Weird problem or crappy PT?

[pdf64]

The higher than normal winding resistance may facilitate a few more uF of reservoir capacitance, as it will act to restrict the peak plate current.
Regarding the plate and screen grid voltages, what is the resistance of the OT primary?
If that's higher than normal too, it would tend to have that effect.

Maybe trying different brands of 5Y3 could help to tweak the HT voltage, or possibly a 5R4 if it can be squeezed in?

[g1]

I just picked up a JJ and an older sovtek 5Y3. Running in an amp with a pair of 6V6. Also compared a UOS. The sovtek put up a 40V increase in B+. The JJ agreed with the UOS.

[Chuck H]

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

You did use a 4 ohm replacement, correct?

Yep.

[Chuck H]

The higher than normal winding resistance may facilitate a few more uF of reservoir capacitance, as it will act to restrict the peak plate current.
Regarding the plate and screen grid voltages, what is the resistance of the OT primary?
If that's higher than normal too, it would tend to have that effect.

Maybe trying different brands of 5Y3 could help to tweak the HT voltage, or possibly a 5R4 if it can be squeezed in?

I do have a 5V4 on hand... But I'm saving that for me and there's no budget in this project left after the speaker (which cost me $21 for shipping

I don't see how the lower voltage is a deal breaker for volume. It's not THAT low. I think I'd replace that pathetic OT if I could though.

[Chuck H]

I just picked up a JJ and an older sovtek 5Y3. Running in an amp with a pair of 6V6. Also compared a UOS. The sovtek put up a 40V increase in B+. The JJ agreed with the UOS.

I've read, here, long ago, about some Sovtek 5Y3's on the market that weren't really 5Y3's and folks were having some trouble with high plate volts because of them.

[elipsey]

I've read, here, long ago, about some Sovtek 5Y3's on the market that weren't really 5Y3's and folks were having some trouble with high plate volts because of them.

I have heard this as well. I have one here, and it gives me higher Vp then any of 5Y3 or 5U4 series recto tubes in my junk drawer. It seems have about the same voltage drop as 5AR4 in my small push pull amps. Sample size n=1, so this observation might be useless.

I have found that building new clones of small fender designs seems to result in way less sag, and thus higher Vp than I expect, in a way that doesn't seem accounted for by wall voltage difference; that is the slope of voltage/load is shallower than I expect. I seem to recall reading (perhaps from Mark Baier of Victoria, but I can't find original source, so maybe it wasn't him?) that most new clones of those PT's usually sag much less then the originals. This seems to me to imply that the originals were higher impedance or DCR, and a bit inefficient. Sorry if I'm repeating unattributed BS here

[Enzo]

Chuck, your plate and screen voltages come from the same supply node, yes, but you are ignoring the resistance of the OT winding. That is why the plate voltage is lower than the screen. The plate voltage does not come from teh B+ node directly it flows through that resistance. Yes, the voltage at the plate end of the 1k resistor will be highe3r than teh screen voltage, but not at the tube plate due to the widning resistance.

[ThermionicScott]

A silverface Champ running at 380V rather than 420+ is something to celebrate, not "fix."

Similarly, it's no big deal if the screen idles a couple volts above the plate. Hardly any screen current flows at idle, so the screen dissipation is negligible. But if the spirit moves you, replacing the 1K/1W with a 1.5K would help filter out hum going to the screen and further down the line...

- Scott

[Chuck H]

A silverface Champ running at 380V rather than 420+ is something to celebrate, not "fix."

Similarly, it's no big deal if the screen idles a couple volts above the plate. Hardly any screen current flows at idle, so the screen dissipation is negligible. But if the spirit moves you, replacing the 1K/1W with a 1.5K would help filter out hum going to the screen and further down the line...

- Scott

The screen voltage remains higher than the plate voltage all the way up until the wave form starts to compress...

This is my problem. The screen remains high to the point of clipping. By my figures it will take about 3.3k between the OT and screens in the HV rail to keep the screen below the plate. In a class AB amp I would be somewhat concerned about the sag. I don't know that this is a problem with class A.

Because actually playing the amp seems to be distorting early in volume regardless of my 1k test putting out 5W I decided to sweep the frequency on the test. Distortion is increasing with frequency. The more I increase the frequency the quicker the onset of distortion. At 5kHz the amp can only put out about 3/4 watt undistorted. There's no way to get my 5W across the speakers range. So the amp is limited to a very low volume for clean tones. I've read about people having trouble with low volume regarding Champs. This is the first time I'm experiencing it for myself. Sweeping the OT proves it out to be a real loser for performance.

I need to talk to my guy and convince him it needs to go. So far he's been alright but a mutual acquaintance tells me he's tighter than two coats of paint and I should get money in hand before handing the amp over. No problem... Been there.

[ThermionicScott]

This is my problem. The screen remains high to the point of clipping. By my figures it will take about 3.3k between the OT and screens in the HV rail to keep the screen below the plate. In a class AB amp I would be somewhat concerned about the sag. I don't know that this is a problem with class A.

Because actually playing the amp seems to be distorting early in volume regardless of my 1k test putting out 5W I decided to sweep the frequency on the test. Distortion is increasing with frequency. The more I increase the frequency the quicker the onset of distortion. At 5kHz the amp can only put out about 3/4 watt undistorted. There's no way to get my 5W across the speakers range. So the amp is limited to a very low volume for clean tones. I've read about people having trouble with low volume regarding Champs. This is the first time I'm experiencing it for myself. Sweeping the OT proves it out to be a real loser for performance.

I need to talk to my guy and convince him it needs to go. So far he's been alright but a mutual acquaintance tells me he's tighter than two coats of paint and I should get money in hand before handing the amp over. No problem... Been there.

I'm not surprised that the DC component of the plate voltage would always be a couple volts lower than the screen voltage in this design, but so what? It's only when the plate voltage swings close to the diode line that the screen dissipates any meaningful power. If you're worried about that, then adding a screen stopper would limit the instantaneous dissipation nicely, just as Fender did on most amps bigger than a Champ or Princeton.

I think you're on the right track with the OT, though. Got a spare you can wire in temporarily?

[Chuck H]

It's only when the plate voltage swings close to the diode line that the screen dissipates any meaningful power. If you're worried about that, then adding a screen stopper would limit the instantaneous dissipation nicely, just as Fender did on most amps bigger than a Champ or Princeton.

I think you're on the right track with the OT, though. Got a spare you can wire in temporarily?

I did add a 1k stopper to the circuit. Unfortunately I don't have a suitable OT on hand.

Edcor makes a couple of models for just $19. A stellar price, but they have a really crappy shipping policy so it costs $21 for shipping!?! I called them about that and asked why they couldn't just ship USPS ground. The semi illiterate lady on the phone babbled about problems they've had and finished with "So now we just charge a three pound shipping weight minimum. Sorry we can't help you." Really? She never addressed why they won't ship at a level below "priority" or why their website automatically allocates one of three shipping "options" (all over $20) but doesn't let the buyer actually opt for their choice of the three. A really cumbersome affair overall that made me want to shop elsewhere. But I've historically been very happy with Edcor products and service. Those would have been bigger, more expensive iron where the shipping cost seemed appropriate. They even made a primary voltage modification to one of their models for me at no cost! Oh well. You take the good with the bad I spose. In the end $40 for the OT and shipping is still the best price for a 10W unit twice the size of any "Champ" OT so I'll probably buy it.

[Chuck H]

Moving to a new post for this next matter...

I had a funny thing happen. I'm not the most technical guy on the forum by a stretch, so I didn't see any immediate problem with trying a 12v/5W zener between the screen filter node and the screen stopper resistor. This seemed to work fine, until I drove the circuit into clipping. Then the plate current dropped and the screen current increased dramatically turning the 1k screen grid resistor a nice golden brown. Why did this happen?

[ThermionicScott]

Edcor makes a couple of models for just $19. A stellar price, but they have a really crappy shipping policy so it costs $21 for shipping!?!

That sucks!

It occurred to me later on, while in a meeting, that you wouldn't need a "Champ" OT to tell if it's the problem. If you wire up some generic 4KΩ 40W OT and get more than a watt or two out of it, that should be evidence enough, right?

[ThermionicScott]

Moving to a new post for this next matter...

I had a funny thing happen. I'm not the most technical guy on the forum by a stretch, so I didn't see any immediate problem with trying a 12v/5W zener between the screen filter node and the screen stopper resistor. This seemed to work fine, until I drove the circuit into clipping. Then the plate current dropped and the screen current increased dramatically turning the 1k screen grid resistor a nice golden brown. Why did this happen?

What's the rating of your 1kΩ screen stopper? An EL34 (I keep forgetting that that's what you have in the amp) can be expected to draw more screen current than a 6V6.

P.S. I dug through some EL34 datasheets, and they expected a healthy voltage drop across the OT primary/ies. Supply voltage of 265V, and only 250V at the plate in their single-ended example. No wonder.

[Chuck H]

What's the rating of your 1kΩ screen stopper? An EL34 (I keep forgetting that that's what you have in the amp) can be expected to draw more screen current than a 6V6.

The stopper is a 3W unit. 5W might be a better choice, but shouldn't be necessary. I don't think that was the problem as this resistor has been fine until the zener was placed behind it in the circuit. When clipping hard the plate rose 12V and the screen dropped 40V! I didn't have the opportunity for further testing because, seeing that this was a clearly dangerous circuit, I stopped experimenting with it.