6888 – A Pentode in Hiding

Guest Writer: Bill True, WA9 ASD

 

surplus-sales-pentode

There are a number of tubes that appear to be ‘under the radar’ of tube equipment designers for no apparent reason. One such tube is the Sylvania 6888. Originally designed as a core driver in first generation vacuum tube computers, it is practical in several other applications.

Multiple versions of a one or two tube AM Broadcast Radio project might be responsible for bringing this tube back in demand. We found a half dozen worthy construction challenges on the Internet. The 6888 is used for an AM transmitter to feed a signal to antique table radios. Using modern input sources, we think there is an un-tapped potential as a low power audio application like a headphone amp or personal stereo as well. If availability or cost were deterrents in the past, we now have enough to go around. With over 10k in stock we offer a 1 each price of only $3.95 and it drops to $2.50 ea for a whole case of 100 at a time. See our tubes at SurplusSales.com

 

Let’s skip the paradoxical complication of a wireless connection and the questionable sound reproduction of antique radios and go right from source to speaker. No limitation of 5 Kc on the top end that AM stations are subject to. No misaligned IF section that misses the detector pass band. No risk of leaky filter condensers and torn paper speaker cones. Let’s just put a little gain and some impedance matching between the source and some good, wide-range headphones or speakers. Worst case; hide the good speaker in the antique radio.

While the 6888 is kind a power hog on the filament side it is definitely a pussycat on the plate side with moderate if not low voltages and current required. Because of this it shouldn’t need some exotic hand-wound high plate impedance transformer to match the load. A couple of thousand Ohms ought to do it.

As a voltage regulator, another great idea for this pentode, it should be nearly indestructible and will probably out-last the resistors in the circuit. It would have considerably more current capacity than a VR-150. We will be happy to post on this site any project using the 6888 and give credit to the author.