Guitar Tube Amp

Step 12 Output Transformer, Part 1

Figuring Load Resistance Mathematically (WARNING: Math Content)

We have an educated guess, using the datasheet (see the previous step):
1833 ohm for two tubes.

The alternative way is to use the formula for output impedance:

Zout = Va/(Pa/Va)

Va = Anode voltage (185V)
Pa = Maximum plate dissipation (10 watts -- from the datasheet)

Load resistance for one tube:
3422.5 = 185 / (10 / 185)

Or half that value, 1711 ohms for two tubes.

For 190V (we're somewhere between 185 and 190):

3610 = 190 / (10 / 190)
Or, 1805 ohms for two tubes.

Whether we use 1833, 1711, or 1805 doesn't matter. Tube characteristics probably vary by AT LEAST that much, any of the three figures is fine.

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OK, first--I bought an 8 inch Weber 4 ohm speaker for the amp (although I wish I'd gotten at 10 instead.)

Important when choosing an output transformer: The speaker's impedance has a definite impact on the output transformer's impedance. As you can see from the chart below, twice the speaker impedance (4 ohms to 8 ohms, for instance) will also double the input impedance for any transformer. As mentioned in the last step, the target primary impedance of the output transformer is 1711 ohms.

In fact, output transformers by themselves don't have a set impedance, but instead possess a turns ratio (10:1, 20:1, etc.) Rather, the impedance of the speaker(s) is reflected backwards from the secondary to the primary. It's that reflected impedance that actually forms the primary impedance.

This isn't an ideal value when searching for off-the-shelf guitar output transformers--they tend to be higher impedance. It's reasonable, however, and there are options. Bigger, higher-wattage amps will use "iron" with lower input impedance, but they are generally PP transformers.

Some OT options for this project:

1) --Hammond single-ended output transformers (125SE Series)

These are great transformer for SE amps, but don't have an official "spec'ed" input inpedance below 2500 ohms. But they have multi-tap output coils, and they could be wired for an appropriate value. For instance, connecting a 2500 ohm primary OT with an 8 ohm secondary to a 4 ohm speaker (halving the load) also halves the primary impedance--to 1250 ohms. Now, 1250 ohms is a little low for two tubes (but probably not for three ;-)...time to plan V2 of this project!)

Also, the transformers would be out-of-spec; a larger-wattage transformer would probably made it a safe choice. Still...

2) --Edcor GXSE Series

Edcor makes some great guitar amp transformers, many of which would be excellent for this project!

These are a nearly perfect match to our requirements:

GXSE10-4-1.7K  (10W 1,700 4 ohms)
GXSE15-4-1.7K  (15W 1,700 4 ohms)

3) --Hammond "universal" SE/PP output transformers (125 Series)

Although touted as having both SE and PP capabilities, perhaps not the best choice. Single-ended and Push-Pull transformers have other differences beside wire--the cores are different. This prevents SE transformers in Class A amps (which are always drawing current) from reaching saturation.

The Hammond universals have multiple output taps, so they have many different input impedances. And they come pretty close to our target value:

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(Due to the fact that many formatting features at Instructables have been revoked for non-pro members, this text-based chart has been removed. It was unreadable.

Refer to the Hammond chart in the images below.)
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For a 4 ohm speaker, either 2100 or 1500 ohms are close to the target of 1711 ohms.

The 125 series is supposed to be usable in both SE and PP configurations. But in reality, they couldn't possibly do both well. I expect it's the single-ended performance that suffers, since it's so easy to saturate an output trannie in SE mode. So, they aren't the best choice, but they are a choice...

EDIT: Hammond no longer claims these are usable as SE transformers, and are now marketed as push-pull only...