Time for a new project- Gibson EH-125 =

[yorgle]

I've got tubes and a field coil speaker and was thinking about building an EH-125 replica using this schematic from the library  https://el34world.com/charts/Schematics/files/Gibson/Gibson_eh125.pdf
I have a nice Zenith 350-0-350 90ma PT.  Does that seem like a reasonable PT for this amp?

[shooter]

You'll be close, but should make it, IF the PT can handle the filaments.  It's getting cold here so it's always nice to have a hand-warmer handy 

[Ed_Chambley]

Here is a handy tool.


https://thesubjectmatter.com/calcptcurrent.html

[tubeswell]

The FC is in series with the B+, so assuming your speaker is made like the original (for this schematic) was intended, the FC will drop about 100V and so you will end up with 280V on the 6V6 plates and screens.


Bear in mind that FC speakers were designed for specific amp typologies so that the impedance of each particular speaker worked with the FC electromagnet to match the required impedance load for the output tubes that were being used in the circuit (taking into account the operating characteristics - plate voltage and plate current etc - of the out put tubes). Will your speaker work with that circuit and that PT? Who knows?

BTW, That schematic is all over the place with resistor value notations.

[yorgle]

Slukey:  The PT pumps out a solid 4 amps of filament juice (came from a chassis that had 10 or 11 tubes) so no issues there.   

Ed:  thanks for the link.  That’s an awesome tool.

Tubeswell:  I see what you mean about the resistors... I’m assuming “m” really means “k” and that “meg” is just what it says.  You would think that would have bothered the guys who drafted these schematics back in the day. 

[shooter]

why, that was normal, K woulda been ab-normal 


that said, probably NOT a 200k for the Rk on the 6V6's, now that shoulda bothered the drafters, but they just wanted to go have a beer and who were they to question an EE

[yorgle]

Got a good start this week on what will be more of an "inspired by" than an actual eh-25 clone.  The PT's girth necessitated deviating from gibson's 6 in-a-row tube arrangement.  I used some textured brown Rustoleum that gives an almost wrinkle-coat finish on the chassis. The volume dial is a water-slide decal on a thin piece of stainless. 

[yorgle]

Close up of the texture.

[bmccowan]

The chassis and volume dial are real nice - looks like a BR-6 chassis, only nicer. Did you buy that waterslide decal - or print it on decal stock? I love those old individual dial faceplates, but they seem to be unobtainium these days. I bought some decals from Radio Daze, but have not tried them yet.

[yorgle]

The chassis and volume dial are real nice - looks like a BR-6 chassis, only nicer. Did you buy that waterslide decal - or print it on decal stock? I love those old individual dial faceplates, but they seem to be unobtainium these days. I bought some decals from Radio Daze, but have not tried them yet.

I laser printed the dial design onto decal paper using MS Paint.  I ran a bunch of variations using various fonts so I’m good for another 20-30 amps!   Spent some time tonight wiring up the power section, heaters and pilot light. 

[yorgle]

Is there any downside to mounting the output transformer inside the chassis instead of on the speaker?  I’ve got plenty of room and I like the idea of keeping all the big voltages inside the box.  What do you guys recommend? 

[sluckey]

There are plenty of amps that put the OT inside the chassis. No problems. But, aren't you using a field coil speaker? Still gotta run B+ for that.

[DummyLoad]

ditch the field coil speaker and mount the OT inside or on the chassis. unless, you're building into a sealed cabinet.


--pete

[yorgle]

There are plenty of amps that put the OT inside the chassis. No problems. But, aren't you using a field coil speaker? Still gotta run B+ for that.

Yeah, your right.  Must have been inhaling too much soldering fumes when i posted that. 

[yorgle]

Got 'er up and running last night and was amazed to hear no buzzing, hissing, humming or other noises, just glorious vintage field coil tone--- and LOUD!  I'm a bit baffled by the volume control with this schematic- assuming I've got it wired up correctly, only the mic input is controlled with the volume pot on the amp.  The other two (presumably instrument) inputs have to be controlled by the volume pot on the instrument.  Now I need to clean up some lead dress underneath and then it's time to start building a cabinet.

[sluckey]

That looks really sharp!    Just begs for some coke bottle 6L6s.

BTW, a trophy shop could make those dials for you at a reasonable cost.

[shooter]

Realy Nice work, you might give Sluckey a run for "cool looking" 
your PT looked familiar soI looked back and you said you had a "nice Zenith PT"
used one in one of my early builds


called it "copperhead" (GA-8pSE)

[yorgle]

Thanks, I picked up an old Masco PA amp and it had the two 6l6's and the rectifier- they were just too cool not to use.  The PT doesn't seem to care- it barely gets warm. 

[yorgle]

I kinda hate to hide this thing away inside a 125-style cabinet.  Maybe I'll build something art deco that will show off the tubes a bit. 

[bmccowan]

Super nice build - I certainly agree with not hiding it away.

[yorgle]

Changed to all big tubes to make bigger sound.