Another question RE Kay 703 iso tranny

[chuggy]

The Kay 703 has no power tranny but does have an isolation transformer as shown in the schem below.  Looks like the iso tranny isolates the b+ circuit but not the heaters.  Is this sufficient for safety?  If not, can the heater circuit be moved to the iso tranny secondary?  

Why would the original design not put the entire power supply behind the iso tranny?  I have a stout replacement iso tranny that can be substituted if the original is not sufficient  to run the entire power supply.  

http://www.freeinfosociety.com/electronics/schematics/audio/kay703b.pdf

Thanks-
Jim R

[eleventeen]

What you have there is 98.5+% of the way towards the safety provided by a normal power trans. I wouldn't mess with it, myself. Put it this way; the only real danger from "line volts on chassis" would arise if one of the wires in the heater string physically broke away from a soldered connection and flipped over to touching the chassis. Possible, but **extremely** unlikely. What is the MTBF of a solder joint?

As to placing the heaters on the secondary, I suppose you could research (using part numbers and tracing to some mfr's specs) whether the tranny had the current capability to do that, but I'd be very skeptical right off the bat whether you could throw an extra 150 mils (= current rating of those heaters) onto a dinky little tranny without overheating it destructively. That would be maybe quadrupling, quintupling the current draw on it. Not a good bet, IMO. OTOH, if you're a big fan of power supply "sag" you could probably induce a world-class case of it, LOL. Well, at least until the smoke starting coming out.

[jjasilli]

The standard solution solution is to pjut abn isolatioon tranny in front of the PT.  There are numerous old posts if you search the forum.   

[phsyconoodler]

"pjut abn isolatioon"

Wow! I'm learning a new language!

[jjasilli]

Sorry, I oftern post while at work, and am a poor proofreader.  Firefox has a spell checker, but it runs slow on my computer.  This is something like plexi50's barley post!   

[PRR]

> the only real danger from "line volts on chassis" would arise if one of the wires in the heater string physically broke away from a soldered connection

OR... internally shorted to cathode.

Which is a fairly common fault.

It's just a skim-coat of alumina clay.

> Why would the original design not put the entire power supply behind the iso tranny?

Plate power is 50mA. Heater power is 150mA. Which is cheaper: 200mA for everything, or 50mA for just the plate supply?

> can the heater circuit be moved to the iso tranny secondary?

If they cudda, they probably wudda... the safety advantage is obvious.

So I assume the stock transformer WON'T carry the whole load.

Don't fool around. Don't cheap-out. There are house-fire and self-electrocution risks here. This was cheezy for 1962, when we drove hi-lead cars with no seatbelts and wrapped pipes with asbestos. Today it is just unreasonably risky.

Use an iso-former. Leave T1 in there: it now does little good but does no harm, and it keeps the Vintage Kay as original as possible.

[chuggy]

Thanks all for the info.  

I am inclined to replace the original iso tranny with a stout one that will handle all of the current.  Is there any electronic reason (hum etc) to not connect the plate and heater circuits to the new iso tranny secondary?

I got around to looking at the my iso tranny- rated at .3A.  PRR assessed the current draw at .2A, but how?  Each of the tubes in this amp draws .15A, but they are in series so I guess, the heater current is..... .15A, right?  So, my .3A tranny is suitable (?).


Any particular considerations for installation of a line cord fuse?  Suggestions for fuse rating?

Jim R

[chuggy]

The schem shows one side of the iso tranny secondary that supplies the plate V is grounded to chassis while the heater supply connected to the iso primary is not grounded.   If a single suitable iso tranny is used with both the plate supply and supply circuit connected to the secondary, one leg of the heater circuit becomes grounded to the chassis.  Is this OK?

Kay 703 schem:
http://www.freeinfosociety.com/electronics/schematics/audio/kay703b.pdf

Thanks-
Jim R

[jjasilli]

It is not clear that the Kay PT is actually an isolation tranny.  Other schematics of that circuit, like my Alamo Capri, show it as a step-down tranny from 110 > 98 volts.  Sometimes a 1:1 tranny is loosely referred to as an isolation tranny.  But absent an internal Faraday shield, and high voltage ability to guard against failure, it provides no more isolation than an ordinary tranny.  It is doubtful that a bargain basement circuit built by a bargain basement manufacturer would spring for the extra cost of a true isolation transformer.  And the schematic does not draw the PT as an iso tranny with the Faraday shield.

The Kay schematic does show the heater circuit as ungrounded.  To be certain you need to check the actual wiring in the amp.  I suspect that the heater circuit is lifted from ground to keep 60 cycle hum out of the rest of the amp*, as you say.  So you should probably keep the lifted circuit.

*Because the heaters are in series, there is no humbucking effect to keep the 60 cycle hum out of the filaments, where it will induce itself into the signal path inside the tubes.  However, it works well enough.

[jjasilli]

It further occurs to me that the stock PT may be under-rated, have high DC resistance, and high induction under load.  And that the amp was designed with these "features" in mind.  This will tend to reduce secondary voltage; and contribute to sag and general tonality.  If you substitute a "better" tranny, you may have an over-voltage condition to the heaters and maybe plates, etc.  If so, this can be corrected with power resistors of the correct value - mimicking the stock resistor in the heater supply.  

It would be interesting to see if tone changes.  Again, all this can be avoided by placing an iso tranny in front of the stock PT.  Mine is mounted to the floor of the combo box.

[HotBluePlates]

The schem shows one side of the iso tranny secondary that supplies the plate V is grounded to chassis while the heater supply connected to the iso primary is not grounded.   If a single suitable iso tranny is used with both the plate supply and supply circuit connected to the secondary, one leg of the heater circuit becomes grounded to the chassis.  Is this OK?

Grounded "to the chassis" is really an asusmption on your part that the chassis is used and the ground return path.

Okay, so yeah, they would cheap out and actually use the metal chassis, but they don't necessarily have to. The ground symbol could be replaced by a drawn line connecting to every other ground symbol. We don't do that because it makes a schematic harder to read.

But for good performance, the loop formed by the PT secondary, rectifier and first filter cap has to be as reasonably short and heavy as it can be. Soldering the PT secondary wire to the chassis right next to the cap can negative lugs, which are soldered to the chassis, makes this short and heavy connection.

I'll say upfront, I don't know why Kay did what they did. I'll venture a guess, but I think PRR can tell ya for sure.

There may be a step down in the PT. The filament string seems to want 117-120v. Notice the 150 ohm resistor in series with the filaments. The first number in the tube name is the approximate needed voltage; 35v + 50v +12v = 97v, which leaves 20-23v left over. 20v/150 ohms = 133mA, and 23v / 150 ohms = 153mA. All of these tubes draw 150mA, and the resistor drops the excess voltage.

The part I'm not sure about:
The heaters are a.c. They don't need a reference to chassis ground, as long as there is a closed loop for current to flow. The 2 sides of the wall plug wiring provides that closed loop. Tubes have a rated maximum voltage between the heater and cathode. Since a.c. varies form a peak in one direction to zero to a peak in the other direction, having no hard reference to "ground" of the amp circuitry (specifically, the tube cathodes) keeps all of the tubes close to a lower average voltage difference between the heater and cathode (this is the part I'm not certain about).

And PRR is right that it would cut transformer cost to not have the PT rated for the extra heater current, which is 3 times what the whole rest of the amp draws.

Do you actually have the Kay? Will a higher-current tranny actually fit?

[PRR]

> PRR assessed the current draw at .2A, but how?  Each of the tubes in this amp draws .15A, but they are in series so I guess, the heater current is..... .15A, right?

OK.

Now what is the plate current draw?

We don't know; I have a feeling KAY didn't care. But we know some stuff.

Power tubes run 30mA-70mA. Preamp tubes run much less.

In this case the 680K preamp plate resistor _can't_ pass over 110V/680K or 1/6th of a mA, probably half that. 0.08mA? Forget it.

The 50L6 datasheet shows several conditions. We are closer to 110V on plate and G2. The datasheet says 7.5V bias for 50mA+10mA cathode current, a 125 ohm cathode resistor. The KAY plan shows 150 ohms.... like they forgot to compute G2 current to cathode, or just had a bucket of 150 parts.

So the 50L6 sucks less than 60mA. 45? 50? 55? 59? Since we also have 150mA, no real precision is needed. Call it 200mA total.

> not clear that the Kay PT is actually an isolation tranny. ...absent an internal Faraday shield, and high voltage ability to guard against failure, it provides no more isolation than an ordinary tranny.

Yes, there's basic isolation and extreme isolation.

We want a Basic Transformer. We want good paper insulation, not filament clay. We don't need shielding and stuff: few amps feature Faraday and this is not supposed to be a high-feature amp.

The voltage ratio is a good question. Working from 35Z5 data, it appears the secondary is very close to 117V. I don't see any reason to care if it is 110V or 125V; everything is inside specs (C1 is marginal but surely too old to keep).

We DO keep the KAY's original T1. The 35Z5 NEEDS series resistance to limit surge current (remember there's >>100A short-term AMPS available in a wall-outlet). We don't know that a 35VA transformer has the right resistance; we do know the KAY lived this long with T1 limiting it. And T1 is vintage original and already in-there.

We do change to 3-pin plug with ground wire hard-connected to all finger-touchable metal (chassis).