K-502 upgrades

[EWBrown]

A couple years ago, I got one of the small K-502 tube amp kits from AES, and it went together easily and soundsed OK (not great, but not bad, either). The original kit had cheapo plastic tube sockets for the 4 11BM8s, which I never really cared for. This weekend, I decided to replace them with ceramic sockets, and while I had it off the metal chassis, I decided to try out the Hammond 1609 OPTs that I recently got from AES. The sound was much improved over the original cheap OPTs, both bass and high freqs were significantly improved. Next, I decided to try out the UL taps.
Cut 4 etches, to free up pin 7 (screen grids) on the 11BM8s, and wired in the UL taps. Oh, yeah, I added a CL-90 in series wuith the power trannie primary. Now teh 11BM8s don't flare up white-hot on startup.

WOW!!!!!!!!!!!! :shock: :shock: :shock: :shock: The sound was vastly improved, even though the output power was slightly reduced, but now it is a significant upgrade, and clean all the way up to "11" on the volume control. So, a $140 "ugly duckling" kit amp, a $1 chassis (from a flea market find) and 2 $35 OPTs make for a nice sounding, though not overly powerful, amp. The DIY-35 will still blow its doors off, but for a second system, or one for a kid, it's hard to beat. My SWAG is that it went from 8 harsh watts at full output, to 5 or 6 very clean and sweet watts. Those UL taps really do make a difference! I'll bring this one to work, and bring home the "doctored" SCA-35 to donate iron to the next DIY35 project.

/ed brown in NH

[Shannon Parks]

WOW!!!!!!!!!!!! :shock: :shock: :shock: :shock: The sound was vastly improved, even though the output power was slightly reduced, but now it is a significant upgrade, and clean all the way up to "11" on the volume control. So, a $140 "ugly duckling" kit amp, a $1 chassis (from a flea market find) and 2 $35 OPTs make for a nice sounding, though not overly powerful, amp.

Great post, Ed!

Shannon

[EWBrown]

Found this while looking at the previously mentioned KT88 SE amp:

http://diyaudioprojects.com/Tubes/K-12M_bh/index.htm



More mods, etc for the K-12 (11MS8/10GV8) and K-502 (11BM8/16A8)

/ed B in NH

[jduffy]

Mr. Brown, which do you prefer, the K-12M or K-502?

I've been trying to get my hands on a vintage amplifier but I keep missing out on bidding or getting there to late. So I've decided to satisfy my building jones and I'd like to try one of these two amps.

Thanks for any advice you can give and for continuing to post about these less expensive kits.

[EWBrown]

My own preference is the K-502 (11BM8) available from AES. It can be run in UL mode and easily modded (needs an extra filament trannie) to use 6BM8s or 16A8s (which are readily available at low cost). I did build a pair of the 11MS8 monoblocks, they are very good in their orogonal pentode mode, but really fall flat in UL mode.

FWIW the small toroid power trannies on this site:

http://audio.engineeringvista.com/catalog.htm

Toroidal transformer suitable for use in our PS-1 power supply and Model 1 amplifier

Primary: 115/230V (50/60Hz)

Secondary 1: 190V, 0.2A

Secondary 2: 26V, 0.7A

Max. dimensions: 3.4" (dia) x 1.7"

are a good alternative power trannie for running the K-502 with 16A8s.
The 26VDC winding has to be FWB rectified and filtered (4,700 to 10,000 uF / 50VDC cap), and the 16A8 filaments arranged in a series-parallel circuit. This eliminates the nasty induced hum from the AC filament power etches running underneath the coupling caps on the K502 boards. The B+ voltage runs a little higher with this trannie, but should present no problems. Conceivably, four 6V filament tubes could be connected in series and run from AC, though this could generate some unwanted hummmmmmmmm,,, Since the trannie is also 50Hz rated, a little extra current capacity can be gained from using 60Hz AC.

Boris also has UK-made Westinghouse branded 16A8s available, they are good but cost more than the Raytheon (Japanese) 16A8s often found on e-bay for $10 per sleeve of 5.

/ed B in NH

[TerrySmith]

On my K12 I cobbled up a simple DC supply fron the stock PT for the filaments, since it has a CT I ended up with just over 6 volts. You could then use 6GV8 tubes, the 6.3v variant of the 11MS8. I tried some Soviet 6GV8's and they sounded really good, better than the Jap 11MS8's.

[EWBrown]

The one nice thing about the 11MS8 is the ability to substitute several budget-priced "plinkers" into the circuit, 6GV8s, 9GV8s, 10GV8s (and 16GV8s?). The K502s have some alternativs, but not as many easily available and cheap.

The 11BM8s and 11MS8s seem to get a bit of a "premium" price progably nbecause of the K-12s and K- 502s. I think someone referred to this as the "McNally Effect".

For the K502, the Russian 6BM8 equivalents (6F3Pi) are a good reasonably priced alternative. These same tubes get re-branded as Sovtek or Svetlana 6BM8s and the price goes up, up, up... I suppose it's only a matter of time before "Mullard" and "Tung-Sol" Russki rebrands start showing up Yellow_Light_Colorz_PDT_04 The Chinese 6F3s are similar, but stay away from them... Yellow_Light_Colorz_PDT_09 Certain Chinese tubes are quite good (mostly triodes) and some are just plain poo-poo...

/ed B in NH

[quackard]

Ok, with the new kits up, I'm back to be interested in modding my k12. I have the 11ms8 version, and I have done many of the recommended mods from voltsecond and others (snubbers, upgraded caps, etc). Always wanted to upgrade the OPTs and try UL mode, so I guess I"m going to try to figure that out BUT my real question:

How do I convert the filament power to 6.3V so I can drop in different tubes? The added bonus here is being able to add on the preamp and tone controls to my existing k12. An email from the friendly folks at s-5 told me that I should be able to use the same B+ no problem, but as is mentioned, the heater filaments on the new line need to be 6.3V.

I realize this may be addressed elsewhere, but I couldn't find it quickly....directions help! Also, I thought sharing the idea of being able to add the new tone control/preamp to an existing k12/k502 were interesting.

I would love to purchase the new amps, but money and WAF say that adding another amp to the house is unnecessary. Adding a preamp and a tone control walks that fine line Yellow_Light_Colorz_PDT_02 . Would the preamp be an acceptable way for me to play my record player through my little k12?

[EWBrown]

The easiest way to add 6.3V filament power is to add a separate 6.3VAC filament trannie, I used a similar approach with the 11BM8 version, so that it could use either 11BM8s or 16A8s. I used a 16VAC, 2A "Signal" brand PC mount trannie, and added a DPDT selector switch (located under the chassis) so that either type can be used.

Now, for another alternate possibility, though I have not tried it, is to connect each channel's two 6.3V tube filaments in series across the existing filament winding, this should work, though it may (or may not) cause some residual induced hum problems. They should draw the same current as two 11MS8s or 10GV8s in parallel. A little bit of etch cutting and patching would be required to do this, but it is rather simple to do.
This doesn't directly accomodate the added preamp or tone control board, a small 6.3VAC 1A trannie will be needed, probably something suitable can be found at Radio Shark (or use a 6V 1A wall wart, either DC or AC).

The 11MS8 can be directly replaced with 10GV8s, or two 6GV8s' filaments can be connected in series. The UTK power trannies actually have a 12VAC filament winding, and they push the 11V (10.6V) filaments rather hard, and it is at maximum capacity with the existing filament current loading... :o

Using the *GV8 tubes gives you more real "tube-rolling" choices, as 11MS8s were a rather late entry into the TV tube field, and most were sourced by Matsushita (Panasonic) and one or two other Japanese manufacturers, though they may have "US" brand labels. Not to "knock" Japanese tubes, just they are from only one or two manufacturers. As far as I know, there is no direct Russian or other new manufactured direct replacement tube, for the "other than 6V filament" tubes.
There are Russian 6GV8 equivalents, though I've never tried them, nor know their designation for these.

/ed B in NH

[quackard]

Well, not sure if I know everything I need to know in order to do this, but hey, that's how I learn. I'm definitely interested in adding the tone control, and don't think I'll have any problem hooking up a 6.3V filament transformer for that...but can I just wire that same filament trannie up so I can use the 6GV8s instead of the 11ms8's, too? Do I need to get a transformer that offers more current if I'm going to do that? Or get a seperate transformer (they're only $12 or so)?

I know some purists are anti-tone controls and things, but as I think some people have said, the universal DIY questions starts with "What if....?" Thanks for the help so far. Hope to post some pics/results when I manage it.

[emuffler]

I have been silently lurking the various sites concerning k-12m kit w/ all the upgrades and I have finally tried a couple things, including my first ever post to a forum (shy I guess). I have an older kit with 11BM8's.
Upgrades include:
-Hammond Power Transformer 261M (hybrid power supply w/ 5v filament tranny)
-Hammond outputs 1609 (no ultralinear yet)
-Tube rectified (5v4, 5r4, 5u4 yielding various voltages 195-224vdc) into
-16uF oil motor run cap 360vac feeding into 8.5 henery choke then thePCB
-Input caps removed and replaced with soild copper jumper
-WIMA .22 400v for coupling caps
-silver mica in-place of ceramic disc
-Matched quad of NOS 6BM8's and 6.3 vdc on filament supply.
What does it all mean? I am no audiophile I just enjoy music and creating things. The stock unit is well documented for sounding better than it should for the money. I agree, and it provides a platform for tweaks! NOW=DEAD QUIET! Tight bass! Just breadboarded now. pics if interested as I build new chasis. :)

[emuffler]

Just to clarify the Hammond 261M has a 215V(269mA) no center tap secondary and a 6.3v (4A)no center tap filament supply. The 5v tranny I mentioned is a stand alone unit for the tube rectifier. http://www.lundahl.se/hybrid_pow.html = something like this.
Ed

[EWBrown]

I'm familiar with the 261M6, I snagged a couple of them off e-bay years ago, before I even knew what a K-502 was. I'll have to dig them out of the "buried treasures" down in my cellar, dust 'em off, and put them to some good use.

At the time, I had a Radio Craftsmen (RCA) RC-2 6V6 PP 10W amp, which had a cooked power trannie, and I had considered using one of these 261M6 as a "bandaid" to get it working. Later on I got a good clean working RC-2 for cheap, and have since traded that to Gary Kaufman, so he now has a pair of these. The dead one got "parted out" long ago...



Sounds like a nice re-build and upgrade project!

/ed b in NH

[emuffler]

Ed,

The 261M has a little higher current rating so it runs much cooler than the stock power tranny. Somewhere along the way you had mentioned a good deal on a toroidal transformer that would fit the bill for a k-12m. Did you ever implement?

Just as a note the 261M with ss full wave bridge developed too high of a B+voltage for the k-12m (which is what I wanted) to just be a drop-in replacement. With Tube Rect--->oil cap-->choke--->to PCB I can get 195-225vdc with different rect tubes.

I have ST-35 board that I need to get built up. From the sounds of it, it is a big step up from the k-12m. I have also built a Darling, SE 6BM8. I have parts for KT-88 Simple SE and a VTV 6SN7 line stage. Too Much!!!! I am in DEEP! The 6B4G looks awesome to. Thanks to all that have helped me and don't even know it - especially all the saftey warnings! What a great hobby. Yellow_Light_Colorz_PDT_01

Ed

[EWBrown]

Another way to reduce the high B+ voltage is to use a smaller value cap (usually around 5.6 or 6.8 uF, Solen or similar) between the rectifiers and the choke. and use the larger value caps downstream. The tube rect approach is also excellent, and it eliminates having to build in a "soft start" circuit.

ANother approach I've seen, is to use a four diode bridge on the (non-CT)power trannie, then feed its DC through a 5Y3GT or 5AR4, with both plates tied together, this acts as the slow starter and also drops around 12-17VDC in the process. Since the 5AR4 in this usage doesn't encounter the higher AC and PIV voltages, it should have a fairly long and easy service life.

This allows the inductance of the choke to "absorb" some of the 120 Hz ripple voltage, and thusly reduces the final B+ voltage. It's better than just using watt-wasting wire-wound resistors to do the job...

/ed B in NH