cathode bias bassman question

[SLW]

After never really bonding with my TOS I decided to rebuild it.  I built it into a 5F6A preamp with the TOS cathode biased power amp.  I just finished tinkering with it.  It works but has a resonance (i don't know any other way to describe it) that seems to be associated with the low "A" note and the notes nearby.  It does not happen when I play notes higher in pitch.  When it happens it will slowly die off after I stop playing.  I hope somebody has experienced this before and can point me in the right direction.

My teenage kids said it was loud. I was very happy to have made an impression.   

Here are my voltage readings.  http://www.el34world.com/charts/valve/ValveData.php?e=view&f=14471 

Schematic attached.

[HotBluePlates]

...  It works but has a resonance (i don't know any other way to describe it) that seems to be associated with the low "A" note and the notes nearby.  ...

Please try to describe it further.

Do you mean these notes are louder than others? Or do you mean there is buzzing on these notes? Or... ?

[tubenit]

IF you have ruled out the guitar neck and fret buzz, .... then it sounds like oscillation issues? It does this on other guitars also with same note/s?

Put a capo on the neck where it buzzes & then carefully/safely use a wooden chopstick to gently move wires around while periodically plucking on that guitar string.  Does that resolve it?

If not, then look for lead dress and poor solder joints .............. layout design ............. or under filtering? 

Also somewhat wonder if the VVR could be a factor in this for some reason?  Can you bypass the VVR if the other things don't work?

With respect, Tubenit

[SLW]

...  It works but has a resonance (i don't know any other way to describe it) that seems to be associated with the low "A" note and the notes nearby.  ...

Please try to describe it further.

Do you mean these notes are louder than others? Or do you mean there is buzzing on these notes? Or... ?

I am sorry.  Words are not my thing, I have to work at it.  It is not hum like you get with get with single coils or a buzz that you would associate with poor PS filtering.  It is not buzzing from the fret or guitar.  The guitar is fine on another amp.  This is a head with a separate cab.  This has a note-like quality to it.  It reminds me of low pitched feedback.  If I play an "A" (open fifth string or on the sixth string) it is really loud.  Notes nearby (G below to the E above)  have this noise to a lesser degree.  The pitch of the noise does not change.  It does not happen when I play triads on the first three strings up the neck. 

It also happened when I was checking voltages.  I don't remember at this moment if it was when I was checking the voltages on the PI tube or the PA tubes.  No guitar was plugged into the amp at that time.

I have tubes that I swap around.  I have a pair of 6L6's and 6V6's as well as different preamp tubes to try.  At this point I think it has something to do with the PA but I may be wrong.

Also somewhat wonder if the VVR could be a factor in this for some reason?  Can you bypass the VVR if the other things don't work?

I can do that simply enough.  I didn't bother to change anything about that part of the circuit.  I was having fun hacking away at the preamp.

Thanks for the help.

[smackoj]

did you try 'chop sticking' it to see if you move wires and it stops?

[sluckey]

I would disconnect the NFB loop to see if that helps.

[SLW]

I removed the VVR with no difference.  I lifted one end of the NFB resistor and the problem was gone.  As usual Steve was right.  I am grateful for the help.  Now I want to know why this helped.  Can you tell me what this means?  I am all ears.

Boy this thing is loud. I need to switch to 6V6's but that is another set of questions.

SLW

[tubenit]

I had an amp  (a few yrs ago)  with the OT wires "backwards" that made an oscillation on some notes that did not do the typical unearthly squeal.   Switching the wires fixed it. 

Maybe that is what Steve was getting at?  Hopefully, he'll explain.

With respect, Tubenit

[sluckey]

Disconnecting the NFB loop fixed the problem because the loop is probably sending the wrong phase signal back to the PI. This results in positive feedback which turns the power amp into an oscillator rather than reducing gain and improving stability and fidelity like NFB should do.

Many amps use a lot of feedback and if the phase is wrong, they will oscillate wildly, producing a very loud howl or scream. The Fender AB763 is such an amp. Get the feedback phase wrong, and the howl is obvious. The AB763 uses a feedback divider ratio of 820:47, or 820:100, depending on the particular amp.

Your amp uses a feedback divider ratio of 6800:47. Much less feedback than the AB763. So a reverse phase will not necessarily howl constantly. But it may quietly howl on certain notes, or it may just have an overall dirty sound.

The cure is the same for either amp. You must reverse the phase of the feedback signal. Reversing the OT plate leads is usually the easiest way to do this. Then reconnect the NFB loop. Hopefully the ghost resonant notes will dissappear and the amp will have a more refined sound.

A good rule to remember when building an amp with a NFB loop is to plan to have the phase wrong and be prepared to reverse the primary plate leads as a test even if it was right to begin with. Then you will be absolutely sure the feedback phase is correct. Never trim the primary leads until you have done this test. A lot of people learn this the hard way. Me too!

[SLW]

Thank you Steve.  I feel like you have taught me to fish.  I understand it now that you point it out.  I had only every heard about the constant howling.  I will switch the wires.  I am going away for a few days to visit family.  When I get back I will let you know how it goes.

SLW

[Geezer]

And now I am wondering if this is why you never bonded with the TOS....... It never would have sounded right with the incorrect phase feeding the NFB....

[SLW]

I suppose that is possible.  While it was a TOS it did not exhibit this exact behavior.  I would hear a boomy "A" note on occasion but I always thought it was just my hack playing.   This amp makes the sound every time I get any where near the "A" so it seemed different to me.  I enjoy the building as much as the playing so it is all good too me anyway.

[SLW]

I switched the OT plate leads and the issue is fixed.  The amp is alive and kicking.  I would like some of you kind folks to check over the power section of the amp.  I have updated the voltage readings for the amp that is posted at the top of the thread and an updated schematic is attached to this post.

I have 456VDC on the plates.  I have 39.8VDC across the cathode resistor.  The cathode resistor is 360ohms.  That gives me a total cathode current of 111mA with each tube giving half of that or about 55mA.  I am assuming all of it is plate current.  A simple check of W=IV gives 25W.  I think I need to lower the dissipation to get below the 23 watts max in the data sheet.  Is this correct or have I missed something in my beginners understanding of things?

[PRR]

> I have 456VDC on the plates.

AND 40V on the cathodes. The tubes only feel 456V-40V= 416V. Times 0.111A is 46 Watts. In two tubes, so 23 Watts per tube.

In 6L6-type tubes the G2 current is about 5% of total cathode current, so actual plate dissipation may be 22W each.

Any modern good-brand 5881 "should" loaf at 22 Watts.

7.5KCT is a good load. If you are literally using a 5F6a Bassman OT, it's honking hard and not giving full power; there's not enough idle current to swing lower loads to full voltage.

[Colas LeGrippa]

PRR, I don't understand your calculation.

The tubes only feel 456V-40V= 416V. Times 0.111A is 46 Watts. In two tubes, so 23 Watts per tube.

416 divided by .111 equals 3.747k

7.5KCT is a good load.

I see that you multiply the load by 2 to find a ''good load'' , why ?

Suppose you run a 6L6 at 22 w , 550V, 50V at the cathode, you get 22/500=  .044 A each tube so .088 for both. Plate to plate load is: 500 / .088 = 5.6 k  so your calcul;ation would ask for the double , 11.2k for a good load.  In the hammond transformer catalog, the max primary load we see for an OT is 10k to develop 10W ( a pair of el84 or so ). In that catalog, output transformers for 50 w or so ( a pair of el34 / 6L6 ) show a primary impedance of 3.4 to 6.6k, half of what you say. Please clarify.

Colas[/color][/b]

[SLW]

7.5KCT is a good load. If you are literally using a 5F6a Bassman OT, it's honking hard and not giving full power; there's not enough idle current to swing lower loads to full voltage.

Thanks for the clarification.  It makes sense.  I am using the 2 6L6 "2-4-8 ohm" that Hoffman sells. I bought it for my TOS build.  I don't know how it differs from an actual 5F6a OT. 

[HotBluePlates]

I see that you multiply the load by 2 to find a ''good load'' , why ?

In a push-pull stage, one side is dropping to a minimum while the other is rising to a maximum. So each side is doing only half the work, and contributes half the power.

That's the easy-to-visualize part. A little harder is that while the tubes are running class A, each side does not see the entire plate-to-plate impedance of the primary, but from plate-to-CT. So each side sees half the total impedance, and is why you came up with half-impedance when you multiplied.

Suppose you run a 6L6 at 22 w , 550V, 50V at the cathode, you get 22/500=  .044 A each tube so .088 for both. Plate to plate load is: 500 / .088 = 5.6 k  so your calcul;ation would ask for the double , 11.2k for a good load.

Yes, but your approach assumes a class A output stage that idles at half-maximum current. 500vdc would require an increased OT primary impedance as you figured, but it's a poor use of these tubes.

So hammond assumed that as you raised the B+, you'd drop the primary imepdance, get much bigger peak current, and keep the tubes from melting at idle by biasing them to class AB.

I said the load above was half-plate-to-plate impedance for class A. Once you move to class AB, the load is half-primary until the "other side" cuts off. When that happens, the effective load is primary/4 and a bigger current peak (or double current to make up for the tube that's off).

So you would not bias class A with the increasing primary impedance for same-power at higher voltage. Instead, you'd take advantage of the higher voltage by using a lower load and class AB operation to get more power. And depending on how long each side cuts off, you may have to switch to fixed bias (cathode bias gets shaky when tubes are cut off a large percentage of the signal cycle).

[Colas LeGrippa]

Hi bro,

I can see what you mean but I just can't take it  :icon_biggrin. Question : Is it unsual to have 2 X EL 34 @ 500V that draw .070 mA ???????? Answer: not at all. Now, 70mA at 500V gives : 7.14k . Now, both of you ( PRR and you ) say I'd need a 14k primary impedance OT ( 7.14 X 2 ). I have never seen a 14k OT. Please link me to a 14k OT specs, if it exists, 'cause that's what I'll need and I can't find one.

Colas

[PRR]

> Is it unsual to have 2 X EL 34 @ 500V that draw .070 mA ????????

Fix-bias, no.

SELF-bias, yes.

[Colas LeGrippa]

all right ! now, you say I'd need a 14k PP OT for best load ( according to the fact that we hook up the right speaker load to the corresponding tap ) if my PT yields 500V to the plates of the  2 XEL 34's drawing 70mA, which is not unusual, we agree on this point. I ask you now to tell me what OT do I need, what model number and make. Link me to the right  output tranny with a primary of 14 K. You just can't because it doesn't exist. So what's next ? I hook up a 16 ohm speaker to the 8 Ohm tap of a 7k tranny ?

Colas.

[frus]

I believe you misread PRR's answer
He said it IS unusual to have 2 X EL 34 @ 500V that draw .070 mA in cathode bias arrangement

[Colas LeGrippa]

You misread my answer Frus,  I asked PRR to tell me the kind of PP OT that had a 14k primary.

[Colas LeGrippa]

May you take a look at some Fender OT specs. The transformers suitable for 50W show 4k primary impedance. Whys is a so big difference between your results and theirs ?

http://www.unclespot.com/FenderXFMRs.html


here is a chart I have prepared:

PLATE VOLTAGE          POWER        70%           CURRENT      LOAD

600                          2 x 30          42 w            70 mA         8.6 k
575                          ------------------            73 mA        7.9 k
550                          ------------------            76 mA        7.2 k
525                          ------------------            80 mA        6.6 k
500                          ------------------            84 mA        6   k
475                          ------------------            88 mA        5.4 k
450                          ------------------            93 mA        4.8 k
425                          ------------------            98 mA        4.3 k
400                          ------------------           105 mA        3.8 k
375                          ------------------           112 mA        3.3 k

and so on. 

Sorry if I insist, but I just can't see how I could use a 14k primary OT for a PP of 2 X 6L6 / 2 X eL34. According to my calculations and Fender / Hammond specs. I don't want to argue for fun, I want an answer that will satisfy my need of knowing. Thanks.

Colas

[HotBluePlates]

You're killing me Colas. I started to write a reply 4 times last night, and deleted them all; I think you keep posting before you fully grasp what we're saying.

Read, and let soak in, each of the points below:

PRR's calculation applies to class A operation of the output tubes.

The current used in the calculation is not the current of both tubes, but the peak current of one-half the output stage (eith the "push" or the "pull" side). You've been confusing that with the idle current of the entire output stage.

Instead, in class A the peak current of one side of the output stage is double the idle current (and the negative peak is 0mA). That is where you see double the idle current in the other calculations.

You are right that if we tried to run a pair of EL34's in class A at 500v B+, that to keep the operation in class A (with a peak current of 70mA, double that side's idle current of 35mA), we would need a 14k impedance at the OT primary.

But as you found, 14kΩ plate to plate is an awkwardly high impedance. So amp designers do not keep running the output stage in class A while raising the B+ to such voltage levels (really, 400-450v B+ is probably a practical limit).

Instead, they operate the tubes in class AB, use a smaller primary impedance, and have a higher peak current for one half the output stage. Class AB allows this in part because the effective load is not plate-to-plate impedance/2 (as in class A, which is why this number is used in the earlier calculation), but plate-to-plate impedance/4. That's a higher peak current, and in combination with the higher supply voltage results in greater output power.

This is also why class AB has to idle at less than 100% dissipation, because the current peaks are so much greater.

In fact, keep calculating with your 500v, 14k impedance example, and find power output. The EL34 won't be able to pull its plate below 50v, so you get 450v peak voltage swing at best, and one tube has a 70mA peak current, double its idle of 35mA. 450v*0.07A = 31.5w peak. 31.5w / 2 = 15.75w RMS

So trying to keep the EL34's class A, while raising B+ above some level, and raising OT primary impedance to maintain class A operation results in less power output than you'd get with the same tubes in class A at lower B+ voltage and lower primary impedance. You should be able to get 25w with a pair of class A EL34's and properly chosen B+ and OT impedance.

So you see, you're calculation isn't wrong (but you got the right answer for the wrong reasons/assumptions)... it just proves why class A isn't used at high supply voltages. Instead, class AB is used, idling the tubes cooler and getting higher power output power with bigger voltage and current peaks.

here is a chart I have prepared:

PLATE VOLTAGE          POWER        70%           CURRENT      LOAD

You're wrongly applying the approach demonstrated to 70% dissipation, presumably related to class AB. So the whole chart is invalid. Peak current in class AB is never double idle current (cause then it would still be class A), but very often 3-4 times idle current. The power you listed is also idle plate dissipation, not output power found by multiplying peak voltage swing by peak current, and divided by 2 to convert peak watts to RMS watts.

There's actually a similar approach, but with very different assumptions used to determine a useful load and power output in class AB. Review the Radiotron Designer's Handbook 4th Edition, Chapter 13, Sections 3-5. You can download a pdf of it in the Library of Information.

All of this is shown in the chart below for a 6L6, which shows that as OT primary impedance is increased beyond a certain point (generally as a result of increasing supply voltage while trying to maintain class A operation), power output drops.

[Colas LeGrippa]

You're killing me Colas.

Well, your last post shows you're still alive and well !


I sure need some time to fully understand what you're telling me before replying some stupid prompt answer out of the blue !

Have a nice week-end

Colas.

[HotBluePlates]

It wasn't a stupid answer. You were just putting words in our mouth:

Question : Is it unsual to have 2 X EL 34 @ 500V that draw .070 mA ???????? Answer: not at all. Now, 70mA at 500V gives : 7.14k . Now, both of you ( PRR and you ) say I'd need a 14k primary impedance OT ( 7.14 X 2 ).

We didn't say that, you said it.

But more to the point, do you need to select an OT for a pair of EL34's operating with a B+ of 500v? If so, I'll show you how to select it.

[clyde]

[/quote]


But more to the point, do you need to select an OT for a pair of EL34's operating with a B+ of 500v? If so, I'll show you how to select it.
[/quote]

Yes please!  And thank-you.

[Colas LeGrippa]

Good morning,

HBP,  I always used my formula to determine a good OT load, which is not exact, as I can see. It has always given good results, though. Last night I have found this formula on the net, in some sort of obscure electronic forum chat, where there is only one reply to a single post..................and now I doubt of everything concerning this particular subject ( primary impedance ). Here it is: RP= EP / 1.8 X IP .  The amplifier I am concerned about is a PP, 2 X EL34, 404 V on the plates, fixed bias , biased at 46mA each tube, hooked up to a 4.3k primary impedance Hammond transformer ( 1650N ). According to my calculations, the match is perfect: 4.3K vs 4.3K .

If so, I'll show you how to select it.

Please !!!!!!!!! 

Thanks, bro !
P.S.: in case Hoffman hasn't got my right e-mail ( 'cause I've changed it) :  jacqueswprevost@gmail.com

[JPK]

Just a note to the tube guru's here, never underestimate the number of green horns like me who are reading this thread and learning. Probably lots more like me. Very educationg. Thanks.  

PS: It was this thread that taught me to reverse the OT primaries to make NFB work on my new P-P 18 Watt Plexi clone. When I got to work (...not howl), I didn't like the reduction in output volume, and tone. I liked the higher output, looser feel,  low end, and high end, with it off. So I'm putting a switch on the back to enable and disable the NFB feature. I have a NFB pot and Presence pot on the front of the amp.

[PRR]

> what OT do I need, what model number and make. Link me

14K is unusual, and suggests you are off-track.

> 404 V on the plates, fixed bias

FIXED bias is DIFFERENT.

You can't know anything from the idle condition.

In SELF bias the peak current is pretty-much twice the idle current, so you CAN design the load from the idle condition.

> RP= EP / 1.8 X IP

I won't argue if the number is 2 or 1.8. We never work to such precision.

However I do not understand that formula or the number you got from it. I say, for SELF bias, (Ep*2)/Ip.

[SLW]

I have been tinkering some more this morning.  I put the VVR back in to give me control over the voltage in the amp.  I have put 6V6(from 1954  )  in place of the 5881's.  I have been concentrating on understanding the cathode biased output stage.  After warming up for half an hour I have 400V on the plates and 29 on the cathodes.  My cathode resistor is 360ohms.  That gives me a total current of 80mA.  That gives me a total power of about 30 watts, 15 watts per tube.  I know the screen current is a few percent (I don't know a real number) but I am sure that will not bring me below the 12 watts max dissipation for the tube.  The way I see it I can lower the voltage or increase the cathode resistor to get me below the maximum plate dissipation. 

My question is what point do you chose for operation conditions?  ( I understand this is a BIG question)  My thought is I choose a voltage and then set the bias so that I have some percentage(70%) of max dissipation to give a good place for the tubes to live at.  Is that how you would go about it?(PRR, HBP, Steve?)  How much attention do you pay to the max voltage ratings of the screen?

Thanks for all of your patience.  I am slowly getting it.

SLW

[sluckey]

My thought is I choose a voltage and then set the bias so that I have some percentage(70%) of max dissipation to give a good place for the tubes to live at.  Is that how you would go about it?  How much attention do you pay to the max voltage ratings of the screen?

I typically use 80-95% for cathode biased amps. I don't pay any attention to screen max voltage ratings. Fender blew up those chart numbers years ago. 

I would not be comfortable using old 6V6s in that amp unless the B+ was limited by the VVR and/or the cathode resistor was increased. I would limit the VVR by putting a series resistor on the top side of the VVR pot so the pot could never be adjusted to full B+. 'course, if you put the 5881s back in everything would probably be happy, right?

[SLW]

Thanks again.  I have had the old 6V6's for a couple of years and they are new as far as I can tell.  I have been wanting to build something that used them.  The 5881's are too loud for my use.  I am going to develop the output section to use the 6v6's.  I will increase the cathode resistor to use with these tubes.  What is the max anode voltage that you would run 6v6's at?  Once I get the output section better I will likely move onto tinkering with the preamp.  I will likely move in the plexi direction. Who knows where I will end up. 

I am going to turn this amp into my "tinker" amp.  I will use it to experiment with things.  When I decide I like something I will rebuild it into a "real" amp.  I have taken the knob off the VVR to remind me to not mess with it.  I would like to move it inside the chassis but there is little room.  I do have a really big chassis laying around.  Hmm....

[Colas LeGrippa]

Clyde,

it's is all written in  a tube data, under PP ab1, different plate voltages corresponding to different loads ( for 2 tubes ). Ex 500V = 3.6k load.( for a pair of given tubes ). As simple as that. No math formula.

Colas

[HotBluePlates]

I promise I haven't forgotten this thread. I've just been doing some extra reading and gathering material so I don't half-remember anything.

Should have the example posted this weekend.

[Colas LeGrippa]

I thought I had definetely killed you

[PRR]

70% is for fixed-bias amps.

SELF-biased amps, worked for "high" power, must be run at 90%, 99%, 110% of rating (AND loaded appropriately). Anything less leaves precious power un-sucked and thus un-delivered.

6V6 are safe at 12W if you do not meter every unit you build and ship (mass production). They are safe at 14W *if* you meter every unit you build. They are known to live a long time at higher power, but at your own risk.