Kay 503A questions...

[gmoon]

Hi! First-timer here!

I've a old (1961) Kay 503A amp that needs some upgrading. It's a low-wattage practice amp with 'radio' tubes--12AU6, 50L6, 35Z5. It has good tone, but is fairly noisy at times, and has some (unpleasant) breakup at higher volumes I can't quite blame on the speaker.

It's all original (including tubes), so I'm planning the the usual stuff: recap, new tubes, speaker replacement. But I've a couple questions first...

--Never done a 'recap' job. Is it customary to leave the old multi-cap 'tin can' in place for looks? (From reading here and other places, I'm aware of the dangers of adding too much capacitance in the rectifier filtering.)

--Should the filter caps voltage rating be upped? I ask because the previous owner had already bought caps and they are rated 150V (highest voltage in the circuit is 130V, I believe.) While the original caps were rated 150V, this seems a little skimpy to me.

--The amp circuitry is isolated from the line, but not the heaters. They're connected directly to the line, in series with a 150 ohm 5 watt resistor. I know it isn't ideal, but it's better than no iso transformer at all. Is this reasonably safe? Or should I build a separate power supply for the heaters? (DC?)

--The amp appears to have an error in the original wiring (The three inputs each have an inline 22K resistor. Between them and the .05 cap should be a 22K res to GND. Instead the 22K resistor is connected to GND BEFORE one of the input resistors on INPUT 1.)

Since I need to correct this anyway, I've found several schematics for amps with nearly identical tubes, but some wiring variation ('Alamo Fiesta', 'Airline GDR-8511A', 'Harmony 303A', 'Kent 2189', 'Silvertone 1430, 1448' and more...) I plan to try some of those variations in the preamp stage. I've done enough experimentation to see that the amp is capable of a much 'brighter' sound. Any thoughts?

[gmoon]

No comments, eh? (would have liked to hear opinions about the AC heater circuit, at least--from a safety perspective.)

So I posted a project about the rebuild here:
Tube-Amp-Rebuild-and-Mod

'Course, I wouldn't be surprised if there were errors. Kind of new to the workings of the magic tubes....

[Bruce / Mission Amps]

No comments, eh? (would have liked to hear opinions about the AC heater circuit, at least--from a safety perspective.)

So I posted a project about the rebuild here:
Tube-Amp-Rebuild-and-Mod

'Course, I wouldn't be surprised if there were errors. Kind of new to the workings of the magic tubes....

Most of us don't like the idea of AC/DC radios and the like and would prefer some kind of an isolation tranny but I personally don't know of anyone has been hurt by one.
I guess if you are worried about the filament string, you can use an extra inline fuse with the right fuse link to blow if anything get shorted.
It is already protected by your mains fuse though and you did ground the chassis... still...?
Just remember that the full 117v-128v line voltage is applied across that circuit so, what ever additional load (less ohms = more load) or short that is accidentally is on it (less ohms then the total resistance of the filaments), will suck up a ton of current, which is only protected by a 1a-2a fuse....and you could have an issue.
Also... drop that high cut tone control and use one from a tweed Princeton 5F2A.
You only need a 1m audio pot, one 500pf cap and one .0047uF cap.
You'll like it much better.
Ignore about 98% of what self appointed amp gurus tell you about O'drops vs any other decent plastic cap.
The tweed tone control mod is all you need to do extra for this amp.

[gmoon]

@ Bruce:

Thanks for the comments! I'll take a look at the tweed Princeton tone circuit next.

Your 'take' on the heater circuit is appreciated...Adding an additional fuse sounds like a good idea (and I doubt I'll do much more to this project after those two changes.)

I've learned a thing or two from the rebuild. Maybe not enough yet to rebuild my old Ampeg Gemini II, but a step in that direction....

[Carlsoti]

Those AM5 circuit radios can be tricky AND dangerous. I've got no issues digging into and of my other amps (including a Fender 400 PS I used to own), but I stay away from the AM5 stuff. I've got a Kay model 703C that I had an aquaintance fix. You seem to be somewhat knowledgable, though, so just remeber to keep one hand in your pocket. Sorry I couldn't be of more help.

[boyt_bpc]

I just bought one of these off evil bay. These circuits don't have an isolated power transformer, but you can install a small isolation transformer in them very easily.

[boyt_bpc]

Also... drop that high cut tone control and use one from a tweed Princeton 5F2A.
You only need a 1m audio pot, one 500pf cap and one .0047uF cap.
You'll like it much better.
\

Hi Bruce,

From this schematic

Would I just remove C2 and the pot that is in there and install the tone control you said? If so, is there a schematic that shows it?

Thanks,

[gmoon]

I just bought one of these off evil bay. These circuits don't have an isolated power transformer, but you can install a small isolation transformer in them very easily.

boyt, as you can see from the schematic, only the heaters aren't isolated. With a proper 3-prong plug mod and a fuse or two, it's not too dangerous.

If I every played this outside my living room, I'd probably fully isolate it...

[boyt_bpc]

boyt, as you can see from the schematic, only the heaters aren't isolated. With a proper 3-prong plug mod and a fuse or two, it's not too dangerous.

If I every played this outside my living room, I'd probably fully isolate it...

The chasis isn't grounded in these, so a 3-prong isn't doing anything. You need to put an isolated tranny in it.

[gmoon]

The chasis isn't grounded in these, so a 3-prong isn't doing anything. You need to put an isolated tranny in it.

Look at the schematic you posted yourself. The plate voltage / signal path is completely isolated. Only the filament wiring is connected to the mains. That's not ideal, of course, in case of a short. That's why Bruce recommended fuses on both sides of the incoming mains as additional protection.

Of course, there are variations on this amp that don't use a tranny at all. Maybe you have one of these.

There's isn't a proper earth ground for the chassis until you add the 3-prong, which I have done (that's what the green wire is for...)

[boyt_bpc]

Look at the schematic you posted yourself. The plate voltage / signal path is completely isolated. Only the filament wiring is connected to the mains. That's not ideal, of course, in case of a short. That's why Bruce recommended fuses on both sides of the incoming mains as additional protection.

Of course, there are variations on this amp that don't use a tranny at all. Maybe you have one of these.

There's isn't a proper earth ground for the chassis until you add the 3-prong, which I have done (that's what the green wire is for...)

I'm new to reading schematics, but I'll go over it again and see where I screwed up in my reading.

EDIT: So, in the schematic that I posted, replacing with a 3-prong and putting a fuse before R3 and after the 12au6 is what you've done?
Sorry for being such a noob.

[gmoon]

EDIT: So, in the schematic that I posted, replacing with a 3-prong and putting a fuse before R3 and after the 12au6 is what you've done?
Sorry for being such a noob.

Pretty difficult to read something that's not there (the modifications...)

The fuse should go between the plug and the on/off switch, on the Hot (black) incoming wire. The center prong (green) should be bolted directly to the chassis. That's the earth gnd.

Place another fuse on the incoming Neutral (white) wire, if that's something you want... As I wrote--if I was using this amp around other audio eq (or someone else's main sockets, extension cords, etc.), I'd fully isolate it.

Everyone's a noob at some point, and I'm no expert. Be very, very cautious poking around inside the amp--it can can kill you. Please read all the disclaimers, and info about discharging the filter caps, unplugging the cord, etc.

[Bruce / Mission Amps]

Hi Bruce,

From this schematic

Would I just remove C2 and the pot that is in there and install the tone control you said? If so, is there a schematic that shows it?

Thanks,

Well, no.... not quite.
Leave the 1m audio pot there but connect a 500pF cap to the top lug and the .005uF cap to the bottom lug.
The other side of side of the 500pF cap goes to the wiper of the volume pot (middle lug) and the wiper of the tone pot goes to the top lug of the volume pot.... OK.. here's a better idea, go look at the way the tone control, the two capacitors and the volume pot of a Fender, tweed Harvard 5F10 or 6G10 is wired up....

[boyt_bpc]

Well, no.... not quite.
Leave the 1m audio pot there but connect a 500pF cap to the top lug and the .005uF cap to the bottom lug.
The other side of side of the 500pF cap goes to the wiper of the volume pot (middle lug) and the wiper of the tone pot goes to the top lug of the volume pot.... OK.. here's a better idea, go look at the way the tone control, the two capacitors and the volume pot of a Fender, tweed Harvard 5F10 or 6G10 is wired up....

Thanks Bruce.

I'm learning as I go. Luckily I have a good friend who is a hi-fi nut to help me along. I'm redrawing the schematic for the Kay to include the tone mod you recommended and gmoon's mods.


On the 50L6 which Pin do I jumper to ground to drain the caps? I think it is pin 3, but I'm not 100%.

[yakfisher]

I have this same amp and it obviously needs new tubes and caps. Although I've never rebuilt an amp, I dealt with electronics in the Navy for 10 years. Would this be a suitable first project amp?

Thanks for the guidance,
Jay