EL34 self cathode biased amp hum pre volume/gain control

[Studiogearlover]

In absence of a full schemo, it's hard to tell. How much is it showing? Does it show same when you swap probes?

will try and make a full schema...what I am attaching here is very similar to what I have in concept. What is different is:

- The V2. Only half of the V2 is used and the eq section is discarded. Its not there.
- small value differences on tone caps and minor differences on some of the resistors

The PSU and the output transformer is the same.

The red drawing is NOT there...

 

[Studiogearlover]

don't want to detract if someone is helping you and yes the wiring (please please pardon me for sounding rude) looks dangerous.

I just wanted to address, if you pull all the tubes (assuming your filter caps are rated to take a jump in voltage) is the hum still there?
alternatively, if THE INSTANT you turn it on is all the hum there? or does it appear when the heaters warm up?  These measures may point to the output transformer being maximally energized by the flux of the power transformer. The solution would be to try to turn the OT for starters.  If this was a heavy modification of an already laid out configuration, then you hope they already addressed this issue in construction.

thanks and best wishes.

Hey. Thank you and I do appreciate your help... the wiring around the tubes are... well old... and these resistors are also at least 50y old i guess...

when I turn the amp on the hum gradually gets louder as soon as the heaters comes alive and start to get warmed up... 

 

[abbey road d enfer]

I just wanted to address, if you pull all the tubes (assuming your filter caps are rated to take a jump in voltage) is the hum still there?
alternatively, if THE INSTANT you turn it on is all the hum there?

Not likely, since the rectifier is a GZ34.

 

[Studiogearlover]

In absence of a full schemo, it's hard to tell. How much is it showing? Does it show same when you swap probes?

I have to apologize, but now I do not have continuity.. i had it for a few times and moments and now there is none. Both pin 3 to ground is measuring a 3M ohms and increasing. If i switch my dmm to continuity mode, there is "OL." and no continuity beep.

 

[scott2000]

drawing above is giving 404 error?

 

[abbey road d enfer]

I have to apologize, but now I do not have continuity.. i had it for a few times and moments and now there is none. Both pin 3 to ground is measuring a 3M ohms and increasing. If i switch my dmm to continuity mode, there is "OL." and no continuity beep.

That's the capacitor discharge you are seeing. Nothing to be concerned with.

 

[scott2000]

Heater circuit is... the 6.4V starting from the transformer going to the EM 87 and  the pilot lamp, then to the two EL34s. These are twisted leads until the ecc83 (the phase splitter?) and then from that point a single wire goes to all the heaters pin 4-5 till the EF86 V1.  At the phase splitter tube is the first point of ground connection to the chassis.

Having a hard time understanding this completely? Is there a drawing ? I can't open the recent one...

 

[abbey road d enfer]

The more I think about, more I suspect the B+ rail is not clean. Are you sure C25 is good? Is it even connected?

 

[Studiogearlover]

Hi

Thanks guys for the reply. Please try this attachment. Is this working now? It seems to be connected, the values are different and I can not locate the C20 for example.. but this schematic was not made for this unit but maybe used as a reference...The bad filter cap I had t here was a 32+32 uf and replaced with a 48+48uFs and that is the C25 and C21.

The C25 I took it off from an amplifier which was good practice amp and only used for a couple of hours. That amp was silent. Then this cap was in my drawer for a year and prior to that was brand new.

The only difference I see at the GZ34 that the pin 8 and 2 is reversed, so the cap and wires for the heaters is connected to the pin 8 and on the schematic the pin 2 is going to the C25.

Should I re wire the whole thing and replace the valve holders?

I've went through again with a chop stick pressing all the tube sockets, the grounding points and the components. Nothing changed the hum. The only change I'heard is when I touched the R7 and C6 and the pin 6 at the V2  (where is a 3300pf cap goes to V3 pin 7, and again this is different to the drawing) so when I am touching these, there is a bass low end hum appears besides the hum.

I am trying to describe what I am hearing, it is difficult to understand through texts I know.. but thought I will try

Thanks guys again!

 

[scott2000]

Please try this attachment. Is this working now? It seems to be connected, the values are different and I can not locate the C20 for example.. but this schematic was not made for this unit but maybe used as a reference...

It's working . Thanks.

Not sure about how useful it will be in the end without it being accurate to what you have in front of you... I don't see a magic eye and I don't think you  mentioned the 300ohm hum pot??.... you bring attention to other differences too...

Everything matters .

Could be running around in circles and it may be something simple. But it's  tough when you're not 100% sure what is going on.

Being certain that this drawing is a good representation by checking against your unit and editing it would be helpful in general.
But you may get lucky in the meantime....

Could even start by editing these small changes with cap values you mention....etc....

Does this thing work?  Maybe you can link the thread you mentioned of it previously...

 

[Studiogearlover]

It's working . Thanks.

Not sure about how useful it will be in the end without it being accurate to what you have in front of you... I don't see a magic eye and I don't think you  mentioned the 300ohm hum pot??.... you bring attention to other differences too...

Everything matters .

Could be running around in circles and it may be something simple. But it's  tough when you're not 100% sure what is going on.

Being certain that this drawing is a good representation by checking against your unit and editing it would be helpful in general.
But you may get lucky in the meantime....

Could even start by editing these small changes with cap values you mention....etc....

Does this thing work?  Maybe you can link the thread you mentioned of it previously...

hey

thanks for your suggestion..i will do an edit today with everything what I have ( the hum pot is not in the unit and the drawing does not indicate the EM87 and there are two pilot lights on the drawing but I have one pilot light).

Will do the changes and come back with an accurate drawing.

 

[abbey road d enfer]

Measure the DC voltage across the 140 ohm resistor.
Measure the DC voltage on plates.
Measure the DC voltage on screens (pin 4).
Measure the B+ DC voltage on C25

Then:
Put the meter in AC, measure the B+ voltage. There should be anything between a few volts and a few dozen, so set the caliber accordingly.

 

[Studiogearlover]

Measure the DC voltage across the 140 ohm resistor.    --->  17 (its a 150ohm actually not 140 my bad)
Measure the DC voltage on plates.                                            ----> 401  and 399
Measure the DC voltage on screens (pin 4).                          ----> 267  and  267 
Measure the B+ DC voltage on C25                                          ----->  405

Then:
Put the meter in AC, measure the B+ voltage. There should be anything between a few volts and a few dozen, so set the caliber accordingly.    B+ AC from the transformer 353 VAC and 353 VAC before the pair of the 120 Ohm resistors

I did draw and changed what I have here in front of me and edited the schematic I have shared previously.  The V2 Anode 1 is not in use at all.

 

[scott2000]

Then:
Put the meter in AC, measure the B+ voltage. There should be anything between a few volts and a few dozen, so set the caliber accordingly.    B+ AC from the transformer 353 VAC and 353 VAC before the pair of the 120 Ohm resistors

B+  voltage is after the rectifier..  So take the AC voltage measurement after it..

 

[Studiogearlover]

B+  voltage is after the rectifier..  So take the AC voltage measurement after it..

:-[ sorry...    just measured it. it is total zero.  0.000 VAC on the dmm

And one weird thing at least to me... is that when I plug in the AC plug into the wall socket, amp off, the negative probe is clipped to a good ground, the positive probe of the dmm is not connected to anything and I see 5.6 VAC on my dmm.

Thank you for your help.

 

[abbey road d enfer]

:-[ sorry...    just measured it. it is total zero.  0.000 VAC on the dmm

And one weird thing at least to me... is that when I plug in the AC plug into the wall socket, amp off, the negative probe is clipped to a good ground, the positive probe of the dmm is not connected to anything and I see 5.6 VAC on my dmm.

Thank you for your help.

It's not weird; it attests presence of an electrostatic field resulting from the electric installation that does not use shielded cable (typical of a domestic installation).

 

[Studiogearlover]

It's not weird; it attests presence of an electrostatic field resulting from the electric installation that does not use shielded cable (typical of a domestic installation).

I see... I did not know that ! Than you ... learned something new again

I did the measurements you've asked previously.... and made a new drawing ....just to exclude the filter cap I did try with another one...no changes... checked the soldering again and tapped everything ...no changes... this hum  is gradually there as soon as the heaters are coming alive after the switch...

I did double check this again  I was wrong.. this is a 100 Hz...and not 50 Hz hum.

I am lost.. really lost .....what this can be... i have all new set of tubes and tried each of them...  :-\

 

[abbey road d enfer]

I did double check this again  I was wrong.. this is a 100 Hz...and not 50 Hz hum.

That's what I thought. Now two other experiments, one simple, one more difficult.
The simple one first: short the grid leak resistors R31 & R34 with a piece of wire. Does hum change?

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.

 

[Studiogearlover]

That's what I thought. Now two other experiments, one simple, one more difficult.
The simple one first: short the grid leak resistors R31 & R34 with a piece of wire. Does hum change?

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.

Thank you so much for this.

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.

With regards the two other suggestions measuring the idle current... the first option i would skip for the reason is that the anodes are connected tight under a rail. So in order to access the anode pin for soldering, i have to remove the rail and everything connected to it at this output stage.

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?

thank you

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

 

[Walrus]

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?