I was a little too fast getting excited about the hum. There is hum, and there is too much of it. Definetely 50hz cycle hum. I have tried elevating the heaters center tap to 40 vdc. I have tried alternate grounding suggestions from the build guide. Tubes have been swapped. No difference. Maybe I should try dc heaters in the preamp section?
There are too things I need to ask though:
1: My chassis is steel, and not aluminum. I know it calls for an aluminum chassis, but I thought I would give it a try with steel, because it was what I had. The differences in current flow? Could this cause 50 hz hum?
2. When I turn on the power switch (not the standby) there is an audible acoustic hum from the power transformer. Not too loud, but it is there. Could this sort of hum find it´s way to the speaker?
There are too things I need to ask though:
1: My chassis is steel, and not aluminum. I know it calls for an aluminum chassis, but I thought I would give it a try with steel, because it was what I had. The differences in current flow? Could this cause 50 hz hum?
2. When I turn on the power switch (not the standby) there is an audible acoustic hum from the power transformer. Not too loud, but it is there. Could this sort of hum find it´s way to the speaker?
Yes, hum can be an elusive solution. I see you mentioned 50 Hz. So you are not in the US, wasn't sure of your location.
My tube amp has an aluminum chassis, I do not have any hum. I believe it worked out well that I carefully arranged my circuitry as best I could to keep low level signal paths as far as possible from typical hum sources. And I used a star ground scheme and kept high current sections from flowing in my preamp ground circuits. Each dual triode has its own wire connected to my star. Besides appropriate star connections for the outputs and etc.
Your AC line cord connection does have a secure ground to the chassis right beside the AC entry point ? That one does not go to the star.
I used 6.3 V AC to my heaters and a grounded center tap of the heater winding.
Some builders shun the star ground and use ground buses or a combination, there are different ways to attempt to minimize hum.
I referred to several star ground articles and papers. Here is one: http://www.geofex.com/Article_Folders/stargnd/stargnd.htm
And another: Grounding
There are more to be found on the internet.
You have tried elevating the heaters with DC and tried new tubes.
The hum from the power transformer might be loose laminations or not quite tight to the chassis...I don't think that will introduce hum heard in the speaker.
If the power and output transformers have marginal shielding and are oriented to couple in the magnetic field around the transformers, that can be a source.
Curious - you decided to stack zeners on your B+. There is an appropriate resistor in the main B+ feed before the zener stack to ground ? Just making sure. If not, the zeners would have probably not lasted long from high current flow and the heating while avalanched.
If you have an O'Scope, perhaps look at the B+ and bias ripple amounts.
Are your input and output jacks isolated from the chassis ?
There are many ways for hum to appear.
Do you have hum with just the output tubes installed and remove the preamp and driver tubes ?
Insufficient bias making the outputs sink excessive current ?
An unbalance in the output section ?
It is hard to narrow down without having it in front of me.
I imagine many of the members can add a lot to what I have said.
Lots of experience and highly qualified people on this site.
carry on
Deric
Zener clamping the voltage will increase the hum...
Try only inserting a resistors to drop voltage without the Zener. Without input signal is not aproblem the voltage sag so you can try to indentify the hum source.
Try only inserting a resistors to drop voltage without the Zener. Without input signal is not aproblem the voltage sag so you can try to indentify the hum source.
Hi again
Unfortunately removing the zeners does not affect the hum.
I´m not sure what you mean by " appropriate resistor in the main b+ feed before the zener stack" - but I have attached a layout of what I did...the zeners still work, and aren´t that hot😉
When I pull the first tube, there is a little hum
When I pull the second tube, there is a little hum
When I pull the phase inverter, there is nothing to be heard
Unfortunately removing the zeners does not affect the hum.
I´m not sure what you mean by " appropriate resistor in the main b+ feed before the zener stack" - but I have attached a layout of what I did...the zeners still work, and aren´t that hot😉
When I pull the first tube, there is a little hum
When I pull the second tube, there is a little hum
When I pull the phase inverter, there is nothing to be heard
I have read that NOS tubes are a must in a trainwreck, but wanted to wait until I had it working before spending too much money. I just remembered that I had a Sylvania NOS 12ay7 and 12au7....unfortunately no 12ax7s
I put the 12ay7 in v1 and used the 12au7 as phase inverter. I know there is much less gain in these tubes, but still,I find it a bit surprising that the hum is absolutely gone? There is a bit of hiss, even if I dime all the controls, but that seems fair enough. The breakup happens later on the volume control, but other than that it sounds ok....but it has not got the "express vibe" obviously.
I put the 12ay7 in v1 and used the 12au7 as phase inverter. I know there is much less gain in these tubes, but still,I find it a bit surprising that the hum is absolutely gone? There is a bit of hiss, even if I dime all the controls, but that seems fair enough. The breakup happens later on the volume control, but other than that it sounds ok....but it has not got the "express vibe" obviously.
Use a scope and find the source of hum.
The hum it is not generated by the tube.
The only difference is that some tube are more immune to coupled noise, some not.
The hum it is not generated by the tube.
The only difference is that some tube are more immune to coupled noise, some not.
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