Two preamps into one phase inverter (check), but PT question

[pbrommer]

I have a quick question. I think I know the answer, but I'm not exactly sure (I'm just a slight step above tube noob).

When I look at the Trainwreck Songwriter (parallel 12ax7) and Rocket preamps, they are not in phase. Correct?

If so, that means if I wanted to do a two channel preamp with each preamp I would have to feed them into separate sides of the PI. Correct? Otherwise, I'd have to bring them together through mixing resistors and then take the signal to the PI.

If I am correct with needing to feed the signal of each preamp into separate 'sides' of the PI, does it matter which side the signals are placed?

I've got an idea to combine both preamps into the same PI/power amp (rocket poweramp) with 2 EL84. If I'm on the right track, then I can get moving on this project. Thanks in advance.

Patrick

[Leo_Gnardo]

As long as you're not expecting to run the same signal thru both preamps at once, who cares about in/out of phase. Gazillions of Fenders can't be wrong - you can use a simple resistor mix, say a pair of 220K's and off you go.

IF it's your aim to run the same signal thru both preamps at once and combine for a satisfying result, then you could do a simple inverter - one low-gain triode with signal taken off the plate, followed by a voltage divider to bring the overall gain back to unity - and you're good to go. OR like the late 50's Guild 100G I was working on last night, run the output of each channel to a grid of the output dual triode 12AX7 drive tube, and simplify the drive circuit - no feedback, no presence. The venerable Guild had a 1100 ohm 1st common cathode R, a pair of 1M5 to ground from each grid, and a common 68K to ground. Never seen the like - it flummoxed me at first - no schematic of course plus a factory miswiring - but in the end I figured out what was going on and it worked just fine. FWIW the Guild preamps are in-phase but they weren't planning to have anyone use both chan's to run the same signal. Guitar in one channel, accordion in t'other. Now that's MUSIC. Hand me that accordion. And that can of lighter fluid...

[pbrommer]

Thanks for the response. If it's that easy, then that should make things simple.

What I'm aiming for is using the 2 preamps (Songwriter and Rocket) in a style like the DC-30. Two jacks on the front panel - one for the Songwriter, one for the Rocket (plugging into whichever sounds good that day, maybe a footswitch a/b).

Still in planning mode - I have the board and parts ready. Just trying to make sure I execute correctly.

Patrick

[Leo_Gnardo]

Then it looks like the Fender-style mixer will do. One thing that's nice about it is you can sizzle up the hi frequencies on one channel if necessary with a hi-bypass cap across one of the mix resistors. Also now you don't need to add an inverter. Whew! It's good to be lazy. Sometimes.

[pdf64]

If the 2 channels create signals of opposing polarity, then as long as there's no global negative feedback loop, why not send them into either side of the long tail pair, as with the Vox AC30, to get them into the same polarity?
Just to save you wondering about the ABY option, if nothing else.
Pete

[matt h]

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[JazzGuitarGimp]

Thanks for the throwback, matt h. Back in my college days (late 70's), "tasty" was the buzz word, uttered by all the jazz rats who roamed the hallways of the music building. "Oh man, that was TASTY!"

[pbrommer]

Looks like I'll just have to omit the NFB from the Songwriter circuit. Not that there's an issue with that....

Thanks guys! Glad to know that I'm on the right track. Just need to track down the transformers (and chassis, and other parts, etc.) and I'm on my way.

Patrick

[matt h]

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[pbrommer]

Okay. So I'm trying to find a transformer that will work for this build. Since I want to build it as a combo amplifier (because I have a 1x12 ax84 October build in one that sounds awesome), I'm trying to find a laydown PT. For some reason, the vertical mount PT scares me in a combo.

Anyways......... I think I may have found one, but I keep finding conflicting information on it. The Hammond 270DAZ http://www.hammondmfg.com/pdf/5c0026-27.pdf looks like it would work, but I've seen it's current maximum at 90mA and 104mA. From what I've gathered, with 3 12ax7s and 2 EL84, I'm going to need about 95mA or so. Do you think this transformer will work? Otherwise, I need to find a suitable laydown style PT about 260-0-260 (which I'm having no luck).

Slowly, I'm getting parts for this build. I've got some ideas, just need to execute them. I think it'll be fun (and loud).

Patrick

[matt h]

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[pbrommer]

only catch with the hammond is that there isn't a 5vac or second 6.3vac winding if you were thinking of a tube recto. not that I'd go tube recto... but i'd be looking at a decent size sag resistor. works for dropping voltage and giving a lil liveliness. without it, even the 520VAC of the hammond seems a bit too high for el84s to be really happy. (500VAC, solid state recto, cathode biased is better.)

I'm looking at solid state (2 1n4007/UF4007 per secondary side (Full wave rectifier, correct...I think). I'd love to find a 250-0-250 horizontal mount transformer, but I have no clue where to look. If I find one, the mA rating is way too low. Ideas on a transformer source?

Patrick

[matt h]

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