viking line amp questions

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hitchhiker

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I have these line amps from record/playback channels by Viking ( maybe they were an RCA division?)
Before dismantling I auditioned the mic pre with the headphone output that is taken from an additional winding on the OT. I liked what I heard.

I was thinking I could use this line amp for the output of the Pultec EQP1A but I have a few questions above my pay grade.
Please see my reversed schematic I've attached.

#1 How much gain is this amp providing?
#2 What is the input impedance as drawn and should I customize the amp to better suit the Pultec?
#3 Am I right that this is a gapped OT?
#4 Most likely ratio of the OT is?

Thanks in advance for all your answers!

 

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PRR

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Viking was its own boss, later absorbed by Telex.

Some bits of that drawing look dubious. And even if accurate, gain and output must be largely guesswork. If it is in your hand, measure it.

The most likely use would be Language Lab and other institutional installations involving tape machines. Viking decks were good but not studio-class. It is what it is. Try it.
 

hitchhiker

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thanks PRR,  I did re trace the card and I couldn't find a mistake.  What part of the schematic do you doubt?

I'll check it out as a line amp with a 1:1 input transformer.
 

ruffrecords

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If I read your schematic correctly, and if there is enough open loop gain, the gain of the two transistors is about 3 (150K divided by 47K). However, the overall gain will be further modified by the turns ratio of the transformer which we do not know.

Input impedance will be about 47K

The transformer should definitely be gapped since it has dc flowing in it. The dc resistance of the transformer primary will be small so the collector of the 2N2053 will be almost at 25V. So its base must be 25 x (10/110) volts = 2.27, so it emitter will be about 0.65 volts less or 1.6V. As the emitter is connected to 0V via a 47R resistor, the current through the 2N2053 must be 1.6/47 amps = 34.5mA which seems about right for this kind of stage. The transistor will dissipate nearly 900mW so I hope it is on a heat sink.

First stage seems to be an emitter follower biased from output. Emitter looks to be about 18 volts so stage current though 10K will be about 1.8mA. Second stage has a gain of around 50 (100K/10K) but as mentioned before, closed loop gain is only about 3.

I would guess it is a line driver stage with about 10dB of gain with a 1:1 output transformer.

Cheers

Ian
 

hitchhiker

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Thanks for that Ian!

  If this amp has a decent sound I was then thinking of using it with the Gyraf Pultec.  I'm thinking I could use an interstage 1:2 step up transformer into the 47K input impedance of the Viking line amp to add to it's 10 dB make up gain.  Does the Pultec filter output need to see close to 10K to work right or is that a minimum?

http://www.gyraf.dk/gy_pd/pultec/pultech.gif
 

Tubetec

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I remember the language lab set ups from school alright , each desk had its own headset/mic and associated enclosure  , everyone was linked back to the teacher who could eaves drop any of the mics she wanted  via a switchboard , or start howling and roaring over her own mic at pupils who didnt participate , it was like learning 'French' Nazi style. 

Maybe what you have are simple bullet proof head phone amps ,that run off a single supply  rail.


 

ruffrecords

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hitchhiker said:
Thanks for that Ian!

  If this amp has a decent sound I was then thinking of using it with the Gyraf Pultec.  I'm thinking I could use an interstage 1:2 step up transformer into the 47K input impedance of the Viking line amp to add to it's 10 dB make up gain.  Does the Pultec filter output need to see close to 10K to work right or is that a minimum?

http://www.gyraf.dk/gy_pd/pultec/pultech.gif

Although Gyraf's schematic shows an interstage transformer, the orginal schematic shows a Tirad HS-29 which is a 20K:80K transformers as shown here:

https://img1.wsimg.com/blobby/go/0e07cc59-2f5a-481f-967d-12c7363b86fa/downloads/1ctuphrpq_834605.png?ver=1555576524756

The thing is the secondary of the transformer is very lightly loaded by the tubes. As the transformer also has a high inductance, its load on the Pultec EQ is also relatively light (probably many times 10K). Notice how in Gyraf's SRPP version, he only loads the EQ with 470K at the input to the tube stage. another very light load.

Bottom line is am an not sure how loading the EQ with the 47K of you amp will affect the EQ performance. I suspect it will particularly affect the performance of the LF controls.

Cheers

Ian
 

hitchhiker

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OK!  I guess we'll find out. I've seen at least one member here post a schematic where he used a Neve BA-283 after a 50K pot, that's why I thought this Viking amp might work. Have to admit I don't remember the follow up comments of his on how well it worked. 
 

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