New Output transformer - Need to Change Tube Bias ?

[HaroldBrooks]

I have a suspicion it might, but please school me on this, as my understanding of output transformers is limited.

Recently I replaced a stock National 1224 Valco output transformer with this one : http://www.classictone.net/40-18002.html the original transformer was a 2 ohm with twin 4 ohm speakers running in parallel. This transformer is an 8 ohm so I run both 4 ohm speakers in series to make the output ohms match.

All seems well and the amp sounds great, but I did notice the power transformerget a bit hotter than I remembered previously at an idle, as if the idle current draw had increased. Used to be 118 degrees Fahrenheit and now leveled off around 125 degrees at the hottest point. This is within my statistical measurement error I am sure based on where I tested the temp and the room temperature to start, but it still got me to thinking that maybe the 6.6k to 8 ohm specs of this transformer might be different than the original transformer and caused a change in output tube current draw.

I've seen a lot of other push pull 6V6 type transformers starting around 7.5k ohms.

Does this make a difference in plate voltage / current enough so that the output tubes should be re-biased ? I don't see any red plating at all, but I know the tubes in this amp are biased up to the max plate dissipation.

Thanks for your help ! I would like to understand output transformers better than I do currently !

[Helmholtz]

Output transformer dissipation at idle depends solely on idle current(s) and DCR of the primary halves. Transformer temperature depends mainly on thermal contact to chassis and to a lesser extent on its surface area, ventilation and ambient temperature

[HaroldBrooks]

Output transformer dissipation at idle depends solely on idle current(s) and DCR of the primary halves. Transformer temperature depends mainly on thermal contact to chassis and to a lesser extent on its surface area, ventilation and ambient temperature

Sorry I probably wasn't clear and mixed two concepts together. The new output transformer is running very cool, it's the power transformer that seems to be running a bit hotter at an idle.

Just wanted to know if there was a power tube voltage or current interaction resulting from ohm changes in an output transformer. Never did this type of change before.

Thanks for your help Helmholtz.

[g1]

Did you have a number for the output tube idle current before? Has it changed?
Like Helmholtz said, differences in DCR on primary side could slightly affect idle current.

[Helmholtz]

Just wanted to know if there was a power tube voltage or current interaction resulting from ohm changes in an output transformer. Never did this before.

Certainly not at idle. Except if there is oscillation caused by wrong OT wiring (meaning wrong phase if the amp employs global NFB) of the new OT.

[HaroldBrooks]

Certainly not at idle. Except if there is oscillation caused by wrong OT wiring (meaning wrong phase if the amp employs global NFB) of the new OT.

I do have a confession to make that may explain a little more... There is a "Fourth" original Green primary side wire on this amp that get's hooked to the positive green speaker terminal...

Not sure what that wire does, but on the first go around I just wrapped it around the post and forgot to solder it ! Caught the mistake and soldered the wire, and that seems to have dropped the temp back.

You are Sharp Helmholtz !

Here's a pick of the Fourth wire on the right of the terminal strip. It's not on the schematic for this amp (at least not being connected to the green speaker terminal :

Is that wire for Negative feedback ? I can't see it going straight to ground like the schematic indicates. I haven't traced it yet. I may have caused the issue with that !



Schematic

https://elektrotanya.com/valco_natio.../download.html

Thanks for the help !

[HaroldBrooks]

Did you have a number for the output tube idle current before? Has it changed?
Like Helmholtz said, differences in DCR on primary side could slightly affect idle current.

I did measure it when I first got the amp, but it didn't get logged as it was ok and I was in a rush... mistake not to do that.

[mozz]

You can make a simple bias tester out of a old tube, a tube socket, and a 1 ohm resistor. The more you work on amps, it will come in handy.

A simple trick to reduce a tiny bit of current is to wire up a LED pilot lamp or use a 120v pilot lamp instead of the usual 150ma 6.3v lamp. Every bit helps in a old amp.

[Helmholtz]

I haven't traced it yet.

Please do! (Both ends of the green wire.)

Do you have a scope?

What was wrong with the original OT?

[Helmholtz]

I did measure it when I first got the amp, but it didn't get logged as it was ok and I was in a rush... mistake not to do that.

Sorry, I did not mean to say that OT primary DCR influences idle current. It does not.

[Chuck H]

There is a "Fourth" original Green primary side wire on this amp that get's hooked to the positive green speaker terminal...

Wait, what? When you say "primary side" it sounds like your referring to leads from the OT primary.?. Not to mention that there should only be two wires from the OT to the speaker, not three or four. Now, I'm no mathematician but you seem to have two wires too many involved, unless...

If the amp uses a "field coil" speaker then the extra wires would be that circuit. But that amp doesn't use a field coil speaker. It uses a pair of 6x9's, right? Not to mention that if it were a field coil wiring there would be four connections to the speaker.

Not sure what that wire does, but on the first go around I just wrapped it around the post and forgot to solder it ! Caught the mistake and soldered the wire, and that seems to have dropped the temp back.

I'm baffled and intrigued by this now. So the new OT had an extra green lead from the primary that you connected to the positive speaker terminal!?! No way. I've been trying to look up the speaker wiring for these amps and finding no clear answers. But here's some images. I zoomed up the speaker terminal/OT connection and I believe I see the "extra wire" is coming from the amp and not the OT, right?

[HaroldBrooks]

Wait, what? When you say "primary side" it sounds like your referring to leads from the OT primary.?. Not to mention that there should only be two wires from the OT to the speaker, not three or four. Now, I'm no mathematician but you seem to have two wires too many involved, unless...

If the amp uses a "field coil" speaker then the extra wires would be that circuit. But that amp doesn't use a field coil speaker. It uses a pair of 6x9's, right? Not to mention that if it were a field coil wiring there would be four connections to the speaker.



I'm baffled and intrigued by this now. So the new OT had an extra green lead from the primary that you connected to the positive speaker terminal!?! No way. I've been trying to look up the speaker wiring for these amps and finding no clear answers. But here's some images. I zoomed up the speaker terminal/OT connection and I believe I see the "extra wire" is coming from the amp and not the OT, right?

No Field coil on this amp. Three wires from ouput transformer to the amp, and two from the output transformer to the speakers... But as you see there is another wire, a green one that runs from one of the speaker terminals back to the amp, and I don't think it's too a ground, because there is a ground tang on the terminal strip right there and the green wire is not connected to it but to another tang, so my guess is it's some type of NFB, and I've never seen that on my other Valco amps, and I'm surprised and a bit intrigued it would be used on an old valco amp like this, they must have been copying Fender on this one.

Sorry if I confused everyone. The output transformer is the typical 3 on the power tube side, and two on the speaker side.
The difference with this amp is that additional green wire that runs from the speaker terminal back to the amp (never touches the output transformer, but runs with the same spiraled wires.

You got the right picks, that it, and here's how it looks at the terminal strip on the amp (the green wire on the most right) :


The chassis on this one is a pain in the butt to take out, and I had it out twice already to make changes to filter caps and again for cathode bypass cap and bigger coupling caps, hence I wanted to see first if anyone knows what that wire is attached to at the amp. I must be a NFB loop, otherwise my guesses are done.

[HaroldBrooks]

Please do! (Both ends of the green wire.)

Do you have a scope?

What was wrong with the original OT?

I have to get a scope ! The one end of the green wire is to one of the first speaker terminal, the other end is to the strip that connects the output transformer to the plates and the center tap connection.

I will have to take the chassis out again or maybe I can use a dental mirror to see where the green wire is connected to.

The original OT transformer works just fine, it's just that it is riveted to the first speaker, and while that speaker is still working, the cone has seen better days with a couple of cracks around the suround. I didn't want to upset the originality of the amp by pulling the output transformer off the old speaker.

I know it's silly at this point, but it's something I didn't want to do just the same incase I wanted to sell the amp again.

I bought a slightly larger transformer (20 watt) in hopes that it would add some more late stage bass to the 6 x 9 vintage 47 replacement speakers, and it may have... or it may have done nothing. LOL. Hey it made me feel better anyway !

I wish Valco would have indicated that green wire on the schematic !

[HaroldBrooks]

UPDATE :

Ok, left feeling pretty stupid with this one !

I just checked the green wire attachment to the amp again using an inspection mirror and a flashlight, and I believe it's connected to another short wire soldered to the same terminal strip tang and that short wire grounds to the metal of the rectifier tube socket (in effect grounding to the chassis in close proximity).

As another test, I connected an alligator lead from the speaker terminal with the green wire attached, directly to the chassis ground and it doesn't change a thing in the gain or sound.

So I no longer believe the mysterious green wire is part of a negative feedback loop, but just a chassis ground for the speaker.


Question : Are speakers generally grounded through the chassis ? May sound stupid considering I've worked on tube amps for a few years now, but I always thought the speaker voltage was just carried through the low volt side of the transformer after the excitation of the ouput transformer in a loop, and that the voltage was insulated from ever interacting with the chassis ground !


As to the output transformer temp rise... well let's just say I made a mountain out of a molehill on that one. Everything is ok after playing the amp for about 1 hour straight, transformer a cool 118 degrees Fahrenheit.

Boy, I still have a lot to learn about amps !!!

[Enzo]

You can see in your photos that the secondary of the OT is wired directly to a speaker, and then two wires put the other speaker in parallel. That is sufficient for operation, after all, a hand held transistor radio needs no ground. But they do want the speaker frames grounded for various reasons, so they ran a wire to the speaker frame from the amp chassis.