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hi-mu triodes: 75, 7B4, 7B6 + 7F7

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Alternative hi-mu triodes in place of my favorite octals

The type 75 is the grandaddy of all hi-mu audio triodes culminating with the ECC83/12AX7.


Grid-capped 75s in the JE Labs Simple 45/2A3 with Noguchi PMF10WS OPTs.

Loctal or Loktal (as trademarked by Sylvannia) tubes were developed concurrently with octal based tubes in the 1930s. But loctal tubes were aimed at the then emerging car and portable radio market - hence, the need for a locking-ability in the tube socket, which required a metal base that gave the additional benefit of better shielding.

The 7B6 is a loctal version of the 75



I originally built the je2A3 mono integrated amp with 7B6s in the phono stage. Right now I am running 7B4s.



The 7B4 has slightly lower Rp than the 7B6. NOS samples are still very affordable so try them both ;)

I built this stereo preamp for the November Hi-Fi Show 2016. It uses 2 x 7F7s for the phono section which are direct loctal equivalents of the 6SL7. The line stage sports 2 x grid-capped 6F8Gs…

Harrisburg Symphony Orchestra

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Tchaikovsky, "Romeo and Juliet"
Harrisburg Symphony Orchestra
Stuart C. Malina, Conductor and Music Director
Dress rehearsal, April 8, 2016

I played with HSO from 1999-2008 and glad to be back this season.

Radio Shack

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In February of 2015, Radio Shack filed for Chapter 11.

So I was delighted to see this Radio Shack store announcing a comeback in midtown Manhattan during a visit in the summer of 2016.

When I was a student in NYC in the 80s, a neighborhood Radio Shack store was just around the block if I needed a resistor or capacitor for a project on a Sunday afternoon. By 2008, I was in the suburbs and Radio Shack was in a strip mall just a mile and a half away. Now I have to drive 35 miles to the next town ;(


In the early 90s, many SE amp DIYers started their foray into efficient speakers inspired by this book by David B. Weems. The speaker projects used various raw drivers from the Radio Shack catalog.

40-1354A

Radio Shack no longer had 2A3 triode tubes on their shelves when Sound Practices started publishing in 1992,  but one could still buy this 5 1/4" full range driver listing for less than $10, IIRC. This speaker was efficient enough to be driven by an SE2A3 amp to satisfying levels.


I u…

Altec small format diaphragms: Aluminum vs. Symbiotik

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I've been playing around with Altec drivers for almost 20 years and have never questioned the dictum of using only aluminum diaphragms for high-fidelity applications until now.

Altec Aluminum Diaphragms

The 16 ohm Altec 20275 is the original diaphragm fitted to my cherished green hammertone 802D. This is the Altec diaphragm I am most familiar with and to my ears its tonal balance favors the midrange more than the later 34852, which is known for better high frequency extension.

I speculate that late 40s to 50s wrinkle charcoal gray/brown and late 50s to 60s hammertone green 802s, 804As and 806As command premium prices IF they are fitted with original and intact 20275s, which are now unobtainium. By early to late 70s, black 802s and 806s were fitted with either the 16 ohm 34852 or the 8 ohm 34647.

My Altec 605B Duplex should have been originally equipped with a 20275 but by the time I acquired it, this 34852 was already fitted. Maybe that's why I still got it cheap ;)
As previo…

Vostok Bezel Mods

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From NATO'ed to Bezel mods

Vostok Amphibia 110640 + Bandukh Bezel

Murphy 1050 bezel + Seiko 'Pepsi' insert
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Vostok "mod" Amphibia 420432