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Be careful with this, I am not sure the power supply is designed for the voltage and current you will need. Be prepare to scrap everything but the chassis and perhaps the power transformer (I seem to remember that Rosenblit likes using 115V/230V transformers, though). Check current capability.

MY pal Jim in Lewes is probably looking to sell his Tubes4HiFi SP14. This is a wonderful 6SN7 pre based on John Broskie's Aikido preamp. He bought it used and I upgraded it with PS and output caps, ladder attenuator and some Duelund wiring. I'll ask if he's ready to sell it. (He's using an Elekit 300B integrated now and likely doesn't need the SP14.)

I built my own version using the Tubes4HiFi board and transformers and I absolutely love it. Open, honest, utterly linear and musical. If you're interested let me know and I'll ask him. Probably about $1K. I made a provision for swapping in output caps, and it is eminently upgrade-able in other ways.

http://tubes4hifi.com/SP14.htm

Grover,

That is another good idea. An SP14 kit runs $1150 complete with chassis, so Jim's unit would be a good value. A 6SN7 Aikido is a good sounding line stage. If Shashi wanted to do a DIY, he could also get boards and PS from Broskie as well and reuse his other parts.

David

A single board Octal Aikido w/Power Supply and parts and no tubes is $200 from TubeCAD. Just putting into perspective. You have to bring your own chassis, source selector, and volume control. And the board is in stock!

David

That's not a cathode bypass cap! That's AC grounding the center tap of the heaters. The heaters are connected to AC.I'm not a fan of AC on the heaters at line level, but maybe they sound better.

In my 12B4 preamp, which I currently use, I built a common mode choke filtered DC heater supply.

For plate loads a cascade DN2540. For plate supply, current source/shunt regulator using 10M45S and gas tubes. Do NOT use gas tubes without a current source as they will be noisy.

In the cathode circuit, LED bias.

I built the circuit on a 1/4" sheet of Teflon. The Teflon is mounted onto a 1/2" thick Delrin sheet, which is inside the metallic enclosure. Input/output jacks are on 1/4" sheet of Delrin.

Attenuator is an LDR setup.

Rectification is vacuum tube.. There is a choke in the HV supply.

Also, used damping rings on the tubes.

All the efforts at damping: rings, Delrin, Teflon, etc. are very important because 12B4 can be microphonic. Buy a bunch of those tubes (I bought at least 20) from someone like Vacuumtubes.net, they are affordable and you should get plenty quiet ones.

I built mine in a Dyna PAS3 chassis. Mistake, as room is extremely tight. If you are buying an enclosure, make it 17" wide and 12" deep, 4" high.

Remember what you are going to drive with it. The lower the input impedance of your load, the bigger the output cap needs to be. Boutique output caps are HUGE!

Wish I could provide more details, but I'm recovering from knee surgery right now and I'm 100 miles from my rig.

I LOVE the 12B4 preamp.

If you would like to chat, send me a PM or email and I'll send you my phone number.

Best of luck,

Stuart

Shashi, I'll just echo what has been said here. A nice single stage preamp is a great way to experiment. I have a beat to hell chassis with lots of iron that has been a 76, 6SN7, 6BL7, 6SX7, VT137 etc preamp. Also a great way to experiment with the sounds of caps, resistors, circuit settings due to the simplicity. Changes can be easily detected.

Thanks Grover.
I will try single stage 12b4 DIY first. I will also try Glassware Aikedo kit.

I replaced the supplied transformer with 2x200V 100VA toroid. Connected the two 200V winding in parallel. The PS circuit has CLC smoothing filter. 470uF caps and 10H choke.
https://www.antekinc.com/as-1t200-100va ... ansformer/

It also has a 48V transformer which can be used for biasing the tubes.

As the transformer has 2 6.3V 3A windings, each heater can have its own power source. How much DC smoothing is needed for the heaters?

Any suggestions on soft start?

Shashi,

Some good reading on how to design a single tube line stage (for a 6V6 triode preamp) is the following:

https://wtfamps.com/sofrito-preamplifier/

This is an alternative to the 12B4 single tube preamp (some favor the 6V6 over the 12B4), but the important thing is how to find a good operating point and the overall design methodology, including sizing of the output capacitor.

There are lots of alternatives out there. If you want to use a single, two section, tube, you can use a 6BX7, for instance, though it is not as linear as a 12B4. It too has a low plate resistance so works as a single stage preamp.

David

Well, with a 200V transformer and a bridge rectifier into a cap input filter, you can get a raw supply of 280VDC. There are couple of power supply design pathways.
1. Use a CLCLC filter with low R filter chokes, if you can keep the total drop in the filter to around 20V, it should put you in a good place for simple plate resistor loading of the 12B4, assuming you are dropping around 150V across the plate resistor.
2. Use a CRCRC filter with CCS loading of the 12B4. A DN2540 (or similar) cascode current source needs around 25V or more for good operation, so if you have 120V at the plate of the 12B4, that means you want to have no less than 145V at the top of the CCS. You need to drop about 130V across your filter network. Actually, the CCS is perfectly happy (given sufficient heatsinking) to absorb some of this voltage. That leaves more than enough voltage to add a shunt regulator for each channel, also fed off a CCS, something like a 0D3 or 0A2. So this looks to be a great upgrade path. Plan for two sockets for the regulator tubes. And space for heat sinks for the CCSs.
3. If you use a choke input filter, the raw DC voltage will be around 0.9x200V = 180V into the filter network. This is good for a choke loaded or transformer coupled 12B4, assuming your filter network drops the voltage down. May not be enough B+ for regulation, though.

So this transformer can definitely work.

I would not bother with fixed bias, it is an unnecessary complication. You can do fixed bias at the cathode of the 12B4 via a string of LEDs, you only need 10 to 12V, so that is 6 Red LED in series, or two 5.6V Zener diodes with a bypass cap for noise.

I do not think soft start is necessary for a preamp, particularly if you are NOT using separate fixed bias arrangement. Fixed bias via the cathode should be OK.

David