50W Build - First Startup, Bias Issue

[sluckey]

Swap the OT primary plate leads and put the PI tube back in. Any better?

[Cathode Ray]

Swap the OT primary plate leads and put the PI tube back in. Any better?

I can try that, but the leads from the output transformer are color-coded, and I have verified that I do have them connected correctly. I realize you're trying to resolve oscillation.


Not wanting to go off on a tangent here, but I'm starting to look at a couple of changes I made to the original circuit when building this project:

I have a question about how the presence control affects the bias readings.. I noticed when I move the knob(s) associated with the presence control circuit, the bias mV readings go off wildly.

I added two changes to the presence circuit while I was building this project; I replaced a fixed resistance on the turret board with a pot to create a 'variable NFB' control. Also, I used a pot with a cap across it to replace the 220K/.0047uf network after the NFB resistor to create a 'resonance" control.

It's easy enough to simply dial the pots to the value of the (former) fixed resistances and it's as if the stock circuit was in place. However, while measuring the bias test points with theses two pots set to the original fixed resistance values, the absolute lowest I can get on one power tube with the bias adjust control is 87mV and on the power tube socket that is always 'high' it's 140mV.

I only ran this for a few seconds. Just long enough to take those readings. I don't think swapping OT primaries will fix that. I think I have something else wrong here.

I don't have enough knowledge about this to know if that's normal to be able to wildly swing the power tube bias around with these supposed "tone shaping" controls. Intuitively, I think this may be my actual problem.. but I can't make sense of why it only affects one of the two power tubes.

[sluckey]

Everything you just said supports the idea to swap the OT primary plate leads. I bet if you plug in a speaker you'll have a loud squeal or howl.

[Cathode Ray]

Everything you just said supports the idea to swap the OT primary plate leads. I bet if you plug in a speaker you'll have a loud squeal or howl.

So it's normal to be able to swing the power tube bias around by adjusting the Presence control knob (or Resonance, or Variable NFB)

Also, I can only get one power tube to bias correctly at 37~38mV with the bias adjust knob at the very end of one extreme of it's travel. This doesn't seem right ..

[sluckey]

So it's normal to be able to swing the power tube bias around by adjusting the Presence control knob (or Resonance, or Variable NFB)

No. But if the NFB phase is wrong both the presence and variable NFB will affect the howling, which is causing the tube current to go wild.

Just do it! If I'm wrong I'll say so. If you will connect a speaker instead of that dummy load then your ears will tell you if the NFB phase is wrong. You're gonna damage your output tubes if you continue like you are. Only takes a couple minutes to swap those two wires.

[Cathode Ray]

Just do it! If I'm wrong I'll say so. If you will connect a speaker instead of that dummy load then your ears will tell you if the NFB phase is wrong. You're gonna damage your output tubes if you continue like you are. Only takes a couple minutes to swap those two wires.

Not trying to be difficult. Just trying to learn something along the way.

You are right. I connected a 4x12 cabinet and as soon as I switched the standby on, I got an ear-piercing howl through the speakers.

[sluckey]

You are right. I connected a 4x12 cabinet and as soon as I switched the standby on, I got an ear-piercing howl through the speakers.

Well, SWAP THE OT PRIMARY PLATE LEADS.

[Cathode Ray]

Well, SWAP THE OT PRIMARY PLATE LEADS.

Ok, I swapped the output transformer connections to the power tubes pins 3.. same whistling/howling sound thru 4x12 cab.

Now the output tubes bias test points both seem to be closely balanced and within a manageable range. I can adjust the bias between 20mV ~ 90mV.

[sluckey]

-56.5v bias on pin5 is way too much for 6CA7s. Adjust the bias pot to increase the mV readings at your test points.

I suspect you need a fresh set of 6CA7s. Those have had an extreme workout.

[Cathode Ray]

-56.5v bias on pin5 is way too much for 6CA7s. Adjust the bias pot to increase the mV readings at your test points.

I suspect you need a fresh set of 6CA7s. Those have had an extreme workout.

With the bias adjusted to 36mV I now measure -47v on pin 5 of the output tubes.

[bepone]

do you have grid stoppers on output tubes? pls install 3k3 or some other value close to pin 5 on output tube sockets

[bepone]

also you can share as built pic how it is now

[Cathode Ray]

do you have grid stoppers on output tubes? pls install 3k3 or some other value close to pin 5 on output tube sockets

Grid stoppers are 1.5k Ohms 2%.

[Cathode Ray]

also you can share as built pic how it is now

[Littlewyan]

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