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Just another K-12 Build
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Author:  gener8r [ 12 Apr 2012, 12:29 ]
Post subject:  Re: Just another K-12 Build

After reading everything on this message board, could someone please tell me exactly between at what two points would a B+ Bleeder resistor be inserted? I'm brain-dead today. I cannot figure out if the tubes bleed off the charge, or if the resitor is supposed to go to ground, or something else. Thank You.

Author:  Les [ 13 Apr 2012, 01:13 ]
Post subject:  Re: Just another K-12 Build

The purpose of a B+ bleeder resistor is to discharge the HT filter capacitor(s) when power is removed. Insert it across the last filter capacitor (looking at the schematic, that would be C9).

The bleeder resistor is actually a "safety" device. Since, after powering off, the HT filter capacitors will not discharge once the tubes stop conducting, a high residual remains on the capacitors. The bleeder resistor will continue to discharge the capacitor, making it safer to service the amp in a short period. You should, of course, meter the B+ before attempting to work on the amp.

Author:  gener8r [ 19 Apr 2012, 20:39 ]
Post subject:  Re: Just another K-12 Build

Thank you everyone for the info provided here.
I just finished the mods on a 16SL. I incoporated:
1. HV input mods outlined in voltsecond schematic
2. Moved HV rectification off board and used UF stealths.
2. C7, C10 and C11 replaced with 330uF and bypassed with 3.3 films (with a 330K bleeder accross C11)
3. 1uF film/snubbers on all 10 tubes' filaments
4. 330uf bypass of R12 and R25
5. Inputs direct to pin 7 of drivers (with 100K resistor to ground for impedence)
6. Replaced four .22 coupling caps with .33 PIOs.
7. Unit built with PRP resistors.

As it is now this is a COMPLETELY different amp than the stock unit before the mods. I could live with this.
It's fairly quiet but is sensitive to wire placement behind the unit. What little hum (60 cycle?) there is, is not volume dependent.
I changed the tubes for some 1960's 6AQ5s and also 6005s, but I couldn't notice much difference, which could change with more burn-in.

Possible Other mods:
1. Replace power tranny with an ANTEK AN-5T240 torroid which should eleiminate the power tranny as a source of noise.
2. 100 ohm (?) five to 10 watt resistor bypassed with a 1000uF/25V (?) between tube's output/pin 5 and input of OPT.
3. Rectify Filaments to DC.

I built this 16SL first and have now started on a K12 ............

Author:  gofar99 [ 20 Apr 2012, 13:58 ]
Post subject:  Re: Just another K-12 Build

Hi, Sounds like a bit of improvement there. Without going back through all the posts I presume you replaced the output transformers. If not that would make a big improvement over the stock ones. Try tube shields connected to the chassis ground (not signal ground) on the driver tubes to reduce the pick up of stray hum. The amp should not really be very sensitive to EMI, but it can happen. Some brands of tubes are more sensitive to EMI than others as well.

Good listening

Author:  gener8r [ 08 May 2012, 18:31 ]
Post subject:  Re: Just another K-12 Build

I have not replaced the OPTs. Looking at the forum, folks suggest either the Hammond 1608 or the Edcor. I'm liking the specs of the Edcors a little better, but not sure this upgrade is worth the cost? And then to decide to go UL or stay stock? Suggestions based on experience would be appreciated.

Anyway I just finished a K12, driven by a line stage/preamp (SP12 from tubes4hifi) all assembeled in one chassis. Came out nice as an integrated amp. Into some Bose 301s it is fairly quiet and a good match with the Bose's lower SPL. Sounds better than the good SS Marantz it is replacing. I'll post photos soon.

The 16SL into some 97+ SPL full range drivers have a little hum, but nothing you can hear seated a few feet back.

I did however add a 1000uf 35V electrolytic (Fine Gold nichicons) with a 10watt 100R bleeder between the tube (pin 5?) and the input (plate) of each OPT. It certainly didn't hurt.

S5 raised the prices on their kits since I last bought a kit from them a few weeks ago and they will not sell just the PCBs. If i do another it'll be point to point and I'll buy the trannies myself.

Author:  gofar99 [ 08 May 2012, 20:22 ]
Post subject:  Re: Just another K-12 Build

Hi, I have used both Hammonds and Edcors on this an prefer the Edcors. In any case the values should be for 8-10k on the K-12. I use U/L and it IMHO makes a much cleaner sound at a rather slight expense in output power. Trannies rated at 10 watts are fine on this amp. A real bargain is the Edcor GXPP10 series. They are supposed to be designed for guitar amps, but are quite excellent and we (Oddwatt Audio) use them in all series one "Poddwatts" which is a project posted on the forum elsewhere. They have a single 6 ohm output but work very well with either 4 or 8 ohm loads.

Good listening

Author:  gener8r [ 09 May 2012, 12:17 ]
Post subject:  Re: Just another K-12 Build

Thank You for taking the time to respond. It is appreciated.
Okay, the Edcor GXPP10 series for the K12. I assume the GXPP15 series for the 16LS?
I have been reading on here to use a 5K primary, but from your post I assume its okay to use either the 8 or 10K primary too?
I see Edcor has a choice of 4 or 6 or 8 ohms out, and your above post suggested the 6 ohm version. If I know for sure I'm using only an 8 ohm speaker, can I go with the 8 ohm version? (I know the 6 or 4 ohm version will work with an 8 ohm speaker)
Lastly, I'm a little confused about where to tie in the two addtional wires for UL operation. Is there a schematic or photos you know of I can look at somewhere, and addtionally, are the tie-in points the same on the K12 and 16LS, or are there differences there too?
Appreciate the help.

Author:  gofar99 [ 09 May 2012, 16:21 ]
Post subject:  Re: Just another K-12 Build

Hi if you are sure you will only be using 8 ohms then the GXPP10-8-10K would be my choice for the K-8 and K-12 with the GXPP10-08-5K for the K-16. The transformers are rather conservatively rated and in the U/L mode will be fine. The extra wire go to the screen grids on the tubes. You will need to cut the traces that already go to them. The modification of the K-12 project I posted quite some time ago on this site shows the changes for a K-12. Right now I'm traveling and don't have access to the rest of my files on this. The PCBs for the three amps are different so maybe someone else has a photo fo how to do the U/L mod to them.

Good listening

Author:  gener8r [ 11 May 2012, 19:31 ]
Post subject:  Re: Just another K-12 Build

Introducing the "12s" This is my first attempt at an integrated tube amp. It uses the S5-K!2 amp kit and the tubes4hifi-SP12 preamp PCB kit.

Built for the shop with just two inputs: a 3.5 on the front and a pair of RCAs in the back toggled by the DPDT in the front.

Unfortunately when laying it out I got the K12 PCB oriented 180 degrees off, Consequesntly the power and OPT wiring has much longer runs than they would have, and I'm sure this is contributing to hum. But in the shop matched up to some Bose 301s with their lower SPL rating; not bad at all. This thing gets loud.

One gaol was to keep the entire project under $500.00 which was accomplished. The SP12 is too much Preamp for the K12 amp, and if I do it again I'll go with the tubes4hifi SP10 to further lower costs as I don't believe it will affect the "sound quality".

The S5-16SL amp to me is a better and quieter sounding amp, especially when driven by a good preamp.

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