Two La2a's In One Rack? And A Lot Of PSU Questions?

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Vikki

Well-known member
Joined
Jun 5, 2004
Messages
276
I've managed to get hold of some more racks from a vibrarion tester, one of these is a 3 unit high rack and i was wondering if there was any point in building 2 La2a's into the one rack. I think i could just about squeeze both in with the 2 chassis running front to back. I don't know if it is best to run each one of a seperate power supply or build one higher current supply to feed both units,i think that would mean both units would have to be powered all the time?(i guess the psu voltage would rise if one units ht was switched off.)
Another question, does the mains transformer need a centre tap on the ht and heater? There seems to be a few mains transformers around quite a bit cheaper that do not have a centre tap on the ht or heater supply. I notice a few valve amps have a non centre tapped heater supply but have a pot across the 6.3v with the centre to earth to balance any hum. I think a full wave bridge would be needed if the ht supply had no centre tap? From what i've read the ht voltage is dependant on the ht current drawn which drops voltage across the resistor in the Pie smoothing circuit, can anyone tell me the ht current drawn by one la2a? Hey no one done a PCB La2a yet?
Sorry for ranting for so long.
Vikki(uk) :grin:
 
Am i thinking along the right lines here? Ht1 feeds one La2a and Ht2 feeds the other. The resistors have to be calculated depending on the transformer voltage and the current drawn? Is it neccesary to have the pot on the heater line?

la2apsu.jpg

Vikki(uk)
 
Yes your ideas are right. You can use a 250v winding if you use full wave bridge. The 6.3v winding can be centre ground referenced with two 100ohm resistors to ground or the pot is better because you can trim the hum a bit better. Also if the high volt power rails end up a bit low you can decrease R1 and increase the filter capacitor sizes. If this ends up with audible hum still in the output change R1 to a TIP50 transistor two resistors and a cap to make a capacitor multiplier. This will make the LA2A power rail super smooth and higher volts and the audio out will be quieter than a real LA2A which is a good thing.

Joe

www.jlmaudio.com
 
Thanks Joe
Could anyone give me a rough idea how that TIP 50 circuit would look. I was looking at the G9 ht regulator is it a similar arrangement to that?
Vikki(uk)
 
Thanks Jacob
:idea: Maplin are selling a mains transformer with 6.3volts 1.5 amp and 240 volts at 100ma which i think would do to run one (or maybe 2) la2a. How do you calculate the component values for this voltage compared to the 48v phantom supply on the G9? From what i understand the bias on the base of the transistor controls the current flow between collector and emmitter :oops: by this is getting a bit deep! Go on help me out fellas :grin:
Vikki(uk)
 
Hi Vikki

Try this.
I used one of those Maplin transformers - full wave with one of these
to make a 105v ht for my U47 clone.
I have made a pcb design for it as well using larger caps as he suggests.
 
:grin:
Many thanks that looks something like!
Many Thanks
Vikki :grin:
 
Replace R1 with either of these circuits if you cannot get high enough voltage when reducing R1 value without getting hum at the audio output of the LA2A. I prefer the first circuit which is a capacitor multipier and the final voltage can be trimmed easily by making the 330k resistor 270k with 100k 25turn trim pot in series. For this circuit to work properly the voltage drop across the TIP50 must be a few volts more than the 100 or 120Hz ripple on the 350v input side.

HVreg.gif


Joe

www.jlmaudio.com
 
Thanks Buttercup and Joe. I'll get some parts and build the psu up. Joe any idea about the value of the filter caps, is 47uf too low?

Vikki(uk) :green:
 
> Maplin are selling a mains transformer with 6.3volts 1.5 amp and 240 volts at 100ma which i think would do to run one (or maybe 2) la2a.

The 240V AC 0.1A AC winding is good for 330VDC at 50mA-60mA.

Power demand of an LA2a:

12BH7 - 4mA to 8mA
6BQ5 - 15mA
12AX7 - 1mA each * 4
meter - 0.5mA
=============
28mA total at 280VDC

heaters
12AX7 - 0.3A * 2
6BQ5 - 0.45A
12BH7 - 0.6A
=============
1.85A at 6VAC

You don't have enough 6VAC to run even one LA2a. Actually it may work for one: the 6V winding will run a little low and hot while the HV winding will run only warm, so it won't burn-up. It sure won't light two LA2s for very long. One of these (using just the 240V winding) and a separate 6V 4A filament transformer (not a specialty part) would run fine.

> Joe any idea about the value of the filter caps, is 47uf too low?

They used 40uFd in 1959. But the price (and quality) of caps is VERY much better now. Unless you want an exact reproduction (humm and all), take advantage of Japanese cap prices and use MUCH bigger caps.

The 12BH7 stage hase not-much ripple rejection, and ripple here appears on the output just 5 times lower. But feedback will reduce ripple. You want not much more than 1 milliVolt ripple at this point inthe power supply. The signal 12AX7 has another stage of R-C, so is probably clean enough. Ripple on the sidechain and meter hardly matters.

The original has about 0.5V ripple at the first cap, about 100:1 attenuation in the 4K7 and second 40uFd. 5mV of ripple here is (neglecting NFB) about 0.5mV ripple at the output. Just 68dB down from +4dBm. NFB may push that down to -80dB, which would have been quite acceptable in 1959 but seems high today.

Also this was made for US 60Hz power, and I gather you get 50Hz power. That alone suggests caps 6/5ths bigger.

100mFd seems minimal, and at today's prices I might go for 220uFd. Or buy a 10-pack of 100uFd 400V to get a price-break, and double-up. With 200uFd, 4K7, and 200uFd you do NOT need any anachronistic TIP50 ripple-smoother. There are times to use judo (transistor gain) and times to just pour a heavy foundation. All the old tube designers would have loved 220uFd 400V caps cheaper than a martini; today we have them, use them.
 
Thanks PRR
I look around for a better rated transformer, there's a guy selling these on ebay could be a bit better proposition although the heater current is still not enough.

But these could be a better quality unit, does that c core make any difference?
Vikki(uk)
 
That's actually lower-rated.

It is heavy-duty military, and will probably beat the Maplin for surviving desert heat and arctic chill. It is also probably VERY big and heavy. I don't think you want it.

Where are you, the UK? Come on UK guys, where do you get tube-unit transformers? (Doesn't help for me to look around: US mains voltages are different, and US-UK shipping is killer on heavy iron, even before the taxman sees it.)
 
Well i've been digging in the garage and come up with a transformaer that may be o.k. Its in an old valve amp(2x EL34), i don't know the voltage rating yet but it has quite a few tappings. Its on the radiator at the moment thawing out, maybe too high voltage. I notice Maplin do a larger valve type mains transformer but i can't find any details as its out of stock until next week, well see. I'll remove the transformer tonight and see if anything can be done with it, its a bit of a beast.
Thanks For All Your Comments And Encouragment.
Vikki(uk) :grin:
 
http://www.bluebellaudio.com/
http://www.bluebellaudio.com/transmain-mains.htm

UK price for good-old-Hammond iron is about 40 to power one LA2, 56 to power two LA2s.
 
Sadly its 350v 0 350v at 150ma, plenty of heater current 6.3v at 3 amps. I'll find something this week. :green:
Vikki(uk)
 
Just tracked down a transformer in the uk that i'm hoping will run 2 off LA2A, does it look feasable? Plenty of heater current but high voltage winding is single ended 350v at 250ma and heater 6.3v centre tapped at max 7 amps. With a full wave bridge what d.c will that give me? Is it 1.414 x 350? And i think the smoothing caps will boost this a little too? I'm at work at the moment but i remember the ht rail for the LA2A is 275v can we push this up a wee bit. Whats the best way to get the 350v ac rectified down to say 300v?
Anyway here's the transformer spec, not a bad price at 29.99 uk pounds, if it will do the job.

http://img.photobucket.com/albums/v703/vikki220561/maplin.bmp

Vikki(uk) :grin:
PS - Sorry cannot seem to get the link any bigger
 
With a full wave bridge what d.c will that give me? Is it 1.414 x 350?

Yes

Plenty of heater current but high voltage winding is single ended 350v at 250ma and heater 6.3v centre tapped at max 7 amps.

I`ve used over specified heater secondaries before & they can sit up a bit voltage wise if the load isn`t that big on them.

When I built my La`2a I used a transformer that is roughly similar to Maplins cheaper/smaller valve PSU transformer. It has a 250v secondary for the HT, which will be easier to tame than a 350v one. It works well & 2 of them are slightly less cost than the transformer you are talking about. It might be worth considering. Just double check that the heater tap has enough current capabilities for all 4 valves.
 
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