B15N Circuit has problems

[htrboy]

Hello! My first time posting here.

I've built myself a bass amp based on the Ampeg B15N (Rev C. for whatever that's worth). I just built one input channel with one jack instead of the 2x2 inputs that the originals had, and for various reasons, I'm using a 5U4GB instead of a 5AR4 .

I've built one other tube amp - a hi-fi type from the RCA manual which turned out pretty well, and I've repaired several tube amps and radios in the past.

This build however has a big problem, which I think I've narrowed down to the phase inverter circuit. With the PI tube in the circuit, there is a HUGE amount of hum, enough to make the output at the output jack look like a square wave. When I take the PI tube out of the circuit, the output from the first stage is reasonably quiet (.01v p-p with no input), and the output from the power tubes is also quiet. I've checked and rechecked the schematic and my wiring but can't find anything wrong. I'm sure I need a set of unbiased (haha) eyes, and help from people more experienced with tubes.

For my chassis, I'm reusing an old Wurlitzer organ amp (and PT) that had 6L6 outputs and the 5U4GB tube. My lead dress is NOT pretty, compared to some of the amazing builds I've seen on here and other forums, but compared to the rats nest I pulled out of the Wurlitzer amp chassis, it ain't bad.

Any ideas and suggestions welcomed,

Scott

[Reeltarded]

Post a picture of the guts. I just sweep and take out the trash. Boss will be along shortly with a great answer!

[selloutrr]

things to check

cold solder point

a resistor value is not correct

Is you PI tube tested and confirmed good?

grounds are solid and correct?

lead dressing

[htrboy]

Thanks for the reply's! I do need to look at lead dress, and I'll recheck my component values, but I think when I said hum, I used the wrong word. This is MASSIVE. It's buzzing so badly with the PI tube in the circuit that I can actually hear it in the components.

I have a Sencore tube checker that tests for emission, grid leakage and shorts, and they tested good, for what that's worth. Plus when I swap the PI tube for the input tube (both 6SL7's) the problem is the same.

I'll try to get a pic posted today.

Best regards,
Scott

[selloutrr]

is it any better if you put a tube shield or dampening ring over the PI tube?

have you tested your caps for leaking?

[Structo]

I would suggest it is a wiring error.

If you can rule out the tube, that is about all that is left.

Post some pictures and also, trace each and every wire.
Compare it to a layout if you have one and verify each wire and connection.

[da Geezer]

Just to eliminate a commonly overlooked problem with new builds..........

Does your 6.3v filament/heater supply have a center tap (connected to ground)?
If not, do you have a 100ohm resistor from each leg of the 6.3v secondary to ground?

Geezer

[htrboy]

On my filament supply I used a 47 ohm resistor from each leg to ground, thinking that it was approximately the same as each leg of a 100 ohm pot - should I add more resistance?

I'm convinced that it's a wiring/grounding issue. Today I lifted the input wire from the phase inverter and the waveform at the output jack dropped to a relatively quiet 13v p-p. There is virtually no noise on the input wire coming from the first stage into the PI. Does it make sense that there could be something affecting the ground side of the long tail pair and it's feeding back somehow into the PI's first stage grid...?

I'm looking as hard as I can at all the wiring, and looking for dc getting through the caps. Getting a photo up will take a while (sorry about that). I appreciate the suggestions - thanks!

-Scott

[Structo]

How is your screen voltage supply to the 6L6?

I guess maybe show us what you have on the plates of all the tubes.

At first I was going to say something about the differences between the 5U4 and the 5AR4 since the 5U4 drops about 30 more volts but I see it is an organ donor amp so that power amp should all be good, right?

The first thing I do when I power up an amp that I am working on is to ground my black probe on the chassis then touch the red probe to the plate resistors, starting at the phase inverter.
You should get some decent pops from the speaker as you go across the amp.

If you get to one that is quiet, check out that tube and or associated circuitry.

Also, when troubleshooting and you don't know where the problem lies, start at the power tubes and work towards the input.

Check each stage as you go for proper voltage readings or oddball readings.
Of course this assumes that the amp worked properly at one point in it's life.

For a new amp, this can be tougher.

For a new amp that isn't working other than a faulty tube, the error is almost sure to be a wiring error.

Best course is to print out a large layout of the amp as you built it.
Then trace each wire and component and mark them on the layout as you go.
It's most likely something very simple but is preventing the amp from working.

If you have been working on it for long hours, the best course is to put it away out of sight for a day or two, that way you can come back to it fresh without preconceived notions about what it might be.

Good luck!

[htrboy]

Well, I've made a little progress in that I got something to change...

Some background: For grounding, I have two star grounds, one for the power supply filter caps and the output tubes, and one for the input stage and phase inverter tube, then they both tie to the chassis at one point.

My output transformer output leads go to an ohm selection switch and then to an isolated output jack, and the ground wire goes directly to the sleeve of the jack, with a small wire that connects to the second star ground (input stage, PI). When I lifted the small wire from the jack to the star ground, the hum (for lack of a bigger word) instantly went away, but there is still some weirdness going on!

With the tubes finally in some semblance of an idle state I measured the plate voltages:

V1a (input stage) plate: 181v
V1b (input second stage): 184v
V2a (PI top): 213v
V2b (PI bottom): 216 (this is supposed to be lower than V2a!)
V3 & V4 (6L6GC) Plates: 392v Screens: 388v Grids: -49v
V3 current draw at cathode (voltage drop of 1 ohm resistor): 13mA
V4 current draw at cathode: 8.9mA

With no signal on the input, there is a small 60Hz wave on the output with the volume at minimum (after lifting the wire as stated above). when I crack the volume the output immediately drives into a square wave. When I put a signal on the input however, (.3v p-p, 400Hz), it behaves a little better, so I'm thinking there has to be something wrong with the negative feedback and output transformer connections...??

[davent]

Hello,
I didn't notice it mentioned but DC heaters are often recomended for 6SL7's. I have 6sl7's for the PI's in my hifi amp and that circuit uses DC heaters, that amp is super quiet. I have a guitar amp based on the 5F2a that has parallel preamp sockets for auditioning 12a_7's, 6eu7's or 6s_7's, this amp uses AC heaters. I can take a 6sl7 from the very quiet hifi amp, plug it into the 5F2a and found any 6sl7 is far noisier then it was in the hifi and far noisier then any 12a_7 or 6eu7 samples i've tried.

dave

[htrboy]

Booyah! Well, I don't know exactly what rework I did that made the difference, but the amp is now quiet and passes a signal all the way through. I got pretty frustrated and ended up taking apart and redoing several complete circuits.

My question now has to do with fine tuning some of the voltages (maybe I need a new thread for this?). My pair of 6L6GC tubes seem pretty far apart in cathode current, 20mA vs 12mA. I'm wondering about building a balance circuit at the bias supply point - what type of pots are required and what is a 'safe' to build (fails gracefully)?

[azatplayer]

Interested in a pic still. I built this amp ages ago and developed a layout from the schematic, great amp! Like to see how you laid it out