OT's question - teachable moment

[TweeDLX]

Lately, I've installed larger OT's in a couple of my smaller amps (5E3 clone & SFPR) with great sounding results. I also attempted using a 50 watt Peavey Classic OT on my Allen Accomplice, but did NOT like the end result. It broke up way early, and didn't have the tone I was after. Interestingly, it wasn't much bigger than the Allen/Heyboer 35 watt OT I was replacing. So, in this case, bigger wasn't better. I know that you need to match the primary impedence of the OT with the tubes (or make adjustments at the secondary level), but what other considerations would I want to be aware of when looking at replacement OT's? Also, I noticed in every amp that my plate voltage changed (upwards). Inquiring mind wants to know... Thanks for your edification!

Mike

[donnyjaguar]

Audio transformer design is a science unto itself. If the plate voltages went up that tells me the resistance in the primary windings is lower. This could be because there are fewer turns or the wire used is of lower loss. The only way to achieve lower loss is with thicker wire or using a metal of higher conductivity. Given most transformers are wound with copper you can pretty much preclude the latter unless the replacement transformer uses silver-plated wire. Some hi-fi transformers do, but we're talking high-dollar stuff here.

Probably the biggest factors in how a transformer sounds is the magnetic permeability of the core and the inductance of the primary. If either are less than optimum the amplifier will have trouble working in the lower frequency (bass) regions. NFB can compensate for much of this but there are limits. I'm guessing the Peavey is suffering from this. I'm not trying to bash Peavey, but hasn't their business plan always been to sell lower-priced gear? From my experience (granted limited) Peavey guitar amplifiers are not up to the same standards as most other brands.

[TweeDLX]

Thanks Donny! That adds to the picture. I also notice a lot of replacement OT's have specs that say something like: "primary impedence of X.x K @ xxx ma". Care to expound?

Mike

[Prairie Dawg]

Originally Posted by TweeDLX

Thanks Donny! That adds to the picture. I also notice a lot of replacement OT's have specs that say something like: "primary impedence of X.x K @ xxx ma". Care to expound?

Mike

Size really does matter.

F'rinstance the old Vibrasonic (aimed at the steel guitar players) was about the same as a brown Pro, but it had a twin reverb output transformer. I recently acquired a Bantam Bass, which has had the wimpy 30w output upgraded to a twin reverb output, and it delivers real clean power to the bottom end. I loooooooooove 15 inch speakers.

[donnyjaguar]

Originally Posted by TweeDLX

Thanks Donny! That adds to the picture. I also notice a lot of replacement OT's have specs that say something like: "primary impedence of X.x K @ xxx ma". Care to expound?

Mike

The impedance ratios are determined by the number of turns on primary and secondary. The primary being wound for a specific impedance input that normally matches that of the optimum operating point of the tubes they are used with. ie: 5000k is typical for a pair of EL34 tubes. If the transformer were 2500k then it would be more suitable for 4x EL34 tubes. As for the current, this is generally the maximum amount of current the primary can handle and would be chose with the output tubes in mind. ie: for a pair of EL34 the rating could be 50ma per tube or 100mA total. Generally you don't run them this hot but its nice to know if you do it won't smoke the winding.

Of course neither of these specifications will tell you how well the transformer will perform, as you've learned.

[TweeDLX]

That adds a lot more to my understanding. Thanks! I wonder how a TR OT would sound on a Fender Concert II? Are there any other limits on how big you could go besides primary Impedence and current handling?

Mike

[Structo]

Yes, to all the info, you have to match the OT to the amp!
Impedance and power rating are very important to operate safely.

You should match the impedance to the annode resistance specs in the data sheets for the tubes.
Well you should anyway, Marshall is known for going a little low there and maybe some of that Marshall crunch is a result of that.

Some will say a smaller tranny saturates more quickly and gives a cool sound.
So I would say in conclusion the most important spec to pay attention to is the power rating of the OT compared to the output tubes you are using.

[TweeDLX]

Originally Posted by Structo

Some will say a smaller tranny saturates more quickly and gives a cool sound.

Thanks for the info Tom. I've been working at making sense of tube charts. Sometimes they seem clear, but they're all different!
I'm aware of that early saturation thing, but I've found that in most cases, I prefer the big sounding cleans over a saturated sound for rhythm work. When I want crunch for a solo, I use a pedal.

Mike