PSU questions - adding la2a to tube pre

[BluegrassDan]

Hey folks,

I want to add a stripped down La2a circuit (minus stereo, side chain, and only GR metering) to my tube pre (Hamptone-inspired with changes). My questions are regarding the PSU to power both.

La2a transformer Hammond 369JX ---->  Secondaries: 500v (CT), 50v (bias tap), and 6.3v ----> PSU: 275vdc B+, 65vdc (I believe) opto circuit, and 6.3VAC heaters.

Tube pre transformer toroid ----> Secondaries: 250v, 50v, 12v ----> PSU: regulated 225vdc B+, 12.6vdc heater, and 48vdc phantom.

So, which is the better PSU arrangement? Adapting the LA2A PSU to also power the pre, or vice versa?

My only preference is that I prefer to run the heaters DC.

Thoughts?

 

[ruffrecords]

The first thing that comes to mind is the la2a has quite a few tubes in it so its heater requirements will be significant. - are you sure the tube preamp toroid can supply the required current?

Cheers

Ian

 

[BluegrassDan]

Since the tube preamp is going down to one channel instead of two, the only addition will be one 12BH7 and one 6AQ5.

The toroid:
12v – yellow (2 AMP)
50v – white (50 MA)
250v – blue (200 MA

 

[BluegrassDan]

Total tube count will be:

Preamp - 12ay7, 12au7
LA2A - 12ay7 (mod), 12ax7, 12bh7, 6aq5

 

[rock soderstrom]

Looks good to me, if you change the 6aq5 for a 12aq5. The total current is ~1,2A @ 12,6V.

 

[BluegrassDan]

So the 12aq5 is identical to 6aq5 - OTHER than heater voltage? It's wired the same way?

 

[rock soderstrom]

Wire pins 4&5 and just use one half of the 12aq7?

12AQ7?  Are you talking about the 12BH7 or the 12AQ5?

All the tubes you have listed a 12V types except for the 6AQ5. So i think its a good idea to take a 12AQ5 and to use your preamp transformer with the 12V/2A heater winding for a 12V DC filament.

 

[BluegrassDan]

I spoke too soon before looking up the tube. Edited my post!
;D

 

[BluegrassDan]

Okay, so now my last question is what circuit can give me two separate B+? One at 275 VDC and the other 225VDC? Is stepping up better than stepping down?

Current B+ circuit is attached.

 

[rock soderstrom]

So the 12aq5 is identical to 6aq5 - OTHER than heater voltage? It's wired the same way?

100% identical, except of the filament. Pinout and wireing is the same.

https://www.radiomuseum.org/tubes/tube_12aq5.html

If you got problems to find that tube, you could use the 12V6 octal tube, the 12 V brother of the 6V6.

 

[rock soderstrom]

Okay, so now my last question is what circuit can give me two separate B+? One at 275 VDC and the other 225VDC? Is stepping up better than stepping down?

Current B+ circuit is attached.

Adjust you zener diodes for 275VDC and than ad a further RC element with the right resistance after the 275VDC tab to bring down the B+ to your needed 225VDC.

 

[BluegrassDan]

Excellent. Thanks SO much guys!

Now...(for the mathematically illiterate - I'm just a musician)

Where can I look up info on how to properly adjust the diodes and RC values? That is an area that I don't understand properly.

 

[johnheath]

Hi...

I have built a pair of LA2A with this transformer:

https://www.don-audio.com/Audio-Toroidal-Transformer-Pri-2x115v-Sec-250V-48V-12V

I might mention that I use a 12AY7 as V1 but it has the same heater current draw as the original 12AX7 as V1

I use DC for for the 12AY7, 12BH7 and 12AX7... and tap VAC for the 6AQ5 with a pair of  10W resistors to bring it down to 6,3VAC +/- 0,3VAC.

The first preamp tube is the most sensitive for AC hum and this arrangement works like a charm.

Heater current for the three first tubes are 450mA and "remember" that the 6AQ5 draw ~450mA. That is half what the transformer is rated for... no noise there.

Best regards

/John

 

[BluegrassDan]

Excellent. Thanks SO much guys!

Now...(for the mathematically illiterate - I'm just a musician)

Where can I look up info on how to properly adjust the diodes and RC values? That is an area that I don't understand properly.

Any insight, fellas? Thanks in advance.

 

[bluebird]

D31,D32,D33 are all 75V zeners. That adds up to 75X3=225. So those zeners define your regulated voltage. You can just add another 51V zener to the string and your regulated voltage will be 276V. Or use four 68V zeners. Whatever values equal 275V.

Then put a nice 5K two watt resistor from the 276V point to a 10uF 350V cap to ground and hook up the rest of the circuit to that point. Adjust the 5K resistor till you get the 225V you want. I cant calculate the correct resistor value because I dont know the current draw of that stage. So you'll just have to start with 5K and experiment with the value. 10uF should be enough filtering for the 225V stage. You can use 22uf or 50uf if you want.

 

[rock soderstrom]

D31,D32,D33 are all 75V zeners. That adds up to 75X3=225. So those zeners define your regulated voltage. You can just add another 51V zener to the string and your regulated voltage will be 276V. Or use four 68V zeners. Whatever values equal 275V

Yes and if we assume that your Hamptone tube pre draws 20mA than you will need a 2,5K resistor.
The math behind that is quite easy. Meet your new best friend, Ohms Law! 

R=U/I  Resistance=Voltage/Current

You want to get rid of 50V, and the current is 20mA or 0,02A.

R=50V/0,02A=2500R or 2,5K

Power Consumption of the 2,5K resistor.

P=U*I    Power=Voltage*Current
P=50V*0,02A=1Watt

double up for safety margin = 2W or bigger

Now, you are a real Jedi!

 

[BluegrassDan]

You guys are awesome! Thanks for teaching me how to fish.

 

[bluebird]

One last thing, You may have to make R30 smaller. You want the voltage right after it (top of C30)to be about 300V with the new zener string. Now that your new regulated voltage is higher (275v) you will want to have at least 20v higher B+ coming into the regulator circuit.

 

[PRR]

> Is stepping up better than stepping down?

Building a dog-house. Is lengthening a board better than shortening a board?

Also: does the dog really care 225mm or 275mm boards? (OK, that's maybe more a mouse-house.)

 

[BluegrassDan]

One last thing, You may have to make R30 smaller. You want the voltage right after it (top of C30)to be about 300V with the new zener string. Now that your new regulated voltage is higher (275v) you will want to have at least 20v higher B+ coming into the regulator circuit.

The voltage just before R30 I am measuring at 295 v.
Voltage drop across R30 (3.6k) is measuring -49 v. (Gives me a current draw of 13.61 mA, right?)

So, in order to keep the voltage at the top of C30 closer to 300v, should I just remove R30 entirely? Or should I put a very small value, like 38 ohms? Or, should I do something else?