• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

stancor williamson?

thanks dave. not even one out of 5 in the right sockets is something to keep that seller off the roulette table.

i've only had a few minutes with the amp, being very bummed it arrived with the tubes were broken. it traveled close to 3k miles to get here so luck of the draw. when the bench clears a bit i will look closer now that i have the schematic.

the 2 toasted grey resistors on v2 would be 47k 2w in the acrosound schematic. it looks like maybe a fer of the carbon comps have been changed from the first build. i guess it is time to check the trannies to make sure they are still good with the seller's tube position mix up. hopefully she just plugged them into the wrong sockets for photos but the toasted 2w 47k make me think she plugged it in to line.

assuming the xformers are ok, if you had this amp, would you restore this to the original circuit? also what would have been the original plan for a tone stack? i am assuming that in the original design perhaps that was another module/preamp?
 
gadget73 said:
Nonsense. All the holes are the same size, therefore it doesn't matter where the tubes go. They're all really the same anyway. You can't fool me with these fancy numbers and such :no:
Click to expand...

that bottom shot of the chassis is the dead give away. too bad the seller didn't look.

but allot of musicians think tubes are like shoes. if they fit you're cool---well till the smoke that is. i once got the "12ax7 is cheaper than an ef86, and i had one so i used it instead". also swapping the rectifier with the output tube on a fender champ happens all of the time. luckily nothing bad happens except for minimum bench charge for a 2 minute tube swap.
 
Gotta love Gadget's sense of humor -- and logic!!

Music -- I would restore the unit as original, if only because it is quite a piece of history in the audio world. While it can be argued that Hafler and Keroes of Acrosound were not the originators of tapped screen output tube operation, none the less, they were the ones that really experimented with the idea, discovered the best screen tap position(s) to use and what the attributes it provided were, and took the risk to manufacture some of the best output transformers ever made to market what they coined "Ultra-Linear" operation.

Before Ultra-Linear, the best you could do in the hifi world was about 7-10 real watts RMS of high fidelity power from a triode Williamson amplifier that was often rather unstable. Hafler and Keroes changed all of that by taking their Ultra-Linear output stage and adding it to the Williamson amplifier, and included some very basic stability improvements to create the very amplifier you have. It at least doubled the power that the original Williamson could create, and produced lower distortion to boot. As such, it turned the audio world on its ear (pun intended), becoming one of the biggest single advances to last the duration of the vacuum tube audio era.

As a piece of history then, I'd restore it and enjoy it for all that it represents.

Good luck with it!

Dave
 
dave, after that concise description of the acrosound contribution to the williamson idea and hi fi in general i would feel very guilty doing anything but restore it to original. why waste such good transformers and topography on a guitar amp. it should actually do exactly what i want for my synth circuit's amp, in that i will get lots of precise "ultra linear" headroom.

one question i still have (actually several). do you think that amp was coupled to another pre amp gain stage/tone stack? it seems so strange that this chassis doesn't even have an on off switch let alone anything but a vol control. and that bias setup is unlike any i have seen. is it like a balanced cathode bias using the 100k 2w pot to balance the output tube's cathodes to ground? i'm not even sure what i would measure in that circuit other than to see the sides balance. if i get to understand the theory behind this elegantly simple amp (parts count etc) i will take my tube addiction to the next level. arrrrgh---so much to learn. glad i joined this group of knowledgeable tubies. though i feel like i drove my 42 dodge flathead to the Lamborghini pot luck luncheon. hope you guys don't mind beans and franks till i catch up a little.
 
It may have had something upstream, but the Stancor 2 chassis one I had certainly didn't need any more gain. Mine had a volume control on it, and with any line-level source it would easily drive to full power. No tone controls either, just flat room-filling sound. I'd have kept it except I never found a twin even after several years of casual looking and I decided I'd rather have the space and the cash instead of one more shelf decoration.



You got the bias scheme exactly right. Most of these have 1/4" phono jacks that allow you to plug in a meter to directly monitor the cathode current. The usual deal was you went back and forth between those jacks, tweaking as you went, in order to get the tubes pulling equal current.
 
In the hifi world, it is quite common for the better systems to split the power amp and preamp into two separate units. This typically allows for the lowest noise levels to be obtained, and each unit in general to be designed best for what it ultimately does. In the beginning, the power supply was often on a separate chassis as well, because the early power supplies typically contained a couple of healthy chokes -- this because high capacitance electrolytic caps were not yet available. Therefore, small capacitance oil caps were used with chokes. If the power supply with its transformer and chokes were contained on the same chassis with the amp with its transformer, the whole affair could exceed a reasonable weight level, or what a chassis could structurally hold -- ergo, two chassis. As higher capacitance electrolytic caps became available, chokes gradually disappeared, and power supplies and their power amplifiers were all integrated into one chassis. However, for best performance, the preamp is still kept separate for lowest noise levels.

At best, basic power amplifiers may have and on/off switch (typically do), and a gain control (to equalize the sensitivity of the power amplifier to that of the preamplifier it is used with), but not necessarily. Power on/off is usually provided on the preamp unit with a switched AC outlet to plug the power amp into.

The world of hifi is similar in some ways to the guitar amp world, but there are many differences, even though basic circuit concepts may be the same. Welcome to vacuum tube hifi!

Dave
 
gadget and dave, thank you so much for your responses and the ideas. if i were going to push a pair of very nice old EV 12's with a tweeter, i think if i read you right, i could go right from a mixer into the acro williamson and get my eq at the mixer. i have a few nice little mackies that i use. if i needed more gain on the front end, which it sounds like might be unnecessary, i could experiment. i actually have a rack mount 12ax7 preamp i haven't used much in the studio that would be worth experimenting with.

so in the hi fi world, less is more too. i like that philosophy.

all the best and thanks again.
 
The Acrosound UL Williamson has an input sensitivity of about 1.2 vac to develop full power output. Sounds like you can put it to good use!

Dave
 
from looking at the schematics in this link :
http://www.oestex.com/tubes/acro.html it looks like i could use 6550's with very little change when comparing the schematics. the pc8413 stancor tranny has 400-0-400 250mA, 5v@4a, nd 6.3v@5a. it looks like there is just enough fil V for a pair od 6550 and the 2 6sn7.

i have several 6550's and no one here uses them so they would be good choice for me. 4 tubes seem to dominate guitar amp output. 6l6, 6v6, el34 and el84 any opinions on the 6550? i have heard they are powerful and clean.

i tested the transformers and they are working fine.
 
hi again amigos--

just lost my post trying to post it and the back button took me back to a blank message box. i'll trim it a bit and hope it posts this time.

3.5 inches of rain in an hour and a half last night---monsoon season in the tropics--at least it was cooler today.

i had a chance to spend some time under the hood of my acro-willies and i think i can have it ready for use quickly. i have a few more questions i hope you have time for. thanks for helping a guy new to hi fi circuits understand a bit more.

the 2 watt wire wounds (22k and 47k) are supposed to be matched but are 8 and 10% different respectively. i have 2w ceramic metal oxide resistors that match to 2 decimals in both. any reason a wire wound is preferable here?

in the schamatic for the TO 300 in this link http://www.oestex.com/tubes/acro.html there is a hum balance if i interpret it right, on the 6.3v secondary in the way of a 100k pot between the 6.3 legs. the stancor has a CT and that is connected to a tab on a strip and not connected to anything else. effecively taped and not used. the wiper in the schematic says "TO CATHODES OF OUTPUT TUBES" . can you help me figure this out. it isn't like a hum balance of a similar nature i have seen on guitar amps, particularly post cbs fenders. not to be confused with the cbs failed attempt at bias balance as opposed to conventional fixed bias adjustment. would adding this pot be worth the effort in the A_W i am restoring?

please help me understand the negative feedback if i see this right, between the 16 ohm tap of the OT and the cathode of V-1A. i am used to seeing negative feedback come back in just before the PI in most guitar amps.

the electrolytic caps all test and look fine but as old as they are i feel i should replace them. usually 10-15 yrs is max in my book even if they look good. i usually beef up the first cap (s) in a guitar amp for better response and bass. there are 3 x 20mF and 2 x 10mF. the 10's are in the first 2 positions after the rectifier. i have lots of 20's and 22's in stock and also have a 40-20-20-20 jj can and could use a jj 20 to make up for the last one. the 8's and 10's i have are blue atoms and are big. i also am a believer in a standby switch and often put a reservoir on the rectifier of the standby switch to aid in charging the downstream caps. this has doubled tube life in my 18 watt builds where i used to use no standby switch at all. especially in hobby musicians amps where weekly use and a cold start with a solid state rectifier and discharged. caps. all of my gigging musicians exclaim about much longer tube life now. anyway what do you think about the cap array i describe above? or should i bite the bullet and order a 10-10 can. no way will i find one here and if i did it would be really old NOS.

i want to get this working well since it will be my first hi fi amp of quality components. i have a few heathkits and an old stromberg carlson, 2 graflex and a moviola that come to mind on the projects pile and a rauland 3075 i can't find hardly any info on waiting for me in tucson but this A_W is the first to really peak my interest and cater to a need for a really good clean amp for my synth guitar. and it really needs very little work.

thanks again and buenas noches.
 
Where are the 22k and 47k resistors in the circuit? If they are in the power supply, you can use the MOX with no problems. Doubt it would matter anywhere honestly but if its somewhere inside the feedback loop, its vaguely possible that the change in inductance might cause some odd stability stuff.

The center tap on the transformer would go to one of the cathodes to get some DC on it. This helps reduce hum over just grounding it and certainly over leaving it float. The cathodes just happen to be a handy source of clean DC voltage that really isn't doing much else. If you use the pot instead, (100 ohm BTW, not 100K) the center wiper becomes your "center tap" and you can tie that to the cathodes instead. If the pot isn't already there, I'd probably just use the center tap on the transformer and call it good.

You got the feedback scheme right. Something you may not notice though, V1B is actually your phase inverter so the feedback is coming in at the normal spot. V2A and V2B are just gain stages post-inverter to get enough drive to get full power from the output tubes.

Should be no problems running all 20uf caps. Just watch the first one off the rectifier to keep the rectifier from arcing over due to inrush loads. I don't really trust ancient caps either, so I'm with you on changing them out.
 
musicamex said:
hi, i am a tube nut but have mostly played around with guitar and bass amps. i just snagged an old chassis on ebay, mostly for the 2 old british coke bottle 6l6 output tubes. those broke in transit due to bad packing (a cannon ball and a lightbulb should never meet in transit). the seller refunded a small amt and i guess in my sadness for the lost tubes i had some relief.

not being a hi fi guy, i was really impressed albeit somewhat confused with the iron. the pt is super clean, huge almost new looking stancor pc8413 and the ot is an acrosound ultra linear to300. there are what appear to be 2 chokes, one mounted under the chassis and the other, a real honker (as big as some guitar pt's but with only on pair of black wires so i guessed a choke or inductor of sorts) mounted on top. in the ebay photo i thought the big one might be a driver like on old gibsons at first.

the tube compliment is 2 tungsol sn7gt, 1 radio sun coke bottle(?) 5u4, and the 2 broken 6l6 (british but not mullards). after a bit of snooping on the wonderful internet, i think this might be a "stancor williamson ultra linear circuit". being of the guitar amp world i could use a little help in identifying this amp. the chassis is aluminum with no holes to mount it to something else. it has one pot on the front and one on the top of the chassis like a bias pot. there is the speaker connections and an rca for the input on top. i think it is a factory build since the wires are all tied with bits of string throughout. it is very clean for it's suspected age and i'm pretty sure this thing is all original except for the power plug. so i plan to replace the filters and anything else suspect.

i play guitar synth and this might be the ticket for getting tube sound without the pitfalls of coloring/changing what i build using a headset like a guitar amp does.

i'd appreciate any ideas or help. i can send photos. i have had it for just a few hours. thanks amigos.
Click to expand...

The Acrosound 300 is a 30 Watt ultralinear.. 807/1625 should be a good impedance/power match for it. The 330 was designed for the 6L6G/GC if I remember my Acrosound data.

What you may actually have is a demonstrator chassis. The Williamson was developed in Britain originally, and Acrosound was the first US company licensed to produce the ultralinear transformers starting in 1947.


Noiw that I have caught up to the end here, the wirewound resistors are just more stable and less likely to shift in value than carbon comp resistors, the metal oxides you have would work fine in the spot you are wondering about. The wirewound resistors do have some inductance, but in that location, that really is not a factor. Yes, I will add to that chorus- change the caps. With something of that vintage, I would seriously consider adding some soft start circuitry just to protect the whole system. I am not trying to step on anyone here, but it is rare to see any acrosounds surfacing these days, and having sold some a decade ago (mismatched), I would not mind finding a pair that I could afford. So please pardon the figurative drool.
 
Last edited:
Trio -- The TO-330 was designed for push-pull parallel operation of cathode biased KT66 tubes, or push-pull operation of fixed biased 6550 tubes.

Music -- you would want to EITHER connect the slider of the 100 Ohm hum balance pot to the output tube cathodes, OR the center tap of the heater winding to the cathodes -- but not both. Use one scheme or the other. The hum balance pot is really only effective if the power amp is also used to power an unpowered preamp (common in those days), but will be of little effect with the power amplifier only.

Hifi amplifiers (i.e. reproduction amplifiers) uses much more feedback that guitar amps (i.e. production amplifiers) do, since they are not trying to create a sound, but accurately reproduce a sound that has already occurred. Therefore, the feedback in hifi amplifiers is often carried back over more stages to effectively increase the feedback level, and reduce distortion in the process. It also helps to flatten frequency response and extend the power bandwidth as well. In guitar amps, you intentionally want very little feedback -- usually only just enough to help protect the amplifier, and provide some level of speaker damping -- but that's about it. As a result, feedback in guitar amps -- what little there is of it -- is often only carried over two stages at best.

Dave
 
thank you very much for the tips and explanations. i must admit, reading your replies i realize what a caveman i am. we have a saying here--"el el valle de los ciegos el tuerto es rey". in the valley of the blind a one eyed man is king. are most hi fi amps so different from guitar amps that i might need allot more understanding to navigate/understand them? i was a chemist. metal sculptor, boat builder before a tubie came to the surface as a necessity of my musical interests here in non tech machete land. i had to repair my stuff as it otherwise came back much worse. that led to a very rudimentary knowledge and i modified and built several guitar amps from scratch. all have had pretty much tubes 101 first week topography. signal to grid, plate out to next grid etc (except for a cathode follower on some guitar amps). although this A_W chassis has a low parts count for how it is touted as being so great, it is confusing me more than it should.

i wish i could invite some of you over to my "cave" and get you to 'splain the subtleties and differences to me over several cool ones. i typed williamson schematic into google and it seems i have lots of homework. any suggestions to sources to understand the basics of tube hi fi would be appreciated.

bottom line is i am thrilled to have a chassis worthy of drooling over trio. and a pair isn't necessary for most of what i do. sure i can get allot of cool effects using a pair of stereo powered studio monitors. but that would be lost on the margarita hour(s) crowd here anyway. so please drool on trio, it makes me feel like i blindly bought right.
 
You must log in or register to reply here.
Back
Top Bottom