Airline 9028A project progress report

[j_bruce]

So the parts I needed came in and Today I started getting this thing done.

I took out the old stuff:

https://drive.google.com/open?id=1-Low6SYg2JzhMGmJuX8A1Wt0SBtIzqct

Put in the isolation transformer, a main power switch and a fuse holder, I will have to remove and reinstall the fuse holder to mount it but have not decided exactly where yet.

https://drive.google.com/open?id=1kELV-P4477V6KZm4Ch1STy2JQ6-ERCAu

Put in the new caps and removed the dangerous stuff:

https://drive.google.com/open?id=1cZMhWDG0BT4CGDP1hnCfTxMcsvdlMBOS

https://drive.google.com/open?id=1p0H5_vT3o_MYBZz_wtBFcZaqGv8XE2kx

I then plugged into a dim bulb and found that there is no current with the main power switch on and the original power switch off. Good news.

But I also found that the PT is reading 140vac on the secondary with no load. Should I be concerned about that?

Refresher, schematic the only thing not to this schematic (barring an error on my part) is the original switch(pot mounted)
 is still in place:

https://drive.google.com/open?id=1VNwLP_LHKNTMti_QPjzhmjXjtZSOJNNm
 
I have to go to work for a while now so I won;t be able to go any further until later tonight.

Cheers!
Jerry

[sluckey]

But I also found that the PT is reading 140vac on the secondary with no load. Should I be concerned about that?

Keep it in the back of your mind and recheck after you have everything hooked up and the amp is working.

[66Strat]

Put in the isolation transformer, a main power switch and a fuse holder, I will have to remove and reinstall the fuse holder to mount it but have not decided exactly where yet.

https://drive.google.com/open?id=1kELV-P4477V6KZm4Ch1STy2JQ6-ERCAu

I have to go to work for a while now so I won;t be able to go any further until later tonight.

Cheers!
Jerry

From the picture, it looks like you may have the switch configured before the fuse. The fuse should precede the switch.

[j_bruce]

From the picture, it looks like you may have the switch configured before the fuse. The fuse should precede the switch.

Thanks for noticing, I will get that right when I disconnect and mount the fuse holder.
Jerry

[j_bruce]

Well it works!

has a heck of a 60 hz hum though. I tested the frequency with my guitar tuner app 60.502 hz!.

any way, I plugged in a squire strat that I have here at the house and it sounds damn nice on the center pick up and the hum goes away, it is dead silent? on any other pick up you get some amount of the hum. No change in the hum when you touch the guitar or release it.

With no input it hums like the dickens, and the hum changes when you touch the chassis.

I am going to read up on the troubleshooting pages here and then take another look in the morning.

At this point I am calling it a win, learned a lot already.

Cheers!
Jerry

As a side note I bet this thing will sound really good with my les paul!

[sluckey]

With no input it hums like the dickens, and the hum changes when you touch the chassis.

That's likely because the cheap input jacks do not ground the input when nothing is plugged in. Can you kill the hum by turning the volume pot all the way down?

[j_bruce]

With no input it hums like the dickens, and the hum changes when you touch the chassis.

That's likely because the cheap input jacks do not ground the input when nothing is plugged in. Can you kill the hum by turning the volume pot all the way down?

Funny thing, the input jacks do not ground the input when nothing is plugged in so you nailed that, But the volume knob I just discovered has NO EFFECT on the volume. NONE>

Now the schematic shows one side of the pot connected to the input, the other connected to ground, and the wiper is the signal out to the pre-amp tube. the thing that bothers me is there in no connection on the third leg that should be going to ground, and the meter reads OL when I check that leg to ground no matter the knob position. The third leg of the pot looks like it has never been connected to anything.

The input leg shows 100k to ground no matter the position of the knob, 100k seems like the guitar, so when I disconnect that it shows me OL.

I checked the voltages as shown on the original schematic and they are all close, within a couple of volts so I can't be too far off.

I am skeptical of the pot at this point but also feel like with the third leg not grounded I will get no change in signal to the pre-amp because all the pot is doing (if it is working right) is changing the input impedance and not dividing the signal.

Jerry

[sluckey]

Pot third leg must be connected to ground to work properly.

[j_bruce]

I found a bad solder joint, look at the pot end of the signal wire here:

https://drive.google.com/open?id=1p0H5_vT3o_MYBZz_wtBFcZaqGv8XE2kx

Also the electrolytic had a questionable solder joint at the pot end and made noise if I moved it with a stick.

got those fixed up and that is now much better, and the volume is working like it should.

Still has a bit of hum but not as bad and it moves with the volume now. I unplugged the guitar and grounded the input and it's all dead silent.

I am going to swap out the pot and eliminate the switch on the pot, but will have to order one and wait for it to show up.

Cheers!
Jerry

[66Strat]

Why not use the switch that you have in this picture?

https://drive.google.com/file/d/1kELV-P4477V6KZm4Ch1STy2JQ6-ERCAu/view

[j_bruce]

I have been waiting for a new volume pot to arrive for this thing and reading a variety of pages on related topics including one linked to on a thread here about grounding by valvewizard. That is a very nice page, moved on to read more and found his page on sources and cures for hum.

One of the primary hum sources he mentions is heater hum caused by high heater voltage. So I got to checking and the 50c5 calls for heater voltage of 50vac, I measured 60.2vac clearly borderline in respect to valvewizards advice that heater voltage should be maintained within +-5% for minimal hum. So clearly I need to address that. Then I recalled that our power company which is a small municipal utility (and thus in Michigan is largely unregulated) is know for very high voltage mains. I checked it to be 127.2vac last night, on the secondary side of the isolation transformer I got 137vac.

So clearly I have some work to do on the power supply side of this thing. It currently has a 100 ohm 5 watt power resistor in series with the rectifier tube I would like to have that resistor receive 120v when the supply is 120v.

My game plan is:
 1.  borrow a variac from work and dial the voltage down to 120v since in most places I would not have the very high voltage that is found here.
 2.  Measure the current at the power resistor.
 3.  compute the resistance needed to get 120v at the power resistor and install that resistor in series.
 4.  check heater voltage (and hum) again, if still out of spec place a resistor in series with the heater to adjust so that the heater is at 50v.
 5.  add a virtual center tap to the transformer

Still waiting on the volume pot. . .

re: 66Strat I planned use that switch and eliminate the pot mounted switch.

[PRR]

> mains. I checked it to be 127.2vac last night, on the secondary side of the isolation transformer I got 137vac.

Turn the transformer around.

[tubeswell]

The fact that it was dead silent on your guitar’s middle pickup, means that it is signal hum.

[j_bruce]

> mains. I checked it to be 127.2vac last night, on the secondary side of the isolation transformer I got 137vac.

Turn the transformer around.

I might just do that, thanks for pointing that out as an option.

[j_bruce]

The fact that it was dead silent on your guitar’s middle pickup, means that it is signal hum.

Well that should have been obvious to me! Thanks for the insight.

On the bright side, I swapped out the volume pot for the new one that came yesterday, grounded it properly with the input jacks, replaced the #1 input jack with a switching jack so it is grounded with no cord plugged in and bypassed the (now gone) switch on the volume pot.

Now the darn thing works pretty well!

I think the pot was a big part of the problem. it gave me odd readings on the ohm meter at all dial settings.

There is still a slight 60 hz hum all the time now, but the bad hum is now gone. It is entirely usable as it is at this point. Thanks to all for the helpful advice. I learned a lot on this little project.

Cheers!
Jerry

[j_bruce]

I got together with the owner of this little amp last night and his daughter brought along her vintage Epiphone Crestwood guitar to test this amp out a bit. He brought a Crate 12" cabinet with a Celestion speaker and the little amp sounded even better with this speaker than the one I had tested it with. When cranked up it is a tone monster! Crazy good tone and smooth crunchy drive. I was amazed at the volume and tone coming from that little amp.

For 3.5 watts it sure surprised us all.

Which leaves me wondering where all that tone was coming from? I may have to build another one from scratch and play around with it.

I also tried it with harp but did not get the drive to come through. I was using am sm57 with a passive impedence matching device and got a lot of feed back unless I kept the volume very low or stood very far away. when I stood on the far end of the room and cranked it all up still not much drive, great clean harp tones though.

I have a feeling that this amp will get some game time in this young ladies future she played for an hour on it just trying out different settings on the guitar.

If I see any Airline amps for sale around here I will grab them and give them a shot. We decided that running that amp through the cheap 8 inch speaker that probably came in it new was a dis-service to the circuit and is the likely reason nobody like these.

[shooter]

running that amp through the cheap 8 inch speaker

I consider the speaker 50% of the amps "sound", I typically run mine through 3 different speakers n cabs before I start tweaking parts