12 Series Build

bwgintegra · Post by **bwgintegra** » Fri Dec 02, 2011 3:28 am

Just started working on this in the past few weeks. One of George's chassis I got from a forum member, Brian H boards, C M transformers and choke, most of the stuff I have purchased from Valvestorm.

Take a look at the pictures and please feel free to make suggestions or point out problems. Already going to make a few changes and not real sure where I going with the build yet.

bwgintegra · Post by **bwgintegra** » Fri Dec 02, 2011 3:42 am

bwgintegra · Post by **bwgintegra** » Fri Dec 02, 2011 3:53 am

vanhalen5150 · Post by **vanhalen5150** » Fri Dec 02, 2011 7:11 am

Pretty good so far. You want to keep your OT primaries and secondaries apart though. I'd move your secondaries over to the right between V6 and V7. I'd put that yellow wire unerneath everything as well. The Purple NFB wire should also be as far away from everything else as possible.

vanhalen5150 · Post by **vanhalen5150** » Fri Dec 02, 2011 7:14 am

"Feel free to make suggestions".

vanhalen5150 · Post by **vanhalen5150** » Fri Dec 02, 2011 7:23 am

At this point from my own mistakes, I'd check for continuity with the heater wires before adding everything else. You could take a 9 volt battery and attach to one end of the chain. Set meter to vdc of course. Make sure your testing the pins themselves and not directly the wire or solder. It will save a from a big headache.

Strat78 · Post by **Strat78** » Fri Dec 02, 2011 9:16 pm

The wiring looks nice! Everyone has there own signiture style, especially the heater wires, cool. Solder the grounds from the pots at this point. Yeah, check each solder joint at least with a magnifying lamp. Make sure the wire insulating hasn't melted into your soldure joint preventing contact like some people seem to do around here

+1 on the primaries and secondaries needing a little elbow room. I would use an external variac and not hook up the dual voltage switch, just use a standard on off and a standby. Like the grid stoppers though some of the old timers around here think that is not as safe as the other way. What OT did you get from Chris, 132? What are you using to populate the board?

bwgintegra · Post by **bwgintegra** » Tue Dec 06, 2011 10:56 am

Thanks for the information guys. Not going to use the dual voltage switch, got to find a good variac. For the main board I am just using the basic stuff from Valvestorm. This is my first ground up build and learning a lot. I don't know where you get the good NOS resistors and caps. I am using the C1998 1.5 stack OT.

Question: The two metal oxide 10K 2watt resistors by the preamp filter, can I just use regular 1 watt carbon film resistors in those spots?

vanhalen5150 · Post by **vanhalen5150** » Tue Dec 06, 2011 6:51 pm

Personally I'd use 2 watts there. If any problems occur, your going to go back and change them as a precaution anyways . When in doubt stick with what has already been proven I guess.
I like those boards!

Strat78 · Post by **Strat78** » Wed Dec 07, 2011 12:17 am

2W would give you a little more security, but I've had 1W Pihers in Ben's amp with no problems yet. Piher and Iskra 10k 1W and 2W resistors are hidden here and there on ebay. You should start collecting them now because it can take months to gather all the right NOS parts.

Valvestorm carries some pretty cool stuff too like the Lemco caps, Rifas and mustards etc, plus pink wire!

vanhalen5150 · Post by **vanhalen5150** » Wed Dec 07, 2011 7:08 am

I've been looking at some of the 10k 2w iskra's on the bay. $16 for 2. Seems steep to me. Then again so is $250 transformers.

bwgintegra · Post by **bwgintegra** » Sun Dec 18, 2011 3:37 am

I picked up some EH 6CA7's for my build. Question: what value of bias resistor are you guys using when using these tubes with a variac and what are you setting the bias at. Anybody know where I can find a good variac? Haven't worked on the amp much lately but hope to get sometime during the holidays. Thanks

vanhalen5150 · Post by **vanhalen5150** » Sun Dec 18, 2011 11:52 am

It will really depend on your individual amp for the bias resistor. You have to play it by ear and see what's going on when you drp the voltage with the variac. I just bought one of the cheap Chinese variac'd for about $50 online. Had it 2 years and it works fine.

Strat78 · Post by **Strat78** » Sun Dec 18, 2011 9:29 pm

for the bias resistor, have values on hand that measure around 27k, 33k, 47k or 56k. Start with a 33k and see where that lands you as far as having enough play with the trem pot. You really need to get a variac befor running those tubes. The EH's seem to dislike voltages above 400v. I keep the plates at around 350 to 385. Ben's amp is at around 350v. According to the formula the bias should be around 46, but it sounds pretty killer up around 51. Another factor is a Drake tranny clone verses the Dognal. The Drake's "132" seem to perform much better with the lower voltage and high bias, where as the Dognals seem to start farting out.
You want a variac with 120v input with 7 to 10amps, 0 to 120 or 140v output. I like the old Ohmites for obvious reasons, but they only come up once a month or so on ebay, and chances are that I will outbid you.

vanhalen5150 · Post by **vanhalen5150** » Mon Dec 19, 2011 8:34 am

What seems to be the direct relationship with the bias as you go up in resistor value? I havent played with this a lot.

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12 Series Build

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