My first build, What should I do before thuning it on?

[M Fowler]

Does this start as soon as the tubes are warmed up or any specific time frame? Can you isolate the noise by removing one tube at a time starting with the power section. I wonder if one of the preamp tubes is bad.

[Mont]

it starts when i flip the standby to on...

[Mont]

I will try other preamp tubes and see what happens..

[paulster]

Just disconnect the negative feedback wire temporarily from the impedance selector and see if the howl disappears.

If it does then your output transformer primaries are round the wrong way.

[Mont]

OK , I switched & twisted theOT primary wires and that took care of the loud whistle, now all I get is a low fequency hiss. The bias was set at -43, when i put the tubes in it went to -20. the tubes did not redplate as you call it so i took readings. V1 pin1 230 pin3 2.5 pin6 222 pin8 2.0 v2 pin1 360 pin3 4.0 V3 pin1 230 pin2 40 pin 6 250 V4 pin3 430 pin4 450 pin5 -20 V5 pin3 440 pin4 430 pin5 -20 when I tried to adjust the bias to -33 the amp would only go to -26 and then I would read nothing on pins 1 on the preamp tubes. all i have is a cheap $30 VOM, Should i go ahead and pug in an axe and see what I get?

[marcoloco961]

OK , I switched & twisted theOT primary wires and that took care of the loud whistle, now all I get is a low fequency hiss. The bias was set at -43, when i put the tubes in it went to -20. the tubes did not redplate as you call it so i took readings. V1 pin1 230 pin3 2.5 pin6 222 pin8 2.0 v2 pin1 360 pin3 4.0 V3 pin1 230 pin2 40 pin 6 250 V4 pin3 430 pin4 450 pin5 -20 V5 pin3 440 pin4 430 pin5 -20 when I tried to adjust the bias to -33 the amp would only go to -26 and then I would read nothing on pins 1 on the preamp tubes. all i have is a cheap $30 VOM, Should i go ahead and pug in an axe and see what I get?

I am guessing these voltages are without the limiter???

Some of the voltages seem a little high, but they are within 10%. You might try changing the tube in V2 with another new tube as it is the only one that is way out of spec. On V4 are voltages on pins 3 and 4 reversed? Pin 4 should probably be lower than pin 3.
You can get the bias lower by changing the 220K R29 resistor in the layout below to a 180K or a 150K.

180K would probably get you there. The important thing is to keep the current per power tube below 45 mA at idle, standby on , no input.

Does the amp have bias test points?

I would change the R29 to a 180, re-check my bias voltage, set it to about -32 and plug in my guitar. Keep an eye on the tubes for red-plating.

[Mont]

Sorry the voltages for pins 3 &4 on output tubes are reversed I don't have bias test points. I willchange the resister and get back to you, Thanks....

[marcoloco961]

Sorry the voltages for pins 3 &4 on output tubes are reversed I don't have bias test points. I willchange the resister and get back to you, Thanks....

With any luck, you will be telling us how much you love your new amp in the next post.

[Mont]

HOLY FRAKIN SHIT. This thing is a frakin monster. I can only get the bias to -30 even with replacing R29 to a 150k. so i said what the hell and plugged in my (86) prs and WOW. Question is do i have to get it to -33. will it hurt the amp if i leave it at - 30?

[Greg]

...and away we go!

[marcoloco961]

HOLY FRAKIN SHIT. This thing is a frakin monster. I can only get the bias to -30 even with replacing R29 to a 150k. so i said what the hell and plugged in my (86) prs and WOW. Question is do i have to get it to -33. will it hurt the amp if i leave it at - 30?

Heheheh,


I think I usually end up around -31 so you should be close enough. It is really a matter of taste from here.
You might be a little over the mA suggested for the tube, but not so much you should hurt anything IMO. You might sacrifice a little tube life if it's too hot, but it's not like the tube will blow if it's a couple mA over. Just check it every so often to make sure the plates are not glowing red. Many people bias their amps a little on the "hot" side and sacrifice some tube life because they like the sound. Experiment a bit and see what you like best.

If you want to hear what it would sound like with the bias a little colder, take 2 test leads and hook one on each side of the 180K bias resistor and hook the other two ends of the test leads to a 1Meg resistor. a 180K and a 1M in parallel comes to about 152K. that way you dont have to change it out then change it back if you dont like it.

Have fun, glad to see ya got it runnin. Good call on the limiter Mark.

[M Fowler]

Good call on the limiter Mark

marcoloco961, lets just say its not the first time a limiter has foiled a first time builder.

I use a variac to keep things on the down low.

Mont I am Glad that baby is up and running, life to a new amp. Next you need to build the Rocket.

[geetarpicker]

Very cool!

[Mont]

You were right about the bright switch. every clone i've heard sounded too bright, that's why i left it out. running thru a vintage marshall cab it needs that switch.

[geetarpicker]

Yep, with G12Ms in an old 4x12 my amp is downright muddy without the switch.