A few la2a q's...

[daArry]

Hey,

For the valves, is it okay to use ones that are marked without the 'A' suffix at the end? So for instance would 12AX7, 6AQ5 be okay?

For the i/o iron would a Peerless 15095 and Triad A-10J be cool?

Are standard 1/4w metals okay? - bar the ones marked out (goin on rons layout), except r16 which i read should ideally be 1W...

Is c10 (100uF/25V) a non-polar electrolyctic type? or would a regular standard polar be okay?

For the 470pF cap (going across DS1/NE2) would a silver-mica type be coo?

That's all i can think of for now!

Cheers :thumb:

D

 

[cayocosta]

Found on the net:

12AX7: This is the original version of this tube. The earliest ones were all made in the USA by the usual manufacturers like RCA, Sylvania, GE and Tung Sol. Very early ones have rater large size plates. The "plain" version without the "A" suffix can only be used in parallel filament circuits, not series. This usually isn't a problem as series circuits are only normally found in TV sets. All of the tubes will have odd voltages other than 6 and 12. If all of the tubes in your set start with 6 or 12 (except for the 5 V rectifier) , then it is a parallel circuit

12AX7A: This later version can be used in series or parallel filament circuits. Many manufacturers produced plain 12AX7's well into the 60's and 70's.

 

[cayocosta]

(I never heard that before about the filaments.)

Can't help with the iron, but I imagine metal film resistors are preferred, the reissues use CC. C10 is polar in the original - sorry should have marked that.

The neon bulb acts as a voltage regulator for the meter in GR mode - I understand the cap is there to stop flicker and noise - mica should be fine.

 

[CJ]

What are the specs on those transformers?
(I should know, but brain fade)

The non-polar 10 uf is considered an upgrade.

12AX7 will be a little bit noisier in my experience.

You can use a 6V6GT in place of the 6AQ5 if you want.

The octal socket is much easier to wire.

 

[Rob Flinn]

Over here in the UK an ECC83 is equivelent to a 12AX7 & an EL90 is equivelent to a 6AQ5/A

 

[daArry]

Hey,

CJ, soz shudda added this to start with:

Triad A-10J miniature Line input transformer:
Primary: 150/600R split winding. Secondary: 60K.

Peerless 15095 plate-line output transformer:
15K primary (no DC) to 150/600R split secondary.

They had similar specs to the A10/24 which is why I went for em...

The non-polar 10 uf is considered an upgrade.

Are you short of an '0' there? - didnt mention a 10uF cap in the 1st post...

...but I imagine metal film resistors are preferred

Cool, and 1/4w ok ye? (except the ones mentioned/marked otherwise)

C10 is polar in the original - sorry should have marked that.

np, just to be clear, negative side still goes to ground there then ye?

12AX7 will be a little bit noisier in my experience.

noted, what about 12AX7LPS?

You can use a 6V6GT in place of the 6AQ5 if you want.

noted, just i've already plunged for a pair of RCA 6AQ5 of ebay... :roll:

Cheers guys, this is my 1st project (other then the prr vari-mu) with valves n p2p (which is why i've had it on the backburner for time) so I'm not sure about much of this stuff - i should read up on the basics n such but gettin hands on and just building the ting is the route I seem to settle on :green:

Best...

 

[cayocosta]

CJ was talking about upgrading the 10uf electrolytic coupling to the output transformer.

1/2 watt resistors.

Yes on C10 - negative to ground.

Here's the schematic:

http://home.earthlink.net/~cayocosta/diy_files/la2a/la2a_1968.jpg

 

[daArry]

thanks...

any leads on a source for that 50-380pF vari-cap? can't find one that'll stretch to those values from regular stockists...

 

[Scenaria]

dont worry about the variable...

grab a 370pf cap to get things going.. then just swap it out with different values until you find the HF bump your looking for..

 

[cayocosta]

Type 46, 75-380pf (close enough) right here:

http://www.web-tronics.com/passive-electronic-components-capacitors.html

 

[cayocosta]

I pity the fool don't have no La2a.

 

[CJ]

Yes, 10 uf coupling I was referring to. 100 uf must be lytic.

12AX7LPS? Sovtek?

A 5751 is the premium version of the AX7 if you happen to have one or know where to get one cheap.

A 6072 is a mil grade version of the 12AY7. This would be my first choice for the LA2 input stage.


From Tube Lore : "A" has lower hum and microphonics"

What is the db rating on that Peerless? If it is 16db or better, go for it. They made awesome iron.

The Triad is going to really drive the input stage, but thats why they have the gain control. You could actually get away with a 1:1 10k:10k bridging x-former, but it will still work. Use it and swap it out later if you get to much overdrive. Also, I used a 12AY7 for the first stage, which gave me a pad effect without all the noise. Less gain. And most AY7's sound better than the harsher AX7.

Also, NYD has a cool mod for the BH7 output stage.

Let me know if you guys want a 12AX7 history mega post.

 

[electronaut]

[quote author="cayocosta"]The "plain" version without the "A" suffix can only be used in parallel filament circuits, not series.[/quote]

I don't understand how this could be. The "12" in 12AX7 indicates 12 volts for the heaters, so it seems the opposite would have to be true -- that series heaters were the requirement in the earlier types. Adding the center tap on the heaters as an afterthought would have then been a smart upgrade, allowing use of the 6 volt configuration.

[quote author="CJ"]Let me know if you guys want a 12AX7 history mega post.[/quote]

Maybe this will set the record straight. (In another thread, maybe?)

BTW, CJ, your parts dissection and historical posts are fantastic! Thanks!
I'm beginning to think that cloning related, design & theory, and HISTORY could all have their own forums! The history stuff is super fun when you've got a few minutes to kill...


-E. Lectreau

 

[cayocosta]

Searching the net, it appears that many NOS tube merchants are copying each other and all incorrectly describing the difference between the 12ax7 and 12ax7a.

Here's a rec.audio.something post from Ned at Triode - he would know:

"The RCA tube manual states specifically that the "A" denotes that the tube has "controlled hum & noise characteristics" Suffixes have different meanings depending on the tube the suffix is applied to, in some cases the "A" merely refers to the heater warmup...

However, after the mid-60's, pretty much all 12AX7's were made as 12AX7A/7025 (look at some GE 12AX7's), so the suffix is moot. European tubes were made to the Philips ECC83 standard,no one has ever said that Telefunken or Mullard 12AX7 were worse than the RCA 12AX7A or 7025 (yet another RCA scam). Basically, unless we're talking about tubes made in USA in the 1950's, the "A" suffix is moot."

 

[fazeka]

[quote author="CJ"]
A 5751 is the premium version of the AX7 if you happen to have one or know where to get one cheap.
[/quote]

Not to nitpick :shock:, but a 5751 (mu = 70) will also have less gain than a 12AX7 (mu = 100).

 

[daArry]

Okay, i'll worry about the valves later and aim to get the beast built n ready for em when the time comes...

CJ, I'm not sure of the dB rating on the Peerless.. :roll:

Cheers ppl!

 

[CJ]

Fazeka is right on the 70 mu 5751. But trannsconductance and plate res for 1 volt bias are about the same. db wise you won't here much difference.

Looking for Peerless info. Fazeka, you got that p/n in any of your catalogs?

 

[jhaible]

[quote author="electronaut"][quote author="cayocosta"]The "plain" version without the "A" suffix can only be used in parallel filament circuits, not series.[/quote]

I don't understand how this could be. The "12" in 12AX7 indicates 12 volts for the heaters, so it seems the opposite would have to be true -- that series heaters were the requirement in the earlier types. Adding the center tap on the heaters as an afterthought would have then been a smart upgrade, allowing use of the 6 volt configuration.
[/quote]

I think this doesn't refer to 12V vs. 6V on a single tube.
Series heating means connecting the heaters of many tubes in series, feeding them with a higher voltage. This means that (a) all these tubes must have the same current rating instead of voltage rating, and (b) they all must heat up in the same time. Resistance changes a lot with temperature, and if one tube in series connection is too slow in building up its nominal resistance, the other ones will get too much voltage and might be damaged.

JH.

 

[fazeka]

CJ, sorry I got no info on the 15095. :sad:

Maybe it's listed as some other catalog number as I think that was an Altec number? :?

BTW, did you survive the deluge? :shock:

 

[daArry]

Hey,

Right, so both the Triad A10J and Peerless 15095 are here now. Is there any testing I can do to be sure theyre working? (got from ebay). Also, I need to be sure of winding hookup...

On the peerless (oktal type plug) theres this info on the can:

So, wiring on the primary side (in la2a hookup here) - gnd to pin 8, and from the 10uF cap to pin 7. Then on the secondary we want 600R so pins 3 & 4 are linked and we go from pins 1 & 6 to the XLR? Is pin 1 the negative and 6 the positive?

The Triad meanwhile has wires coming out the can...the scrathed up lable reads as follows:

600R PRI
USE: Brown AND Blue
TIE
: Red TO Black & Red

Is Brown Hot?

The 15K secondary is on the Green and Black wires - I assume green is the signal and black goes to ground?

There's also an uninsulated wire coming out - is this just tied to chassis ground?

Cheers!