power tube screen/plate bias vs master volume performance?

[ALBATROS1234]

my last build has been a bit of a a pain, since i tweaked my screen voltage much lower than my plate voltage on my power tube the volume started not giving much volume increase from 0/off to 12noon ...after i pass up the noon position it gets very loud quickly about 3/4 up it starts to break up slightly and only when  both volume/gain knobs are cranked do i get a decent overdrive and tone. one volume/gain is after the first triode stage and one after(master) i have no tone control at present i have been tweaking this build for 3 weeks + .it usually doesnt take so long. it started as a valco/supro spectator clone but it was quite anemic sounding due to many variables that make it not similar enuff to what i wanted so component swaps began. i made breakthroughs with the tone when i dropped the screen voltage to about half the plate with a fat resistor. i noticed the volume control change right away but since it was sounding much better i kept with it now i wonder why is it doing this? should i change to a 500k pot instead of 1meg? i kinda assumed that there was some impedance shift when i dropped the voltage which caused the weirdness. as it stands i got it at around 13watts plate dissipation which is ideal for the tube but i cant crank 13 watts everytime i want to play the thing.and 400volts is too much for me old 6v6s to deal with it has to be the 6L6 or 5881. its like the only way the thing sounds great is totally cranked though.
i did experiment with a lower voltaged pt kinda crappy spare one i have which sounded basically the same albeit punchier because i used a bridge rectifier it still does the same thing with the volume control described above.it was better in one way as at that low voltage i was only making about 4 watts so cranked wasnt horrible in the living room. i just dont get why this is my 5th amp build and the other ones had a few tweaks but worked out well rather quickly. is it the 6x5? is there a 6 volt rectifier that would supply more current .do some builds just never work out right?

questions....do i just scrap this thing pull the parts and build something else? do i pull the transformer and put a lower voltage xfrmr with a bridge rectifier and just deal with an amp that only has one sound which is cranked to the max?. do i continue to twist this things arm until it submits and gives me something i am happy with? and whats with the volume weirdness do i change the pot value or try to tweak the screen higher which had me running crazy high plate dissipations. this build is starting to become irritating.

[jjasilli]

my last build has been a bit of a a pain, since i tweaked my screen voltage much lower than my plate voltage on my power tube. . . i made breakthroughs with the tone when i dropped the screen voltage to about half the plate with a fat resistor.


That's not a "tweak"; that's wreaking havoc.  See Hotblue's comments:  http://el34world.com/Forum/index.php?topic=12281.0\
It's not OK to drop screen voltage 50% with a resistor.  You don't state if this is at idle or under signal conditions.

The points made, such as plate voltage, screen voltage, fat resistors, plate diss are presented in stream of consciousness fashion.  Great practice for a James Joyce novel, but does not help us help you.

If you are seeking technical help: Post a schematic with voltages stated and components ID'd.

[shooter]

do i just

I built 14 amps like you just did, I considered it my 2yr AS degree in amp technology 

The 5c1 - e1 amps only have 1 settling, dimed, into a 4X 12 >90db cab and every player can "remember when......."
You cannot take it apart til you understand both the DC and AC operation, record the finding, archive it under, 1    Then, take that knowledge, spare parts, new parts, and start amp 2

EDIT:
Here's a screen shot, each folder has a schematic, note's, who I sto, borrowed from, tranny types, etc. if there's subfolders, they have scope shots, maybe freq plots etc, fun geek stuff
take the time now, you will save much later

[ALBATROS1234]

I wasn't asking for specific help I was trying to get general held opinions on possible ways foward. After I posted this I thought maybe I should have tacked this on the thread from last week so the schematics would be there at least initial version because i have swapped out this and that many times in the attempt to get this thing to sound right in my opinion. I hesitated to do so cuz I feared that people who had checked the thread before might not Check it again hence the title of this posting.

That said I have read conflicting things about the screen voltage value in relation to plate value . All I know is when the screen was higher ,equal and slightly under my plate dissipation was far too high for a 6v6 and 6L6 power tube and when I lowered it this changes to an acceptable level. There also seems to be other variables which cause the volume issue because I went back a couple steps after posting last night and without changing screen&plate value I got a partial improvement in both headroom and  1st half of volume controls effect. Below is my initial schematic but I will have to pick through tonight to give an updated version tonight.

[ALBATROS1234]

Also I wired up a 9 pin socket and tryed a 12ax7 which gives it a nice flavor. I as well tryed to add a gain stage last week after the 6sl7 and before the master volume I added 1/2 of a 12ax7 and it was a disaster .a screeching nightmare. I can't seem to get a 3 stage preamp working at all as I tryed before on other builds. Even using similar values to existing schematics. There's just so many variables it's hard to balance my own semi original designs.

[shooter]

for guitar builds, unless your B+ gets close to the 500vdc range, a simple current limiter on G2 seems to be "the standard". 
when you experiment with "how big", monitor your tubes current, and sound, you will notice a point where things just avalanche down, loss of power, anemic sound etc. G2 also controls somewhat, amount of drive, which can sound ugly if you already have a hot signal in.

[jjasilli]

Albatross:  The way forward is to go backward.  Our beloved tube amps are 1940's technology, perfected in the 1950's.  There is no point in reinventing the wheel by building any circuit which pops into your head.  You have gone off on, what we call in the law, "a frolic of your own".  You are wasting time, money & effort in needless research & development which has all been done before.  That which works has all been published. 


Moreover, the subject of reduced screen voltage is a recurrent topic on this Forum.  As sluckey just mentioned, a large series resistor is a bad way to significantly drop screen voltage, because it also limits current which is needed for proper tube operation.  The best options are:  UL; separate PT tap; zener voltage reduction in the screen supply; voltage divider in the screen supply (the last being a poor choice also, due to current loss and poor voltage regulation).


At the risk of drawing hate mail, low screen voltage is more of a hi-fi, Public Address amp, or bass guitarist thing.  Paying fair homage to UL lovers (you know who you are), most electric guitarists prefer the tone of the "standard" power tube circuit which sluckey describes.

[66Strat]

I have reviewed your schematic and added some suggestion notes that may help to remedy some of the problems that you are having.  I have also added a 6V6 datasheet for reference.

Note 1
The maximum grid resistance for a 6V6 in cathode bias operation is 500K ohms. You might consider swapping this 1 Meg volume control pot with the 500K ohm tone pot.

Note 2
The tone control as drawn does not provide the grid a reference to ground. The triode will not function properly if implemented as drawn. See the suggestion for implementing the tone control.

Note 3
The 220K resistor is not needed, unless you are using it to drop plate voltage supply to the 6SL7 plates. If you are trying to drop plate supply voltage, capacitor to ground from the juncture of the two 300K ohm resistors and the 220K ohm resistor should be added to decouple the two gain stages.

Regarding screen voltage on the 6V6, unless you are operating the 6V6 at very high plate voltages, severely reduced screen voltages are not needed. High plate voltages (above 350) in a single ended 6V6 prevent the tube from being operated in center biased Class A operation. I have added suggested voltages that will allow the amp to function properly.

[ALBATROS1234]

Thanks guys for the helpful replies. I an not trying to reinvent anything it's just done in the spirit of experimentation and learning. This started as a valco 510 /spectator clone but when it was built it sounded weak and muddy nothing like the amp I was trying to make. So I threw a master volume in because that had worked in other builds for getting a nice classic rock grit tone due to being able to crank preamp drive .a big problem on this build has been controlling the voltage. Right now it sits at plate 400ish and screen 200ish. The 220k was put inline to drop the voltage to the preamp tube as it was sitting at 220 to 240vdc originally too high in my limited experience  I have had good success with my 6sl7 voltages between 70 to 140vdc so I was trying to get close to that. Perhaps I need to redesign the power supply as following the original schematic was inadequate in multiple ways. Sluckey helped by suggesting an extra cap and revised input resistor which killed the hum issue I had originally. Maybe I should back off of this one and build my next project? I want to perfect a circuit which suits me and my style hence the experimentation.  Also hands on component swaps show direct results if action.  The problem seems to be as I tweak to fix the tone of this thing it gets closet and farther away from perfection at the same time.then I get to the point like last night when I am like this is just not sounding good. Then I undo a few things and it gets better then I change this and that and as it seems to get better it's actually getting worse.

[jjasilli]

Everyone has the experience of thinking their sound system or guitar rig sounds fabulous one day, only to feel that it sounds substandard the next day. 


There are numerous amp designs in existence.  As amp builders its our job to know how they sound.  E.g.'s:  Fender clean; Tweed overdrive; Vox; early Marshal; Hi Gain; old bargain basement amps for slide guitar; etc. 


You are not presenting a clear concept.  It sounds like you're concocting circuits, tossing them together and expecting some vague but wonderful result. Could you imagine if someone made dinner like that!?!


Decide on the tone you want and build the amp that produces it.  Then tweak it to your satisfaction.  N.B.:  The definition of a tweak is a small adjustment made to something in order to improve it.  This does not include starving the screen or raising the value of a power tube grid leak resistor by a factor of 5.

[dennyg]

My 2 cents is that the power amp should be designed for maximum headroom/output and focus your tonal preferences on the pre-amp.  If you can come to terms with that, using a small pentode in later stages (or IMHO, the last stage is best) of a pre-amp design can provide tons of flexibility with tone based on  screen voltages.  I used Tubenit's TOS as a baseline then tweaked the operating points to yield one of the most tonally unique (in a good way) amps I've ever built. 

The problem with reducing Vg2 in a PA is that you basically are reducing headroom and output with little tonal variation/impact. 

[Ritchie200]

At the risk of drawing hate mail, low screen voltage is more of a hi-fi, Public Address amp, or bass guitarist thing.  Paying fair homage to UL lovers (you know who you are), most electric guitarists prefer the tone of the "standard" power tube circuit which sluckey describes.

You don’t know me!  You don’t know my life!!!!  😎  Nice dance counselor! You sound like a professional. I be watchin’ you....👀

To the original poster: To expound a little further on the opinions...  These tubes like a certain set of rules to allow them to play well with other tubes and associated parts.  These rules of operation are tried and true and have been wrung out for decades.  You saw Shooter’s list of wandering around the pasture - similar to what you are doing now. It’s very hard to take something that violates the very rules of operation and “fix” what’s wrong as they greatly intertwine. That’s why the suggestion was made to build something proven, get it working, and THEN start making changes.  This way you know the cause and effect of the change compared to the baseline. Nobody here is saying that you shouldn’t experiment.  Just try your ideas on something that already works.

Jim

[jjasilli]

I can't believe i actually agree with this guy 

[Ritchie200]

I want to perfect a circuit which suits me and my style hence the experimentation. 

I hope you have plenty of spare time in the next 20+ years!!!!  That’s why all us addicts gather here away from our families and friends to share our secret old fashioned  speak, our frustrations, our victories, bash odd shaped guitars (teles), all so we can say “Yup, I built that!”  It makes all the “Mom! Dad’s on Hoffman AGAIN!” complaints worthwhile. 😎

Jim

[ALBATROS1234]

Thanks guys I appreciate the guidance as I am new to tube building. Played tube amps my whole life and done electronics and electrician work most of my life. That's kinda what I did though guys I built a valco supro clone and it sounded shite so I started moving stuff around.when I had the 23 watt plate dissipation due to plate and screen being at 400 plus I started trying things to remedy that. I read a lot of stuff on forums and gear blogs so I try them to see what happens. When it seems progress has halted I post here for advice because I am a huge novice with tube albeit I absorb things quickly have no kids or wife to deal with so I have hobby like this where I come home every day and read and experiment.  I will be able to grab the why's and why note quicker if I have done this or that and had success or failure. It's all in fun and the pursuit of knowledge all of us are that level of nerd if we enjoy plating around with high voltage to make our guitars have a different texture palette of tones right. Besides amps and playing guitar I built ww1 scale aircraft models as well as tanks and armored vehicles from ww2 and I detail the cap out of them. It's all fun to me

[ALBATROS1234]

As far as what I want is an amp which has a ton a tonal character and texture is thick as molasses. Creamy as Belgian butter but yet bites like a rabid dog. I like and have owned and used old marshals ampeg peavey 70s tube stuff as well as newer tweed classic. Love 50s and 60s valco grit. Like old fenders somewhat but not cranked as they seem floppy in the bottom end but I have mostly experienced Silverdale quad and twins as early years stuff just remember them farting out if you got enough bite.

I at this moment don't need anything super loud I don't want to have to crank a 30watter to have a good tone. I suppose single ended is how I should proceed and I want to experiment with some of these pentodes in preamp also a 45 power triode I got from a 20s.radio. part of the reason I started playing with tubes is I can't think of an existing amp that is perfect for me. I hear demos and from what I play and played thru some sound very good but there's always been something I wish was slightly different in performance controls features and tones so maybe I can get closer to my personal tonal nirvana.i am not in a band atm  but have played and written songs since I picked up a guitar 35 years ago.

[sluckey]

Have you tried a modeling amp? I bet Guitar Center has exactly what you want.   

[ALBATROS1234]

i had a modelling effects processor before katrina which i ran through an 1971 ampeg v4 200watt tube bass amp w/ a marshal 4x12 it sounded ok because it had a tube power amp warmth with all the different modeling patches it was a decent gigging rig but nah i want a little tube machine.

 i tried to add a third triode stage but it sounded shite blocking mess. i have a bunch of used and nos tubes i think next i will go back to my original 1st tube amp build concept but instead of a 6SJ7 into a 6SL7 SE into a 6F6 which was a horrid squealing blocking noise factory i may try a 12SQ7 which as i am sure you know is 1/2 of a 12ax7 with a couple of diodes i will just ground into the 6SJ7 SE not sure which power tube maybe my nos EL84. or perhaps try a supro model 24 as i lucked out on evilbay an got a pair of used but strong testing original 6973s for $12.50 plus shipping. only problem with this is i dont have any voltage refs on the model 24 so i will have to figure out what to do about the pt.i have another pt which is around 360-0-360 with a 5 and 6 volt taps which is humongous in size. one big problem with this last build is it was a struggle to get the voltages where i wanted them as the pt is like 365-0-365 so rectified it was higher than i wanted.

as for this last build i did some more playing around last night and i got it sounding ok except the fact that in order to get nice grindage i have to blast 13watts in my face. i guess its just gonna be a cleanish amp unless its blasting.

[sluckey]

or perhaps try a supro model 24 as i lucked out on evilbay an got a pair of used but strong testing original 6973s for $12.50 plus shipping. only problem with this is i dont have any voltage refs on the model 24

The original S-6424 schematic has voltages listed. So does my S-6424. You can find the original schematic right here in Hoffman's schematic library, or you can find both on my website.

[ALBATROS1234]

thanks sluckey ,i was just looking at your site and that schematic. i like the one input channel switch capability much cooler than physically jumping channels. of course i started thinking and what i would like to try is to omit the tremolo because i dont really care for them and make it a one channel amp but sound as if it were the 2 channels jumped. would i just wire it as the v1 a/b were parallel as in split the input signal and hit each grid then re- combine the plate outputs before the coupling caps? probably not that simple knowing my luck. also i was wondering why in your mv mod there are 2 500k pots instead of one "master" volume? also is the high voltage ac output 320-0-320 or 305-0-305?

[sluckey]

would i just wire it as the v1 a/b were parallel as in split the input signal and hit each grid then re- combine the plate outputs before the coupling caps?

You could do that. Or you could simply connect both plates together, both grids together, and both cathodes together. I would probably just omit the channel switch and connect both grids together, with no other changes. Then you have two volume and two tone controls for a greater variety of sounds.

i was wondering why in your mv mod there are 2 500k pots instead of one "master" volume?

That's a dual ganged pot. Two pots operated with a single knob. Notice the dashed line between the two pot wipers? That indicates a mechanical connection that turns both pots together.

also is the high voltage ac output 320-0-320 or 305-0-305?

My amp has a Hammond AO-24157-1 PT from an AO-43 amp, and has 320-0-320 voltage. But I also listed the Classictone 40-18062, 305-0-305V PT because it's advertised as a replacement for the S-6424 Supro and other similar Valco amps.

[brewdude]

Have you considered trying a VVR?


I have had great satisfaction using a VVR on simple low wattage (10-20 Watts). 

[ALBATROS1234]

vvr? variable voltage resistor? is that a type of attenuator? i know of their existance but no i have not until now thought of using something like this. is this external or incorporated into the build. i know thaere are amps that have a switch that cuts wattage usually from my limited experience it seemed to suck the tone out of the amps i tryed them on but if there is a component or device that i can put on my amp so i can crank it for tone then drop the volume i am interested thanks.

[brewdude]

VVR is an internal addition to the circuit that will lower the B+ voltage and reduce volume.  It can be implemented to scale the entire amp or just the PA.  I feel it sounds better than a speaker attenuator. 

[ALBATROS1234]

thanks sir, i need to check that out, i really have no need to be insanely loud atm (besides it being fun as hell) but i got neighbors.

[HotBluePlates]

... it started as a valco/supro spectator clone but it was quite anemic sounding ...

... This started as a valco 510 /spectator clone but when it was built it sounded weak and muddy nothing like the amp I was trying to make. ...

You may be interested to know I've got an original Supro Spectator (early 50's?).  At anything less than full-up the amp is perfect for copping the song "Sleepwalk" using a Tele's bridge pickup.

I don't think I've measured voltages to compare them to a schematic, but with the small cab & small speaker, it's pretty much all midrange.

[ALBATROS1234]

Thanks I tryed my 5x7 speaker from a 1951 revere r2r and it gives that type of old timey mid rich tone .I guess it's how they made them back then. I just got a 1928 rca electrodynamics it's either a 10 or 12 inch that I want to second cuz it's shredded. I don't know the exact model etc yet though.