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 Post subject: ampex 620 suitcase amp & speaker
PostPosted: Jul Mon 30, 2018 1:01 am 
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Joined: Aug Sat 16, 2008 10:42 pm
Posts: 180
Location: tucson az usa 85745
Hi: Is there a whole cult surrounding these things? My friend has four of them but they have been modified by previous owners and now we are trying to decide best course of action. Near as I can determine, these had some kind of special high boost EQ in the feedback circuit to offset the speaker rolloff. The idea in some versions was to provide a flat output to an external speaker maybe via a quarter inch phone jack. I am advocating converting the feedback to a linear type using a similar ampex monoblock design and sticking a tweeter inside to provide the upper octaves. Owner leans toward restoring complete original EQ network at the speaker side and keeping original speaker driver with no tweeter. What does the rest of the world think about this or who has been down this road? Thank you. --Tom H

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Mon 30, 2018 1:38 pm 
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Location: Sunbury, Ohio 43074
My take on this, and other similar items, is the perceived "cult" following for these is driven by hype, mostly over on that certain auction site, and in Aduiophile circles where snake oil is the coin of the realm.

This was, indeed, a very excellent powered speaker, possibly the premier one of it's time for it's intended purpose. I used them, and other previous and subsequent Ampex models, for traveling multimedia road shows as PA amps. You could more or less take 4 or 6 of them and create a pseudo-surround feeling with just plain old stereo, and easily fill a large auditorium or hotel ballroom with crystal clear sound.

As to what you, or the current owner, want to do with it ... I'd say since some sort of modification is in order, it' not going to be "original" so turn it into whatever you want to turn it into. But realize that it will then be, forever, a modified Ampex Suitcase Powered Speaker, not an "all original one". Otherwise, restore it to all original.

If the intent is to USE it, that makes no difference. If the intent is to sell it at a profit, that would, as always, depend on the buyer :)

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Mon 30, 2018 4:01 pm 
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Location: tucson az usa 85745
This is most helpful and enlightening, thank you. The owner's goal is to set up a quintet of these in a 5.1 system. He is not a speculator or flipper. He is an audiophile who does crave and appreciate wideband sound but seems blind to the failure of these ampexes to provide that at least in their current configuration. I did not want to rain on his parade but feel more confident now in guiding him toward a true hi fi solution. Refitting them to original does not seem consistent with that.

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Mon 30, 2018 4:33 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
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Location: Sunbury, Ohio 43074
If the intent is to use them in any kind of critical listening environment, a few further thoughts.

Since they will be used a lot, I would replace every capacitor in them except perhaps micas, after testing even those. Then do a full test and make sure that it is at least performing as per original design specs. You may have to replace some resistors as well but I, personally, am not of the "shotgun" replacement mind. As far as resistors are concerned, I replace only those that need it after testing.

Note that, while an excellent speaker system for its time, it is not currently going to be able to reproduce 20Hz - 20KHz.....not even close. The original spec was 65Hz to 10KHz. So if that's the goal here, I'd find some other solution, or you will be facing some rather extensive modifications which are probably a, not worth it and b, unlikely to fully succeed regardless.

How about some nice Voice of the Theater systems :) Perhaps some AR3 speakers with a Mac amp??

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Mon 30, 2018 4:59 pm 
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Location: tucson az usa 85745
The owner is in love with the suitcase self contained aspect and will not entertain addition of big speaker cabinets. Actually the wife is the main opposition to anything like this. So to make these viable within that constraint, I am advocating the addition of a tweeter coaxially if possible. My plan for the amps is to remove the designed-in EQ from the back end and replace it with the same simple linear negative feedback found in later ampex monoblocks of basically same topology. I have a pair and they are quite linear way beyond 10K. I am assuming the stock speaker in this 620 is the biggest single limiting factor. Complicating all this are the multiple versions produced over time. There are at least three variations of the thing; sams print does not agree at all with the real thing we have on hand here. But another print from elsewhere on the web is much closer.

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Mon 30, 2018 6:08 pm 
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Location: pensacola fl
You have a big job. First you could get a coaxial speaker to get the tweeter in there but it doesn't stop there. The am has freq response issues at both ends of the spectrum as was stated earlier. To fix the low freq response you will need new output transformers designed to go down to 20hz and up to at least 20khz. These will take up more room and that chassis is small. Then the filter caps will have to be increased and maybe even the power transformer if the B+ drops at those low freqs. The coupling caps will also have to be changed as well as the negative feedback network.


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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Mon 30, 2018 8:44 pm 
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Location: tucson az usa 85745
Here we have a classic comparison between a PA amp and true hi fi one. It would seem unrealistic to re-engineer the thing to get down to 20hz; the speaker might be some sort of holy grail to the cult but it has zero prayer of making anything happen below 50 or 60 so I would not care about that. Owner would use a powered sub anyway. I have some ampex monoblocks that use 6973 outputs with a bigger triad OPT than this thing has. The component values surrounding the EF86 and 12au7 drivers are identical on both these amps. The 620 has 6v6 outputs in PP just like in the 6973 with cathode bias. On a wing and a prayer, I would throw the three negative feedback passive parts shown on the 6973 into the 6v6 and stick a tweeter on the output with a NP cap in front of it and see what we have. I bet I get another 7K on the top end and this would be a somewhat happy ending that would cost $20 for each amp. I know: you are thinking why is this guy asking questions on the forum if he already has made up his mind? Well I was hoping to find somebody who did this; blazing a new trail is fun but if it is known to be futile, that would be of some value. Thank you.

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Mon 30, 2018 9:52 pm 
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Joined: Apr Thu 14, 2016 8:25 pm
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Location: pensacola fl
If you are not too concerned about the low end you probably can bring up the high end some. The feedback loop will figure in on that as well as the transformer. You have the advantage that ampex over built a lot of their equipment. There just was not as much reason to go down to the low end for it's intended purpose. It may make 15khz flat or even beyond if it is not driven too hard in which case it would distort. Give it a try.


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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Mon 30, 2018 11:00 pm 
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Joined: Aug Sat 16, 2008 10:42 pm
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Location: tucson az usa 85745
Encouragement is appreciated! I shall try to convince owner of wisdom of my idea. Thank you.

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Tue 31, 2018 4:31 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
Posts: 2016
Location: Roanoke, VA
thomash85715 wrote:
It would seem unrealistic to re-engineer the thing...

As one whose professional experience with the Ampex 620 goes back nearly 50 years I agree fully.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Jul Tue 31, 2018 5:09 pm 
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Joined: Aug Sat 16, 2008 10:42 pm
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Location: tucson az usa 85745
Dale wondering if you feel it is wrong to put in the simple linear negative feedback bits used in the 6973 monoblock ampex amp? My quote you pulled from my previous post had to do with re-engineering the entire amp to gain deeper bass octaves. It was that specific task I was loathe to undertake. But I still hope the grafting of later linear negative feedback components from otherwise identical ampex monoblock power amp will be a good thing. Thank you for views on this.

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Aug Wed 01, 2018 1:38 pm 
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Location: Utica, NY 13502 (USA)
The 620 is an accessory amplifier/speaker originally intended to be used with an Ampex model 601 reel-to-reel recorder, a mainstay in radio station newsrooms in the '50's. On location, it could be used as a PA amp while recording or to listen during playback as the 601 had no speaker of its own. I wonder what the "cult" attraction of this simple system is?

Dave


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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Aug Wed 01, 2018 1:43 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
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Location: Sunbury, Ohio 43074
Quote:
I wonder what the "cult" attraction of this simple system is

It's old, and cute :) a rare combination. Also, it's got ..... wait for it ..... tubes :roll: And a handle <shrug>

who knows. I'm not sure there really IS a cult attraction .... in t his particular case, seems to be just one guy that loves them.

EDIT: Oh yea, it says "Ampex" on the nameplate :)

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Aug Wed 01, 2018 2:03 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
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Location: Roanoke, VA
thomash85715 wrote:
Dale wondering if you feel it is wrong to put in the simple linear negative feedback bits used in the 6973 monoblock ampex amp?

The use of, and type of, feedback depends upon basic topology and needs. As a rule I am loathe to modify circuits, which in general I feel should be done by someone with academic qualifications and experience at least comparable to the original designer(s). I cannot tell you how many times in my career I have had to remove an "improvement" made by an unqualified person that degraded some important aspect of performance (such as stability or reliability) which had not been sufficiently considered by the "improver."

The 620 does its intended job quite well, and, IMHO, should not be modified to try to make it do a job that it was never intended to do.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Aug Wed 01, 2018 2:27 pm 
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Location: Sunbury, Ohio 43074
I suppose one could replace both the speaker and the electronics completely. Call it a “ modification. Who would ever suspect?

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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Aug Thu 02, 2018 11:24 pm 
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My advice would be to add a small dome tweeter in front of the full range speaker and use a proper crossover.

Then do what you can to make it basically flat up to 16KHz if that is possible.

For the bass a powered sub would be necessary.


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 Post subject: Re: ampex 620 suitcase amp & speaker
PostPosted: Aug Fri 03, 2018 9:58 pm 
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These are actually excellent monitor systems for home/studio recording and make very pleasant mixes...I used to use a pair for some time, now I only have one original 620, although the filter and most paper caps have been/were replaced in both when used, otherwise left alone..
They have also been widely used and revered over the years in recording studios, for recording bass guitar and /or electric guitar when miking the cabinet.
I've not heard much about modern day hi fi (If you want to call anything digital that?) stereo use,
but I've often thought if I had sell all my other hi-fi setups someday, that could be a way to go on a budget with 2!


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