diytube.com

http://www.gibson.com/Files/schematics/EH-185%20Amp.pdf

This is an interesting little beast. A friend picked one up recently and has fallen in love with the tone.

It has 3 6J7s that are triode strapped and 6N7 as a phase invertor. The tone control is a neat little feedback circuit. Very cute, I'd love to put it on my bench and see what it is all about!

Don

What's the vintage of that amp, Don?
From the use of "M" instead of "K" on resistors, and the two
16 mfd filter condensers, I'd say early 50's?

... tom

From the tube line-up of 6J7s and 6N7s, etc, it looks more like a 1940s design. From what part of the schematic I could receive, it looks like all metal tubes, except perhaps the rectifier.

For some strange unknown reason, the schematic downloads about halfway down the page, and then everything just flies off to la-la-land

FWIW, a 6N7 in Class B makes for a dandly little 10W PP amplifier. It needs an interstage transformer, and the IT's secondary CT is grounded, as this is a zero-volts biasing arrangement.

Another 6N7, wit the two triodes connected in parallel can serve as Class A driver, delivering about 400 mW into the IT's primary. Class B operation draws grid current, so normal R-C coupling will not work in this arrangement. The Hammond 124B or the Triode Electronics IT would be good choices here.

I'll look up the actual circuit, it is in one of the older 1940s or 1950s ARRL radio amateur's handbook.

/ed B

I've been told that it is ~ 1939.

Don

I had no problem downloading it tonight, dunno what was going on last night Either my ISP or their site was experiencing a bit of a SNAFU

Field coil speaker and no fuse, that's REALLY old school , mid to late 1930s, to just before WWII... With 275V B+, they're running each of those 6L6s pretty lightly, around 12.5 W combined , Plate and SG dissipation.

Some of its circuitry looks like it was lifted right out of RDH3...

"M" is the really old school way of expressing " X 1000 Ohms", must have been using the Romal numeral M, instead of the more modern "K".

The notion of picofarads and nanofarads had not yet entered the field of electronics design, so they used MF or MMF, and a thosand ohms could be expressed with either M or K. So, M could mean "a thousand" or "Micro", depending on when and where they were applied. The lower case "u" micro had not yet been universally adopted. Confusion reigned, I'm sure

Frequency waw still expressed in Cycles, Kilocycles and Megacycles, microwave did not uyet exist, and in its early days, those were expressed as KiloMegaCycles rather than today's "GigaHertz".

/ed B

EWBrown wrote:"M" is the really old school way of expressing " X 1000 Ohms", must have been using the Romal numeral M, instead of the more modern "k".

I'm glad you explained the M and K thing. I was trying to imagine how hard it would be to locate a source for a handful of 500 meg audio taper pots.

..Todd

Hey Todd!

TAJ wrote:I'm glad you explained the M and K thing. I was trying to imagine how hard it would be to locate a source for a handful of 500 meg audio taper pots.

At least in a size that'll fit in an average amp

I did see one in an industrial catalogue a while back for 10KV or some rediculous voltage.

Cheers!

Many years ago (like back inthe early 1970s) I bought a defibrillator at Eli Hefron & Sons, in Cambridge Mass.

No, I wasn't thinking of starting up my own cardiac "jump start" business (it didn't come with the paddles) , it had a nice 12VDC to 600V inerter, using a Triad TY-84 toriod transformer and some hefty germanium transistors, which I used in a mobile tube 2 meter transmitter. That was based on a WWII surplus 100-156 MC AM VHF aircraft transmitter. Final output stage was an 829B, it originally used an 832A, but I "upgraded it" for more RF firepower

The voltage multiplier charged up a really HUGE oil cap to a maximum of about 2500VDC, and to measure the charge, it used a 470 Megohm resistor in series with a fairly standard panel-mount analog meter. The resistor was about 8 inches long, and 5/16 inches diameter, with a 1% tolerance rating. Unfortunately, this giant resistor didn't survive my many moves over the past 40 years...

I later traded the xmitter when I got my first PLL synthesized 144-148 MHz transceiver, though I later wished that I'd hade kept it

/ed B

6J7 do sound great but I would be willing bet that at least some of the lovely tone is due to the field coil speaker.

My favourite little suit case style amp is a 6J7 pentode to 6J7 triode strapped to 6V6 single ended with an 8 inch field coil speaker. Rectifier is an 80. In that case I know where the design came from, a quite famous name - W.N. Williamson and the design is dated 1942. It is also published in Radiotron Designers Handbook in the Tone Control Section, uses a "nifty" feedback based tone control around the 6V6 output tube.

Cheers,
Ian