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> Inconsistent/ up and down B+
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Wanabejimi
Junior tube assistant
Posts: 41
(2/15/04 6:56 pm)
Inconsistent/ up and down B+
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I'm working on a big bass amp. When the PT is not under load (no power tubes installed), it has a b+ of 548 vdc. When power tubes are installed, the voltage on the plates, the screens, and the bias jump up and down by 20 volts. For example, the voltage on the plates can go from 507 to around 527. Of course, there is corresponding noise/motorboating with every jump. The amp will pass some guitar signal, but it also "produces" more motorboating sound....In other words, the guitar signal becomes motorboating. I should mention that all of the filter caps have been replaced with the same results. I am using an old PT....is the PT bad? Help, please.
Wanabejimi
Junior tube assistant
Posts: 42
(2/16/04 7:46 am)
Re: Inconsistent/ up and down B+
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Just wondering out loud.... I checked the AC side of the rectifier with the PT under load and the ac was only going up and down by a single volt. I'm just wondering that IF the caps in the bias circuit are not smoothing out the bias voltage (which is, in fact, jumping around by 20 volts) would that throw of the rest of the B+. I'm thinking that it would, but I'm not an expert. So how about it, you gurus of tubes and tone?
GroundhogKen
Forum Moderator
Posts: 2268
(2/16/04 8:19 am)
Re: Inconsistent/ up and down B+
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That's a good thing to check.
Did you install the bias supply caps with the (+) positive end to ground? That's how it needs to be.
Ken
Wanabejimi
Junior tube assistant
Posts: 43
(2/16/04 8:42 am)
Re: Inconsistent/ up and down B+
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Yea, the caps are installed with + to ground. I've got two 10uf at 100 vdc caps in there now....with a bias voltage of around -60. That is, until I install power tubes...then it jumps all over the place.
GroundhogKen
Forum Moderator
Posts: 2270
(2/16/04 8:45 am)
Re: Inconsistent/ up and down B+
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What happens if you install the tubes one at a time?
Check the B+ each time you add a tube.
Yank out the phase inverter tube to make sure that isn't messing up your bias voltage.
Ken
Wanabejimi
Junior tube assistant
Posts: 44
(2/16/04 10:05 am)
Re: Inconsistent/ up and down B+
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I'll try all that when I get home this evening...
However, just for the sake of learning something new...how would the PI mess up the bias voltage? That has me very curious.
GroundhogKen
Forum Moderator
Posts: 2271
(2/16/04 12:29 pm)
Re: Inconsistent/ up and down B+
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It wouldn't unless it was producing some sort of oscillation.
I'm just trying to rules out many possible problems as quickly as possible.
Ken
Wanabejimi
Junior tube assistant
Posts: 45
(2/16/04 5:07 pm)
B+
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Dear Mr. GroundhogKen,
I am very thankful that you use your powers for good instead of evil!!! Amazing. It IS the PI and not the power tubes. I'm going back down in the basement to look at the PI circuit for the bazillionth time. If, in the meantime, you happen to read this and know specifically what I'm looking for...I'd appreciate any help.
Thanks
Wanabejimi
Junior tube assistant
Posts: 46
(2/16/04 5:55 pm)
B+
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OK....armed with the idea that the PI is oscillating, I went and looked at the lead dress....and I'm thinking you are right... The leads from the PI to the grids of the power tubes are awfully long. I had no choice because of the layout of the amp carcass I was using. This amp has 4 power tubes. I have 4 wires going from the bias/PI to the grids. Should I cut that down to 2 wires and kinda "daisy chain" the pairs of power tubes together? Would it help if I used shielded wire (if so, could I ground the sheild to pin 8?)? Should I increase the value of the resistors on the grids. ...and should I take Mavis or Stella to the Jr. Prom? Thanks for your patience and help!!!!
GroundhogKen
Forum Moderator
Posts: 2273
(2/16/04 9:39 pm)
Re: B+
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Use grid stopper resistors ( say 1.5K or 5.6K ) on the power tube grid pins.
Run short wires to connect the grid stoppers on each half of the push-pull circuit. (daisy chain--as you described)
Then run two wires back to the PI coupling caps and bias feed resistors. If these two wires are long, twist them together like filament wires.
These two wires carry a differential audio signal. If you twist them together, any noise signal that is picked up by one is picked up by the other equally, so then the noise is canceled by the push pull circuit.
You can also put a 47pF or 51pF cap across the plates of the PI. I'm assuming it's a long PI.
What sort of amp circuit is this? What Power tubes? Four of 'em?
Take Stella to the prom, but make an earlier date with Mavis. Tell Mavis that Stella had already asked you to the prom so you felt you had to go. Figure out which girl like good tunes and weigh that against which one has larger breasts and which one has more money.
Ken
Wanabejimi
Junior tube assistant
Posts: 47
(2/17/04 4:56 pm)
B+
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Thanks Ken for all the great advice! Particularly about Mavis and Stella! I'm actually 41 years old, so I know good women advice when I hear it. Unfortunately, it seems to me that I am a sophomore, at best, at tube amp building.
Here is the circuit:
www1.korksoft.com/~schem/...owramp.pdf
The preamp seems fine, so we'll forgo that. I wired up the amp as you suggested on the grids. Louder, faster motor boating. That seemed to quiet quite a bit after awhile, strangely enough. I also switched the OT primaries with no change. The voltage on the plates and screens is much more stable, but as far as I can tell, they are the same (both are going up and down by a volt). The bias voltage is less jumpy, but still going up and down by about 8 volts give or take. The amp is not effected by guitar signal at all (nor does it produce any). Turning the volume knob or the tone stack pots have no effect at all. Pushing the grid wires around with a chopstick...nothing. I have not put all 4 power tubes in at once...just two at a time. Let me know if you need any more info...or if you can help me at all. I'm thinking it would make a very fine boat anchor. I've been trouble shooting for weeks and the amp will give me no love or tone. Thanks for your time and thoughts!
GroundhogKen
Forum Moderator
Posts: 2278
(2/17/04 7:52 pm)
Re: B+
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OK, it's a V4.
Does the grid voltage ( and all the power tube voltages ) remain stable with the PI tube removed?
Disconnect the negative feedback wire for now to keep that question off the page.
Pull V3 (12DW7) to make sure that it really is the PI tube causing the problem.
Measure the resistance from PI pins 3 and 8 to make sure you have 1.5K ohms to ground.
Ken
Wanabejimi
Junior tube assistant
Posts: 48
(2/17/04 9:14 pm)
B+
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Yes, the voltages remain stable with the PI out. In fact, the voltage stabilized by improving the lead dress. The nature of the motor boating was changed when he lead dress was improved...it got faster (and louder at first) and it is no longer effected by the guitar. I should also mention that the motor boating is not effected at all by biasing the tubes hotter or colder.
I pulled V3 and the motorboating got a little softer, but that is the only change.
Pins 3 and 8 are tied together on the tube socket and measure 1.5 K exactly.
Plans for tomorrow are to re-do the lead dress on the grids (again) and see if I can do a little better. I think I'll increase the grid stoppers a bit (from the 47K that are on there now) Will keep you posted.
Wanabejimi
Junior tube assistant
Posts: 49
(2/18/04 12:51 pm)
progress
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The good news: I found a resistor gone bad in the PI circuit. I actually got some nearly, almost decent, but with buzzing, guitar signal out of the amp for the first time!! However, I've got some pretty major oscillation going on. The bias on one "side" of the tubes climbs really high, and starts motor boating as I turn up the volume on the amp. This happens just as any decently strong guitar signal begins to pass. The bias remains stable as long as there is no guitar signal. This is only happening on one side of the power section. The other side remains stable. If I only play with 1 power tube, the amp is stable, but buzzes. If I pull the PI, the amp is stable. Will try and have pics posted tomorrow, but any ideas are appreciated today.
David
Wanabejimi
Junior tube assistant
Posts: 51
(2/19/04 5:18 pm)
B+
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This just ain't my project. When I went to attempt to take pics to post...I found that my digit
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