Federal Am864/U limiter attenuator problem

[gary o]

Hiya iv made a DIY Federal Am864 limiter, not finished yet but have been testing it, & it sounds great however, I was just about to solder up a 20 position Ladder type attenuator that should be 500K according to the schematic, & have realized the one I knocked up on a bread board for testing was only 500R & yet it worked great?,

would we expect it to still work at 500R anyway?

I then made a 500K ladder atten on the bread board & now I have lots of popping noises & more as I speak thru the mic,I think its the control voltage coming thru ?

could the schematic mean 500R instead of 500K ?
Iv seen 2 schematics both say 500k
The input is similar to a BA6A input, same tubes & the BA s attenuator is 25K.

heres the AM864/U schematic

http://www.waltzingbear.com/Schematics/Misc/AM-864.htm

Any thoughts anyone....many thanks.

Gary O.

 

[rafafredd]

I don´t think it would be a good thing to terminate the input tranformer with 500R POT... But well, if it worked better... It´s a 600:10k transformer, so I would use two 5k pots to have a true 600ohms input impedance on the primary. That should work nice. You can always try higher vaules, like 50k or 100k, but I think 500k is really way too much to make this transformer happy.

Also, I think it has more to do with the transformer used than with the compressor circuit. So, if you are using the BA6A input transformer, you should try the same 25k pots used in the BA6A...

Let´s hear what others have to say.

 

[gary o]

Hiya rafafred thanks for your thoughts im afraid I dont know enough about the theory,
the input tran is 1:4 600r to 10K recomended to me by Brian Sowter for the Ba6A its also the one I believe for an LA2A, iv also got a Sowter 9045 1:5 better quality input tran that iv tried in the AM864 & that makes the same noises,
if my tran is 600R /10K...should be right, my atten is 500K should be right {according to diagram} the tubes in my case are 12SK7s should be right...can tubes go a bit funny causing them not to work properly in the correct conditions?
I did buy them from a junk box 1 pound each they are RCA look unused tho.
i'll have a play around with the attenuator values for now also might be able to test the 12SK7s in the BA if I change the heater voltage.

many thanks..... Gary O.

 

[rafafredd]

Just try the 25k pot and it should work.

 

[gary o]

Its weired the more resistance I add{ started at 500R} to the attenuator the more distortion I get, then if I add more up to the 500K area I get constant popping plus more distortion on top.......dont understand.

Cheers...Gary O.

 

[Rob Flinn]

Could this perhaps be that the tolerances between the 2 wafers of the pot are not very good ???

 

[gary o]

Hiya Rob I see what you mean but im useing resistors & crock clips on a bread for testing, good job I did really got hold of nice 20 pos 2 pole switches & was about to solder 40 resistors onto one to make the 500K daul atten.

Its strange the thing works so well with only 500R atten, its sound on a parr with the BA6A in my opinion & is more flexible I love it,perhaps I should leave it as it is,tho I would like to understand what is going on

Cheers Gary O.

 

[gary o]

Its been in my junk box for about 5 years now....but now it lives again , I never understood my attenuattor problem & never really wired the  project up permernantly so i could use it properly & times since I wired it up I couldnt get it to work as nicely as first it did.....But I think Im learning and understand more now I have it working a treat again & am gonna make it usable....when I first made this little thing there was no info or pics on the web so I just followed the schematic & point to pointed the components under a aliminium platform, shes not a looker  but wow she sounds cool now.... I found Ht to be critical.....she runs quiet & is quite flexible ratio compression wise.....Im using a sowter input tran 250K daul ladder pot & sowter output tran thats designed for the BA6A plus output attenuattor....dont no if the transformers match properly but it sounds really really good....

Am going to finish wireing & am considering adding a gates mic pre on the same chassis will post more findings soon...

 

[substitute]

Keep posting please, I just stumbled across the manual and would love to build one of these.  I'm psyched you got yours working. 

 

[gary o]

Im psyched that your psyched that its working................well did work back in 2005 trouble was & still is I didnt have all correct parts....its working great now Im over the moon

Since 2005 Iv followed with interest threads of fellows cloning these down to the nuts & bolts or fixing original units sadly lots of times with limited results (no pun intended) problems with humming noise various nasties I read somewhere that the circuit must just be just bad & noisy ( nope)...component layout has been blamed ....for the record mine was kinda thrown together at the time had no pictures or manuals to look at and just basic knowledge of layout from lookig at old valve radios....I basically put the four tubes (not PSU rectifier) close together on ali platform and soldered components between tube sockets no turret boards even, messy but direct, Im often laughed at & told off in a nice way about my messy sometimes scary DIY and I understand & some the works of art I have seen here look amazing.......but sometimes they have to be ripped apart as they dont always sound amazing.....heres my top tip for tube DIY (not that Im DIY guru but this has always worked & educated me) build & try as you build, build PSU then a stage of audio carefully crock clip it get some audio running thru it,if all sounds good build next stage and so Ive learnt so much this way & its fun.

getting back to this limiter my power supply for this isnt same as original its not on same ali platform as the rest of the audio this must help to keep the humm away Iv built all my tube DIY this way.

For the record I tried couple of supplies I have made for other DIY one was even simpler than the original federal design, as mine had no choke.... tho I settled on using a supply that did have a choke in line tho it doesnt seem any quieter or anything it was just easier to use right now....the supply put out about 295 volts & worked like this with a little hum the other supply I had , had bleeder resistors so I could try other lower voltages with lower Ht voltage the hum went away, but too low cause some distortion while compressing...so I went back to my other supply and tried some resistors to drop the power from 295 ....I found a sweet spot the voltage now measures 220 volts which I think I read is what its meant to run on.....lower & distortion come higher output level rises & humm starts to be heard.

I have 2 newish 12SK7s not matched but maybe I got luck as I have no added balancing pot

Im running those on 12 VDC from another supply I have its just a wall wart in a box.

Other tubes are run on 6.3 Volts from the Ht transformer

Input tran is sowter 1:4 ratio expensive but good feeding a 250K daul pot (for now)

Output tran is Sowter very very expensive and is meant for a BA6As 6V6 tubes its very good quality but wonder if can be improved as is not meant for this circuit.......

Ive been wondering if the cheaper (very cheap) transformers used in other attempts at the this compressor could have caused problems with hum & or noise......Ive been reading about problems with Edcor transformers & Im very keen to try these as they are so cheap (not sure if I can get them cheap in UK tho) Im hoping they work it would be great to get a few different models to try.

Anyway I hope this helps any fellow DIY ers I love this little limiter what a great simple fun and bloody useful project il post any more activity soon.

 

[mushy]

This is on my list as well. Any chance you could post a few picks?

 

[substitute]

heres my top tip for tube DIY (not that Im DIY guru but this has always worked & educated me) build & try as you build, build PSU then a stage of audio carefully crock clip it get some audio running thru it,if all sounds good build next stage and so Ive learnt so much this way & its fun.

that's a good one.

my power supply for this isnt same as original its not on same ali platform as the rest of the audio this must help to keep the humm away Iv built all my tube DIY this way.

Not quite sure what you mean by ali platform, I understand the purpose of isolating the power supply, but could you explain how it's set up in your build? 

and yes, let's see some pics, maybe with a crocheted  "bless this mess" hanging in the back ground

 

[gary o]

Bless this mess I like that....I will put some pics on soon, as I say not much to look at. theres also some pics here somewhere of other DIY am864s that look fantastic.

Sorry what I mean by ali platform is aliminium platform....cant spell it & its not the best way to discribe it....chassis would be a better way, I just bolted the tube sockets on top & soldered the components underneath the chassis onto the tube sockets, birds nesting is the term I have read.

Im gonna try for fun to add a full wave daul diode to the plates of the 6sn7 like most other vari mu limiters just to see if it works & how it compares to its unusual half wave diode...maybe even add an extra buffer stage...

 

[gary o]

Hmmm just tried adding a dual diode 6AL5 stage.......cant seem to get any compression wasnt expecting great results but maybe something interesting ............unless is my 6AL5s I will be able to test them later

 

[guavatone]

Thanks Gary I am trying to put together a PCB project for this baby.

Was your 220 VDC for B+ measured after the choke?

right now, mine sounds great until it starts limiting.  When limiting it is popping and fuzzing like mad.

 

[gary o]

B+ 220V after choke as I say my PSU isnt same as federal schematic after choke I get 295V I then have a 2K7 resistor so ends up 220V after choke and resistor for the record it works on 295V without resistor too but hums a little.....Im not very good at theory as I say but it sounds like yr circuit not balanced good and is letting control voltage come thru....What input tran are you using ? and are yr 12SK7s matched ? do you have the balancing pot mod ? Is it worth putting on a PCB so few components Im wondering if it may create more trouble there are pics in the manual thats here of the originals turret boards....

 

[Joechris]

In the manual it says 200VDC. I guess 220 is in the ballpark.

 

[guavatone]

Thanks, I think I've got it now.  I had the input atten. pot wired so an uneven resistance was being dialed in on each side of the input secondary.  Now it is wired so that the resistance is close (as much as an alpha can do).  I plan to try a fixed 500K resistor on both sides of the secondaries to CT.  I have a daven before the I/P trannie but one could use the 1176 600R PAD.

Now it just distorts if pushed hard enough to level out a clean elec. guitar arpeggio mixed with chords.  I mean that when it is totally squashing the part it crunches.  But if is just leveling off the signal a bit to catch transients it sounds quite good. 

The input is fairly balanced and shows 3 mV across grids as per NYDave's recommendation.

I tried both Edcor 2.5W and sowter 9900 1:4 but I cannot answer any preference since the latest try with sowter was distorting and then was distorting less after correcting the input pot.

Next for me is to suss out a good output variable Bridged T-Pad.  I think a 10-30 dB pad with 2dB steps may be the ticket.

Sorry to Hi-jack. Have a great New YEars everyone!!!!

 

[gary o]

Got my AM864 wired for use, she not that pretty at the but will be experimented on further before beautifing a little I never like to declare my projects finished as they are always ongoing, yr prob wondering where transformers are.......they live next door on my DIY RCA BA6A theres a switch that switches powers from external PSU and switches In/out Transformers....to complicate matters Ive just wired into service a VK 1/ 436C  vari mu comp that I built ages ago too & connected that to the input transformer as well .....when both powered up 2 side chains act upon the compression tube, so I can have AM864 with its own side chain or with VK1s side chain or both at once also I can have VK1 with its own side chain or AM864 side chain or both....to further complicate things the BA6A can be added to the VK or the other way round..... its very interesting the daul and triple action type of compression I can get by using different limiters and side chain combinations......So at the mo the is a little bit of realestate left on the AM864, something could live there.....got Gates pre amp I might bolt on there, we'l see