Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > General Off Topic Forums

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #1  
Old 02-12-2018, 02:57 PM
benman94's Avatar
benman94 benman94 is offline
Resident Lunatic
 
Join Date: Jun 2011
Location: Detroit, MI
Posts: 1,089
6AS7G Amplifier Design

Hello all,

Having been snowed in here in Detroit, I decided to sit down and draw up a new amplifier to build. I'm dissatisfied with my EL84 amps and want a change. In any case, I recently acquired beautiful a beautiful RCA LC-1A and an RCA 515S1 in a trade. I've been meaning to put together a kick ass mono system for early LPs, 45s, and of course, 78s.

I decided I'd try to build an amplifier using what would have been near state of the art circa 1948-1950, when my speakers would have been new.

This is what I played around with:

Starting at the front end, a Van Scoyoc "cross coupled" input stage and phase inverter. There are technically superior inverters out there, but this one was period correct, pretty damn popular in the '48 to '50 time frame, offered a reasonable degree of balance at a reasonable level of distortion, and made use of one of my favorite small signal tubes: the lowly 6SL7.
This was followed by a driver stage, admittedly lifted almost component for component from a wonderful high fidelity design published by RCA in the summer of 1948. Both sections of a 6SN7 are RC coupled to the grids of the output stage delivering significant driving voltage at negligible distortion.
The output stage was again lifted almost component for component from that same design from RCA design. A single 6AS7G is used in push pull with itself. A 6AS7G contains two triodes of 13 watts plate dissipation each. The 6AS7G's only major failings are that you MUST use cathode bias with it if you expect any sort of reliability, and with a mu of only 2, the 6AS7G requires an ungodly grid drive. Other than that, the output stage is capable of incredibly good damping factor, better than a push pull pair of 2A3s even, and lower distortion than a comparable 2A3 stage. You can reasonably expect about 10 watts of real RMS power at only 2% THD into a 5000 ohm load, with the third order distortion by far dominant.
Finally, the power supply is a rather conventional type using a 5V4G. Nothing special here aside from a rather large 20 Henry choke to help quash the 120 cycle hum.

The resulting schematic is shown below:



If any of you have any potential improvements or notice any major errors on the schematic, please, let me know.

If you accept the general conclusions reached by Harry Olson at RCA, 10 watts is more than enough power, including reserve power, for the efficient loud speakers of the day. Also with the RCAs, which were revealing and low distortion drivers for their time, and are still quality full range speakers by even today's standards, it was thought by Olson, and borne out in his psycho-acoustic experiments in the late 1940s, that most people would not be able to detect ~0.5% to ~1% THD from the amplifier, as any speaker is producing much more distortion by virtue of the mechanical limitations involved.

Compared to a Williamson, which was sweeping the globe around that time, and famously offering 0.1% THD with gobs of global negative feedback, this design offers 10 watts at 2% THD, 8 watts at 1% THD, and 5 watts at 0.5%. In addition, it affords good damping factor, and something no Williamson could ever deliver, unconditional stability into near any load. Despite the inferiority on paper, I strongly suspect a real working amplifier of this type would sound pretty decent in the real world.

I promptly simulated the design in LTSpice using Ayumi's extremely accurate tube models, and it compares well with expected behaviors as found via paper, pencil, and calculator.

I then remembered that LTSpice offers the ability to feed a circuit with a WAV file, and then export the voltage signal as a WAV file from any node in the circuit. LTSpice uses the WAV file as essentially a very complicated piecewise function, for generating a voltage at a voltage source. This is conceptually no different than what any DAC does, except that LTSpice should theoretically be able to do this to a much higher degree of precision because it is just generating a table of values that represent instantaneous voltages. This is effect offered me the ability to "hear" a reasonable facsimile of the amplifier's output before even warming up the soldering iron.

I fed the simulated amplifier a short 15 second sample of Clifford Brown performing "What's New", and "recorded" the output at the 8 ohm load resistor (the "stand in" for the voice coil) as a WAV file. The simulated amplifier, under this test, clipped a bit and reached a peak THD of ~2.6% to ~2.7%

I am not claiming that simulation is an acceptable substitute for actually building an amplifier. Nor do I claim that the final built version of this amp will sound exactly like the result of the simulation. I will assuredly deviate from some of the schematic values, in particular all the coupling caps need to be tweaked as of yet. In addition, the simulation has many theoretically perfect components, perfectly matched tubes, etc. The real result will have a bit higher distortion, etc. Yet, I still find it interesting that the non-linearity of the active elements in the simulated amplifier did show a realistic level of distortion, with characteristics you would expect in such a design. This has proven yet again to me that LTSpice is an incredibly useful tool in design.

Below is a link to the two WAV files, input, and output:
https://www.dropbox.com/s/w6iq4b7ro9...Files.zip?dl=0
Reply With Quote
  #2  
Old 02-12-2018, 04:26 PM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 3,810
How does V1b function?

jr
Reply With Quote
  #3  
Old 02-12-2018, 04:33 PM
Celt's Avatar
Celt Celt is offline
Peanut Head
 
Join Date: Nov 2002
Location: Paragould, Arkansas
Posts: 1,690
I likes 6V6GT's as outputs....they are wonderful run in Class-A too!
__________________

Let me live in the house beside the road and be a friend to man.
Reply With Quote
  #4  
Old 02-12-2018, 05:09 PM
benman94's Avatar
benman94 benman94 is offline
Resident Lunatic
 
Join Date: Jun 2011
Location: Detroit, MI
Posts: 1,089
Quote:
Originally Posted by jr_tech View Post
How does V1b function?

jr
Both halves of V1 function as cathode followers as V2 needs to be fed from an extremely low impedance source. It helps to understand the circuit if you actually ignore the presence of V1 and imagine an input transformer in its place.

Now, the Van Scoyoc circuit can be used in one of two ways, the single ended configuration as I have used or with a balanced push pull input. If you tie the grid of either V1A or V1B to ground, the circuit functions as a phase inverter with a tremendous amount of gain and relatively good AC balance, differing only by an approximate factor of u/(u+1) where u is the mu of V2 (70 in the case of the 6SL7 shown). If you use it with a push pull input, you would have two separate grid leaks to ground and apply a signal of some amplitude to V1A, and a signal of the same amplitude but opposite phase to V1B.

In practice, the grid of V1B is often not connected directly to ground, but is connected to a very small resistor (about 100 ohms) which is tied to ground. Then this resistor is used as the bottom leg of the voltage divider formed by the feedback resistor from the secondary winding of the OPT in designs that use global feedback.

It is really a shame that this circuit wasn't used by D.T.N. Williamson in his famous amplifier. Whereas a Williamson has three RC time constants that must be managed to prevent instability with the application of feedback, replacing the first 6SN7 in a Williamson with this circuit would give only two RC time constants to manage. Since the Williamson is on a knife's edge in terms of stability to begin with, this would have likely prevented a lot of damaged loud speakers...

Here's a link to Van Scoyoc's original article from 1948:
https://pearl-hifi.com/06_Lit_Archiv...e_Inv_Comp.pdf

There are some downsides to this circuit. You essentially "waste" a 6SN7 that provides no increase in gain and serves merely to feed the signal to the 6SL7, and it is very, very sensitive to the amplitude of the input so it MUST be used as the very first stage in an amplifier. This isn't necessarily a bad thing however, in that it forces the designer to make the rest of the amplifier push-pull in nature, and that in turn causes the natural consequence of better power supply ripple and noise rejection.
Reply With Quote
  #5  
Old 02-12-2018, 05:12 PM
benman94's Avatar
benman94 benman94 is offline
Resident Lunatic
 
Join Date: Jun 2011
Location: Detroit, MI
Posts: 1,089
Quote:
Originally Posted by Celt View Post
I likes 6V6GT's as outputs....they are wonderful run in Class-A too!
The 6V6 is a rather nice tube. Like most beam power tubes, it is fairly benign in terms of its distortion characteristics. Not as nice as a triode of course, but still better than a true pentode like the 6F6.

Last edited by benman94; 02-12-2018 at 06:41 PM.
Reply With Quote
Audiokarma
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 01:27 AM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
ęCopyright 2012 VideoKarma.org, All rights reserved.