All things LA2a related

[matta]

Hey Guys,

Think may sound like a strange question but is it normal for the LA-2A to emit a low hum even when the gain is fully CCW?

I've not owned a single piece of Valve/Tube gear before the LA-2A but am aware that they tend to be more prone to EMF/Hum than solid state stuff... or so I've heard and this might be normal thing.

I made a clip of it with a few second of with the gain all the way down and then I slowly turned the gain knob up to max and as you can hear the floor noise of the maxed out Gain knob is WAY louder than the slight hum, but just wanted to know if it is normal.

I recorded it straight into Pro Tools with the faders set to 0dB and bounced it down and you can hear it below... like I said it is soft so you may only hear it on a set of studio monitors, headphones.

Matt's LA-2A Hummer

I bought my first little tube guitar amp today, a 5W Fender Champ Reissue and it seems to have a similar/same hum even with the volume all the way down, maybe it is Valve/Tube thing?

Please excuse my ignorance if this is normal, but if it is not, I'd like to fix it and willing to take suggestions.

Other than this is operates fine, compresses well, sound great, just worried in the quieter passages the hum might show itself up.

Cheers

Matt

 

[objectx]

thanks cj I will check those tonight (i'm on the other side of the world) .I think everything is ok wiring wise now... I fixed a few dubious solder joints and quadruple checked my wiring against the schematic. Actually I think I may just not be giving the comp enough juice at the front end to start compressing... if i really crank my input volume from my mixer I can only get 35v AC max on pin 5 of the t4 when the gain reduction is max but then I noticed I had my vlz mixer out put set to mic level... dumb ass. I haven't had a chance to test with the higher level yet. It seems to be a common mistake after reading this thread in its entirity. I've checked the t4b resitance under a lamp and with the cover on and it seems to check out resitance wise so I don't think its the t4b. stay tuned....

 

[CJ]

that line amp is almost a mic amp, so i don't know if level is the issue.

Matt, short the input to the 100 x and compare.

 

[bradb]

Here's why:

It's marked like this because greg only puts a SPST swith on the hot, as is standard. if you had your house wiring reversed, you would only be switching the neutral and you can still have current flow between that and ground (perhaps not between neutral and ground)

Ideally you want to have a DPST as a power switch, so you can break the connection between the neutral and the hot.

This is my understanding, hope someone will chime in, i saw that noone was answering this question.



[quote author="Purusha"]

Because of sloppy wiring which does occur occasionally, the hot and neutral in an outlet may be reversed. The appliance will still operate.

I am aware of this. So there is no bad effect on the sound if the hot and neutral are reversed
in tube audio units? If this is true than I wonder why Greg marked hot and neutral on the PCBs...[/quote]

 

[AnalogPackrat]

Matt,

I don't hear much, if any, hum on your sample. Am I deaf? I hear some slight hiss (normal).

On the Champion 600...I looked around for schemos and only found a few photos someone had taken HERE.

Looks like a standard SF/BF champ with SS rectifier, fixed tone controls (each set at slightly more than half), and a 1k cathode R on the 6V6 (which is good). They flew the heaters off-board so that's not the problem. I bet it's the same thing that all champs have which is noisy B+ on the output transformer. Pop it open and measure your B+ at the connection to the output transformer. You should be able to hack an extra R-C stage in there between the rectifier and the output transformer. The R will depend on your raw rectified voltage, but the C should be 10-22uF. I ended up using 1.2k and 22uF on my SF VibroChamp (but I wanted to drop the voltage, too--maybe you want more like 330R or 470R).

If you really want to hack it up, reduce the feedback (increase the R that goes from speaker output to the cathode of the second preamp stage), or put in a switch to disable the tone stack for more gain. Or adjust the tone stack "presets--a setting like that has a pretty big mid dip for my tastes. Or, for that matter, replace the fixed bass and treble Rs with pots like a the BF/SF champs.

Have fun!

A P

p.s. Search here for Vibro Champ and I'm sure you'll find my posts and the helpful comments I got when I modded my VC earlier this summer.

 

[AnalogPackrat]

Oh, and I almost forgot--try it through a real speaker and you'll be amazed. I am using a big 2x12 cab with my SF VC and it is a great little tone machine. No lack of bass and plenty loud for recording, driving off wildlife, rattling the parts drawers, annoying the wife, etc.

A P

 

[CJ]

good god what isphired you to do that?
petty freaks are weird.
can you pleas spell inspired for me?

 

[AnalogPackrat]

What inspired me to play the VC thru a real cab? I dunno. Maybe it was because I found this hacked 2x12 pseudo-Fender cab at Showcase Consignment for $50 a couple of months ago. It's a big Showman sized thing and it had one semi-cruddy Eminence in it (the one used in the DRRI), but the previous owner had sawed most of the original baffle out and put in a 1x12 + 1x10 particle board one in there. Fugly mess.

I removed it, made a new 2x12 baffle, converted it to semi-open backed, and stuffed it with an Eminence Red Fang (Alnico pseudo-Celestion Blue) and an Eminence Wizard (pseudo-G12H30). Now it eats other 5W amps for breakfast :grin: My recently refurbed SFDR sounds nice thru it, too...

Didn't you make a FrankenChamp of your own?

:sam:
A P


p.s. Oh, and "freak" coming from a guy who stays up all night to hack up antique transformers doesn't mean much, eh? :razz:

 

[CJ]

yes. KT 66 single ended for a "melted butter on your french toast" sound.

Chuck Berry like the with the 2-15 Showman cabs.

A bit tough to fit into a Honda> back then, that Caddy had wings.
And a trunk that would hold the Nebraska Corn Huskers, Larry, the cable guy's favorite team, but whats that have to do with anything?

Hey, what about these coated strings?
14 bucks a set, for the acoustic.

what if you don't like th gauges, just get out the dikes and snip off 14 bucks?
ouch.

Running a plain third on the acoustic, killer sound, but the bridge needs a tweak if you play above the fifth fret.
But good players adjust chord intonation by tweaking individual notes with minor bends.

Can anybody here bar an F chord and make it sound tight?
You need hands like a capo.

 

[objectx]

well it seems that I have had a working la2a all along... just wasn't giving it enough at the front end... I now have good deflection on the meter and nice compression on the sound. A little bit of hum which i'll get the chopstick on to, thanks everyone for your help.

 

[CJ]

oh, cool!
try reversing the heater leads.

 

[John Peacock]

Well I ordered a cheap 6aq5 tube and tried replacing the one in there. No change.

[quote author="AnalogPackrat"]
Did you adjust R3 and R37 per the LA2a manual? Have you changed them since it last worked? There's always the possibility that you wired one or both backwards (I do that fairly often). If you have the "stereo adjust" (R3) turned the wrong way (electrically), it grounds the grid of the 6aq5 and you won't get anything out. R37 adjusts the high frequency response of the sidechain and won't cause things to completely stop working. [/quote]

I hadn't changed anything since it worked. Checked the wiring in those areas - looks okay. Turned them the other direction (clockwise) - no change. Swept them while running audio - no change.

[quote author="AnalogPackrat"]
Is R38 soldered to the center lug of the 7 pin socket? Looks dry. I don't see anything else right off the bat.
[/quote]

I redid the solder around the center pin. Again, no change.

I also swapped out C11 and another .01 cap (maybe C8?), since they were subpar components. No change.

Then I decided (since I was ordering parts and had the thing open) to replace R29 and R36 with values from cayocosta's layout instead of the conflicting values included in the blue kit (which combined multiple revisions). What this did for me was get some measured values much closer to what they were supposed to be. What it did not do is give me a working compressor.

So what about these other pins at the 6AQ5? What should I be looking for? Working upstream, where does the audio that goes to the 6AQ5 come from? I still think our issue is something with the 6AQ5 section of the circuit, but I don't want to rule out anything else.

Thanks.

-Jp

 

[CJ]

the aq5 is the smallest socket, so typcally, that socket is where things can get off by a pin.

also make sure you count clockwise when counting pins from the bottom of the socket, opposite of the side that the tube plugs in to.

this is really easy, you just find where you have good signal, follow it til it disappears, and thats where your problem is.

finding the culprit of the disappearing act is a bit hard, but logic will beat out hunt and peck, by a wider time margin, so use the noggin.
and the scope.
or a DMM set to ac volts.

 

[John Peacock]

When running audio into the unit and metering around 0 on the +4 VU, I am now measure a little more than 1.4 volts AC coming off of pin 5 of the 6AQ5 (heading towards C11 and the TB4, yes?).

Okay, so then when I get to C11, on the 6AQ5 side, I still have a little more than 1.4 V AC. On the TB4 side, I've got next to nothing (like < .01 V AC). So I now believe there is an issue with either the tb4 circuit or the wiring around it. And since back on page 110 of this thread I found that the light emitter on the TB4 was not generating any light, the problem in the circuit is somewhere between C11 and the light emitter.

Where do things go after pin 3 on the TB4. Feel free to refer me to some other place for details.

My new guess is something is grounding out in the TB4 circuit. Does this seem reasonable? What about everything else I just stated? Reasonable? So where to next?

-Jp

 

[AnalogPackrat]

Remind me again--is this a T4b from the Bloo kit or a JBL buy or something else?

Pull the T4b and measure resistance between pins 2 and 3 (across the EL panel). What do you get?

Check the wiring on the T4b socket. When you made the cross with buss wire, did you remember to solder the intersection between the two crossing bits? Pins 2, 4, 6, and 8 should show continuity (nearly zero ohms reading resistance on your meter) to ground.

A P

 

[CJ]

you need 100 volts off that plate.

 

[John Peacock]

[quote author="AnalogPackrat"]Remind me again--is this a T4b from the Bloo kit or a JBL buy or something else?[/quote]

Bloo kit TB4

[quote author="AnalogPackrat"]Pull the T4b and measure resistance between pins 2 and 3 (across the EL panel). What do you get?[/quote]

OL. Is that bad? I am hoping the answer is yes, because this is where I think we should have the problem. Assuming that is bad, what should I be looking for once I pop open the TB4? I actually just went over the solder joint between the bus wire from pin three and the Bloo TB4 PCB. It didn't look great, but now it does. No change in operation, though.

[quote author="AnalogPackrat"]Check the wiring on the T4b socket. When you made the cross with buss wire, did you remember to solder the intersection between the two crossing bits? Pins 2, 4, 6, and 8 should show continuity (nearly zero ohms reading resistance on your meter) to ground.[/quote]

Yes, I soldered that one correctly. I double checked and I am getting next to zero resistance (.3 ohms) at pins 2, 4, 6, 8 and 1, as well as the cross and the grounding post connected to pin 1.

[quote author="CJ"]you need 100 volts off that plate.[/quote]

Where do I put my probes to measure that?

Thanks, guys. I think we're are almost there.

-Jp

 

[AnalogPackrat]

You should see an open across the EL panel--it's basically a capacitor. Seems like something is wrong before the T4b. As CJ said, you should be seeing at least tens of volts (more likely over 100V) AC at the output of the 6AQ5 which is the plate. In case your meter doesn't like measuring AC on top of a couple hundred DC, you should measure between ground and the other side of the output coupling cap, C11. If you don't have significant AC there, then trace back to the grid of the 6AQ5.

I believe CJ posted a nice AC voltage trace diagram on this thread some time ago (at least a year back). Ask him nicely and maybe he'll send you a link in case it's been moved.

A P

p.s. Christian Sugar also posted a really nice "LA2A Deconstructed" schematic that makes it much easier to see what's going on. Search for that and if you don't find it ask Christian.

 

[hodad]

So, I got my la2a (cayocosta stylee) put together this aft. Audio passes fine, vu seems to be doing great. But no compression.

So far I've
Swapped tubes
Swapped t4b's
Checked wiring around the t4b & 6aq5
Checked voltages.

Here's what I've found: I had one wrong resistor--R34 was 22K instead of 10K, which was causing the plate voltage to be lower than screen voltage on the 6aq5. I fixed that; however, the voltages on plate and screen are quite high--190 & 158 volts respectively. I think my line voltage here may be a bit high as almost all the voltages are measuring 10 to 25 volts high, but these seem out of whack.

I measured the voltage on pin 5 of the t4b--8 volts. Don't know if that's okay or not.

Also, when I run audio through, at various points when the signal is hot I get distortion--ugly distortion. Maybe it's on peaks and transients, but I'm not sure.

Any thoughts on where to go from here are welcome.

Tom

 

[CJ]

Put a signal thru, click on my sig line, print it out, a pen, and compare volts, call me in the morning.
10K/22K is not it, nor line voltage, tubes very very uncritical of plate supply.
Screen runs hot in LA2.

And don't forget the easy to forget r3 stereo adj!

Got me the first time even, and I am supposed to be joe stud trouble shooter.
At least thats what I do to keep from dumpster divin.

I'm gonna shrink Lemmy, he looks too much like Waveborn, who looks too much like small tree!

:razz: