68 12,000 with 2" high impedance OT

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emmjaydubya
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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Sun Feb 03, 2013 1:39 am

No, 13.65 watts is the plate dissapation, with 17.5 watts (@70% of 25) being the goal, meaning the bias is cold. From what I gather, this method works backwards from the other methods. We're trying to calculate the tube wattage.

Again, instead of using the target wattage as part of the equation, the wattage ends up being the goal, correct? In other words, you take the [OT volts; variable depending on bias] divided by [OT resistance, in my case 45 ohms] times [plate voltage; also variable depending on bias] EQUALS the target wattage of 70% of 25 watts plate dissapation of EH 6CA7s which is 17.5 watts.

Again, the equation from VH junkie:

OT_VOLTs/OT_RESISTANCE * plateV = target wattage (17.5w @ 70%)

Again, calculating the plate dissapation wattage. If it's less than 17.5, you're cold. If it's more, you're hot. Is this right?

The resistance on my OT with power off is 45 ohms, each side, not 17 like you had suggested. Is this because my OT is 2.5k instead of 1.75? This is where I got confused and remeasured a bunch of times. But after using this formula, I got close to the 17.5 watt target, so I assumed I was right.

Funny thing, the bias keeps falling each time I play the amp for 15 minutes or so (fallen 3 times now). Right now, I have the bias pot cranked all the way and my my plate dissapation wattage ends up around 15 watts, under the 17.5 goal, so, tubes are biased cold, right?

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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Sun Feb 03, 2013 2:56 am

Since the last few pages of this thread have been either desperate cries for help or boring math, I thought I'd post some finished build pics (till I start changing things), taken in the 72 degree twilight of Austin, Texas. Everyone likes amp pictures, right? :lol:

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Last edited by emmjaydubya on Sun Feb 03, 2013 3:05 am, edited 2 times in total.

emmjaydubya
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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Sun Feb 03, 2013 3:02 am

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Re: 68 12,000 with 2" high impedance OT

Post by rgorke » Sun Feb 03, 2013 9:29 am

Beautiful work and I love all those Iskras!!!
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.

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Re: 68 12,000 with 2" high impedance OT

Post by vanhalen5150 » Mon Feb 04, 2013 6:44 am

Looks good. :thumbsup:
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Re: 68 12,000 with 2" high impedance OT

Post by Strat78 » Wed Feb 06, 2013 9:11 am

A work of art, magnificent! Keep going with this one, like using truss string instead of the plastic ties. We need to find the formula for that original red solder shield paint. You can buy the bright red stuff these days, but it is more like rubber than the blood red lacquer consistency of the old material. Ralle made me start thinking about the correct wire like 7/30 22awg verses the 16/30 pre tinned stuff that is widely used these days. I don't have my amps set up at the moment, could someone measure the voltage that the pots see. You can get pink 7/30 22awg NOS wire but it is mostly rated for 300v vs. 600v which seems to be the standard. Perhaps 300v is fine for the pots?

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Re: 68 12,000 with 2" high impedance OT

Post by rgorke » Thu Feb 07, 2013 12:50 pm

I just noticed that 330 or ? Erie cap in V1A. Where did you get that? I don't think I have seen one before.

Frankly, this looks better than many of the original marshalls I have seen pictures of.
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.

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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Thu Feb 07, 2013 2:17 pm

Thanks so much guys, it was a fun build. It took about 9 months to get everything, not just finding the parts but mostly PAYING for them. I'm just glad I was lucky enough to find this stuff to solder together.

The Erie cap in V1A is a 100uf 25V that I thought I'd try first, you can find them on ebay from comtex. I have read many times in different threads that NO ONE uses 330 or 220 because of the sub harmonics that they let through, yet NO ONE EVER says what they like to use here! Neil advocates a 25uf but I was wondering if it would shelf too much bass? Once it's broken in, I plan on trying a 330, a 220, the 100, a 33 and a 25 to see how they all compare since I've never done this. I also want to try the same in V2 over the .68/820 to check out what the fat cap buzz is all about.
Strat78 wrote:A work of art, magnificent! Keep going with this one, like using truss string instead of the plastic ties. We need to find the formula for that original red solder shield paint. You can buy the bright red stuff these days, but it is more like rubber than the blood red lacquer consistency of the old material. Ralle made me start thinking about the correct wire like 7/30 22awg verses the 16/30 pre tinned stuff that is widely used these days. I don't have my amps set up at the moment, could someone measure the voltage that the pots see. You can get pink 7/30 22awg NOS wire but it is mostly rated for 300v vs. 600v which seems to be the standard. Perhaps 300v is fine for the pots?
Thanks Phil, I definitely cheesed out on the zip ties! :shock: I considered lacing cord on the leads for about 5 minutes, but the zip tie tray was staring me in the face and I caved, haha! :hide:

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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Thu Feb 07, 2013 9:17 pm

I played it on and off for the last few days, at 90V (350V B+) it sounds really compressed and gainy at about 9, and it screams! A little more definition at 95V. I haven't dimed all the controls yet to see what that does since I'm still watching for red plating. It has that 'thing' that I've heard many times on here, tight, artuculate, with lots of harmonics.

The only humbucker guitar I have is my '57 Les Paul Historic gold top with a '57 in the bridge. I've had this guitar a long time so I know all of it's characteristics, and though it does rip through this amp, it has a very distinct sound. The next thing to build is an ash bodied, humbucker, 6-screw trem strat. That is gonna roar.

Last night I took it to band practice and right away I noticed how the amp was losing dynamics at 95V (363V B+) with the band. So I bumped it to 100V (384V B+) and re-biased. Sounded even better so I'll probably keep experimenting with input voltages till I find the sweet spot.

Now the issues. The bias keeeps dropping on me after playing. I've checked, upped bias, and played probably 20 times in the last few days, the bias keeps gradually slipping. Since I've been upping the input voltage, I haven't had to change the bias resistor, so far it has been staying in range, which is why I'm confused. The bias is falling and it shouldn't, right?!?!

Plus, what kind of noise floor do you guys have on these amps? Mine is pretty noisy when it gets up around 9. Marshall hiss of course, but with some dull crunchy noises mixed in, sort of dull static in the background. I poked around in there with the amp on but couldn't get it to change. Pre-amp tubes maybe??

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Re: 68 12,000 with 2" high impedance OT

Post by vanhalen5150 » Fri Feb 08, 2013 9:11 am

I've noticed the snap crackle pops on mine as well. They tend to go away after the amp has been on for a few hours. Keep in mind you have a lot of old parts in there. Thats the double edged sword of NOS.
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Re: 68 12,000 with 2" high impedance OT

Post by axeman » Fri Feb 08, 2013 1:14 pm

A work of Art indeed.

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Re: 68 12,000 with 2" high impedance OT

Post by Strat78 » Sat Feb 09, 2013 9:02 pm

EH tubes seem to drift quite a bit in the beginning, I buy them eight at a time and match them myself after they have burned in. Also use those aluminum covers for the 12ax7's, that might stop some microphonics. Yeah, that resistance of 45 ohms on the tranny threw me off, but I have to get my amp fired up so I can try your method. Again, beautiful work!

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Re: 68 12,000 with 2" high impedance OT

Post by SteadyEddie » Sat Feb 09, 2013 9:42 pm

vanhalen5150 wrote:I've noticed the snap crackle pops on mine as well. They tend to go away after the amp has been on for a few hours. Keep in mind you have a lot of old parts in there. Thats the double edged sword of NOS.
Not to derail, but I also have snap crackle pops. I think it's moisture in my carbon comp resistors.

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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Sun Feb 10, 2013 2:34 pm

Strat78 wrote:EH tubes seem to drift quite a bit in the beginning, I buy them eight at a time and match them myself after they have burned in. Also use those aluminum covers for the 12ax7's, that might stop some microphonics. Yeah, that resistance of 45 ohms on the tranny threw me off, but I have to get my amp fired up so I can try your method. Again, beautiful work!

Thanks again! And it isn't my method, it came from VH junkie:

OT_VOLTs/OT_RESISTANCE * plateV = target wattage (17.5w @ 70%)

I'm hoping I have it right, I've mainly been measuring off of V4, but the tubes seem happy so far so I'll stick with it. Just wondering when the bias will quit falling, you think it's the tubes, huh? I haven't played it in the last four days so I'll see where it's sitting and I'll try putting on the alum pre covers to see what that does. Also gonna mess with input voltage some more.

Probably have to live with the noise since it is, after all, a Marshall circuit with old parts giving everything it has, and it sounds like it! It has a really sweet glassy quality when dialing back the volume on the guitar but also has a ton of gain full out, really digging this amp and it isn't even close to breaking in yet.

Next I think I want to rip off Shannon and build a box with 4 speaker motors. Except with a line out (for around the house) AND put the Variac inside too, along with 120V fan to keep everything cool. Totally self contained live rig: guitar, head, cab, ammo box, attenuator, GO!!

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Re: 68 12,000 with 2" high impedance OT

Post by Jeremy1283 » Sun Oct 19, 2014 1:59 pm

emmjaydubya wrote:Since the last few pages of this thread have been either desperate cries for help or boring math, I thought I'd post some finished build pics (till I start changing things), taken in the 72 degree twilight of Austin, Texas. Everyone likes amp pictures, right? :lol:

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This build is really nice! Great work.
How does it sound?

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