All things LA2a related

[QNote]

FWIW, I did just notice something on the original LA2A schematic that I feel may be contributing to the confusion...the little + symbols next to the diode cathodes. What were they thinking? These ARE NOT the positive sides of the diodes, but it IS where the positive DC voltage resides after rectification. Scenaria, was this what was throwing you off?

Oh, well. This is still not the issue in my case. To be continued...

 

[QNote]

UPDATE!...

The FRED's were breaking down from reverse voltage, or PIV. Since they were 4 amp, there wasn't enough current to evidently destroy them before popping the fuse. Again, my fault from overlooking this detail when ordering them. I should have known better!

Replaced them with standard 1N4007's and all is well--at least the basic, no-tube voltages look good! Now for the final test and alignments...

 

[QNote]

Well, the tubes and T4 are in anf the unit passes audio and compresses! BUT...

The frequency response of the unit sucks! I have about a 10dB/octave roll-off up at 1000 Hz (high pass filter), so there is a huge reduction in the low-end. The high-end (above 1000 Hz) is pretty flat. No problems on linearity above 1KHz.

Adjusting C4 seems to do little--if anything. All voltages around tubes line up with Scenaria and Jahnsen's book. I have also gone back through the unit verifying resistor/cap values, placement, etc. I can zero the meter and it shows gain reduction when compressing. This problem also exists without the T4 module inserted. I'm using Sowter transformers, and I have the two grid-stopper resistors on V2 (12BH7A).

Any ideas? Time for some serious o'scoping, huh? BTW, I don't have an extra 12BH7A tube--wish I did. Originally, I found I was missing the link between A11 and A14. Could this have damaged V2? I did swap the two 12AX7's with no luck.

 

[Scenaria]

check the caps you used for C13, C9, C11, C1

im not talking the value rather the voltage rating.. If I remember correctly keith and I experienced a similar problem with lack of low freq... found that the caps we used for those designations were simply too small..

 

[QNote]

C13 - 500V
C9 - 400V
C11 - 600V
C1 - 400V

They are all Orang drops except C13 (standard poly).

 

[Scenaria]

I would scope it starting from the input on the wiper of the input level pot... move forward from there to see where its rolling off..

 

[QNote]

I think I found it! :grin:

0.00 uF reading on C5. It's not shorted, but open. I was loosing the lower freqs between it and the output transformer primary. Sure wish I had another one handy...

Oh well, maybe Mouser will send me a replacement.

 

[Scenaria]

c5 is the coupling cap right?

the 10uf/450v one?

just run down to radio shack and grab something equivelent to see if it works..

you can use a 20 or 30 uf too if you cant find a 10uf..

 

[QNote]

Yep! It is the coupling cap. I did have a 47uF at 400V, so I put 'er in there and...voila! Everything is great!

I put her on the analyzer and the specs are quite impressive. Flat frequency response from under 20 Hz to about 22 KHz. Noise floor better than printed specs. THD+N peaks at 0.46%, and that is with 3dB of reduction at 1 KHz. Interestingly, THD+N peaks at most frequencies when there is about 3dB of compression--lower and higher reduction values have less THD, for some reason. Nominal THD+N stays around 0.15% on average. Spectrum activity shows the lovely 2nd order and 3rd order harmonics (vary with peak reduction)! :thumb:

Thanks to everyone in the forum. Great info here. I will post a link of the pictures in the next few days (when I have them uploaded). Now, off to plan the next DIY project...

 

[Scenaria]

well congrats!

I know you have been waiting a long time to finish this one :green:

cant wait to see some pix

 

[SSLtech]

Do try to "wash" the T4b. leave it with a signal across it for a few hours and you should find that the THD+N goes down.

The '+' end of the diode is the end with the white band. If the white band is at the 'B' end then everything is hunky-dory. There might be some argument over which way current flows, but basically if the white bands are pointed towards the front panel, everything is hunky-dory.

:thumb:

Keith

 

[QNote]

Well, here are some pictures...

http://www.quarternotestudio.com/LA2A.html

 

[Scenaria]

wow!

looks good

<-- little biased

but it still looks good... nice and tidy

 

[CJ]

This is pretty funny:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7337593537&category=23793&ssPageName=WDVW&rd=1

 

[Bluzzi]

OK, I got all my audio transformers and meters and I just ordered my passives but I still can't locate a few parts.

One is the 2 trimmer capacitors at 50-580pf. Can I go for less range?
Any sources in Canada or U.S.?

The other is C13 at 510pf. I found it but it comes in Ceramic or Mica. Does it matter?

My pocket is empty again but when I get the passives and turret boards at least I can start to solder! Hopefully my pockets will fill again for the balance of parts by then. I've never been so patient in my life and its killing me!

Thanks

Jim

 

[rove]

[quote author="SSLtech"]
The '+' end of the diode is the end with the white band. If the white band is at the 'B' end then everything is hunky-dory. There might be some argument over which way current flows, but basically if the white bands are pointed towards the front panel, everything is hunky-dory.

[/quote]
Is this right? I thought it was the negative end on the diodes that had the white band...
thanks.

 

[SSLtech]

It is absolutely dead-serious right.

Keith

 

[AnalogPackrat]

[quote author="Bluzzi"]
The other is C13 at 510pf. I found it but it comes in Ceramic or Mica. Does it matter?

Thanks

Jim[/quote]

I used silver micas for the pf range caps (except for the one trimmer). No problems...

 

[rove]

[quote author="SSLtech"]It is absolutely dead-serious right.

Keith[/quote]

Thanks!

 

[dissonantstring]

hi,
i read on past post that a 100pf cap works well instead of using the trimmer caps. i'm wondering because i'm having the same problem as Bluzzi finding a trimmer cap with enough range. what is the minimum voltage of the trimmer cap if used?
thanks,
grant