All things LA2a related

[Kid Squid]

SOUNDS THE F**KIN' BOLLOCKS :thumb: :thumb:

Pics Here
http://www.groupdiy.com/index.php?topic=18913

Nice one Fellas :guinness: :guinness:

Steve

 

[matta]

Hey Guys,

Here a question for all of you LA-2A builders... Resistors...

In the Uaudio Reissue they have gone with Carbon Comp Resistors, I guess to keep in step with the 'original' design.

I've been looking into reasons, or rather trying to understand WHY a particular resistor type was used and over another and I've found through Google a few bits of info and I can rightly/wrongly sumize the following...

Resistors suffer from 3 types of noise: thermal noise, shot noise and contact noise (through there are are others). In tube amps thermal and contact are the main players.

All resistors have thermal noise and this noise is independent of type so for example a 1K Metal Film will have the same amount Thermal noise as a carbon comp Resistors of the same value with the same voltage across it.

Contact Noise (1/f Noise) is the result of fluctuations in the conductivity of the Resistors materials, particularly bad in Carbon Comps, this increases in volume as you increase the current though the resistors. So if we increase the current across it 5 times, we increased the noise 5 times.

In order of least noisy you have Wirewound, Metal Film, Carbon Film, Carbon Comp etc.

I think the discussion for using Carbon over say Metal Film is that they 'sound more musical', but that for means the 'noise' is more musical, not the sound passing through the Resistors, and in that case if you used Metal Film, you would have less noise, which seems better than musical noise?

So why use Carbon Comps in something like the LA-2A? Does it make a big difference in tone if you put in metal Film instead? I'm sure you get a much bigger difference in sound say swapping the transformers rather than the resistors, or the tubes you use, is the tonal quality effected by using Metal Film over Carbon Comps?

I've heard people say it sounds 'cleaner' with metal film, what does that mean? Are they referencing 'cleaner' being less noisy with 'dirty' referring to the sound they get from noisier Carbon Comps?

Has anyone had the chance to A/B Metal Film vs Carbon Comps in the LA-2A project and noticed a perceivable audible difference?

Thanks in advance for your comment. Tube/Valve stuff is all kind of new to me and this is my first HV project having wrapped up COUNLESS transistor ones.

Cheers

Matt

 

[Bluzzi]

Matt, I can only tell you I used Metal Film in my first one and using same in my second one. It sounds great! Very quiet (not even a hiss) except for an almost imperceptible (except at full blast) 60hz hum which I haven't bothered to do away with. It sounds that good that I just want to use it now.

I'd be surprised if anyone finds that much of a difference in using carbon type. Everything does make a difference but how much and is it worth it to start worrying about it. Use good tubes and soldering technique, and proper grounding. The rest was done for us.

If there is anything to improve in the LA2A (now that I've buit one) I'd say its the input transformer ratio. Its too high. Maybe even the output transformer could be a bit tamer. I'm using original UTC ones and I have enough for 1 more LA2A but if I ever make more I would use less gain on the input.

I was worried that my LA2A was not going to sound good (it was my first DIY in decades let alone tube one). I can't wait to have my second one finished! Its not the most versatile compressor out there but so far on some vocals it has magic.

You are going to love making and using this project.

jim

 

[guavatone]

mat, I did mine with CC's based on Caycosta's notes. He has a parts list with the R #'s and they correspond to this PCB. Dunno if it make's a diff. since I never heard the same with films, but mine is super quiet.

 

[CJ]

I like the old resitors for the plate circuit, that's about it.
Use the 1 or 2 watters.

Lousy tempco might mean usefulness in regulating a pwr supply.

I wish I knew what the V72/76 type resitors were made out of.
I do not think they are carbon comp.

 

[matta]

Hey Guys,

Thanks so much for all the info and encourgament. I'm slowly pieceing it all together, have all the Tubes, Sockets and the Caps are on order from Mouser.

It makes sense to me to use 1% Metal Film where one can except in the PSU and where 'lousy' Tempco can be used to ones advantage.

I like the old resitors for the plate circuit, that's about it.
Use the 1 or 2 watters.

CJ,

I assume looking at the schematic that those resistors are R34, R32, R9, R13, R15, R17, R18 assuming you are talking about JUST the resistors on the plates?

Cheers

Matt

 

[CJ]

Thats right, on second thought, I do not think the EL panel cares about carbon comp mojo, so standard metal there.

Save the carbons for the Fender.

 

[Kid Squid]

Hiya Matta,

I think it will sound great, whatever resistors you use :thumb:

no, in fact , I knows it will :grin:

Seriously, I haven't got another la2a yet, but i built mine with metal films, and it sounds F**kin' terrific :wink:
just my two-penneth on the matter, Matta :grin:


regards
Steve :thumb:

 

[CroatianSensation]

hi all!

I've been hanging around this forum for some time but haven't posted anything...until now.
I'm nearly done building a Bloo La2a and everything seems to be dandy save for 2 voltage readings.
A20 is a mere 10v '' and A19 is a whopping
274v ':shock:' with the tubes in place.
I have yet jam in the T4B.
I've looked over my work repeatedly. It's not the neatest I've seen but I cannot find any mistakes...any suggestions?

 

[CroatianSensation]

Awwright.
Well, I checked C10 for orientation, the cathode ground and then-the most obvious thing-the tube! ':roll:' Lo and behold the 6AQ5a had crapped out.
Luckily a buddy of mine had one laying around so now I'm back in business.
Here are new improved readings along with what they should be:

A1: 54v/60v
B21: 352v/360v
A20: 82v/131v
B20: 278v/275v
A14: 230v/216v
A17: 82v/90V
A19: 124v/105v

Are these well within range to pop in the T4B?
The only ones that weird me out are A20 and A19.
Any suggestions? Comments?
How was the turkey?

 

[CroatianSensation]

hello?

 

[CJ]

The turkey was great, now plug in the T4.

 

[CroatianSensation]

All systems are go.
It hasn't blown up yet, it's passing audio and seems to be compressing.
SWEET!
The two voltages are still off but that doesn't seem to be hurting things.
This is my second DIY project....well my second successful one.
I was just a little apprehensive when it came to jammin' in the T4.
Thanks for the vote of confidence CJ. Your posts and this forum have been a great help.
Time to squash some stuff!

-Jason

 

[CJ]

Good going!
Remember that tube voltages can be 20-30 percent off, and everything will still work fine, unlike transistor circuits.

 

[emx]

Ok quick question I think I may possibly have a bad power tran it seems to be giving off a bit more voltage than I'm needing and what it's rated at. I'm getting this at these points.....

A22 +23 Main set off of Power Tran =572VAC supposed to be 500
A24 +25 Heaters =6.84VAC Supposed to be 6.3 I think


Should I pull the Power Tran out and double check it off the Turret board?
I would think that It would be unnecissary, I used the Allied power supply rated at 500VAC, heaters are supposed to be 6.3 I believe.

 

[emx]

Edited post

 

[emx]

Ok Pulled both the Red and Green wires from the turret and am getting these voltages......

582 VAC off of the Main Reds
7.3 VAC off of the Heaterwires

I'm wondering if Allied does returns this long after the sale(5 or so months)
I'm also wondering if I can just adjust the voltages by replacing say R34 or R29, Probably I should go with just waiting for a different power tran though.

 

[AnalogPackrat]

Was your first measurement (572V and 6.8V) with all the tubes in? If so, yes, it seems like you might have a mislabeled transformer. Is this a Bloo kit or something you're doing from scratch?

A P

 

[emx]

The last voltage readings were with both the primary and heater wires removed from the circuit, so they are being checked outside of the circuit. The readings from before were with the wires still attached to the turret, with the tubes in. I'm doing this completely from scratch. I'm going to contact Allied and see if they will do a replacement.

 

[emx]

hmmmm, just bought another Allied power Tran and am getting close to the same readings off of the new one wich makes me think that my Rat Shack tester may be off on it's readings. It Irks me to think I have possible been holding back on running the whole unit due to a crappy radio shack multimeter.