Tag Archives: tube limiter

The 1959 Ikegami-Tushin Limiter Inspired by the RCA BA6A

NHK_D_1Reader S. Komiya recently contacted us with some information regarding the Ikegami-Tushin limiting amplifier, an RCA BA-6A inspired piece that was built for Japanese broadcaster NHK in 1959.

SK has been so kind as to provide the schematic for this obscure device, as well as some background information.  I am posting the schematic full-size, so you can control-click it and download it for detailed viewing.

00_schematics_ikegami-limiter1959NHK_E1The photos in this post come from this Japanese auction website; the device pictured here recently sold for just Y30,000 ($300 USD).  And in working condition. 

600x450-2015011900002Here’s what SK has to say about the Ikegami-Tushin Limiter:

‘(It is) very much inspired by the RCA BA6A for sure.  It even looks a bit like it.  The tube format is very similar:  just change 6sk7 to 6ba6, 6j7 to 6au6 those goes into 6v6 PP and transformers between 6au6.  6ba6 were popular and cheap in japan because we made those a lot in japan in the 1950s and 60s.

6sk7 and 6j7 were never made in japan.  This unit also has an extra gain stage before first stage, which is pretty neat.  The components seem very high-end and some are custom made for this.  When i was gathering info about the ba6a in old tube shop, an older ham radio guy told me that he DIY’d one of these a long time ago…’NHKH_1NHKF_1*************

*******

***

20141230_123551SK also recently built his own BA6A from scratch.  It is depicted above, and you can hear audio samples of it at his soundcloud page.  SK has also scratch-built the Federal AM864 tube limiter, and he has this to say about the projects:

“The sound you will hear in soundcloud is a good comparison with the fed864.   The fed has good high open but compressed sound,  the ba6a has low mid, ton of low mid. I love them both.  The first time I used them was at a studio in Chicago back in 90s when I was living in US.  At that time I was using LA-2A mainly, but that studio had a Fed864 and BA6A.  They blew my mind, and since then I wanted them so bad!’

SK also provided some build-notes on his BA6A project; if you are planning on building you own BA6A, you might find these useful:  S_Komiya_RCA_Ba6a_DIY_notes

A Simple One-Tube Compressor Circuit c.1953

1954_DIY_limiterReader Bill W recently sent us a few interesting ancient DIY audio projects that we had not seen before.  Today’s post is from RADIO-ELECTRONIC ENGINEERING and was written by one Ed C. Miller.

DOWNLOAD THE COMPLETE ARTICLE:Constant Output Broadcast Amplifier

The article details the operation and construction of the above-illustrated broadcast-remote mixer/amp, which includes a limiter.  While the overall unit is probably not something you’d want to ‘F’ with, the limiter component could be added to a DIY tube mic preamp (perhaps with a true-bypass switch).  One caveat: as the article warns, there are apparently ‘thumping’ artifacts inherent to this circuit, which was acceptable in its intended operation since it was designed for voice-frequency.

1954_limiter_SchemAnyhow, might be a fun project for a rainy day – build a pair of these into box with cheap 1:1 input transformers and simple line output stages?  Crush some room mics?  Anyone?

Federal AM-864 Limiter Clone: Build Notes: update 1

Federal_AM864_clone_w_mic

Note: I performed extensive frequency, level, and actual studio tests on the 864 clone today, and several interesting details were revealed.  Text has been edited to reflect that. 

From 1954 through at least 1963, the Federal Television Corporation built an audio limiter called the AM 864/u for the US Air Force and US Army.  The 864 is a simple, rugged device that accepts 600 ohm balanced or unbalanced line-level signal, offers a single front-panel input-attenuator control, and compresses the output level at a 10-to-1 ratio once the threshold point is reached.  The output is also 600-ohm balanced or unbalanced, and it offers a maximum 36db of gain.  The rear panel of the unit displayed the threshold and ratio controls, although these are confusingly referred to as (respectively) CURRENT and THRESHOLD in the manual and schematic.  Attack and release times are fixed, and the manual indicates them at .05″ and 2″ respectively.

AM864_hookupAs you can see in the diagram above, the 864 was intended to be used as what we call a ‘broadcast limiter’ – the final step in the signal chain before the broadcast transmitter.

Download the original 1963 manual for the AM864 (apologies to whomever did the epic work of scanning this 55pp document; I have long forgotten where I got this file from)

DOWNLOAD: Federal-AM-864-U-Manual copy

Federal_AM864_on_benchAfter having scratch-built an Altec 436 compressor years ago, I wanted to try building an 864.  The circuits are very similar, although the 864 uses the older 1940s-era octal tubes and uses a feedback circuit from the plates of the input tubes (rather than the output tubes, as in the 436) for its compression control signal.  More importantly, though, the 436 remains a bit of an oddball underdog in the vintage-compressor market while the 864 enjoys a very strong reputation.  Anyhow, like the 436, the parts cost to build one of these things is negligible, so I figured what the hell.

This is going to be a very long + detailed +technical article, so I’m going to ask y’all to please click the link below if you dare to READ-ON,,,,

Continue reading Federal AM-864 Limiter Clone: Build Notes: update 1