• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

A Neophyte's 12B4 Build Thread

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#41
thereverend said:



So what exactly does this red LED treatment do? Also, why would changing the anode to 6k be helpful? Sorry for my confusion here. I guess I don't understand why using an LED would give the same effect as a capacitor. I can see that a charged cap would stop current in all directions, and I know that an LED stops current in the backwards direction, thus giving a forward bias.

Next question for now as well: Lat night we talked about the input impedance from my B&K. Now just speaking in a general case, what design parameters are effected by the input impedance? In other words, what, in the case of your schematic, is a function of input impedance of the power amp?

Thanks!
Colin
Click to expand...

For cathode bias, the resistor sets the relationship between the grid and cathode at DC. Normally, the grid must be negative relative to the cathode. How much negative determines the bias current. The capacitor allows the AC to bypass the cathode resistor. An LED conducts at a specific voltage and has a very low dynamic impedance. Which is to say that the voltage doesn't change much with changes in current. So it's basically a constant voltage device. In this circuit, the combination of resistor and parallel capacitor is essentially a constant voltage device at audio frequency.

If you are going to play with tubes a bit, get a copy of "Valve Amplifiers" by Morgan Jones. It does an excellent job of explaining basic principles, using practical examples.

Sheldon
 
#43
Each tube is pulling 25mA, so that's 50 there. If you use the OD150 tubes then about 20mA there too (shared PSU between channels), so I'd go for min 100mA. The Hammonds are fine. Details of which particular model will need to be calculated once the final design details of the circuit and PSU are in place.
 
#44
Alright, this one is the one. 130 mA sound like a good safe number. I am working up a decent BOM here, I will be posting it in the next couple of days when I finish it up. Thanks for all of your help.

What was that about the OD150? I remember talking about the VR150 and how it would only give 40mA, but not the OD150.

EDIT: ahh nevermind, I think you meant OD3A
 
#45
OD3A/VR150. Tomato/tomato.

No experience with the Hammond open frame stuff, but should be OK. More experienced users may want to confirm though. Rating is fine. Except where weight, size or costs are an issue bigger is almost always better with transformers, IMO.

Second the suggestion above that you buy both of Morgan Jone's books, and read everything on tubecad whilst waiting for them to arrive.
 
#47
I have been reading through the really large thread about this project and I am concerned that my PS is not going to be regulated well enough. Am I just being worried about nothing? It sounds like over there they have all sorts of neat ideas with their fancy VR150 or LM317, should I be trying to incorporate something like that into this design?
 
#48
Brett said:
Each tube is pulling 25mA, so that's 50 there. If you use the OD150 tubes then about 20mA there too (shared PSU between channels), so I'd go for min 100mA. The Hammonds are fine. Details of which particular model will need to be calculated once the final design details of the circuit and PSU are in place.
Click to expand...


shunting just 20mA isn't good idea ;
this construction needs dedicated string of ODs for each channel , so - consumption will be in range of 25+20 mA per channel .
 
#49
Back in post 14 Arnold said that I shouldn't use the VR150 since it will give me only 40mA as opposed to the required 50mA. I would happily insert more glowing things into the design if it were feasible and I am curious what it would take to build a PS using the VR150 to achieve the required 25mA /channel.

Best,
Colin

EDIT

how about this one? http://www.diyaudio.com/forums/attachment.php?s=&postid=910671&stamp=1147048038
 
#50
thereverend said:
Back in post 14 Arnold said that I shouldn't use the VR150 since it will give me only 40mA as opposed to the required 50mA. I would happily insert more glowing things into the design if it were feasible and I am curious what it would take to build a PS using the VR150 to achieve the required 25mA /channel.

Best,
Colin

EDIT

how about this one? http://www.diyaudio.com/forums/attachment.php?s=&postid=910671&stamp=1147048038
Click to expand...

yes - this is schmtc I drew for Mark ,according to his tubes and wishes .

exactly what I meant in previous post; you need almost same amount of shunted current as in active stage,to preserve functionality even in cases when mains are in 10% boundaries.... nothing else- just engineering rule,habit or praxis...whatever ;

my advice (without thorough looking at previous posts,really) :

use plate choke instead plate resistor ;
change Ua accordingly (in 150V range,so- just one OD is needed per stage , also xformer voltage is lover) .

more info you'll find in that thread,also pointing to adequate PT
 
#51
How much current is this line stage actually going to need to swing? 2Vrms into 25k? 20Vrms into 2k? A common supply will be fine. Use GAS regs, passive sectioned RC stages a la Jones or SS regs,:whatever way it'll be excellent.
 
#52
I think Brett's approach is better suited as a starting point for this situation - first time, budget conscious builder. Leave room to go the Zen Mod route if you want to build up to the ultimate. Changes along the way are an opportunity to hear different approaches.

And Rev., while you are waiting for Morgan's books and pondering your way through John Broskie's terrific and dense site, check this one out: http://members.aol.com/sbench101/
It's not so polished, but it's full of good stuff, some basic and presented for the beginner, and some sophistocated and novel design stuff.

Sheldon
 
#53
That first schematic should be updated, labelling a DC-coupled grid voltage -9.25 Vdc is seriously misleading.


SY said:
LEDs have a very low AC impedance .... that dynamic impedance stays low and relatively constant well into the hundreds of kilohertz.
Click to expand...

Hmmm, sounds like someone's been measuring.
 
#54
So what do most people use for switching between sources with a project like this? Should I put a switch onboard, or should I build a more universal offboard passive switchbox? Could I still use my NAD's tape out without introducing a bunch of active circuitry (I assume not). I can't imagine building a switch being particularly hard to do!
 
#55

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#56
Your Resistors on your power supply should be more than 1/2 watt...I built a similar 12B4 pre (I have two blue LED instead of Cathode resistor/Cap) and my resistors were 220 ohm 5Watt...

Calculate and you will see that 1/2 watt will not do...I think you meant 5W not .5W...actually you were looking at 50 - 70ma current total? Your over 1W at 70ma (.070*.070*220) and most of us like to have safety factor of at least 2 so you may be near a couple watts at least...

Otherwise you are there...did you have tubes on the list? 12B4 may be a little hard to find...

I would also advise DC heaters - my original build had AC heater connected had too much hum - just add a single 10,000uF cap, a .5ohm 3W resistor and four more diodes (I don't think the ones you ordered will work as we would be dealing with higher current for the heaters)

BTW, Do you have transformers for the filament (heaters)? you need 6.3VAC or 12.6VAC, Don't know if your Hammond has another secondary for the filaments...
 
#57
hey-Hey!!!,
I'd suggest using standard intensity red LED's. Examine the I/V curve( if it is published ), and you'll probably discover that they're the lowest impedance group/type of LED. You need more of them, but that's not such a big deal, eh?
cheers,
Douglas
 
#58
john65b said:
Your Resistors on your power supply should be more than 1/2 watt...I built a similar 12B4 pre (I have two blue LED instead of Cathode resistor/Cap) and my resistors were 220 ohm 5Watt...

Calculate and you will see that 1/2 watt will not do...I think you meant 5W not .5W...actually you were looking at 50 - 70ma current total? Your over 1W at 70ma (.070*.070*220) and most of us like to have safety factor of at least 2 so you may be near a couple watts at least...

Otherwise you are there...did you have tubes on the list? 12B4 may be a little hard to find...

I would also advise DC heaters - my original build had AC heater connected had too much hum - just add a single 10,000uF cap, a .5ohm 3W resistor and four more diodes (I don't think the ones you ordered will work as we would be dealing with higher current for the heaters)

BTW, Do you have transformers for the filament (heaters)? you need 6.3VAC or 12.6VAC, Don't know if your Hammond has another secondary for the filaments...
Click to expand...

I figured that the .5W resistors would be a problem. To be honest I didn't do any calculations, I knew someone would pipe up. Sorry for my laziness.

Right about the tubes, I don't really know where I am going to get them yet, but they seem reasonably inexpensive.

The Hammond does have a 6.3VAC filament winding. I was not really sure what that was for, and I don't really know what exactly it does for my amp. Also, I don't really know what you mean by adding those parts, where exactly would I add them? Sorry for my newbie questions, I really appreciate yours and everyone's help with all this.


-Colin
 
#59
The Hammond does have a 6.3VAC filament winding. I was not really sure what that was for, and I don't really know what exactly it does for my amp. Also, I don't really know what you mean by adding those parts, where exactly would I add them? Sorry for my newbie questions, I really appreciate yours and everyone's help with all this.
Click to expand...

There is a thread entitled "On Line Tube Learning for newbies...." here: http://www.diyaudio.com/forums/showthread.php?s=&threadid=38278 You should read through some of it before you go any further. Specifically, read the NEETS manual. This will answer almost all of your questions, including the ones you haven't asked yet.

This is basic stuff, and with your electrical knowledge, you should get through it in an afternoon.

Second, read the thread on working with high voltages: http://www.diyaudio.com/forums/showthread.php?s=&threadid=30172 Glancing through this thread, I didn't see anyone telling you that your power supply can kill you yet, and that seems worth knowing something about.
 
#60
Also, for your BOM,

- you can get the IEC at Digikey which cuts out one shipping charge

- you can get basically everything else not from Digikey from tubesandmore.com -- save yourself some shipping.

- if you are using a FC to bypass Rk, don't bother with the Auricaps. You will swamp their goodness with the badness of the FC. I'd just use a Solen as the output cap for now. Buy the good caps once you eliminate the hum.

- why are you buying so many different bypass caps? The difference between 56uF and 68uF is nothing. In fact, if you insist on using a cap in this spot (which is right in the signal path so quality matters) bigger is likely better. Try this part number at Digikey (604-1058-ND) it is a much better sounding cap and doesn't have steel leads.
 
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