low end and how to reduce it?

[daz]

I understand how it works. It's just semantics. It allows whatever frequencies it passes to move on by any resistance while the rest of whatever is left in the signal is attenuated. One part goes thru unaffected, the other is lowered. It's however not the point.

The point is that when it's done that way the bass is still being reproduced the same as before, but now there are treble frequencies that are louder IN RELATIONSHIP to the rest of the bandwidth. So what happens as i said before is now i have more high end, but i have to let so much high end thru to even make the bass seem a bit better (and barely any better) that the tone is now a piercing treble fest with a huge super low (i'd estimate 30 Hz) flab tacked onto it. (Picture a graphic EQ with a huge V shape with the low part around 500Hz and dipped 20 DB). The only options left if using the filter method then are to use different caps to alter the frequencies that are allowed to pass, whether up or down. That doesn't work. I won't keep going on, but suffice it to say, been there done that to death, and it's not the answer. I'm not tech, but i'm also not stupid and i know whats going on here. The bass is SO low in frequency, and theres SO much of it, that using filters like that is not an option. Thats a way to tailor a NORMAL tone to taste. The tone here is NOT normal. There is a flaw in it and i need to find the source. Band aids won't work. I've done every filter trick everywhere in the circuit in many different ways ad nausium. The source of the problem needs to be found and rectified, period. i think i may have found it but i won't know till i can turn it up to normal listening levels tomorrow.

[Liam]

I understand how it works. It's just semantics. It allows whatever frequencies it passes to move on by any resistance while the rest of whatever is left in the signal is attenuated. One part goes thru unaffected, the other is lowered. It's however not the point.

The point is that when it's done that way the bass is still being reproduced the same as before,

No, it's electronics rather than semantics. The bass is NOT being reproduced in the same way as before. It's attenuated at about 3 dB per octave, and where that starts depends on the cap and resistor you use. (Have you guessed how much I love the high/low pass filter?)

If you've got a problem at 30 or 50 Hz then something must be letting a reasonable portion of those frequencies through. I must say I liked the NFB theory posted earlier, like a cap blocking the bottom end of the NFB from the PI.

I sense your frustration, and would love to hear where the problem turns out to be.

[daz]

What i mean by "it's semantics" is that the net result sonic ally is the same. When i did those tweaks the result sounded exact like i simply added a lot of highs but the low end did not change, at least to to human hearing. And the point being it just didn't work. however, i now know the reason it didn't....it was ALREADY done to death ! See, i was told while asking questions on tweaking the amp that i should reduce the gain pots from 1 meg. So i had 250k and 500k pots for the 2 gains. Acting themselves as filters, it gave me that low end apparently because i reverted back to 1 meg the other day and this morning had ny first chance to hear it turned up. the low end is gone ! However, now i have to go back to some other values too. Ones i had changed to fight the lows. But this should be easy because i'll only have to season it to taste instead of "fixing" an issue.

I'll tell you one thing....i have learned more about theory tweaking this thing than i have in all the fiddling i have done in all my proceeding years of playing with electronics. Good fun too.

[Bruce / Mission Amps]

... See, i was told while asking questions on tweaking the amp that i should reduce the gain pots from 1 meg. So i had 250k and 500k pots for the 2 gains. Acting themselves as filters, it gave me that low end apparently because i reverted back to 1 meg the other day and this morning had ny first chance to hear it turned up. the low end is gone ! However, now i have to go back to some other values too. Ones i had changed to fight the lows. But this should be easy because i'll only have to season it to taste instead of "fixing" an issue...
.

Filters? Well... I understand your use of generic terms (semanitcs and all that rot) but I don't know about the gain/volume pots being tuned filters Daz.
Yes, the coupling caps from the plates, or where ever, need to be larger when driving a lower impedance load.
Regardless, is this the same basic layout we discussed a little while back?
The only pot that I can remember telling you to reduce to 250K or so was the gain pot at the very first triode stage. The others were 1M audio taper pots.
That 100K to 250K value was not to reduce or add bass/treble, but instead just to make sure there wasn't so much gain, that by the time the signal go to your tone controls, three stages later, they were not trying to handle fully clipped and distorted signal... (at an early volume control setting).... so much that the EQ portion of the amp could become "relatively" less effectual.
If I remember correctly, this was an EL84 amp. A pair of EL84s don't require as much gain or P-P signal on the grids to be driven into full blown power tube distortion as say, 6V6s, 6L6 or EL34. etc.
Also, without having to go back and read every thread, can you tell us what speaker(s) and cabinet type this is being tested with?
That can positively or negatively impact the tuning of a fresh chassis build too.
Sorry I have to run and can not add much right now but give us a little cabinet speaker info too.

[Alex/Tubewonder]

Are you sure the amp is guilty and not speaker cab?

[daz]

Bruce,

the speaker, cab, and tubes are all moot points because i have used 3 or 4 different speakers, 2 different cabs, and many tubes. The cab i'm using to tweak the amp is a known good sounding one that i have used with many amps over the years. It's always sounded great and sounded a lot better than the others i tried with this amp. It's loaded with a 12L thats been my main speaker for about 20 years. I also tried a couple celestions, a all-tone, jensen, and fender blue. This is why i know it;s not the speaker. another reason i know is that i have had this amp sounding so good i didn't want it to change even slightly except for one thing......that huge low end that is impossible for the iron to handle in many situations, (the more gain or vol of course, the worse it gets) even with the bass pot on zero. But the point is, the speaker and cab and tubes are fine and the amp has sounded incredible.

Then i F'ed it up ! I have no idea how. But friday i finished building a new board to replace the old one that was not a horrible mess from all the tweaking. it was sounding like sheer perfection except the bass when i took it home . the notion the pots act as filters was something someone here told me. possibly in this thread. so i though maybe thats the answer since i have literally tried everything you can imagine. So i put the 1 meg pots back in. I also had all new pots i ordered with the board parts because the pots i has in it were all old ones i had laying around when i built the amp. So i put those in too and even removed the 4 shielded cables that were very cheap cable and relaced them with very high quality teflon.

long story short, i brought it to work today and cranked it up and the bass was no longer an issue, but the tone was super bright and hard and that squishy complex tone was gone. So one by one i replaced the pots with the ones i had before so that all the values were the same. (excet a couple where i left the new pots but of the same value as they were before) And i had removed some parts i had done in fighting the bass issue since it seemed the issue was now gone. So i put all those back too. so now it is the same as when i left work friday but it doesn't sound the same ! i don't get it. The only thing that remains different is that teflon cable and a couple pots that are different ones but same value, but i can't imagine thats causing it. In any case, my brain is spinning and i'm getting real tired of this. I have nailed that squishy complex tone several times now, but i always end up losing it somehow trying to resolve some issue like this bass issue. so frustrating.

[Guitarist]

Hey, why not try adding 2 parts: a cap and resistor in series to make a conjunctive filter. Try a .022 600V plus a 10K at 10watt. Unless you've got a Mercury Magnetics OT it will tone out those obnoxious highs. Good enough for Dr. Z good enough for me. Really easy to do. Somehow, think that you'll like it.

What other specific features are unique to your amps?

I probably have a couple of original designs that are unique to my amps, and they were all evolved from a circuit that's called a conjunctive filter. It's a filter that goes across the primary side of the output transformer. The Carmen Ghia has a very traditional conjunctive filter, or corrective filter, as it's described in the RCA Receiver's Handbook. It affects the primary impedance of the transformer and allows frequencies to be very flat, or balanced. From say, 100Hz to 3K, the amplitude is the same. So from the high E to low E strings if your pick attack is the same, you'll get the same volume from the notes. The volume of individual notes isn't frequency dependent. So going back to what we were saying earlier about the touch dynamic of our amps, all that is related to the conjunctive filters that we use, and you're really in control because of them.

[daz]

I'd try if you could describe it to me. Are you saying put the cap and R together in parallel and series them with the wire that goes from the primary to the PSU?

[Bruce / Mission Amps]

I'd try if you could describe it to me. Are you saying put the cap and R together in parallel and series them with the wire that goes from the primary to the PSU?

No, across the two OT primary leads but with the resistor and capacitor in series.
Like if it was a 6V6, EL34 or 6L6 push pull PA with only two power tubes,
from lugs 3 to 3 of each power tube socket.
I've seen this in a few hi-fi project amps and old public address sized audio amps.
Not sure how valuable the conjunctive filter will be for you because I suspect this won't be the cure for your amp.
If it is, it would be a brilliant stroke though.

[daz]

Thanks. Maybe it won't, but it's worth a shot if i can find a 10k 10w around here. (or a 5w for testing purpose at least)

This does however sound like a similar thing to the cut control that i am using and helps a lot. But thats more of a band aid on the real issue, and i think it's always best not to as engineers say, "fix in the mix". In other words, fix the problem, don't try and mask it. That always works best if it's possible.

[daz]

Wow, that worked ! I'm not saying i'm done and the amp is perfect, but it went a long ways in tightening the bass and removing the ear splitting high mids. I may just remove the cut control altogether and start tuning the amp with this filter in place. I'll have to get a 10k 10w tho because i used a 3 watt R for testing. (all i had on hand)

[daz]

I've been pounding on it for the last 1/2 hour and theres no doubt this trick worked great. I still need to fine tune the amp, but it's close. the cut control did a similar thing, but it also kinda put a blanket over the tone. Great, but not a perfect solution, plus the volume of the amp went way down. With this filter it's loud !

What i don't get is this.....i'm certainly no amp tech. And it makes me wonder why Dr z who apparently IS one used this because the only way it would be good is like with my amp....if it has a horrid amount of high mids and huge low flabby bass. If used with a other wise normal sounding amp i suspect this filter would deaden it. So it makes me wonder if the good doctor had similar problems as I and used this to fix it. Since it's supposedly not used in any other guitar amps (right from his own mouth) i would think it's only needed as a fix for a otherwise bad sounding amp. just seems odd he'd need this trick if he's a good designer/tech.

Well, whatever, i don't care. This may be a bandaid as i mentioned in a earlier post, but it seems to leave no side effects so i'll keep it. The amp is still a bit bright and the bass isn't perfect, (tho nothing like before) but i think some tuning will take care of all that.

[Bruce / Mission Amps]

I've been pounding on it for the last 1/2 hour and theres no doubt this trick worked great. I still need to fine tune the amp, but it's close. the cut control did a similar thing, but it also kinda put a blanket over the tone. Great, but not a perfect solution, plus the volume of the amp went way down. With this filter it's loud !

What i don't get is this.....i'm certainly no amp tech. And it makes me wonder why Dr z who apparently IS one used this because the only way it would be good is like with my amp....if it has a horrid amount of high mids and huge low flabby bass. If used with a other wise normal sounding amp i suspect this filter would deaden it. So it makes me wonder if the good doctor had similar problems as I and used this to fix it. Since it's supposedly not used in any other guitar amps (right from his own mouth) i would think it's only needed as a fix for a otherwise bad sounding amp. just seems odd he'd need this trick if he's a good designer/tech.

Well, whatever, i don't care. This may be a bandaid as i mentioned in a earlier post, but it seems to leave no side effects so i'll keep it. The amp is still a bit bright and the bass isn't perfect, (tho nothing like before) but i think some tuning will take care of all that.

It is mostly about swamping the Q of the OT.
It can make a so-so OT act like a much better one.
You have to keep in mind that the primary impedance is actually an impedance ratio, a reflection of the secondary.
Yours... somewhere around 1000:1.

If your speaker's impedance (8 or 16 ohms, or whatever)... is only that impedance for a certain part of it's frequency bandwidth, then all the other frequencies will show the OT a different impedance and subsequently your power tubes will see a vast array of different impedances too.
In other words, the 8 or 16 ohms is only for a small portion of the bandwidth (the zed of most guitar speakers goes way up in bass freqs and high mid treble freqs).
This variation would cause your El84s to see all kinds of mismatch as some frequencies and the conjunctive filter has a tendency to smooth those mismatches out quite a bit.
Typically you would use 1.3 times the OT/power tubes normal impedance and around .022uF to .047uF at 630v to 1000v.
It does use up power, as you'll find out if your resistor is too small of a wattage.
I don't think Dr.Z says he invented this and most of us who have been around for a while have seen it used many times.
I think the last time I saw one of these in an old production amp was when I refurbished a 4x KT88 Orange PA amp head where it was also used as a high power bass head.
It had two sets of RC.
A 15 watter with cap from each outside leg back to the B+ feeding center tap.
That amp made 180 watts all day long.
This would had to have been way before Dr. Z was using them as the Orange amp was an older, vintage 70's model

[daz]

Both trannies are heyboer. Is it possible these are not as good as people say? I was told elsewhere it could also me the power amp's "operating point". Are they saying the plate voltage? If so, which way would typically sweeten things up, and would i want to change the 1st dropping R at the 32/32 cap can to alter it? (its at 355 v now)

[kg]

Both trannies are heyboer. Is it possible these are not as good as people say?

every amp is different.

if you want the best tone you have to hand-tune it.

and even then you'll probably end up retuning it a little while later.

enjoy the journey!