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Step 13Output Transformer, part 2
Ok, so I already had a Hammond 125C, which is an 8 watt, "universal" OT.
Much of the testing with this amp was done with the Hammond. But it's not ideal--it's better used in the push-pull configuration, especially this close to it's rating. Single-ended transformers generally reach core saturation quicker than PP OTs--SE (Class A) amps draw current throughout the operating cycle. That has a reactive effect on the inductor. consequently, SE and PP output transformers are designed slightly differently (so the validity of a "universal" OT is in the eye, er, ear of the beholder...)
Technically, the 125C is big enough, since this amp is approximately 8 watts....but in practice this was not so...
(Is this really less than 8 watts? Ignoring the "typical" example on the datasheet, the charts indicate that a single 6DG6GT can produce upto 4.4 watts..this is probably an 8 watt amp...)
Some general notes about the Hammond 125C :
--I tried both 1500 and 2100 ohm primaries. The lower impedance was definitely more musical, bluesy and overall the better tone. But the 2100 primary gave the amp a more gravelly, overdriven sound. Some might prefer it.
--At both hookups, the Hammond OT was definitely getting warm. Not really hot, but I didn't play power chords for three hours straight, either. Probably too small for the amp, especially as a SE OT.
--Since the transformer was right at it's limit, the core (over) saturation gave it a very "Cream" sounding distortion...unfortunately, chords and comping leads pushed it over the edge, and there was no definition, just muddy, nasty distortion.
So I sprung for the Edcor GXSE15-4-1.7K, with a 1700 ohm primary. This thing is MASSIVE compared to the Hammond; easily 3 or 4 times as heavy. The 10 watt version probably would have worked as well, but many people are putting Hammond 15 watters in "Champ" projects, and those are only 5 watts (vs. 7 or 8 for this one.)
Plus, as noted on the previous page, 1700 ohms is an almost exact match to the theoretical output impedance of two 6DG6GT tubes at our supply voltage (yay!)
The Edcor has specified inputs for the B+ and Plate, as well as Screen (Not sure "screen" would work with the 6DG6GT's, but I already had the screen voltage tap on the power supply.) And there was a discernible tonal difference between the two secondary / speaker wiring options (swapping the order of the speaker wires.)
Also, the GXSE15-4-1.7K is too large to mount on the chassis, OR the speaker! But it fits fine in the cabinet. Doesn't seem to cause any noise issues.
The difference between the two transformers is marked! The amp now handles chords and multi-note leads just fine. Single notes are fat; the overall tone is warm and full. The Edcor was right choice....
The Edcor GXSE15-4-1.7K lists for $30 USD, and shipping was $9. The 10-watt version (GXSE10-4-1.7K) costs $10 less... Either price is very reasonable, for such excellent output transformers. Find them at Edcor Class X Guitar Single-Ended Audio Output Transformers...
Sept. 2008
Note: I've been using this amp almost daily for 4 months. The chassis gets a little hot. The power transformer gets a little hot....the Edcor output transformer is always as cool as cucumber. This is an excellent OT!
Much of the testing with this amp was done with the Hammond. But it's not ideal--it's better used in the push-pull configuration, especially this close to it's rating. Single-ended transformers generally reach core saturation quicker than PP OTs--SE (Class A) amps draw current throughout the operating cycle. That has a reactive effect on the inductor. consequently, SE and PP output transformers are designed slightly differently (so the validity of a "universal" OT is in the eye, er, ear of the beholder...)
Technically, the 125C is big enough, since this amp is approximately 8 watts....but in practice this was not so...
(Is this really less than 8 watts? Ignoring the "typical" example on the datasheet, the charts indicate that a single 6DG6GT can produce upto 4.4 watts..this is probably an 8 watt amp...)
Some general notes about the Hammond 125C :
--I tried both 1500 and 2100 ohm primaries. The lower impedance was definitely more musical, bluesy and overall the better tone. But the 2100 primary gave the amp a more gravelly, overdriven sound. Some might prefer it.
--At both hookups, the Hammond OT was definitely getting warm. Not really hot, but I didn't play power chords for three hours straight, either. Probably too small for the amp, especially as a SE OT.
--Since the transformer was right at it's limit, the core (over) saturation gave it a very "Cream" sounding distortion...unfortunately, chords and comping leads pushed it over the edge, and there was no definition, just muddy, nasty distortion.
So I sprung for the Edcor GXSE15-4-1.7K, with a 1700 ohm primary. This thing is MASSIVE compared to the Hammond; easily 3 or 4 times as heavy. The 10 watt version probably would have worked as well, but many people are putting Hammond 15 watters in "Champ" projects, and those are only 5 watts (vs. 7 or 8 for this one.)
Plus, as noted on the previous page, 1700 ohms is an almost exact match to the theoretical output impedance of two 6DG6GT tubes at our supply voltage (yay!)
The Edcor has specified inputs for the B+ and Plate, as well as Screen (Not sure "screen" would work with the 6DG6GT's, but I already had the screen voltage tap on the power supply.) And there was a discernible tonal difference between the two secondary / speaker wiring options (swapping the order of the speaker wires.)
Also, the GXSE15-4-1.7K is too large to mount on the chassis, OR the speaker! But it fits fine in the cabinet. Doesn't seem to cause any noise issues.
The difference between the two transformers is marked! The amp now handles chords and multi-note leads just fine. Single notes are fat; the overall tone is warm and full. The Edcor was right choice....
The Edcor GXSE15-4-1.7K lists for $30 USD, and shipping was $9. The 10-watt version (GXSE10-4-1.7K) costs $10 less... Either price is very reasonable, for such excellent output transformers. Find them at Edcor Class X Guitar Single-Ended Audio Output Transformers...
Sept. 2008
Note: I've been using this amp almost daily for 4 months. The chassis gets a little hot. The power transformer gets a little hot....the Edcor output transformer is always as cool as cucumber. This is an excellent OT!
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