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 Post subject: Mystery Guitar Amp
PostPosted: Dec Tue 05, 2017 1:57 am 
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Joined: Feb Fri 03, 2017 5:28 am
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Location: Mikwaukee, WI 53207
Anybody recognize this amp?
It's in rough shape but I may take a shot at restoring it.
It may be a Dynacord.

Thanks,
Joe Y


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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Tue 05, 2017 3:58 pm 
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Location: South Jersey East of Philly
Looks like it was made by Danelectro or Valco, based on the pressed wood cabinet, and sheet metal chassis. The guts looks very much like some early 60's Danelectro/Sears amps I've had.


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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Tue 05, 2017 6:41 pm 
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Location: Fayette County, Pa
Agree regarding Sears. I had a Silvertone come through here recently that I made operational for a guy who bought on FleaBay. Looked just about like it with tube placement, etc. No real issue getting it to play; just the usual change caps, clean sockets / controls, etc.


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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Wed 06, 2017 3:06 am 
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Joined: Feb Fri 03, 2017 5:28 am
Posts: 86
Location: Mikwaukee, WI 53207
Well, fellas', I appreciate the info, but after looking at several hundred photos and many schematics, I have not found a match.
Here is a list of the amp makers that I have searched so far:
    Valco
    National
    National-Dobro
    Sears
    Supro
    Airline
    Gretsch
    Harmony
    Oahu
    Alamo
    Sano
    Dynacord
    Rex
    Danelectro


I guess it doesn't really matter, it's a pretty simple amp that was built for the low-end market.
The tube complement is: 50L6, 35Z5GT, 12AU6 and 12AV6.
I may just go ahead and draw my own schematic and go from there. I don't think there's much value in keeping this thing "original".

Edit: Right after I posted this, I found an amp schematic using these tubes. It from an Alamo Fiesta. There is also an Alamo Capri that appears to be very similar.

Thanks,
Joe Y.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Wed 06, 2017 5:19 am 
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Joined: May Sat 06, 2006 4:03 am
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Location: ZIP 23831 South of Richmond, VA 25 miles down the pike.
Those knobs look to be Magnatone and I need two. But the cover looks sears.
Bill J.


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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Thu 07, 2017 1:45 am 
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Congrats on finding a schematic. You can expect to see some minor variations, but it will be a generic circuit design. Sure, call it an Alamo. 8)

1) Install a 3-prong grounded cord and make sure all your outlets are wired correctly.
2) If there is no power transformer, find a small isolation transformer to eliminate the shock hazard. I see your amp has a small power transformer.

Some of the amps with that power tube had a small isolation transformer for the B+, the plate volts, but used the house current directly for the heaters. LectroLab is one company that used that system. I guess you were a bit safer, but still vulnerable. What you have is not a LectroLab, different chassis, but the size of that transformer makes me think it isn't feeding the heaters.

I had a Symphonic years ago that used a 50L6. It had a little power transformer with a 50 volt tap. The 50L6 heater was wired in parallel with the other tubes, which had their heaters in series. The mystery there was why not a 6 volt tap? Maybe they bought a lot of cheap tubes, early 60's, radios were no longer using the old octals.

3) One more thing. If you polarize the power cord, you don't need the "Death Cap". Clip it out so the chassis has no connection to the circuit, except where the jack connects it to your guitar input.

I enjoyed the 50L6 output because at 2 watts, it is easy to keep the volume down. You can sub an 8 ohm resistor for the speaker and run a line out into another amp or whatever for recording and the like. Lots of fun.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Sat 09, 2017 5:05 am 
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Joined: Feb Fri 03, 2017 5:28 am
Posts: 86
Location: Mikwaukee, WI 53207
westcoastjohn wrote:
Congrats on finding a schematic. You can expect to see some minor variations, but it will be a generic circuit design. Sure, call it an Alamo. 8)

1) Install a 3-prong grounded cord and make sure all your outlets are wired correctly.
2) If there is no power transformer, find a small isolation transformer to eliminate the shock hazard. I see your amp has a small power transformer.

Some of the amps with that power tube had a small isolation transformer for the B+, the plate volts, but used the house current directly for the heaters. LectroLab is one company that used that system. I guess you were a bit safer, but still vulnerable. What you have is not a LectroLab, different chassis, but the size of that transformer makes me think it isn't feeding the heaters.

I had a Symphonic years ago that used a 50L6. It had a little power transformer with a 50 volt tap. The 50L6 heater was wired in parallel with the other tubes, which had their heaters in series. The mystery there was why not a 6 volt tap? Maybe they bought a lot of cheap tubes, early 60's, radios were no longer using the old octals.

3) One more thing. If you polarize the power cord, you don't need the "Death Cap". Clip it out so the chassis has no connection to the circuit, except where the jack connects it to your guitar input.

I enjoyed the 50L6 output because at 2 watts, it is easy to keep the volume down. You can sub an 8 ohm resistor for the speaker and run a line out into another amp or whatever for recording and the like. Lots of fun.



I really appreciate all the info.
I have made a little progress in my research - I pulled the amp out of the cabinet and found this small, hand written message on the side of the cabinet:
Attachment:
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Tag_Small.jpg [ 20.11 KiB | Viewed 94 times ]

With this clue, I was able to fine a picture of a Lindell amp from the same era:
Attachment:
vintage-tube-lindell-tremola-guitar_1_20642f83ce88a9d2c0bc330ee986f918 (1).jpg
vintage-tube-lindell-tremola-guitar_1_20642f83ce88a9d2c0bc330ee986f918 (1).jpg [ 20.65 KiB | Viewed 94 times ]
Attachment:
vintage-tube-lindell-tremola-guitar_1_20642f83ce88a9d2c0bc330ee986f918.jpg
vintage-tube-lindell-tremola-guitar_1_20642f83ce88a9d2c0bc330ee986f918.jpg [ 23.62 KiB | Viewed 94 times ]


Although it's not the exact amp, it has the same panel layout, suitcase handle, tilted front, 20" width and 9 1/2" depth. It also has a Tremolo. It would be nice to come up with a Lindell schematic, but in the mean time, I'm tracing out my circuit. It is indeed very similar to the Alamos and others of that era. I'll post it when I complete it (takes me a while).
The filaments are all wired in series, straight off the mains. It's nice the way the math works with this combo (35+50+12+12=119V) but I'll work on getting an isolation tranny for these guys.
Many of the components are grounded to the chassis in what appears to be a totally random fashion. Looks like they just grabbed whatever grounding lug was convenient and went with it. AC, DC, signal,filaments and mains are all on the chassis.
It shouldn't be too difficult to correct these issues. I'll be pestering you guys for details soon.

Thanks,

Joe Y

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Sat 09, 2017 5:23 am 
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Posts: 2047
Location: New Port Richey, FL, USA
Kasanay wrote:
westcoastjohn wrote:
Many of the components are grounded to the chassis in what appears to be a totally random fashion. Looks like they just grabbed whatever grounding lug was convenient and went with it.


That's very normal with tube equipment. Any unused tube pin is fair game. It can sure make it fun trying to trace a circuit on a schematic when you see a resistor soldered to a tube socket, but the schematic doesn't show the resistor going to that tube. Only to find out, it's an unused pin on that tube.

It can make things interesting at times too. The 35Z4 and 35Z4 are both rectifier tubes that are identical except that the 35Z5 has a center tapped filament. The Z4 wasn't nearly as common but it's out there. So if you have a radio set that has a Z4, and the unused pin was used for some tie point, you can't drop a Z5 in it's place.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Sat 09, 2017 9:14 pm 
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Right, the tube pins may be used, but check to see if they are actually bonded to the chassis. The 7 and 9-pin sockets may have a bonded center post. But tube pins may be floating. Google the tube names to check pinouts.

To improve safety, a copper bare wire could be added, isolated from the chassis, and all B- connections could be made to that. The idea is to separate the guitar cord jack from any of the energized circuit.

The isolation transformer won't help if there is a bond from the transformer secondary to the chassis. :x

A 1 amp slo-blo fuse is a good thing to add.

And read the story of Yardbirds Keith Relf.

Not familiar with Lindel. I suppose there was a badge glued to the grill cloth at some time.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Sat 09, 2017 11:27 pm 
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Location: Boston, MA USA
westcoastjohn wrote:
The isolation transformer won't help if there is a bond from the transformer secondary to the chassis.

Of course it will! The idea is to isolate chassis ground from line hot, which the isolation transformer will certainly do.

-David


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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Sun 10, 2017 2:10 am 
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westcoastjohn wrote:
To improve safety, a copper bare wire could be added, isolated from the chassis, and all B- connections could be made to that. The idea is to separate the guitar cord jack from any of the energized circuit.

The isolation transformer won't help if there is a bond from the transformer secondary to the chassis.


On tube guitar amps with a power transformer, the center tap of the secondary is your B-. So if you try what you are saying, you won't have a complete circuit. But that is still isolated from the wall outlet, and in order to get shocked, you would have to also touch one of the hot wires under the chassis.

Generally it is recommended to put a 3 prong power cord in, with the ground wire to the chassis. In that case you loose your isolation, but it's safer that way. Just remember is you have to service such an amp, to run it on a variac with no ground connection in the outlet.

westcoastjohn wrote:
But tube pins may be floating. Google the tube names to check pinouts.


None of the tube pins should be grounded except maybe the cathodes, depending on the bias that's used. But looking up the pinouts won't tell you that. You have to look at the schematic.

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Last edited by TPAairman on Dec Sun 10, 2017 3:36 am, edited 1 time in total.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Sun 10, 2017 2:16 am 
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Accidental double post.

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Last edited by TPAairman on Dec Sun 10, 2017 3:36 am, edited 1 time in total.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Sun 10, 2017 2:31 am 
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dberman51 wrote:
westcoastjohn wrote:
The isolation transformer won't help if there is a bond from the transformer secondary to the chassis.

Of course it will! The idea is to isolate chassis ground from line hot, which the isolation transformer will certainly do.

-David
I suppose you are right. I was thinking it would be best if the chassis was isolated from the B+. An insulated guitar jack would be good, but they can still come loose and short to the chassis from wear.
The isolation transformer will prevent the most likely shock hazard, which is from the guitar strings through the player to ground, such as the basement floor. The metal bridge is bonded to the guitar jack.
What about making adjustments with the right hand while holding guitar in the left? Well, I suppose that would usually be ok, no exposed wires.

This will become more clear when we can see the schematic.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Sun 10, 2017 3:35 am 
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westcoastjohn wrote:
I was thinking it would be best if the chassis was isolated from the B+. An insulated guitar jack would be good, but they can still come loose and short to the chassis from wear.
The isolation transformer will prevent the most likely shock hazard, which is from the guitar strings through the player to ground, such as the basement floor. The metal bridge is bonded to the guitar jack.


An insulated jack is a nice thought, but as you pointed out, the stings are connected to ground anyway. But this is why some amp guys will convert amps like this to one with a power transformer, though it does require changing the tubes. When you add the power transformer, the chassis is connected to the secondary center tap, but unless you get ahold of that and a hot lead, it's not a problem. And if a hot wire did touch the chassis, and for some reason the fuse never blew, it still can't shock you just from touching the chassis.

westcoastjohn wrote:
What about making adjustments with the right hand while holding guitar in the left?


Absolutely something to keep in mind, but not just that, you can get shocked by touching the strings and touching your lip to a mic. If everything is wired correctly, none of this should be a problem, but as well know, amps do develop faults and some club owners like to do their own electrical. So one of them changes an outlet, and wires it backward, well, yeah. Keep your lip back from the mic a tiny bit.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Sun 10, 2017 3:48 am 
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Kasanay wrote:
The filaments are all wired in series, straight off the mains. It's nice the way the math works with this combo (35+50+12+12=119V) but I'll work on getting an isolation tranny for these guys.


I forgot to mention this part. That's no coincidence. Those are the filament voltages, and each drops that much of the total voltage. So, it's done intentionally. This was found in quite a number of guitar amps. It's also the same as what you find in AA5 and similar radios. AA5 stands for All American 5, and was a common thing after WWII. It saved production cost by not having the power transformer.

But before you go buying a transformer, you have to take a couple things into consideration.

Here's a video that address just this (D-Lab coming up in the YouTube search again, but he knows his stuff on this)

https://www.youtube.com/watch?v=IuNOJUim8Ww&t=761s

You could buy a regular isolation transformer, such as this: https://www.jameco.com/z/ITR300-Power-Transformer-Isolation-120VAC-120VAC-300Va_181315.html

If you did that, I'd mount on the bottom of the cabinet, so it's as far away from the chassis as possible. The magnetic field from the windings can induct things like the output transformer. In fact, I'd set it in there first, and run the amp, and turn it to make sure you don't get hum.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Mon 11, 2017 7:39 pm 
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Here's a video that address just this (D-Lab coming up in the YouTube search again, but he knows his stuff on this)

https://www.youtube.com/watch?v=IuNOJUim8Ww&t=761s

You could buy a regular isolation transformer, such as this: https://www.jameco.com/z/ITR300-Power-Transformer-Isolation-120VAC-120VAC-300Va_181315.html

If you did that, I'd mount on the bottom of the cabinet, so it's as far away from the chassis as possible. The magnetic field from the windings can induct things like the output transformer. In fact, I'd set it in there first, and run the amp, and turn it to make sure you don't get hum.[/quote]


Thanks for all the great advice!
The D-lab video was very encouraging, as I have been considering adding a 6.3V transformer and switching to all 6V tubes (6x5GT, 6AV6, 6AU6 and 6W6GT). That way, the existing power transformer would take care of isolating the B+ voltage and the added 6.3V tranny would do the same for the heaters.
Of course I'll still add a fuse and change the cord to a three prong.

I took a shot at drawing up a schematic of the amp (I used Paint). It's a work in progress and I hope to finish it in the next couple days.
Here's a peek:
Attachment:
DIY LindellSchmatic_Draft.jpg
DIY LindellSchmatic_Draft.jpg [ 92.85 KiB | Viewed 16 times ]

I would appreciate any constructive criticism.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Mon 11, 2017 11:00 pm 
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Nice schematic. I think C10 should have the 2 flat bars symbol of a non-polarized cap.
I see the heater string is not isolated, as we suspected. Brand new, there was probably minimal hazard there, but since the heater wiring is usually run along the chassis to reduce 60 cycle hum, there could be some hazard if the wire insulation is brittle.

Now for your modification plans:
The 6X5 rectifier is a problem tube. Google it for info or see this discussion below:

https://canadianvintageradio.com/cvrs-f ... atic-tube/

That amp will work just fine with silicon diodes. B+ will be a bit higher, so more headroom. All the flap you hear about tube rectifier sag is non-existent in a single-ended amp. The tube does draw the B+ down a few volts, but that is steady, not the same as when there is push-pull output.

When choosing your 6.3 transformer, make sure it can handle the heater current by looking up the specs of all the new 6 volt tubes.

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 Post subject: Re: Mystery Guitar Amp
PostPosted: Dec Mon 11, 2017 11:07 pm 
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Kasanay wrote:
I took a shot at drawing up a schematic of the amp (I used Paint). It's a work in progress and I hope to finish it in the next couple days.


Why does it show the tube filaments off the primary side of the transformer?

By the way, if you convert to a regular power transformer with a 6.3 volt filament line, you can still use your 12 volt tubes if you wanted. They have a center tapped filament, so the two halves can be put in parallel. Just connect both ends to one side of the filament line, and the center tap to the other. But remember that it will draw twice as much current, so don't forget to account for that when you buy your transformer.

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