Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid.....

[Looneytunes]

Hi all,

I'm a total newbie. I’ve never built a thing that made sound. But, I'm fascinated by the whole idea of it, and I'm fascinated by tubes.

I found the Etude preamp on the internet and thought that it might be a good way to get started in tubes. Audio circuits can't get much simpler than this, right? I'm also thinking that this project might teach me a thing or two about power supplies. When I saw Mark's posts comparing the 12B4A with the GG preamp (with the 12B4 winning big time), that did it for me. I'm going to build one!

I have a lot to learn, both about tubes and electronics in general. I have already bought an electronics book (kind of basic), a Beginner’s Guide to Tube Audio Design, and I'm reading your posts. Things are slowly starting to make a little sense, but I still have a long way to go. Unfortunately, I don't know any of the tricks. I hope you all can help me out in that department.

I need to start buying some parts to play with... tubes for starters. I found both GE and RCA 12B4As on the internet for only $4.10 each (pretty cheap, I'd say). Can anyone tell me which is better? Also, how many should I buy for a project like this? I know that I only NEED two. But, should I buy a bunch extra so I can find match pairs, or to have some handy to replace the ones that meet an (ahem) untimely demise?

Any and all suggestions and/or tips on how to get started would be very much appreciated.

Hopefully, I won't electrocute the cat!

Thanks

 

[Mark A. Gulbrandsen]

You would probably have really good luck with JAN 12B4's more so than run of the mill 12B4's. JAN(Joint Army Navy) tubes were built to much higher standards than every day tubes were. I have only tried out JAN tubes so far because others have had problems with noisy/microphonic 12B4's. Jan tubes should only cost you a little more than the regular tubes. I paid 10.00 each for my NOS 12B4's.

Above all don't electrocute yourself or the cat. If you're not familiar with High Voltage safety then you really should become familiar with it way before you apply any voltage to any circuits.

Good luck!

Mark

 

[Bryan]

I can second the JAN 12B4A. I've tried a BUNCH of different makes (RCA, Sylvania, JAN, and a few others that escape my mind) and the JAN were the least microphonic. Tubes within the same manufacturer will vary as well, so at 4.10/tube, I would buy a bunch and find a few pairs that are good. When I purchased my 12B4A's, I was paying about 1.80-2.20/tube. This made searching out pairs relatively affordable.

In my pre, I use a LCLCLC - VR - CCS to feed the plate, and I have to say it is glorious. I run the 12B4A at about 20-25mA. It's a very resolving and detailed set-up! I wont go back!

BK

 

[VT67]

In my pre, I use a LCLCLC - VR - CCS to feed the plate, and I have to say it is glorious. I run the 12B4A at about 20-25mA. It's a very resolving and detailed set-up! I wont go back!

Do you mind sharing the schematic of this PS with us?

Regards

 

[Bryan]

Sure, but I need to draw up the schematic as it currently stands. I will try to do so this evening...

Briefly, it is a hammond 175VCT Transformer, with each leg of the secondary feeding a 12AX4 damper diode. There are two schotcky rectifiers off the plates of the 12AX4, forming a tube hybrid rectifier. The B+ is filtered through LCLCLC where all chokes are 20H Hammonds. The first cap is a 50uF unylytic (only b/c I ran out of real estate), the second is a 50uF ASC motor run, and the final is a 60uF ASC motor run. From the third cap I use an appropriate dropping resistor to series connected OA2 and OB3 VR tubes to ground, each bypassed with very small ASC cap, which regulates B+ at 190V. This feeds a Pimm self biased CCS which in turn feeds the plates. The amp is complete dual mono.

Sorry if this is more confusing than helpful. I'll get the scheme up tonight...

 

[VT67]

Thanks Brian.
I almost get the picture of the PS. What I also understand is that it will take up quite some space with all those chokes and cap's. If someone can come come up with a deicent SS power supply for this amp I would realy appreciate it.

Regards

 

[Bryan]

This is VERY true...

I have two chassis, one for PS and the other for audio. The PS chassis is COVERED with iron. 2 HV tranies, 4 damper diodes, 4 12.6 volt tranies, and 6 chokes... Fortunately, the last cap is housed in the audio chasis, close to the plates. It is NOT light, but I can say it was well worth the trouble...

 

[Mark A. Gulbrandsen]

If someone can come come up with a deicent SS power supply for this amp I would realy appreciate it.

I left a link to what should be the perfect solid state supply for this line stage...., a bit complex but not expensive to build... back in post 60... Let me know what you think....

Mark

 

[Looneytunes]

Thanks Mark, Thanks Bryan.

I think I've hunted down some JAN 12B4A tubes. I'm planning to buy 8 of them. I hope I'll find one close pair in the group. If I'm being overly optimistic about my odds, someone please let me know and I'll buy more. Besides, who knows where this is going. I may get adventurous and use the extra tubes for to build a balanced circuit.

Brian, I look forward to seeing your power supply. I saw you working on it in another thread, but I don't think I saw the final schematic. I'll bet there is no noise or grain at all.

And Mark, I was kidding about the cat. I can actually be very careful. I learned a very long time ago (when wearing tennis shoes) that the shortest path between an electric fence and ground is through your dog's nose touching the back of your leg. Double-YEEEOW! & (The pup kept his distance for a long time after that.) Luckily there wasn't enough current to toast the both of us. I plan to study-up before I plug this in, and there won't be any pets or anything else around that might add to the element of unpredictability. I also plan to enlist the help of a buddy that works with high voltage electronics to help me get me started.

At last, there is a question in all this. Are there any breadboarding parts that I should buy before I start? I don't want hot wires flapping in the breeze. If there are, can anyone recommend a good source for them?

Thanks for the help.

 

[dhaen]

I learned a very long time ago (when wearing tennis shoes) that the shortest path between an electric fence and ground is through your dog's nose touching the back of your leg. Double-YEEEOW! &

That's really funny (from a distance....)

 

[planet10]

Are there any breadboarding parts that I should buy before I start?

Clip-leads... you should see some of the creations made with clip-leads....

dave

 

[Sherman]

... I plan to study-up before I plug this in, and there won't be any pets or anything else around that might add to the element of unpredictability. I also plan to enlist the help of a buddy that works with high voltage electronics to help me get me started.

At last, there is a question in all this. Are there any breadboarding parts that I should buy before I start? I don't want hot wires flapping in the breeze. If there are, can anyone recommend a good source for them?

Thanks for the help.

I'm with Dave on the clip leads, but if you buy cheap ones from a surplus outlet be sure to check them all out for solid connections and continuity! I wasted 15 minutes once trying to figure out a problem with a circuit when it was just a bad clip lead!

When I turn on a new project for the first time I usually plug it into a three-prong (earth grounded) extension cord which is plugged into a switched outlet. That way I can stand across the room and turn it on. The displays on my meters are large enough for me to see from that distance so I can monitor voltages and currents.

 

[Bryan]

Hey Guys,

Here is a rough drawing of the PS as it is in my amp. I need to double check the VAC output of the power tranny, and confirm the dropping resistor for the VR tubes is 2.7K.

Never the less, this should give a clear idea of what I have done.

Hope it helps....

Bryan

 

[Mark A. Gulbrandsen]

Any tubes the glow are to me worth trying out . I've always been a fan of gas regulators but have never messed around with them very much, always went the solid state route. Does your power supply have a fairly low output impedance and how is the impedance at different frequencies? Also, how "quiet" are gas regulators compared to the solid state circuit I posted alink to in post#60? Keep in mind that the op amp in post 60 operates at unity gain so it doesn't generate much noise of its own, if any.

Thanks,
Mark

 

[Zen Mod]

Any tubes the glow are to me worth trying out . I've always been a fan of gas regulators but have never messed around with them very much, always went the solid state route. Does your power supply have a fairly low output impedance and how is the impedance at different frequencies? Also, how "quiet" are gas regulators compared to the solid state circuit I posted alink to in post#60? Keep in mind that the op amp in post 60 operates at unity gain so it doesn't generate much noise of its own, if any.

Thanks,
Mark

gas shunt stabs (when properly implemented) are dead silent;

feeding both halves in some lalala RCA style RIAA stage without hiss is good enough ?

 

[Zen Mod]

Hey Guys,

Here is a rough drawing of the PS as it is in my amp. I need to double check the VAC output of the power tranny, and confirm the dropping resistor for the VR tubes is 2.7K.

Never the less, this should give a clear idea of what I have done.

Hope it helps....

Bryan

in my xperience - with shunt stabs -best way is when you burn AT LEAST same current in reg as your active stage ;
takin' in account that OA3 is capable of some 40mA max and calculating that 12B4 needs atleast 30 mA to sing,you are pretty close to margin

try to analyze few old books for paralell connecting of gas stab tubes or-even better - wire shunt resistor for 30mA in parallel to OA combo,and let them to burn just 20mA
(old trick )

 

[tja]

Ok, this Glassware article on a H-V regulator is very interesting to say the least and it might be the cats meow for use with the 12B4. It features low output impedance over a very wide range and it could easily be setup to deliver the 320 volts thats required by Brian's 12B4 line stage. Any input on using this circuit would be appreciated.... I was thinking of an IRF740A as the pass device, but any other suggestions for a similar but higher voltage device would be appreciated as the 740 is rated right at 400 volts.

Thanks,
Mark

IRFP450? (is rated at 500V, 180W).

Take

 

[Bryan]

As I designed the PS, the VR tubes tap 20mA, and the 12B4A between 20-25mA, so the load is equally shared. I have not done any serious measurements, but I can say that the supply is quiet.

If I was doing this again, I would decrease the B+ a little bit, as I am wasting a lot of volts dropping the B+ before the regs...

I do have a very slight amount of hum, but this is caused by the physical proximity of all the iron of the PS chassis (and maybe a little bit of messy wire which needs tidying up in the audio chasis). I cant hear noise from the reg tubes, and my hearing is pretty darn good

Bryan

 

[240z4u]

Well I am convinced.

I spoke to mark via email and thats the final straw. I was looking at audio research, conrad-johnson etc.. unfortunately at my budget (350 or so) I really could not afford anything too exciting.

Looks like I am going to give this a go. I have had some experience fixing solid state amplification so I am somewhat confident.

Any thoughts as to stuff I should read before doing this project besides the newbie pages? I have read most of them but have trouble putting a finger on things without getting dirty.

I am also very aware of the danger of high DC voltages, and understand that sipping on a cocktail whilst piddling in tube equipment probabally is NOT the best idea

I will continue to watch the progression. I am not sure whether to do solid state regulation or tube. I guess I will cross that bridge when I get there!

This site rules, and thanks alot for the great information!!!!

Evan

 

[mach1]

I use the Tubecad PSU linked at post 60 on my phono preamp and it sounds great. If you build it use the 'design trick' discussed. It keeps frequencies above approx 250Hz (I can't remember the exact figure) out of the feedback loop so the sound 'breathes' a lot more. You still get a rock solid bottom end with lots of slam.

You can experiment with different value feedback resistors to vary the feedback cutoff frequency to get the sound you like. The higher the frequency the 'tighter' the sound.

If you want another great sounding cheap psu for the linestage try the following:

Obtain a transformer - rectifier combination putting out around 350V. Place a high quality 6-10 uF cap after the rectifiers (yes only 6uF ! pio are great in this application) followed by an IXCP10M45S wired as a CCS set at around 75mA (reasonably heavy heatsinking required). Feed this into a shunt reg consisting of 2 x 0D3 (or 0A2) regulators (assuming you want a plate voltage around 150 - 175v) bypassed with only 0.01uF, followed by a an IXCP10M45S CCS for each 12B4 (set to approx 25mA - small heatsink required). No iron, tiny amount of capacitance, three chips and two reg tubes. For an upgrade swap the 0D3s for a tubecad style shunt reg and use Gary Pimm style CCSs.

have fun