EL34 self cathode biased amp hum pre volume/gain control

[Studiogearlover]

Is it my imagination or is that nut (with the black wire and solder tag) loose?
Have you made sure that all nuts and screws like that are tight?

hi Kevin, I did check... it is super tight... i think the angle of the pic and the photo is misleading a bit... it is with an extra washer. That is the connection point of the negative side of the main filter capacitor..this screw is also holding the main transformer metal frame. (one out of 4)

cheers

 

[abbey road d enfer]

I did short the two 800k resisitors R31 and R34. The hum is significantly less... like at least 60-70% if not more but it is still there.

It seems to indicate ripple on B+.

The second option with the resistors ....will need to check if I have the resistors... this should go between cathode pin 3 and the bias resistor right?

That's correct.

just checked, sadly I don't have 10 Ohms resistors ...is it worth it measuring without these?

You may try with lower value, e.g. with 4.7 ohm, the factor would be 4.7mV per mA.
You may also parallel several resistors, e.g. two 22r resistors result in 11 ohms, so the scale factor is 11mV per mA.
Five 47r resistors in parallels would give 9.4mV per mA.

 

[Studiogearlover]

You may try with lower value, e.g. with 4.7 ohm, the factor would be 4.7mV per mA.
You may also parallel several resistors, e.g. two 22r resistors result in 11 ohms, so the scale factor is 11mV per mA.
Five 47r resistors in parallels would give 9.4mV per mA.
[/quote]

Thanks...  sadly my resistors are starting at 1k ....  can I use by any chance a fixed axial inductor? I  have a pair measuring 4 ohms (rated at 2,7?) and 0.205A 47uH?  ... i think the answer  will be a big NO  ;D

 

[abbey road d enfer]

You may try with lower value, e.g. with 4.7 ohm, the factor would be 4.7mV per mA.
You may also parallel several resistors, e.g. two 22r resistors result in 11 ohms, so the scale factor is 11mV per mA.
Five 47r resistors in parallels would give 9.4mV per mA.


Thanks...  sadly my resistors are starting at 1k ....  can I use by any chance a fixed axial inductor? I  have a pair measuring 4 ohms (rated at 2,7?) and 0.205A 47uH?  ... i think the answer  will be a big NO  ;D

Are you sure they're exactly 4 ohms?  What do you measure when you short the probes? I guess it's not zero... I would tend to believe the mfgr.
2.7 ohms would read about 100-130mV, which is probably close to the minimum your meter can accurately measure.
Anyway some deviation from nominal is not a big issue, particularly because the goal is to compare bias currents to make sure or not they are balanced.

 

[Studiogearlover]

Are you sure they're exactly 4 ohms?  What do you measure when you short the probes? I guess it's not zero... I would tend to believe the mfgr.
2.7 ohms would read about 100-130mV, which is probably close to the minimum your meter can accurately measure.
Anyway some deviation from nominal is not a big issue, particularly because the goal is to compare bias currents to make sure or not they are balanced.

hmm... that's strange... now my meter says 1.4 ohms on those inductors...ahh.. its tricking me i guess... sorry for confusion. I guess I can not use these then right? Will order some 10 ohm 1w resistors...that is the only way I can do the test sadly...

 

[abbey road d enfer]

hmm... that's strange... now my meter says 1.4 ohms on those inductors...ahh.. its tricking me i guess... sorry for confusion. I guess I can not use these then right? Will order some 10 ohm 1w resistors...that is the only way I can do the test sadly...

You won't regret it because it makes bias measurement a breeze.

 

[Studiogearlover]

You won't regret it because it makes bias measurement a breeze.

Understood, thank you so much for your help. Ordered the parts. Will be able to do the test in the next couple of days... will come back with the reuslts.

Have a great weekend and stay safe

 

[Studiogearlover]

The more difficult one: measure the idle current of each EL34.  You can do that in several ways:
[list type=decimal]
[*] First turn off the amp,disconnect the wire that goes from the output transformer primary to the anode of one EL34, insert meter in current measurement mode then turn on the amp  (expect about 30-70mA). Repeat for other EL34. Readings should be reasonably similar (+/-10%). BEWARE OF HIGH VOLTAGE
[*] Insert 10 ohms resistors between  each cathode and the 150 ohm resistor. Measure the voltage across each resistor; divide reading in millivolts by 10 for the current value in milliamps. Typically 500mV->50mA. The big plus of this method is that the resistors can be left in place and measurement /balancing bias can be simultaneous using two meters.
[/list] Note: if left in place, these resistors should be at least 1W.
[/quote]

Hey, i hope you are well ! Just got the 10 Ohms resistors delivered. Quick question on the task you've asked with regards this.

The two resistors should go from each pin 8 cathode right? There is a rail here which is going to the bias resistor of each cathode pin8 and pin one tied together. Should I disconnect the rail with the resistors or just solder the resistors there leaving the rail? and the pin 1 and 8 still tied together?


attached a quick drawing...  but I think I need to remove the rail right?

many thanks

 

[scott2000]

If this is what you are saying....yes

 

[Studiogearlover]

If this is what you are saying....yes

Roger that, thanks Scott. Will go now and do some soldering work and then will post everything here...  cheers

 

[Studiogearlover]

Okay.

Soldered in the 10 Ohm resistors.  Voltages in mV was OL so I had to put my DMM to a higher DC setting in order to measure this.

0.634 and 0.520 VDC


So this means 520mV and 634mV as 52mA and 63.4 mA.

I assume this is not equal and this should be biased to be a close match?  I have never done this so I am a bit of a clueless here what to do with this and I do apologies for that

Please help me out... is this why this amp is humming like crazy?

Thanks so much.

Oh and one thing... these EL34s are not grounded at pin 1... just mentioning in case if this making some trouble ?

 

[abbey road d enfer]

I assume this is not equal and this should be biased to be a close match?  I have never done this so I am a bit of a clueless here what to do with this and I do apologies for that

The difference is not very significant; in any case it does not explain the hum.

Oh and one thing... these EL34s are not grounded at pin 1... just mentioning in case if this making some trouble ?

Pin 1 (g3) should be connected to cathode (Pin 8).
Perhaps this amp was designed for 6L6's, which have g3 and k connected internally... It's not a good idea to have g3 floating...

 

[Studiogearlover]

The difference is not very significant; in any case it does not explain the hum.
Pin 1 (g3) should be connected to cathode (Pin 8).


Perhaps this amp was designed for 6L6's, which have g3 and k connected internally... It's not a good idea to have g3 floating...

Thank you.. this amp came with the EL34s ...i only can rely on that... should I ground the socket with pin 1 to the chassy?

What should I do next? I am a bit lost... :-[

 

[abbey road d enfer]

Thank you.. this amp came with the EL34s ...i only can rely on that... should I ground the socket with pin 1 to the chassy?

No. Pin 1 must be connected to Pin 8.

 

[Studiogearlover]

No. Pin 1 must be connected to Pin 8.

Thank you. Understood...  is this hum I am experiencing is not a ground loop right? as that should be 50hz...? should I exclude any grounding issues? or just rewire the whole thing with new sockets?

 

[abbey road d enfer]

Thank you. Understood...  is this hum I am experiencing is not a ground loop right? as that should be 50hz...? should I exclude any grounding issues? or just rewire the whole thing with new sockets?

Ground loop can cause 50Hz or 100Hz depending on position.
I don't think replacing sockets can solve the problem.
Have you connected the EL34 g3's?
If this is not the cause of the problem I'm afraid this will take more knowledge than I can pass over the internet and more test equipment; an audio millivoltmeter and an oscilloscope.

 

[Studiogearlover]

Ground loop can cause 50Hz or 100Hz depending on position.
I don't think replacing sockets can solve the problem.
Have you connected the EL34 g3's?
If this is not the cause of the problem I'm afraid this will take more knowledge than I can pass over the internet and more test equipment; an audio millivoltmeter and an oscilloscope.

Thank you, yes the pin 1 and pin 8 is connected.  I understand...I actually have an old scope, a tube based one which is from repair shop and it seems in working condition.. thank you so much for your help so far.

I think this will be beyond my knowledge, would love to learn more but I guess I need to start at the basics..

 

[scott2000]

Maybe something hokey with the way the heaters are grounded?? It seems ok from what little I understand but, the way that 1/2 12ax7 is wired up is bugging me for some reason......??

 

[Studiogearlover]

Maybe something hokey with the way the heaters are grounded?? It seems ok from what little I understand but, the way that 1/2 12ax7 is wired up is bugging me for some reason......??

Thanks Scott...i wanted to write this actually... i have been poking around that 1/2 12AX7 tube...  anode 1 and grid 1

if i go near by this hum and some high pitch static noise is increasing... if i touch that junction where at the grid where the 10nf and 1M resistor meet, my speaker nearly exploding... this is the same with the anode connected to the 3300pf and a 3k resistor to ground... uuh i just wanted to burn it.  ;D but ... what the hell is going on with this...

 

[scott2000]

Exploding speakers doesn't sound fun. I hate when that happens... so does one of my dogs...

abbey mentioned very early in the thread, about your heaters and, I too am curious about how they are grounded. Your description is confusing and, I'm not sure your drawing is 100% right. I make little errors all of the time when trying to trace things out...

There's always something when I go back and look again it seems.....

Anyway, it would be nice to see how the circuit is wired with respect to the heaters at least.  I'm not sure how it affects everything with the way that 1/2 ecc83 is grounded ...  if the other tubes aren't? maybe someone else knows...

I was thinking pin 9 may not need to be grounded but rather complete the circuit since 4&5 are tied....maybe ground the entire heater circuit with a couple caps or resistors...or pot (like the original drawing)..... Guessing it's wired for 6.3v....

Also, I'm not sure what the school of thought is on how to treat the unused triode.... I see the anode is floating... Don't think it matters but, the other pins are sort of wired strange....like an after thought.... The grid and cathode resistors etc still there.....

That 60k resistor is confusing too.... I still struggle with load lines so, even if I could figure out what is going on  in the drawing, I wouldn't know if it's optimal.....

Hopefully you'll get some more help....

I know it stinks to have to redo things but, in the end, it really doesn't take too long and you'll feel better about making sure everything is right.  I have some hokey looking stuff too and, although I know it's safe and works, I'd be hesitant to share or sell anything to anyone I didn't know or trust not to hurt themselves......