Bias circuit help

[Megachunk]

Hello guys. How's things?
Tubenit is helping me with a build for myself and it's coming out great. Thanks Sir Tubenit!
I am having trouble figuring out the bias circuit, as I have a Fenderish hardware setup but want to make a Marshall type bias since I'll be running EL34s. Can anyone help me with this? I was wondering if I have to run the 47K resistor from the 220K junction to the center wiper of the bias pot and the other lug closest to the resistor/cap combo. Also, do I ground the remaining lug? Any help is greatly appreciated and thanks mucho in advance. Feel free to draw lines in my photo.
Megachunk

[sluckey]

Connect the dangling end of that 15K to one end lug of the pot. Also run a wire from the junction of that 15K and pot end lug to the junction of the two 220Ks. Now connect the center lug and the other end lug of the pot together and connect a 47K from the center lug to ground. That's all.

[ThermionicEngine]

Hello MC,

I'll give you a lawyer answer first- It depends.  I am presuming by the 220K resistor that you are taking the bias supply from the B+ and not a bias supply tap on the transformer.  The Marshall set up calls for a wire from the 15K to the junction of the two 220Ks in the PI output.  The 47K goes between the 15K and left lug of the pot.  The wiper is connected (as a rheostat) to the right lug and the right lug goes to ground.  Do not wire the pot as a voltage divider (like a volume pot in a guitar).

If the bias source is a dedicated tap or winding off the transformer (usually about 70V), then you have an entirely different set of component values to install.  Let me know if this is the case and I'll revise my suggestions.

I would consider using a 27K pot (or something close to it.)  Since you are building it for yourself, you should be able to set the bias without frying the tubes but I wouldn't use the 50K in a customer build.

I hope this info helps.  I stand ready to be corrected by wiser tube gurus.

Regards,

Larry

[ThermionicEngine]

Oops!  I didn't know Sluckey posted to this.  (I walked away from my post for a bit before I sent it).  I think electronically they are the same but I yield to the senior member.

Larry

[tubenit]

Just reinforcing what has been said.  Here is Hoffman's Plexi 5ow bias circuit layout

[Megachunk]

Thanks everyone!
It's coming along nicely. Be done wiring in the next day or 2 and will post voltages.
Thanks all!
Megachunk.

[Megachunk]

Doing my startup and measuring the negative voltage with no tubes in at the 220K junction. I'm only getting just over -8 volts on this. Could my pot value be too high at 50k?
Other voltages in the preamp are running too high as well. Ive got a 325-325 power trannie and am trying to bring the rail voltage down. Afraid I may need a choke. I'll measure all votages and post later tonight or tomorrow. Got a gig tonight. Thanks all.
Megachunk

[Megachunk]

Here are my voltages with no tubes in yet. Preamp seems way too high. Can post my B+ rail and transformer values if it helps. Bias neg voltage is too high as well at -8V. Bias range resistor is a 220K. Any help is very appreciated. Thanks! Link to voltages below:

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10871

Megachunk

[LooseChange]

What is the AC voltage you are pulling of the PT... Do you have a Bias tap or running off a HT tap?
What is the high and low voltage as you turn the pot from one end to the other?

[Megachunk]

325V-325V. Bias tap. Range in bias pot from about -4.3v to -8v to -8 or close to that.
Thanks man.
Mega

[LooseChange]

What is the Bias tap voltage? Around 60vac?
And what is the value of that resistor between the bias tap and the diode?

[ThermionicEngine]

Hello MegaChunk

From my previous post, replace the 220K resistor before the diode with a 27K.  Replace the 50K pot with a 22KL or 25KL potentiometer.  Wire it up with the pot in series with the 47K and ground.  Either of the two sugggested variations should work.

Please let me know how the voltages range.

Larry

[Megachunk]

Thanks guys.

The bias tap voltage is about 51 vac. That's a 220K resistor between the tap and diode. I'm going to try the 25KL pot and 27K resistor unless someone has another idea, but I'm real concerned about the high voltage on V1 and V2 mostly. Have to bring that down and not sure if I'm able to use a resistor or have to put a choke there.

I'll repost when I get the bias in range. Thanks TE and LC.

MikeG
Megachunk

[sluckey]

The bias tap voltage is about 51 vac. That's a 220K resistor between the tap and diode.

That 220K resistor is the value to use when you connect to the 325vac HT winding. The easiest fix is to just disconnect the 220K from the bias tap and reconnect it to the 325vac HT winding. Tape off the bias tap wire.

Or, leave it connected to the bias tap but change the 220K to 1K/1W or maybe even 470Ω/1W. If you do this, you may also have to tweak some other resistor values. I'd just take the easiest way out.

[Megachunk]

Already got the 27K in place of the 220K and am going to the Electronic storeto get another 25K pot. I always take the hard way! A little confused as to how to wire up the pot in series with the 47K to ground.

Sluckey did you mean take the 220K to one of the HV taps going to the diodes to the stby switch? Wouldn't that bypass the bias pot? I'm new, I admit.

Mike
Megachunk

[ThermionicEngine]

Hello MC-Assuming you have committed to the hard way, here's how it should go:

The Bias supply tap is now connected to the 27K.  The 27K goes to the striped end of the diode.  The other end of the diode is connected to the 15K where it is also connected to the negative lead of a 10uF cap.  The positive lead of this cap goes to ground. The 15K is connected to negative lead of the second 10uF cap. The positive lead of this cap also goes to ground.  The 15K is then connected to junction of the two 220K resistors at the output of the PI.  The 15K is also connected to the left lug of the bias adjustment pot.  The wiper is connected to the right lug.  The 47K resistor goes from the right lug of the pot to ground.  Check the voltage- you should be able to dial in -30 to -45 VDC.

Let's see what you get.

Larry

[sluckey]

Mike, decide which way you want to go with this. We're presenting two different approachs. Either is fine, but you must choose. The method I used in the pic was based on your statement about wanting a Marshall type bias since you were using EL34s. Then, when I realized you were feeding the bias circuit from a low voltage bias tap rather than the HT winding (Marshall style), I suggested simply feeding the bias circuit from the HT winding rather than the bias tap. That was a one wire change.

I'll back off to keep the confusion down until you decide which way you want to go.

[Megachunk]

Installed the 25k pot and I tried the 27K bias resistor and it only got it down to about -26. Put in a 10 K and got it to -35. Is this enough or do I need more neg voltage? If this is OK, I have to work on the voltage going to V1a and b. Still too high even with a 22k across the nodes.

Mega.

[Megachunk]

Can I blow a power trannie if there are no tubes in it and I run it too long? It's blowing fuses now and humming loudly when I power it on. Smell a little electrical... Hope not!

UPDATE: Yep I did. unsoldered the primaries and measured with the tone setting and it rang out, and displays 3 ohms. F***! Well, it's the first one I've ever blown on my own, so I consider myself lucky. Oh well. Is there a limit to how long you cna run them w/o tubes for testing? It wasn't on any longer than 5 minutes for a couple times.

Megachunk

[ThermionicEngine]

MC,

You can run it for a long time with no power tubes in it.  The issue is running the tubes with no load on the secondary of the OT.

I have questions, but on second read, I figured out what you did.  Your transformer is not likely blown.  It is not an extraordinary reading across the primary to get 1 to 3 Ohms.  This is where the big wire is and DC resistance is minimal.  You will almost always get continuity reading on the primary of the PT.

It doesn't mean you are out of the woods.  If you smell wiring burning and it's blowing fuses, look for a short. The high voltage on V1 and V2 indicate to me that something is mis-wired, but it could be several other things.  Shorted windings in the secondary could cause higher voltages but if you had 325-0-325 on the HT, it is probably not that.  I would look elswhere.

My typing is very slow. I'll PM you my phone #
Larry

[tubeswell]

, I have to work on the voltage going to V1a and b. Still too high even with a 22k across the nodes.

Mega.

Did you say those voltages were unloaded (i.e. without tubes)? You need the tubes in to drag the HT voltages down. Otherwise there is no current flowing through that part of the circuit, and everything on the B+ rail will be sitting at the same maximum.

[Megachunk]

Well it seems the transformer didn't blow and I'm currently hooking up each thing till I get to the one that blows things. Primary and heaters hooked up. doing the bias tap and lines to the stby switch next. TE thinks I probably have somthing touching ground; I'll buy that, since this is the tightest build I've done to date.

Didn't have tubes to try it out. Got some yesterday so I'll put them in and remeasure when I get it working. That makes sense that the voltage is the same throughout all the sockets. Thanks Sluckey for all the input. And thanks to you too, TE. The call helped me out greatly. A little worse for wear after the gig last night (was a raging good time!) So I may need to rest a bit more before I dig around in some high voltage.

Thanks you guys, it's great to know you got my back. Report back soon.
Mike
Megachunk

[Megachunk]

Well I got it all hooked back up and it’s doing the same thing: LOUD hum and fuse blown. Not until I hooked up the HT taps to the diodes that run to the stby switch, though. All other connex are OK. Checked all connex and they’re clean. I also noticed that when using the tone part of my meter those points make sporadic sound or have some continuity to ground. When I put the stby switch to play, it goes away. Doesn’t seem right. Going from the stby switch down the line to the 1st node on the rail, there is sporadic tone, but gone when I switch into play. Could there be a short in the HT windings? Bad switch?

Any help is greatly appreciated. Thanks in advance.
MikeG.

[sluckey]

Do you have a schematic and/or layout?

[Megachunk]

Hey Sluckey, thanks.

Yes I do, but you know me, I'm not really following it to a T. I'll PM it to you. I did manage to get the HT taps connected and working again. I replaced the switch and diodes and it worked. I'm off to get a 10K pot and try it the old Fender way - Only getting -36VDC on that 220K junction. Still no tubes yet, but will get that going after I clean up a few things first and get sufficient negative voltage.

Megachunk

[sluckey]

I'm looking at the pic in your first post... corner of the board that shows the two 220K resistors. Is that heavy buss wire connected to the junction of those two 220Ks? Surely not. But, if so, take it off. Don't put any tubes in until you get the power supply working right.

[Megachunk]

I figured that out - I fixed that already. It was like that on the layout. Got a 470ohm in the bias resistor, a 10K pot and a 27K resistor. Getting about -44VDC now. Thinking maybe the 47K back in there on the back of the bais pot will get me in range if I'm not already there. Will let you know how it goes.
Mike

UPDATE: Put in the 47K and I'm at -51VDC on those 220k's. I may be ready for tubes.

MikeG.

[sluckey]

Check for -51v at pin 5 on the output tube sockets before plugging tubes in. Still blowing any fuses?

[Megachunk]

No more blown fuses. Put in a new stby switch and diodes to the switch and things were happening again. Got -51VDC on the output tubes pin 6.

Thanks again, Sluckey.
MikeG.

[sluckey]

Got -51VDC on the output tubes pin 6

Needs to be on pin 5.

[Megachunk]

DOH!!!

Pin 5 I meant.

[sluckey]

Sounds like you're ready for the big moment.

[Megachunk]

OK!
Got tubes in and turned on - Made noise right out! Sounds strong. But a few things need tweaking...

Here's my valve data with tubes in:
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10887

Preamp tubes too hot and cant get enough current through output tubes. Only avg of about 10mA when bias pot is fully cranked. Need to tame the preamp tubes and get more current, or be able to adjust the bias pot to get more in range. -50VDC  neg voltage. Also, the EQ (treble and bass controls) doesn't seem to do much. Hard to tell what its doing. Will this get better with less voltage to preamp tubes?

Thanks all! I'm on my way!
MikeG
Megachunk

[tubenit]

It was like that on the layout.

Because the layout was for cathode biased not fixed bias ........ the 220k resistors went to ground.


with respect, Tubenit

[tubenit]

What are the dropping resistor values between nodes C & D  ............ and nodes D & E?

With respect, Tubenit

[Megachunk]

Hi Tubenit!

Thanks for straightening me out on that bias thing. I've never done a cathode bias amp.

The values are as shown in the diagram you posted. Between C&D it's a 22K and between D&E it's a 10K.

Thanks and much respect,
Mega

[Megachunk]

Hi All! Back to the amp today - Hope to get it finished up, but have some voltage issues on the tubes.

Once again, here's my valve data with tubes in:
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10887

I'm trying to find a larger value 5 watt resistors for the B+ rail. Or do I need to look for a choke now? A little confused as to what to do there. Also, to get my mA higher on the output tubes, do I need to change the bias resistor or look into another pot/resistor-on-the-back-of-pot combo?

Any advice is greatly appreciated. Thanks in advance.
Megachunk

[tubenit]

A choke is just different and not necessarily "better", in my experience and I've built amps both ways. Typically, my experience with chokes is that they offer some quietness at idle, maybe a little bit of a compressed feel to them. However, I typically have found a 1k-2.2k resistor dropped more B+ than a choke did.

Try a 22k/3w resistor between nodes B & C and then repost the V1 & V2 tube volts.  That should lower volts to both V1 and V2.

I'd like to see you get somewhere between 150-180v and close to 160v on the V1 tube.  And somewhere around 200-240v on the LTPI tube.

If you have a copy of the schematic handy and are OK with sharing it, I'd post that for others to chime in.  Remember the schematic I sent you was for cathode and not fixed bias.

With respect, Tubenit

[Megachunk]

Thanks for the response, man.

I put on the 22K in-between B & C and it came down to 230v on the PI and still at roughly 200v on V1. but I think I'm going to try a 15 or 20K 5W there as well. With the 50K bias pot with the 27K on the back and a 470 Ohm bias range resistor, I'm getting about about 23 mA on the output  plates. It ranges nicely, but need more current I think.

Here's an updated schematic and photo that is as close as I could get. It's LOUD! EQ controls aren't that responsive, but it sounds good and clean and beefy when you need it. Nice sound. Pots are in upside down or something because the volume and controls work backwards; turning to the right turns it down & vice versa. Oh well, I like to solder!

Thanks again.
Megachunk

[tubenit]

Mike, yes .....  the tone pots aren't that responsive. You have to think of the midboost and the PAB as part of the tone controls. The switches change the tone as much or more than the pots do. Looking at the pots PLUS switches, you can get alot of different tones, IMO.  And when the PAB is used, the bass pot is rendered completely out of the circuit.

If it's too loud, you could always go with cathode bias. Not trying to get you to change from fixed, just simply mentioning an option.

With respect, Tubenit

[Megachunk]

Would cathode bias make it a bit quieter? I'm having trouble with both volumes being real sensitive, like going from 0 to the wife coming out and yelling at me between 1 and 2. Even my 50 watt marshall seems to have more control and seems not as loud. It sounds good though, and I can get a lot of tonal options as I've been messing with the switch... I'll be tweaking a few things here and there, changing the pot positions and putting it in a closed back cab. Having a friend make that for me next.

Thanks,
Mike

[tubenit]

Yes, cathode bias would make it somewhat quieter.  I converted a JCM900 into a Plexi 50w and then the owner wanted it converted further into cathode biased which is what he prefers.

It sounds like the volume pots are wired odd?  You should be able to dial the volume to 7-8 and then slowly increase the master volume to get alot of overdrive with the mid boost and PAB on.

Going from zero to wide open at 1-2 doesn't sound right  because it appears that you have a 1MA pot wired backwards.

Change that wiring by switching the outside lug connections. The 1MA pot turns pretty evenly up til maybe 7-8 and then changes rapidly at the end.  Wiring it backwards, you have it changing rapidly at the start.

Does that make sense to you? 



With respect, Tubenit

[Willabe]

and putting it in a closed back cab. Having a friend make that for me next.

You don't mean the amp in a closed back cab, just the speakers right?


                  Brad         

[Megachunk]

Just the speaker. The head part will be in an open top, like a Traynor Guitar Mate; closed speaker back and open back top shelf with vents on top.

They are 1M audio. I'll run with this bias for now. Actually with the pots wired right-side up, the volumes work much better. I can dial stuff in easier. I think the only thing I'd change is to make the bass and treble have more of a noticable effect. It sounds great now, but I'd like more control of bass and treble. Going to run some pedals through it next. After playing for a while, there's 450V and 30-32mA on the plates. Still 200V on V1 and 245V on V2. But it sounds pretty sweet!

Thanks,
Mega.

[tubenit]

Mike,

Look at this layout.  I think you might have wired the pots like in the "blue".  Instead of correctly (red)

That would explain having all the volume so quickly by "2". 

With respect, Tubenit

[Megachunk]

Thast's exactly what I did. I had the pots all reversed and now it's the way it should be. It sounds great and adjusts easier.

Thanks man!
MikeG.