Heathkit A-9c

[DummyLoad]

i have had this old heathkit for a couple of years now and i was getting tired of kicking it around, so to speak. i tried to sell it on craigslist, here, etc.. but no interest i guess b/c it is/was fairly beat up (one side was bashed in) and the transformers cases were rusted pretty bad, and i refuse to sell stuff on fleabay.

whomever built it did a horrible job; the resistors were butt end to tie points and nearly every solder joint was gray and crusty. some components were simply wrapped around a terminal once or twice and left unsoldered. i couldn't justify the expense of waiting for a fleabay deal to own a pair of a not so decent heathkits, so i did the sacrilegious thing - i gutted a hi-fi amp t make a guitar amp. now before you guys get all up in a tither about it, this amp was at best, in donor amp condition. since none wanted it for the paltry sum of 50 bux, i gutted her gleefully.

yes, that's a rat-shack 12.6VCT transformer used in the 12VDC supply.

i built into her carcass...
 

[PRR]

37V across 200 ohms and 463V supply looks like 40 Watts dissipation in each plate?

If you have 37V/200= 0.185mA of pure cathode current, get RID of the RadioShed iron and silly-state junk, power V1 V2 heaters with the waste heat in the power cathodes.

[DummyLoad]

37V across 200 ohms and 463V supply looks like 40 Watts dissipation in each plate?

yes. stock trim PT too. as far as i can tell the 5V4G is what's specified - circulated copies of the schematics are hard to read. near 40W, is what i calculated as well, actually 39W and change, 40W round up. the sylvania 6L6GC are holding up - no red plate - 5932s red-plate and so do 5581s. the original Rk resistor was a 195ohm 7W for the 6L6s. i used 2 x 100R 5W WW in series - ahhh... but what was g2 dropping R? i don't have a clue - wanton and reckless dykes... :(

likely that this PT is old enough to carry a primary rating of 110V and why my B+ is so damn high. maybe i should have used that rat-shack tranny for a buck-tranny. ha! curse you hindsight!

the OT is rated at 8K Z pri. link here. which falls in line with the published 6L6 plan for AB1 self bias 24W, 8K5 Z pri., 250V g2, 400V plates. i believe that this amp is rated @ 20W.

the solution may just simply be to get the screens down to <=350V or thereabouts.

not up to gutting the 12VDC power supply. it works just fine. the plan you propose is nice but too much work as i'd have to hack out some of the filament wiring in a point2point built amp - that would be a major PITA, however, if all other attempts to tame Pdiss fail and i have to make use of the space for a bucking tranny i will.

as always, much thanks.

--ISODummy.

[PRR]

Heath didn't run 6L6 at double rated Pdiss.

125V wall is not enough to get that far out of spec.

http://ampslab.com/SCHEMATICS/HeathkitA9C.gif

340r from rect to first cap.

3750? from first cap to screen node.

>  in line with the published 6L6 plan for AB1 self bias 24W, 8K5 Z pri., 250V g2

250V G2 is very different from 418V G2. It amounts to double current (418V/250V)^1.5 which is near where you arrived.

110mA would be a happy idle for 440V on plates and 8K load. (Sadly this kills the idea of heating the small tubes with big tube waste.)

Or drop 6550 in there.

[DummyLoad]

250V G2 is very different from 418V G2. It amounts to double current (418V/250V)^1.5 which is near where you arrived.

i gathered that is what was happening. the schematic copy i have is all but useless, should have reverse engineered the damn PS before i got cut happy. hindsight. again. :-\

stuck a 5U4GB in it B+ dropped to 435V - 430V g2. 35V Vk of output tubes - or Pdiss of 35W ea. better but still not good. sounds really nice in this trim. 5v reto fil. winding measures 5.1V assume it is still happy w/ 5U4GB.

125V wall is not enough to get that far out of spec.

+10% from wall is global +10% no? 400VDC becomes ~440VDC, right?

second hack... moved fuse holder to make room for a couple large resistors 180R @ 25W and ( i wanted 270R or more - it's all i had in stock) + 3K 10W for g2 dropping R. results 400V @ first filter & OT CT, 365V @ g2 w/ 30V Vk for 150mA. Pdis is ~28W / tube now.

sounded better with the 430V B+. still, overall it sounds pretty good. but then again, we are persnickety. i guess leo figured out that the harder you push tubes the better they sound... the schematic i'm posting has telemetry taken with 430V B+. i'll grab some telemetry of the current PS config and update the schematic later.

it's a nasty mess - i've built much, much better and neater; this one just didn't want to go together without a fight...  

much thanks,

--DL

http://i2.photobucket.com/albums/y11/pmitchel/Heathkit%20A9-C/2011-06-23_22-04-15_742.jpg

http://i2.photobucket.com/albums/y11/pmitchel/Heathkit%20A9-C/2011-06-23_22-04-37_375.jpg

http://i2.photobucket.com/albums/y11/pmitchel/Heathkit%20A9-C/2011-06-23_22-04-55_148.jpg

http://i2.photobucket.com/albums/y11/pmitchel/Heathkit%20A9-C/2011-06-23_22-05-09_415.jpg

http://i2.photobucket.com/albums/y11/pmitchel/Heathkit%20A9-C/2011-06-23_22-06-23_783.jpg

http://i2.photobucket.com/albums/y11/pmitchel/Heathkit%20A9-C/2011-06-23_22-06-09_602.jpg

http://i2.photobucket.com/albums/y11/pmitchel/Heathkit%20A9-C/2011-06-23_22-05-49_969.jpg



schema:

[DummyLoad]

this is done... now it's off in a corner gathering dust. :-\ attached final schema with last mods and updated telemetry.


i think i'll tackle the glo-champ this weekend.

thanks PRR - yer a good egg.    

--DL

[DummyLoad]

this came back from odessa, tx, with a fried PT. 800V secondary shorted. guess i loaded it up too much? i found another PT in friends scrap pile and installed it, although i'm having second thoughts. the output stage is idling 148mA. i have a NOS hallardson 800V @ 200mA PT X-mount that i can work into place with some minor machining.

thoughts? run with the A9-C replacement or hack in the hallardson?

TIA,

peace.

--DL

BTW, the schematic is incorrect: C1a is located after the 180R 25W resistor - i'll correct it l8r.

[PRR]

Why can't you bias for something nearer 110mA?

[DummyLoad]

Why can't you bias for something nearer 110mA?

did just that - got close anyway... i stuffed a 270R 25W under the cathodes. ya, i know 25W is ~18W overkill; it was that or a 5W which would run rather warm.

31V / 270R  ~115mA.

predictable it would - B+ popped up.  it's now 410Vg2 and 430V to plates.

much thanks!

--DL