The Amp Garage

Audio transformer turns ratios are always given. 4K @ 8 ohms is the impedance ratio and that’s the turns ratio with a little math. They don’t tell you the actual number of turns of wire or the wire gauge and they don’t tell you the winding geometry. That’s the “secret stuff”.

Slowly removing NFB from a LTP is not going to show you anything about the imbalance of the phase inverter outputs unless it’s already fairly imbalanced to begin with and the feedback is helping to correct the balance. It WILL remove the NFB and show you more gain. Replacing one plate resistor with ...

This is a very general question. The transformer unloaded produces nearly 400-0-400. 397-0-397. It’s spec is 375-0-375 @ 200mA draw so it’s stout. It’ll make well over useable voltage for current production EL34’s and 6L6GC. KT88’s or 6550’s would live well and be quite a nice probably 75 watt amp w...

… really want to know the pattern of the Drake 784-103 jtm45 output transformer. You’d need to get a 200ohm speaker load rigged up to use all its secondary (100V) :) I’m not concerned with using the 100v tap but I’m curious if it’s wound with a typical Drake secondary pattern ( turns per tap) or if...

No, I understood you completely. He is the one trolling. No worries. There’s still a lot I have to learn about transformers but I’ve also been successful with what I’ve learned and produced. I have trouble finding information. I also need more equipment but I’ve been lucky with my designs and really...

No one is going to create an output transformer that works with any success without quite a bit of mathematical calculations. If one simply clones an existing transformer from a tear down and cleanup and rewind, you definitely don’t need any calculation for the assembly. I spend way too much time cr...

You’re asking the million dollar question. Without complete analysis of the specific transformer, there’s no way to speculate.

Remember also that I think we’re talking about a guitar amp and transformer. We generally prefer distortion in this case. I’m not talking about clipping but simply a difference between input and output. Phase shift also. In high fidelity, we want little to no distortion, maybe just a touch for warmt...

It really depends on how it’s wound but most guitar amps output transformers are just a series connection where the 4 ohm tap is half the secondary, 8 ohms is 1.14 times the 4 ohm and 16 is the full length. The important thing is that when series wound, the full secondary conducts all the current an...

If it were me, I’d feed 100 volts into the full primary and measure the output voltage from the speaker leads but that’s just the way I do it.

I’ll almost bet the filament voltage will be acceptable. Measure it unloaded. It’s already a high current winding and because it’s going to be a really low DCR it’s probably about 6.6 volts unloaded and the draw from 4 EL34’s is going to pull it down.

I think those come out of a Sound City Mark 4 120. If this is what they are, the schematic shows 350 volts AC from the power transformer, great for your application. The schematic shows 375 volts DC on the B+ at the plates of the EL34’s. Wondering what the impedance is for the output transformer. Th...

What do the phase inverter outputs do on the scope? Previous stages? Work back into the preamp and ensure that the waveform is good.

RFT 12ax7’s are a little bit aggressive sounding if you’re going to push the preamp. Mullard I61 are my favorite for Marshall stuff. I don’t use current production preamp tubes.

I use old Mullard I61 and I63 preamp tubes in my Marshall builds. From time to time I find a tube that crackles at power on. Sometimes it goes away and sometimes it persists. I also notice sometimes the new amp has some tonal behavior that seems off. I play the amp for a while before making any chan...