50W Build - First Startup, Bias Issue

[Cathode Ray]

Is that the nfb going to the two pots next to the PI valve?

That is my attempt at a "Resonance Control".

In place of the RC network on the board, I placed a 250K pot with a .0047µF cap across it in a hole meant for the effects loop return level, as I did not want nor did I install the effects loop.

I'm guessing that is the cause of the oscillation ? Should have used shielded wire for that ?

[bepone]

also you can share as built pic how it is now

for the start, redo PI wiring, your anode (red) and grid (blue) leads are flying around make them short and straight and see if is something better..

[bepone]

also you can remove all aditional presence resonance options, leave basic connections only, you can add this later when the amp will be ok

[bepone]

also i dont like output tubes grid wires pin 5 , they are too long , and are going with speaker wires together, reroute them also, or you can use shielded wire for them and see if will be better

[Cathode Ray]

also you can remove all aditional presence resonance options, leave basic connections only, you can add this later when the amp will be ok

Will do.

Many thanks

[Littlewyan]

Just a point on what sluckey said about the OT Primaries. You said they were wired correctly but your grid wires for the output valves may not have been. So to put the phase right you can either swap the grid wires or swap the OT primaries, whichever is easier. In this case it sounds like you've sorted it by swapping the OT Primaries so we're good there. Few questions on the remaining oscillation, does it occur if you remove the PI valve? If not, does it occur if you remove the other preamp valves but leave the PI in? Just want to make sure it's happening in the output stage and not the preamp. Your NFB wire is pretty long right now so I'd say it could be in your output stage.

[Cathode Ray]

Just a point on what sluckey said about the OT Primaries. You said they were wired correctly but your grid wires for the output valves may not have been. So to put the phase right you can either swap the grid wires or swap the OT primaries, whichever is easier. In this case it sounds like you've sorted it by swapping the OT Primaries so we're good there. Few questions on the remaining oscillation, does it occur if you remove the PI valve? If not, does it occur if you remove the other preamp valves but leave the PI in? Just want to make sure it's happening in the output stage and not the preamp. Your NFB wire is pretty long right now so I'd say it could be in your output stage.

Yeah, I have the grid wires correct according to the layout. This was a "kit" I bought from Malaysia, so early-on in this build, there was some fun stuff with the wiring on the PT being reversed from all the hi-res images and the layout (there is no schematic provided - prolly because it's a clone of the Friedman BE-50).

So it's not really a surprise that the OT may have also been shown with wrong polarity/orientation.

Swapping the OT connections to pin 3 of the output tubes solved the bias issue (thank you Luckey )

I will try these things you mention here after I make the other recommended changes & corrections to my lead lengths.

[Littlewyan]

Something you should get which is an incredibly useful tool, is a wooden chopstick. If you have one you can use it to push wires about inside the amp while it is oscillating to see which wire affects it, thus showing you where the problem is. It's also a great tool to have to check for loose connections or solder joints.

[bepone]

also thin green wire from speakers (feedback=?) is also having wrong path, pasing to PI sensitive wires.. if this pot on the back side need to be kept, then this wire must be shielded.

so oscillations how i see are probably started from not so great PI/OUTPUT/ NFB wiring / lead dress.

+very very important, crucial thing! where is speaker jack grounded=? i dont see that, need to be at PI tail ground point...

[johnnyreece]

Yeah, I have the grid wires correct according to the layout.

The reason it was adamantly suggested for you to swap the primaries is, most of the time, the wiring colors are a crapshoot. I've had several builds that were wired correctly according to the layout, but I got an awful squeal. Swapping the primaries fixed it. It happens more often than you'd think!

[Cathode Ray]

I surely appreciate everyone's help

I am making these changes right now.

As suggested, I am abandoning the "Resonance" and "Variable NFB" controls for now and placing those components back on the turret board (like the stock amp).

Shortening leads to the phase inverter.

I'm looking for my ground wire from the speaker output jacks to ground.. Embarrassed to say it looks I missed that.

Re-routing the grid wires as well.

[bepone]

I'm looking for my ground wire from the speaker output jacks to ground.. Embarrassed to say it looks I missed that.

??
not possible to have "feedback voltage" feed back with only one wire? this feedback voltage also have "plus and minus connection" means need to be returned to PI in opposite phase. good is to keep this two wires twisted, they are going to the same place, PI circuit. so route must be similar. not to put ground wire one route and feedback from terminal other route.

ground point for speaker negative terminal and this feedback voltage is where is PI tail ground point..

[Littlewyan]

In that case, I would just ground the speaker jacks for now and see if that solves the issue. If it does then look at rerouting some wires to tidy things up a bit, but you may be ok. Do the test of running every control on 10 (without a guitar plugged in), the amp should be stable. Run it into an attenuator if you can in case it does feedback.

[Cathode Ray]

In that case, I would just ground the speaker jacks for now and see if that solves the issue. If it does then look at rerouting some wires to tidy things up a bit, but you may be ok. Do the test of running every control on 10 (without a guitar plugged in), the amp should be stable. Run it into an attenuator if you can in case it does feedback.

I finished making all the recommended changes before I saw these last two posts.

Shortened PI anode and grid wires.
Removed "Resonance" and "Variable NFB" controls. Put these circuits back on turret board (just like stock amp).
Grounded speaker output jacks.
Re-routed grid wires away from speaker wires.

All controls set to "0" and speaker jack connected to dummy load I biased the output tubes to 36mV.

When I connect a 4x12 cab, as soon as I flip the standby switch I get a high-pitched squeal.

With PI tube removed, no squeal.

Pi tube back in, 4x12 cab, poked around with chopstick. Nothing made the squeal worse or better or any change that I could tell.

[bepone]

impossible
questions:
try now to swap primary wires and see if is something better.
if not:
from this point and further on, until you get good working amp, remove all the preamp tubes out from the circuit, leave in only PI tube.

can be some fault or wrong connection in the amp , need to start from the output stage alone.