Heater Voltage is 0???

[mpayne]

I've put together my first amp and the tubes are not heating up.  I pulled a 6550 tube and measured the a/c voltage across pins 2 and 7 and it's 0.  I've verified that the pins are connected to the transformer.  I measured the d/c voltage between J3 and J4 and it's ~470, so that part of the transformer is working.  Anybody have this problem?  Any suggestions on what I should check next?  I knew I would have some glitches here and there in this build, but not being able to heat the tubes!?  The power transformer is a 290GX.  I've attached the schematics.

Thanks,
Mike

[phsyconoodler]

Something is amiss with your wiring.How can we diagnose without a picture?Try lifting the heater wires off where they come out of the transformer and measuring there.
  Don't run the amp anymore or you might burn the heater windings out.You may have a poor connection.

  Go over your work one step at a time from the transformer out.

[mpayne]

Got it.  There were 2 black wires on the secondary side, one common between the two 345 and one that was for the 3.15.  I had them swapped.  I assumed since they were on the same side of the transformer and the same color they would be the same voltages.  I was wrong.  phsyconoodler, thanks for the reply.

- Mike
 

[plexi50]

I was going to say that is an odd color combination for heater wires (Orange & Black) from your schematic. Real nice schematic to. Usually they are the same color. Glad you got it. NEVER QUIT!!!!!

[mpayne]

One thing after another.  I'm measuring my bias and it's 1.5 ma.  I adjust the pot but nothing happens.  I check the voltage at J3 and J4 and it's now around 30.  I'm confused.  It was around 470v last night, but that was with the other black wire as the tap.  I measured the voltage at the fuse F1 and noticed that when I turn on the amp it slowly ramps up to around 90v then starts going back down.  I figure that's a bad thing so I turned the amp off.  Not sure what going on.  Any ideas?

Thanks,
Mike

[JayB]

One thing after another.  I'm measuring my bias and it's 1.5 ma.  I adjust the pot but nothing happens.  I check the voltage at J3 and J4 and it's now around 30.  I'm confused.  It was around 470v last night, but that was with the other black wire as the tap.  I measured the voltage at the fuse F1 and noticed that when I turn on the amp it slowly ramps up to around 90v then starts going back down.  I figure that's a bad thing so I turned the amp off.  Not sure what going on.  Any ideas?

Thanks,
Mike

Your not using a slow blow fuse for F1? Slow blow will act funny on the other side of the transformer.

I would pull all tubes and check. You might of burnt out a heater or two in a one or more tubes. I have done it before. Crossed one B+ and fried a tube heater.

If it persist, I would disconnect all your secondaries and check for continuity between all your secondaries. 2 pairs of 3 should have continuity between them. I'm not familiar with that transformer though. They all should be fairly close between the center taps for each winding. If not, could be fried. Do thid with the amp unplugged of course.

[plexi50]

Just a reminder: Be very careful with the high voltage leads. Steve is very good at walking you through any amp issues as are many others here on the forum. You have good friends here. I just got this feeling you are lucky that you did not do any damage to your PT and yourself. The secondary windings can be very dangerous so go slow and if nothing else use a DVM to confirm the voltages present on your PT secondary leads before attaching them to anything. Dont want to sound like an old grandma waving her hand about. Just be careful and use your meter /A concerned friend!

I can send this guy over if all else fails /

Im wondering if LC had any luck with him? How about it Dan?

[mpayne]

Thanks for everyone's help, and plexi50 your not sounding like a grandma waving her hand, I want to her every concern you might have.  This is my first build and I want to make sure I do it right.

The fuse is "fast acting"  I got it from digikey part F2389.

I pulled the fuse and measured the a/c voltage going into D4 and D5.  It was 756.  Sounds right to me.  Then I measured the d/c voltage going into the fuse (with out the fuse inserted).  It was 572.  I put the fuse in and re-measured the voltage while I turned it on.  It slowly ramped up to 110 d/c then started going back down again.  So I have something wrong after the fuse.  I continue to carefully diagnose.  Any advise is welcome.

- Mike

[plexi50]

WRONG! You should not have 756 volts on D4 & D5. Hammond says 704 with no load. It dose not seem that you have the CT center tap to ground on the secondarys for the two red high voltage leads. Hammond says 690 VCT. If the CT was grounded it should read half of 690 VDC. That would be 345 VDC on either D4 & D5. Across D4 & D5 you would read about 700V. Unplug from the wall. Take your meter and clip one lead to one red secondary lead and the other to the black lead or leads until you get a tone from your DVM. This will confirm that it is your CT center tap for your secondary high VDC leads. You may have to unwire the heaters from the PT just to be certain. You have 2 black leads on your secondary and one of them is for the heaters and the other your HV CT to ground. Maybe i am not interpeting you right here. What is your voltage at D2 with your black meter cliped to chassis ground?

[mpayne]

Good catch Plexi50.  My com was floating.  My PCB layout had it going to ground but I guess the PCB fab messed up.  I fixed that and now when I turn the amp on, after about 10 seconds I can hear my guitar through the speaker and it sounds great, then after about another 5 seconds it starts to ring very loudly and I have to turn it off.  I guess I've got another problem to fix.  Any ideas what to check next? 

Thanks for everyone's help,
Mike
 

[mpayne]

Also, my bias current is through the roof.  It's jumping to like >80ma even with pot R36 set to max 22K ohms.  I apparently suck at building amps.  What should I check next?

Thanks,
Mike

[HotBluePlates]

If you have a negative feedback loop running from the speaker to an earlier stage (usually at/near the phase inverter), unsolder 1 end of that connection. If the ringing stops, swap either a) the wires running from the output tube plates to OT primary, b) the wires from OT secondary to speaker jack (if you only have 1 secondary tap; if multi-impedance, don't swap here), or c) from phase inverter output to power tube grids. Each of these swaps should accomplish the same thing, unless there are complicating factors in your particular build. Often, one method of swapping is much easier than the others. ONLY do 1 swap; don't swap at multiple places.

What you're finding out with the initial test is if your intended negative feedback is sctually the wrong polarity and causing positive feedback (ringing, howling). If it stops with the feedback disconnected, then you confirmed wrong polarity, in which case swapping wires will correct the polarity.