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 Post subject: A little help with Magnavox transformers?
PostPosted: Sep Tue 28, 2021 4:26 pm 
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I bought a Magnavox AMP-111 tube chassis for parts on eBay, mainly for the transformers to use on guitar amps. The PT seems healthy and strong, putting out about 390-0-390, with the other secondaries coming it right where I'd expect for an unloaded transformer.

One question I had was for an odd set of secondary leads (Brown) that put out ca. 7.25VAC. Anyone know what that one's for? I don't need it for a guitar amp, just like to know.

Also, when I checked the output transformer, there were more windings than I'm used to seeing on guitar amp OTs. The wire colors were also quite faded so I had to work out the winding pairs using a continuity meter. I'm not certain I got that right, but I when used a variac to input a known voltage on the primary, I got some voltage readings on the secondaries and was able to calculate the winding ratios. (Thanks, Uncle Doug!)

BUT--Using an 6000 ohm output impedence for the 6L6GC tubes, I calculated speaker output impedances of 2.02, 2.73, and 3.68 from what I think are the three output secondaries.

Question 2: Is that in the ballpark for these old Maggies? Did they wire up speaker combinations at 2 ohms and 4 ohm? Or are my readings so far off the mark that I got the secondaries wrong? Any suggestions or knowledge to help with identifying the leads correctly?



Any help with these would be greatly appreciated. I'd like to use the OT in a project if I can just get a comfort level about how to wire it.

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 Post subject: Re: A little help with Magnavox transformers?
PostPosted: Sep Tue 28, 2021 5:20 pm 
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I looked up the AMP-111 here https://www.tubesandmore.com/sites/defa ... pg18-3.png

For the power transformer there was one winding (Brn-Brn) that just went to an Auxillary socket and was not labelled as to the purpose or voltage. However I suspect it is intended to be able to supply 6.3 VAC under load (that's just my guess). You could just leave it unconnected.

Y-Y is 5 VAC for supplying a pair of 5U4 rectifier filaments
G-G is 6.3 VAC for all other filaments
R-RY-R is HV-CenterTap-HV
BLK-BLK is 115 VAC Primary Winding

Curtis Eickerman

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 Post subject: Re: A little help with Magnavox transformers?
PostPosted: Sep Tue 28, 2021 5:29 pm 
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thanks Curtis,

You have confirmed my suspicions about the Brn-Brn pair on the PT. I will cap them off (separately) for my application.

Do you have any thoughts on my other question about the output transformer and the common impedances Magnavox designed into their speaker complexes?

Thanks again!

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 Post subject: Re: A little help with Magnavox transformers?
PostPosted: Sep Tue 28, 2021 5:42 pm 
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Lectroid wrote:
Do you have any thoughts on my other question about the output transformer and the common impedances Magnavox designed into their speaker complexes?
I am not sure what they were doing exactly with regard to speaker impedances. There is one note in the documentation that says that the internal cabinet speakers were 15 Ohms and the external speakers were 3 Ohms. Sorry, that's it.

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 Post subject: Re: A little help with Magnavox transformers?
PostPosted: Sep Tue 28, 2021 6:17 pm 
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Manufacturers were all over the place with speaker impedances and output transformers.

You can reasonably expect to find some combinations of speakers that work out to 2Ω or a bit less, and others that would be higher.

They also did not always operate output tubes at their "tube manual" design specs, but could have chosen higher or lower plate load impedance on the primary to suit their needs. There is some tradeoff in power output and distortion depending upon which direction you deviate from the center of specs.

Sometimes an extra secondary winding was used only for negative feedback and did not have speakers connected to it, but this Magnavox did not have that feature.

Since AMP-111 uses four 6L6's in P-P-Parallel, one would expect the desired primary impedance to be half of the tube manual specs for two in P-P, or more on the order of 2500Ω to 3000Ω. You would not realistically run four of them with a 6000Ω load.

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 Post subject: Re: A little help with Magnavox transformers?
PostPosted: Sep Tue 28, 2021 11:57 pm 
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https://www.tubesandmore.com/sites/defa ... pg18-2.png

http://www.nostalgiaair.org/Resources/download.asp?FN=\M0010303.pdf


The Brn, Brn pair were the filament leads for the tuner / or preamp

This magnavox had a electro dynamics speaker , so you will most likely want to add a choke to the power supply to keep the hum to a minimum , and it had one or more woofers and horn speakers depending which model it was .

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 Post subject: Re: A little help with Magnavox transformers?
PostPosted: Sep Wed 29, 2021 3:12 pm 
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Thanks all. All of this was a big help to me. I'm now thinking this is not worth repurposing for a guitar amp.

@battradio,

I am assuming that by "electro dynamics" you meant the speaker(s) had a field coil?

Would the choke be added to the B+ going into the OT, or would it filter the screens tap?

Would a value of 5H be in the ballpark?

Also, the OT had a weird lead coming out of it. There were two solid uninsulated wires about 22ga together inside a stiff brownish outer tube about 1/8" dia. Any idea what that was for?

Thanks again!

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 Post subject: Re: A little help with Magnavox transformers?
PostPosted: Sep Wed 29, 2021 9:46 pm 
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Lectroid wrote:
Thanks all. All of this was a big help to me. I'm now thinking this is not worth repurposing for a guitar amp.

@battradio,

I am assuming that by "electro dynamics" you meant the speaker(s) had a field coil?

Would the choke be added to the B+ going into the OT, or would it filter the screens tap?

Would a value of 5H be in the ballpark?

Also, the OT had a weird lead coming out of it. There were two solid uninsulated wires about 22ga together inside a stiff brownish outer tube about 1/8" dia. Any idea what that was for?

Thanks again!

A 5 hy coke would be fine in the B+ , the two solid uninsulated wires are for electrostatic grounding and the speaker had a field coil . The amp is about 40 watts output ,when it was new it had 6L6G's in the amp or metal 6L6"s and four of them would do about 40 watts . The Driver transformer in it would make it good for a bass guitar amp.

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 Post subject: Re: A little help with Magnavox transformers?
PostPosted: Sep Thu 30, 2021 12:59 am 
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There are two kinds of dynamic speakers. Electrodynamic speakers have a field coil. Permanent magnet (or PM) dynamic speakers have a permanent magnet instead. Except for the type of magnet, both kinds are the same.

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