Something's wrong between the PI and output, but what?
Moderators: pompeiisneaks, Colossal
Something's wrong between the PI and output, but what?
Hi guys, a the title implies, I am having trouble tracking down the reason why my amp is working strangely. I have traced the signal through the entire amp and everything is fine up to the signal grid of the power tubes. The tubes glow when powered up and the voltages on the screen, plate and heater pins all check out. I also checked the OT and it seems fine (measured plate to plate resistance, plate to +ve resistance and plate to chassis resistance, all fine). The tubes are cathode biased. I have tried everything I can think of (admittedly not much), but I don't have a spare set of tubes to swap out. One thing to mention, it was working just fine the other day, then it crackled some like static and stopped working. I just don't know what this could be. Any ideas?
Exact science is not an exact science
Re: Something's wrong between the PI and output, but what?
It would help to know the amp with the problem.
When I have problems like this I inject a signal where you think the signal is getting lost and listen. Take some type of player, mp3 player, CD etc, and connect the output hot lead to a .1uF 400V cap, ground to chassis ground. Touch the other lead of the cap to the point where you think the signal is getting lose, I.E. the power tube grid and listen.
IF you get sound on all power tubes this way, move back a stage to the PI. Touch the cap lead to the PI grids.
At least this will let you know were the problem is.
When I have problems like this I inject a signal where you think the signal is getting lost and listen. Take some type of player, mp3 player, CD etc, and connect the output hot lead to a .1uF 400V cap, ground to chassis ground. Touch the other lead of the cap to the point where you think the signal is getting lose, I.E. the power tube grid and listen.
IF you get sound on all power tubes this way, move back a stage to the PI. Touch the cap lead to the PI grids.
At least this will let you know were the problem is.
Re: Something's wrong between the PI and output, but what?
If the signal is ok all the way up to, but not including the OP tube grids, it would be bad grid stoppers. Do you see the signal on the other side of the grid stoppers? If you are not using grid stoppers, do you see the signal on the plates of the PI ? (Make sure you are blocking DC of course.)
If it says "Vintage" on it, -it isn't.
Re: Something's wrong between the PI and output, but what?
Crackle/static makes me think it could be the first filter cap, a broken/cold solder, or resistor that's been fried.
Re: Something's wrong between the PI and output, but what?
Cathode voltage please?Ripthorn wrote:the voltages on the screen, plate and heater pins all check out.
Re: Something's wrong between the PI and output, but what?
The plate voltage is 110V (submini tubes), screen voltage is 99V and cathode voltage is 11.4V. I hear the signal on the grids, but not on the plates (there is a very faint signal when I have it fully cranked, but barely audible). I guess that only really leaves tubes, doesn't it? If so, looks like I'm off to buy some new tubes...
Exact science is not an exact science
Re: Something's wrong between the PI and output, but what?
That's just bazaar. The fact that you have cathode voltage says that the tubes are conducting. Do the grids have a ground reference resistor?Ripthorn wrote:The plate voltage is 110V (submini tubes), screen voltage is 99V and cathode voltage is 11.4V. I hear the signal on the grids, but not on the plates (there is a very faint signal when I have it fully cranked, but barely audible). I guess that only really leaves tubes, doesn't it? If so, looks like I'm off to buy some new tubes...
This doesn't strike me as bad tubes. How about a schem posted, pics?
Re: Something's wrong between the PI and output, but what?
I think it's gotta be a bad solder joint now, as I was going through and double checking voltages, it started working again. So I am going to retouch all solder joints and see what happens.
Exact science is not an exact science
Re: Something's wrong between the PI and output, but what?
Yep, sometimes you just don't see a bad joint so touch up the joints with a tiny bit of fresh solder.
Also check your grounds very carefully, make sure they are tight and making good contact with the chassis.
If you have a good sensitive ohm meter check the continuity and resistance on your ground runs.
On one amp I had used solid hookup wire with PVC insulation.
Well, I had been in and out of the amp fixing things and moving wires here and there.
Well, one plate wire had pretty much broken but it was hidden just inside where the insulation started.
I must have troubleshooted that amp for a week on and off before I found that break.
From then on I always use stranded wire and prefer the Teflon insulated stuff because it doesn't melt.
Sometimes the problem is stupid simple and other times you may never know what it was exactly if it was a cold solder joint.
But what a relief it is when you finally get it going and it works as it supposed to!
Also check your grounds very carefully, make sure they are tight and making good contact with the chassis.
If you have a good sensitive ohm meter check the continuity and resistance on your ground runs.
On one amp I had used solid hookup wire with PVC insulation.
Well, I had been in and out of the amp fixing things and moving wires here and there.
Well, one plate wire had pretty much broken but it was hidden just inside where the insulation started.
I must have troubleshooted that amp for a week on and off before I found that break.
From then on I always use stranded wire and prefer the Teflon insulated stuff because it doesn't melt.
Sometimes the problem is stupid simple and other times you may never know what it was exactly if it was a cold solder joint.
But what a relief it is when you finally get it going and it works as it supposed to!
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Something's wrong between the PI and output, but what?
Since you have signal at the grids, you can check at the grid and plate directly on the tube, eliminating the solder joint.Ripthorn wrote:I think it's gotta be a bad solder joint now, as I was going through and double checking voltages, it started working again. So I am going to retouch all solder joints and see what happens.
If I had a nickle for every time I screwed up a connection, I'd have a bunch of nickles.
