounds pretty sweet—even though it sounds like the speaker is naked and needs a cabinet. ; )

Also, all 3 tubes are original Silvertone tubes—still kicking. I'm thinking about replacing them and storing them with the original speaker because I want to really use this thing for recording. It has such a unique tone compared to my other Fender amps.

I'll order a new 6" speaker and try to figure out how to move that transformer that is riveted to it? I almost want to leave it intact and find a new transformer for that new speaker. I don't know how to spec it though. Any suggestions on how to determine what the primary to secondary ratio is?

============
Also, thanks for all the curcuit update suggestions. I'm not very comfortable with modifiying anything to that degree, but I'll take a look at it all when I have the schematic and amp in front of me tomorrow.


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 Post subject: Re: Silvertone 1448
PostPosted: Aug Tue 13, 2019 11:50 am 
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Posts: 20207
Location: Warner Robins, GA
Basically the 150 ohm and 10 ohm resistors will go in series and replace the 100 ohm resistor in the heater string.

Or if the existing resistor is wirewound just put a 56 ohm 5 watt resistor in series with it.


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