i built a 5f1 champ clone, used a fender transformer and an eyelet board and it worked upon first fire up... there's a slight problem... when the volume is set low it sounds good, when it's turned all the way up i get the nice distortion tone.... if i put the volume pot towards the middle i get a very blatty distortion out of my amp... it makes a sputtery mis-biased type of sound but i have no idea why... any clues?
Well, as you roll off the volume the ration of highs to lows changes..more lows in relation to highs. Low freqs are going to sound ugly when distorted. Did you use a bypass cap on the preamp stages? If so, try lifting that (the original champ did not have them). If that fixes it, try using a smaller value bypass cap (or none). (1 or mF instead of 20 mFd)
maybe a dirty or spoiled (2nd hand/desoldered) pot?
well it's not a bad part, i bought everything new... i used the original champ layout, i didn't do any mods or anything that's why i'm a little flustered.... it doesn't have the bypass cap in the preamp stage... any more clues?
interstage coupling caps are hi-volt types?
(don`t know the schem... :wink: :wink: )
it's basically this design withoug the first 25-50 cap in the preamp section... http://www.tubegarden.com/champ/schematic.JPG
temporarily solder a 1M resistor from grid of 2nd triode to ground,
and rotate the pot again, listening if the sound at the middle setting has changed (improved).
before you do this: does the amp sound equally bad at medium (acoustical) output-volume, when you turn the amp all the way up,
and control the "loudness" with your guitar-knob?
If it does, you need not do the gridresistor thing...
tried the guitar volume and it didn't improve anything untill it was very very quiet... i tried the grid resistor too with no imporvement... i've switched out tubes, tripple check the parts values, tried it through different speakers... the only other thing i can think of that seems kindof odd is that the large caps don't really store anything after i shut it down... they seem to discharge them selves somehow but i don't know how... i dont know if that could be a problem or if it's just something they do...
seems I have misinterpreted the description of the distortion...
maybe s.o. else got better imagination?
Have you checked the tubes? Maybe you have a funky tube...
yeah... i've checked the tubes... one thing i noticed is that my transformer is putting out 420v instead of the usual 380v in the champ amps... i notice a spike in the power when i turn it on, it goes over 500v... my power caps are 475v... could the spike damage my caps so where they are still working but sound crappy and discharge themselves? are they one of those things that either work or don't? i'm at a loss...
Yes, always try a different tube. But how about setting your meter to DC and checking the grids of each tube as you turn the pot?
I wonder if somehow something is leaky?
Your filter caps are probably fine. When I first built my Firefly, I connected the center tapped PT up wrong and had 585V instead of the ~300V that it was supposed to be. My caps are rated for 500V. Everything was fine, nothing blew up. I only had my amp on for a short time, though.
It sounds to me like the starting B+ voltage is a little too high. I think that could be a large part of the problem. I'm not really familiar with the Champ, but it very well could be that an additional 40V could be too much for the circuit as designed.
Aron has a point, too. Maybe you have a coupling cap that is leaky...
My Champ has a high voltage on it too. I think it's close to 400V as well but I can't remember. That's how mine was stock - it's an old silver face.
1.)Something's screwing up your V1b grid driving voltage. That's what puretube was getting at with the 1 meg grid resistor to ground...(I think).
The signal needs to swing fully back and forth otherwise the interplay between the cathode current and the grid voltage gets goofed up and that new waveform shows up at the plate. If all else fails come straight off the coupling cap into the grid with the 1 meg resisitor from grid to ground just to see if the pot's messed up.
2.) The other part of this is the cathode. Check your NF loop setup. It could be affecting the cathode/grid interplay. You might want to just disconnect the loop and listen... maybe the voltage drop across that loop resistor is screwing with your cathode's operation. If all else fails run the loop configuration of the AA764... 3 extra parts. This loop comes up through the lower end of the 1k5 instead of the top. (double check all your parts values). This route takes the cathode and feedback paths through roughly the same IR drop point which might better sync the degeneraton. I don't know... maybe puretube will speak to this.
3.) The filter caps are supposed to discharge when turned off. That's one of the reasons for bleeder resistors across the power rail... for quicker, steadier drain down.
4.) At 420 Ep you definitely want to check your current draw across the 6V6... any more than say, 20 ma (I don't know what tube you're using), you'll want to watch carefully for redplating. Increase your bias resistor for starters if so. If no redplating then go for it...you might have a short tube life but it'll sound great while it lasts.
QuoteThe other part of this is the cathode. Check your NF loop setup.
Good one! I missed that. Yeah, something could be happening there for sure.
I put a heavy-duty variable (clamp) power resistor for the cathode. Works well.
(a) have you read the Tube Amp Debugging Page at GEO?
(b) blatty distortion comes from two sources, bias problems and oscillation.
(b1) Does your volume pot have DC on it? Check with a meter.
(b2) Does any grid have a positive voltage on it?
(b3) Are your coupling caps rated for 600V?
(b4) Are there grid stoppers on any grids?
csj: my Qs sure were about possible DC at the grid, or missing groundreturn thru a bad spot in the pot (possibly in turn burnt by leaky coupling cap - wanted to exclude this without measuring - which now seems to become neccessary. :)
Wouldn`t start messing with the feedback at this point, except disconnecting the 22k from the OT for a moment, and see what happens...
(can get loud and hummy, but maybe that "halfway distortion vanishes").
Can we assume, that all values, esp. cathode resistor of 2nd triode are correct??
A scope would say more than 1000 words esp. regarding oscillations.
Mixed up OT secondary windings will be excluded by lifting the FB-resistor
(and would have caused loud, audible oscillations anyway...).
That leaves us to R.G.`s wise words (b4), concerning inaudible (HF or RF) oscillations (where a scope would tell us immediately...):
assuming your wiring/layouting is properly done (you noted having copied the original...), there`s still a chance the tubes and its close surrounding are swinging on an unhealthy frequency at large amplitudes, which screws up the idle settings, and interfere/modulate with the audio;
to exclude this, route a 1k to 22k resistor in series with the grid of 2nd triode,
by disconnecting whatever is running there (the pot`s wiper in this case), and inserting the resistor between what`s running there (the wiper) and grid.
Solder this resistor, with the lead connecting to the grid
cut really short, directly to the tube-socket`s appropiate lug.
If it hasn`t helped yet, leave it there & do the same on the 1st triode,
if it hasn`t helped yet, leave it there & do same on the 6V6.
If it still hasn`t helped, insert a 470 ohm 1W res. (short-cut lead) directly at the screengrid lug on the 6V6 socket.
If this helped, you can remove the first "gridstoppers", (which btw. form a lowpass with the tube-inherent capacities, to roll off high frequencies...)
one by one, until distortion re-appears. Put that last removed one back in.
Go.
oh yes: of course oscillations are being supported by long wires and improper layout - if there are lengthy loose wires messing around, you can wiggle and shove them around with a long isolated nonconducting (plastic) tool, while the amp is on, and listen for (positive) changes...
hmmm... the consensus seems to be leaking dc...
well, whatever it is I hope he gets it fixed soon.
I can hardly wait for the group hug at the end...
Group hug at the end? Is that like "reach out and e-touch someone"? :lol:
My thought is that with B+ being ~40V "too high" is that it could be messing with the bias some, possibly causing the nasty distortion, or at least contributing to it. I wonder if improper grounding could have anything to do with it? If that were the case, though, I would imagine that there would also be a nasty hum. A DC leak somewhere does seem like the most likely candidate...
ok.... no dc on the pot... the grids on the preamp tube is fine... the power tube i get 0v when the volume is turned down, if i turn the vol all the way up i get a +.089 .... i don't know if that would do anything... i haven't tried the grid stopper thing, i'll have to do that when i get some more time... disconnected the 22k feedback resistor and it didn't really help at all... if there is a dc leak what would be the culprit, bad parts or bad builder...? :wink:
What about lead dress? Have you tried seeing if any the wiring could be causing the problem? It could be a problem with oscillation. Maybe pushing some wires around could make the problem disappear. I use wood chopsticks, as they are non-conductive.
bad parts...
(assumed the values of the parts are correct,
and wired according to the schematic)
got it: (or not...)
the schem doesn`t show a connection from heater supply
to the general ground.
If you followed the original layout, it should be somewhere.
Is one of the heater-windings connected to circuit ground,
or are they both "floating"? *
If floating, hook one of the 2 filament wires to ground;
check if any hum should occur, if grounding the other
wire sounds better;
if hum is too loud, connect 2 100 Ohm/0.5W resistors in series,
and hang them between the 2 filament wires, and hook
the middle of the 2 res. to ground...
*: this concerns only the 6V3 audio-tube filaments (green wires)
of the 12AX7 & 6V6,
NOT the rectifier filaments (yellow)!!!
8)
read edit on bottom of last post!
ok... tried the grid stoppers, no help there... the only thing i didn't do is insert a 470 ohm 1W res. (short-cut lead) directly at the screengrid lug on the 6V6 socket. i'm not exactly sure what's the screengrid lug?.?... also just for fun i re-routed the heater wires to see if that was messing anything up, no help either... i've moved wires around, cut some leads shorter... we're running out of options here... :(
the screen-grid is the grid close to the plate,
which goes to the node of 10k & 8µ/450V & 22k
on the hi-volt side.
did you ground one side of the filaments yet?
like mentioned: it is not shown in the schematic,
but very usual practice...
yeah... i grounded one of the green wires... i'll try the 470ohm next and see if that helps...
oops, re-read the old posts, and seemed to have missed the HV details:
420V is very high for 6V6...
320V is more of what they like.
(Fender seems to love taking it to the limits... :roll: )
Is the cathode resistor (470ohm) of the 6V6 a hi-watt type (>1W)?
Tell us more on the voltages (with no input signal):
1st, 2nd, & 3rd hivolt electrolytic;
plate & cathode 6V6;
plate & cathode 1st & 2nd triode;
Quote from: R.G.
(b) blatty distortion comes from two sources, bias problems and oscillation.
I have to go with R.G. on this one..probably oscillation. especialy since it happens halfway on the volumepot, where the impedance to ground or signalsource is largest. try to re-rout the cable going from the centretap of the pot to the tubegrid. it is not unlikely that it is picking up the signal in phace from one of the other stages...
Johan
Oscillation could be happening here. Although, I still think the power tube might be getting pummeled from the higher voltage being supplied to it compared to what is specified on the schematic. I think doing something to lower the B+ voltage a little bit might help (bigger series resistor), and checking the power tube bias again to make sure it is right with the lowered B+ voltage.
Quote from: JohanQuote from: R.G.
(b) blatty distortion comes from two sources, bias problems and oscillation.
I have to go with R.G. on this one..probably oscillation. especialy since it happens halfway on the volumepot, where the impedance to ground or signalsource is largest. try to re-rout the cable going from the centretap of the pot to the tubegrid. it is not unlikely that it is picking up the signal in phace from one of the other stages...
Johan
this kind of oscill. wouldn`t be the one we tried to remove with gridstoppers, but have tried to exclude under "proper wiring/chopstick"...
but worth checking once more...
BTW: are the wires running to the filaments twisted tightly (mostly for hum-reduction, though...)
Next thing from my side would be wanting to hear a soundclip/seeing some pics of the layout/scopetrace of output... :lol:
ok... here's my voltages...
from tranny: red 376v yellow 426v
1st hivolt elec 426v(16uf-10k)
2nd hivolt elec 375v(8uf-pin 4 of 6v6)
3rd hivolt elec 250v (i upped the 22k resistor to 68k to get the voltages down on my preamp tubes but i still had the problem before i changed that resistor...
from the 470ohm-25uf-pin 8 6v6 is 23v...
6v6 pin 4 366v, pin 3 418v
12ax7 both at 153 volts...
i don't have a scope so i can't do any of that stuff.... i tried poking and moving stuff for about 15 mins last night but not a lick of difference there...
ok... more notes... i rerouted the pot to see if that would help... it didn't... i lowered the voltages by changeing the 10k resistor just to see if that could make a difference, it didn't... i've tried the chopstick thing with no avail... i tried a 6l6 in the circuit also to see if the voltages or anything was the problem but it still does it... i've also noticed that it still makes the sound with the volume all the way up but it's not as noticable as when it's at 5-6, maybe the natural distortion was fooling my ears for a minute... what should i check next?
i've read a few things about old fenders and it seems that quite a few were at the voltages that my trannys putting out... so i don't think the voltage is the problem... is there other stuff that i could definetly rule out as the problem... could it be the output tranny? my caps? ect... i know it's not the tubes, i've changed them a few times to check... i don't believe it's the pt from what i've tried and read about old fenders... i've rechecked my resistors with my voltmeter to see if they were the right values instead of just a visual check and they all read right... the heater lines usually only cause grounding problems as well as the way you ground the actual circuit, correct? it sounds almost identical to a misbiased tranny in a pedal but coming from my amp... do you think it could be a bias problem with the tubes?
With out scopin the thing guessing is not a good choice, but you for sure got a DC voltage messin around with your AC signal, you look close at your bypass cap's to make certian that they are + side to the Tube ?,You might try another .02 coupling cap that feeds your tone pot, could be leaking, also tug on those bypass resister's to ground and make sure non are loose or cold soldered, if it was a voltage problem disconnecting the 22 K neg. feedback resistor would change it in someway, you did use Axial cap's for the bypass cap , you can use radial's but they look funny stickin up on the board. maybe if you post some voltages that you can read on the pre amp and phase inverter someone here will figure it out, 2 head's are better than 1, Sometime's :lol:
Johnny
after reading all your test's you've run i only see 1 thing that catche's my eye, on most Fenders i've worked on pin's 3 and 4 will have close to the same voltage reading, your pin 4 look's a tad low for some reason and also, fender ran most of there 6V6 amps over 400, some around 390 on the B+, that's what makes the B/F Delux very costly to buy, it do sing on 425v's
Johnny
this version got the screen grid to B2, which is lower.
other versions got it right to the OT, where it`s closer to plate voltage.
It appears to me, that the screen grid voltage has 2 values:
366V @ pin4;
375V @ 2nd el. cap;
in the schem, it`s a direct connection...
oscillation here? (screengridstopper, 470 Ohm/1W at socket).
BTW: is the OT a single-ended suitable type? (air-gap?).
Quote from: marrstiansok... here's my voltages...
1st hivolt elec 426v(16uf-10k)
2nd hivolt elec 375v(8uf-pin 4 of 6v6)
3rd hivolt elec 250v (i upped the 22k resistor to 68k to get the voltages down on my preamp tubes but i still had the problem before i changed that resistor...
from the 470ohm-25uf-pin 8 6v6 is 23v...
6v6 pin 4 366v, pin 3 418v
12ax7 both at 153 volts...
ok, here`s Aspen Pittman`s Tube Amp Book (4th ed.) corresponding 5F1 voltages:
1st cap (B+) 340V;
2nd cap (B2 = screengrid) 295V;
3rd cap (B3) 250V;
Uk[6V6] 18V;
Ua[12AX7 a/b] 150V;
Uk[12AX7 a/b] 1.5V;
though it says all voltages +/- 20%, your readings look close to/over
the edge of SOAR from my databook.
Someone else here will guide you to re-bias things into the green zone, if that is the problem (and if nothing else, or the combination of some, helps).
:oops:
I still think the power tube is getting pummeled. One thing that you have to remember when looking at these data sheets on the 'net is that they are 30-40 years old and created around the time when tube manufacturing technology was at its peak. I am not so sure that new manufacture tubes live up to the old standards. This situation could be a case where this is true. And the bias current is probably messed up, adding to the nasty distortion sound.
(just looked up "pummeled" in the dic: just what I had expected...)
so, Paul, we`re thinking the same: there might even be an
arcing-over to be seen inside the 6V6 in the dark?
My (old) databook says: Ua-max: 315V; Ug2-max: 285V;
Qa-max: 12W; Qg2-max: 2W;
Uk=23V div. by Rk=470ohms equals: Ik= 49mA thru the tube;
now if we took only the measured 366V at the screen-grid
multiplied with Ik= 49mA (which is Ia+Ig2) we get 18Watts;
if we calculate more realistic Ig2=6mA@366V equals 2.2W,
plus Ia=43ma@426V equals 18.3W, sum= 20.5W.
That`s 46 % overload for a "good old 6V6".
am I overlooking something?
my output tranny is from hoffman amps... it says it's a champ eq.... the only thing with the power is that i tried changing the first 10k resistor to a higher value which significantly dropped my voltages but it still made the same sound... unless when i droped the voltages the bias was still off because i didn't change any of the bias resistors?>?
I`d like to see the first HV (B+) at the plate lower - the others will follow down - I`d try a 1k or 1.2k resistor (5W) between rectifier output and first el.-cap - this would lower the B+ by roughly 50V.
this is not a recommendation, but what i would do, if it were my amp...
Yep, that's what I was getting at puretube.
I also was thinking B+ was a little too high, as I mentioned earlier.
My bet is on the power tube is overloaded. 8)
There is a lot of talk about how to drop voltage after the rectifier on this site:
http://www.firebottle.com/fireforum/fireBB.cgi?cfg=dlx&enter=go
search on "zener diode" or "dropping voltage"
You don't want to use a resistor as its behavior is voltage dependent - you'll get LOADS of sag with a resistor. Zener diodes have a flatter behavior and are preferred in this case. They DO get hot, be warned....
sag is no big problem here:
a.) R is before 1st reservoir cap;
b.) class A PA: quite "constant" current flow;
hey... tried the screengrid resistor... i think i did it right... i just put a 470 ohm resistor in series with pin 4 of my 6v6 right? that didn't help at all if that's what i'm supose to do.... i'll have to try and find a 1k 5watt resistor somewhere... the local store only had 1 watt, i don't think that would be safe... so i might have to wait a couple of days.. this problem really sucks but i've learned more about amps and the terms used with them in the last week than i ever did reading crappy books... thanks to everyone that's trying to help....
five 4.7k/1W in parallell...
What about pictures of the inside? Maybe that would tell us something?
yup, puretube, good point on the class 'A' bit, a resistor would work. Either way, this thing will make a great room heater....
alright... i finally got that 1k5w resistor and put it in series with b+... it dropped my voltages down alot, my b+ is almost right at the 360v that it's supose to have... it helped a little bit but it's still doing the same mis-biased sounds... i'll try and take some pics this week but i'm running on fumes with ideas now... i'm about to send it to someone else to try and figure it out... :cry:
well... i think i may have got it.......... i started the whole process over with the new lower voltages... after i got a 22k resistor on gird 2(pin 7) of my preamp tube it seemed to go away... my question is why did this seem to work and why wouldn't it work with the old b+? thanks to all who helped out with this thread... :D
well... i think i may have got it.......... i started the whole process over with the new lower voltages... after i got a 22k resistor on gird 2(pin 7) of my preamp tube it seemed to go away... my question is why did this seem to work and why wouldn't it work with the old b+? thanks to all who helped out with this thread... :D