Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rafaelctt on January 04, 2023, 04:54:05 am
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Hello! Do you consider these voltage values in normal 6V6 GT?
(Seems a bit high to me according to Datasheet)
Thank you
(https://i.postimg.cc/CLwLYDbq/6v6-gt-medidas.jpg)
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math says ~~ 14W so it's
"in the ballpark"
my last PP self-biased was running 13+W
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Look like typical Fender AA764 voltages when run on "modern" wall voltages.
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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.
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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 (https://el34world.com/charts/Schematics/files/Fender/Fender_champ_cbs.pdf) 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.
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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
(https://i.postimg.cc/mgVpBFbw/se-al-altavoz.jpg)
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math says ~~ 14W
I make it 12.5W (after deducting screen current).
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Your scope is showing a 16kHz signal. Disconnect the feedback loop from the OT secondary. Any better?
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This measurement is with the Feedback disconnected. When I connect it the measurement is the same.
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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
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Remove the tube. Any better?
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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?
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(https://i.postimg.cc/RZt6tqVz/939f132f-d8e0-46d8-8772-05c0b3896878.jpg)
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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.
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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
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(https://i.postimg.cc/RZt6tqVz/939f132f-d8e0-46d8-8772-05c0b3896878.jpg)
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.
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^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
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These are the measurements obtained
(https://i.postimg.cc/NjSt23Q9/medidas-totales-en-6-V6.jpg)
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These are the measurements obtained
(https://i.postimg.cc/NjSt23Q9/medidas-totales-en-6-V6.jpg)
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.
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-2.9V on the control grid is wonky.
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Screen grid drawing nearly twice as much current as the plate! Something very wrong! How did you determin the screen current? Be specific.
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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
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^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...
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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.
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... 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.
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How could I isolate the ground from the speaker? The entire aluminum chassis is grounded.
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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?
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(https://i.postimg.cc/0jd2VykB/ESquema-actual.jpg)
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What’s the voltage drop across the 1k supply resistor between B+ (A) and B+ (B)?
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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
(https://i.postimg.cc/L6rmtp4F/Medidas-8-1-2023.jpg)
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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.
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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
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Look at the power supply. If B+B is 364v then B+C cannot be 361V!!!
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Look at the power supply. If B+B is 364v then B+C cannot be 361V!!!
a:408 b:400 c:369
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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).
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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! :BangHead:
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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! :BangHead:
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.
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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).
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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
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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?
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with screen current so high, the tube is a GREAT place to make 16Khz
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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.
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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
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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
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Did you pull V3 to see if that would kill the 16KHz oscillation?
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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
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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?
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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
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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?
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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!
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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!
That means the oscillation is coming from the preamp or the reverb circuit. Remove V1 and remove the ground clip from V3 pin 7. This removes the pream from V3. Does this kill the oscillation?
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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!
That means the oscillation is coming from the preamp or the reverb circuit. Remove V1 and remove the ground clip from V3 pin 7. This removes the pream from V3. Does this kill the oscillation?
With V1 removed there is oscillation.
With V1 and V2 removed: there is oscillation
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So, the oscillation is coming from the reverb recovery circuit.
Turn the reverb pot to zero. Does this kill the oscillation?
Use a gator clip test lead to ground pin 2 of V3. Does this kill the oscillation?
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So, the oscillation is coming from the reverb recovery circuit.
Turn the reverb pot to zero. Does this kill the oscillation?
Use a gator clip test lead to ground pin 2 of V3. Does this kill the oscillation?
I'll try it as soon as I get home. Thank you!!
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And by the way, please: How do you call your terminal boards? They look like fun but i don`t know how to google them.
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And by the way, please: How do you call your terminal boards? They look like fun but i don`t know how to google them.
https://www.tube-town.net/ttstore/tube-town-ez-board-60-x-300-mm-fr4-2-mm-schwarz.html
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And by the way, please: How do you call your terminal boards? They look like fun but i don`t know how to google them.
I have tried grounding points A,B,C individually and in all cases there is still oscillation. Only when I connect pin 7 of V3 to ground does the oscillation disappear.
(https://i.postimg.cc/Gm2SrJwr/PRUEBAS-A-TIERRA.jpg)
NOTE: I have not connected the speaker to ground, nor the feedback
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I just verified that by injecting the signal from my guitar (passive pickups) into pin 7 of V3 I already have a fairly high signal. If I feed it into the V1 preamp, there's a lot of distortion and I have to turn the signal all the way down.
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Measure resistance from V3 pin 7 (probe directly on socket) to chassis. What do you have?
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Measure resistance from V3 pin 7 (probe directly on socket) to chassis. What do you have?
471K
Measured with revervb pot to ground
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Resolder that red wire on the wiper of the reverb pot. Looks suspicious to me.
Can you post a better high-resolution pic of your amp? I would like to be able to clearly see the color bands on your resistors.
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Resolder that red wire on the wiper of the reverb pot. Looks suspicious to me.
Can you post a better high-resolution pic of your amp? I would like to be able to clearly see the color bands on your resistors.
(https://i.postimg.cc/bNNX2gRK/e3779f63-b482-4ed5-b739-e5f98b148018.jpg)
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(https://i.postimg.cc/kgsLMb87/DSC09399.jpg)
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I also send you the draft that I made at the beginning
(https://i.postimg.cc/267ZgrM9/Nueva-placa-de-torretas-PARA-5-W-V6.jpg)
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That's a much better pic, but now I can't see the front panel pots.
Did you check the wire on the reverb pot?
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(https://i.postimg.cc/02VDn51V/22b02e15-17dd-48ea-b7a4-b78131167b5b.jpg)
(https://i.postimg.cc/0yrmBg8P/75c2b9cb-26a6-4c9d-be02-15a1fd3ab670-1.jpg)
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Use some gator clip test leads to connect a 470k resistor directly to V3 pin 7 and chassis. Does this kill the oscillation?
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Use some gator clip test leads to connect a 470k resistor directly to V3 pin 7 and chassis. Does this kill the oscillation?
Sorry, I'll have to do it later, I'm already working. To do that test, I disconnect everything that is connected to pin 7 of V3? Or do I connect the resistor without disconnecting anything?
Thanks
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Just connect the resistor without disconnecting anything.
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Just connect the resistor without disconnecting anything.
ok, when i do i tell him
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Just connect the resistor without disconnecting anything.
When putting the resistance, very annoying noise is produced and the oscillation continues. With or without the resistor, when I vary the value of the reverb potentiometer (100k) the oscillation goes from 16Khz to about 60KHz
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When putting the resistance, very annoying noise is produced and the oscillation continues. With or without the resistor, when I vary the value of the reverb potentiometer (100k) the oscillation goes from 16Khz to about 60KHz
OK. Now remove the wire from V3 socket pin 7 and connect a 470k resistor directly to V3 pin 7 and chassis. Does this kill the oscillation?
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When putting the resistance, very annoying noise is produced and the oscillation continues. With or without the resistor, when I vary the value of the reverb potentiometer (100k) the oscillation goes from 16Khz to about 60KHz
OK. Now remove the wire from V3 socket pin 7 and connect a 470k resistor directly to V3 pin 7 and chassis. Does this kill the oscillation?
if there is oscillation
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if there is oscillation
Not sure what you mean???
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Connecting pin 7 of V3 to the chassis through a 470K resistor, there is still oscillation.
(https://i.postimg.cc/Pr5cxJXj/oscilacion.jpg)
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We're dancing all around the problem but can't see the elephant in the room. The elephant is most likely that crazy layout. Sorry, but I'm out of ideas.
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I'm going to build on a separate chassis, just the part that's on that schematic. In this way I think we can have a clearer idea of the problem since it will not depend on the design. do you think it will be a good idea?
(https://i.postimg.cc/HxqYmN8r/construccion-por-partes-1.jpg)
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a well-designed circuit with an il-designed layout will result in a poor outcome.
you have High voltage DC running parallel to sensitive, low voltage AC, a perfect environment for cross-coupling signals. Signals that will meander throw-out your amp, causing all manner of chaos.
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ok, I'll touch up the whole design and I'll give you results. For now I'm going to do just that part on a separate chassis.
Thank you!!
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I have simulated the schematic circuit on a separate chassis and the result is the same.( there is still oscillation)
connecting pin 7 of V3 to ground, I measured pin 3 of V4 (anode): 381 V and pin 4 of V4 (screen): 388 V. Is it normal for a higher voltage on the screen than on the anode?
I'm thinking that the fault could be in the 6V6, can it be?
(https://i.postimg.cc/WzVcFZzm/construccion-por-partes-1v2.jpg)
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Would it be possible to replace the 6V6 with an EL84 (polarizing it correctly)? I say this because I have an EL84
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screens draw less current so there's less drag on the voltage.
the tube could be bad, did you swap it with another one?
you are most likely picking up something in the environment and coupling it into your circuit. LED lights, routers, wall-warts for charging...
try putting a resistor with a value of 5K to 10K from pin 5 of the 6V6 to the junction of R22 and C8.
how are you viewing the oscillation?
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... I'm thinking that the fault could be in the 6V6, can it be?
Would it be possible to replace the 6V6 with an EL84 ...
The 6V6 is not causing your trouble. The layout of your circuit is causing your trouble, and it cannot be fixed without building the amp with a entirely different layout. Sluckey proved this by stepping you through tests to kill the oscillation.
It might be wise to pick a ready-made kit that is a Fender amp with reverb, and build it. Then you will have a known-working layout.
After building such an am with a known-working layout, compare it to your present layout. You will see that what Fender lays out in a straight line, you have bent into a U-shape, putting output next to input and crossing over itself. This fact plus long grid wire length is why your present layout is suffering problems.
I'm sorry this is true, because you obviously put a lot of time and care into your amp build. But it is like a finely-crafted building on a foundation of sand: it will fail unless the core problem of the layout is fixed.
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... I'm thinking that the fault could be in the 6V6, can it be?
Would it be possible to replace the 6V6 with an EL84 ...
The 6V6 is not causing your trouble. The layout of your circuit is causing your trouble, and it cannot be fixed without building the amp with a entirely different layout. Sluckey proved this by stepping you through tests to kill the oscillation.
It might be wise to pick a ready-made kit that is a Fender amp with reverb, and build it. Then you will have a known-working layout.
After building such an am with a known-working layout, compare it to your present layout. You will see that what Fender lays out in a straight line, you have bent into a U-shape, putting output next to input and crossing over itself. This fact plus long grid wire length is why your present layout is suffering problems.
I'm sorry this is true, because you obviously put a lot of time and care into your amp build. But it is like a finely-crafted building on a foundation of sand: it will fail unless the core problem of the layout is fixed.
Sorry, I don't think I have explained myself. I have made a very simple design from scratch where there are hardly any threads or crosses. Only two 6V6 and Ecc83 valves connected and the power comes directly from the transformer and diodes. It can no longer influence that design that I made at the beginning. And still there is oscillation.
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Would it be possible to replace the 6V6 with an EL84 (polarizing it correctly)? I say this because I have an EL84
You can do that. But if you suspect the 6V6 why not just replace with another 6V6?
Show us a hi-rez pic of the test circuit on a separate chassis.
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(https://i.postimg.cc/HW6v5424/Whats-App-Image-2023-01-16-at-17-54-43.jpg)
(https://i.postimg.cc/7ZLdCt0k/IMG-20230116-WA0006.jpg)
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screens draw less current so there's less drag on the voltage.
the tube could be bad, did you swap it with another one?
you are most likely picking up something in the environment and coupling it into your circuit. LED lights, routers, wall-warts for charging...
try putting a resistor with a value of 5K to 10K from pin 5 of the 6V6 to the junction of R22 and C8.
how are you viewing the oscillation?
I have put a 4k7 resistor as you have told me and this is the result in the speaker output:
(https://i.postimg.cc/gk2CVDzj/Whats-App-Image-2023-01-16-at-21-05-45.jpg)
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what's it read for frequency? I think it's 100Khz but I might have slipped a decimal.
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what's it read for frequency? I think it's 100Khz but I might have slipped a decimal.
Yes, 100Khz
Very weird
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it's there when you un-ground pin 7, or always?
did you remove all external noise sources from the room you're working in, or move the amp to a "quite room"?
If the 100khz is always there, ground the neg speaker lead to chassis with a gator-clip, any change?
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I think we are on the right track! I have changed location, I have removed resistance between pin 5 of V4-C8/R22. I have connected new chassis to ground. And in loudspeaker I get 20mV at a frequency around 50Hz(oscillating). What I notice is a lot of sensitivity since as soon as I get close to pin 2 of V3 with my finger, the signal goes up a lot. I have connected pin 2 of V3 to ground and there is still 50 Hz noise (oscillating) and the signal goes up to 400mV.
I think there is too much equipment in my shop that was affecting
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It’s not only the ‘U’ shape of the layout with input close to output that’s problematic for unwanted cross-coupling, it’s also the long leads from the board to the sockets, where you’ve got grid and plate (and cathode) leads all running parallel closely bundled, and long leads everywhere threaded through the board passing closely to other components, (and, although it’s not evident from the pictures, I’m guessing they’re similarly routed higgledy-pigeldy with regard to signal voltage strength and phase, behind the board). The higher voltage wires will be making stronger EM fields that get inducted into the lower voltage signal wires, with signal phasing all over the place. = Recipe for oscillation.
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It’s not only the ‘U’ shape of the layout with input close to output that’s problematic for unwanted cross-coupling, it’s also the long leads from the board to the sockets, where you’ve got grid and plate (and cathode) leads all running parallel closely bundled, and long leads everywhere threaded through the board passing closely to other components, (and, although it’s not evident from the pictures, I’m guessing they’re similarly routed higgledy-pigeldy with regard to signal voltage strength and phase, behind the board). The higher voltage wires will be making stronger EM fields that get inducted into the lower voltage signal wires, with signal phasing all over the place. = Recipe for oscillation.
Please read the Reply #83
Thank you
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It’s not only the ‘U’ shape of the layout with input close to output that’s problematic for unwanted cross-coupling, it’s also the long leads from the board to the sockets, where you’ve got grid and plate (and cathode) leads all running parallel closely bundled, and long leads everywhere threaded through the board passing closely to other components, (and, although it’s not evident from the pictures, I’m guessing they’re similarly routed higgledy-pigeldy with regard to signal voltage strength and phase, behind the board). The higher voltage wires will be making stronger EM fields that get inducted into the lower voltage signal wires, with signal phasing all over the place. = Recipe for oscillation.
Please read the Reply #83
Thank you
I have made a very simple design from scratch where there are hardly any threads or crosses. Only two 6V6 and Ecc83 valves connected and the power comes directly from the transformer and diodes.
The photo in your revised preamp driver and output tube layout is still getting its input signal from the preamp in the original layout is it not?
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No, there are only two valves (V3 and V4)
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No, there are only two valves (V3 and V4)
It’s not clear from the photo what is happening at the input. Where is the input?
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No, there are only two valves (V3 and V4)
It’s not clear from the photo what is happening at the input. Where is the input?
No input connected yet. We are looking at output oscillation or noise in the absence of a preamp signal. It's like testing the circuit in phases from the end to the beginning
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No, there are only two valves (V3 and V4)
It’s not clear from the photo what is happening at the input. Where is the input?
There is no input in this partial circuit. Refer to the schematic in Reply #79. The problem is a 100KHz oscillation on the speaker. Pulling V3 will kill the oscillation. Put a ground on V3 pin 7 will kill the oscillation. Put a ground anywhere else in the V3A circuit will NOT kill the oscillation. What is the solution?
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No, there are only two valves (V3 and V4)
It’s not clear from the photo what is happening at the input. Where is the input?
There is no input in this partial circuit. Refer to the schematic in Reply #79. The problem is a 100KHz oscillation on the speaker. Pulling V3 will kill the oscillation. Put a ground on V3 pin 7 will kill the oscillation. Put a ground anywhere else in the V3A circuit will NOT kill the oscillation. What is the solution?
I think that the oscillation was caused by some external equipment that was interfering. I took the equipment to the terrace and the only thing there is now is the 50Hz of the network. I imagine that the circuit is open at the input.Reply #90
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No, there are only two valves (V3 and V4)
It’s not clear from the photo what is happening at the input. Where is the input?
There is no input in this partial circuit. Refer to the schematic in Reply #79. The problem is a 100KHz oscillation on the speaker. Pulling V3 will kill the oscillation. Put a ground on V3 pin 7 will kill the oscillation. Put a ground anywhere else in the V3A circuit will NOT kill the oscillation. What is the solution?
There must be some positive feedback going on somewhere, or the 1st stage is acting like a radio antenna. What about grounding Pin 2 of v3? (A normal guitar amp has a ground tip switch on the input Jack, or an instrument plugged in.) Or running a meter over the PS rail to check the filter caps are properly decoupled?
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No, there are only two valves (V3 and V4)
It’s not clear from the photo what is happening at the input. Where is the input?
There is no input in this partial circuit. Refer to the schematic in Reply #79. The problem is a 100KHz oscillation on the speaker. Pulling V3 will kill the oscillation. Put a ground on V3 pin 7 will kill the oscillation. Put a ground anywhere else in the V3A circuit will NOT kill the oscillation. What is the solution?
There must be some positive feedback going on somewhere, or the 1st stage is acting like a radio antenna. What about grounding Pin 2 of v3? (A normal guitar amp has a ground tip switch on the input Jack, or an instrument plugged in.) Or running a meter over the PS rail to check the filter caps are properly decoupled?
That is what I find strange, that putting pin2 of V3 to ground, there is a signal in the speaker. Even putting to ground the union of R19 with C12, there is still a signal.
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signal implies you are providing it, is that the case, you're injecting a signal?
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No, there are only two valves (V3 and V4)
It’s not clear from the photo what is happening at the input. Where is the input?
There is no input in this partial circuit. Refer to the schematic in Reply #79. The problem is a 100KHz oscillation on the speaker. Pulling V3 will kill the oscillation. Put a ground on V3 pin 7 will kill the oscillation. Put a ground anywhere else in the V3A circuit will NOT kill the oscillation. What is the solution?
There must be some positive feedback going on somewhere, or the 1st stage is acting like a radio antenna. What about grounding Pin 2 of v3? (A normal guitar amp has a ground tip switch on the input Jack, or an instrument plugged in.) Or running a meter over the PS rail to check the filter caps are properly decoupled?
That is what I find strange, that putting pin2 of V3 to ground, there is a signal in the speaker. Even putting to ground the union of R19 with C12, there is still a signal.
Have you measured the signal frequency? The universe is abuzz with electromagnetic radiation at all sorts of frequencies (some of it from outer space, some of it man-made) and an unshielded amplifier will pick up whatever is out there. (The more gain stages there are, the shorter the aerial needs to be to amplify it). If grounding the grid of V3b kills it, its coming from before the grid of V3b. Is there a radio transmitter/repeater station in your neighbourhood? You might be picking up 'raw' AM or FM carrier waves (that haven't been demodulated through a superhet) or something. :dontknow:
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signal implies you are providing it, is that the case, you're injecting a signal?
no, I mean the 50Hz that appears in the speaker with levels between approximately 30 and 400 mV. Tomorrow I will try to inject a signal
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signal implies you are providing it, is that the case, you're injecting a signal?
no, I mean the 50Hz that appears in the speaker with levels between approximately 30 and 400 mV. Tomorrow I will try to inject a signal
That is just power supply hum.
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Do you think this simplification of the circuit could work?
(https://i.postimg.cc/ZKRZpbHY/ESquema-parcial-2.jpg)
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"Work" like play? Probably. Work well? I bet with a complete AA Champ, no NFB or tone, volume, and then a THIRD triode gain stage, it will be a bee's-nest at any knob setting, and overload or scream past "3".
Have you tried a faithful copy of a Fender Champ? Especially the layout! When I tried that, I realized there are hard-learned unwritten lesions in how Leo placed those parts and wires. I was able to get it clean pushing wires around with a 'lead' pencil, but I had 90% stolen Leo's layout so I was closer than I knew.
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Do you think this simplification of the circuit could work?
NO!
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I have watched this thread with interest. I applaud the patience shown.
Having swum in Lake Stubborn more than a few times myself, I think this is going to go one of two ways.
- Chasing rabbits down multiple holes until exhausted; disassembling the amp and rebuilding with a proven schematic and layout.
- or; proceed right to - disassembling the amp and rebuilding with a proven schematic and layout.
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I have watched this thread with interest. I applaud the patience shown.
Having swum in Lake Stubborn more than a few times myself, I think this is going to go one of two ways.
- Chasing rabbits down multiple holes until exhausted; disassembling the amp and rebuilding with a proven schematic and layout.
- or; proceed right to - disassembling the amp and rebuilding with a proven schematic and layout.
Not the idea. The circuit I'm trying to make is proven. The problem is that I'm trying to simplify parts to make sure it works and build it little by little to learn how it works. Thanks to this and to you I am learning how these amplifiers work. My idea is to learn what amplifiers I have to spare. Thanks for your help
NOTE: Sorry for the Google translation
(https://i.postimg.cc/HxJNLtX1/Mi-Dise-o-2.png)
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Do you think this simplification of the circuit could work?
(https://i.postimg.cc/ZKRZpbHY/ESquema-parcial-2.jpg)
Regarding V3b -
A DC path between a grid and its cathode is essential. It’s normally achieved by a grid leak resistor.
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Do you think this simplification of the circuit could work?
(https://i.postimg.cc/ZKRZpbHY/ESquema-parcial-2.jpg)
Regarding V3b -
A DC path between a grid and its cathode is essential. It’s normally achieved by a grid leak resistor.
Yes, I totally agree. It's a mistake. In fact I was going to do it with a 220k-470k resistor. Thanks
(https://i.postimg.cc/TwCQZH4j/ESquema-parcial-2b.jpg)
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Hello!
I have redone the circuit by shortening cables and following the layout of a Fender as precisely as possible.
It is made as in the circuit of the image.
The measurements I get now seem correct to me except that:
1- They seem a bit high in value to me
2-In pin 7 of V3B I have a voltage of -4.1 V
3- In pin 4 of V3 I still have higher voltage than in pin 3
Are they normal values?
(https://i.postimg.cc/kGyNSmyR/composicion-V3-PARA-MEDIDAS-30-1-2023.jpg)
tank you
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The measurements I get now seem correct to me except that:
1- They seem a bit high in value to me (Quite normal for that Hammond 372BX)
2- In pin 7 of V3B I have a voltage of -4.1 V (should be zero. Maybe it's really -4.1mV but you misread your meter?)
3- In pin 4 of V3 I still have higher voltage than in pin 3 (pin 4 is filament. What's your point?)
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The measurements I get now seem correct to me except that:
1- They seem a bit high in value to me (Quite normal for that Hammond 372BX)
2- In pin 7 of V3B I have a voltage of -4.1 V (should be zero. Maybe it's really -4.1mV but you misread your meter?)
3- In pin 4 of V3 I still have higher voltage than in pin 3 (pin 4 is filament. What's your point?)
I meant V4
in the 6v6 gt pin 4 is screen
(https://i.postimg.cc/qv0mRp8r/6v6gschaltung.png)
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I meant V4
Sorry. I'm not a mind reader. :wink:
It's very possible that your OT has been damaged by that BIG high frequency signal you had in the beginning.
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Is there a way to check it without replacing it?
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Is there a way to check it without replacing it?
Use your ears. How does it sound? If not sound good, replace the OT. No guarantees.
I have redone the circuit by shortening cables and following the layout of a Fender as precisely as possible.
Show us a hi-rez pic of the redone amp.
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3- In pin 4 of V3 I still have higher voltage than in pin 3
Are they normal values?
I meant V4
Overall, having 6V6 screen voltage a bit higher than the plate voltage doesn't matter. And it happens due to voltage drop across the output transformer primary.
Your diagram shows 406v in the power supply, and 386v at the 6V6 plate: 20v dropped across the transformer primary.
Hammond says the primary has a resistance of about 360Ω (https://www.hammfg.com/files/parts/pdf/1760C.pdf). 20v / 360Ω = about 55.6mA of plate current.
Your diagram shows 24v at the cathode, across a 460Ω resistor: 24v / 470Ω = 51mA
We can see that either the cathode resistor is lower than 470Ω, or your 1760C is higher than 360Ω.
6V6 Dissipation: (386v - 24v) x 56mA = 20.3 watts ---> way too hot.
Reduce the 6V6 dissipation:
Assume the plate voltage will land at 393v
14w / 393v = 35.5mA
Add 3mA of screen current: 35.5mA + 3mA = 38.5mA
Calculate cathode voltage: 470Ω x 38.5mA = about 18v
Using the triode curves in a 6V6 data sheet (https://frank.pocnet.net/sheets/135/6/6V6GTA.pdf) (page 5), I estimate the 6V6 needs a screen voltage of about 275v to pass 38.5mA with a bias of 18v (I can show the method some other time if you like).
Your diagram shows a voltage drop from "B+ B" to "B+ C" that indicates preamp current is about 5mA.
We'll estimate our 3mA 6V6 screen current plus 5mA preamp current flows through R20: 8mA
We have 406v at "B+ A" and need 275v at "B+ B": (406v - 275v) / 8mA = about 16kΩ ---> use an 18kΩ resistor for R20
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3- In pin 4 of V3 I still have higher voltage than in pin 3
Are they normal values?
I meant V4
Overall, having 6V6 screen voltage a bit higher than the plate voltage doesn't matter. And it happens due to voltage drop across the output transformer primary.
Your diagram shows 406v in the power supply, and 386v at the 6V6 plate: 20v dropped across the transformer primary.
Hammond says the primary has a resistance of about 360Ω (https://www.hammfg.com/files/parts/pdf/1760C.pdf). 20v / 360Ω = about 55.6mA of plate current.
Your diagram shows 24v at the cathode, across a 460Ω resistor: 24v / 470Ω = 51mA
We can see that either the cathode resistor is lower than 470Ω, or your 1760C is higher than 360Ω.
6V6 Dissipation: (386v - 24v) x 56mA = 20.3 watts ---> way too hot.
Reduce the 6V6 dissipation:
Assume the plate voltage will land at 393v
14w / 393v = 35.5mA
Add 3mA of screen current: 35.5mA + 3mA = 38.5mA
Calculate cathode voltage: 470Ω x 38.5mA = about 18v
Using the triode curves in a 6V6 data sheet (https://frank.pocnet.net/sheets/135/6/6V6GTA.pdf) (page 5), I estimate the 6V6 needs a screen voltage of about 275v to pass 38.5mA with a bias of 18v (I can show the method some other time if you like).
Your diagram shows a voltage drop from "B+ B" to "B+ C" that indicates preamp current is about 5mA.
We'll estimate our 3mA 6V6 screen current plus 5mA preamp current flows through R20: 8mA
We have 406v at "B+ A" and need 275v at "B+ B": (406v - 275v) / 8mA = about 16kΩ ---> use an 18kΩ resistor for R20
Thank you very much for your explanation. Understanding that the power consumed would be 1, 152w, with which a resistance of 18k and 3w would be enough
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3- In pin 4 of V3 I still have higher voltage than in pin 3
Are they normal values?
I meant V4
Overall, having 6V6 screen voltage a bit higher than the plate voltage doesn't matter. And it happens due to voltage drop across the output transformer primary.
Your diagram shows 406v in the power supply, and 386v at the 6V6 plate: 20v dropped across the transformer primary.
Hammond says the primary has a resistance of about 360Ω (https://www.hammfg.com/files/parts/pdf/1760C.pdf). 20v / 360Ω = about 55.6mA of plate current.
Your diagram shows 24v at the cathode, across a 460Ω resistor: 24v / 470Ω = 51mA
We can see that either the cathode resistor is lower than 470Ω, or your 1760C is higher than 360Ω.
6V6 Dissipation: (386v - 24v) x 56mA = 20.3 watts ---> way too hot.
Reduce the 6V6 dissipation:
Assume the plate voltage will land at 393v
14w / 393v = 35.5mA
Add 3mA of screen current: 35.5mA + 3mA = 38.5mA
Calculate cathode voltage: 470Ω x 38.5mA = about 18v
Using the triode curves in a 6V6 data sheet (https://frank.pocnet.net/sheets/135/6/6V6GTA.pdf) (page 5), I estimate the 6V6 needs a screen voltage of about 275v to pass 38.5mA with a bias of 18v (I can show the method some other time if you like).
Your diagram shows a voltage drop from "B+ B" to "B+ C" that indicates preamp current is about 5mA.
We'll estimate our 3mA 6V6 screen current plus 5mA preamp current flows through R20: 8mA
We have 406v at "B+ A" and need 275v at "B+ B": (406v - 275v) / 8mA = about 16kΩ ---> use an 18kΩ resistor for R20
Thank you very much for your explanation. Understanding that the power consumed would be 1, 152w, with which a resistance of 18k and 3w would be enough
Are you using language translation software?
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Hello!
I have redone the circuit by shortening cables and following the layout of a Fender as precisely as possible.
It is made as in the circuit of the image.
The measurements I get now seem correct to me except that:
1- They seem a bit high in value to me
2-In pin 7 of V3B I have a voltage of -4.1 V
3- In pin 4 of V3 I still have higher voltage than in pin 3
Are they normal values?
(https://i.postimg.cc/kGyNSmyR/composicion-V3-PARA-MEDIDAS-30-1-2023.jpg)
tank you
Based on this schematic and these reported voltages, the 6V6 is dissipating 16W
(I get 51mA cathode current, i.e., 24V/470R, including 6.9mA* screen current)
So, 44.2mA plate current x 362V plate-to-cathode = 16W plate dissipation
*based on the voltage drop across the supply resistors. i.e. 12V across 1k (R20) minus 92V across 18k (R21). This is way too much idle screen current for a little ole 6V6. In order to lower the screen current, you need to lower the screen voltage by about 50V. So try 10k (instead of 1k) for R20. This should bring the 6V6 back into more normal operating conditions. (Note also that when you increase R20 to 10k, you may want to decrease R21 to something between 4k7 to 10k if you want to maintain the preamp plate voltages around 250V)
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3- In pin 4 of V3 I still have higher voltage than in pin 3
Are they normal values?
I meant V4
Overall, having 6V6 screen voltage a bit higher than the plate voltage doesn't matter. And it happens due to voltage drop across the output transformer primary.
Your diagram shows 406v in the power supply, and 386v at the 6V6 plate: 20v dropped across the transformer primary.
Hammond says the primary has a resistance of about 360Ω (https://www.hammfg.com/files/parts/pdf/1760C.pdf). 20v / 360Ω = about 55.6mA of plate current.
Your diagram shows 24v at the cathode, across a 460Ω resistor: 24v / 470Ω = 51mA
We can see that either the cathode resistor is lower than 470Ω, or your 1760C is higher than 360Ω.
6V6 Dissipation: (386v - 24v) x 56mA = 20.3 watts ---> way too hot.
Reduce the 6V6 dissipation:
Assume the plate voltage will land at 393v
14w / 393v = 35.5mA
Add 3mA of screen current: 35.5mA + 3mA = 38.5mA
Calculate cathode voltage: 470Ω x 38.5mA = about 18v
Using the triode curves in a 6V6 data sheet (https://frank.pocnet.net/sheets/135/6/6V6GTA.pdf) (page 5), I estimate the 6V6 needs a screen voltage of about 275v to pass 38.5mA with a bias of 18v (I can show the method some other time if you like).
Your diagram shows a voltage drop from "B+ B" to "B+ C" that indicates preamp current is about 5mA.
We'll estimate our 3mA 6V6 screen current plus 5mA preamp current flows through R20: 8mA
We have 406v at "B+ A" and need 275v at "B+ B": (406v - 275v) / 8mA = about 16kΩ ---> use an 18kΩ resistor for R20
Thank you very much for your explanation. Understanding that the power consumed would be 1, 152w, with which a resistance of 18k and 3w would be enough
Are you using language translation software?
Yes, I'm sorry, I'm writing from Spain
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Based on this schematic and these reported voltages, the 6V6 is dissipating 16W
I agree.
But didn't I mention I don't exactly trust the numbers provided? Because I don't exactly trust the numbers provided. :laugh:
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I have made a series of measurements of voltages and currents. I have done it with resistance R20=1K and with R20=10K. In both images you can see the measurements obtained.
with R20=1k
(https://i.postimg.cc/855KkBdJ/composicion-V3-PARA-MEDIDAS-3-2-2023-A.jpg)
with R20=10k
(https://i.postimg.cc/sxnZj7cX/composicion-V3-PARA-MEDIDAS-30-1-2023-B.jpg)
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I think that with the 10k resistor the measurements are more or less good. The problem is that I have a lot of distortion and too much gain with the volume at minimum. The reverb also gets really loud with the volume all the way down. How would I measure the gain of each triode? I have a signal generator but it barely injects a level above 100mV when I plug it in. (no load if it goes up to a few volts). It must be the low impedance it has (600 Ohm)
(https://i.postimg.cc/nLyvQ4Bw/generador-de-se-al.jpg)
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I think that with the 10k resistor the measurements are more or less good.
I agree
How would I measure the gain of each triode?
Inject a 1KHz 100mV zero volt offset signal at the input jack (If you don't know what offset is then use a .1µF cap between the FG and the amp). Set scope to AC coupling on input. Use scope to measure signal amplitudes at V1A grid and plate. Divide plate signal voltage by grid signal voltage. This resulting number is the gain for V1A. Repeat for other triodes. For example, if you measure 100mVpp at the grid and 2Vpp at plate, the gain would be 20.
I have a signal generator but it barely injects a level above 100mV when I plug it in. (no load if it goes up to a few volts). It must be the low impedance it has (600 Ohm)
The input impedance of your amp should be 1MΩ. That's almost like "no load" to your FG. There's either a problem with the input wiring of your amp ***OR*** a problem with your FG.
To confirm your FG is good... Set the FG for 1KHz sine with zero offset. Connect a 1MΩ resistor across the 600Ω output of your FG. Now measure the signal voltage at the output of the FG. If you can still only get 100mV signal on the scope then the FG is bad. If you can get several volts on the scope then you have a wiring error between the amp input jack and V1A grid.
You can confirm the input is wired correctly by measuring the resistance between V1A grid and chassis. You should measure 33K with nothing plugged into the input jack. Now connect a guitar cable to the input but DO NOT connect the other end of the cable to anything. The resistance between V1A grid and chassis should now read 1MΩ.
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...I have a signal generator but it barely injects a level above 100mV when I plug it in. (no load if it goes up to a few volts). It must be the low impedance it has (600 Ohm)...
FWIW: Looks like a 3311 series.
"Ten V p-p into 600 ohms (20 V p-p O.C.). This output may be attenuated by >30 dB by a variable attenuator and offset by ± 5 V."
https://calright.com/product/agilent-hp-3311a-function-generator-1-mhz/
https://engineering.case.edu/lab/circuitslab/sites/engineering.case.edu.lab.circuitslab/files/docs/Hewlett_Packard_3311A_Function_Generator_Operating_and_Service_Manual.pdf
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These are some captures with the oscilloscope with high volume.
The current through R20 is increased to 16 mA. Therefore the resistor has to dissipate almost 3w.
(https://i.postimg.cc/T1RbzsbS/medidas-osciloscopio-4-2.jpg)
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The current through R20 is increased to 16 mA. Therefore the resistor has to dissipate almost 3w.
I suggest replacing R20 with a 10W resistor. I'd probably use a 5KΩ 10W resistor.
Have you plugged a guitar into the amp? How does it sound?
What did you do to resolve the low output from your FG?
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In order to work optimally with the FG, I put a 0.1 capacitor as you indicated. From then on I was able to do the measurements without problems. He was injecting me with a voltage of 7 volts DC.
Tomorrow I'll try the guitar but I think I'll have distortion.
Something strange that happens is that a sound is generated in the output transformer with the same frequency as the input. I am doing the measurements without a speaker and with a load of 8 Ohms 15W. Even so, the frequency of the input of the FG is heard in the OT.
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Something strange that happens is that a sound is generated in the output transformer with the same frequency as the input. I am doing the measurements without a speaker and with a load of 8 Ohms 15W. Even so, the frequency of the input of the FG is heard in the OT.
That's very normal. The OT laminations are vibrating. Doesn't mean anything is wrong. You need to turn the FG output down to 100mVpp.
He was injecting me with a voltage of 7 volts DC.
The DC offset knob controls that dc voltage. Read the manual to learn how to use it.
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As expected, the sound with a minimum volume already sounds distorted and the reverb sounds too much with the P5 potentiometer turned down. How can I lower the gain of the reverb?
Thank you
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I definitely don't know as much as the rest of the folks on this forum and have learned A LOT lurking here and reading.
In my humble opinion I suggest you connect a 50k resistor from pin 7 of V3 to ground.
I built an amp like this a while back and although I can't find my notes I remember looking at schematics for other Fender Reverb amps that have higher gain phase inverters, like the long tail pair phase inverters, had some gain reduction through the 50k tremolo pot. The princeton reverb has a cathodyne phase inverter which has a gain of less than 1, so it doesn't have the 50k resistor. I think your last gain stage has a lot more gain than a cathodyne phase inverter. I believe/hope this will also help with the reverb.
I'm sure other folks here can better speak to all of these points or help to correct what I have said here.
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After second thought I think most of what I said is flawed, but I still maintain that some sort of grid leak resistor at v3 pin 7 will help you tame the gain.
The 50k in the other fender circuits comes after that gain stage, but before the phase inverter.
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R19 and the Reverb pot are the grid leak for V3 pin 7. If you put a 50K between V3-7 and ground you will create a voltage divider (66:1) with that 3.3M. That will almost destroy the signal arriving at V3-7.
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Thanks for the correction, Sluckey. That makes a lot of sense.
What's up with the 220K resistor in the other AB763 circuits? Would that be that more appropriate?
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I inject a 90 mV rms signal into pin 2 of V1. With the volume at maximum (P1) I have 30 volts rms on pin 5 of V4, isn't it too much? The wave is very distorted on its positive side.
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These are the rms measurements injecting 97mV 1Khz
(https://i.postimg.cc/wxPzpx95/composicion-V3-PARA-MEDIDAS-RMS-5-2-2023.jpg)
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What's up with the 220K resistor in the other AB763 circuits? Would that be that more appropriate?
Only Leo knows. :icon_biggrin:
I can speculate... Maybe it's to provide a dedicated ground reference for the tube grid. Maybe the signal was too hot so the 220K was added to drop the level a bit.
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These are the rms measurements injecting 97mV 1Khz
I don't see any alarms.
You have analyzed this amp to death with your FG. IT'S TIME TO PLUG IN A GUITAR!!!
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These are the rms measurements injecting 97mV 1Khz
I don't see any alarms.
You have analyzed this amp to death with your FG. IT'S TIME TO PLUG IN A GUITAR!!!
I connected the guitar and that's why I commented that it distorted a lot and that the reverb immediately had too much level
Reply #132
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Hello rafaelctt, did you make any more progress on your project? I have been really appreciating your perseverance and I think you're probably very close. I appreciate long threads with people who are dedicated to finding solutions. I know I've burned a lot of candles chasing down silly mistakes or even sometimes faulty components.
did you double and triple check that the reverb pot ground lug is definitely connected to ground?
what kind of settings did you have your eq on when you plugged your guitar in? were they all the way up?
are you using humbuckers or single coil pickups?
can you share any high resolution photos of your latest work?
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I inject a 90 mV rms signal into pin 2 of V1. With the volume at maximum (P1) I have 30 volts rms on pin 5 of V4, isn't it too much? The wave is very distorted on its positive side.
These are the rms measurements injecting 97mV 1Khz
Your diagram shows 33.8v RMS on the 6V6 grid.
The diagram in this post (https://el34world.com/Forum/index.php?topic=29768.msg329236#msg329236) shows your cathode (bias) voltage is 19.8 volts. That means the 6V6 cannot accept more than about 18.5v peak before it draws grid current & distorts heavily.
"The wave is very distorted on its positive side" because 6V6 grid current is lopping off the top of the wave. The 6V6 is receiving 33.8v RMS x 1.414 = 47.8v peak, or about 2.6x more than it can handle cleanly. PRR told you this would happen back in this post (https://el34world.com/Forum/index.php?topic=29768.msg328766#msg328766).
You might be able to knock down some of the signal level by making R19 smaller (reduces the Dry signal strength, but makes the Reverb stronger), splitting R14 into 2 resistors to reduce the drive into the Reverb Driver 12AT7, and use a 100kΩ Audio taper for the Reverb pot so the reverb is not so strong at low settings.
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I wonder what it sounds like if you just turn the volume control down?
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It looks like he had the volume control down when he tried it with the guitar... from reply #132: "As, expected, the sound with a minimum volume already sounds distorted and the reverb sounds too much..."
Maybe the circuit is "proven" as mentioned earlier by rafaelctt in this thread, but this isn't a commercial circuit... or is it??
maybe the extra gain stage is just overdrive right out of the gate? :dontknow:
maybe its worth incorporating some gain reduction techniques?
an amp I built a few years back for my GF is also single ended with reverb. almost exactly the same as this except I used the fender tweed style tone control. The tweed tone control doesn't bog down the signal much and... holy moly it had way too much gain at the start. I definitely used some resistance to bring the signal down to a reasonable level and may have removed a bypass cap somewhere too. I'd have to pull the chassis, because I can't quite remember and have no idea where that notebook is.
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> The 6V6 is receiving 33.8v RMS x 1.414 = 47.8v peak, or about 2.6x more than it can handle cleanly.
It would be reasonable, for most genres and styles, for the amp to be on the edge of overload when dimed and 20mV input. Since he has 97mV input when he has gross distortion, he's not far off; unless this is already turned-down some.
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Maybe the circuit is "proven" as mentioned earlier by rafaelctt in this thread, but this isn't a commercial circuit... or is it??
I and others questioned the circuit several posts back. It seems that the basic circuit is proven, but there are multiple added gain stages. In addition, the subject of the non-standard practice layout is still out there. Requests for pics of the revised layout have not been answered making this a frustrating thread.
Were I to work on this amp, I would temporarily disconnect the reverb and the extra gain stages and get the amp working producing clean sound. Then I would start adding things back in, testing and adjusting along the way.