HELP WITH TRANSFORMER

[Pete Farrington]

I don’t see any purpose in a switch; just wire each cathode to an appropriate cathode resistor.

 

[Marcomel79]

I don’t see any purpose in a switch; just wire each cathode to an appropriate cathode resistor.

I forgot to mention that they will share the cathode bypass capacitor, so then i will have to use a switch. Right....?

 

[Pete Farrington]

Right, but to me it seems a bit weird to use a switch to save on a cap?

 

[Marcomel79]

Right, but to me it seems a bit weird to use a switch to save on a cap?

Man, and i thought i was so smart with my switch also i dont have the cap at hand and im impatient... sounds like im making excuses... i might be..

 

[Pete Farrington]

Oh well, if you’ve got a spare switch but not a cap, then go for it

 

[PelliX]

Oh well, if you’ve got a spare switch but not a cap, then go for it

I was thinking the same thing, but wasn't going to say it

Some future tech in 50 years is *really* going to scratch his head when he opens this one up...

 

[Marcomel79]

I was thinking the same thing, but wasn't going to say it

Some future tech in 50 years is *really* going to scratch his head when he opens this one up...

Well well well, what do we have here....

 

[pietro.castelli]

As promised @PelliX here are some pictures of my scratch build. Im waiting for the 6V6 that should come in today i hope. Except for the filter caps and a couple resistors, everything is recycled in the circuit. The chassis i found in the electronic waste bin at workView attachment 113048View attachment 113049View attachment 113050View attachment 113051View attachment 113052

Instead of a 5Y3, the new transformer came with a 4V tap for a AZ31. I read that its basically a 5Y3 that runs on 4 volts. Voltages look in the ballpark with just the 12ax7, well see after i get the 6V6.

If it works, im gonna be a happy man!

Terrific work!

 

[Marcomel79]

Hey @Pete Farrington one quick question: the scrapped champ is all done and i have to say it sounds amazing! Very quiet. I have a AZ31 rectifier which has its on 4V secondary and does the job. I Also have a bunch of ez80 and ez81 and since im a tinkerer i was wondering if i can use the 6.3V filament for it since it has the separate cathode.... i know the PT can take it

 

[Marcomel79]

It certainly looks as though you are making progress and it will soon be making noise. You have a variety of resistors there. Did you have a specific tolerance that you were willing to accept before you would replace one and how did they fare? The 290-0-290 PT should be more than enough as long as it is 50-60ma or better. Watch the 470r cathode resistor for the 6V6 it may need to be more in the ballpark of 650r-1k to bias in a more acceptable range. Looking good, keep us posted.

Hey @Dblgun, i have a 2W 470ohm cathode resistor (more like 464ohm) for the 6V6 and i think i could even use a 220ohm... for the EL84 (i have wired the sockets in parallel, just to get a different flavour) i have a 270ohm cathode resistor but i could probably have 180/150ohm... but the amp sounds amazing so i let my ears be the judge of that.

I did not have a specific tolerance for the resistors. I used what i had from gutting old radios and reel to reel recorders that were not in working condition

 

[Pete Farrington]

i was wondering if i can use the 6.3V filament for it since it has the separate cathode.

In theory, yes, but probably best to restrict that the vintage production, as there’ll be a lot of DC voltage between the rectifier’s heater and cathode, putting the insulation under stress.
I’ve seen reports of current production EZ81 not coping with that.

If you do try it, it may be a good to elevate the heater circuit, eg to 80V, to ease off some of that voltage stress.

 

[Matthews Guitars]

I'm late to the party that happened in the first two pages after Ken gave his unvarnished opinion. (Given that this started with a damaged transformer winding, that's almost a pun.)

I kind of think he's right. Maybe he could have been a bit more subtle about his statements, but from an academic standpoint, he's not wrong.

I'm formally trained in electronics and have a great deal of post-school training and experience as well. Granted, I'm not the grand master of vacuum tube amp theory, and have never claimed that title.

But while I know how to stitch up a wound and apply a band-aid, I am not a doctor.

You may know how to follow a diagram and wire up an amplifier that works. Congratulations, so can all the people working on the assembly line at Marshall, Fender, Peavey, and Mesa/Boogie. But that doesn't mean you really KNOW what's going on in the circuit and how to troubleshoot any problem. To really have that knowledge, you have to know the circuit theory. And it starts with understanding the types and functions and values of basic components. Know Ohm's Law, Thevenin's Theorem. Know how to calculate an RC time constant and how to select resistor and cap values for a tone stack. Know vacuum tube theory. Know how to trace signal flow through the circuit. Know how to read a schematic and understand what it's telling you. This doesn't come from the assembly instructions that come with an amp kit.

If you're serious about working on amps, you need a certain level of formal education. You need to UNDERSTAND the signal flow in a schematic and explain what each part in the circuit does. You're not going to be able to fake it when a really WEIRD problem comes in, and it turns out that there's one critical resistor in the circuit that's literally just way off value, or mis-marked as to its value. That simple little problem will separate the men from the boys, so to speak.

Years ago I tested for an electronics tech job and they tested me by putting me at a test station with some basic instruments, an example of a board type I'd be expected to troubleshoot, the schematic, and told to find the problem. I NEARLY got a perfect score. I did better than any other person who tested for the job. It only took two tries. First I identifed a transistor that probably wasn't working. A new one was installed, that didn't fix it. Then I got deeper into LOOKING and READING the schematic, and found that the resistor feeding the base of the transistor was not of the right value. When I announced that to the engineer overseeing me, he said, "You got it! You did EXCELLENT. I'd be happy to have you work for me. Nobody else has gotten it so fast. One guy wanted to replace every part on the board. "

The company elected not to even open the position and fill it internally, unfortunately for me. But I appreciated the cofirmation of my troubleshooting skills on a ciruit I'd never even seen before.

There is a difference between being able to build something and being able to really UNDERSTAND it, which opens the door to fixing any problem it might have. Or, even innovating something new.

If you are in the first category, I ENCOURAGE you to continue your education to join the SECOND category. Life should be an endless sequence of new things to learn and new adventures to experience.

I"m NOT putting down those of you who are not formally trained electronics technicians with comprehensive DC and AC electronics theory under your belt. I'm saying you've got a basis to build on. So build on it. Learn that theory. Get that knowledge. It'll make your work more fun and rewarding.

 

[thetragichero]

6V6
a: 296V
k: 15.2V

Voltage drop on 6V6 cathode is 15.36V. Divided by 460ohm resistor (measures a bit lower) gives me 33mA cathode current.

you're biased at like 61% of tube dissipation. way low for single-ended class a (90%+++). when I'm just throwing a cathode biased amp together, i just tack solder the power tube cathode resistor because even with some calculations there's a good chance it'll be wrong. for testing purposes try another 470r (or 680r) tack soldered to your existing cathode resistor, remeasure, and plug in figures here: https://robrobinette.com/Tube_Bias_Calculator.htm
once you've found an appropriate parallel resistor, calculate its value and replace with the proper value

120hz hum likely comes from single-ended amps' inherent lack of common mode rejection that you get with push pull output. this is where a pi filter (and extra capacitor and choke before the plate supply node) comes in handy
if 'twere my build I'd do away with the tube rectifier and go solid state for the increased output (and likely a happier-running power transformer)

 

[Marcomel79]

Thank you Pete, i do have some vintage Mullard ez81s...

I'm late to the party that happened in the first two pages after Ken gave his unvarnished opinion. (Given that this started with a damaged transformer winding, that's almost a pun.)

I kind of think he's right. Maybe he could have been a bit more subtle about his statements, but from an academic standpoint, he's not wrong.

I'm formally trained in electronics and have a great deal of post-school training and experience as well. Granted, I'm not the grand master of vacuum tube amp theory, and have never claimed that title.

But while I know how to stitch up a wound and apply a band-aid, I am not a doctor.

You may know how to follow a diagram and wire up an amplifier that works. Congratulations, so can all the people working on the assembly line at Marshall, Fender, Peavey, and Mesa/Boogie. But that doesn't mean you really KNOW what's going on in the circuit and how to troubleshoot any problem. To really have that knowledge, you have to know the circuit theory. And it starts with understanding the types and functions and values of basic components. Know Ohm's Law, Thevenin's Theorem. Know how to calculate an RC time constant and how to select resistor and cap values for a tone stack. Know vacuum tube theory. Know how to trace signal flow through the circuit. Know how to read a schematic and understand what it's telling you. This doesn't come from the assembly instructions that come with an amp kit.

If you're serious about working on amps, you need a certain level of formal education. You need to UNDERSTAND the signal flow in a schematic and explain what each part in the circuit does. You're not going to be able to fake it when a really WEIRD problem comes in, and it turns out that there's one critical resistor in the circuit that's literally just way off value, or mis-marked as to its value. That simple little problem will separate the men from the boys, so to speak.

Years ago I tested for an electronics tech job and they tested me by putting me at a test station with some basic instruments, an example of a board type I'd be expected to troubleshoot, the schematic, and told to find the problem. I NEARLY got a perfect score. I did better than any other person who tested for the job. It only took two tries. First I identifed a transistor that probably wasn't working. A new one was installed, that didn't fix it. Then I got deeper into LOOKING and READING the schematic, and found that the resistor feeding the base of the transistor was not of the right value. When I announced that to the engineer overseeing me, he said, "You got it! You did EXCELLENT. I'd be happy to have you work for me. Nobody else has gotten it so fast. One guy wanted to replace every part on the board. "

The company elected not to even open the position and fill it internally, unfortunately for me. But I appreciated the cofirmation of my troubleshooting skills on a ciruit I'd never even seen before.

There is a difference between being able to build something and being able to really UNDERSTAND it, which opens the door to fixing any problem it might have. Or, even innovating something new.

If you are in the first category, I ENCOURAGE you to continue your education to join the SECOND category. Life should be an endless sequence of new things to learn and new adventures to experience.

I"m NOT putting down those of you who are not formally trained electronics technicians with comprehensive DC and AC electronics theory under your belt. I'm saying you've got a basis to build on. So build on it. Learn that theory. Get that knowledge. It'll make your work more fun and rewarding.

thanks man i appreciate it. I do understand a simple circuit like the 5F1 and how vacuum tubes work. I started reading and building out of necessity, i cannot afford a real Marshall. So this is the only way i could get close enough...

 

[Marcomel79]

you're biased at like 61% of tube dissipation. way low for single-ended class a (90%+++). when I'm just throwing a cathode biased amp together, i just tack solder the power tube cathode resistor because even with some calculations there's a good chance it'll be wrong. for testing purposes try another 470r (or 680r) tack soldered to your existing cathode resistor, remeasure, and plug in figures here: https://robrobinette.com/Tube_Bias_Calculator.htm
once you've found an appropriate parallel resistor, calculate its value and replace with the proper value

120hz hum likely comes from single-ended amps' inherent lack of common mode rejection that you get with push pull output. this is where a pi filter (and extra capacitor and choke before the plate supply node) comes in handy
if 'twere my build I'd do away with the tube rectifier and go solid state for the increased output (and likely a happier-running power transformer)

I clipped a resistor in parallel with a total resistance of 214ohm now. Thats 44mA, which brings me to 11.3W dissipation. I think i like the cold bias better. Well see..

 

[Marcomel79]

there’ll be a lot of DC voltage between the rectifier’s heater and cathode, putting the insulation under stress.

Which insulation are you talking about Pete? Sorry about my ignorance. I do have some vintage mullard ez80 and ez81 if thats better

 

[Marcomel79]

UPDATE:
Ok i fit in a socket for the EZ81, wired everything and took some measurements.

EZ81 EL84 12ax7
B+ 326V
Second node 274V
Third node 241V

V1:
pin 1: 164V
pin3: 1.1V
pin6: 160V
pin8: 1.2V

EL84:
pin7: 310V
pin9: 238V
pin3: 8.6V

Cathode resistor: 224Ohm
-------------------------------------------------
EZ81 6V6 12ax7
B+ 328V
Second node 277
Third node 244

V1:
pin 1: 164V
pin3: 1.1V
pin6: 160V
pin8: 1.2V

6V6:
Pin3: 319V
Pin4: 274V
Pin8: 16.5V

Cathode resistor: 465Ohm

I get about 11.3W dissipation for both the EL84 and the 6V6. Ill have to test the sound but i suspect i liked it better before. More chimey and less stiff from the little i played but i might be wrong....

 

[Pete Farrington]

Which insulation are you talking about Pete? Sorry about my ignorance. I do have some vintage mullard ez80 and ez81 if thats better

Inside the valve bulb envelope there’s a cathode. Inside thd cathode there’s a heating element. In the EZ80/81, the heater and cathode are electrically isolated from each other. Between them, to enable that isolation, there’s insulation, with a design centre rating of 500V!
Which seems a stretch, when it’s usually at the most 180V for most of the receiving type valves we use.

 

[Marcomel79]

you're biased at like 61% of tube dissipation. way low for single-ended class a (90%+++). when I'm just throwing a cathode biased amp together, i just tack solder the power tube cathode resistor because even with some calculations there's a good chance it'll be wrong. for testing purposes try another 470r (or 680r) tack soldered to your existing cathode resistor, remeasure, and plug in figures here: https://robrobinette.com/Tube_Bias_Calculator.htm
once you've found an appropriate parallel resistor, calculate its value and replace with the proper value

120hz hum likely comes from single-ended amps' inherent lack of common mode rejection that you get with push pull output. this is where a pi filter (and extra capacitor and choke before the plate supply node) comes in handy
if 'twere my build I'd do away with the tube rectifier and go solid state for the increased output (and likely a happier-running power transformer)

Check my update with a different rectifier!

 

[Marcomel79]

Inside the valve bulb envelope there’s a cathode. Inside thd cathode there’s a heating element. In the EZ80/81, the heater and cathode are electrically isolated from each other. Between them, to enable that isolation, there’s insulation, with a design centre rating of 500V!
Which seems a stretch, when it’s usually at the most 180V for most of the receiving type valves we use.

Thank you for clearing that up Pete. We learn every day