EL34 self cathode biased amp hum pre volume/gain control

[Studiogearlover]

Hey Guys

I have an EL34  amplifier with cathode biased output. I have been working repairing this for quiet a while.  It hums like crazy when the volume/gain control is fully down and this hum does not change if I turn the controls.

Would you suspect the EL34s ( i have no history of these tubes). ? They are not red plating but if I poke one of the tube with my finger I hear something is buzzing inside the tube.

OR a bad ground connection somewhere?  I've poked around with a chop sticks and no change.

New rectifier tube does not change anything. Preamp tube swapped, no change.

Before I spend money on a new set of EL34s could you please guide me what else to check?

1. filter cap changes
2. film caps changed
3 resistors checked
4. switches and transformer checked
5. soldering checked
6. tube sockets tightened up and cleaned
7 bias resistor and coupling cap changed.
8 heater wires are twisted

Its a home built amp (not by myself) schematic is not accurate what I have for it. The amp has: GZ34, EF86, 2x ECC83 and 2x EL34 output tubes.

thanks for your suggestion

 

[abbey road d enfer]

Hey Guys

I have an EL34  amplifier with cathode biased output. I have been working repairing this for quiet a while.  It hums like crazy when the volume/gain control is fully down and this hum does not change if I turn the controls.

Would you suspect the EL34s ( i have no history of these tubes). ? They are not red plating but if I poke one of the tube with my finger I hear something is buzzing inside the tube.

OR a bad ground connection somewhere?  I've poked around with a chop sticks and no change.

New rectifier tube does not change anything. Preamp tube swapped, no change.

Before I spend money on a new set of EL34s could you please guide me what else to check?

1. filter cap changes
2. film caps changed
3 resistors checked
4. switches and transformer checked
5. soldering checked
6. tube sockets tightened up and cleaned
7 bias resistor and coupling cap changed.
8 heater wires are twisted

Its a home built amp (not by myself) schematic is not accurate what I have for it. The amp has: GZ34, EF86, 2x ECC83 and 2x EL34 output tubes.

thanks for your suggestion

Could be many things...
Could be bias unbalance. What happens when you pull out the phase-splitter?
Is the heater circuit properly grounded? There should be either two resistors (about 100 ohms) to ground or a hum balance pot.
Is the amp properly grounded? With a 3-core cable and 3-prong plug, and the earth connected to chassis?

 

[Studiogearlover]

Could be many things...
Could be bias unbalance. What happens when you pull out the phase-splitter?
Is the heater circuit properly grounded? There should be either two resistors (about 100 ohms) to ground or a hum balance pot.
Is the amp properly grounded? With a 3-core cable and 3-prong plug, and the earth connected to chassis?

Hey Thanks

I have pulled out the last ecc83 tube before the power tubes. The hum is lower but still there.

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.

What I found really odd is that the output to speaker + and - (chassis) has continuity...? somewhere the +ve signal path is shorting? or this is a silly question of mine?

No sign of any resistors in the heater path and the amp has a 3 prong cord, fused and grounded.

 

[abbey road d enfer]

Hey Thanks

I have pulled out the last ecc83 tube before the power tubes. The hum is lower but still there.

can you measure the bias current?

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.

Do you mean the heater circuit is grounded there?

What I found really odd is that the output to speaker + and - (chassis) has continuity...? somewhere the +ve signal path is shorting? or this is a silly question of mine?

Not surprizing. Very often, the secondary is grounded - that is necessary if using NFB; so on one side you meaure zero ohms, and on teh other side you measure the DC resistance of teh secondary, which is about a fraction of ohm.

 

[Studiogearlover]

can you measure the bias current?    uuh will try... i can not get to pins with my DMM only I can measure and have more space at the EL34s

Do you mean the heater circuit is grounded there?  Yes, there is a star ground point and that is the first ground point there to the chassis.

Not surprizing. Very often, the secondary is grounded - that is necessary if using NFB; so on one side you meaure zero ohms, and on teh other side you measure the DC resistance of teh secondary, which is about a fraction of ohm. Noted, thanks

attached a pic or two, there are already new components ( the amp had exactly the same hum before anything was replaced there or changed , besides the squealing, which was caused by the  microphonic EF86 V1)

Many thanks for your help.

 

[abbey road d enfer]

attached a pic or two, there are already new components ( the amp had exactly the same hum before anything was replaced there or changed , besides the squealing, which was caused by the  microphonic EF86 V1)

Many thanks for your help.

I've never seen such a rat's nest, looks like it's been wired with a Gatlin! The fact it produces any sound at all is a miracle.
If I was to work on it, I would start with cleaning up the mess. The panel potentiometer is not even wired.

 

[Studiogearlover]

I've never seen such a rat's nest, looks like it's been wired with a Gatlin! The fact it produces any sound at all is a miracle.
If I was to work on it, I would start with cleaning up the mess. The panel potentiometer is not even wired.

Yes, good point. The transformers and Siemens tubes there a really good quality...I hope it is worth it. The resistors there also old Siemens its a mix of 0.5w and 1w ... 1w is not necessary for this application right? I can get away with 0.5w values? the space is very limited there... will order new tube holders too.

Thanks again!

 

[abbey road d enfer]

Yes, good point. The transformers and Siemens tubes there a really good quality...I hope it is worth it. The resistors there also old Siemens its a mix of 0.5w and 1w ... 1w is not necessary for this application right? I can get away with 0.5w values? the space is very limited there... will order new tube holders too.

Thanks again!

You need to draw the schematic. Annotate the schemo with indications where the grounds are located. For example indicate where the cathodes of the EL34 are grounded in relation to the main smoothing caps. ATM I don't know if the EL34's are self-biased (with one common or two separate resistors between cathodes and ground) or fixed-bias (with a negative voltage provided by a separate winding, or a tap in the main winding).
Then we can try to work out why it hums.

 

[Studiogearlover]

You need to draw the schematic. Annotate the schemo with indications where the grounds are located. For example indicate where the cathodes of the EL34 are grounded in relation to the main smoothing caps. ATM I don't know if the EL34's are self-biased (with one common or two separate resistors between cathodes and ground) or fixed-bias (with a negative voltage provided by a separate winding, or a tap in the main winding).
Then we can try to work out why it hums.

Roger that. It is self biased with one resistor and cap... this is how it looks like at the end stage attached... (i did post this amp in another thread  but not with the hum problem you've might seen this)

Values are slightly different, the bias resistor is in this amplifier is 150 Ohms

I am waiting also for another set of EL34s I think I would need that to exclude that the current ones are not faulty...

Many thanks for your suggestions! It is helping a lot...

Additionally, just got hold of another pair of EL34. Regrdless how I mix all of these 4 tubes and swap them around, same hum is happening. It is 50Hz and not responding to any of the gain/ volume controls.

I dont see that the EL34s are grounded anywhere....

 

[abbey road d enfer]

i did post this amp in another thread  but not with the hum problem you've might seen this

No I haven't. What thread?

Values are slightly different, the bias resistor is in this amplifier is 150 Ohms

The connection between the center tap of the mains xfmr and the negatives of C25 and C24 must be as short and sturdy as possible. Is that the case?

 

[Studiogearlover]

No I haven't. What thread?
The connection between the center tap of the mains xfmr and the negatives of C25 and C24 must be as short and sturdy as possible. Is that the case?

I don't see that the center tap is connected to any of these points... the C25 negative has a starground to chassy with a vey short lead and the C24 negative goes to another star ground with several other resistors and from this, a lead is going to the AC connector earth.  :'(

The center tap has another ground point which at the fastening screw of the rectifier tube socket.

 

[abbey road d enfer]

I don't see that the center tap is connected to any of these points... the C25 negative has a starground to chassy with a vey short lead and the C24 negative goes to another star ground with several other resistors and from this, a lead is going to the AC connector earth.  :'(

The center tap has another ground point which at the fastening screw of the rectifier tube socket.

Check any oxidizing. Anyway I would make a strong connection with solid wire between these points.

 

[Studiogearlover]

Check any oxidizing. Anyway I would make a strong connection with solid wire between these points.

Thank you. I did that, the stargrounds and soldering are new.  The hum is really loud tho... like very loud..not a slight 50Hz hum ...will re check again...

 

[scott2000]

Merlin's grounding info has some good stuff in it
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwjkpo6p8uzoAhVhm-AKHe3rBkkQFjAAegQIAhAB&url=http%3A%2F%2Fwww.valvewizard.co.uk%2FGrounding.pdf&usg=AOvVaw0Si1Yp7qjRr72et6IMZHFs

Aiken amps has a good page on grounding as well

Ian has some grounding info....

may just want to familiarize yourself with good practices then the obvious issue might stand out more?

 

[abbey road d enfer]

Thank you. I did that, the stargrounds and soldering are new.  The hum is really loud tho... like very loud..not a slight 50Hz hum ...will re check again...

You didn't tell me the results of checking ground conductance for the heaters.

 

[Studiogearlover]

You didn't tell me the results of checking ground conductance for the heaters.

I am sorry I have missed that... heather conductance to ground? yes. There is. I hope I am answering this correctly..there is continuity between the heaters and the ground. Measured to the ground connection point where the heater wire first hit the ground coming from the EL34s. 

Please forgive me as I am not an expert, still figuring out some stuff here...

The plates has also continuity between the chassis  .. the pin3's from the EL34s... ?

 

[abbey road d enfer]

The plates has also continuity between the chassis  .. the pin3's from the EL34s... ?

Is that real continuity; I mean is the indication stable? Plates go the output xfmr than to the smoothing caps. One usually sees the capacitor discharge there.

 

[Studiogearlover]

Is that real continuity; I mean is the indication stable? Plates go the output xfmr than to the smoothing caps. One usually sees the capacitor discharge there.

it seems so ... my dmm showing me clear continuity... is this normal?

 

[abbey road d enfer]

it seems so ... my dmm showing me clear continuity... is this normal?

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

 

[andyfromdenver]

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.