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dude

Sixty Cycle Blues

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Is that a amp or something used by nasa in the 50's

damn thats funny........................... i like it. God rest his soul this man helped me learn a lot about tubes and such i wish he was still kicking around

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That is a BEAST of an amp.

I remember you talking about the compressor idea on my last amp thread (when I REALLY had no idea what I was doing). It sounds like a pretty nifty idea. I don't think I'll use it on this first amp, but I may have to look into it in the future.

I've got a bunch of triode-pentodes in noval packages lying around from old TVs. I guess they're intended to drive the electron beam in a CRT, but hey, they'll probably be just fine for audio. I think I'll use one of them. I'm thinking I'll drive the tone stack as is typical with a dual triode (one stage direct coupled at the plate to the driver stage, which is used as a cathode follower to drive the tone stack). the triode will drive the tone stack, and the pentode will drive the triode. I think I'll put one 12ax7 stage in front of that (possibly in a two-channel arrangement), and then follow the tone stack with a phase splitter and a power amp.

For the power amp this time I think I'll use a pair of 6L6GC tubes, though sometime soon I want to try this one tube I recently found (6080) which is a dual power triode. Each triode is good for something like 12W, so you could pretty reasonably build a 15-20W push-pull amp with a single power tube. Pretty neat.

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Quick update: Circuit design is complete. It will be 2 channel (because I've got an extra half of a 12ax7 sitting there, and I've always liked the whole "jump-the-channels" thing on the old Marshalls), with one tone stack (Baxandall) and one volume per channel. Tubes include 2 12ax7s, 1 6au6, and 2 6L6s. Ordering parts tomorrow. Hopefully she'll be operational in a couple of weeks.

...I'll take pictures

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Ask and ye shall recieve:

schematic.png

the 6L6s aren't pictured, but they'd go right after the phase inverter. For now I just have resistive loads on each of the triodes there to show their output signals in simulation. Also missing is the second channel, but that will be mixed in at the volume control, so the gain will actually be reduced somewhat from what I've simulated, which is fine.

I have my pots represented as resistor pairs at the moment as well.

Edited by dude

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i think i would drop the second channel for now and do a cathode follower in to the tone stack and then run that in to the 6au6. but thats mainly because i have no idea how the 6au6 is gonna react feeding a tone stack.

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since your using a baxandall [my favorite btw ] you should bread board a simple inverting/non-inverting mid-range control and check it out you might be surprised. you can wire it up similar to a presence knob using a bandpass filter from the OT. back into the amp depending on where you put it you will get a negative or a positive phase relationship. and since it has all the saturation and characteristics of the output section it will deliver much more harmonic content. i put mine on a switch. that way i could get the best of both worlds. you can also do two pots one for total resistance ie volume of the filter and one for filter resonance. [resonance might be the wrong word choice but it felt right.] ie using the capacitor reactance table and your pot with a fixed resister you can set the bandwidth of the the lowest and highest point in between.

also you could incorporate a dual ganged pot for a movable filter since technically this is a high pass filter strung back into negative/positive phase you can do a movable version of the rats freq/gain control.

i think i have mine drawn up some where.

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Driving the tone filter with a follower is probably a good idea. Even passive filters start to get weird with high-amplitude signals, and reducing loading is always a good thing. The 6au6 will probably get enough "juice" after the follower to sound nice and distorted still. And hell, I'm simulating this all so I can even check on that.

Ansil, I actually decided to go with a baxandall after you suggested it a while ago. I looked into them and they seem like a very good choice. I think I'm going to keep things simple for this build (I've been on a pretty big strip-down-and-simplify kick lately. You should see my project guitar), but I am interested in seeing the diagram of the tone stack you've just described. Sounds cool.

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Question for you two (or anyone else with experience): what, sonically, are the differences between doing a long-tail phase inverter (as is seen on most big amps with two triodes) and a simple phase splitter (configuring a single triode with balanced anode and cathode resistors and taking the signal across each)? I'm playing around with some configurations, and one of the ones I'm rather fond of lands me with an extra half of a dual triode.

Also has anyone ever tried using a 6n2p instead of a 12ax7 for your typical high-mu dual triode purposes? I've heard they're voiced in more of a hi-fi fashion (less mids), and break up a little less smoothly, but are much clearer than a 12ax7. Since this amp is (save for the pentode) intended to be mostly clean, I'm considering trying them out (I'll probably put jumpers on my pcb for easy swapping from 6n2p to 12a_7). Also they're pretty much free.

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No on the tube but I use pentodes not triodes anyway. I only use triodes if they come with a pentode or if I am ckoning an amp. a pentode has more gain but more I guess the word would be headroom. As well so when it does break up it does so in a different fashion. A long tail is more commonly used imho when presence is required and adequate negative feedback. Personally I do not like it in my designs except for clones

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Also I might add a pentode has enough gain to drive an eq stack and still hit the phase inverter with enough juice to saturate it look at done if dr z designs. Not a fan if his particular tube choice but it its his amp design not mine. Mine are overly simple and complex at the same time

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Good information, Ansil, thanks. I'm particularly excited that the pentode has more headroom.

In that case I have a new circuit design. Once I get it mocked up I'll post it. Planning on doing 2 channels as before, with a triode in front cascaded into a pentode for each channel. The pentodes will be direct coupled through a resistor network to a triode cathode follower driving the Baxandall. Then I'll have a single triode (the other half of the follower) acting as the phase splitter. I may or may not use negative feedback. I had a switch to turn it off on my old project and I loved the openness of the sound without it. We shall see.

I was concerned before that the single tube splitter might have less headroom or something, but I guess it always sounded okay on my old project.

I ordered parts last night. Lots of them. So I can change my mind on the design quite a bit until they get here without any problems ;)

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I have an opinion question for y'all:

Currently I'm playing my amps through the 4x12 from a "Crate Flexwave" solid state stack. As far as I'm aware it is the cheapest 100W+ halfstack on the market, and I don't think the speakers are very good (though I've not compared them directly to any others). Would it be worth my while to invest in new speakers/a new cab so I can properly test this (and future) projects?

Okay, well I already know the answer, and its probably "yes". My follow-up question is which speakers to get. This is, of course, subjective, but I had a thought: My cab allows stereo play, so I could get two sets of different speakers with drastically different sounds so I can do a wider array of testing. I was considering a pair of greenbacks and a pair of vintage 30s (I haven't looked at non-celestions yet, but I will).

Any thoughts on this?

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have you seen pyramids 6.5" speaker that is 100 watts rms. i was playing around with one of these. its 8 ohms and it has the same freq response as a celestion. well at least that is what the freq graph they had showed but it was quite nice. it started tapering off at 3.6khz i know its not a 12" but have you ever heard a high power smaller speaker in a larger cabinet. jesus christ...........

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i like the ted webber sig series fairly decent speaker and not too badly priced.

i think it falls on the type of music you play for clean and bluesy type tones i prefer atleast a 10" speaker.

with higher gain stuff you need a different style speaker somthing that wont breakup nearly as bad.

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keep in mind that open vs closed back cabs will alter the sounds quite a bit. also there is a formula for how much air a speaker can move.

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Yeah, speakers are a whole new can of worms aren't they. Well, my current design strives to be as crystal clean as possible, but still open (minimal negative feedback), so an open back is probably a good way to go. On the other hand, a circuit with no negative feedback will be "tamed" somewhat by a closed cabinet ... so there's that option too.

Perhaps I should throw some webers into my current 412 and perhaps get a 212 at some point to run open-backed. Or just take the back off my cabinet sometimes. I do plan to build a JMP clone at some point as well, so there's that to consider as well.

One thing I always wondered is what would happen if you connected two speakers in a closed cabinet in reverse polarity. Do they resonate with each other then (since two would be pushing while the other two are pulling)? I'm guessing this would be dangerous, as it would essentially be abusing the cones of the speakers with twice the power it would on an open back. Could be louder - could oscillate and break your cones. I'm sure somebody has tried this (on purpose or otherwise.

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One thing I always wondered is what would happen if you connected two speakers in a closed cabinet in reverse polarity. Do they resonate with each other then (since two would be pushing while the other two are pulling)? I'm guessing this would be dangerous, as it would essentially be abusing the cones of the speakers with twice the power it would on an open back. Could be louder - could oscillate and break your cones. I'm sure somebody has tried this (on purpose or otherwise.

it wont hurt the speakers at all. the only thing it will do is kill the bass response and the mids will be muffled the highs will be effected but not as bad as the bass and mids. it happens all the time to guys doing car and home audio its just a simple out of phase problem but you will think your amp is dieing.

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Another acoustics question: I've always heard that for HiFi speakers, rectangular cabs are bad, because you get standing waves inside, and hence a very nonlinear frequency response and weird harmonics. And yet virtually all guitar cabs with a closed back are big, 90-degree boxes. Bass cabs tend to be ported and such, but guitars pretty much have stayed the same since the 60s. Is this simply tradition, or is there a different reason for this?

I'm considering having a pair of cabs built for 1x12+1x10 speakers with more conical shapes, no parallel walls, and possibly a small port on the 12-incher, just to see what happens. I'd do it myself but I have no woodshop. Thank god for craigslist.

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Look at all recording engineers speakers. They have been rwctangles for years. I have cabinet that has 2" speakers in it and its tuned for low end response. But there is like 12 in there and its only 4 inches deep on top and 6 on bottom

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Hifi audio is ment to reproduce sound. Guitar cabs are used to produce sound and there for are used as part of your tone shaping just like pickups wood and amp. Studio and hifi stuff will also be made of mdf where guitar cabs will be plywood or solid wood. Most of your speakers are aimed to emulate the speakers from the 50's and 60's to be honest leo fender probably just baught the cheapest things jensen made the same thing that went in to home sterios and tvs but as technology progressed so did speaker quality and guitar players liked the way the older speakers shaped there sound so at some point makers had to back up and make them like they used to.

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Hifi audio is ment to reproduce sound. Guitar cabs are used to produce sound and there for are used as part of your tone shaping just like pickups wood and amp. Studio and hifi stuff will also be made of mdf where guitar cabs will be plywood or solid wood. Most of your speakers are aimed to emulate the speakers from the 50's and 60's to be honest leo fender probably just baught the cheapest things jensen made the same thing that went in to home sterios and tvs but as technology progressed so did speaker quality and guitar players liked the way the older speakers shaped there sound so at some point makers had to back up and make them like they used to.

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