A new amp with new tubes

[KramerBassFan]

Welcome all!

This is a build i've had in the works for awhile now.

A salvaged power amp from an old Univox 1236 with re-done power supply, and the Transformers from a Univox.

BUT, the Preamp is totally different.... I'm using 6AC7's. No, I don't mean 6CA7's, i mean 6AC7 the old school high gain eight pin tube that was used in WWII aircraft radio's. Best part is NOS 6AC7's are $3 a pop.



Also going to have orange tolex, black metal faceplate, and my signature, an absurd number of strategically placed handles. No one strap handle on top of an old 70lb Amp here!

Enjoy!



Handles:


Recessed handles and chassis


Chassis with turret board and tape, ready to be marked out and drilled.

The three tubes:
I'm using three 6AC7's, one as the first gain stage, one as the Treble boost/cut, and the third is the Phase Inverter. (Five pictured because I bought five just to be on the safe side)

And my two coke bottle Chinese 6L6GC's.




I also found a stash of Thomas Organ Solid State VOX Era style knobs out in the knob bin that will be used on this thing.


Questions? Comments? Flames? Post away!

Thanks for reading.

[LGT]

Whoa, this is gonna be sweet! Do you have any idea how it's going to sound?

[GregC]

I don't know enough about amp building to ask a good question. But I really like the looks of this so far.

[KramerBassFan]

Probably like a Marshall, except a bit more gain.

[My name is Mudd]

Sub'd...how much power?

[KramerBassFan]

The Univox 1236 was labelled as 50w (well 100w, but thats "Peak music power" ) And the dual 6L6GC's could definitely do 50w, but i'd imagine with the old style (Read: underdesigned Univox) power supply it'll probably be closer to 40-45w. Still plenty of headroom for what it will be doing.

[knigel]

Nice! Looking forward to progress pics/sound clips. I'd love to build an amp someday, but I have a feeling I'd end up like this guy:

[JimmyM]

you're not done yet? let's go, dude...chop chop! we're not getting any younger here!

[KramerBassFan]

Yeah yeah, i'm still waiting for all my caps, resistors, pots, and jacks to get here.

[faceinbass]

Originally Posted by KramerBassFan

The Univox 1236 was labelled as 50w (well 100w, but thats "Peak music power" ) And the dual 6L6GC's could definitely do 50w, but i'd imagine with the old style (Read: underdesigned Univox) power supply it'll probably be closer to 40-45w. Still plenty of headroom for what it will be doing.

Awesome man! I can't wait to see how this turns out!!!

fwiw I think Univox Bass amps are total sleepers and am glad they can be found for cheap. I have 2 of them and love them both.

[JMac4strngr]

Very cool. Sub'd

[MAMMOTHvolume]

Originally Posted by KramerBassFan

Welcome all!

This is a build i've had in the works for awhile now.

A salvaged power amp from an old Univox 1236 with re-done power supply, and the Transformers from a Univox.

BUT, the Preamp is totally different.... I'm using 6AC7's. No, I don't mean 6CA7's, i mean 6AC7 the old school high gain eight pin tube that was used in WWII aircraft radio's. Best part is NOS 6AC7's are $3 a pop.



Also going to have orange tolex, black metal faceplate, and my signature, an absurd number of strategically placed handles. No one strap handle on top of an old 70lb Amp here!

Enjoy!



Handles:


Recessed handles and chassis


Chassis with turret board and tape, ready to be marked out and drilled.

The three tubes:
I'm using three 6AC7's, one as the first gain stage, one as the Treble boost/cut, and the third is the Phase Inverter. (Five pictured because I bought five just to be on the safe side)

And my two coke bottle Chinese 6L6GC's.




I also found a stash of Thomas Organ Solid State VOX Era style knobs out in the knob bin that will be used on this thing.


Questions? Comments? Flames? Post away!

Thanks for reading.

just wondering what phase splitter your gonna use? also what're microphonics like with the 6ac7s?

[KramerBassFan]

Originally Posted by MAMMOTHvolume

just wondering what phase splitter your gonna use? also what're microphonics like with the 6ac7s?

I didn't design the preamp, but it should be one of the 6AC7's.

Microphonics? I'd imagine some of them are microphonic, a lot of metal can tubes are. But thats why i got a bunch, and they are so cheap anyways i can keep rolling until i find a good one or two.

[BassmanPaul]

There was a thread on the 6AC7 a little while ago. It's a sharp cut off pentode. While it does show up in my old RCA tube manual there's very little actual information. I would not expect it to be suitable as a phase slitter. Before you go too far with the design process I would breadboard the input stage and see how it responds. I also would limit the B+ supply to around 200V.

[KramerBassFan]

Will do - thanks. Like I said, i didn't design it, i just offered to be a guinea pig and build it. I'll let the friend of mine who designed it know that.

These help any?
http://www.shinjo.info/frank/sheets/093/6/6AC7.pdf
http://www.shinjo.info/frank/sheets/127/6/6AC7.pdf

[BassmanPaul]

They recommend a DC supply for audio use. That could be achieved using the TO3 version of the 7805 regulator with a couple of diodes in the ground path to lift it by 1.2V. That will give 6.2V out which is close enough.

[KramerBassFan]

Once my rather large Mouser order gets here, i'm going to go on and load up the turret board before i drill the chassis any.

I also sent you a PM - tell me what you think, if you don't mind.

[faceinbass]

Please post more pics as you go!

[KramerBassFan]

I ran the PI scenario by the fellow who designed it, and he said that:

As far as the phase inverter goes, if you remember some of the original conversation we had I had questioned using the 6AC7 as a PI. I poured over a bunch of data sheets to compare values and decided it would probably be suitable. Here's why, common PI tube choices include many of the 12A*7 family. A 6SJ7 triode strapped has very similar characteristics to a 12AU7.
NJ7P Tube Database Search
NJ7P Tube Database Search
You'll note the 6AC7 is listed as a substitute for the 6SJ7. It's higher gain, which we want.

So, according to the sheets, if we Boot strapped the cathode PI to boost the gain, then a 6AC7 is similar to a 12au7.

Any other thoughts?

[KramerBassFan]

All here! Chassis almost done being drilled, turret board getting loaded up as we speak.