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First-time DIY scratch amp is done! PICS!!

Thanks again for the kind words guys, I sure appreciate it!

Hey, hey Craig!

Whaddaya know, I DID it! Firing that thing up and hearing music come out of it was about the proudest I've been of myself since college graduation day.

You don't need those specs of yours to see what you're seeing-there are a few mechanical connections. If you need a closer look though, you can click on the thumbnails for a closer view-but even then the images aren't huge.

I went back and forth on the barrier strip for the pwr supply-I don't like the idea of stranded wire anywhere, and the 10 or 12 gauge secondaries of the filaments were also big enough to be a pain to route cleanly and solder to the tube pins.

In the end, neatness, terminating and keeping the standed wire out of the circuit trumped soldered connections-but I appreciate the advice and will keep an eye on them. I'm using an old severly mis-matched quad of red-headed-stepchild 7591s that are mismatched enough to not quite balance perfectly even at the end of travel for one of the DC balance pots.

I'll be ordering some new 7591s next week so when I re-balance I'll be able to tweak those connections.

Thanks for taking a look Craig, means a lot to me.
 
hi mojo,
i see you finally got the amp all finished up and running.now go grab yourself a handful of cd's sit back and enjoy the fruits of your labor.it's already been said but ill say it again that amp looks beautiful especially the wiring :thmbsp:
chris
 
jcmjrt,

Thanks to you as well for the kind words and taking some time to have a peek; I appreciate both.

Heya Chris,

See, I told ya I really WAS building an amp! :thmbsp: Thanks for stopping by to take a look, you know it's appreciated.

And speaking of projects, when are we gonna see that Fairchild dual mono pre-amp lighting up a dark room?

Best,
John
 
And speaking of projects, when are we gonna see that Fairchild dual mono pre-amp lighting up a dark room?
Click to expand...
john,
hopefully soon.as you may have seen from some another post titled"i hate cleaning"i finally got around to organizing and making some room.i will now be able to set up a small table and dig into the fairchild.so hopefully not to far into the distant future ill have it up and running to show off to my fellow ak'ers.i will keep you posted on the progress.
chris
 
Wow!

Fantastic work and your attention to design details is commendable! You will have a lot of enjoyable hours remembering the work as you listen to your favorite cuts. :thmbsp:

Paul
 
Man o man, that amp is gorgeous! I'm just getting together the info, knowledge, and equipment to tackle my first his winter. It won't be anything like yours, much simpler SE amp, but after seeing this I'm jonesing big time to get started.

Shane
 
mojojojo said:
Well the DIY amp is finally done!
Click to expand...

Let's see, which one inspires confidence in the builder -

This one:

insideampRTchannelCU.jpg


Or this one:

P1050012.JPG


Found here:

Morrow Audio

Let me think about that and get back to you.

:scratch2:

Really nice work for your first time out!

:thmbsp:
 
Bump!

I think this spectacular DIY project needs to be revisited. Workmanship like this bares repeating IMO!

My dad can build shortwave tube receivers of this same calibre...alas, I do not poccess such skills but I wish I did.

Magnificant work, John. Have you obtained the new 7591 quads yet?
 
Having just bought my first piece of used tube gear, a factory-assembled Grommes 502M receiver I plan to work on myself, I am in awe of this project. Thanks for re-linking to it; it's inspirational.
 
I'm curious mojo... did you design your amp from scratch or did you start with another amp and improve on the design?

Scott
 
Gosh, thanks for all the kind words guys-much appreciated!

1stcav,

I did indeed get the JJ 7591s-they are a fine tube. After about 40 hours of burn in time, they now hold cathode bias exactly at .899-.9v. AC balance is steady at 485v.

In the meantime, I've dressed and trimmed the speaker connections-in the previous pix they were left long because I wanted confirmation from Heyboer which colors matched which secondaries before cutting to fit. They are now short and neat like the rest of the circuit.

I also changed the dropping resistors to 680ohm 25 watt to get my B+ down to 485v since it was initially @ 520. Ouch!

I may replace the Spragues on the 7591 plates with some solens since the Spragues are rated for 500v and the solens are rated at 630v.

Other than that, no problems to report! Sound is great, better than any tube amp I've heard so far. I recently bought a digital O'scope since my freebie Knight-kit scope bit the dust and hope to take some measurements and post some more pics.

Craig,

This project would never have happened without your time, patience, knowldege and help. I am certainly in your debt!

I recently took the amp to a local shop which sells moderatly expensive gear. One of the salesman is a friend and fellow vintage tube amp enthusiast.
We AB'd the amp against some pricey SS stuff and used a pair of Magnepans for the comparison. The amp managed to (subjectively) beat the SS stuff all across the board, but most surprisingly in the low-end; better control, deeper response, just fuller, beefier, tighter bass.

Scott,

The layout and implementation of the circuit is an original design, but I didn't create the circuit itself-that came from an old issue of Electronics World.

The idea of striving for neatness and right angles came from looking at old military/ham gear my great uncle owns.

Mainly it was form following function: I knew I wanted a symmetrical layout on top and I wanted to make all connections with componant leads. Other than B+, I came close! But between those two considerations the layout for the amp presented itself through trial and error and experimentation.

The ground however was a solution that evolved over time. In the end I opted for a "true" star ground with every single grounded componant having its' own dedicated ground wire running back to chassis ground to minmize any chance of ground loops.

I read somewhere that the true test of amp layout is to not have to use shielded wire to avoid hum and interference. I think I did OK since I didn't use any shielded wire anywhere and I've got zicho hum or interference.

Then again, if it was just beginner's luck-which it may very well be-I'll still take it!

Wornears,

I myself, just as many others before me, started with a restoration-a 299C to be exact! I then restored a 299B and lastly a 296 with-once again-a great deal of help and patience from that "all-around-smart-guy, helpful-son-of-a-gun" Craig (NOSvalves)! Funny how his name pops up so much!

If I didn't know any better, I'd think that guy may actually know what he's talking about! :thmbsp:

Good luck with your resto: Keep the forum posted, there's so much combined knowledge and friendliness that you're almost guaranteed to get your amp up and running in no time.

Beware: once you do-you're hooked! But then, that's kind of the point... :D

Best,
mojo
 
Thanks for bumping this to the top, I didn't see it first time around.
That's amazing. It's a piece of functional art. :yes:
 
How's the old girl doing these days?

Any changes?

Still looking great and your awsome workmanship still holding up?

Great looking work and piece!
 
mojojojo said:
Gosh, thanks for all the kind words guys-much appreciated!

1stcav,

I did indeed get the JJ 7591s-they are a fine tube. After about 40 hours of burn in time, they now hold cathode bias exactly at .899-.9v. AC balance is steady at 485v.

In the meantime, I've dressed and trimmed the speaker connections-in the previous pix they were left long because I wanted confirmation from Heyboer which colors matched which secondaries before cutting to fit. They are now short and neat like the rest of the circuit.

I also changed the dropping resistors to 680ohm 25 watt to get my B+ down to 485v since it was initially @ 520. Ouch!

I may replace the Spragues on the 7591 plates with some solens since the Spragues are rated for 500v and the solens are rated at 630v.

Other than that, no problems to report! Sound is great, better than any tube amp I've heard so far. I recently bought a digital O'scope since my freebie Knight-kit scope bit the dust and hope to take some measurements and post some more pics.

Craig,

This project would never have happened without your time, patience, knowldege and help. I am certainly in your debt!

I recently took the amp to a local shop which sells moderatly expensive gear. One of the salesman is a friend and fellow vintage tube amp enthusiast.
We AB'd the amp against some pricey SS stuff and used a pair of Magnepans for the comparison. The amp managed to (subjectively) beat the SS stuff all across the board, but most surprisingly in the low-end; better control, deeper response, just fuller, beefier, tighter bass.

Scott,

The layout and implementation of the circuit is an original design, but I didn't create the circuit itself-that came from an old issue of Electronics World.

The idea of striving for neatness and right angles came from looking at old military/ham gear my great uncle owns.

Mainly it was form following function: I knew I wanted a symmetrical layout on top and I wanted to make all connections with componant leads. Other than B+, I came close! But between those two considerations the layout for the amp presented itself through trial and error and experimentation.

The ground however was a solution that evolved over time. In the end I opted for a "true" star ground with every single grounded componant having its' own dedicated ground wire running back to chassis ground to minmize any chance of ground loops.

I read somewhere that the true test of amp layout is to not have to use shielded wire to avoid hum and interference. I think I did OK since I didn't use any shielded wire anywhere and I've got zicho hum or interference.

Then again, if it was just beginner's luck-which it may very well be-I'll still take it!

Wornears,

I myself, just as many others before me, started with a restoration-a 299C to be exact! I then restored a 299B and lastly a 296 with-once again-a great deal of help and patience from that "all-around-smart-guy, helpful-son-of-a-gun" Craig (NOSvalves)! Funny how his name pops up so much!

If I didn't know any better, I'd think that guy may actually know what he's talking about! :thmbsp:

Good luck with your resto: Keep the forum posted, there's so much combined knowledge and friendliness that you're almost guaranteed to get your amp up and running in no time.

Beware: once you do-you're hooked! But then, that's kind of the point... :D

Best,
mojo
Click to expand...

Mojo,

I missed this post all together. Thanks for the kind comments but really it was my pleasure.

I do however have a few question about your bias setup and AC balance.

What value cathode resistor are you measuring to bias to .9V ?? These seems scarry high for a single 7591.

You mention AC balance in DC volts? AC balance has nothing to do with DC volts. AC balance is the AC signal produced by each output tube for the channel (which should be equal for lowest distortion). You check this with a function generato and a scope at near full power. You can do it either entering the grid of the output tube or at the plate of the output tube. Its best to check it at various frequencies.

Craig
 
Hey Kegger!

Well, it's working great-but boy did I have one HECK of a time chasing down a gremlin or two!

There's been a few changes. I'll try to post a few pics in the next day or two.

First: The single 40uf 500v Sprague filter caps on the plates of the outputs have been swapped for a pair of 100uf 450v Sprague caps in series, with 200k voltage dividers in parallel. Schematic lists 500v caps there with 475v on the plates-probably a solution looking for a problem, but running 25v (probably a bit more, the Spragues are pretty conservative) below max rating makes me nervous. Now with a over-rating of 425v, no more worries! :thmbsp:

Next up: Bias. The circuit creates bias by tapping off the HV secondary and dropping voltage with a coupling cap in series with the HV before going to the diodes. Article suggests using a TV transformer (which most likely doesn't have a 50v bias tap-hence the solution above).

When I had the transformer built, I figured there was some kind of special reason for doing it that way. If I knew then what I knew now, I would've just included a 60v bias tap! Oh well, live and learn... :D

At any rate, I couldn't find any kind of coupling cap that would survive for long in that circuit-even 3000VDC/1500VAC rated caps! I got a crash course in dissipation factors and eventually scrapped the caps completely and installed a small 120VCT 100ma bias transformer instead.

That seemed to solve the issues I was having, but the amp would still behave erratically-voltages and current levels never seemed to land in the same place twice... :scratch2:

After double checking and triple checking the schematic against the build, touching up all my solder joints, checking caps and resistors I was just plain befuddled. I was frustrated because every time I ran it up on the variac it would work just perfectly. Plug it in the wall and who knew what the voltages would measure.

Seems obvious now, but I suddenly realized the issue may not be the amp at all-so I checked the wall voltage: 142 volts!!! :yikes:

There was the problem-most likely the issue all along, the caps in the HV secondary for bias were never a great solution, but they most likely would've worked fine at a 120v. Now I've been using the amp with my metered variable AC power supply and things are just peachy. An electrician is coming tomorrow to fix the unbalanced common leg in the building, but I think I'll continue to use the AC power supply anyway.

One thing is for sure: This has been the BEST!!! education for tube circuits I could ever imagine-experience really is the best teacher!

Best,
mojo
 
Heya Craig-nice to hear from ya! Hope things are going well for you.

Each 7591 has a 27ohm cathode resistor at the test points for DC balance:

.9/27=33ma per tube.

You're spot on about the AC balance thing, I've got test points on each plate of the output tubes and a trimpot on the plate of the triode section of the phase-splitters. AC balance is achieved by adjusting the pot until I read equal plate voltages in the 7591s. So it's kind of a "back-door" method for testing and adjusting for equal AC. :thmbsp:

Always a pleasure and an educational experience Craig-I always look forward to your posts and your feedback.

Take care,
mojo
 
Mojo,

Okay the designers were using a fairly high value cathode resistor to add some feedback. So your bias figure is correct good.

Again though you have the AC balance thing really confused. The AC balance has nothing to do with balancing the DC voltage on the plates. This setting has to be done with a scope and function generator. You can kind of get it close using a test tone CD and then measure the AC volts on the plates and set them to an equal amount. That is why it is so appropriately called "AC Balance" :scratch2:

I still say that amp is one of the most professionally looking builds I have seen out of a DIY'er then factor in it was your first attempt! Another big pat on the back is deserved.

Craig
 
Wornears said:
Having just bought my first piece of used tube gear, a factory-assembled Grommes 502M receiver I plan to work on myself, I am in awe of this project. Thanks for re-linking to it; it's inspirational.
Click to expand...

Indeed inspirational as I await the parts to rework my preamp. I have been considering a rebuild of 1 of my 2 Conn organ tube amps and that will now be my next project. Superb work and craftsmanship. I hope my projects look half as good as yours.

mac
 
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