bias voltage range for a 4x6L6 amp question

[Shack]

I am now at the stage where im putting amp together, and I am testing each phase before moving on, to try and avoid stupid mistakes. 

I just built the bias circuit and  unloaded , the pot sweeps from -48v to -58v at the end of the 220k resistors that feed the grids.

The original Peavey schematic shows -55v coming out of the pot, so was wondering if that range  sounds right for this stage of the build. If I need to change the bias range resistor, would rather do it now , but can wait, ...at least I am assured that everything works up to this point....and only preamp and PI left to build.

Thanks

[sluckey]

That's probably pretty close but I would feel better with a range of 40 to 60. Now is a good time to tweak it. Doesn't matter that it's unloaded because the load is usually very light, if any.

[Shack]

thank you Sluckey.....I am using the 5F6A bias circuit from here. It has a 1k bias range resistor, and a 47k resistor going from bias pot ( 50k )  to ground , if I understand correctly....the 47k is what keeps it from going any lower in case the pot failed or I turned pot down too far, and the 1k sets the highest point of the range?


The Peavey used a 470 ohm for bias range, a 10k pot and an 82k from pot to ground..... I would guess though that I should go lower with both resistors , like 470 ohm and maybe 33k or so, but that would be me guessing , and I dont trust me that much,lol.

Had to edit, wasnt from wiper to ground, and I forgot about the 15k that goes from middle pin of pot to the diode, and I hope thats not the one to change, it was hard to fit in there.

[sluckey]

Refer to Hoffman's schematic. Change R33 from 15K to 10K. Then experiment with the value of R35 (up and down). Can you now get a wider range? Maybe something close to 40 to 60 volts?

[Shack]

changed R33 to 10k, damn I hoped you didnt say that, was tough, but now I have -52 to -60 , so now I have to work on R35......thanks again Steve

[Shack]

now I got -40 to -57 volts with a 22k resistor for R35.....but, I think the bridge rectifier that I bought died.....not getting the DC volts now for some reason. filament voltage and bias voltage are working good. getting about 368v AC between the red wires of the rectifier, but the DC volts on the standby switch arent there . I wonder if the self contained rectifier isnt as strong as using diodes to make it.

[Shack]

thanks for the help Steve, but I think my insulator under filter board slipped out and I fried either the rectifier, main 100uf caps or both.....but bias voltage seems to be in a good range 

[sluckey]

We need full disclosure. Can you show us your working schematic?

[Shack]

Steve, this is a project that I can only ask for help in a given area, because I took the Peavey chassis and kept main filter board.....wired a bridge rectifier that I bought here, to terminal strips....and wired up the power tubes like a twin reverb. All of that is good, and I replaced all caps on board, all that was good. but somewhere when I was trying to get bias right before moving on, I believe that the insulator under filter board may have slipped and I dont get B+ on standby switch or main 100 uf caps, but the 22uf caps had voltage on them lol, cause I felt it.


I cant do anything until I replace the rectifier and caps for now. I was working on bias circuit and that worked well, as I said, as well as the filament circuit. I just wanted everything to be as expected before moving on, and I wasnt getting DC voltages where I expected them, but tranny isnt hot so I suspect caps or rectifier.



here is original schematic

[Shack]

Im just a little discouraged right now, but I believe the power tranny is good, im gonna put a smaller fuse in, and replace a few components before moving on.

This has turned into a scratch build, just using the Peavey filter board, and wiring up the output sockets like a twin reverb. and using the bias circuit from Dougs bassman layout. I got a respectable voltage range doing as you said, but need to get back to the same b+ voltages I had before what I think is the insulator coming out from under filter board while doing this. Unless my meter just sucks.

[Shack]

Add that schematic with the bassman schematic, and thats where im at

[Shack]

So I put it aside overnight to calm my frustration, and decided to test the power supply and this is what I have

AC secondarys ..385 volts AC

DC out of bridge rectifier.....385 volts DC

A , B, C, and D nodes all read about 525 volts

this is a schematic I drew showing all that is built thus far ( resistors and diode in power supply are from original amp)

[sluckey]

I think that -42v to -58v bias range will be fine. And you know how to tweak it if you need a different range.

[Shack]

yes, thank you very much

[Shack]

everything is good now, but I will have to find a way to make a better filter board before I fire this up again....the Peavey design leaves too much chance for something to touch ground now that ive changed the way the can is mounted.

I need to learn to make turret boards maybe

[2deaf]

this is a schematic I drew showing all that is built thus far ( resistors and diode in power supply are from original amp)

That's not how Doug has it drawn.  He has the bias coming off of the wiper and you have it coming off of the top of the pot.  Same bias voltage either way, but Doug's goes to the coldest bias upon wiper failure open and yours goes to no bias and serious red-plating.

[sluckey]

Referring to your schematic... I suggest you put a jumper between the pot wiper and the top terminal. Then you will be safe if the wiper fails.

[2deaf]

Doug's Plexi 6V6 and Plexi 50 could also benefit from a jumper between the pot wiper and the top terminal.  In those, wiper failure leaves the bias circuit unloaded and it will charge up until the over voltage destroys the filter capacitors.

[sluckey]

  Ahhh! I never noticed that until now.

[Shack]

I  drew that part wrong, because I have a free terminal on the pot like Dougs layout......is it safe for that dinky pot to unsolder it and add a jumper? and it would go from wiper to the unused terminal? 

I build this stuff to further increase my knowledge, but every time you guys chime in, I realize how much I dont know still.

[sluckey]

There is no "unused" terminal on your schematic???

[Shack]

there is on Dougs, and I said I drew that part wrong somehow.....sorry for that

[sluckey]

So how do you really have your pot wired?

[Shack]

Sorry it took me so long......like this

[sluckey]

OK. Now put a jumper between the pot wiper and the unused pot lug. You'll sleep better. 

[Shack]

OK, im working on it now thanks Steve :)

[Shack]

should that jumper change my negative voltage range?

[sluckey]

You aint put that jumper on yet? My, my...

You tell us if it changes anything.

[Shack]

I did and it seemed to change the range, thats why I asked Sensai

[sluckey]

Well, you had -42v to -58v bias range before. What is it now?

[Shack]

I read -36 to -50 , then I realized id been using my dim bulb tester,lol.

Now I just read -40 to -56.7

So I  continue......I am going to order an eyelet filter board from Doug and make that area safer than the filter board that came with the amp. then ill sleep better at night.

Thank you again for helping me with the bias circuit, I never really paid enough attention to that part of the amps.

[Shack]

My amp is built....sans the filter board , and I suspect that I really need to do it.....can I make a new build thread with pics and such or just keep it here.


I am using old tubes to test and only 2 output tubes for now. Current limiter is cool until I turn on standby and then I get a buzz through speaker and it seems to not dim down, I suspect that filter board.

I am not sure why the voltages are the same on all B+ nodes (unloaded)  with the dropping resistors, and they are high, as far as im concerned,

Is there a possibility that I changed the phase from the original amp to this and need to just reverse the output transformer primaries?

I will look over everything to make sure all is wired up correctly cause ya never know, but im past the bias question and I can stay here or make a new thread on the build.

[sluckey]

I am not sure why the voltages are the same on all B+ nodes (unloaded)  with the dropping resistors, and they are high, as far as im concerned,

Unloaded means no current flow. No current flow, no voltage drop. All B+ nodes will read about the same. That will change when you plug in the tubes.

Is there a possibility that I changed the phase from the original amp to this and need to just reverse the output transformer primaries?

That's always a possibility when you build an amp with NFB from the speaker. But you're not far enough along to know if that's a problem yet.

[Shack]

ok, then with tubes plugged in, albeit only 2 output tubes, I cant test anything because of the buzzing through the speakers. I will delve into it tomorrow

[sluckey]

ok, then with tubes plugged in, albeit only 2 output tubes, I cant test anything because of the buzzing through the speakers. I will delve into it tomorrow

Maybe the NFB phase is wrong. You can reverse the OT plate leads to confirm. This is something you eventually need to do. Or, you can just disconnect the NFB loop and deal with NFB phase on down the road.

[Shack]

OK, ill disconnect it tomorrow first......and ill post pics too, cause it did come out well. I still think ill redo the filter board, cause I want it as bullet proof as possible if I use it live

[Shack]

So I unhooked the feedback wire from the speaker jack, and the buzz is gone, so I will have to reverse the primaries.


While looking everything over I noticed that the filament wire going to pin 4 of a preamp tube wasnt soldered in and fixwd it

I put in tubes ( a mix of 4 different 6L6 and 12ax7) to see if I could get the bias in the right area and listen for sound. While trying to measure for plate dissipation I noticed a 6L6 starting to red plate so I shut her down and looked over the socket and sure enought , the grid resistor going from pin 1 to 5, wasnt soldered to the socket, so I fixed that.

Then I measured the current going through each pair from the 1 ohm resistors, and they are way off from each other, like 90 ma in one pair and 65-70 in other pair......maybe from the mismatch of tubes? The tubes arent even 6L6GC, so I probably shouldnt even be testing with them, since plate voltage is still like 500 volts...so before shutting it down, I plugged in a guitar, and there isnt any sound, except the hiss from the amp itself, but im not too worried, since im pretty sure that with a guitar jack plugged in, I shouldnt be reading continuity between the tip and ground, so ill fix that next.

I am running out of solder lol.

Here are a few pics of it, I am almost there. 

[Shack]

I think that I wont be able to try to get sound out of it until I figure out the power supply.....This amp puts 470 volts on the plates of the preamp tubes. 500 on the output tubes. Glad im using old spare tubes to test it with....If I kill them I will survive.....maybe I already did, ill test them in another amp.

[shooter]

This amp puts 470 volts on the plates of the preamp tubes

did you verify the tubes were conducting by measuring Vdc across the cathode R's?

Do you have another tap besides the 385v?  something around 280v with bridge will get you around 400 at the rail.

[Shack]

I will check

[Shack]

no other tap

[shooter]

Got my 6L n 6V confused, but the original schematic shows like 390 at Pre tap, dropped through 200+ plate R should keep you close IF the preamp tubes are conducting, otherwise they WILL read high.

[Shack]

So besides altering my power supply with bigger dropping resistors.....maybe thats why the Peavey schematic used larger plate resistors.....because of the huge B+ voltage of the power transformer

[shooter]

So besides altering my power supply

if your pre tubes are conducting and you have 470 at plate, you probably ain't gonna get there from here.  The peavey shows like 390vdc before the plate R's.

does your PT have a CT for the HV winding?

[Shack]

nope, thats why a bridge

[PRR]

> 368v AC between the red wires of the rectifier

This should give a bit over 500V DC at the first cap. You got this.

It will also give 470V-500V at the small tubes' plates WHEN the small tubes are not installed or their heaters are cold (not sucking).

So do you have small tubes that are glowing?? Or are they missing/dead?

[Shack]

All three preamp tubes and all 4 output tubes filaments are working and glowing. 6.5 volts AC between all filament pins.

I have 500 volts on plates of 6L6, 497 or so on screens, 475 on phase inverter plates and 470 on preamp tube plates.

I am using the complete power supply from the Peavey...only the bias supply is different.

All I can think with my limited knowledge is that maybe the 6C10 could handle higher plate voltages ( havent looked this up tho ) , and maybe thats why the Peavey had larger plate resistors.

[sluckey]

Sounds like your preamp tubes are not conducting. Measure voltage on the cathodes of all your preamp tubes.

[Shack]

ok, thats a possibility, cause they are all spare chinese tubes.....ill let yall know

[Shack]

so I put meter on DC volts....black lead to ground and red lead to the pos side of all 12ax7 cathodes ....nothing.....was that right?think I got voltage on PI tube, but not on the preamp tubes