Federal Limiter AM864 thread

[emrr]

[quote author="sinestaraudio"] and a small little hum only when it is releasing from compression...but its not there at idle.. its fine as its subtle but i would like to know what is is..

pete[/quote]


Pete, I would suspect better tube matching or balancing would help this. Did you add a balancing circuit, or means to test current draw in each side?

 

[user 10827]

Hi Doug,

OK, no i did not do any of that, how does one go about that? i have seen a drawing some where there is a pot between the heaters? would that be it?

im not sure i understand how to balance them..but id love to know.

:thumb:

 

[schmidlin]

Great work all of you! I have a Federal clone on the bench myself (two actually), only still waiting on the Edcor trannies (this week, they say). I will post pics asap.

First, I offer some help. Edcor has great trannie for the power supply, but it's not listed on their website. 175-0-175 at 100 ma (enough for a stereo unit if you are so inclined), with 5v and 6.3v ct. Around $43. Btw, they have hundreds already designed, and only want $20 to design anything if they haven't already. So glad I found them, and thanks to you guys.

And Pete, I would suggest a balance pot in the center of the plate resistors of the first stage. Use some math to keep it at 47K/side, and a 2W pot or more would be best. Balanced plate votages are good start to balancing tubes. Personally, I bought a slew of the cheap 6sk7s to get close-enough tubes.

Yet, I'm still wondering what all of you are using for the meter: I have little experience with these, but have many cool vintage ones that could be great candidates.

Also, I was wondering: I am using 2 x .47caps for C1, the release cap. If I use a switch to cut one off (with a 10M bleeder to prevent pop), would that quicken both attack and release? At least that's my guess. Some local producers claim much of the tube stuff doesn't attack fast enough when you slam it hard. This may be a nice/easy option.

Thanks again and keep up the great work.

 

[MHanson]

[quote author="sinestaraudio"]here they are...

I had to load the output with a 600ohm resister, change R3 & R4 to 12K from 47K, also added a switch to half the release time.

[/quote]

Tell me more on the release time switch. I've got an original that I'm pulling back out to do some work on and I've love to do this.

Thanks,
Michael

 

[user 10827]

HI chaps,

Re my release switch, as i had no capacitors at the right value, i switched in a 1M resistor in parallel with the 2M, this approximately halves the release time.

i experimented with further reducing the resistance value but it had some undesirable results so 1M was the best fit IMO

the half release is much better for drums, the normal is great for bass etc...

the meter i used was this, i think Cannakin has used this meter too.

http://www.surplussales.com/Meters/MtrVU.html

it works backwards, but its cool looking meter!

thanks for the plate balancing trick....i will try that out.

:thumb:
pete

 

[cannikin]

hey guys read this thread:

http://www.groupdiy.com/index.php?topic=27372&highlight=

 

[schmidlin]

Thanks guys, most helpful.

I found this Collins that has the plate balance option:
http://www.waltzingbear.com/Schematics/Misc/Collins_26U1.htm
It also has a textbook attack and release mod. Scratch your head a little and the theory shows itself.

Incidentally, I tried the cathode balance on my first Sta-Level clone, and it was not enough, and switched the balance to the plate. I must have gotten a pre-rejected 6386. (no surprise)

Pete, your link went to a whole page. I assume you meant the only backwards meter

On the cannikin link, any theory on the purpose of the variable screen grid voltage? Current controller? Not sure how to adjust with the cathodes going to ground.

As for hum, I am sold on the Sta-Level style small cap from the 6.3v ct to ground. So far the easiest way to get the DC heater potential above any cathode voltage, IMHO, which I suppose could have an added wiper to ground for versatility.

Thanks again!

 

[user 10827]

opps! yes the meter link, bottom left corner of the page is the meter i used.

Best
Pete

 

[tardishead]

What is the approx max power output from the 6SN7 push pull output stage? I see the original output transformer was rated for 6mw. If you use transformer with higher rating can you get a bit more out of the 6SN7 stage before say 5% distortion or is this approx the max headroom?????

 

[tardishead]

Having a few problems with this circuit on my clone.

I am getting motorboating through the CV circuit. If I change the release cap from 1uf to 1.5uf the motorboating is a lot better but still apparent at the maximum gain reduction settings. I have to bring back the threshold a bit from max to get rid of it entirely.

On my input transformer I noticed that the secondaries are not perfectly balanced - one is 227 ohms the other is 231 ohms. Is this enough to make a problem?????
I have tried swapping and changing 6sk7 tubes - does not seem to make much difference.

I changed the plate resistors on the 6sk7 tubes from 12k to 47k but this did not make a difference - so I changed back to 12k -  only changing the release cap to bigger helped this problem. I am monitoring the CV voltage on my scope and somehow even with 1.5uf as a release cap the release seems to be in order of 1 second and not 2 seconds as it should be. Why would I be getting a faster release than normal? I have checked all the components in the CV chain and they all seem to be good. The only one I have not checked yet is the 0.01uf cap that sits between the 6Sq7 plate and the anodes of the two 6SQ7 diodes. If this cap was bad would this cause a problem???
Can anyone shed any light on this??????

Oh yes aswell - I am using an old skool bench tube power supply to get 200 volts. Could this have any effect on release times etc?

 

[emrr]

trans balance is better than most.  Not unusual to see 50-60 ohm imbalance with PP output transformers totaling only 400 ohms DCR. 

Bench supply; nothing to do with release times.  But do we have any local filter caps at the amp chassis, and are you certain the chassis and B- are tied to true ground at the supply?

Release times as quoted back then usually reference a time that is longer than the majority of the release range, so may not be noticeable.  Like the final 10% takes 2 sec, maybe. 

 

[tardishead]

It looks like the full 100% releases in about 1 second on the scope.
It is almost as if there is something capacitive in series with the CV thereby reducing the release time. Can anything become capacitive - I had it wired up wrong to start off with and then I realised the problem and corrected it.

Now I have totally got rid of the motorboating but I am using 2.5uf release capacitor.

 

[tardishead]

OK - I just checked my ground bussing and actually there is a 5 millivolt potential difference between one ground point and the real ground. This ground point is the one which the release cap is grounded to.
Is this bad enough to cause problems???

 

[tardishead]

OK OK OK
I think I might have found a problem. When I take out the 2 6SK7 tubes and the 6SQ7 and power up - I am still getting a couple of volts on the CV wiring. Surely that would point to a leaky cap C6 not blocking the DC thru the 27k resistor.

I tried another cap and still had the same problem, They were both WIMA 0.01 from a bag - they might all be out of spec now they are old or maybe they are designed for another purpose where a bit of leakage is not the end of the world - surge protection??????

All my other coupling caps are brand new and completely block the DC fed to them.

This must be the problem.

 

[emrr]

Almost any cap is going to show DC leakage potential with no tube grids present to act as load, and bleed down any trickle charge. 

Do you have any filter cap present, or is it direct drive from the bench supply?  Sometimes this can be a problem. 

Maybe I missed it, but what is your input trans, and are you using the stock impedances for transformer and input attenuator? 

Have you checked the 2M release resistor? 

 

[tardishead]

Direct drive from power supply bench supply. Which I think is the issue. It must be a different impedance from the original because although I am feeding in approx 200VDC - the voltages I am reading at the plates of all the tubes are higher than what the manual indicates.

When I turn down the voltage from the supply to about 175VDC the motorboating disappears - compression seems to sound right and everything is a little bit more stable.

The manual shows 140VDC on the plate of the 6SQ7 so I turned down my supply to match that and it seems to work a lot better.

What do you think does that answer a few questions???

 

[skal1]

Hi lads,

just got an original am 864/u all the tubes are missing  .


what  tube brand did they supply the unit with when it came out the factory and do you have to go with the 6sn7wgt,6sk7gty to get the original sound or can i just the 6sn7and 6sk7.


And can any recommend a tube store which will have these tube and how have agood rep..


thanks

skal1

 

[sintech]

LOL! you beat me on the ebay!

 

[skal1]

i would say beat  ;D , lots of tubes missing  do not really know too much a bit this unit hopes it  is the 30hz to 15khz  not the 300hz to 15khz  how  do you tell them apart?


skal1

 

[sintech]

Mmm.. not really sure but you'll know when you hear it, bought Sinestaraudio's clone for parts price, and it's totally fab! super silky midrange  vocal parallel blend.