AA764 Vibro Champ build (power supply and grounding)

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,001
Location
Salt Lake City
Just on other build question, how important is it to place the screen resistors on the tube socket? I like to use the unused pins of the 6V6 socket, but with a grip stopper, a screen resistor and 2 100Ohm resitors to the cathode it will get pretty cramped there. I was considering placing the screen resistor on the circuit board.

Board is fine, though the ideal stopper spot is right on the pin. It's not that hard; the secret is thinking vertically. Here's my 5E3 with grid stoppers and screen resistors on both sockets, plus artificial CT on the right socket:

1641397268367.png
 

MichelB

TDPRI Member
Joined
Dec 30, 2020
Posts
65
Age
43
Location
Dordrecht, The Netherlands
The build is finally underway.

Most of the parts have been delivered, amp knobs unfortunately made for 6mm shafts :(

First time building with eyelet board, have to say, this works nicer then PCB soldering.
20220114_173413.jpg
Hopefully this weekend more soldering....

Michel
 

MichelB

TDPRI Member
Joined
Dec 30, 2020
Posts
65
Age
43
Location
Dordrecht, The Netherlands
Gentlemen,

While trying to fit the JJ cap can into this chassis I've noticed that the solder lugs are dangerously close to the edges of the chassis. There is upwards of 300V on these lugs and mounting them with just a mm of room to the chassis, while only secured by a clamp system makes me feel uncertain.

The chassis hole is 28.5mm, the cap can has 40mm diameter, the solde lugs have around 27mm outer diameter.

How have you solved this? Enlarging the chassis hole? Bending the cap can lugs inwards?

Just putting axial caps inside really feels like better solution, but what to do with the ugly hole in the chassis?
 

Linkjr

Tele-Meister
Joined
Aug 16, 2017
Posts
205
Age
52
Location
Uk
I think i have the same chassis as you i enlarged the hole, its a bit rough but it was a risk i didnt want to take, i also thought the bolt holes for the PT on my chassis were too close to the cutout.
 

MichelB

TDPRI Member
Joined
Dec 30, 2020
Posts
65
Age
43
Location
Dordrecht, The Netherlands
The amp is sort of put together. The clamp for the JJ cap can is in backorder, so I had to improvise, also the tone stack raw switch is now made in a temporary style.

20220126_143005.jpg

The amp works and the first impression was a very big smile. However there are some things not completely correct.

First some changes to original layout.
- There is an additional power filter stage before B+, with 470 Ohm resistor.
- I've placed an extra 20uF Cap on the circuit board. This is the preamp power supply and linked the ground to the preamp bus.
- preampbus is connected to input2, only to chassis.

1: the amp is not completely quiet, there is hiss. The grounding is now done via a ground bus (although my solder iron is only 35Watts I had difficulties flowing the solder on my ground bus)
2: The amp is too clean. With the tonestack in place I can't get any distortion. If I break the connection from bass pot to ground there is a huge increase in volume.
3: The amp doesn't seem loud enough, full volume with tonestack is a nice living room volume, with tonestack defeated I'll get frowns from the neighbours though. The fact that there is so much more volume change on full volume lets be to believe that with the tonestack I'm not fully driving the powertube.

After this I've started some measurements in the amp.

B+ = 370V
B1+ = 359V
B2+ = 336V

Voltage drop over primary OT = 17V
Prim OT resistance = 403 Ohm
Voltage drop over Cathode resistor is 21.8V
Cathode resistor = 470 Ohm
The output tube is biased a bit hot with 13.9 Watt plate dissipation.
Besides from that, the screen voltage is around 4V higher than plate voltage.

Now onto the strange measurements around V1.

Plate voltage V1a = 203V
Cathode voltage V1a = 1.69V
Plate voltage V1b = 278V
Cathode voltage V1b = 3.75V

The second stage is drawing much more current and I feel that this might be where things are going off. Any idea where the problem might be? Plate resistors are in spec (100k) cathode resistors as well 1.5k)

PS: swapping the V1 tube did not make any change.
 
Last edited:

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,591
Location
Cincinnati, OH, USA
Plate voltage V1b = 278V
Cathode voltage V1b = 3.75V

The second stage is drawing much more current and I feel that this might be where things are going off. Any idea where the problem might be? Plate resistors are in spec (100k) cathode resistors as well 1.5k)

PS: swapping the V1 tube did not make any change.

I assume V1B is the stage you we worried about. Yes, it looks off, but it appears to be too little current, not too much. I would suspect the cathode resistance is too high. Measure resistance from the cathode pin on the socket to chassis. Does it come out to 1.5-1.6K? Confirm the value of that NFB tail resistor. Did that by chance end up 4.7k instead of 47? Those low ohm (<100) resistors are really easy to misread.

The other option is that the plate resistance is too low and it indeed would draw more current then, but the cathode resistor should keep it relatively in check. My sense tells me you have the plate correct and the cathode is off.

Looked at your photo—a lot of those eyelets look really low on solder. You might go back and touch those up.
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,297
Age
62
Location
Finland
Just measure voltage losses over voltage drop RC filter resistors and their resistances and calculating currents is very easy. Voltage drop divided using resistance. They become milliamps dividing using 1000

Depending pre amp tride gains explain how anode voltages settle!

You can verify anode, screen and cathode currents as well. It is called Kirchoff Current Law :)

If idle voltages/currents do not match there possibly is leaking coupling capacitor which scews circuit current or high frequency oscillation?
 
Last edited:

MichelB

TDPRI Member
Joined
Dec 30, 2020
Posts
65
Age
43
Location
Dordrecht, The Netherlands
I assume V1B is the stage you we worried about. Yes, it looks off, but it appears to be too little current, not too much. I would suspect the cathode resistance is too high. Measure resistance from the cathode pin on the socket to chassis. Does it come out to 1.5-1.6K? Confirm the value of that NFB tail resistor. Did that by chance end up 4.7k instead of 47? Those low ohm (<100) resistors are really easy to misread.
If I make my calculation V1b is dissipating 0.69Watts (3.75/1500*275) while V1a is dissipation a great deal less.

Both cathode resistors of V1 are 1.5 kOhm.
 

peteb

Friend of Leo's
Joined
Apr 25, 2003
Posts
4,793
Location
Cascadia
You are right, the current on v1 is right and the current on v2 is too high.

the cathode voltage should be in the range of 1.5-2.0 volts.

1.5/1500 = 1 mA
2/1500. = 1.33 mA

3.75 / 1500 = 2.5 mA


your bias is high (cold), -3.75 versus -1.5.

that bias should be choking things down but it is not.




I am guessing that the current in v2 has an easier path to ground than it is supposed to.


and you have to consider the possibility of a faulty tube.
 
Last edited:

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,591
Location
Cincinnati, OH, USA
Why do we think it's not choking things down? The voltage drop over the 100K plate resistor is only 58V which indicates 0.58mA of current. Either the plate resistance isn't actually 100K (it's lower), or the cathode resistance isn't 1.5K (it's higher). Those are the only two options that I can see. Since there's more to mess up on the cathode, I would guess the cathode is off, but I could certainly be wrong on that end.
 

David Barnett

Doctor of Teleocity
Ad Free Member
Joined
Mar 17, 2003
Posts
17,330
Age
65
Location
The Far-Flung Isles of Langerhans
So that is my answer why I propose to lower B+, i dont need the headroom. Also feeding the OT and plated from 20uF rather than 40uF is because of that. However, just rewiring the cap can gives me options to tailor the sound later on.

The change to 40uF also audibly reduces hum.

In my opinion that's why Fender did it in the first place. And they did it pretty early in the BF run, my '65 Champ-Amp came with 40-20-20.
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,591
Location
Cincinnati, OH, USA
If I make my calculation V1b is dissipating 0.69Watts (3.75/1500*275) while V1a is dissipation a great deal less.

Using the plate load resistor for the calculation (58/100000*275) I get only 0.16W. So do you see why I think even though you have the right value resistors, something is still off? Measure the actual resistance from cathode pin to chassis and confirm it's correct. Measure the actual resistance from plate pin to B+3 supply node and confirm it's correct.
 

peteb

Friend of Leo's
Joined
Apr 25, 2003
Posts
4,793
Location
Cascadia
Why do we think it's not choking things down?
The normal bias, -1.5 volts, and the regular cathode resistor, 1500 ohms, should result in 1 mA of current. 1.5 volts on a 1500 ohm cathode is also 1 mA by ohms law.

what I am saying is that -3.75 volts of bias is a far colder barrier to current flow than -1.5 volts, assuming a 1500 ohm cathode resistor.


we can see the current is higher, but -3.75 volts, grid minus cathode, should prevent current from flowing a lot more than the -1.5 does.
 

MichelB

TDPRI Member
Joined
Dec 30, 2020
Posts
65
Age
43
Location
Dordrecht, The Netherlands
Using the plate load resistor for the calculation (58/100000*275) I get only 0.16W. So do you see why I think even though you have the right value resistors, something is still off?
That is exactly what bugs me. Voltage drop over plate resistor is limited, hinting low current draw, yet voltage drop over cathode resistor gives me a huge dissipation???

If I check the voltage drop over the 10k dropping resistor between B1+ and B2+ I also see a "normal" current draw for V1.

I did check the cathode resistor, but actually measuring from pin to chassis is a better check... I'll report with findings.
 

MichelB

TDPRI Member
Joined
Dec 30, 2020
Posts
65
Age
43
Location
Dordrecht, The Netherlands
I did check the cathode resistor, but actually measuring from pin to chassis is a better check... I'll report with findings.
Both cathode resistors measure 1490 Ohm, measured from tube pin to ground bus. The plate resistors both measure 100k, measured from pins to power node on final filter cap.

Faulty tube was my first thought ss well, changing that didn't accomplish anything usefull :(
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,591
Location
Cincinnati, OH, USA
Both cathode resistors measure 1490 Ohm, measured from tube pin to ground bus. The plate resistors both measure 100k, measured from pins to power node on final filter cap.

Faulty tube was my first thought ss well, changing that didn't accomplish anything usefull :(

Very well then. Sounds like all is in order there. Let’s consider a faulty solder joint.

These in particular look suspect.
1643226107527.jpeg
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,591
Location
Cincinnati, OH, USA
Whoa… I didn’t even realize the tremolo connects to the cathode until right now. Sorry about that. 😖 Repeat your measurement with the intensity control turned completely down. Any change?
 
Last edited:




Top