Built a quick mock-up of the amp today since all of my parts finally arrived. Went with the Transcendar "budget" 5k:8 5W output transformers and Hammond power transformers. Other parts were Vishay/Dale, Elna Silmic II, Nichicon UPW, and Panasonic EZP.
Decided to use the #76 tube as input. Schematic is attached and could probably use some tweaking... B+ is a little higher than predicted by PSU Designer, and my first heater supply capacitor is running right at its 16V limit. Hope it holds up until I can get a suitable dropping resistor in there (my junk box 2W, 1.8-ohm resistor smoked after a few seconds). But, it makes sound! And surprisingly good sound, at that.
I do have the dreaded tweeter buzz, though. No hum that I've heard, just the buzz. What might be the cause of this? It is present on both channels. I've used a star ground, so I don't suspect a ground loop.
Decided to use the #76 tube as input. Schematic is attached and could probably use some tweaking... B+ is a little higher than predicted by PSU Designer, and my first heater supply capacitor is running right at its 16V limit. Hope it holds up until I can get a suitable dropping resistor in there (my junk box 2W, 1.8-ohm resistor smoked after a few seconds). But, it makes sound! And surprisingly good sound, at that.
I do have the dreaded tweeter buzz, though. No hum that I've heard, just the buzz. What might be the cause of this? It is present on both channels. I've used a star ground, so I don't suspect a ground loop.
Attachments
hi needtubes,
did u elevate the heater potential to ~50v higher than the cathode voltage of both the driver/power tube? i find that it helps with hum/buzz.
did u elevate the heater potential to ~50v higher than the cathode voltage of both the driver/power tube? i find that it helps with hum/buzz.
Fixed it... I had forgotten to reference the heater supply to anything at all. 
Tied it to ground and the buzz is gone. Quite a lovely amp. Does it sound better than my 300b monos?! 😱
Tied it to ground and the buzz is gone. Quite a lovely amp. Does it sound better than my 300b monos?! 😱
Should I use a thermistor to limit inrush current to the heaters? They are at full glow mere seconds after turn on.
Also, my 269GX gets quite warm and buzzes, although I am not running it close to maximum. I guess this is normal? I should tie the Hammond transformer body to earth ground, right?
Also, my 269GX gets quite warm and buzzes, although I am not running it close to maximum. I guess this is normal? I should tie the Hammond transformer body to earth ground, right?
I would not worry much for small tubes like 1626 but if u wanna proling the life, why not. Did u measure the voltage at the filament?
Warm as in what temperature? Buzzes is not normal for good quality trannies. Is yr mains having dc components? Dc at mains will saturate the transformer core n cause hum. I m using jandkaudiodesign transformers n i m not seein such probs.
Usually mine r grounded to the chassis. Is yr chassis grounded?
Warm as in what temperature? Buzzes is not normal for good quality trannies. Is yr mains having dc components? Dc at mains will saturate the transformer core n cause hum. I m using jandkaudiodesign transformers n i m not seein such probs.
Usually mine r grounded to the chassis. Is yr chassis grounded?
As tubeworld sayd its normal that the tubes light up when heating starts.
But if you don´t like it a thermistor will work.
Also you can try to switch a little resistor in series with the heater.
When the heater is cold the res is about 6Ohms - when its hot its about 50Ohms.
So if you switch a 4,7Ohm res. in series your cold current will be only the half and when the heater is warm you will have only about 1Volt less wich is tolerable.
For your mainstransformer - yes it has to be grounded with a separate wire connected to chassis-earth.
And for the buzz try to set it on rubbergrommets.
Good luck with your darling.
Hilmar
But if you don´t like it a thermistor will work.
Also you can try to switch a little resistor in series with the heater.
When the heater is cold the res is about 6Ohms - when its hot its about 50Ohms.
So if you switch a 4,7Ohm res. in series your cold current will be only the half and when the heater is warm you will have only about 1Volt less wich is tolerable.
For your mainstransformer - yes it has to be grounded with a separate wire connected to chassis-earth.
And for the buzz try to set it on rubbergrommets.
Good luck with your darling.
Hilmar
There is very little DC on the line (~20mV). I don't know how hot the transformer gets since I don't have a thermometer, but it is definitely warm to the touch and feels hot after touching it a few seconds. Not burning hot, but quite warm.
Seems the buzzing starts when I turn on the high voltage supply. Also, the transformer is not currently grounded since the amp is in the breadboard stage.
Seems the buzzing starts when I turn on the high voltage supply. Also, the transformer is not currently grounded since the amp is in the breadboard stage.
Maybe I am overloading it? The specs claim 225-0-225 at 75mA and 6.3V at 2.5A.
The heaters draw 0.8A from the 6.3V lines after the voltage doubler (which should be good for a bit over 1A) and I am pulling 50-60mA off the HV lines.
The heaters draw 0.8A from the 6.3V lines after the voltage doubler (which should be good for a bit over 1A) and I am pulling 50-60mA off the HV lines.
if you can have your fingers on the power trans for 5-6 seconds, then it should be fine. normal power trans work in such fashion unless you're getting the audiophile trans that are over-spec and uses better (lower loss) cores like the Z11.
i had used ordinary cores before and compared it to the properly designed (custom made) Z11 cores and it makes a huge difference in terms of electrical and mechanical properties (not to mention better sound). normal cores are very hot to touch (5-6s rule) and Z11 is barely warm to the touch.
as for the hum, you can try mounting it securely on a piece of plank + ground it to see if it helps. if it does, probably when you mount on the chassis, it will be fine. if not and if it disturbs you, maybe you can consider getting a better power transformer. humming transformers are annoying, i do agree. sometimes humming could be the power transformer construction/design itself.
check out this website on transformer design. the owner shares quite a lot of information on transformer design. it might help you understand the problem you are facing: J&K Audio Design
i had used ordinary cores before and compared it to the properly designed (custom made) Z11 cores and it makes a huge difference in terms of electrical and mechanical properties (not to mention better sound). normal cores are very hot to touch (5-6s rule) and Z11 is barely warm to the touch.
as for the hum, you can try mounting it securely on a piece of plank + ground it to see if it helps. if it does, probably when you mount on the chassis, it will be fine. if not and if it disturbs you, maybe you can consider getting a better power transformer. humming transformers are annoying, i do agree. sometimes humming could be the power transformer construction/design itself.
check out this website on transformer design. the owner shares quite a lot of information on transformer design. it might help you understand the problem you are facing: J&K Audio Design
After some modelling in Duncan Amps PSU Designer, I think I found the issue...
As it stands, my voltage doubler seems to be pulling ~3.2A from the 2.5A-rated winding, even though I only have 0.8A worth of filaments. This means everything in my heater supply is working too hard. Even adding some series resistance before the diodes can only bring the current draw to 2.8A if I want my regulator to function.
This is bad news... I don't want to change transformers since I already have a top plate with this transformer's mounting holes on order.
Is there anything I can do to get it to run at a lighter load?
As it stands, my voltage doubler seems to be pulling ~3.2A from the 2.5A-rated winding, even though I only have 0.8A worth of filaments. This means everything in my heater supply is working too hard. Even adding some series resistance before the diodes can only bring the current draw to 2.8A if I want my regulator to function.
This is bad news... I don't want to change transformers since I already have a top plate with this transformer's mounting holes on order.
Is there anything I can do to get it to run at a lighter load?
Do you have room under the chassis to add a separate 12.6 volt transformer for the filaments? Or a 6.3 volt filament tranny and keep your doubler circuit.
Try this trick before you throw in the towel, put a forward biased diode ( --|>|---) in series with the output of the regulator and the filaments, see if that can get rid of the inrush problem.
You might also try the filament circuit without the voltage regulator. That's how I run mine using that same exact transformer. I use a 12SL7 as a driver and it's pretty quiet. My transformer runs cool.
That might be better. How much capacitance do you have in the doubler? Maybe I am overtaxing the transformer with the ultra-low impedance Nichicon UPWs.
Mine uses a pair of 2200 uF @ 25 volt caps for the doubler and then straight to the filaments with no further filtering.
Hm. I will try it that way. Looks like the current requirement will be below 2.5A if I put a small resistor in series with the 6.3V line and use one smoothing cap after the doubler (according to PSU Designer). Guess regulation isn't really necessary here.
Did some more modeling with PSU Designer... Looks like my high voltage supply is pulling 105mA RMS from the transformer which is rated at 75mA. I have not confirmed this in real life since the amp is currently in pieces awaiting the arrival of my top plate.
So, did I just make really bad design decisions or is PSU Designer lying to me?
So, did I just make really bad design decisions or is PSU Designer lying to me?
- Status
- Not open for further replies.