All things LA2a related

[CJ]

OK, here is a voltage chart for the LA2. I used my own machine, which utilizes a 12AY7 for V1, but that should not make that much difference for troubleshooting purposes. Just expect more gain from V1 from the 12AX7a.
Also, my 6AQ5 is really a 6V6GT, the exact same tube, different bottle, so that should be ballpark also.

I am using an external pwr supply, so the B+ comes straight into the box, thus, I did not include pwr supply ac volts.

File is a bit large thus I am linking it. Filp it left or right to get it to print on one page. Colors will be lost on B/W printout.

I injected real music from a CD player, set the GR on 5, set the gain pot so I was reading 0 db on the meter output. GR registered about minus 20 on the meter. My VU meter is not perfectly calibrated, so allow 20 percent difference in all of these readings, which is normal for a tube circuit anyway.

Now go get that problem, Paul! Rework be damned!
:thumb:

http://vacuumbrain.com/The_Lab/TA/LA2/la2_voltages.jpg

 

[rove]

[quote author="CJ"]I used my own machine, which utilizes a 12AY7 for V1, but that should not make that much difference for troubleshooting purposes. Just expect more gain from V1 from the 12AX7a.
Also, my 6AQ5 is really a 6V6GT, the exact same tube, different bottle, so that should be ballpark also.
[/quote]
Can I just put in a 12ay7 or 6072 in place of v1 without any other alterations?
thanks.

 

[CJ]

Yes. Drops right in.
A lot of those min dual tri's have a mantra that you should recite:

plate- grid-cathode----plate- grid- cathode---
if you memorize that, it will save you a lot of time fetching the tube manual.

the heater wires divide the two sections, so an id of elements is quick and easy.


12AT7, AY7, AX7, all the same.

 

[Paul B]

[quote author="AnalogPackrat"]Paul,

So the distortion sounds similar both with the T4b in and with it out? That is, the T4b doesn't change the character of the distortion, right? If so, it seems like something is awry in your audio path (maybe a bad joint [seeds?] as CJ pointed out). What kind of caps did you use for C1, C2, and C3? Can you make the distortion go away with really low gain settings or is it always there?

A P[/quote]

The distortion is the same with t4b in or out. C1-3 as well as both C8's and C9 are orange drops. The distortion goes away at low gain levels. The basic problem remains: Little/no compression because input signal is weak (more than 320mV or so at the input and it starts distorting even at low gain levels).

Now on CJ's chart (thanks for the chart, CJ) he's got 4.0V at R5-R6. Does that mean the input level pre the A-10/HA100X is about 1.0V?? Is that 4.0 a peak or an average (or something else?)

Everything seems whacked at the input tranny. The signal is weak when I come in thru the input jack and when I wire straight in the signal is stronger but must be kept weak to avoid distortion.

CJ, how far did you turn the Gain up to reach 0db?

 

[AnalogPackrat]

Paul,

Well, maybe there is something amiss with your input tranny. Those orange drops should be fine, though (assuming they're the 400 or 630V ones). The HA100x is wired as a 1:10 on the original, I believe. I'm guessing that's how CJ has his set up. So he's actually got more like 0.4V on the primary. He said he used music for a signal so you'll have to ask him whether it was Black Sabbath or classical harp music :grin:

I still haven't managed to get my test equipment set up this week like I promised . Today is my wife's birthday. Maybe tomorrow evening...

A P

 

[CJ]

Yes, HA 100X wired 1:10, so about 0.4 on the input jack. I was using music so the signal moed around a bit, but was stable enough to get an average reading on a digital voltmeter. I also double checked with the scope at the same time.

Everybody says their LA2's distort at about anything past 5 on the gain dial with a hot (modern cd signal input, so that part of your project might not be a concearn. The 12AY7 will help that out a lot.

It sounds like you have a problem with the input trans and/or input jack.
Make sure you both are wired correctly.

I think I remember the gain knob being set at about 3 or 4 to get o db out.

How is your compression when you straightwire the thing?

Fill out that voltage chart along side my numbers so we can have a gander. I can post a pic if you can get someone to take a digi pic.

Just for the heck of it, you might want to make sure that your nrgative feedback is reducing the gain of your amplifier. This can be done by disconnecting the wire that leads from R11 (68 K) to the cathode of V1a.

Do an A/B test with the amp running. No high voltage at this part of the circuit.



cj

 

[Paul B]

I filled in my readings next to CJ's. How do I post the chart?

I did the readings with a 750mV input signal. I could only turn the Gain to about 15 before it got to 0dB and really started to distort. BUT it is a strong enough input to get about a 20 dB reduction when I turn the PR to 50. When I crank PR it REALLY gets quiet!!

Some of the voltage readings are completely whacked. I can understand why some of the AC readings are low because my gain was only at 15, but the higher AC voltages I dont understand.

It appears the unit will function just fine at these higher input levels, once I can tame the output. By the way, the distortion is more of a rolling warm tubey distortion with this higher input level.

Pls tell me how I can get the chart up. It's a .gif format. thanks.

 

[AnalogPackrat]

Hi, Paul. Just open a free account at photobucket, upload your picture and then post the link here. I think CJ is out kayaking this weekend, but I have a couple of hours this evening to help you out. As a bonus, I dug out some of my test equipment so we can try to trace a nice simple sine wave input for comparison. My Bloo also uses Sowters, so we should be good. You changed your 220k plate resistors on V1 to 100k, right? We'll have to keep that difference in mind in our comparisons. I have to eat dinner at some point, but we should be able to make some progress on this thing.

A P

 

[Paul B]

OK let's see if this works....

http://i8.photobucket.com/albums/a5/PaulB_/PaulsVoltages.gif

 

[AnalogPackrat]

OK, I'm checking vs. your gif. I have 750mV in, gain at 15 and PR at zero. Set to compress. At the input transformer secondary I get 2.89VAC and between R6 and R7 I'm getting 1.25VAC. At Output +4 meter setting I get -4.3dB. Right off, it looks like you have a problem somewhere around R5, R6, or R7.

A P

 

[AnalogPackrat]

Or an alternate theory is that your EL panel is lit even with PR at 0 and that is killing your input. Can you pull the cover off, dim the lights and see if the EL panle is on at all?

 

[AnalogPackrat]

Interestingly, I have to turn my PR up to 80 to match your 0.268VAC after R7. Are you sure you don't have a linear pot for PR?

 

[Paul B]

[quote author="AnalogPackrat"]OK, I'm checking vs. your gif. I have 750mV in, gain at 15 and PR at zero. Set to compress. At the input transformer secondary I get 2.89VAC and between R6 and R7 I'm getting 1.25VAC. At Output +4 meter setting I get -4.3dB. Right off, it looks like you have a problem somewhere around R5, R6, or R7.

A P[/quote]

are we all comparing the same thing, tho? My reduction from 2.86 to .345 across R6 is about 8.3 times difference, where CJ's is 6.7 times difference, yours is 2.3 times difference....

yep, I still have the 100k's in for R9 and R13...

also, The EL panel is dark when PR is turned down.

 

[AnalogPackrat]

Sorry for the confusion. My measurements were with PR = 0. To match your number at R6-R7 I had to turn my PR up to 80. I think we should try to debug this thing with the simplest setup. PR = 0. Let's make sure your amp (v1 & v2) works. I'm not sure it does looking at your voltage readings near the back end. Can you set you PR to 0 and measure at R6-R7? Then turn your gain up until you get CJ's voltage at V1 pin 2. I need to eat dinner. Be back in about 30 minutes.

A P

 

[Paul B]

AP, what VAC did you get at the output with that setting?? I had my PR setting at 50. was getting about 15-18 db reduction.

 

[AnalogPackrat]

I get 0.225VAC out with gain at 15 and PR at 80. Look, we're not going to be able to easily match up if we don't simplify. Our pots aren't going to be the same. Let's set PR to 0 and try match our voltages at the input and go from there, OK?

A P

 

[Paul B]

With Gain at 15 and PR at 0, I got 2.91VAC at R5-R6 and 1.14 at R6-R7. The VU meter was a hair away from pegging, +3db (at +4 setting)

 

[AnalogPackrat]

OK, I just traced mine at gain 15, PR 0.

750mV, 1kHz input
2.89VAC at input transformer secondary
2.86VAC at R5/R6
1.24VAC at R6/R7
0.015VAC at V1 pin 2 (grid)
0.1VAC at V1 pin 1 (plate)
3.55VAC at V1 pin 6 (second plate)
0.345VAC at A14 (V2 pin 6)
3.22VAC at C5
1.08V at output jack

Try to set your gain so that your V1 pin 2 voltage matches mine. We'll SWAG the AX vs AY gain differences...

 

[Paul B]

[quote author="AnalogPackrat"]OK, I just traced mine at gain 15, PR 0.

750mV, 1kHz input
2.89VAC at input transformer secondary
2.86VAC at R5/R6
1.24VAC at R6/R7
0.015VAC at V1 pin 2 (grid)
0.1VAC at V1 pin 1 (plate)
3.55VAC at V1 pin 6 (second plate)
0.345VAC at A14 (V2 pin 6)
3.22VAC at C5
1.08V at output jack

Try to set your gain so that your V1 pin 2 voltage matches mine. We'll SWAG the AX vs AY gain differences...[/quote]

Here's what I got:
750mV, 1k input
2.91 at R5-R6
1.14 at R6/R7
.033 at V1 pin 2
turned gain down to 12 to match your .015 at V1 pin 2
.1VAC at V1 pin 1
2.83VAC at V1 pin 6
1145mV at A14 (!!)
2.57 at C5
.84VAC at output w/ gain at 12 , gain at 15, output = 1.28VAC

where exactly is the input transformer secondary? isnt that the same as R5-R6??

 

[AnalogPackrat]

I think your 12BH7 may be oscillating. Can you describe how you wired your grid stoppers? They're 1k's right? Let me go check one other voltage on the 12BH7. Can you measure ACV on Pin 6 with the current setup? Be right back...

Yeah, R5/R6 = input trafo secondary. I somehow managed to take that measurement twice