Kay 803 Tube Confusion & Ground Issue

[plexi50]

I have a Kay 803 amp that is using different tubes than what is on the only schematic i have been able to find.Amp works fine and was silent until i added a 3 prong cord ground to the chassis. Lifted the ground and silent and working again.
My tubes are #1 (36AM3)  #2 (60FX5)  #3 (18GD6A) / Not like the Kay 803 schematic at all.
Kay schematic shows 12AU6 / 50L6 / 35Z5. This amp has an isolation transformer. OT is mounted on the speaker.A few pic's.The PS capacitors were in the amp as i opened it up.
I found this schematic on this forum yesterday. Again different tubes.
 

[plexi50]

Model 803C

[tubeswell]

The amp in the schematic is 'optimised' for 120VAC wall voltage (you can work it out by adding up the heater string i.e.: 12.6V + 35V + 50V across the tubes and 22.5V across the 150R resistor - with 150mA being drawn through the string.

Your amp's tubes drop 114VAC on the heater string on their own. So if you assume a 120VAC wall supply, you'd be looking to add a resistor that gives you a 6V drop in series with that string (which I guesstimate to be about a 60R resistor - assuming 100mA* draw). Or, if you've got 115VAC at the wall, then a 1V drop would be needed across the resistor, which is like a 10 Ohm resistor). YMMV


*i.e. the draw of an 18GD6A https://frank.pocnet.net/sheets/049/1/18GD6A.pdf

Whatever you've got at the wall (that is, at your isolation transformer secondary), I think it'd be preferable to have some additional series resistance in the heater string to add a bit of current limitation, as long as your overall heater current is within the ballpark of what your tubes are designed to draw.

[tdvt]

That is a departure from my norm, especially the heater setup. I will have to try to digest Tubeswell's explanation.

Wondering about the added hot/neutral labels in red pen..? Is hot on the chassis ground? Seems like they should be swapped.

[plexi50]

To be honest i don't even know what i am looking at here. Nothing makes sense.The power cord line is going to Pin 6 (36AM3) Rectifier Tube.I need to take some better pictures of the power cord going to the tube.

[AlNewman]

It's like an all american 5 radio, except luckily you have an isolation transformer.

The heater string is in series, using tubes that all run on the same current.
The Kay amp heaters run at 150 mA current.
The Kay schematic uses 12v + 50v + 35v + 150R resistor.  The resistor drops 22.5V @ 150 mA.

Your amp uses 36v+60v+18v which equals 114V
Looks like Tubeswell has done his homework, and your tube heaters are drawing 100 mA.

The Idea is to match your voltage at your isolation transformer.  IE...Your resistor should drop 6V/.1 mA = 60 ohms to match your wall voltage if it is 120V.

It sounds like as added protection, you could add additional resistance as a safety buffer.

I've rebuilt radios with likely 5-10V extra wall voltage than designed without changing any values and have not had any problems...yet...

[plexi50]

You know what they say! You learn something new everyday! I haven't seen many of these isolation transformer amps over the years.Only a few. But i never have had a problem where adding a grounded cord to the chassis would cause it to hum.Thankyou all for your help and advice.We all want to get things right and improve on them.

[Carlsoti]

I've got a Kay 703C with the same tube compliment and arrangement. It's a widowmaker. They take a little bit more than just a 3-prong to get them "safe."

FWIW, that little transformer listed as "Isolation transformer" is NOT what people are talking about when they say it needs an isolation transformer. This little x-former was the cheapest UL workaround at the time. A "real" isolation transformer goes in between the wall and the amp.

After futzing with my Kay for who knows how many hours, I've come to the conclusion that it's better to have a big isolation transformer on hand, and a clearly marked plug that shows hot/neutral mains to make sure it's plugged in the right way, running it more-or-less as designed. I'm all about finding hidden treasures, but as I gain experience, I find some holes aren't worth digging. I have yet to find a widowmaker that's worth "cloning", though many do have a few interesting tones.

[plexi50]

Look at what i found!!! Yes some amps are a PITA Period. Like this Marshall 6100 i have here now. All new caps and still no sound. All voltages correct. I guess pain and misery is what drives me. No not really, But there is a breaking point to.
So i finally found a schematic with the tubes that are in this 803. I have found 2 different 803 models so far. Mine is a 803C model from what i can see. C?

[plexi50]

Another Kay 803 Model???

[plexi50]

Oh boy!

[AlNewman]

I've got a Kay 703C with the same tube compliment and arrangement. It's a widowmaker. They take a little bit more than just a 3-prong to get them "safe."


FWIW, that little transformer listed as "Isolation transformer" is NOT what people are talking about when they say it needs an isolation transformer. This little x-former was the cheapest UL workaround at the time. A "real" isolation transformer goes in between the wall and the amp.

You're right, my bad I only glanced at the schematic, assumed since it had a transformer the heater string would be supplied from it.

It makes sense that grounding the chassis would create hum.  You could run safety caps from wall leads to chassis which may reduce hum.

But I agree, it isn't something I'd plug a guitar into they are okay with radios because everything is normally insulated from the user by the case.