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 Post subject: 50s RCA 6AS7 project
PostPosted: 11 Mar 2017, 16:47 
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Joined: 10 Mar 2017, 12:21
Posts: 2
Hi all!

First post here; thanks for the aDid.

I would like to make the hi-fi amp presented by RCA on RC-16 tube manual - that one with 6AS7, three 6SN7 and 6SC7. So I need some advice:

1- anyone here know the results of this amp? I know it's not very powerful, but it's for an average -sized room, to play LPs.

2- using an ordinary cartridge can I take off the 6SC7?

3- I have already here some 12AU7 and 12SN7. Can I use them with no trouble instead 6SN7?

I have some experience with constructing tube amplifiers (made a few simple, with 6V6 and EL84) but far to be an expert.

Thanks!


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 Post subject: Re: 50s RCA 6AS7 project
PostPosted: 01 May 2017, 14:25 
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Joined: 29 Apr 2017, 15:14
Posts: 41
I have noted this amplifier schematic, but what you are lookin at, is a phono
pre amp, to use this to full any room space, will require you to add a power
output amplifier to it, yes a 6v6 will serve that purpose, quite well. you must
remember, however, that if you use a 12sn7 tube in that circuit, you will require
a 12 volts for the filament (heater) go ahead build it. consult us, if you have any
questions.


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 Post subject: Re: 50s RCA 6AS7 project
PostPosted: 01 May 2017, 21:43 
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Joined: 09 Oct 2012, 19:43
Posts: 266
Location: Vancouver Canada
Hello Martinsantos. This is a 10 watt amp which will have no problem fulling an average size room with sound. It will depend greatly on the efficiency of your speakers and the size of the average sized room and just what you call a good listening level. 10 watts of power is plenty of power for stereo system. 10 watts of tube power is capable of rivaling most 50 watt SS systems. A few things to keep in mind.

1- 3 db is said to be one perceptible increase in volume. 3 db is a doubling in power. eg. 5 watts + 3 db = 10 watts. 10 watts + 3 db = 20 watts, 20 watts + 3 db = 40 watts. So,,, 1,2,3,4,5,6 on the volume control is really 5,10,20,40, 80, 160 watts of power.

2- Selecting a set of speakers with high sensitivity should be a higher priority than looking for the big watts amp. You can see then a spkr with an efficiency of 98db/w and an amp of say 10 watts // compared to spkr of 83db/w would need an amp of 320 watts to achieve about the same volume level. This is approx but you get the pic.

3- The amp you suggest is a full amp and not just a pre-amp. It has the pre-amp and power amp in one. You will need the 6sc7 though for using a phono cartridge. Here is why. There are 2 basic catagories of signal level for audio at the input stage. Low level signal, (phono carts, microphones) [which need to be amplified to LINE LEVEL] and finally line level signal, (tape, radio/receivers, CD, mp3, comp, etc, etc).

4- each / every amp is made up of various stages. On the outside, the smaller signal goes in and comes out big enough to drive spkrs. On the inside however:
4a- A moving coil cartridge, (MC) outputting in the micro volts of sig will need an active amp or a transformer to boost it to the same level as the regular phono cart, (Moving Magnet MM).
4b- A moving magnet cartridge, (MM) will put out in the millivolts so it needs one stage of amp to bring it up to the standard line level of couple hundred millivolts. 283mV. Which is equivalent to aux, radio, comp, mp3, tape, etc.
4c- from here, any passive tone control will knock the signal down again so another stage of amplification is needed for bass and one for treble to get back up to the original line level sig. This amp has both.
Tone controls take on many faces and are passive or active. Active controls look (on paper) better but are not as pop with people as they introduce noise and distortion where as the passive controls not so much but need to be re-amplified as the components will bring the sig down again.

5- Now just the output stage is needed. These are ether single ended or push-pull. Single ended use one signal to be driven, and, push-pull using one tube to drive each side of the output trans will need a phase-splitter to provide two signals, one driving one output tube, and a second sig, (180 deg out of phase from the original) driving the the second tube. Each of these tubes then drive each end of the output trans, simultaneously.

6- A little FYI, the term (tube), is used loosely in the industry. On the schematic each TUBE is actually a VALVE not a tube. The tube is the casing around the valve or valves. Valves that are designed for smaller signal and smaller current levels usually come 2 valves per tube. The larger output tubes again, usually come one valve per tube. I could be wrong but i think this is due to the larger amounts of current handled by the output tubes and therefor larger amounts of heat generated. Also having two valves in one tube with very high voltages would get a bit tricky and well quit spectacular shortly after turn on for a few seconds at least. Until death due us part as they say.

7- Have a look at the projects listed on the main page here and you will see many examples of single ended and of push-pull amps. You'll catch on. I will also say the amps presented here are designed by people who are well versed in the subject and there are many threads on most of these amps which comes in very handy for advice. The RCA manual has valid sch's vintage in nature but i would not say better than anyone you would see here. Designing a vacuum tube amp takes the same skills now as it did then. Perhaps more now than then, as vacuum amps are not as common and the designers are truly devoted to their passion and deserve full respect for what they do and provide for us not so smarts (of which i am one).

8- finally when looking at tubes numbers the first number is the heater voltage. 6sn7 is 6.3v and 12sn7 is 12v heater. You could use the 12sn7's but your output tube is a 6as7 which has a 6 volt heater so you would need an additional 6 volt regulator to get the 12 volt heater voltage for the 12sn7's then down to 6 volts for the 6as7. Or two different power supplies. Sometimes when the tube count works, one can put two 6 volt tube heaters in series so they can be connected to the 12 volt source. But an odd number of tubes makes it complicated. Easiest to stick to one voltage of tube heater.

I don't know anyone who may have built this amp, although i am sure some have, but this sight has many willing to help, and, has many many amps to choose from which have all been built, tested, and have been running for years proving they stand the test of time.


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 Post subject: Re: 50s RCA 6AS7 project
PostPosted: 02 May 2017, 07:09 
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Joined: 06 Jun 2008, 18:23
Posts: 4835
Location: Australia
Schematic?

_________________
Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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 Post subject: Re: 50s RCA 6AS7 project
PostPosted: 02 May 2017, 13:18 
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Joined: 09 Jul 2012, 14:49
Posts: 39
Have a look here in section 2 1945-54, There are a number of 6AS7-G circuits here with analysis and criticism.
Some are much simpler than the RCA one you mention.

http://lilienthalengineering.com/100-am ... -chapter-1

John


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 Post subject: Re: 50s RCA 6AS7 project
PostPosted: 02 May 2017, 15:17 
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Joined: 06 Apr 2009, 10:08
Posts: 1394
Location: US Pacific Northwest
mwhouston wrote:
Schematic?
Attachment:
6AS7-10W-1950.jpg
Looks unduly complicated. Typical 1950s design.


This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.


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Matt
It's all about the Glass!
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 Post subject: Re: 50s RCA 6AS7 project
PostPosted: 02 May 2017, 18:17 
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Joined: 04 Jun 2008, 20:59
Posts: 3552
Location: Arizona, USA
Hi, I'm with Matt, there are simpler designs that will do the job better. I believe I would start with an amplifier that would accept line level signals and use an external phono preamp. The reason being that many builders trying to get both in the same chassis end up with rather poor signal to noise levels on the phono section. Since decent phono preamps can be had for under $50 this often is a better solution. YMMV

Good listening
Bruce

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 Post subject: Re: 50s RCA 6AS7 project
PostPosted: 03 May 2017, 11:49 
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Joined: 29 Apr 2017, 15:14
Posts: 41
Every process of learning moves from the simple, to the complex.
if you want to build this simple amplifier, go right ahead, and enjoy the
experience of getting to the complex, remembering, that we learn by fault.
don't get confused by high technical dogmas. and lose that wonderful
experience. we will help you, all the way


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 Post subject: Re: 50s RCA 6AS7 project
PostPosted: 03 May 2017, 17:14 
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Joined: 29 Apr 2017, 15:14
Posts: 41
Woooo, those schematics were just fantastic, what an experience, just went back in time for a hellava long time, had to shake my head to come out.. where did you get all that info? is that a book I can buy? woooo remarkable just remarkable!!!


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 Post subject: Re: 50s RCA 6AS7 project
PostPosted: 14 Jun 2017, 14:51 
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Joined: 10 Mar 2017, 12:21
Posts: 2
Thanks a lot for all feedback, guys!

Well, after looking around for other projects I believe I wanted too early to go to a rather complex circuit. Let's start before with something easier. Got a few troubles starting with a tone control and preamp from RCA (working just perfect! :D). So, anyway, I'm using already the 12SN7s (plus 2x 12AX7 for magnetic cartridge).

Thanks!

Martin


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