Bias trouble.(very popular topic subject)

[garysanders]

OK (DRUM ROLL)

I re-wired and relocated the entire bias circuit( from the pot to the tap )

I also installed the bias check terminals and whatdya know.I was checking the pin 8 bias ALL wrong,like I said guys this is my first build.I got bored with stomp boxes and decided to do something dangerous!

Anyway,now I have another slight problem.with my bias pot all the way down I can only get around 45mA per tube,should I fix it to go down to 35mA?And I know it isnt matched pair cause the other was 52mA.And these were the tubes that I got with the amp when it was a Carvin so who knows?After what Ive put em through,Im suprised they light up.

I played through it for a minute.Only have one channel going. That EQ,Skyline,Deep,Mid boost,Dumbo-blah is INSANE.And dont have my trans for the relays yet.Honestly,I dont even know what channel Im on.I think its OD cause the OD volume is working but not the ratio.I can figure all that out myself.Im not new to electronics,just power tubes.

Whats cool is this thing was built with stuff that come from and old guys TV repair shop.All like NOS 70s stuff.Except the electrolytics,They are new.And I built it on loop terminals.No board.I dont know if that will bite me in the end but,its working so far,its quiet,and I have learned alot.Especially when I hit that plate resistor with the forceps.Funny story to that,I'll tell it another day.

So one more question: How do I get the pot to drop a little more?I need to loose about 10 more milliamps.

[BobW]

Gary you need to increase your bias dropping resistor. Doing so increases the negative bias voltage and reduces the idle current.

Remember a greater negative DC bias voltage = a lower bias current.

I can't wait to hear it.

[garysanders]

I put in a 20K and got it where it needs to be.Now to tackle the EQ glitches.I have decided to do away with the PAB and the deep switch.I will never use that.Just a high mid low and presence is good enough for me.Plus I want to add a reverb driver so that way I will have the poles on the footswitch socket.

[garysanders]

Don't know if this helps but as it relates to the subject.. Marshall style bias supply...

The following graph shows the grid bias voltage versus the ratio RS / RL for the simplified grid bias supply schematic. The curves are for five different transformers: 300-0-300 (upper curve), 325-0-325, 350-0-350, 375-0-375, and 400-0-400 (lower curve).


The ratio of source to load resistance varies from 5 (variable resistor set to minimum) to 3.3 (resistor set to maximum). The middle curve, corresponding to 350-0-350 power transformer, shows that the grid bias voltage can be adjusted between -28 and -42 volts. The ultimate adjustment range will vary depending on the error tolerance of the components and variations in AC line voltage.

Tony VVT

Thanks for that!My PT is a 350-0-350

[Structo]

Don't be so quick, the PAB can be made to work very well.
It is part of the ODS amp.

Do you have channel switching relays in your amp or do you plan on installing them?

That really makes the amp much more flexible for channel switching and or PAB or Mid boost.
Fairly simple to implement as well.

Bob can help you with that I'm sure.

[garysanders]

Youre right.I tried a Fender type tone stack and one like the boogie mark II and either way it sounded pretty rough.Guess it just wasnt designed for that.I have put the PAB and Rock/Jazz switch back in.A little better.For some reason my bass and mid isnt working,the treble is acting like a tone control with a very high top end.I think its something with the way I wired the PAB relayor the Rock switch.The EQ may just be bypassed.It just doesnt look right to me.The over drive sounds awesome,besides the lack of volume.The Level and Ratio controls function backwards of what I had in mind.But that stage of the amp sounds awesome.if only the EQ was working(!!!!!!)

What gets me most is that it has less than half the volume a 50 watter should and its breaking up with the pre volume about halfway up.Like the pre amp is pushing too hard and the power section isnt pushing enough.I can turn the master all the way up and can still hear myself talk.

I also have the OT on 8 ohms and the FB line on the 8 ohm tap.I am running this because the cabinet has an 8 ohm EVM 12L

Please share your thoughts,other than that I am a noob,that is obvious