Kay 703-C isolation transformer

[punkykatt]

http://el34world.com/charts/Schematics/files/kay/kay703c.pdf

Hey Guys, Got a Kay 703-C practice amp on the bench. The amp hums bad so its going to need new filter caps and a 3 conductor grounding power chord. On the schematic they are calling T1 a isolation transformer.  Is this truly a isolation transformer for today`s safety standards?  Thank you  for any input.
Punky

[shooter]

isolation transformer

Looks to be a filament tranny to me, no *input* tranny involved, but I'd wait for someone with a higher pay-grade than me

[Jack_Hester]

No.  A true isolation transformer would have the secondary winding connected to the two points where the AC line input cord is shown.  And, the plug would be fused and connected to the primary side of that transformer.  I have an external fused isolation transformer box made, just for these little amps, and most old radios.  T1 appears to be a filament step-down transformer for the pre-amp tube. 

I'm also guessing that your PS de-coupling caps are combined in a paper tube, beneath the chassis.  That's the way my little Harmony amps have theirs.  I left it in place and clipped/taped the leads.  Then, I soldered in individual caps. 

Jack

[punkykatt]

Thanks Jack.  Yeah, I need to install a fuse too.  Im thinking maybe a 50VA isolation tranny should be enough for this little amp? What do you think?

[PRR]

That amp only "isolated" the MINIMUM amount required by UL at the time; the UL rules soon changed to ban such obvious cheaters.

Overall you (the player) are only "isolated" by 47K+0.04uFd. This may have squeaked-through the UL leakage tests at that time, but gives one heck of a jolt, and might be a no-no today.

You are not the 13 year old finding a "guitar amp!!" under the Xmas tree in days of no seat-belts and whooping-cough epidemics (and polio not far in the past). You survived this long, maybe by luck, but it is high time to do better than KAY did.

[Jack_Hester]

Thanks Jack.  Yeah, I need to install a fuse too.  Im thinking maybe a 50VA isolation tranny should be enough for this little amp? What do you think?

Here's one for a starter: 

https://www.edcorusa.com/xpwr217

Only problem is, there's a lead time of several weeks. 

Another choice:

https://www.amplifiedparts.com/search/node/262?cats[0]=65

These are ready to ship.

Jack

[Jack_Hester]

Here's one like yours: 

https://reverb.com/item/1005469-project-vintage-1964-kay-703-c-all-tube-guitar-combo-amplifier

[punkykatt]

Jack, Thanks for the pics.  The Kay that I`m working on didnt have the schematic in the cab.  No name on the cabinet  except on the underside where the rubber feet  are was a small plate that said 803-C  and a serial #.  I went  to Doug`s schematics and just guessed Kay to start hunting.  Found the 803-C schematic, the tubes were all different and it had only 2 inputs.  I kept checking  the Kay schematics and sure enough the 703C was dead balls on the money.  I guess they were not too fussy with these  amps. 
Punky

[jojokeo]

Punky, I've done a number of these amps. I've got schems and photos if needed? Let me know?

[punkykatt]

Thanks for the  offer Jojokeo,  I`m all set.  I ordered a 50va isolation transformer and the filter caps it needed. The three conductor cord , fuse holder and fuse I had in stock.  The amps sounds pretty good for a little practice amp and no hum, hiss or shocks.  The customer will be pleased.

[jojokeo]

No problemo PK. These amps actually sound pretty good recording, better than the practice amp that it is but, the thing I like is the preamp pentode grind they get which is nice at any volume especially a low watt thing like these. Now my feeling is that they already have an isolating transformer and another isolation isn't needed especially with a fuse and three prong grounding chord properly installed. I have tested these and the fuse does it's job as it should in the event of a possible and remote chance of a short. JMHO...

[punkykatt]

The 50va isolation transformer for this amp cost $16.26 + shipping. Pretty cheap insurance policy. Thanks for the info.  I have had some old Ampeg amps come in the shop that someone changed out the two conductor cords to the  three conductor grounding cords and left the death cap installed. Those death caps were leaking enough to trip the GFCI`s in my shop.

[HotBluePlates]

Now my feeling is that they already have an isolating transformer and another isolation isn't needed especially with a fuse and three prong grounding chord properly installed. I have tested these and the fuse does it's job as it should in the event of a possible and remote chance of a short. JMHO...

As the original amp stands, the rectifier is directly connected to the line, and there's no fuse. Even adding a fuse, there is no way to guarantee line voltage can never be present on the chassis in a fault.

That's why UL won't recognize those amps anymore. It's not the common or likely faults you have to consider... It's whether any condition could ever exist wherein the chassis is energized by the wall outlet, even if you'd have to work to make it happen.

Hell, they even put cages around tubes in combo amps now, because someone unknowing could burn their fingers (usually targeted at very young children).

[PRR]

> rectifier is directly connected to the line

And the input jack shell is only 47K away, even if the 0.04uFd cap fails open instead of short.

While it may have been legal at the time, and maybe today (0.05u is common chassis-grounding in digital stuff), for audio and guitar (bare exposed grounds on connectors) I'd rather be MUCH further away from the line. (Dry sneaker resistance, many hundreds K Ohms.)

> and there's no fuse.

Fuses won't directly eliminate the shock. The smallest common fuse is MUCH bigger than lethal current.

Fuses may forestall the fire when stuff fails.

Fuses could break the power when a live wires falls on the chassis. That's the idea in your dishwasher etc etc: any substantial rise of the case/face above ground potential will pull enough current to blow the fuse/breaker in the cellar.

Punky's GFI is a _VERY_ wise idea. The fact that a stock hot-circuit amp will trip the GFI is IMHO more confirmation that these amps should NOT be used without FULL isolation treatment.

[darryl]

Am I missing something?

Common sense.

The 50va isolation transformer for this amp cost $16.26 + shipping. Pretty cheap insurance policy.

Electrical safety is non-negotiable. I'm sure there are many widows and orphans who bitterly regret the day their loved one made contact with "lowly 117VAC".

[Ed_Chambley]

Looks just like a lot of radios.  I get a good chuckle every time I go hunting at salvage/flea market for radio stuff.  You see a sign on a radio that states "we have the knobs put away" meaning they do not want to get them stolen.  Original knobs are hard to come by.  Then the proprietor notices you and says the radio works and proceeds to come over to demo it for you.

I don't know if Joe is missing anything, but we are much more safety conscience today.  One of the memories of my grandfather was him on cold mornings opening a valve from a propane line in the bathroom and lighting it like a blow torch to warm things up.

The preferred method of updating old 2 prong wiring is to install a GFCI at the first location in the electrical run, even if there is BX cable.  I just got finished doing this and decided to use the GFCI and also the BX cable.

Edison and Tesla both were fairly smart chaps.  Sometimes I wonder if the regulations we have in place today existed in their time if we would have had a great loss of invention.  I am assured we would have.

I am not advocating being ignorant and reckless with electricity, especially like when Edison built the electric chair or used AC to kill dogs to prove it was dangerous.  I also own a lot of collectible radios that have no isolation transformers and never will as long as I own them, but if they do not have knobs on them and I have not found the correct ones yet, they also do not have a power cord connected.

But then again, when in South Carolina I don't wear a helmet when riding a motorcycle.  I guess there are many ways to make widows and orphans.

[jojokeo]

Am I missing something?

Common sense.

 
We're all here to learn are we not? And this was my point or did you  miss it? Thanks for bringing nothing to the table smarta$$

[darryl]

We're all here to learn are we not?

There are relatively inexperienced members of this forum who are here to learn, so advocating dangerous practices in the way you have, on a public forum, is being very irresponsible.

If you wish to "take an amp shock any day" that's your choice, but punkykatt is servicing an amplifier for a customer. He has wisely chosen to make the amplifier as safe as possible before returning it to that customer.

[Ed_Chambley]

We're all here to learn are we not?

There are relatively inexperienced members of this forum who are here to learn, so advocating dangerous practices in the way you have, on a public forum, is being very irresponsible.

If you wish to "take an amp shock any day" that's your choice, but punkykatt is servicing an amplifier for a customer. He has wisely chosen to make the amplifier as safe as possible before returning it to that customer.

I would like to know why Joekeo's proposed method is not safe considering worst case would have to be loss of earth ground would cause a current of 28ma.  I know this has exceeded the "letgo threshold" which maximizes at 22VAC and begins at 3ma.  I would say reducing the 28ma would be desirable.

It is confusing as old telephone service is 50v and working on it recently I felt a tingle fairly strong as I was sweating and rubbed against a copper water line. Did not bother me nor does 12v DC from a car battery, but 12VDC is enough to kill a cat.

In the real world we come in contact with current from static that exceeds these numbers, but the duration is not a factor as there are loads of factors.  I have "Googled IT" as has been suggested by others who don't really know either, but want to act as if they do.

So how much electricity is "safe" since we are trying to insure newer hobbyists remain safe.  It cannot be zero as most of us will touch a 9v battery to our tongue to see if it is dead or not.  Should I really get my meter and test because this not safe?

[jojokeo]

We're all here to learn are we not?

There are relatively inexperienced members of this forum who are here to learn, so advocating dangerous practices in the way you have, on a public forum, is being very irresponsible.

If you wish to "take an amp shock any day" that's your choice, but punkykatt is servicing an amplifier for a customer. He has wisely chosen to make the amplifier as safe as possible before returning it to that customer.

Okay Knucklehead Smith - you are twisting words, taking things out of context, and implying things to support your hubris attitude & opinion while also not reading what had been written prior - notably on item #1 from my response above:
"#1) Disclaimer: I'm not advocating to not exercise safety. "

Heaven forbid this issue be discussed or my god - somebody taking a hypothetical look at something or even to dare think to open a discussion on such things involving "safety" or degrees of safety even? But then you and your PC police ways just have to come along to the rescue to inject your perceived superiority & attitude onto others with the unfortunate pleasure of your presence? What on earth would we all do without a debbie downer to keep us all in line and point out the obvious?

[shooter]

point out the obvious

Isn't he a Captain?

[HotBluePlates]

Fuses won't directly eliminate the shock. The smallest common fuse is MUCH bigger than lethal current.

#1) Disclaimer: I'm not advocating to not exercise safety.
#2) Who here on this forum has NEVER been shocked working on their amps (or in any another way)??? Anyone? Beuler? Also by much higher B+ than lowly 117vac and we're all still here living and breathing fine. Therefore I'd like to put aside the "shock paranoia potential" for just a minute.

Looking at the attached schematic - there is a grounded 3-prong chord with the ground wire bonded directly to the chassis. The so-called "death cap" has been removed. The load current has to go through the filaments of the rectifier, 56r power resistor, power tube filament, through the isolation transformer, then that 47k resistor. Even after all that it would act like a current limiting resistor in the same way it does in preventing an LED from being over-cooked. But even this could only burn open and not short out like a cap. There's only a small current "at best" that can make it even remotely to the guitar player right? Even though we're talking AC voltage 117v/47K=24.8mA this certainly isn't enough to KILL anyone unless it's a very old man with a pacemaker playing barefooted (and somehow the ground wire came loose on the chassis)??? Am I missing something?
Could that 47k resistor be increased to 100k or more w/out harm to the circuit operation?

I've been shocked by my lawn mower, weed whacker, car distributor, jet ski, etc...and I'll take an amp shock any day. 


Reporting jojokeo's post because of dumb error on my part -- HBP

[HotBluePlates]

So how much electricity is "safe" since we are trying to insure newer hobbyists remain safe. 

So voltage, in itself isn't the problem, it's current. Current causes burns and can stop your heart given a suitable path through the body.

The problem is humans are variable, and so even published, authoritative sources differ on the specific quantity of current which is lethal. Most appear to cite 100-200 milliamps as enough to stop the heart, while some also point out that a current as low as 60mA could cause irregular heartbeat leading to death. The sources I looked at cited a.c. as being much more dangerous than d.c., requiring lower current for fatal result.

So the ideal is as low as practically feasible.

A general approach should be to use known best-practice. I say that because "best practice" usually evolves over a long period of time, and probably addresses issues not immediately apparent. I've been in the Navy and the Army; my Navy time was aboard an aircraft carrier, where the motto was, "Navy regulations are written in blood." That phrase meant that the rules, even the apparently silly ones, are most often the resulting of sailors dying due to unforeseen problems with how they typically did something. The actions following a fatality almost always include changing the regulation to prevent a future fatality of the same type.

Looking at the attached schematic - there is a grounded 3-prong chord with the ground wire bonded directly to the chassis. The so-called "death cap" has been removed. The load current has to go through the filaments of the rectifier, 56r power resistor, power tube filament, through the isolation transformer, then that 47k resistor.

The addition of the 3-prong power cord and fuse have improved matters noticeably in the circuit. The neutral, however, is floated off the chassis by the 47kΩ and still needs a cap across it to eliminate noise as the filter caps are "lifted" off ground by the 47kΩ. If the wall outlet is wired incorrectly, the neutral could be live, negating some of the protecting when the fuse blows; one option might be fusing both legs of the power cord. It is such "improper operation" is what you have to worry about, as well as what "uncommon faults" have the chance of occurring.

The 47kΩ should limit current from 120vac to under 3mA. What happens if the rectifier shorts plate to cathode? 120vac is passed to the filter caps, and I don't know if they'll fail short or open when they explode due to the large a.c. Further, the 120vac is no separated from the top of the preamp tube plate and screen resistors by 3.3kΩ; what if the caps failed short, and the outlet is wired backwards? Fault current is limited only by the 56Ω resistor, and could result in a live chassis if there's not a wall outlet breaker (or insufficient current is drawn to trip the breaker).

By now, I'll admit I don't even know where the dividing line is between adequate and insufficient protection/isolation. Another reason why I assume UL and the like created best practice on the basis of long experience, and I'll simply follow what they prescribe.

[jojokeo]

The addition of the 3-prong power cord and fuse have improved matters noticeably in the circuit. The neutral, however, is floated off the chassis by the 47kΩ and still needs a cap across it to eliminate noise as the filter caps are "lifted" off ground by the 47kΩ. If the wall outlet is wired incorrectly, the neutral could be live, negating some of the protecting when the fuse blows; one option might be fusing both legs of the power cord. It is such "improper operation" is what you have to worry about, as well as what "uncommon faults" have the chance of occurring.

The 47kΩ should limit current from 120vac to under 3mA. What happens if the rectifier shorts plate to cathode? 120vac is passed to the filter caps, and I don't know if they'll fail short or open when they explode due to the large a.c. Further, the 120vac is no separated from the top of the preamp tube plate and screen resistors by 3.3kΩ; what if the caps failed short, and the outlet is wired backwards? Fault current is limited only by the 56Ω resistor, and could result in a live chassis if there's not a wall outlet breaker (or insufficient current is drawn to trip the breaker).

By now, I'll admit I don't even know where the dividing line is between adequate and insufficient protection/isolation. Another reason why I assume UL and the like created best practice on the basis of long experience, and I'll simply follow what they prescribe.

It seems that there's a lot of "if's" that need to occur for something really bad to happen? IF it's actually to be considered "really bad" at all? The Load wire has to go through a lot of small cathode and other wires where there's going to be voltage drops at various points. The high current going through very small wire and/or resistances all is going to act as current limiters. Also, with a 3-prong chord, fuse, and the majority of properly wired receptacles in the US in general, it just seems "in reality" that the likelihood is rather very small for tragic death to occur in almost all instances? As I've said before, I've been shocked by so many various voltages by so many things in my life which were almost all un-fused (aside from service panel breakers, I think I've seen/felt what's really bad out there? Now working on Pool and Irrigation pump systems as well as high voltage glass blowing studios, etc... I have not, installing 220vac electric ranges and cloths dryers - guilty, but that's all another story for a different time. Bottom line is I'm still alive and well much to some people's chagrin? 

*I realize that if there's any potential or remote chance of ANYthing to happen (according to Murphy's Law especially) it will. But again, I'm trying to separate the myth or legend or folklore from actual real world reality and likelihood of a realistic outcome on one of these type of amps. I'm thinking that worse case scenario the player gets a zap like one would get singing into a cross-grounded microphone? You go "WOW" and then shut things down?

Also, is there another or best way to wire these little amps utilizing the existing "isolation transformer" without the need to have two isolation transformers which seems overly redundant?

[HotBluePlates]

It seems that there's a lot of "if's" that need to occur for something really bad to happen? IF it's actually to be considered "really bad" at all?

The bottom-line "IF": If the chassis is energized by the power line anywhere prior to a resistor, and a player touches the chassis while being in contact with an independent ground (earth itself, another piece of gear, plumbing, etc), then they will die unless the electrical path manages to stay away from their heart.

That statement is based on the chance current is limited by a breaker (15-20A) rather than a resistor. In risk management, you have to weigh both the possibility of occurrence with the severity of occurrence. Low probability, extreme impact, what degree of control should you use? What if the victim is your child? Will it still not be worth adding a properly-functioning isolation transformer?

My ex never drives with a seatbelt; I don't feel right sitting in a car without being strapped in. She has a head injury and permanent brain damage as the result of a car crash; I totaled a Chevy Tahoe (in a *very* little bit of slushy snow, while in 4-wheel drive, with large offroad tires with more than enough tread depth) and only scraped my shin (and walked away thinking, "I thought that would be a LOT bigger hit...."). First accident ever, and I wouldn't have thought it was possible until it happened...

Maybe we'll just agree to disagree.

[jojokeo]

Maybe we'll just agree to disagree.

To me, this isn't about "agreeing" or not. I seem to be doing a poor job of explaining what I'm thinking? I'll try another tact:

1) What is the difference(s) between an isolation transformer and a normal power tranny we use in a Fender or Marshall amp (for comparison sake pretend these will have the same primary & secondary ratings)?

2) Can either transformer be used in a way for these tube types and provide equal protection?

3) Is the unique and dangerous thing here the fact that these amps are using the same transformer for both the filaments and the B+ voltage, and is what gives these amps (and tubes made for this) simply a Bad Design flaw?

[Ed_Chambley]

So how much electricity is "safe" since we are trying to insure newer hobbyists remain safe. 

So voltage, in itself isn't the problem, it's current. Current causes burns and can stop your heart given a suitable path through the body.

This is comically true.  The reason it is comic is it is like saying it is not the height of the fall that kills you, but it is how fast you hit the ground.  One is determined by the other.  Still you must have a certain amount of voltage to overcome skin resistance which varies to conditions.

My concern with current is exceeding the letgo factor.  The same grandfather who lit the propane pipe lost part of his left hand to an old fuse panel.  Same story, working in the dark, tripped and caught himself with both hands.  Remember one hand in the pocket unless you have mesh isolating.

A battery charger I have is 12VDC, but @15 amps and is certainly enough to kill a small animal like a pet.  Every time I see someone using one I let them know to cover it while charging so a neighbors dog doesn't get too nosy.  Being concerned and knowing humans vary, I don't know if anyone is willing to say what is a safe amount of electricity.

Sure, added protection of isolation is always welcomed. 

Now back to the Kay Schematic posted by Joe. The amp has an isolation tranny, but reading PRR's post I can see where the input is only 47K away from the filter caps.  Could you wire the isolation tranny between the load and fuse and reroute the heater supply from V2.  I guess I don't understand why the isolation trans is in use where it is.

It is a line level amp just like grandma's kitchen radio, just has a isolation trans where the radio uses plastic.

[Willabe]

It is a line level amp just like grandma's kitchen radio, just has a isolation trans where the radio uses plastic.

But, with grandma's radio she wasn't plugged in with a guitar holding on to the steel strings that are connected to the amps input ground.

[Ed_Chambley]

It is a line level amp just like grandma's kitchen radio, just has a isolation trans where the radio uses plastic.

But, with grandma's radio she wasn't plugged in with a guitar holding on to the steel strings that are connected to the amps input ground.

Yea, I know.  That is why the need for the isolation.  I am just looking at the schematic Joe posted to see if there is any way to make it safe using the isolation transformer that is already there.  Just curious because if you took grandmas radio and added a Isolation Transformer you could plug in a steel string guitar.

Follow me?  Like any line level amp that doesn't Isolation, you can add it.  The Kay already has one in it.  That would seem to make it like Grandmas Radio with an isolation tranny.  No biggie, but it seems like you could rework the amp.

[Willabe]

Yea, I know.  That is why the need for the isolation.    Just curious because if you took grandmas radio and added a Isolation Transformer you could plug in a steel string guitar.

Follow me? 

Yes but,

I thought the problem was the Kay 'isolation' transformer is only for the heaters? The B+ is still hooked to the AC wall voltage.

[PRR]

jojokeo, I am not arguing with you. I took all the safety switches off my lawn tractor; we are both old enough (have enough scars) to pick our risks.

This is really for everybody else.

> What is the difference(s) between an isolation transformer and a normal power tranny we use in a Fender or Marshall amp (for comparison sake pretend these will have the same primary & secondary ratings)?

Same thing.

However the Kay 703c plan linked above (or the same-thing plan in post #10) does NOT have transformer isolation on anything except the 1st stage heater. Given the choice between a 24VA tranny for true isolation, or a 2VA tranny for the failure point that UL (at the time) thought most dangerous, they took the low road.

Once you put an isolation tranny on the WHOLE thing it is as safe as any other.

(While that plan computes to ~~24VA, katt's 50VA for $16 is a good choice. Not over-price, cooler running, and the drain of these amps is somewhat uncertain.)

[jojokeo]

> What is the difference(s) between an isolation transformer and a normal power tranny we use in a Fender or Marshall amp (for comparison sake pretend these will have the same primary & secondary ratings)?
Same thing.

However the Kay 703c plan linked above (or the same-thing plan in post #10) does NOT have transformer isolation on anything except the 1st stage heater.

Thank you!

[Ed_Chambley]

Yes, thanks.  One more quick question.  How did you know it is a 2VA tranny?

[sluckey]

How did you know it is a 2VA tranny?

The transformer only needs to light a tube that requires 18V @ .1A.

[jojokeo]

Sorry for all the questions but I've wanted to know this for some time: why are transformers' rating listed as VA instead of W???

[sluckey]

It could just be some convention that stuck, I don't really know why but I do know that W=VA. But think, you rarely see an output transformer with a VA rating. It's usually Watts.

[jojokeo]

but I do know that W=VA. But think, you rarely see an output transformer with a VA rating. It's usually Watts.

Right, but I was more referring to a power tranny's primary ratings. See random screenshot photo:

[HotBluePlates]

You'll need to do a search for "watt vs VA vs VAR".

Bottom-line, watts mean power dissipated in a device or load and typically volts and current are taken as though they have 0-degrees difference in phase angle.

Volt-Amps (VA) is equal to the product of RMS volts and current, and if the power factor of the circuit is 1, is equal to power expressed in watts. However, it allows for the chance volts and current are not in-phase, which leads to a lower effective VA number.

Volt-Amps-Reactive expresses reactive power, or the "imaginary part" of apparent power. Both this quantity and VA are important when the load is reactive, because volts and current are likely out of phase and much higher than the circuit-heating (which corresponds to the real part of apparent power) would lead you to think.

We don't deal with things like tank circuits (where a coil and a cap are in parallel, oscillate and have very large circuit currents for dissipated power) or motors much, where this stuff becomes really important.

[PRR]

If Volts and Amps are out of phase, Power is zero but there is still significant energy sloshing back and forth.

This may be important for some uses of transformers. Indeed the design process treats V and A separately, so it does not matter if the load in in-phase or out-phase.

Heaters are 99% dumb resistance. VA = Watts for all practical purpose.

Loudspeakers can be reactive. However in practice they have dead resistance, any reactance is in addition to the resistance, so large out-of-phase energy doesn't happen. As the users think in Watts, the OTs are rated for Watts.

(However I was just shown a Polish spec-sheet of OTs rated in VA.)

[DummyLoad]

in the schematic attached, the dual secondary PT separates filament power and B+ power. uses the hammond 185d230 43VA transformer. primaries can be wired series for 230V or parallel for 115V operation, transformer is also rated for 50Hz work. i'd probably add another RC filter stage for the preamp pentode.

--pete

[jojokeo]

HBP / PRR - last question I promise 

To determine the primary fuse rating to use would I (using ex above VA=66),  W/E=I   66va/115v=~.57A so use a fuse rated 1A slo-blo?


PETE - very nice schematic. That helps my brain see the circuit much better separating the heaters vs B+ like that. It also helps illustrate what I was "seeing" in my mind in remaking this circuit in a more modern way. An extra filter stage would be easy for V1. This makes for a super easy build if anyone has any of those old tubes laying around OR simply redo the existing circuit with those minor changes since a lot of it is already there in the first place. 

[punkykatt]

I have a question on the new safety mod schematic.(thank you for sharing that)   By removing R7 & C5 and installing a jumper wire, wont that change the bias on the 60FX5 power tube cathode? 

[jojokeo]

I have a question on the new safety mod schematic.(thank you for sharing that)   By removing R7 & C5 and installing a jumper wire, wont that change the bias on the 60FX5 power tube cathode?

No, not at all. It's biased normally like any other SE design by it's own K resistor & bypass cap.

[punkykatt]

When I look at the original schematic I see 150 ohms from the cathode in series with R7 47k to ground.  150+ 47000= 47150 ohms to ground? Removing R7 would be just 150 ohms to ground.  What am I missing?

[sluckey]

The current path for the output tube never goes to/through chassis ground. Current always flows in a closed loop from one side of the source to the other side of the source, which in this case is line hot leg to line neutral leg. See pic...

[DummyLoad]

When I look at the original schematic I see 150 ohms from the cathode in series with R7 47k to ground.  150+ 47000= 47150 ohms to ground? Removing R7 would be just 150 ohms to ground.  What am I missing?

you're not missing anything. the B+ is floating. 150 is connected to the negative side of the main filters. that's the complete circuit. you don't have to ground power supplies for a circuit to work.


--pete

[punkykatt]

Thanks for  clearing that up for me.     much appreciated.  More note for my scrapbook

[HotBluePlates]

To determine the primary fuse rating to use would I (using ex above VA=66),  W/E=I   66va/115v=~.57A so use a fuse rated 1A slo-blo?

Yes, that's reasonable.

A "best method" would be to insert an ammeter in one leg of the circuit to be fused, then measure. A peak-hold function would be nice, because switch-on currents could be a high spike relative to the steady-state current. You might pick a Slo-Blo fuse in that case (fuse not rated for the full switch-on current, but time delay allows for the switch-on surge), which a fast-acting fuse might be better somewhere else where you're certain of the upper limit which shouldn't be exceeded (like Marshall's 500mA B+ fuse).

[PRR]

> fuse rating to use

The line fuse protects the line-cord. (It is not trivial or practical to "protect the amp".)

Depending on tube-set this amp eats 25W-35W out of the wall. If this amplifier ever drew anywhere NEAR 120 Watts (120V*1A), something is very wrong. Any line-cord can carry 1A. Yes, 1 Amp fuse is fine.
_______________________

> change the bias on the 60FX5

If you think it matters, note that 47K self-bias on a power tube will likely cut it hard-OFF.

I agree to disagree about the drawing. Personally I'd rather see the AC power source (T1 secondary) return right TO the first DC filter cap. The drawing implies rectifier return through chassis, which may cause buzz. Now that the AC power source is *fully* floating, we can dump the rectifier spikes where they need to go, "C6c".

(I also think you will use separate caps instead of Kay's 3-in-1 wax lump.)

I fully agree with trying 60FX5. In my experience it is the sweetest little thing. It also saves heater power. The flip side is that if you decide you want the harder sounds of 50C5 et al, you will also have to change the preamp tube (or shunt some current around it) and also re-calc the R9 dropper.

C2 should probably be >150V rating. In use it may run 12V or so (and meter will read lower); but at turn-on it could jump up to 160V for a moment. A healthy "100V wax paper" cap will stand enough 160V jolts to last past next X-mas, when the kid will be whining for a "real amplifier". If this IS your "real amplifier" (for some purposes) then you might want to splurge on a cap that won't die in a season or a session.

C4 sure does not have to be 1KV rated. That may have been common in 1962 but again 150V-200V is ample.

My data for 60FX5 shows 62 Ohm cathode resistor. Is 150r copied from a 50C5/50L6 plan? With 60FX5, cathode voltage is only 3V so "C6a" need not be over 5V rating. The older bottles need 10V rating. 25V is fine, but if you have healthy take-outs caps, use them.

[DummyLoad]

* remove r7 & c5 and install jumper.

look at the schematic i drew; there is is a wire drawn through R7.

ground symbol indicates same tie point, right? if not, when did it cease being so?

what i drew is not wrong. ;-)

--pete