First Metro Build - 12-Series

Moderator: VelvetGeorge

User avatar
vanhalen5150
Senior Member
Posts: 7307
Joined: Wed Sep 10, 2008 3:13 pm
Just the numbers in order: 7
Location: Halifax, Canada

Re: First Metro Build - 12-Series

Post by vanhalen5150 » Tue May 18, 2010 6:48 am

somethin'else wrote:26.7k, and 46.4k on the other bias resistor.

Would that be a place that the overall plate voltage could drop?

Things are slowly settling. I have 480 tonight, with bias between 33 & 35.5 vdc. Not shabby, and loud enough to call the dentist. Just wondering "where that initial 500 B+ goes". I know the tubes soak up quite a bit.
The voltage there should be about 90 (27K). Ive been getting 101. Perhaps not a big deal.
12000 Metro Kit

User avatar
vanhalen5150
Senior Member
Posts: 7307
Joined: Wed Sep 10, 2008 3:13 pm
Just the numbers in order: 7
Location: Halifax, Canada

Re: First Metro Build - 12-Series

Post by vanhalen5150 » Tue May 18, 2010 6:50 am

somethin'else wrote:26.7k, and 46.4k on the other bias resistor.

Would that be a place that the overall plate voltage could drop?

Things are slowly settling. I have 480 tonight, with bias between 33 & 35.5 vdc. Not shabby, and loud enough to call the dentist. Just wondering "where that initial 500 B+ goes". I know the tubes soak up quite a bit.
480 on pins 3/4/6 V4-V7 right?
12000 Metro Kit

User avatar
vanhalen5150
Senior Member
Posts: 7307
Joined: Wed Sep 10, 2008 3:13 pm
Just the numbers in order: 7
Location: Halifax, Canada

Re: First Metro Build - 12-Series

Post by vanhalen5150 » Tue May 18, 2010 6:55 am

The pin holes were crimped together a bit. Tough to fix without damage. Looking at other 12AX7's the Mullards seem to be slightly larger pin wise. Only one of them went in correctly. Works fine but not something I'll leave.
12000 Metro Kit

somethin'else
Senior Member
Posts: 1147
Joined: Tue Mar 18, 2008 10:19 pm
Just the numbers in order: 7
Location: the dirty south

Re: First Metro Build - 12-Series

Post by somethin'else » Tue May 18, 2010 8:58 am

The voltage there should be about 90 (27K). Ive been getting 101. Perhaps not a big deal.
Aw Crap, I lifted them to read them individually, not in the circuit. Dohf. :lol:

Will re-measure tonight, after work, where all day I read the forum!

Yeah, last I left it I had 485 on pins 3/4/6 on V4-V7. Well, 485 on pin3, 483 on pin6. Bias right at 35. Pretty great.

My bud suggested getting some dense rubber grommets for the OT to decouple it from the chassis. I'm gonna try it. The main thing is the Alu chassis sort of magnifies and resonates whatever hum there is, making it perceptively louder. I think my house is suspect. I variac 120 from the wall, but it's older wiring where grounding is hugely suspect, if there at all.

I got nothing grounding to transformer lugs (Larry Grounding) so I'll check it out. Or will this kill any grounding effect of the OT tabs sitting on the chassis. We'll see.

I also saw talk about rotating the PT 180 degrees being a rare solve for the transformer fields' cross talk. THAT would be my last resort, and I don't really need to go there.
dave

somethin'else
Senior Member
Posts: 1147
Joined: Tue Mar 18, 2008 10:19 pm
Just the numbers in order: 7
Location: the dirty south

Re: First Metro Build - 12-Series

Post by somethin'else » Wed May 19, 2010 6:48 pm

I got's 101-102 on the 27k bias resistor, vanhalen5150. These are areas that I need to study more. Why voltage there should be 90?

Measurements at other locations to... I'd love to learn where everything should sit through the circuit. Mains, screens, rectifier, pres, all the way to V1. My knowledge isn't that deep yet.

I know somewhere a slight drop is happening. If I read 507 initially w/o the power tubes in, and now have 478 average on the plates. Maybe that's a perfect settling place?

Then how in the heck does George, and you, stay right around 498? Diffs between '68 & '69 layouts? do you have the 1203-80-ML? The store says the ML does 485/395 and the SL does 495/395. Hmm, maybe a typo on the ML.

Onto the Wiki...

Thanks,
dave

Post Reply