reversing tube PA amp mic-In transformers to act as Outs

[Guitarist]

H, I've rebuilt a Bogen MX30a to sweet condition but never really used it. Now I'd like to go straight into the grid and route the plate cap to the nice, 9 pin transformers. So this would make a really versatile amp or bass pre/amp so I can send balanced to the board etc.

I made some markings on one channel, please review.

Should I nix the "remote sends" and change the plate resistors to 100K? And add cathode Ω + cap. Maybe just on a couple channels for variety. But then pulling that 470k would have "insert" style recording vanishing. Not that I'd foresee using that. From a 4x12 this is PLENTY loud as it is even though the preamp plates are super low volts stock. I think I actually blew an AlNiCo testing. The tone was like caramel. It has its original Mullard 7246 (12dw7) and 7868 (6L6) by the way.

Any other ideas? Thanks.

[Steve Conner]

Mic transformers are designed to operate at mic level. You won't be able to get much output voltage before ugly saturation.

[Guitarist]

Perhaps just the one triode will work though, I hope. Maybe I swap in a lower mu tube. But hey, thank you greatly for the feedback I didn't think of that. I guess I can try it on one channel first...

I found this, "microphone transformers have usually smaller headrom-saturation point (ca +6 to 9dBV) than line transformers. Reversing primary and secondary connection will works if your signal is below this limit."

There is even a preamp that has a post transformer trim, wow. I suspect this one will stay in the sweet spot unless I screw it up.

[soundguruman]

Mic transformers are designed to operate at mic level. You won't be able to get much output voltage before ugly saturation.

Doesn't it make you sick when somebody is always right?

[km6xz]

Steve....I was in your neighborhood last week...I explored around Sterling to Glencoe and stayed on Loch Ard for 7 days. I was there with my sister with the goal of spreading my parents ashes over Lake of Mentieth. Our family is from that region, back to 1328. Although it rained most days for a period, it was stunningly beautiful and I got a lot of good photos with my new Nikon D800 and a new 24-70 2.8. I was surprised some people in several villages and towns remembered me from past visits. The weather was pretty decent in the Glencoe area and got to do a little rock scrambling and hiking for one day. Never made it to Glasgow however but spent 1 night in Edinburgh, one of my favorite cities. Yes, there is life away from the solder fumes...

By the way, the mic transformers are not designed for any DC, so using them after a RC coupling network and divider will work as a line out.....but why?

[Guitarist]

Sorry to hear of your parents–sending some love your way! You've been a good man around here, and you must've got that somewhere. Sounds like a lovely trip you guys had. Thanks for the sharing.

Steve likely figured I add this transformer 4 stages downstream and I'm glad he reminded me of jamming the coils.
I'm heading a band now. We always use a DI on bass so now he could use this amp and have a built in DI. I also have a dual 15" cab collecting dust.
Hey, plus having 3 other tube buffer DIs is a rather sweet trick to have in sleeve. I may add a piezo to the kick drum shell and run that into a 4.7M Ω channel input. I'll borrow Steve's idea to add a weight to one side. And suspend the disk's center a tad. ala: peter rambles on about piezo acoustic guitar pickups and how they work DIY - YouTube I reali

The other deal is that maybe God meant this amp for this such a future as the hard to replace Cannon ends are reversed to convention, which is a royal pain. But not now. I'll might swap in the original Cannon jack. In a couple days —it will enter metamorphosis. And gets its stage time at last.

[Guitarist]

I found this info re. (of this size) mic transformer distortion. The gentleman who is selling some Beyers references them to Peerless and Sure. Above .5 Volts the transformer saturates and this is at 50Hz.

I remember have a Fearn tube preamp for a couple days and drew up the schematic. It was basically a Marshall 800 style preamp with some feedback —well, and nicer components.

I'll do the mods Weds...