Ampex 620

Discussion in 'Ampage Area' started by Racing, Oct 27, 2016.

  1. Racing

    Racing Synchromatic

    521
    Aug 30, 2014
    Sweden
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    Right. So here we go again,dept of odd and peculiar. Ran into this one on one of our locals auction sites. An Ampex 620 used by Swedish national Radio at the time.
    In short...a Williamson setup,from an electrial POW,that resides in a...suitcase.

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    A US made proposition at the time,from the early -60´s. Uses a pair of 6V6`s in push/pull,obviously,run at approx 350VDC stock. I presume that 350 number to jump a bit seing modern day n era voltages at the wall.

    http://ampex.com/images/Legacy/Audio/612man/ampex612.jpg

    There´s the schematic for it. As you can see the tonecontrol for it is...different,to say the least.

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    Yep. A suitcase. To the letter.

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    Strikes me as in a real nice nick to boot.

    This one...simply makes the cool gauge go into orbit...
     
    G DynaTron likes this.
  2. Racing

    Racing Synchromatic

    521
    Aug 30, 2014
    Sweden
    An Ampex showed up alright...

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    This then how you carry the thing. As a suitcase.

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    All said and done it was kind of smaller then i expected it to be. Not tiny,by any measure,but smaller. Mind you the stock speaker is one of the old man JB Lansings own...of 8".
    Weight of it tho..is on the hefty side seing what it is. Haven´t put in on a scale yet but a guesstemate would be around the 12-13kg mark.

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    Sticker residue. Came right off with some washing acetone.

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    These XLR jacks ain´t stock. Most likely a way to use instruments or whatever,in contrast to the RCA plug on the other side of the amp.

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    Yep. An inventory number for Swedish Radio. Our national broadcasting company.
    That jack there in turn..is the mains one and...

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    F-ing idiots. That jack isn´t stock by an means and an adapter plate had been cut rather crudely to make the thing fit. In turn this jack is bolted to a bracket,including the RCA input, that is bolted to the chassis.
    What they´ve done is basically strip a piece of wire and in turn wrap that around one of the M3 screws... *shakes head*. Yeah...right.

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    So i fabbed another adapter in a hurry...

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    ..which made that there fit sans any real surgery. In turn,safety ground now hit the CHASSIS via a solderprong that´s bolted in with an M3 screw...and is soldered at both ends.

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    For the innards then. What you see here is in essence the components used for the "equalizer" when you opt to use an external speaker.
    As you can see the faceplate is out of cast aluminium.

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    This rats nest in turn...is the actual amplifier.

    So? What gives? Well,the tubes are...
    1 EF-86 / preamp
    1 ECC-82 / phaseinverter
    2 6V6 / power
    1 6AX5GT / rectifier
     
  3. Racing

    Racing Synchromatic

    521
    Aug 30, 2014
    Sweden
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    That there then is the business end of the onboard multisection electrolyte. As it turns out tho....

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    ...it wasn´t doing all that well. Ironic...seller told that the amp works just fine as is. Guess not is my reply to that...

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    Same thing went for this one which is the e-lyte across the powertubes cathode resistor. Mark the proximity between the cap and the resistor. That resistor gets REAL hot as you start to load the amp rendering that the cap in turn will cook sky high in notime flat. Ie; there should have been left ample room between the two when the thing was put together.

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    Atop then is the stocker for the cathode resistor of 25µF/25V and the lower one is a new one of 22µF/63V capacity.

    My aim here was clear,to get the thing to work...

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    As this thing was intended for the US market the powertransformer had been replaced,obviously. Seems to be an older job tho..

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    As stated,sticker residue could be made to vanish with some washing acetone alright.

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    The entire amp/chassis is rubber suspended in every way imaginable. For the better if you ask me.

    That said i took to hook the thing to the wall and flicking the switch and.....life. Amp did NOT conform to "the golden rule" so i handled that right off the bat while replacing e-lytes. That said the thing is REAL quiet in idle. Didn´t sound all that hot tho,which was to be expected seing that it was most likely put together with 775mVAC in mind-Ie;line level.
    It was manufactured as an "add-on" for Ampex tapedecks at the time after all,leaving us with the mere fact that the signal out of a guitar comes up a little short-to say the least.
    None the less.
    It was in a word boring.

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    This is a setup i´ve used before in cramped chassis. Putting a "capacitor bank" together like this makes for one rather compact setup seing the capacitances involved.

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    Old teq´s were used to wire the thing together. In turn the entire "bank" came to fit real snug between the screws for the PT. Yeeeeees....enough of the cathode bias setup already. That there is a small "bias board" i put together for it...

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    Uhu. Cleaning the thing out from that old "air castle" takes a wee bit of time TBH. All well that ends well though...
    As you can see i´ve started to tidy the wiring up,considerably. In stock form it was all over the friggin place.
    Gonna keep the HT and heater wires and what not close to the powertube and rectifier socket...

    Zeners and 100 Ohm resistors already installed for g2...

    So. ATM i´m debating how to handle this. Seing the age of the thing i´m to keep it blues/bluesrock. I don´t wanna mess with that old beautiful faceplate anymore then needed so..
    What i´m thinking here is a two stager. However,i want to keep the EF-86...seing it´s a part of the amps heritage,so what i think i´ll do is that i´ll install yet another noval socket and then use that for PI. In turn one socket for that EF-86 aaaaaaand...let that head into ONE of the two triodes of an ECC-83. Ie;still at twostager but with ample capacity as far as gain/amplification factor.
     
    G DynaTron and Random1643 like this.
  4. Tony65x55

    Tony65x55 Gretschified

    Age:
    62
    Sep 23, 2011
    The 'Shwa, Ontario, Canada
    It really does smack the cool factor into the sky!
     
  5. wabash slim

    wabash slim Friend of Fred

    Age:
    69
    Feb 10, 2010
    lafayette in
    I've got one myself. The AC input on yours has been changed, as well as the XLR inputs. The originals have a two prong AC input that is impossible to find, so, an IEC connector will fit the bill nicely. RCA is the standard input. We had this and the matching Ampex 600 mono tape recorder at my theater. They became dumpster fodder. I got the amp, a buddy grabbed the reel to reel recorder. I picked up a lot of speakers heading toward the dumpster---more than I'll ever need. It just seemed wrong to pitch 15"Jensens and Altecs out in the trash.

    So, you noticed that the tone control is "different"? It's a hi-cut, lo-cut kind of thing.
     
  6. loudnlousy

    loudnlousy Friend of Fred

    Age:
    53
    Oct 18, 2015
    Hildesheim, Germany
    Looks fascination. How does it sound?
     
  7. fender62custom

    fender62custom Gretschified

    Age:
    53
    Mar 3, 2012
    Helidon, Australia
    WOW!!!!!!!!!!!!!!!!!!!! super cool Amp!!!!!!!!!!!!!!!!!!
     
  8. Racing

    Racing Synchromatic

    521
    Aug 30, 2014
    Sweden
    What it sounds like stock is...boring.
    As in falling asleep boring. That said,thanx lads!!

    Anyways. On to the build shall we?

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    Right. So while at it i came to rubber suspend both of the first sockets. The EF...is a given. The single used triode of stage two..not so much,but rather safe then sorry i guess.

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    God almighty i hate putting heater wiring down. There´s NOTHING as unsexy when putting a new build together. None the less...centertap is out so humdinger it is,installed to the EF socket no less. Turned the sockets around to get the heater wiring "up n in the corner" as best as i could.

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    Yep. By now all in all three chimneys. For the better. I simply took to reusing the hole for the stock multisection e-lyte and done deal. Notice the solderprongs for the sockets that are suspended. Yessssssss...shielding might come in important. The physical aluminium shield is stock btw.

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    Yeah. This is a used unit alright. That there is a mic transformer. I guess this amp was used along those likes by national radio or whatever..no matter,out it comes.
    POS and weighs a friggin ton too.

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    Bet you guys´s never seen a JBL speaker like this? About as die hard as they come i guess. Since the wiring has been replaced by some more heavy duty one.

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    Now... The stocker is out. Them two XLR/Canon jacks sure ain´t stock...hence the mic transformer...and they´d been like "gorilla´d" in there. So? What to do?
    I certainly need more then two pots up front to make this thing come together so...i opted to fab that there small piece of aluminium sheet to house all in all two pots.

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    Uhu. Made that happen with a set of old used up Marshall knobs,and the whole deal is kind of recessed. For better for worse. That there,in short terms,is the "master" and PPIMV control.
    Reason for that? Simple,i REALLY don´t wanna drill into that beautiful old faceplate anymore then needed.

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    "Compact living",if ever. Brings that i need to fab a shield for them controls at the FP no doubt.

    Right. So it up and running,from an electrical point of view,again. B+ hammers down at 400+VDC...all good. Brings that most likely we´ll see power out of this one along the lines of a regular DLR at LEAST.
    The zeners vs g2 and so forth will do the rest.

    Now for some signal wiring...
     
  9. Racing

    Racing Synchromatic

    521
    Aug 30, 2014
    Sweden
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    Yep. Up and running. Not all sans merit i´d say. Sounds.."raw" for the lack of better words.

    As you can see i moved things around a bit as far as the faceplate. The mains fuse i moved inside the chassis.

    That said i will NOT by any means sign what´s been touted as far as the stock speaker! What´s been claimed is just total BS IMO. To the point where it needs to be replaced with a more contemporary offering i´d say. A newer way higher dB Eminence for instance...

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    Right. I felt that i all in all needed 5 dials to control this SOB. Something that worked out perfect seing the reuse of the holes left by the XLRs. For better for worse i guess.
    Of course i could have opted to use dual gang pots but then again...seing how this whole thing turned out,why.? Nah. This worked out rather well and i STILL haven´t drilled into that beautiful old faceplate.

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    Yeah. Underpinnings turned out rather cramped all said and done. Works and works very very well. Got ONE issue and that is a high freq oscillation as i dime BOTH the gaindial for the EF tube AND the treble one. Guess this comes down to lead dress...we´ll see. As stated that chassis isn´t exactly overly large.

    All said and done what we´ve got us here is basically a sort of heavily driven DLR of sorts. The EF tube makes sure of that.
    See..when we use a regular ECC-83/12AX7 triode the actual triode might have a maximum amplification factor of 100 however...in practice we rarely set them up to handle more then 50-55 times.
    Enter the EF..which has the same max amp factor set to 200...rendering that when we set it up within practical realms we normally end up with 100-120 times amp factor. Makes for all the difference as you can imagine.
    What´s more the EF is a pentode (in contrast to a triode) and that brings a few other things. One is wider bandwidth. Ie;we see a "different" amplification signature out of an EF-86 then do we out of an ECC-83.
    In turn the EF carries a different so called gm. Transconductance,and this translates into that the entry tube,in this case,is more responsive to signal then your typical triode.
    What´s more...the whole package changes our "needs" downstream too.

    Alright? Yeah,it does.
    Them 6V6´s wants to see like maybe 15VAC to reach into compression phase. That is what happens after what´s known as full saturation,for the powertubes AND the output transformer in liason.
    That tells us that anything beyond that number is basically waste,if we´re not into powertube distortion-which is NOT a given by any measure as powertube distortion isn´t anything we really can control.

    So. We´ve got this HUGE ass amplification going on at the entry tube. This is then presented to a regular ECC-83 triode,and that amplitude will shove that ECC-83 triode into distortion in a hurry,no MATTER all them "losses" downstream of the EF (tonecontrol,volume control and so on).
    What that tells is that we really don´t NEED an ECC-83 for phaseinverter as we´ve got ample signal to hand the powertubes anyway.
    Ok?
    Yeah.
    So what i did was that i kept the stock "bugle boy" ECC-82/12AU7. Where an 83 carries an amp factor of 100...an 82 does so to the tune of approx 17 times. IOW way less amplification.
    Reeeeeeight?
    So what´s my point?
    Well. Altho it´s still setup with 83 type component values,this might be changed as it turned out to my liking,it will hand us ALL the amplification factor we could ever ask BUT it will also for instance minimize hiss and what not.
    IOW a tradeoff with a twist. A twist that in this case works FOR us.
     
  10. Racing

    Racing Synchromatic

    521
    Aug 30, 2014
    Sweden
    I tossed yet a few tweaks at it.
    Including the replacement of the "separation resistor" of 470k of the James with a ditto trimpot,and as it turns out i came to dial that way lower,in Ohms,then them 470k´s. I´ll tell you that much.
    Most likely this is due to the EF-86. As stated they bring a MUCH greater bandwidth and...i normally don´t install a James to an EF-86 plate so.. I´d say it figures.
    More then that i also installed a small ceramic capacitor across the plate resistor of it. All to tame top end freq,and i´d say the two tweaks did the trick.

    B+ comes in at just a tad over 400VDC in idle,and idle then being approx 22mA per powertube (it´s adjustable now,remember). ATM there´s a pair of new JJ´s in there buuuuut...uncertain as to if they´ll stick. The stockers are still around after all.

    So.
    Brought it home with me to..ya know,noodle a bit..just around the house. No doubt the stock speaker´s GOT to go. I don´t care how haussed it may be,better putting it up for sale on e-bay then and it might very well be that someone´s got a good use for it in a stocker.
    As a guitarspeaker it´s friggin worthless at least. IOW,no point in killing it due to that.

    Brings that i´ve looked around for another 8" and to tell the truth the offerings are...not exactly a dime a dozen. The Celestions out there,aimed for guitar,are rated at 15 watts...that just won´t do. WGS offerings at 20 watts in turn...still no go...
    PA speakers,but they very very rarely hand us what we need for guitar use. What´s more their dB rating is usually down a LOT as they´ve got to cope with often rather massive power ratings.
    I guess this could be handled by explicitly TELLING the new owner that there´s an underrated speaker in there...and nor the Celestion or WGS will make you rob a bank exactly...
    I GUESS a 10" could be gorilla´d in there..need to look into that. If so it´s on the fabled... Width is there alright...height is not.

    All in all tho i´m happy. Turned into a rather nice sounding single channel jobbie that´ll stand its own,when let to speak out of a regular guitar cab. Ie;might be a good idea to scrap the entire "combo" idea and just hand it over with a stand-alone 112 cab?
    We´ll see i guess...
     
    MTurner likes this.