schematic and question

[daz]

This schematic (hastily edited, sorry) represents my new build with several changes, some temporary. It's sounding fantastic at low bedroom levels or just above. but once i get it a bit loud, much too lous for an apartment say, the tone gets much harder and harsher. Before you tell me thats normal, let me say i agree. however, i have had many amps that do this but not as much. in other words, most amps don't change this much. they change some in the same regard, but onstage in a mix the harsher highs are diluted and they tend to then sound pertty much like they do at that lower volume. And i probably should be mentioning "feel" more than tone, because aside from the extra and harsher high end as it gets louder, it's even more so a feel thing. The strings feel like rubber bands at low volume with a ton of saggy feel and dynamics, while at higher levels they start to feel stiff and no sag to speak of and dynamics become lessened. I'm talking preamp drive here, not output grind by the way. Plate current is at 35 according to the weber calculator.

So while i realize this is to some extent normal, i feel it's exaggerated comapred to most amps, and i'm asking what you might suggest that can counter this issue to any degree. I would imagine the PI would be the likely place to look? anyways, fire away. i'm all ears.

http://img356.imageshack.us/my.php?image=ampzy6.jpg

[Paul P]

Why don't you compare the amps you do like with the one you've built and
see where the differences are in the circuit ? You seem to have a good idea
of what you're after and apparently you've played commercial amps, maybe
not all in one amp but at least spread across several amps, that gave you
what you're seeking. The difference can only lie in the circuits.

Perhaps if you told the experts around here which amps you like they could
say what you should change (you may already have done this ?).

Anyway, I'll get my things in order so I can follow you in your new quest.
I'm bound to learn a bunch of stuff along the way .

Paul P

[daz]

I DO do that. i use the amps i liked a lot as models to a degree. But i don't want them ! i want IMPROVED versions of them, otherwise i'd just buy a production amp. By i've always felt i could build an amp thats better because i knew some of the things that could be changed in those amps i liked to make them even better. And so far they ARE better in most ways. The thing i'm really after is harmonic complexity which i find can be had with home built amps to a degree thats better than most production amps in the style i prefer....master volume Marshalls. in simple terms, the best Marshalls i've had were great but not as harmonically complex as the 2 amps i've built at any volume. My only problem is at higher volumes as i described, the complexity and feel go away to a degree i don't feel it should. My fave amps were all marshall 50's, and i always felt a cathode biased version with a tube rectifier (undecided on NFB) at a lower plate voltage would give me a more harmonically complex marshall sound that wouldn't have to be cranked too high to get the feedback loop happening. It worked too ! I just need to get more of that squishy transparent tone i get at low volumes at stage levels. if i can do that i'll have built an amp very close to what i've always wanted.

[Tubis]

1.Feedback in

2 Lower the value of the the first plate resistor in the PI. How the PI works makes a lot of good shit.

3.What plate voltage do you have at 35mA?
Try setting it to 70% of max plate dissipation. Max 25Watts. That will make 17.5Watts for an el34. If thats what you have now your plate voltage would be 500V?
The lower the voltage, the higher the current. The effects law here. Voltage multiplied by current.

4.Old tubes tend to sound like that. And old as in used a lot, not old in time.

5. Different brand of tubes. Man does those brands ofEl34 sound different.

[Wakculloch]

The 68k ground reference will be loading the anode of V1b at AC.
Have found this can lead to wierdness, and would increase by X 10.

[daz]

Tubis....i'll try a, what, 82k, 68k? (on 1 pi plate) As to the bias, i had it much higher. Closer to 70%, but it sounds much better like this. Someone told me to lower it and it worked. The tubes....tried 2 sets, same thing, except the one i'm using sounds better. they're an old but good set of tesls i kept from my last el34 amp because they sound great.

Wakculloch....thats not is. I did that because i had a 1 meg pot there and the ground ref R sounds worlds better for some reason. And actually, i have a pot there working as a variable R by puttimng both the input wire and output wire to center lug and one lug to ground. It sound best with the pot set as low as possible before it cuts the siound out abrubtly.

[MWJB]

Why no cathode bypass caps on the EL34's?

82K at first PI triode is well tried & tested, maybe go a little larger at the PI tail - 27-33K as you have no global NFB.

The schematic doesn't show specific voltages, we can only learn so much from the schem alone, please list dc voltages for all tubes.

[Tubis]

Well if i´m not misstaken a cold bias and no feedback opens up for crossover distortion. That could be what you experience.

[daz]

Cold bias because of the values MWJB mentioned? I will change that. As far as the EL34 lack of caps, i hated that sound. It sound worlds better w/o them. I tried 100uf all the way up to 2200 uf trying several in between. But w/o any it sounds much better. With it sounds closer to what i asked about getting rid of in this thread...hard harsh and stiff.

[Tubis]

Cold bias as the current thru the el34 in regards to the plate voltage. By the way, how high is the plate voltage on the el34?

[daz]

Someone elsewhere told me i should run them real cold at 31 MA. I was at 41 at the time which is what is average for CB according to weber's page. So i took this guys advice anyways and started adding resistance till i got to almost 31. But i found it sounded best where it is now. It's at 36 according to weber's calculator and the plates are 373 as measured between plate and cathode, or 400 measured plate to ground. I had 270 ohm R's at the cathode and the design followed the matchless chieftain to a T except for the trannies. It's sounds best where it is in any case, so thats not it.

[Tubis]

Someone elsewhere told me i should run them real cold at 31 MA. I was at 41 at the time which is what is average for CB according to weber's page. So i took this guys advice anyways and started adding resistance till i got to almost 31. But i found it sounded best where it is now. It's at 36 according to weber's calculator and the plates are 373 as measured between plate and cathode, or 400 measured plate to ground. I had 270 ohm R's at the cathode and the design followed the matchless chieftain to a T except for the trannies. It's sounds best where it is in any case, so thats not it.

Well its not realy cold then, its in fact a little bit hotter then i usualy setts it to

[Bruce / Mission Amps]

Well its not realy cold then, its in fact a little bit hotter then i usualy setts it to

I agree.
IMHO, it is amazing to me how much internet drivel has been created over the years, with respect to what is "bias", bias voltage, cold bias, hot bias, or high-low idle current. etc., etc., etc.
Or how little many, prolific, Net schooled tweaker/builders really understand bias point or what it all means....
I guess the prevailing accepted motto is "as long as the bias setting it isn't red plating the power tubes, it's OK" .... ah, no.
I mean I am not into Hi-Fi PA tone but I've come to the conclusion that most casual to weekend warrior players don't really understand what is best way to get the greatest sound out of their own gear, and end up latching on to some of the goofiest notions... Oh well...

[Bruce / Mission Amps]

Someone elsewhere told me i should run them real cold at 31 MA. I was at 41 at the time which is what is average for CB according to weber's page. So i took this guys advice anyways and started adding resistance till i got to almost 31. But i found it sounded best where it is now. It's at 36 according to weber's calculator and the plates are 373 as measured between plate and cathode, or 400 measured plate to ground. I had 270 ohm R's at the cathode and the design followed the matchless chieftain to a T except for the trannies. It's sounds best where it is in any case, so thats not it.

My opinion is if you have around 370vdc on the plates, with respect to the cathodes, in this amp with cathode biasing... the best overall vintage PA tone will be with the idle current at around 38ma to 40ma.
And I do not really understand why you don't want the power tube's cathode resistor to be bypassed... it will allow the cathode biasing to be more stable and give you a better "fixed bias" feel to the amp...
If it sounds too gainy, mushy or something, then it is probably in the previous stages you need to be looking.
If you are looking for a Hi-Fi, Class A sound, without cathode bypassing, then you'll need to rebuild the amp for that configuration.

[daz]

If it sounds too gainy, mushy or something, then it is probably in the previous stages you need to be looking

The opposite actually. With caps it sound hard and stiff. remove them and it becomes softer and more elastic, which is what it desperately needs.

the best overall vintage PA tone will be with the idle current at around 38ma to 40ma.

And i am only going by webers calculator, but it was at 41MA and once i added some more resistance a bit at a time, when it got to 36 it sounded best. I added more thinking it would get better but at 36 is where it sounded best. I'll try your suggestion, but at the moment i have it SS rectified. Would 38 to 40 still be a good number or with the extra 23V i got from this mean a different optimum current should be aimed for?Also, how about with those old tesla EL34LS they made till the early 90's? I heard back then they need to be biased differently than other 34's, tho i don't remember in what direction.