voltage on 6v6 GT

[rafaelctt]

Hello! Do you consider these voltage values ​​in normal 6V6 GT?
(Seems a bit high to me according to Datasheet)
Thank you

[shooter]

math says ~~ 14W so it's
"in the ballpark"
my last PP self-biased was running 13+W

[CO_Hoya]

Look like typical Fender AA764 voltages when run on "modern" wall voltages.

[Lectroid]

Look like typical Fender AA764 voltages when run on "modern" wall voltages.

Looks the same to me.  I have a 'Deluxe Reverb' no-tremolo clone based on some old Hammond organ transformers.  The original organ schematic specc's the 6V6 tubes' to run at a B+ of 375V IIRC.   My amp runs at about 410V for the 6V6 tubes' A-node B+.  The tubes themselves are modern JJ's so I don't worry about them at all.

[HotBluePlates]

Do you consider these voltage values ​​in normal 6V6 GT?
(Seems a bit high to me according to Datasheet)

6V6 data sheets never seemed to get an update of their true voltage-handling capability (and plate current with screen volts over 250v) the way it was done with the 6L6-family of tubes.

While I would prefer lower voltage, the silverface Champs ran higher voltage.  Hardly legible, but there's 420v in the power supply just before the OT so nearly-420v on the 6V6 plate.  410v on the 6V6 screen, and around 24v at the cathode.

[rafaelctt]

Please I need help on biasing 6V6 GT. I have a very distorted signal at the speaker output. I don't know if the voltages on the board and screen are correct. The signal to the speaker, no signal is the one in the image. Thank you

[tubeswell]

math says ~~ 14W

I make it 12.5W (after deducting screen current).

[sluckey]

Your scope is showing a 16kHz signal. Disconnect the feedback loop from the OT secondary. Any better?

[rafaelctt]

This measurement is with the Feedback disconnected. When I connect it the measurement is the same.

[rafaelctt]

math says ~~ 14W

I make it 12.5W (after deducting screen current).

Please, could you explain this to me? Measure the screen current and multiply by voltage to obtain that power value? (W=VxI). And what result do you want to get?
Thanks

[sluckey]

Remove the tube. Any better?

[rafaelctt]

Remove the tube. Any better?

My question is, if it is what you have to do to obtain that power and what result is satisfactory once you do it?

[rafaelctt]

[sluckey]

My question is, if it is what you have to do to obtain that power and what result is satisfactory once you do it?

I don't understand this question, but now I see where that high frequency oscillation is coming from.

[tubeswell]

math says ~~ 14W

I make it 12.5W (after deducting screen current).

Please, could you explain this to me? Measure the screen current and multiply by voltage to obtain that power value? (W=VxI). And what result do you want to get?
Thanks

Plate dissipation is the product of plate current and plate voltage*
 *plate-to-cathode voltage = 366


Cathode current (17V/470R = 0.036A) is the sum of plate current and screen current. (So technically, we should subtract screen current for calculating plate dissipation). Screen current in a typical 6V6 at idle is about 2mA. So 36-2=34 (0.034A)


366V x 0.034A = ~12.5W

[pdf64]

You need to understand that parasitic capacitance links everything together, and how that would work with such a layout and lead dress to fatally compromise operation of all but the very lowest gain circuit.

[tubeswell]

^what sluckey and pdf64 said^


In an amplifier, any signal between the input transducer (guitar pup) and the output transducer (speaker) amounts to changes in current. From Michael Faraday, we know that changing current within any wire produces an electro-magnetic field* surrounding each wire, and if another wire is present nearby (i.e. ‘nearby’ enough to be within the same electromagnetic field surrounding the first wire), the changing current (‘signal’) in the first wire can be induced into the second wire through electromagnetic induction.


*and the higher the signal voltage (or the greater the signal current), the bigger the electromagnetic field will be.


Your layout is full of copious amounts of wires that can generate parasitic signal induction coupling between different parts of the circuit where you don’t want parasitic inductive coupling. Could be causing all sorts of feedback loops (including positive feedback resulting in oscillation). Tried and true amplifier layouts (like vintage Fender amps) were deliberately laid out in a particular way to avoid unwanted parasitic induction between different parts of the circuit. If you’re getting HF oscillation, you may want to start from scratch with your design and consider something along the lines of a more traditional vintage amp layout to avoid this. YMMV

[rafaelctt]

These are the measurements obtained

[tubeswell]

These are the measurements obtained

Quite a bit different to your first schematic?

Edit: oops, just noticed your current calculations. If that really is your screen current, then something is wrong with the tube or the power supply connection.

[pdf64]

-2.9V on the control grid is wonky.

[sluckey]

Screen grid drawing nearly twice as much current as the plate! Something very wrong! How did you determin the screen current? Be specific.

[rafaelctt]

Screen grid drawing nearly twice as much current as the plate! Something very wrong! How did you determin the screen current? Be specific.

Do you mean how did I measure?
I did it the same as the others, in series measuring current

[rafaelctt]

^what sluckey and pdf64 said^


In an amplifier, any signal between the input transducer (guitar pup) and the output transducer (speaker) amounts to changes in current. From Michael Faraday, we know that changing current within any wire produces an electro-magnetic field* surrounding each wire, and if another wire is present nearby (i.e. ‘nearby’ enough to be within the same electromagnetic field surrounding the first wire), the changing current (‘signal’) in the first wire can be induced into the second wire through electromagnetic induction.



*and the higher the signal voltage (or the greater the signal current), the bigger the electromagnetic field will be.


Your layout is full of copious amounts of wires that can generate parasitic signal induction coupling between different parts of the circuit where you don’t want parasitic inductive coupling. Could be causing all sorts of feedback loops (including positive feedback resulting in oscillation). Tried and true amplifier layouts (like vintage Fender amps) were deliberately laid out in a particular way to avoid unwanted parasitic induction between different parts of the circuit. If you’re getting HF oscillation, you may want to start from scratch with your design and consider something along the lines of a more traditional vintage amp layout to avoid this. YMMV

All wiring is done the same as my previous AC15. With ground bar. Grounded audio cable for signal, braided cable when alternating, etc...

[Williamblake]

The best way to measure screen current afaik is to measure the voltage drop across the screen resistor. If there is no screen resistor you can only guess and wish you had a screen resistor, not for measuring only but to cut down on noise, too.
The long and tight wiring in your amp is bad practice at least, some claim it worked but nobody really knows what they would call good enough.

[HotBluePlates]

... I see where that high frequency oscillation is coming from.

... signal between the input transducer (guitar pup) and the output transducer (speaker) ...

Your layout is full of ... coupling between different parts of the circuit where you don’t want ... coupling. ...

All wiring is done the same as my previous AC15. ...

The Input Jack is on the same end of the chassis as the output tube.  That creates high risk for coupling the largest signal in the amp back to the beginning of the circuit.

Wiring from early in the circuit crosses over places that have signal from late in the circuit.

So that means the overall layout creates the conditions for the amp to oscillate.  The oscillation very likely will not stop until the amp is laid out differently.

[rafaelctt]

How could I isolate the ground from the speaker? The entire aluminum chassis is grounded.

[sluckey]

How could I isolate the ground from the speaker? The entire aluminum chassis is grounded.

If the amp has a NFB loop from the OT secondary the speaker must be connected to ground. A bigger question might be "Why is the speaker jack located right beside to reverb jacks?" Can you show us a schematic?

[rafaelctt]

[tubeswell]

What’s the voltage drop across the 1k supply resistor between B+ (A) and B+ (B)?

[rafaelctt]

I have disconnected the speaker output from ground and without a jack inserted in input, I have made these voltage measurements. Should I measure something else? How could I only test the 6v6? I think the oscillation problem could be in it.
Thanks

[sluckey]

How could I only test the 6v6?

Pull V3.

In the power supply you show B+B is 397V, but at V4 pin 4 you show B+B to be 364V. One of those numbers cannot be right. I suspect that V4 pin 4 is actually connected to B+C. Double check that.

[rafaelctt]

How could I only test the 6v6?

Pull V3.

In the power supply you show B+B is 397V, but at V4 pin 4 you show B+B to be 364V. One of those numbers cannot be right. I suspect that V4 pin 4 is actually connected to B+C. Double check that.

Disculpe , es un error de trascripción. Es el mismo valor: 364V

[sluckey]

Look at the power supply. If B+B is 364v then B+C cannot be 361V!!!

[rafaelctt]

Look at the power supply. If B+B is 364v then B+C cannot be 361V!!!

a:408  b:400 c:369

[tubeswell]

Look at the power supply. If B+B is 364v then B+C cannot be 361V!!!

a:408  b:400 c:369

So you only have 8mA going through the 1k supply resistor. Between the screen and the 6 preamp triodes, there’s no way there can be 34.9mA on the screen (unless it’s sourcing more current from another power supply).

[sluckey]

Look at the power supply. If B+B is 364v then B+C cannot be 361V!!!

a:408  b:400 c:369

Everytime you post voltage readings they are different!
   
ARGH! I GIVE UP! 

[rafaelctt]

Look at the power supply. If B+B is 364v then B+C cannot be 361V!!!

a:408  b:400 c:369

Everytime you post voltage readings they are different!
   
ARGH! I GIVE UP! 

I think that perhaps it is from that oscillation that causes the voltage to change due to current variation in resistors r20, r21.
I measure again and I have voltages that oscillate very little a:404v b:396v c:367v.
New current measurement: anode current: 22.3 (with small variations). screen current: 28.8-30mA.

[tubeswell]

How fresh is your meter battery?


Your screen current measurement doesn’t add up with the current going through the 1k supply resistor (which is the PS resistor supplying the screen and the preamp).

[rafaelctt]

How fresh is your meter battery?


Your screen current measurement doesn’t add up with the current going through the 1k supply resistor (which is the PS resistor supplying the screen and the preamp).

Totally agree, tomorrow I will measure with a new battery and if not, I will look for a new tester

[rafaelctt]

I have obtained two more measurement equipment and I already think that everything is fine with values ​​a: 401v b: 360v and c: 287
anode current: 21mA , screen current: 30mA.
The problem is the frequency that is generated somewhere in the value circuit: 1/60usec: 16Khz.
Where could the generator of that frequency be found?

[shooter]

with screen current so high, the tube is a GREAT place to make 16Khz

[sluckey]

I have obtained two more measurement equipment and I already think that everything is fine with values ​​a: 401v b: 360v and c: 287
anode current: 21mA , screen current: 30mA.
The problem is the frequency that is generated somewhere in the value circuit: 1/60usec: 16Khz.
Where could the generator of that frequency be found?

In your illogical layout. Wires going everywhere. Components mounted helter-skelter. You broke almost every rule of good layout practices. It's as if you have never seen a successful, logical layout before.

[rafaelctt]

I have obtained two more measurement equipment and I already think that everything is fine with values ​​a: 401v b: 360v and c: 287
anode current: 21mA , screen current: 30mA.
The problem is the frequency that is generated somewhere in the value circuit: 1/60usec: 16Khz.
Where could the generator of that frequency be found?

In your illogical layout. Wires going everywhere. Components mounted helter-skelter. You broke almost every rule of good layout practices. It's as if you have never seen a successful, logical layout before.

Much of that design is not connected. I did it experimentally. To modify later. I have assembled the rest as I did the AC15 (first heating braids, then valve wiring and finally grounds and power. Transformers forming 90 degrees to each other. I could make it more pleasant but fundamentally I think I have followed all the AC15 guidelines. I would appreciate it that you tell me which part you see as the most conflictive on a functional level. Thank you

[rafaelctt]

with screen current so high, the tube is a GREAT place to make 16Khz

Could you tell me how I could lower that current?
Would it be enough to lower the voltage b?
Thanks

[sluckey]

Did you pull V3 to see if that would kill the 16KHz oscillation?

[rafaelctt]

Did you pull V3 to see if that would kill the 16KHz oscillation?

When I disconnect the grid (pin 5)
The screen current drops to 13 mA, with which I understand that the parasitic signal that is entering the grid is very high

When I remove the V3, the oscillation totally disappears

[sluckey]

Did you pull V3 to see if that would kill the 16KHz oscillation?

When I remove the V3, the oscillation totally disappears

I bet the screen current goes way down too. What voltages do you have on B+ nodes A and B when V3 is removed?

[rafaelctt]

Did you pull V3 to see if that would kill the 16KHz oscillation?

When I remove the V3, the oscillation totally disappears

I bet the screen current goes way down too. What voltages do you have on B+ nodes A and B when V3 is removed?

a: 404    b:395

[sluckey]

OK that's good. Now put V3 back in the socket and use a gator clip test lead to ground pin 7 of V3. Does this kill the oscillation?

[rafaelctt]

OK that's good. Now put V3 back in the socket and use a gator clip test lead to ground pin 7 of V3. Does this kill the oscillation?

YES!