What seems odd to me is that the voltage at output stage grids is -35 Volts but it is -43 at the bias pot... that implies grid current flowing and I don't think that should be the case. See what George has to say.
It also seems that the bias was meant to be adjusted simply by measuring the voltage at the bias pot... other amps use a resistor in the cathodes so that an idle current can be dialed in... here you have no such resistor.
To hell with the blue glow for now... I'll bet that goes away when you have an 8 Ohm speaker. Do you have another speaker to try?
It also seems that the bias was meant to be adjusted simply by measuring the voltage at the bias pot... other amps use a resistor in the cathodes so that an idle current can be dialed in... here you have no such resistor.
To hell with the blue glow for now... I'll bet that goes away when you have an 8 Ohm speaker. Do you have another speaker to try?
The Blue was just a secondary concern, The primary concern was that when I hit a cord or note on the guitar, The Screen gets brighter orange. I had originally thought that I was using was a 4 ohm speaker, and had the amp set on the 4 ohm tap. Turns out I had a 16ohm speaker.
So, I am limited to the 16 ohm speaker on the 8 ohm tap until Friday. (UPS Tracking says Friday delivery )
BTW, when the amp is set to the 8ohm tap, the screen glow/pulsations are not as severe though it is still a mis-match.
I was reading another thread here today, A guy has a picture of an EL34 with an isolated blue glowing point, Very much like what I had got.
Tubelab commented that this is common also.
Stands to reason my KT77's are JJ and basically being pushed as an upgrade over the EL34. The point of blue glow is different, but just as isolated.
I will re-check those readings again tonite and post them in the morning.
I have never measured the other working amp at the tube socket, I just set the voltage to -40V and ran the heck out of it,(on correct speakers).
Beats me on that one, I have no experience with this type set-up. I had only built cathode biased amps prior to these.
Maybe I should set it aside until the speakers get here Friday and see if the problem goes away lol
Trout
tubelab.com said:
Voltages confirmed, -44V at pot, -36 at tubes.
Seeing as this now has an entirely new set of caps and resistors, Would it be worth the effort to replace the coupling caps yet again? or just raise the voltage at the pot?
I have plenty of .1 600V coupling caps, but they are all the same brand CDE's from Justradios.com
The transformer is supposed tohave a 50V tap, But, I have no idea what ma rating it has at that tap.
The rectifer used in the bias circuit is an el cheap silicon rect rated 1000V 2.5A
Trout
Gene,
The thing to do at this point is find out why there is grid current, or something that looks like it, flowing in the first place.
Pull out the output tubes, and get measurements at the bias pot and the grid pins again... need to track down that current.
Just for giggles... does these tubes glow when you put them in another amp?
The thing to do at this point is find out why there is grid current, or something that looks like it, flowing in the first place.
Pull out the output tubes, and get measurements at the bias pot and the grid pins again... need to track down that current.
Just for giggles... does these tubes glow when you put them in another amp?
Hi
Your anamalous grid voltage can be accounted for entirely by meter loading as I think Poobah mentioned earlier.
If your meter has an input resistance of 1 Meg, as most do, the voltage divider created by it and the 220K grid resistors and 1K5 stoppers gives (1/1.2215)*44=35.64 Volts.
Taking the same measurement on the other amp would prove the point.
Cheers
Rob
Your anamalous grid voltage can be accounted for entirely by meter loading as I think Poobah mentioned earlier.
If your meter has an input resistance of 1 Meg, as most do, the voltage divider created by it and the 220K grid resistors and 1K5 stoppers gives (1/1.2215)*44=35.64 Volts.
Taking the same measurement on the other amp would prove the point.
Cheers
Rob
Thats the interesting thing, all things being equal(production tolerances set aside)
These 2 amps are identical. I can move the tubes from one to the other, 1 is fine, the other is not.
I have logged no less than 50 hours on the good one. That was basically why I built this one, I figured I had a winner.
OK,
So, I will pull the output tubes, get readings, Then additionally pull preamp tubes and re-check.
Then I will add the 10ohm resistors to the cathodes and get readings.
Maybe I better add the screen resistors also.
BTW It is possible that the other unit is on the edge and not exposing iteslf.
I will pull the voltages off the bias circuit on that one also just to be sure and for comparison purposes.
Back to the basement LOL
Trout
Thats It, its 10:46 PM, I have hit my 60th trip up and 60th down 27 stairs since noon.
The minds willing, but the knees say" What Are You Nutz??"
Fresh start tomorrow, Put in 100 ohm screen resistors, Run to my brothers and get my cat5 crimpers, Put connectors on the cable thats been hanging in my workarea over 2 years and hook up my laptop.
3240 steps since noon, Wow If I ever end up in a highrise fire, I am Doomed!!
Its not the 1620 going up that kills me, Its the ones going down lol
Trout
The minds willing, but the knees say" What Are You Nutz??"
Fresh start tomorrow, Put in 100 ohm screen resistors, Run to my brothers and get my cat5 crimpers, Put connectors on the cable thats been hanging in my workarea over 2 years and hook up my laptop.
3240 steps since noon, Wow If I ever end up in a highrise fire, I am Doomed!!
Its not the 1620 going up that kills me, Its the ones going down lol
Trout
OK,
Taking Tubelabs advice, I installed the 2 100ohm resistors at the sockets, I did not have any 2W so I subbed with 5W. After looking at several other guitar amp schematics that utilize those, Most had 3W or better so this should be ok.
I have not powered it up yet but here is a decent picture of how it sits right now.
Rebuilt Chassis Shot
Those turrets really made life a lot easier. I think next build, I will combo using eyelets for component mounting and turrets where ever I need to run wire off board. Then I will have a few less antennas (turrets) onboard.
Overall its not to messy even after a needless board transplant.
Trout
Taking Tubelabs advice, I installed the 2 100ohm resistors at the sockets, I did not have any 2W so I subbed with 5W. After looking at several other guitar amp schematics that utilize those, Most had 3W or better so this should be ok.
I have not powered it up yet but here is a decent picture of how it sits right now.
Rebuilt Chassis Shot
Those turrets really made life a lot easier. I think next build, I will combo using eyelets for component mounting and turrets where ever I need to run wire off board. Then I will have a few less antennas (turrets) onboard.
Overall its not to messy even after a needless board transplant.
Trout
Well,
I got my new speakers Friday evening, UPS was late but they arrived ok.
The Pulsating screen glow is no longer an issue. I did add the screen resistors Tubelab mentioned, I am not sure if the correct 8 ohm speakers on the 8 ohm tap or the resistors cured it, But its no longer an issue.
I may have a new issue though, I need to run it a bit more to be sure.
But it " Seems Like " when I hit and hold the low E note for example, initially its loud, but 1 second later its even louder before it starts to fade. This seems to only happen at the lowest notes.
But Like I said, I ran it 30 minutes, Basically watching primarily for odd screen glow or pulsating.
BTW, These Jensen Neo speakers are tight!!! Really Tight!! It will be interesting to see how they sound after a full gig rehearsal.
Trout
I got my new speakers Friday evening, UPS was late but they arrived ok.
The Pulsating screen glow is no longer an issue. I did add the screen resistors Tubelab mentioned, I am not sure if the correct 8 ohm speakers on the 8 ohm tap or the resistors cured it, But its no longer an issue.
I may have a new issue though, I need to run it a bit more to be sure.
But it " Seems Like " when I hit and hold the low E note for example, initially its loud, but 1 second later its even louder before it starts to fade. This seems to only happen at the lowest notes.
But Like I said, I ran it 30 minutes, Basically watching primarily for odd screen glow or pulsating.
BTW, These Jensen Neo speakers are tight!!! Really Tight!! It will be interesting to see how they sound after a full gig rehearsal.
Trout
Good news Trout,
Could your volume boost just be a feedback thing in disguise? Try putting some real distance between you and the amp...
It would be interesting to watch the neg bias voltage as you do this... maybe it is sagging?
BTW... that's some really excellent looking work! I can see that you are really putting some thought into the layout of those turret boards... nice.
Could your volume boost just be a feedback thing in disguise? Try putting some real distance between you and the amp...
It would be interesting to watch the neg bias voltage as you do this... maybe it is sagging?
BTW... that's some really excellent looking work! I can see that you are really putting some thought into the layout of those turret boards... nice.
poobah said:Good news Trout,
Could your volume boost just be a feedback thing in disguise? Try putting some real distance between you and the amp...
It would be interesting to watch the neg bias voltage as you do this... maybe it is sagging?
BTW... that's some really excellent looking work! I can see that you are really putting some thought into the layout of those turret boards... nice.
I had a chance to run the amp an hour or so today, Interestingly, I had something odd happen, As I hit the bass note, the entire speaker cab started rocking, 18X22 30 yr old/dry yellow pine box, 80% open back, speaker biased towards a lower corner. I bet if I tried, I could time the note properly to get it rocking enough to tip over!!
That may have been what I was hearing. These new Neo speakers are VERY light weight, This will not happen in my regular cab, But this pine one in my shop was one I use just for testing, Previously it had a Celestion with a huge ceramic magnet that was very heavy.
I still have no idea if I am biased properly, But I can say the tubes display no overheat issues, No odd distortion type junk. A hint of isolated blue flash/glow now and then, but otherwise great.
That turret board worked out fairly well indeed. I think a tiny bit of design tweaking is still in the game plan but its very close to correct. The only thing I really want to do is adjust the component spacing on a couple cap locations, The taller turrets tend to get a bit close compared to the eyelets.
Right now, I have a BIG board layout I am working on that uses turrets and has PCB style sockets onboard opposite the turrets on the bottom of the board!
I have it sized to fit a common Hammond chassis in hopes of making a fairly easy kit type thing.
Should be a bit more durable over a standard PCB and still maintain P-P turret type wiring.
All in all, I am having lots of fun, I have learned a lot and the more I learn, the more fun it gets!
Trout
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