RC-12 Tube Manual

© 1934

All the tubes I am using in my Hi-Fi System were being manufactured by the time the RC-12 was published.

RCA phono from RC30

More info

But I also keep an RC30 copy! 

Jensen Ultraflex redux

A few updates to the Jensen Ultraflex

Revised crossover + EQ/HF boost 

 The fixed resistor attenuator was inspired by the Jensen Duette crossover + recent knowledge gained in boosting frequencies above 7khz using an EQ network in my Altec 2-way XO experiments. Try it on your Heathkit SS1 or Eico HFS1 which AFAIK, used only a 2uf beeswax paper cap + L-pad.

Damping removed
paper in oil caps for
crossover and EQ

Happy Listening!

de Forest 410 Mesh Plate

I've been feeling nostalgic lately especially after unearthing a box of prints and negatives which I've been scanning and uploading in the Arkiv.

PT-6A Horn Tweeter

It's been many years since I listened to 2 cubic foot slanted front boxes loaded with 755s. The 755E was chosen because I have had the least flying hours with them. In the past I used either Altec 3000H or Fostex FT17H to fill in the extreme top end on the 755C and 755E. But the FT17H has been gone in a trade for quite some time. 

While browsing my favorite local electronic parts supply store, I found this pair of plastic horns for a little less than $20/pair. With specs and a price like that it would be worth the fun playing with them had they turned out to be duds.

The model number and trademark printed on the box don't exactly match what are stamped on the unit itself. Back in the 70s-80s, Micro was a 100% Philippine manufacturer of speakers and drivers. It doesn't seem to be that way anymore ;( I am guessing the "K" = Konzert, and "PT-6"A is an allusion to the vintage Pioneer PT-6 from the 60s from which, I speculate, this unit was inspired (or cloned). 

Mylar, phenolic or?

I was tempted to open a unit just to see what was inside and in particular, what material was used for the diaphragm. Unlike compression drivers which have field replaceable diaphragms, vintage horn tweeters can be very tricky, if not impossible, to reassemble without the proper jig and tools. I returned to the store a few weeks later and found out that there were no more PT-6As in stock. So I decided not proceed with exploring the innards of this horn tweeter.

Even if there were no reference points for the claimed 94dB sensitivity, to my ears it was pretty much within ballpark of 1 meter/1W because it did not require much attenuation used above the 755E. A 1 uf paper in oil cap  in series gave the most subtle blend between the PT-6A and the 755E.  

Comparing the PT-6A head to head with the Altec 3000H is pretty much an exercise in futility. The 3000H is a very tough act to follow. Instead I will report more on the fun factor I derive from the PT-6A coupled with my distant memory of the FT17H as super tweeter for the 755C and E. 

The PT-6A performed well beyond my expectations. Just like the Fostex FT17H, it aided the drooping  high frequency response of the 755E without deleterious effects on the midrange performance for which the 755 series is known for. Perhaps it is a tad less extended in the extreme top end than the Fostex but there were no nasty spikes to be heard.  I doubt if I can find a better performing super tweeter for the 755E at this price range ;)

Fuji X-E1

I have been a happy user since October 2013 ;)

Altec 2-way XO update

The Altec N1600C series crossover has been a significant constituent in the evolution of my Altec 2-way horn speaker system. C. G. McProud's 1947 article on page 101 of Audio Anthology, Volume 1, was my first encounter with series crossovers although Western Electric was already using the topology in the 1930s. As an aside, my knowledge of electronics is basic. I don't even own a circuit simulation program and rely on my collection of audio electronic textbooks, old magazines plus what my ears tell me as reliable internet sources for "techie stuff."

JEL/N1600C 2-way Crossover

Based on Earl K's simulations and analysis at the Lansing Heritage forum, Altec configured the N1600C 2-way crossover with staggered filters. In this application, a 2nd order/12dB/octave high pass filter @ ~ 1600 hz is provided by the 4 uf capacitor + 1mH inductor, while the 1mH inductor series connected to a 16 uf capacitor functions as a 1st order/6dB/octave low pass filter @ ~ 1100 hz. The 30 ohm potentiometer is part of an upper midrange frequency attenuator/high frequency EQ boost circuit I derived from reading through Altec 30923 discussions at Audio Karma and at Lansing Heritage. Altec incorporated this circuit in their Model 19 crossover and Jeff Markwart used it in his phase corrected Altec 605 crossover. 

In subjective terms, the combination of staggering the crossover point + upper midrange attenuator creates a "dip" in the crossover region where harshness and congestion can easily be detected by critical listeners, while my Altec 3000H tweeters were made redundant by the high frequency EQ boost.

Outboard crossover built using
paper in oil caps, low DCR air core chokes and
non-inductive resistors