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.
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 used this driver in a compact bass reflex box...
from David B. Weems book
...as well as in a Tapered Quarter Wave Tube (TQWT)
Not quite in the same league of sophistication and refinement as the Altec 2-way, but lots of fun since they can boogie on a couple of triode watts ;)
Time Warp
During a recent "let's see where the wheel takes us" expedition, my wife and I spotted this in Pennsylvania.
40-1310
As I browsed around the store, a pair of 40-1310 super tweeters bearing their original sticker price immediately caught my eye. I told my wife about them and she insisted that I get this pair of NOS Chinese-made plastic version for $60 + tax, so I can time travel.
These drivers developed quite a following in the 90s because the original all-aluminum units embossed Realistic were allegedly OEM'd in Japan by Fostex. The graphs above and below should corroborate this speculation. Later versions with a plastic pod and horn were subcontracted in South Korea and then China.
Fostex FT17H
I didn't join that 90s bandwagon. Instead, I bought a pair of the equivalent Fostex FT17Hs sans pod from Madisound for $70. I used them as super tweeters for Altec 755Cs and 755Es until they were superseded by a pair of superior Altec 3000Hs.
Brand new Fostex FT17Hs are now made in China and can be had for a little less than $100/pair. I still believe they are excellent value for the money. Perhaps the early Japanese all-aluminum 40-1310s can command a slight premium, but beware of unscrupulous eBay flippers asking ridiculous amounts for the plastic version.
Subtlety
Before inserting the 40-1310 into the Altec 2-way, I bypassed the built-in electrolytic cap/choke crossover components. For the past 20 years, I've been using smooth and natural sounding paper in oil caps in the signal path of my preamp, amp and crossover, nothing else.
Altec 2-way + 40-1310
Altec 2-way sans 40-1310
In this application, the 40-1310's positive terminal is protected by a 1uf PIO cap and then connected to the high-pass section of the crossover. The effect of the super tweeter is subtle...just a touch more air.
As I was about to sit back to reminisce the 90s with the 40-1310, an internet friend informed me that Radio Shack just filed for another bankruptcy claim ;(
I've been playing around with Altec drivers for almost 20 years and have only listened to aluminum diaphragms until now.
Altec Aluminum Diaphragms
Altec 20275
Aluminum voice coil
DCR = 9 ohms
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 previously noted in my 605B experiments, I hear a significant sonic difference between a 34852 and a 20275, so much so that I cannot recommend their use as a stereo pair. Jeff Markwart has more 20275 vs. 34852 info if you scroll down to the bottom of this page.
GPA 34852
Copper voice coil
DCR = 11.5 ohms
AFAIK, Great Plains Audio is the only source for Altec diaphragms made to original Altec specifications. Even if they have a website, they prefer to do business the old fashioned way. I always callBill Hanuschak to place an order and never had problems.
I haven't had a chance to compare this GPA 34852 to the Altec 34852 because the 605B Duplex was left behind in the attic;( But there's no doubt that this GPA is the real thing - very dynamic, with clean and clear harmonic overtones up the wazoo!
For kicks, I made an offer to one of the Chinese vendors on eBay and got this pair of 16 ohm diaphragms for about $24/pair, shipped. These clones will tide you over in a pinch. However, even a casual comparison to an Altec original or GPA diaphragm revealed smeared harmonic overtones and compressed dynamics.
Spot the fake
Left picture (clockwise from left) - older version Altec diaphragms used screw terminals on a raised platform, later Altecs and GPAs have tabs, Chinese clones do not have a raised platform for the screw terminals.
Right picture - The raised platform in the Altec uses tapped machine screws whereas in the Chinese copy the screws are self-tapping.
Altec Symbiotik
I recently acquired a couple of unloved 70s era 802 and 808 drivers, some fitted with Symbiotiks.
Two abused Altec diaphragms:
Symbiotik on the left and aluminum on the right
The Symbiotik was Altec's initial response to the 60s demand for a higher power handling diaphragm. As pictured above, due to the rigidity of the aluminum suspension, it can shatter when overdriven. I can only surmise that the Symbiotik diaphragm, which uses a polyimide/mylar type suspension, evolved from a research and development project at LTV by John Hilliard and Jonas Renkus.
Early + Later Symbiotik diaphragms
In early Symbiotiks, the abrupt 90 degree angle voice coil termination (left) also caused failures in the field due to metal fatigue. I repaired this early Symbiotik by carefully flowing solder between the broken ends with a low temperature soldering iron. This issue was rectified in later production Symbiotiks (right).
Uncovered: Altec > Emilar
Top = early Emilar with brown mylar suspension
Bottom left = later Emilar with gray mylar suspension
Bottom right = Altec Symbiotik
I have written favorably about Emilar and Renkus-Heinz compression drivers, and noted that their extreme top end was a bit reticent compared to Altec drivers with aluminum diaphragms. Let's see how the earlier Symbiotik technology stacks up.
Top = later Altec Symbiotik with tabs and
improved termination
DCR = 6.7 ohms
Bottom = early Altec Symbiotik with
screw terminals
DCR = 7.5 ohms
In retrospect, Altec aficionados may have been hasty and/or unkind in their initial reception of the Symbiotik. Even if the Symbiotik rolls off earlier than its aluminum brethren, it still reproduces proper harmonic overtones. It sounds very sweet, airy and open, with none of the smearing and compression I hear from the Asian clones. A Symbiotik equipped 802/808 sounds more organic than its descendants from Emilar and Renkus-Heinz. To my ears, the Symbiotic diaphragm has a niche in Hi-Fi and may well be the antidote to those who find the aluminum diaphragm a bit aggressive.
Unfortunately, there are no modern replacements for Symbiotik diaphragms. You'll have to find original units that did not suffer catastrophic meltdowns. Although it's just a hunch, the Radian replacement diaphragm for Altecs should have Symbiotik DNA because of the Emilar/Plus One connection.
Octave RTA by Onyx
Since affordable audio measurement apps have become available, I got this $5 app for my iPhone as suggested by my buddy J-Rob.
Test set-up
The Altec 414A housed in a 3.5 cu. ft. repurposed Madrid (861) bass-reflex cabinet is driven full-range, J-Rob style, while the Altec 32A metal horn + driver combo is protected by the crossover + EQ below.
Crossover + EQ
I was quite surprised to see pretty decent RTA results since I've been building/tweaking crossovers for over 20 years without a computer simulation program or an RTA.
Altec 802-8D + GPA 34852
Altec 808-8A + Chinese 16 ohm diaphragm
Altec 808-8A + Symbiotic
Emilar EA175-16 (alnico)
Emilar EC175-8 (ferrite)
These graphs pretty much represent how these driver/diaphragm combinations perform except the Chinese clone which looked good butsounded rolled off, compressed and smeared as mentioned above. Unlike human ears, an RTA will measure sound waves but will not discern unmusical harmonic overtones. Even my wife thought the Chinese diaphragm sounded muffled. Although she argues that if I can afford only a $12 diaphragm, I better be happy with it ;(
It goes to show that it is much easier to satisfy audio measuring equipment than human ears.
