Weird problem or crappy PT?

[Chuck H]

I'm working on a Champ right now. The HV seemed to drag down too much with current so I checked everything in the circuit but found no joy. Then I checked the DCR on the HV secondary winding and read 450 ohms!?! 5V and 6.3V windings are fine. HV reads 480 without a load and 380 loaded. Which is about 40V below spec on the schematic. It doesn't run hot and the amp is "working" but the screen voltage is 8V higher than the plates and remains so until the output starts to compress (also weird). I'm scratching my head over how this could happen. Shouldn't it be low from some shorted windings or open with no continuity? Or might this just be how it is? What could cause high DCR on a fixed length of wire? Perhaps that winding was wound with the wrong gauge wire by some ninny and somehow passed inspection? Thoughts and experiences appreciated.

[Enzo]

Horses, not zebras...

Fender doesn't have a guy sit down and wind a transformer as he builds a Champ. It is REAL unlikely they made a run of transformers with the wrong size wire. But anything is possible.

Looking at a 5E1 for example, note the B+ goes to the screen, and then through the OT primary to the plate, so it will always sit there with higher screen voltage. Then compare the 5F1 where the B+ comes off two nodes, and the screen is 10k downstream from the plate, so it will be different there.

If we assume a 1k screen node resistor, and maybe 5ma current through it, that will drop 5v from the B+ supply, but the 6V6 draws a lot more plate current than 5ma, and through that 450 ohms it drops more than 5v, and so will have a lower voltage.

[Chuck H]

Thanks Enzo.

This is a silver face amp. So no choke. I'll try to get my head around it, but it seemed to me that since the plate and screen are both downstream of the winding DCR the screen voltage could only START from there. The OT primary has all of the plate current through roughly 290 ohms. And the screen only 5(ish)mA through 1k rail and a 1k at the socket. I figure this is where the problem is.

Still holding out for any speculation on the high HV secondary DCR.

[Jazz P Bass]

Quote" HV reads 480 without a load and 380 loaded. Which is about 40V below spec on the schematic"

Which schematic are we refering to?
The schematics that I have show 360V on the OT.

[Chuck H]

Sorry... This one:



A bit illegible. The voltages on the plate says 420 and the screen says 410. I think that was wishful thinking. I do know the plate voltage is right there for the same era Vibro Champ.

[Enzo]

I am probably senile, but don't follow you quite.

5ma through 1k to the screen drops 5v, so 420 becomes 415. Or correct for lower voltage loaded. Now what is the plate current through that 290 ohm winding? And so what is the drop across the transformer. Guessing say 30ma, gives me like 9v, so the plates will sit lower. No choke enters into it.

I have no idea what the resistance of that HV winding ought to be. If it is way high, I wouldn't think the wire itself changed resistance, but the winding wire has to be connected to the outside wires, and that connection could be faulty.

[Justin Thomas]

SF Champ, converted to Vibro. Mine is 320 ohms, with the recto tube pulled. Just cuz I was bored & hadn't seen inside an amp in a while, I checked...
Tranny # NSC125P1B, EIA606-030. Huh, I always heard mine was a 79... guess it's an 80, or the PT is replaced...

Justin

[Chuck H]

I am probably senile, but don't follow you quite.

5ma through 1k to the screen drops 5v, so 420 becomes 415. Or correct for lower voltage loaded. Now what is the plate current through that 290 ohm winding? And so what is the drop across the transformer. Guessing say 30ma, gives me like 9v, so the plates will sit lower. No choke enters into it.

I have no idea what the resistance of that HV winding ought to be. If it is way high, I wouldn't think the wire itself changed resistance, but the winding wire has to be connected to the outside wires, and that connection could be faulty.

Sorry Enzo. I hate trying to follow descriptions of circuits too. What I meant was that the plate and screen both follow the HV winding. So I figure the drop there is moot. So, from the 380V node there is the OT primary DCR at 290 ohms dropping 9V, so 30mA. from the same node the rail resistor is 1k between the OT/plate and the screen grid. That resistor is dropping 5V, so 5mA. I think that should make the screen grid 4V higher than the plate. In reality there is an 8V difference because I have an el34 in the power tube socket and the screen now has an additional 1k "screen resistor" off the HV node. I'm sure the math would bare it out If I had finite figures in front of me.

[Chuck H]

Thanks for checking that Justin Since the PT is a replacement I think it's fair to find the DCR is a little different (better?). I'm thinking now about what Jazz mentioned. The Champ right before this CBS design does indeed indicate 360V with the same PT.?. So I'll speculate (entirely unsupported) that the spec for that part number changed at some point and this seemingly high HV wind DCR may be correct for that 360V model, and this amp may have been built during a transition and has that PT.

It IS a class A amp so there isn't any appreciable sag when the amp is played clean. It doesn't seem that bad when the amp is clipping either actually. I've just never seen an HV winding with that much DCR. Usually MUCH less. So I was checking in on the matter. Your HV DCR is actually high by my experience too. So that's what leads me to think this may be "normal".

[pdf64]

Hammond helpfully provide winding resistances; the Champ types seem to be around 330 ohms across the HT Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Power Transformers

[Chuck H]

Hammond helpfully provide winding resistances; the Champ types seem to be around 330 ohms across the HT Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Power Transformers

Thanks. That supports Justin's finding to be sure. The only mystery now is whether the PT I have is a representation of the previous 360V spec for the same part number.?. I think it must be. When Hammond "clones" those transformers I doubt they sample a range of examples. So it's sort of a crap shoot as to which year/variant is represented by their "clone".

Justin, did you measure your plate voltage when you had the amp open? I'd be interested to know if the example with the 320/330R HV winding equates to Champ models that have over 400Vp.

[Jazz P Bass]

Question: Does the amp work o/k?

Not too sure what you are fixing.

[Chuck H]

Question: Does the amp work o/k?

Not too sure what you are fixing.

Well, sort of "runs ok". I'm not fixing anything. I was asking about my findings to see if I needed to. My reference point is limited to bench testing and two previous SF Champs I worked on many years ago. I feel like the amp should be a little louder even though I measure 5W output. I replaced the speaker with an efficient 10" and even with that it's not matching my memory for volume. But memories (and ears) have a way of changing with time The screen voltage remains higher than the plate voltage all the way up until the wave form starts to compress and even played clean there is, what I perceive to be, more intermodulation distortion than I remember for these amps. The OT tests fine. Well, for the crappy little walnut sized thing that it is.

More to the point of your inquiry. The power supply isn't giving me any trouble. PT runs warm, not hot, with a long burn in. All other voltages, 5V and 6.3V windings are a little high as should be expected.

[Justin Thomas]

Not recently, but yes, it's about 425, last time I checked. I put.up a.thread here asking why so high, because the scheme said 360... then someone kindly pointed out that I was looking at a BF schem, and they posted the SF schem... mystery solved. What's the date code & part # on your PT? Maybe it was replaced with an old junker?

Justin

PS - Mine can hang with a polite drummer I n a sort of "punk jazz" way... never measured, but it's not a wuss!

[Jazz P Bass]

Quote:"I replaced the speaker with an efficient 10""

You did use a 4 ohm replacement, correct?