Bought a Mojotone kit off Craigslist...

[AlNewman]

...The impedance ratio of the OT came in at 2300 ohms, so I should be able to run a 4 ohm speaker at about 3-4 watts.

The 6k6 spec sheet says 7600ohms load at 250v as a pentode
Maybe a 8 or 16ohm speaker would be better

But if you are running as triode it says 2500ohms

I was looking at 315v I guess and saw 9000.  Then I divided by 2300 and got close to 4 ohms.  7600/2300 would be between 3 and 4 ohms.  I'm thinking I might be at a higher voltage with new caps as well, which should raise the plate load a bit higher yet, but not positive.  The B+ directly off the rectifier was 288v.

Maybe I'm figuring it wrong?  How do you get 8 or 16 ohms?

[bmccowan]

I wasn't even thinking of using the octal sockets.  Good point though, I could.

Oh yeah, If a chassis has existing octal sockets I always reuse them. There are tons of great octal tubes available - many NOS, but also some new reissues.
Some SE octal circuits worth looking at include:
Fender FC1
Valco/Supro 1947/48 - 6SL7 preamp - my favorite, I have a vintage one and built one
Valco 1610 and 1625 - pretty much as above, but option to run parallel output tubes
Gibson GA-5 Les Paul Junior - pretty similar to the Fender FC1 but 8ohm output and oval speaker.
Gibson early Gibsonette - 2 6V6 in parallel
Magna (before Magnatone) 197-3
The ones with a pentode preamp are all pretty similar
And the ones with a dual triode (6SL7) are also pretty similar.
Octal preamps are underrated these days, IMO.

[PRR]

I first saw one in 1961. Dad had been transferred mid-winter and the motel pool had a dome.

I have never heard of those blow-domes injuring anybody. They weigh so little, they come down slow, and even if collapsed you can lift the fabric and walk out. Normally they blow up again with small repairs. This time it does seem to be worse, maybe irreparable.

[AlNewman]

I wasn't even thinking of using the octal sockets.  Good point though, I could.

Oh yeah, If a chassis has existing octal sockets I always reuse them. There are tons of great octal tubes available - many NOS, but also some new reissues.
Some SE octal circuits worth looking at include:
Fender FC1
Valco/Supro 1947/48 - 6SL7 preamp - my favorite, I have a vintage one and built one
Valco 1610 and 1625 - pretty much as above, but option to run parallel output tubes
Gibson GA-5 Les Paul Junior - pretty similar to the Fender FC1 but 8ohm output and oval speaker.
Gibson early Gibsonette - 2 6V6 in parallel
Magna (before Magnatone) 197-3
The ones with a pentode preamp are all pretty similar
And the ones with a dual triode (6SL7) are also pretty similar.
Octal preamps are underrated these days, IMO.

Nice!  Thanks for the rabbit hole.  I may have to rethink my strategy with that little guy.  It's really one of the few with octal sockets I could use, I have a couple more like boat anchors, and some with 1 and 3 volt tubes, no transformers, etc, so not really feasible.

[AlNewman]

Fired up this guy yesterday.  Electrohome Sonata.  Had a bit of a runaway, the coupling cap off the 12at7 was shorted, so was getting 80v on the grid of one of the 6v6's.  Replaced that and was able to tune in some AM radio.  This is a really cool amp, but the switching and signal chains are making my head spin trying to figure it out.  It would almost be a shame to convert it, but at the same time with no cabinet it's pretty useless as a radio.  Plus, I have a feeling it could be a wicked guitar amp.  I'll try plugging a guitar into it later and hook it up to my super reverb cab, see if I can piss off the neighbors.

[AlNewman]

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[bmccowan]

Did you find these?
http://www.pacifictv.ca/electrohomeschematics.htm#electrohome
There is a manual that has a schematic that might match.
Man, Electrohome sure made a lot of stuff - I had no idea.
It should work out as a guitar amp. Lots of parts you won't need and you may have to bleed off some voltage, eh?
Sympathize on the radio thing, but there are many cool AM radios to be found and brought back to life.

[AlNewman]

Did you find these?
http://www.pacifictv.ca/electrohomeschematics.htm#electrohome
There is a manual that has a schematic that might match.
Man, Electrohome sure made a lot of stuff - I had no idea.
It should work out as a guitar amp. Lots of parts you won't need and you may have to bleed off some voltage, eh?
Sympathize on the radio thing, but there are many cool AM radios to be found and brought back to life.

I linked the schematic in the last post, it got hidden above the picture.  I tried my guitar through it as is, and it sounds pretty dead, so I no longer feel bad about deleting the radio circuit.  I didn't reconfigure the input though, so I'll put a fender type grid leak and grid stopper in.  It did sound reasonably better with a boost pedal infront of it.

So B+ as it sits is 325v, and I'm getting 10.5 watts plate dissipation/6v6.  The RCA tube bible has max plate dissipation at 10 watts, but I've also seen 12-14 elsewhere, so I dunno... nothing's blown up yet.  I'd like to maybe get them down to 9w or so to be safe.

I think the way the switching works, the phono input bypasses everything up to the phase inverter.  So I'll try just plugging into the grid of the 1st 6ba6, and running it straight into the front of the phase inverter as is, and see what that does.  I might have to play around with running it as a triode, and different voltages and biasing, etc...  I'll start there, and if I can find something that works, then I can add the other 6ba6 and a better tone control into the mix.  The fancy 12 position 4 gang switch will have to wait until I have a better handle on what I can do with it.  I might need a big bag of mushrooms in order to wrap my head around that.

[bmccowan]

325-375v is a sweet spot for 6V6s IMO. I like your experimenting, so not trying to divert that. But there are several good 7 pin tubes, 6C4 is one of them. Some of the Lectrolab/Harmony amps used them. Plus you can configure that 12AT7 socket for a tube with more huevos.

[sluckey]

You can't run an audio signal through that 6BA6. That tube is a 455KHz IF amp. The audio circuit begins at the volume control and consists of the 12AT7, 6C4 and two 6V6s. Everything else is radio.

The first half of the 12AT7 is the preamp. Second half 12AT7 and the 6C4 are the paraphase PI. Then the push/pull 6V6s.

[AlNewman]

325-375v is a sweet spot for 6V6s IMO. I like your experimenting, so not trying to divert that. But there are several good 7 pin tubes, 6C4 is one of them. Some of the Lectrolab/Harmony amps used them. Plus you can configure that 12AT7 socket for a tube with more huevos.

Hey, thanks for all your input i really appreciate it.  I do have other tubes I could use, but I'd like to try to work with what I have.  Maybe it's not possible.  I redesigned the input, that helped immensely with clarity and tone.  Then I was poking around and found the plate resistor for the 6ac4 was open, so changed that and it's starting to sound pretty good.  You're right though, I could raise the voltages on the 12at7 and probably the 6ac4, and push the power amp harder.

[AlNewman]

You can't run an audio signal through that 6BA6. That tube is a 455KHz IF amp. The audio circuit begins at the volume control and consists of the 12AT7, 6C4 and two 6V6s. Everything else is radio.

The first half of the 12AT7 is the preamp. Second half 12AT7 and the 6C4 are the paraphase PI. Then the push/pull 6V6s.

Thanks Sluckey.  So is that just how it's set up, or the tube is just incapable of amplifying anything below those frequencies? 

[bmccowan]

You're right though, I could raise the voltages on the 12at7 and probably the 6ac4, and push the power amp harder.

I was not suggesting that you try to raise the voltage. That will happen to some degree as you disconnect the things you do not need.
What I was suggesting is that you could use a 12AX7, 5751, 12AY7 etc in that 9 pin socket, adjusting plate, grid, cathode resistors as needed. The signal path that Sluckey pointed out is the simplest route to a working amp. That volume pot is headed that way.
I'm sure he will answer on that 6BA6 - I know little of such things - but there are hundreds of tubes that are not suited for audio amplifiers - and a lot of them are in my basement

[AlNewman]

You're right though, I could raise the voltages on the 12at7 and probably the 6ac4, and push the power amp harder.

I was not suggesting that you try to raise the voltage. That will happen to some degree as you disconnect the things you do not need.
What I was suggesting is that you could use a 12AX7, 5751, 12AY7 etc in that 9 pin socket, adjusting plate, grid, cathode resistors as needed. The signal path that Sluckey pointed out is the simplest route to a working amp. That volume pot is headed that way.
I'm sure he will answer on that 6BA6 - I know little of such things - but there are hundreds of tubes that are not suited for audio amplifiers - and a lot of them are in my basement

Well I've got a pretty good collection myself.  Somebody should build an audio signal that works with those tubes.

It's strange though..  6ba6 can be substituted with 6au6, and they can be used in preamps.

[sluckey]

Not saying the tube can't be used for audio. Just saying the circuit will not work with audio.

[AlNewman]

Not saying the tube can't be used for audio. Just saying the circuit will not work with audio.

That's what I needed to know.  I figured I'd have to tinker with the circuit, considering all the radio tuners poking up from it.  Thanks for clarifying. 

https://el34world.com/Forum/index.php?topic=20383.msg214556#msg214556

I was looking at this thread, looks like maybe it could work in a low voltage application.  This amp is a good candidate for that.

[scstill]

After the circuit is redesigned for audio, the 6ba6 being remote cutoff might be better served if the 6au6 sharp cutoff is used instead. Seems that they are very close except for the grid voltage needs.

I had a similar issue (discussed in this forum) trying to use a 78 (remote) and decided to replace with 77 (sharp). Haven't finished the project yet (Phirty6) but will try both the 78 and 77 to see how the sound compares with proper bias for each.

[AlNewman]

After the circuit is redesigned for audio, the 6ba6 being remote cutoff might be better served if the 6au6 sharp cutoff is used instead. Seems that they are very close except for the grid voltage needs.

I had a similar issue (discussed in this forum) trying to use a 78 (remote) and decided to replace with 77 (sharp). Haven't finished the project yet (Phirty6) but will try both the 78 and 77 to see how the sound compares with proper bias for each.

I set up the ba6 as a triode at the input, and I'm really happy with the results so far.  By itself, it's enough to drive the power amp pretty well.  I started at around 50v at the plate, and 1.5ma current, and it worked ok, but was still a little dull sounding, so I changed it up to 100v at the plate and around 4.4 milliamps, and it is sounding pretty good.  If I really push it with an eq and overdrive, it starts to break up, although the distortion doesn't seem amazing on it, but for a clean driver it works well. 

I'm putting in a baxandall tone stack in with a volume, and I'll probably use a ac4 or au6 for recovery, and dial in the ba6 to get the response I'd like down the chain.  If anyone has a bunch in their basements, I would suggest dusting them off and trying em out, they seem to make a pretty nice pre amp tube to me. 

[AlNewman]

So I went with the 6ba6 +6c4 in the preamp section with the bandaxall tone stack plus volume in front of the 6c4, and I am happy with the results so far.  I ordered new electrolytics and will rebuild the power section and play with values once I know everything is running efficiently.  right now the plates on the preamp are in the 120-130v range, drawing about 5 mA of current, but from the rca bible there should be some room to push them harder.  I left the b+ running through the radio transformers for now, I figure they act as extra filtering, I'm wondering if that's a good idea or not.  I'd like to leave them on the chassis, they look cool, and make good tie points, just wondering if there's a benefit or danger leaving them in the circuit.

[sluckey]

I'd like to leave them on the chassis, they look cool, and make good tie points, just wondering if there's a benefit or danger leaving them in the circuit.

Just another potential point of failure. Why tempt Murphy?

[DummyLoad]

Sitting here in the dark, going on 50+ hrs w/o power, and I am enamored with that Leslie power amp. I'm thinking split chassis geetar amp. Silvertone/Valco 1485-ish front end, or SuproThunderbolt.


Wanna flip it? 


--Pete

[bmccowan]

Quote from: AlNewman on February 01, 2023, 09:22:11 pm
I'd like to leave them on the chassis, they look cool, and make good tie points, just wondering if there's a benefit or danger leaving them in the circuit.
Just another potential point of failure. Why tempt Murphy?

Well, if they look cool (very important factor) you could disconnect them, leave them in place, and make up a George Santos story about some function they perform. Recycling of electrons or such.

[AlNewman]

Sitting here in the dark, going on 50+ hrs w/o power, and I am enamored with that Leslie power amp. I'm thinking split chassis geetar amp. Silvertone/Valco 1485-ish front end, or SuproThunderbolt.


Wanna flip it? 


--Pete

Hey Pete, I wouldn't be against flipping it, but there is a pretty big caveat on that one...   Apparently when the guy went to test it, he plugged it in and said the motor was seized, so something blew up.  The 10w 75 ohm resistor is split in 2, and I see the can cap has been mangled and is sitting in the chassis.  Soo, I measured across the transformers and didn't find any dead shorts, but I did notice some knucklehead installed a 25 amp fuse, unless I'm missing the decimal point...  Luckily the fuse is in good shape!   

So between that, and the shipping from Canuckistan, it might not be worth the risk, but I am open to the idea, all the same. 

It does have all that vintage sprague orange drop mojo though, so there's that. 

Sucks about the no power thing, tough to build amps rubbing 2 sticks together as a soldering iron.  Hopefully you have a wood stove or a genny or a big dog...something to keep warm.  All the best.

[AlNewman]

Quote from: AlNewman on February 01, 2023, 09:22:11 pm
I'd like to leave them on the chassis, they look cool, and make good tie points, just wondering if there's a benefit or danger leaving them in the circuit.
Just another potential point of failure. Why tempt Murphy?

Well, if they look cool (very important factor) you could disconnect them, leave them in place, and make up a George Santos story about some function they perform. Recycling of electrons or such.

Yes, but I suppose Sluckey's right, why tempt fate?  I'll leave em in for the cool factor, and just unhook em.  Speaking of cool factor, I was thinking of gutting the can cap and dremmelling out some sort of design and putting the pilot light in there, but turns out the guy who sold it to me wants to buy it back!  He's got the original cabinet he turned into some sort of guitar stand...  but he wants to mount the amp back in it and rock out.  So that's cool. 

[PRR]

...wondering if there's a benefit or danger leaving them in the circuit.

I dunno danger. If you are using them as pass-through: these coils commonly went open in old age due to fine wire and dirty air. So it may quit unexpectedly. I guess if the crowd turns on you because the music stopped, that's a danger.

The inductance is about-none at audio frequency and tube impedance. 1mH at 10kHz is under 100 Ohms, and less in the main music band.

[AlNewman]

...wondering if there's a benefit or danger leaving them in the circuit.

I dunno danger. If you are using them as pass-through: these coils commonly went open in old age due to fine wire and dirty air. So it may quit unexpectedly. I guess if the crowd turns on you because the music stopped, that's a danger.

The inductance is about-none at audio frequency and tube impedance. 1mH at 10kHz is under 100 Ohms, and less in the main music band.

Well, it won't quit on me...   and I doubt he'll pack his 300 lb guitar stand to gigs with him, so not much in the way of danger there.  The main danger now I guess would be doing warranty work on used/used 60 year old components and tubes.  So I'll just leave them on the chassis to look good and fill otherwise empty spaces....  Which they do.

It is good to know they don't actually serve much of a function in this application.

[AlNewman]

Well, I think I pretty much have the electrohome amp dialed in.  I MAY just try to push the 6ba6 a little harder, but it sounds really good as is.  Humbuckers and a tele can really make it sing, my 79 strat sounds a little anemic, at least without a drive pedal.  Anyhow, onwards and upwards, she's pretty close, she starts getting juicy about midnight, and at 3 am she gets pretty nasty so I'm gonna call it a win, but as long as it's sitting within reach I reserve the right to tweak.

[AlNewman]

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