LA2A building thread for newbies

[cayocosta]

Hi Arrigotti,

Have you checked your voltages against the JBL schematic?

The JBL schematic shows 135V at pin#5 and 100V at pin#6 of the 6aq5. Unfortunately, I never checked my voltages on the 6aq5, but my B+ rail voltages were all in line - give or take 5%.

Ron

 

[CJ]

Check pin 6.
This is the screen.
It can influence the plate resistance and therefor the voltage on pin 5.
There was some tinkering going on at the screen over the years.
Also check the Appendix of Jahnsen's book.
There are some notes on various changes and voltages over the years.
Your schematic might be a little different.
cj

 

[Arrigotti]

Thanks, I will check out pin 6. If pin 6 is higher will pin 5 be higher or do they have an inverse relationship?

 

[PRR]

> pin 5 of the 6AQ5a. I have 171 volts as opposed to 100 volts

It is a pentode. Plate voltage isn't controlled except indirectly with G1 and G2 voltage. It would be normal to see +/-30% variation.

In this type circuit, plate voltage should probably be "about half" of B+ voltage. Either 100V or 171V might be fine.

Before you go further:

Trippple-check all the resistors around the 6AQ5. You can go nuts trying to find the complicated problem, when it was really just a red stripe where it should have been a brown stripe.

Let the thing cook (on a fire-proof table!) for about 24 hours. Tubes can drift in storage and heal themselves while running. Gas is possible, though your symptoms don't look like gas. It could be leakage in the capacitors, which should heal (or smoke!) overnight.

Then:

Re-check the cathode voltage and resistor value. This should be very close to nominal.

If the cathode voltage is low and the plate voltage is high, the tube is not passing as much current as the chart says. In a pentode, the best step is to increase the G2 (Screen grid) voltage a little. Screen grids fed from a large dropping resistor, as the LA2 does, are not real precise. The plan I have here shows a 220K resistor from +275 to 6AQ5 pin 6. We want more plate current. We want higher screen voltage. So try changing from 220K to maybe 150K.

 

[Arrigotti]

Still tinkering with the LA2A trying to get the Pentode voltages more in line with the 3 schematics I have. I finally ran a bit of audio through it, and it worked quite well. It hissed quite a bit until I realized that I had the pots wired in reverse and the gain an reduction were both full bore.

Regarding the hiss: Would a higher quality potentiometer help quiet it down? I get hiss once the gain is past 1/3 rotation or so.

As far as hum, I can't hear any yet, but I was trying it out on a pair of cheap ass sony powered speakers, so maybe it is there, but I can't hear it.

 

[Arrigotti]

I have been tinkering with pin 6 (screen grid) voltage on the 6AQ5 to bring the plate voltage down some. I tried 150k instead of 220k and then 100k. I was able to get the plate down to 140 volts at pin 5.

The interesting thing is that the voltage at pin 6 is identical to the voltage at the filter cap even though it is going across a 22k resistor. Could this be the sign of a bad or unformed filter cap?

Thanks.

BTW this has been a really fun project....Just what I needed after building 3 guitar amps.

 

[Arrigotti]

So, I gave it the big test and put it through the real recording stuff. It sounds GREAT! I recorded some vocals through it and compared it to some takes without it. It really PHATTENS things up. It sounds really smooth. I like it.

I checked the noise floor and it looks like it kinds bounces between -72db and -62db with the gain and the compression all the way down. It isn't much noisier when the gain is set to unity.

I checked the specs. on the JBL site and it says the noise floor is -75db at +10. I'm not sure how that compares, but it seems that mine is a bit noisier.

I can't hear noise unless I really crank up the track, so that's good, but in the interest of learning, how can I improve this? or is this normal? The pots I used are cheap Alpha's. Would this improve the s/n ration?

Thanks!