Cap value question, and perhaps other questions from a beginner.

[j_bruce]

So I recapped a Kalamazoo Model two a few years ago to use as  harp amp and have been learning about tube amps in general since the lock down stuff began. I have a decent background in physics (BA) and took some instrumentation courses in undergrad which did all the same types of circuits that I see here except with transistors!

So anyway, a friend gave be a garage sale Airline 9028A chassis to play with. I found one schematic online and that is barely readable. I decided to redraw it and put in the values as best I could and then check against the actual chassis (which has been worked on ??) to give a starting point for trying to make this thing go.

Problem is the power filter caps are specked out as 40mF @ 110v but the chassis has shiney new 470mF @ 450v. I know that the voltage rating is no problem, what about the capacitance?

Can someone point me in the right direction? Happy to read an learn so a link to an article is fine.

jerry

https://reverb.com/item/8404631-airline-tube-amplifier-ghc-9028a-8-speaker-similar-to-harmony-mid-60-s-works is the schematic if that is of interest. Mine is almost done.

[sluckey]

...the chassis has shiney new 470mF @ 450v.

Really? I'd like to see a picture of that.

That big red firecracker in the reverb.com link you posted has two caps in it, 47µF and 80µF, both rated at 160V. That's a perfect replacement for the original.

You can buy that exact cap at AES...

     https://www.tubesandmore.com/products/capacitor-ce-mfg-160v-4780-f-electrolytic

[j_bruce]

Really? I'd like to see a picture of that.

OK, but for the record, I had the voltage wrong they are 250v:

https://drive.google.com/open?id=1iFoITZlzAxRCg2nSrti4zk7EDomspqWu

more:
https://drive.google.com/open?id=1ZciAT1Zk7BjPl4HwN8Cw1InQBo_ullhE

That big red firecracker in the reverb.com link you posted has two caps in it, 47µF and 80µF, both rated at 160V. That's a perfect replacement for the original.

You can buy that exact cap at AES...

     https://www.tubesandmore.com/products/capacitor-ce-mfg-160v-4780-f-electrolytic

thanks for that!

jerry

[sluckey]

That's quite a repair job on that amp. That makes a killer amp even more dangerous.   

[j_bruce]

That's quite a repair job on that amp. That makes a killer amp even more dangerous.   

I thought I might take a bit more care on my rebuild.

[Johnt123]

You realize that's a hot chassis amp. You could add a power transformer and rewire it. I think you could likely get tubes that would work in those sockets. 6X4, 12AX7, EL84.

[PRR]

> Airline 9028A

Hot chassis !!! Deadly !!!

You will live, if you get the plug right way, or on a dry floor and no other metal within reach. So many people were seriously shocked by such things that *this* model has two half-assed "safety improvements". (Even so, the whole genre was banned in a few years.)

The only thing saving Mr Dale from possible death is C3. Which was lowest-bid 56 years ago, absolute cheapest wax. WHEN C3 goes short, every gig is Russian Roulette. May as well stick silverware in the outlet.

[PRR]

> tubes that would work in those sockets. 6X4, 12AX7, EL84

12AX7 and EL84 are 9-pin not 7-pin.

The original tubes are NOT hard to get. Often cheaper than their transformer-chassis friends. And a AU6+C5 amp is a change of flavor from the usual run of yet-another-EL84 amplifiers. But as you say the "HOT CHASSIS" death-trap aspect has to be addressed. Generally: add a proper 120V isolation transformer.

[j_bruce]

> tubes that would work in those sockets. 6X4, 12AX7, EL84

12AX7 and EL84 are 9-pin not 7-pin.

The original tubes are NOT hard to get. Often cheaper than their transformer-chassis friends. And a AU6+C5 amp is a change of flavor from the usual run of yet-another-EL84 amplifiers. But as you say the "HOT CHASSIS" death-trap aspect has to be addressed. Generally: add a proper 120V isolation transformer.

Thanks for the fun graphic!

I was actually a little concerned when I noticed that the black wire from the wall went to the ground side of the instrument jack (the pink path in your graphic).

What do you have against Dick Dale?

I have another chassis to play with, I will take a look at it and maybe it will be a better long term project.

Cheers!
Jerry

[shooter]

I have another chassis to play with

a $30 part and you can play with.. the one you have 

[echuta13]

The only thing saving Mr Dale from possible death is C3. Which was lowest-bid 56 years ago, absolute cheapest wax. WHEN C3 goes short, every gig is Russian Roulette. May as well stick silverware in the outlet.

I thought he already shed his mortal single-coil... Too soon?!?

[sluckey]

I thought I was the only one with a yellow strat! 

[PRR]

> What do you have against Dick Dale?

"Guitar player" in Image Search, he was a top hit, AND he was a lefty which made the cord come right for a short connection to the schematic.

[shooter]

he was a lefty

so if a lefty and a righty play through the same deathtrap amp is the neighbor safe 

[sluckey]

Nope. Gonna change the world!

[dude]

Hey, is that duck tape on that amp above with the "yellow" strat ....?


Dam, last time I saw an amp like that was in Jr High, maybe the 8th grade...
Must be a Silvertone..

[shooter]

Gonna change the world

hellava job, world 3.0 welcomes you, please don't hold my hand!!! 

[sluckey]

Hey, is that duck tape on that amp above with the "yellow" strat ....?

Nope. It's silver sparkle roll and tuck Naugahyde. Notice the matching strap.

Dam, last time I saw an amp like that was in Jr High, maybe the 8th grade...
Must be a Silvertone..

Good guess, but it's a Sunn Sonora, my first ever guitar amp. It's actually a bass amp. Nobody was cloning Silvertone in the '60s. You could buy the best Sears had for $149, even cheaper in a sales catalog.

[j_bruce]

> What do you have against Dick Dale?

"Guitar player" in Image Search, he was a top hit, AND he was a lefty which made the cord come right for a short connection to the schematic.

At least you used a renewable energy source to kill him!

[j_bruce]

I talked to the guy who gave me this to play with and he said he would buy the parts so go ahead and give it a shot. he wants to build a cab for it when it's working again.

So I guess it's game on.

[j_bruce]

Here is my version of the schematic redrawn as best as I could read it. There are a couple values that I could not read and I put in the isolation transformer.

any and all comments welcome. Disclaimer: I have never drawn a schematic before so if I did something wrong I would appreciate knowing.

https://drive.google.com/open?id=1VNwLP_LHKNTMti_QPjzhmjXjtZSOJNNm

Thanks
Jerry

[PRR]

Main change I suggest is, since all is now isolated, you don't want that barely-lethal cheat in the preamp feed, but connect the common through like an honest amplifier.

[PRR]

Here's my similar (and deadly) Kent for cross-check:

The 0.05u and 100K is drawn different but is topologically the same.

[j_bruce]

Main change I suggest is, since all is now isolated, you don't want that barely-lethal cheat in the preamp feed, but connect the common through like an honest amplifier.

PRR, I had just finished typing:

"I understand your suggested change.  but if I make the common that way, then I should probably move the switch to the other line on the cord? In household wiring the white is always the common and is the same potential as the ground. So the switch is always on the black, shouldn't I make this the same so that the commons and grounds are all the same potential?"

Then I realized that since the isolation transformer isolates the circuit from the ground once inside the common can be arbitrarily chosen by convenience and there is no worry about which side is the common. Unless there is something else that I am missing?

Feel free to teach me anything, I have very little experience with working on actual devices and designs and I am a good learner.

Thanks for the reply,
Jerry

[sluckey]

I touched up your drawing. Heavier ground lines for higher visibility, included the values for R1 and R9, and "Red X'ed" the two components to remove and replace with a wire.

BTW, nice drawing.

[66Strat]

If it were me, I would put a fuse on the hot leg of the AC coming in to the PT, fuse -> switch.

[Tone Junkie]

Jerry your in good hands to learn here . PRR and sluckey have forgot more about amps than I will ever know. On top of that the group here is very forgiving. Kindness and camaraderie are a staple at this forum.
Peace Bill

[PRR]

> In household wiring the white is always the common and is the same potential as the ground. So the switch is always on the black

In books, yes.

In my last house, 60% of outlets were white/black reversed, one switch was wired to TWO circuit breakers, and all the green wires were cut off.

If we actually KNEW with live/die confidence that 'polarity' was correct it would not be horribly dangerous to omit the transformer. But we don't know. And it is not good odds.

Therefore it must NOT matter if you put the switch and fuse in any specific color wire inside your amp, because you don't know where they really come from.

In restoration, leave it the way it was, don't make arbitrary changes. If restoration extends to major rip-out, make-new any way looks good to you.

> white .... is the same potential as the ground.

No. "More near", not "same as". I can show you 3V of white-green potential from current-drop in my long neutral wire. That's not dangerous. But I have also been around neutral failures, major smoke-bomb. I do not think the white wire is safe.

[66Strat]

AC receptacle testers are readily available to test whether your wall sockets are wired properly.

https://www.homedepot.com/p/Power-Gear-3-Wire-Receptacle-Tester-50542/206212329

[j_bruce]

The parts came in, so time to get the workbench ready. I will start a new thread with progress pics and updates, failures, cheers and tears.

Jerry