Kay model K507 amp

[billcreller]

 A person I know from the airport where I work sometimes, has brought in an old dusty beat-up Kay amp, for me to check out, a model K507.  First thing I noticed is that the PP power tubes are odd balls ( to me) with a Novar type base, namely 7868 tubes.  One of the tube sites shows them, at $45 bucks each !
  I checked the listing for Kay amps on the Schematic Heaven sit, and it's not listed.  It does have a sheet glued inside of the cab with a schematic & parts list though.
  Anyone here ever worked on one of these ?  

 It has two 8 inch speakers, of a brand I can't identify, but have a logo on the magnets.

  And I haven't plugged it in & turned it on yet, since I don't have a fire extinguisher handy in my basement !

[sluckey]

It has one oddball preamp tube. Luckily they're available on ebay. Pretty cheap too. Hopefully this schematic will be easier on the eyes. Looks like some decent transformers.
 

[DummyLoad]

20EZ7 - i have 10 or so in my stash. bill, if you need one, then pay for the postage and you can have one.

interesting ckt for a guitar amp.
20EZ7 filaments are cathode resistor for output stage. seen this in dan electro? PRR mentioned some time ago that this scheme was also used in a 6V6 powered hi-fi he liked very much.
diff long tail summing amp to 7199 pentode gain stage then direct-coupled to triode cathodyne.
g2 bias for pentode tapped from cathodyne bias.
clever. nice. 

--pete

[PRR]

> namely 7868 tubes

Pretty much 7591 in a cheaper bottle.

7591 is also an odd-duck.

If it *had* to be a gigging amp, you could convert to 6L6. Gain would be a hair less, but all gigging guitarists use pedalboards now, you can probably pick-up the lost gain in a pedal.

I suspect the operating point is near the 300V 24W condition. Anyway you wouldn't put the 44W condition into a mere two cheap Eights.

Very strange mix-stage. Looks like they were going to a long-tail, realized that would not do, added the 7199 stage, but retained the lower half of the long-tail.

_I_ think it is a cute parlor amp. If it can play, OK, let it sing a few times a year. If not, it isn't worth costly tube-search or massive re-design.

[billcreller]

Thanks for the responses, and thanks for the schematic sluckey !  It looks just like the page that's glued in the cab, where it's hard to read.

  If I can get it working, I'll just have the guy take it home and out of my hair......


Bill

[silverfox]

I checked a couple of reviews and those could be Jensen speakers.

http://craigslistvintageguitarhunt.blogspot.com/2009/11/kay-507-2x12-tube-amp-in-riverside-ca.html

This amp might surprise if it works.

I had Iron from a 7868 and used it to power EL34's after rebuilding the amp. Not suggesting that- Just saying. I also have two of the 7868 tubes I'd part with if you change your mind and try to repair it if it needs that but no doubt it would need a cap job too.

Silverfox.

[sluckey]

Some pics...

[eleventeen]

Do you know if the speakers are in phase....?

[billcreller]

I haven't got that far with it yet. It's ugly, just like sluckey's pics !!

Not sure if the owner wants to spend any bucks for parts on it yet.

[sluckey]

Post so pics showing the guts if you pull that funky chassis. I bet it's real purty. 

[billcreller]

  I'll get some pics soon.....first thing I noticed about it was the 2-prong cord......no grounding, and I asked the owner about if he ever got a zinger from it.  He told me that his band played a lot of outdoor gigs, and he carried a piece of plywood to stand on, after a few shocks !!

[spacelabstudio]

The construction on that looks just like the old Gibson amps.  Were they using the same people?

Chris

[Shrapnel]

7868 = 7591(A) and almost = to a 6L6. Of course, all these have different pin-outs (and a completely different base in the case of the 7868.)

Some 7591 replacements required a mod because they were actually 6L6 tubes. Their (7581/7868) gain structure is more in-line with the EL84/6BQ5.

Some like the sound, some dont, of them being overdriven. They see more use in the Hi-Fi world than they do the guitar amp world, but they were used, as you can plainly see.

You just might be our only source for a schematic on this one.

[billcreller]

 The inside of the chassis is really clean ( big surprise )....


the other end....


The big cap in the top picture is across a section of the filter can...

And here is a pic of the logo on the speaker mag...hard to tell what it is, but seems similar to the Quam aircraft speakers I've installed.



I haven't spent any time on this yet.....my plate is full, and energy is lower since radiation treatments
began..   almost done though...

[silverfox]

"since radiation treatments began..   almost done though..."

Been there. Great recovery, over 10 years and 3 kids later... Good you got it out of the way now. May not be covered in the future...


Silverfox.

[billcreller]

 I did get the Kay working good.  The pots needed some Doxit spray, and I replaced the cord, removed the death cap & switch. Cleaned the tube sockets, and even the tube pins.  No more scratchy noise when the tubes are moved around. It actually sounds quite good.  The owner is gonna get it back soon....

[Platefire]

That amp is so weard it's cool. The face plate looks like a multi-channel PA system. Platefire

[scrimpus]

That speaker was made by Oaktron.

[billcreller]

Never heard of Oaktron speakers.  These seem to sound decent....

[PRR]

Oaktron was a major minor maker most of your life. Never a big player in retail speakers, mostly bulk contract speakers and also odd-lots in surplus venues. From 1950 on, but after 1990 a part of Mitek (MTX Audio, Atlas Sound, Soundolier....).

http://www.mitekusa.com/culture/OurStory.aspx?page=2
http://www.mitekusa.com/Locations/monroe.aspx