Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on September 29, 2014, 08:08:49 pm
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I’ve stitched together my next Idea from the builds and ideas I’ve found here. The front end is a matchless lightning mated to a powered FX, PI that I used in a recent build and finally either 6v6 or el84 PA section. The numbers aren’t all finalized, but the schematic should be close. The 1st questions I came up with
1. double CF, I found a Mars OD&FX&2CF schematic with notation that indicated this was musically good/ok?
2. on the return side of the FX, the section I copied has a 220k feeding the grid and an 82k to ground. Should those be swapped?
3. The MV on the matchless is like I have on my schematic. My take is it acts like a volume because it phase cancels the signals feeding the PA tubes? Haven’t build any amps with a PPIMV so is that a normal/acceptable way?
One last aside question, is there a "rule of thumb" kinda value for phase shift input to output, the amps I have checked seem to be about 5 - 15 degrees.
This will be a slow build since winter is soooo long!
Thanks again.
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> phase shift input to output, the amps I have checked seem to be about 5 - 15 degrees.
At what frequency??
A "flat" amp will be near zero (or 180) in its flat-range.
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One last aside question, is there a "rule of thumb" kinda value for phase shift input to output, the amps I have checked seem to be about 5 - 15 degrees.
Assuming you're talking about input jack vs. speaker jack, and you're feeding your instrument into only the one amp, your ear can't distinguish the absolute phase difference.
If you apply your one instrument signal to multiple parallel amps/devices each with differing phase shifts, you might hear a sonic change when hearing all their outputs at the same time.
When talking about a feedback loop inside a circuit, phase shift will dictate whether the loop is stable or unstable. However, if you're tinkering a loop any way other than reducing the amount of feedback from a known, proven-good circuit, you should already have enough background knowledge to make the question unnecessary. That, and you'd probably be designing on the basis of "rate of attenuation" because phase shift varies across the frequency spectrum, even for a circuit as simple as 1 resistor and 1 capacitor.
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I was wondering because I read, here I believe, "to much phase shift is not musical". my "normal" testing is done at 100hz, 400hz, 1k and 4k. I didn't think it much mattered in an instrument sense since there really isn't "anything to compare to", your ear hears what the speaker puts out, not a mix of guitar vs speaker. I get the concept from a NFB sense if it's far off, there will be more signal cancelation. It's just been engrained into my head for 30yrs because the circuits I maintained used fundamental frequencies that were phase encoded n those were a bitch to track down.
Anyway, thanks, I can scratch that.
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I was wondering because I read, ... "to much phase shift is not musical".
Yeah, that's not a factual statement, standing alone.
Maybe the writer of that misinterpreted something they saw relating to a hi-fi or stereo amp. You'd like there to not be any serious discrepancy between the phase of left channel vs. right, and phase issues can degrade the sound of a multi-driver speaker system (where it is mostly described in terms of delay for different frequency bands).
Also, some manufacturers occasionally show the phase response of their products, which really shows that where there is a net 0-degree phase shift that there is no significant attenuation across a band of frequencies, which really is implying even amplitude response (such plots often show phase not shifting until something like 100kHz at one extreme, just to show nothing in the audio band is attenuated).
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V2-3 cathode cap of 25uf will sound muddy (I think?), & I recommend a value more like 2.2uf. You could try a 5uf?
You will need either a 250ka return pot where you have the 82k to ground on the FX ........... OR a 250ka level pot after the return triode.
I think the FX will not sound right without the pots. I typically have the pots on my FX dialed around 3-5 max.
With respect, Tubenit
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Thanks guys, Tubenit, I had the pot in the initial sch but took it out since I had the MV? pot "close". My thought was that pot(MV?) would be more for fx when fx was used for vol when fx was out. EDIT, (live) the MV effects both, where fx affects fx only!! got it, pot in also.
I'm shooting for fender twin 0 -4 clean tone so I will make the cap change, have a few 2.2's so that works.
thanks