100 watts preamp tube figures

5150loveeddie · Post by **5150loveeddie** » Tue Jun 15, 2004 9:56 am

All the readings on my pre amp tube checks looks OK except for 3!!!???

it'all on V3
1) V3 pin1=201 (should be 225)
2) V3 pin2=12.8 (should be around 24)
3) V3 pin7=14.1 (also should be 24)

Trannies are MM
tubes are the JJ that come with the kit

What should I do Jon, George. Is that normal?

JF

Post by **VelvetGeorge** » Tue Jun 15, 2004 1:59 pm

JF, the pin 1 reading is a little low. But that could be caused by a lower voltage coming from your wall outlet. It's not low enough to be a huge concern.

The other (2) reading are on grids where your main concern is signal voltage more so than DC. The DC reading here can also be affected by the negative feedback wire. Specifically, which tap it's conected to and what size resistor is used- typically 47k or 100k.

I'd install those 1M resistors (they should arrive any minute now) and fire it up. Let the amp settle in a bit and then see how the readings turn out.

Keep us posted. And one other thing...............

DUDE! YOU BUILT YOUR OWN AMP!!!!

How cool is that???

George

5150loveeddie · Post by **5150loveeddie** » Tue Jun 15, 2004 5:02 pm

That's really cool.............I'll redo it any time and will anyways!!!

And yeah for the 1M resistors, I did install them before the preamp tubes checks!...but they are not from you....I went and buy some here somewhere they are 1/2 watts, maybe they're not good they are smaller in size.......!!!???

Could it be the MM trannies? The tube itseft?....

Anyway I will swap the tubes and see...!!!

JF

5150loveeddie · Post by **5150loveeddie** » Tue Jun 15, 2004 5:06 pm

I forgot,,,the negative feedback is installed on the 4 ohms (purple one...)terminal tap...

JF

5150loveeddie · Post by **5150loveeddie** » Wed Jun 16, 2004 2:32 pm

The amp is working and loud like crazy and you can see that the plexi tone is there

Now have to find how to decrease the humming it is way to hight.....

Hum was there when I bias the amp with no inputs (guitar...) and the settings were all flated out, the hum increased when bias setting was increased, and after, when teble or any other setting was increased on the preamp section. Everything is working well except for the steady hum.

Will redo my heater wires to be as close to the chassis as possible....

Could it be a ground somewhere?

Notice that the AC wire had a bare wire I connected it with the green one(ground) to the chassis.

Any suggestions would be appreciated!

JF

Post by **VelvetGeorge** » Wed Jun 16, 2004 3:43 pm

Hi JF, glad to hear the amp is up and running. Now, about that hum.....

is it a low frequency hum? like 60 hz line noise?

is it more of a hiss?

does it go almost all away with the presence control on zero?

can you try removing the first preamp tube?

still there? remove the second preamp tube. this will help isolate where the hum is originating.

This is a little more dangerous, but with the amp on- clip a test lead to one the ground points on the board. If you ground the other end of the test lead to the chassis, what effect does it have (if any) on the hum?

Let us know.

George

5150loveeddie · Post by **5150loveeddie** » Wed Jun 16, 2004 11:58 pm

When V1 tube is removed no change.......

When V1 and V2 tubes removed NO MORE HUM

When only V2 removed almost no more hum, got even better when mid or teble pot was increased!!!

Swap tubes also on V2 with no result......

Green (grid pin2 of V2) wires are lifted up AMAP

I redoned the heaters on V1 V2 V3 looks great but no change in hum reduction........

So somewhere in V2 is my hum problem!?

BTW the bright cap (500pf) is very effective could it be reduced a little...??

JF

flemingmras · Post by **flemingmras** » Thu Jun 17, 2004 2:09 am

If you run the amp fully dimed, that cap will have no effect. The BEST way to run ANY Marshall!

Jon

5150loveeddie · Post by **5150loveeddie** » Thu Jun 17, 2004 8:54 am

Hey Guys,

was just checking some voltages and the hum got cancelled when I touch the upper lead to the green wire on the board that connect to pin3 of V3 of rather the .022cap or both top leads of those 1M resistors orthe .1cap It's like that section is in need of beeing grounded?

^

I'm so new at this......................

JON, Goeorge??????????????66

JF