Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Need ideas for balanced input vacuum tube audio driver




 
The AM Forum
October 14, 2019, 10:12:26 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Need ideas for balanced input vacuum tube audio driver  (Read 441 times)
0 Members and 1 Guest are viewing this topic.
W8ACR
Contributing
Member
*
Online Online

Posts: 643


Penta 254W


« on: October 07, 2019, 12:54:16 AM »

Hello all,

As you all know, virtually every speech amp circuit found in old radio handbooks has unbalanced input to the grid of the first tube in the circuit. I would like to find a balanced input circuit speech amp to drive a class B modulator using 838's. The operating conditions fo the 838's will be 1250VDC plate voltage, zero bias, LS-66 modulation transformer and LS-102 mod reactor. Final amplifier will be about 400W input. I have an LS-10X input transformer, and an LS-47X driver transformer. I need at least 7.5 watts of audio power to drive the 838 grids. I will be running outboard processed audio into the balanced input.

I do have the schematic of a Gates BC-1G which uses 807's to drive 833 modulators, but I don't need that much power. I have a nice 300V regulated power supply that I'd like to use for the speech amp. That would be good for 6A3's or 6B4's, and those tubes should provide adequate power. I am currently using PP 6A3's driving 811A's in my other Homebrew rig, but that one has a standard unbalanced input.

Bottom line: can anybody direct me to a balanced input speech amp circuit using 6A3's or 6B4's (or something similar) as output (driver) tubes?


Thanks,
Ron W8ACR
Logged

The AM voice of Knox, North Dakota
KK4YY
Member

Offline Offline

Posts: 391


Your best isn't as good as you can be.


« Reply #1 on: October 07, 2019, 04:08:07 PM »

The first amplifier on this page, might do. If not, there are 99 more to look though.  Wink
http://lilienthalengineering.com/100-amplifiers-chapter-1/100-amplifiers-part-2-1945-54
Logged

Fate does not protect its worshipers any more than its deniers.
K8DI
Contributing
Member
*
Offline Offline

Posts: 56


« Reply #2 on: October 07, 2019, 05:29:24 PM »

I am currently using PP 6A3's driving 811A's in my other Homebrew rig, but that one has a standard unbalanced input.

Bottom line: can anybody direct me to a balanced input speech amp circuit using 6A3's or 6B4's (or something similar) as output (driver) tubes?

Hi Ron --

this will sound a little strange as methods go, but if you take that circuit you already have and love, mirror the unbalanced stages along the ground line, bypass/remove the phase splitter stage, and stick a transformer like the UTC A-11 on the input, you'll have your fully balanced/differential speech amp. The A-11 is low impedance to center-tapped secondary; your processing will need to be able to drive an old-school 600 ohm line. There are a fair number of similar transformers out there that would work as well.

Ed

Logged

Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
W8ACR
Contributing
Member
*
Online Online

Posts: 643


Penta 254W


« Reply #3 on: October 08, 2019, 10:19:12 PM »

Thank you both for the replies to my post. Thanks for the link to lilienthal engineering, that is a nice website.

I have added the schematic for my current speech amp below. I'm not sure where I found this circuit, maybe an old Radio Handbook. It does seem to work very well. I have added a few "control" components.

Thanks again,
Ron W8ACR


* img006 copy.jpg (488.63 KB, 2550x3300 - viewed 57 times.)
Logged

The AM voice of Knox, North Dakota
WBear2GCR
Member

Offline Offline

Posts: 3723


Brrrr- it's cold in the shack! Fire up the BIG RIG


WWW
« Reply #4 on: October 09, 2019, 11:41:35 AM »

A pair of 6A3/2A3/6B4 will be sweating hard trying to do 7.5 watts, imo.
Best to double check specs to be sure.

Otoh, driven by a transformer on their grids, you can run A2 and push more on
peaks.

As far as going "balanced", the easiest method is to simply transformer couple each
stage! You already have the LS-10X for the input (overkill!), so you'd need something
between the input tube and the 6A3s. You could make a bunch of gain using a
cascode at the input and maybe only need a transformer to drive the 6A3 grids...

Of course, you'd probably want tubes on each half (positive and negative going leg) of
each transformer stage.

There's two different ways to run the driver to the 833s, one is to use a follower (unity gain)
like Gates does or to use a tube that makes gain. The difference comes in both the
control the driver has over the grids of the 833s and gain needed. The unity gain driver
stage has the advantage (usually) of "caring" less about the transition from drawing
no current, to drawing a LOT of current when the grid of the 833 draws current.

A bit depends on the way the modulator tube is biased... is it Class 1 or Class 2? Some
tubes are "zero bias" so the grids are going to draw current from the git-go. Tubes that
are negative biased will draw nil current until one hits the "zero" and goes positive. So
that looks like a big impedance change. The driver goes from loafing along to grunt. The
cathode follower arrangement handles that well. But you need all of your voltage gain in advance
in the case of the follower 

There are two main methods of doing balanced circuitry. One is transformer coupled,
and then two signal paths in parallel... or using differential pairs (long tail pairs), or a combination
of these.

Oh, it is possible to make all of your (voltage) "gain" in the transformers - in that case the tubes then provide
the current gain to drive the next stage...

You can go balanced without any transformers using the diff pair method.
However making the diff pair actually be balanced (IF that matters) requires more
than trimming resistors on the plates. Also the diff pairs may be sensitive to gain differences
between pairs of tubes or sections of tubes.

Yes, one could transformer couple diff pairs...

Oh IF there is an advantage to going transformer coupled it is that transformers (to the limit of
their internal balance) automaticially enforce "balance" between positive going and negative going
sides, whereas an entirely active signal path needs great care in the design to maintain true balance
and not have an asymmetry. (Of course for ham radio, a bit of less than fractional % perfection is
not important at all - in studio gear, it may be)

So, the main design considerations will be to work back from the output side (drive to the 833 grids)
to the input, and figure out how much gain (dB ---> volts of swing) is going to be needed. Then
fill in that black box... first step is how will you couple to the grids of the 833. Cheesy

Anyhow the older mags and handbooks will have examples of entirely transformer coupled
speech amps...
Logged

_-_- bear WB2GCR                   http://www.bearlabs.com
WA4WAX
Member

Offline Offline

Posts: 283


« Reply #5 on: October 09, 2019, 12:03:19 PM »

Here is something novel........there are several other beam deflection tubes to choose from.

You could run the plates directly to the signal grids of 2A3, 6V6, etc.

https://www.radiomuseum.org/forum/a_curious_one_tube_audio_amplifier.html
Logged
PA0NVD
Member

Offline Offline

Posts: 608


Nico and Chappie (Chappie is the dog...)


« Reply #6 on: October 09, 2019, 12:38:31 PM »

Those sheet beam tubes make exelent high IMD mixers.
Logged
WA4WAX
Member

Offline Offline

Posts: 283


« Reply #7 on: October 09, 2019, 02:33:12 PM »

You meant to say high IP3 I presume.

:-)
Logged
W8ACR
Contributing
Member
*
Online Online

Posts: 643


Penta 254W


« Reply #8 on: October 09, 2019, 04:42:33 PM »

Thanks again for the input. I think I have found the circuit that I will use. I came across the audio circuit used in the Collins 300G transmitter - Balanced transformer input to PP6J5's which are resistance coupled to 6A5's which drive class B modulator grids (810's). 6A5's are roughly equivalent to 6A3's (6A5's have indirect cathode heater). 6A3's are capable of 10-15 watts of audio power in AB1 depending on the plate voltage and type of bias used.

73, Ron W8ACR
Logged

The AM voice of Knox, North Dakota
PA0NVD
Member

Offline Offline

Posts: 608


Nico and Chappie (Chappie is the dog...)


« Reply #9 on: October 09, 2019, 04:48:58 PM »

You meant to say high IP3 I presume.

Indeed, Thats what I meant... Embarrassed
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 7304



WWW
« Reply #10 on: October 10, 2019, 10:58:59 PM »

Thank you both for the replies to my post. Thanks for the link to lilienthal engineering, that is a nice website.

I have added the schematic for my current speech amp below. I'm not sure where I found this circuit, maybe an old Radio Handbook. It does seem to work very well. I have added a few "control" components.

Thanks again,
Ron W8ACR

Good on the 6N7. A great tube and too much overlooked!
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.063 seconds with 19 queries.