SE Champ Design Problem

[Amp Kat]

This is my 1st SE design at least Cathode biased and have a freaky thing going on with the power supply. It's actually an exact replica of a Champ
except there is no tube rectifier. I'm using the CT to one winding of a 450-0-450 CT tranny rectified FWB 1N4007's. That goes to the exact topology of an AB764 Champ Design. The OT is a Mercury FTPO-5 SE and I'm using a 6V6GT tube. The problem is I'm getting 63 volts across the Cathode Resistor and the screen goes higher than the plate voltage. The plate voltage sits around 340 and the screen around 360. Anyone tell me whats going on. I've checked wiring and connections and can't find it and at the moment the load is off the preamp and still no change. What do you think ?

[Tonewood]

This is my 1st SE design at least Cathode biased and have a freaky thing going on with the power supply. It's actually an exact replica of a Champ
except there is no tube rectifier. I'm using the CT to one winding of a 450-0-450 CT tranny rectified FWB 1N4007's. That goes to the exact topology of an AB764 Champ Design. The OT is a Mercury FTPO-5 SE and I'm using a 6V6GT tube. The problem is I'm getting 63 volts across the Cathode Resistor and the screen goes higher than the plate voltage. The plate voltage sits around 340 and the screen around 360. Anyone tell me whats going on. I've checked wiring and connections and can't find it and at the moment the load is off the preamp and still no change. What do you think ?

I have a aa764 vibro. On quick search I didn't find a sch for the ab but mine may be similar to yours. What are the voltage drops across the OT primary and across the first dropping resistor? Mine is almost 15v across each & a bias of about 20v. Maybe check the actual resistance of the cathode resistor. If that is correct, up the first dropping resistor to get the plate and screen closer? The disconnected preamp would drop only a few volts across the 1k dropper.

Let us know what turns up-

[Amp Kat]

Yeah it's a AA764 and very close to the vibro with the only difference is a cap from the 220k to the Cathode that the Champ doesn't have and the negative feedback and vibrato circuit. I checked the Cathode resistor and it's right on 470 ohms. I did find a mistake I made but it didn't clear everything up. The heater CT wasn't grounded so it was running the tubes at 13 instead of 6.3 but now it's right on. Now the Cathode voltage is at 55 volts and obviously red plating like a biotch. I did up that 1st resistor to 3.3k and here are the new voltages. I also reconnected the preamp.

1st cap (OT Primary) 375 vdc
plate voltage (other side primary) 340 vdc
Screen Voltage 293
Cathode Voltage 57 volts

That Cathode voltage should be around 20 volts. What is yours reading across the Cathode ?

[madkatb]

Have you tried another tube in it?

[Enzo]

Well, it's a build, so it has never worked correctly, right? I mean as opposed to it used to work but has failed.

So we cannot yet assume it is put together right. Pull the 6V6, got zero volts on the cathode pin now? Got zero volts on the grid pin 5? Power off, is there the 220k to ground from pin 5? Power on, does grounding the grid pin 5 affect the cathode current? Does breaking the NFB affect the cathode current? Scope it, is the thing sitting there as a power RF oscillator? Does pulling the 12AX7 affect this?


If the tube is conducting hard, which 60v on the cathode resistor says it is, I don;t doubt the plate is falling below the screen, the resistance of the OT primary is dropping voltage if nothing else. WHat is the DC resistance of the OT primary. 280 ohms?

[J M Fahey]

I'm using the CT to one winding of a 450-0-450 CT tranny rectified FWB 1N4007's.

If that's so, you have a +HV of 1.41x450V=635V.
Please draw your PSU schematic.
Thanks.
Or the full amp, if possible.

[Amp Kat]

JM yes I had the wrong specs on the power supply. It's a 325-0-325 so I'm getting 325X1.414= 456 and it's pretty close as I'm getting 448 to the 1st cap. The power supply is the exact power supply in the AA764 Champ except for SS diodes and actually the whole amp is an exact replica of the Champ just in a different chassis and done with terminal strips instead of the fiber bd.

Enzo- Yes if I pull the tube I get zero on the Cathode. Yes after the tube settles in I get zero on pin 5 so the coupling cap is doing it's job but this is where I'm not sure of some things I'll explain in a bit. Yes 220k to ground is there. When I turn the power on and ground the grid it settles in nicely at 22 volts or at least where a normal Cathode biased amp should be at Quiescent which is around 22 volts across the Cathode right or is this not right ? The amp works and sounds right except a little hum and the signal is good and symetrical on the scope and no oscillations. My concerns are at idle this thing is roasting the tube so something can't be right. If I use my tone generator and run the signal through the Cathode voltage gets less as I crank the amp and at full tilt brings it down close to 25 volts so obviously the larger AC signal superimposed is causing the impedance on the grid to drop causing the voltage on the Cathode to swing less positive with respect to ground. I could always fix bias it and prolly make it work but just trying to learn something here as it’s baffling me.

The output transformer DC impedance is 378 ohms

[g1]

Where are you dropping the B+ ? You say you have 448V at the first cap, but 340V at the 6V6 plate, is this correct? If so you are dropping voltage somewhere before the 6V6 plate which is different from AA764.
I believe you want to be biased around 14W so you will need to increase your cathode resistor value. The AA764 shows 19V across the cathode resistor with 350 volts at the plate so you seem to be biased a little hot.

[Amp Kat]

Absolutely g-one and you are right that the only place I can see where I'm dropping the voltage is in the OT and this being my 1st SE build I'm not familiar with how they act so not sure if thats what they do are not but increasing the Cathode Resistor to 1k also increases the voltage across it so it went from 50 to 75. I had read that Fender changed the power trannys to the Princeton which were higher voltage but didn't change the Cathode resistor to compensate for the higher voltage but it didn't work for me and I've combed the amp for mistakes and there isn't nothing there as it's about as simple as you can get. I'm thinking maybe the OT may be causing it because in reality that's where the voltage is dropping. What do you guys think ?

[g1]

Yesterday you said the cathode voltage was 22v, today you said 50V, which is it? If it changes when you ground the grid, then replace the coupling cap.
Originally you said the voltage at the first cap was 448V, then you said it was 375V, which is it? The first cap is connected to the OT primary, correct?
Changing the cathode resistor will change the cathode voltage, but what is important is the current. 50V/470ohm is about .1A (or 100mA). 75V/1Kohm is .075A (or 75mA).
So you can see that even though the cathode voltage has increased, the idle current has decreased. Idle current multiplied by voltage across the tube (plateV minus cathodeV) will tell us the idle dissipation in watts. The target is around 14W which is the max for a 6V6. SE class A amps should be biased to idle at full power.
Can you upload the schematic?

[Amp Kat]

champ_aa764_schem.pdfchamp_aa764_schem.pdf

Yesterday you said the cathode voltage was 22v, today you said 50V, which is it?

It's 50 with the grid ungrounded and 22 with the grid grounded. Changing the coupling cap makes no diiference at all and it's doing the job. There is 180 volts on it and 0 on the other side to the grid.

Originally you said the voltage at the first cap was 448V, then you said it was 375V, which is it? The first cap is connected to the OT primary, correct?

When I 1st power it up it's at 448 then as the tube warms up and starts drawing current it trickles down to 375 at this point it's redplating so I just turn it off. And yes the Primary of the SE OT is connected to the 1st cap.

I agree with you on upping the Cathode resistor will drop the current but it's at a ratio and even though it's less current it's still 75 volts / 1k ohm = 75ma X 350 volts = 35 watts and for a poor little 12 watt 6V6 well you get the picture.

champ_aa764_schem.pdf

[Steve Conner]

The magic 8-ball says it may be a bad power tube. Do you have another 6V6 handy to try?

[Amp Kat]

Yes Steve I have and at this point I've tried several but I'm gonna try a 6v6 black plate tomorrow and see what happens. I've pretty much swapped all of the components that would cause this which is why it's frustrating.

[Steve Conner]

Ok, so here's what I'd do now.

Is that 220k grid resistor really 220k? Turn the power off, let all the caps discharge, pull the power tube and measure resistance between the grid pin of the tube socket and ground. Should be about 220k.

When you replaced the coupling cap, maybe the replacement was bad too, especially if they're some NOS things.

You ground the grid pin, the problem goes away. Ground it through your meter set to the most sensitive DC current range. What current do you see? It should be too small to measure. If it's not, find out where it's coming from.

When the cathode is at abnormally high voltage, the grid must be too. That's tube physics 101. So when measuring some crazy cathode voltage like 50V, you should see say +40V on the grid instead of 0. Do you see that? If not, why not?

[J M Fahey]

There is NO WAY a 6V6 passes 75 mA with -75V on its grid.
Nor with -50V for that matter.
Your grid simply is not connected to ground through those 220K.
Please draw the actual schematic of that power stage, pencil and paper in hand and with good light.
What you actually see.
Do not refer me to the Champ official schematic or any other.
Label parts, such as R1/2 , C1/2, etc, besides the actual part value, for clarity of reference.
Good luck.

EDIT. Let me add, yours is not exactly a Champ but:

Yeah it's a AA764 and very close to the vibro with the only difference is a cap from the 220k to the Cathode that the Champ doesn't have and the negative feedback and vibrato circuit.

Also:

The heater CT wasn't grounded so it was running the tubes at 13 instead of 6.3 but now it's right on.

does not make sense.
Whether you ground or not the filament center tap, the filaments still get the same voltage.