MAXSON CA-1589 Tube Limiter and Tubes

Ok, I'm playing with some tube substitutions here (Matched Nos Telefunken 6BA6's in place of 6AU6's). With the 6AU6's in there the unit works fine, however 6BA6's are supposed to offer optimal results.

Reference the schemo here:

http://www.waltzingbear.com/Schematics/Misc/FAA_CA-1589.htm

The 6BA6 was giving off a pumping sound until I replaced the .001uf caps at C3 and C4 with .1uf Hovland Musicaps. Now the pumping is gone but a very rapid sound in now in place of the pumping. I also changed out C1 and C2 for new .001uf caps. What's going on here? Any ideas?

With 6AU6's in there, I noticed a clearer picture using the Hovland Musicaps right off the bat.

My best guess is that it is a matter of unbalance between the tubes, making a "thump" when GR is controlled, fed back to control, and thus oscillating. The smaller anode transfer caps blocks low frequencies, and thus the oscillation. "Matched" preamp tubes is not an exact science, to say the least.

I would use a trimmer for minimizing CV breakthrough due to input tube pair unbalance:

Mount a 100K or 250K/1W anode balance trimmer between R7 and R8, and feed the 250V to the input stage through this. You may or may not want to make R7/8 smaller to compensate for this added resistance. It probably dosen't matter much, but who knows..

Then remove the sidechain rectifier tube V5, 6AL5, and input a 200Hz, maybe 0dB, tone into the control input (centre of the input transformer).

Heat the beast up up, monitor the output level, and adjust the new anode balance trimmer untill minimum of the 200Hz signal breaks through to the output. This minimises "thumps" when GR is controlled - and thus also the source of the oscillation (or actually "motorboating" from your description). If cancelling does not seem to be good (no, i don't know what is "good enough" in this case), try changing one of the input tubes to see if another one balances better.

Do the adjustment when the unit is properly heated up - allow at least 30-60min for it to stabilize.

And, oh - remember to put back in the sidechain rectifier tube..

When properly adjusted, you may try with the larger anode coupling caps - they might work when the input stage is working in balance - and it will possibly give the unit a much better low-end.

Jakob E.

Jakob what is an anode balance trimmer? A pot correct? A special type?

Thanks for the post

No, it's just a simple pot or a trimmer.

But it's function in this circuit will be to act as an anode balance trimmer.

Make it 1W or so..

Please let us know how it turns out..

Jakob E.

The motor boating only becomes more rapid when the unit sits for a while with no signal being passed. However if I inject signal the motorboating stays as a constant low thumping sound at 1 per second. When I pass some signal I can hear that the results of the new caps .1 Hovland Musicaps at C3 and C4 instead of the .001uf in combination with the Telefunken 6BA6's tubes is going to sound fantastic.

Which caps control the high pass and low pass?

I'm looking for a pot 1W poat now; I think I have spare I used for volume control of my Altec 436b.

Someone along the line removed C5 and replaced it twith three little caps piggy backed onto R21, the values are .1k, .1k and .047k could this be the issue? Or would those equal what was there a .20uf?

Sorry, I have nothing to add regarding the motorboating problem, but...

[quote author="tubemonkey35"]Someone along the line removed C5 and replaced it twith three little caps piggy backed onto R21[/quote]

By "piggy backed" I assume they placed them all in parallel, right? That's close enough to the .25uF shown on the schemo. Shouldn't be a problem.

Peace,
Al.

Alk, thanks for the confirmation; just wanted to be sure.

As I said, the motorboating will probably go away with trimmed anode balance. Try that first.

Jakob E.

An audio taper pot should work correct?

> An audio taper pot should work correct?

No!

Plain (linear) taper!

(OK, audio taper "would work", but you'd have to put it SO far off-center that it would be very twitchy.)

Bad balance can encourage motorboating. But I would also examine all the B+ filter caps.

Before I tried to substitute tubes it would be best to get the frequency response to not roll off below 300Hz.

With C1 C2 C3 & C4 = 0.001uF the audio has permanent HPF which is 3dB down at 300Hz. So cannot understand how this thing could be pumping without low end. This thing was designed for speech I presume. Even with the change of C3 & C4 to 0.1uF the HPF is now 3dB down at 230Hz. Changing C1 & C2 to 0.022uF will fix this and make a -3dB point at 10Hz. Don't go any bigger that that for C1 & C2 as it will just make the Mu comp pump more.

Not sure why you think 6BA6 will be better as it is usually only good as a Mu tube when run as a triode like Manley use it. In this circuit it is being run as a pentode with crazy amounts of gain. They are pin for pin compatible but that is where the similarities kind of end. Have you read this somewhere?

Joe

WWW.JLMAUDIO.COM

Joe, thanks for the info. I was recomended to switch to the 6BA6 tubes by a guy who had one of these; he also recomended switching out the C3 and C4.

I'm going to the surplus store today to pick up a linear pot and I'll grab so .022 caps to try as well.

So the C1 and C2 are the high pass and C3 and C4 are the Low Pass then correct? Making the cap values larger just shifts what freqencies are effected, or does it actually remove the filter?

Could it be that the channging of the 6AU6 to the 6BA6 was because the 6BA6 is a true Vari-Mu tube? Ifthis is the case, then they would have to be rewired for triode correct? I'm guessing here...

I think this unit is going to kick butt when all is said and done.

Joe I re-read your post.

Duh, they (C1,2,3,4) act together to form the HPF; there is no LPF.

Doink!

Just tried the .022uf caps in C1 and C2, however now the pumping is more rapid at the same pitch; this even using the 6AU6 tubes.

Disconnect the wire from pin2,7 of the 6AL5 that goes to C5 & R21 this will stop all compression for now and should also stop the thumping.

Does it?

If the thumping is stopped does it sound like the the audio is now full range ?

If all above questions answer yes. Then Insert a 100k or 500k between pin2,7 and the wire going to C5 & R21 (one wire to a left side tag and the second wire to the centre wiper tag looking from the front of the pot with the tag pointing down). This is now the attack control. With the attack control wound open (fully CW) to 100k or 500k the compressor shouldn't motorboat. As the pot is wound closer to 0ohms there will be a point where it should start to motorboat as attack time approaches 0. Set the pot so it just doesn't motorboat and measure the pot resistance between the two wires with the power turned off. Insert a resistor of the closest value to what you measured in series with the pot if you want to leave the attack control. or just leave the resistor in if you want the fastest attack without motorboating sound effects.

Also if you are putting line level into the compressor the 27dB pad shown must be wired in circuit. Also start with the output pad on +20 so it is out of circuit for now.

Joe

www.jlmaudio.com

Joe, should I try C1,2,3,4 all at .022uf?

The 100k or 500k POT I have is 1watt volume type; oh I'm looking, it's a 250k

Having C1 to C4 at 0.022uF gives a 12dB per octive roll off under 13Hz not including the transformer roll off points which would be good as it will stop the Mu thumping a bit more than with C1 & 2 at 0.022uF and C3 & C4 at 0.1uF.

But for now either will work fine for the tests above. Have you tried any of the above tests yet?

250k log or lin will work fine for now. I think you will find a resistor value of about 10k to 22k in series with the pot will be enough to stop the motorboating.

Joe

www.jlmaudio.com

1watt resistors or do you think 1/2 will be ok? I'm going to try the tests here in a bit.

I dumped those C5 piggy backed caps on R21 as I found an oil cap at the surplus store that was a drop dead replacement for the orignal (.25 at 600v).