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Identifying Old Output Transformers


mach5rider

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Hey guys, I've been servicing my own tube amps for a while now, but am just getting interested in building my own tube amp (5E3?) from scratch. My g-pa was a tv repairman in the heyday of tubes, so I've got all kinds of stuff to root through to find parts. I've got a nice 300-0-300 PT picked out, but I'm trying to figure out how to locate a good output tranny and how to tell if they are designed for PP or for SE type circuits. Any help would be great.

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Hey guys, I've been servicing my own tube amps for a while now, but am just getting interested in building my own tube amp (5E3?) from scratch. My g-pa was a tv repairman in the heyday of tubes, so I've got all kinds of stuff to root through to find parts. I've got a nice 300-0-300 PT picked out, but I'm trying to figure out how to locate a good output tranny and how to tell if they are designed for PP or for SE type circuits. Any help would be great.

 

 

Usually a single ended OT is going to have only two inputs (wires) on the primary side, and a PP amp usually has three.

 

To find the proper value of an unknown transformer you need to detremine the turns ration.

 

See if this helps any:

http://us.f13.yahoofs.com/bc/43c71b6em9c650a2b/bc/Output+Transformer+Impedance++Text+file/OT+Impedance+HTML.htm?BCoQ_cIBDYKOf4e8

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Thanks Casey,

that makes sense after I look at a few schematics - the 3rd wire is for the grid on a PP. FYI, the Yahoo link didn't work, but I've found some helpful info here:

 

 

The third wire, or centre tap, on a push-pull transformer is for the B+ (high voltage) line. The two outer wire are for the valves plates.

An OT does not have any connection to the valve grid unless you mean the screen grid.

Sometimes a PP OT will have five wires, the other two for the screen grids at about 40% of B+, for an ultra linear amp. Most guitar amps will take the screen supply from the choke (or resistor) on the B+ line though as ultra linear usually does not equal good guitar tone.

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Thanks Casey,

that makes sense after I look at a few schematics - the 3rd wire is for the grid on a PP. FYI, the Yahoo link didn't work, but I've found some helpful info here:

 

 

Here is a cut and paste from my link:

 

 

 

Finding Unknown Values for Output Transformers

 

(A little Math is required)

 

If you have an Output Transformer with unknown values and want to find the intended speaker impedance for a given tube type there are several things to consider. An OT reflects the impedance load (the speaker) on the secondary back onto the primary winding.

 

There are a few abbreviations I am going to use for the purpose of this explanation.

 

VR = Voltage Ratio

 

IR = Impedance Ratio

 

TR = Turns Ratio

 

IL = Impedance Load (The speaker)

 

ELR = Effective Load Resistance (from tube manual)

 

ELR Examples for two tubes in PP AB1 or AB2

 

6L6 ELR = 3,800 ohms

 

6V6 ELR = 8,000 ohms

 

EL84 ELR = 8,000 ohms

 

EL34 ELR = 6,500 ohms

 

 

 

The Turns Ratio (TR) can be found by applying a small AC voltage to the secondary and measuring the result on the primary. If you put 1v AC on the secondary and measure say, 31.6v on the primary you have a Voltage Ratio (VR) of 31.6 which means that the turns ratio is 31.6:1.

 

Finding a 1v AC source is difficult unless you have access to a Variac, small transformer or similar device. You can use a 5v rectifier winding from an existing PT or the 6.3v heater winding. You must then divide the answer by the number of volts you applied to the secondary. For example the 31.6:1 TR would measure 158v with 5v AC applied to the secondary.

 

The Impedance Ratio (IR) is the square of the Turns Ratio (TR), in this case:

 

31.6 (sq) is 998.6

 

To determine the expected Impedance Load (IL) or speaker impedance you will divide the known Effective Load Resistance (ELR) by the Impedance Ratio (IR)

 

So: ELR / IR = IL

 

In this example we want to use a pair of 6V6 tubes that have an ELR of 8,000 ohms with this transformer and want to determine the proper speaker impedance (or IL).

 

8,000 / 998.6 = 8.0 ohms

 

So you would use an 8-ohm speaker with this transformer and a pair of 6V6 or EL84 tubes in AB1 or AB2.

 

What about a pair of 6L6 tubes? RCA says the 6L6s have an ELR of 3,800 so:

 

3,800 / 998.6 = 3.80 ohms

 

So you would use a 4-ohm speaker with this transformer and a pair of 6L6 tubes in AB1 or AB2.

 

 

 

 

Casey4s

02/04/01

 

REV. 07/08/01

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Thanks guys, this is helpful info. I'll see if I can measure what I've got tonight and report back my plans.

I was thinking a 5E3 would be a good first build, but last night I was playing my musicmaster bass amp (12 watt 2 6V6 amp) and had it maxed and was struggling for stage volume, so maybe something more like a blackface deluxe AB763 with 22 watts is more inline for what I need. Any suggestions?

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Thanks guys, this is helpful info. I'll see if I can measure what I've got tonight and report back my plans.

I was thinking a 5E3 would be a good first build, but last night I was playing my musicmaster bass amp (12 watt 2 6V6 amp) and had it maxed and was struggling for stage volume, so maybe something more like a blackface deluxe AB763 with 22 watts is more inline for what I need. Any suggestions?

 

 

 

The 5E3 is a pretty simple first build, but if you like the Tweed feel and sound try a 5F6A Bassman. It's also a fairly easy build and a lot more power (35-40 watts) than the 5E3. You do not have to build it as a 4X10 if that doesn't work for you, it could be built as a head, or a 1x12 combo or whatever works for you.

 

The Blackface DR and the PR are both cool amps and of course have the reverb and tremolo.

 

I have built the 5E3, 5F6A, and a BF Princeton Reverb, among others and they are all cool amps.

 

Good luck with your future build.

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Thanks for the help guys. Being a newbie, it's great to be able to ask "stupid" questions and have helpful results. I think have found a couple decent OT's depeding on what I decide to build - I need to play some more amps first. New question: I'm working on a Fender Pro Junior - looked at the schematic:

http://www.ampwares.com/ffg/schem/pro_jr_schem.jpg

why does the red wire on the secondary of the PT show 132 VAC. I would expect it to say 325 VAC or something in that range?

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Thanks for the help guys. Being a newbie, it's great to be able to ask "stupid" questions and have helpful results. I think have found a couple decent OT's depeding on what I decide to build - I need to play some more amps first. New question: I'm working on a Fender Pro Junior - looked at the schematic:

http://www.ampwares.com/ffg/schem/pro_jr_schem.jpg

why does the red wire on the secondary of the PT show 132 VAC. I would expect it to say 325 VAC or something in that range?

 

It's weird 'cos the filament winding is showing half what you'd expect also.

I'd have expected to see something in the 265-295Vac range as a bridge recitifier does appear to increase the volatge a bit when converting to DC. The filament winding should definitley show 6.3Vac for most tube types, 12AX7 included. :confused:

Is this some weird convention that Fender use? Anyone? :confused:

Did AC meters only pick up the positive going waveform for measurement back in the day???:confused:

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The PT I have for this pro junior measures around 320 VAC across the red secondary taps, I'm assuming this is correct, since it's for this amp and seems reasonable, but I was just confused by the schematic.

 

Yup, that sounds about right to me.

Anyone got any theories on the Fender schematic? :confused:

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