Bob-I wrote:It sounds to me like all the stages are not amplifying correctly and gain is being lost. Double check all the voltages and the values of the components on V2. Check the signal going in and out of each stage, there should be an increase in gain at each stage.
I'd second this. I've heard that type of breakup coming from an amp I build where I didn't double check the components on V2.
One of the best solutions I've found that works for me is to print out the schematic on 11x17 paper and tape it in front of your work bench. Go through with your multimeter and check every single connection. Where things test fine, highlight that part of the schematic. By the time the entire schematic is highlighted, you're ready for the successive initial fire up procedures. It can save a lot of troubleshooting down the road.
Structo wrote:Yes, double check resistor values.
Easy to make a mistake on the color coding on certain values.
I use a meter on all resistors now. I got resistors from weber and at least 5 of them were no where near the value indicated by the color bands. Meters are also far more acurate and the resistance includes the two leads.
It's true i've lost my marbles and i cant remember where i put them
Even though I've gotten back to where I can read the codes quickly, it's easy to get say a 470K and a 4K7 ,mixed up if you aren't paying close attention.
The following is aimed at the new builders here.
What I do and I think quite a few other guys do , is to measure each resistor and cap that I will use in a build.
It may seem obsessive and overkill but it can save your bacon down the road.
I actually find it relaxing to do and I can weed out any component that is too far out of tolerance.
What I do is measure each resistor that I am going to use in my amp, then take some heavy stock paper (8.5x11) and tape down the component on the paper and write the measured value beside the part.
Same with the coupling caps.
It can really save a lot of aggravation later on when the amp is not sounding right and you don't know where to start looking for the problem.
Then as I install the part, I just take it off the paper and I know that it is the right value.
hello,
something is wrong definitely.
this grainy sound comes from bad preamp tube? or bad wiring regarding g2 screen on the power tube?
in some special case i've heard this sound on 50W-100W amps where g2 (EL34/or 6L6) was connected on first cap in schematic (C1)- C1 is also output transformer supply... this connection provides too much attack and result are very fizzy mids/highs.. solution was to move screens on C2, where C2 was connected to C1 via 470 ohm/5w+3H choke.
this arrangement slowers attack and resolve problem
case 2- bad solder on gain/volume pot - i had this in my Hi-gain mesa recto build, the same fizzy sound. i had luck in this case because i change gain pot just before the problem comes and i knew where it was..
I was getting some undesirable fizz at one point; during which time the amp sounded also better without a d'lator than with.
Increasing the 220K OD input resistor to 470k helped a bit.
But adding a 47PF cap to ground at the grid of the PI tube, right after the .02 input cap, vastly smoothed out the tone - to where the d'lator also started improving the sound the way it should do.
The 47PF cap also introduced a nice compressed attack and a brassy sustain to my Les Paul's bridge PU that it lacked before.
The improvement seemed to come primarily from the cap whereby I suspect I could have gone back to the 220K on the OD entrance. But the two changes (cap and resistor) seemed to suite my amp's overall nuances.
LPSGME wrote:I was getting some undesirable fizz at one point; during which time the amp sounded also better without a d'lator than with.
Increasing the 220K OD input resistor to 470k helped a bit.
But adding a 47PF cap to ground at the grid of the PI tube, right after the .02 input cap, vastly smoothed out the tone - to where the d'lator also started improving the sound the way it should do.
The 47PF cap also introduced a nice compressed attack and a brassy sustain to my Les Paul's bridge PU that it lacked before.
The improvement seemed to come primarily from the cap whereby I suspect I could have gone back to the 220K on the OD entrance. But the two changes (cap and resistor) seemed to suite my amp's overall nuances.
....and thereby getting rid of parasitic oscillation
Thanks for the great suggestions. I've built a LOT of amps before. Been in the business as a builder and repairman since I was 13, which makes 12 years now. I guess my dad got me infected with this habit at an early age, lol!
Anyways, I've narrowed it down to the oscillation, which I'm gonna try to fix in one of the next days. I've litterally check ALL solder joints, measured and reflowed where I saw the slightest lack of shine. I don't rush my work, so normally everything is spot in, which also was the case here according to my DMM (Fluke 179).
If this doesn't work, I'm gonna borrow my friends scope and check out what is happening.
That being said, all resistors, caps and tubes have been measured before, and are they way they are supposed to. Lead dress is very good as well. I'll keep you posted.
Just for kicks, I placed two 300pF from the anodes to the gates of both halves of the OD-tubes, as I see on some of the amps Howard made - no difference. Shouldn't that kill the oscillation?
jakehop wrote:Just for kicks, I placed two 300pF from the anodes to the gates of both halves of the OD-tubes, as I see on some of the amps Howard made - no difference. Shouldn't that kill the oscillation?
solder these 300pF from anode to cathode, not to screen
jakehop wrote:Just for kicks, I placed two 300pF from the anodes to the gates of both halves of the OD-tubes, as I see on some of the amps Howard made - no difference. Shouldn't that kill the oscillation?
solder these 300pF from anode to cathode, not to screen
my second post, electronic and tube fanatic, from croatia