New 4X1 X 4X1's Rig - Problem with audio spikes - Questions - Pictures

[AB2EZ]

Tom

Regarding grid current and self biasing resistors:

I think it is best to think about this by starting with your principal objectives

A. You want to achieve a specified output power
B. You want to operate the tube(s) in a manner that is consistent with their rated plate dissipation
C. As a matter of pride, "greenness", and the cost of electricity...you want your high power rigs to run at reasonable efficiency when they are putting out high power.

The following things are not objectives... they are just things you need to do to achieve your objectives:

a. For the tubes you are using, you need to drive the grids positive with respect to the cathodes for a (small) portion of each cycle. If you don't, you cannot achieve the objectives above

b. Given the choice of where you want to bias the tubes (to achieve the efficiency objective and to meet the plate dissipation constraint) there is a certain grid current pulse (amplitude and shape) that will occur each cycle.

c. Each grid current pulse has a certain area (units of Amperes x seconds = Coulombs). The average  grid current is just the area of a single pulse of grid current x the number of cycles per second (units = Coulombs per second = Amperes)

Thus the average grid current you need is determined by: the tubes you are using, how you bias them (for efficiency), and how much output power you want to obtain.

Separately, there is a question of what combination of self bias and fixed bias you decide to use to bias the tubes.

If you want to depend entirely upon self biasing, then the grid resistor value you need is completely determined from a) the average grid current you need... as described above, and b) the bias voltage you need... as described above.

If you want to use some amount of fixed bias (or cathode bias), then the remaining bias voltage and the average grid current determine the needed resistor.

In my Ranger, given the choice of a 6146 as the output tube... Johnson designed it to be biased in Class C (of course), at around -70 volts grid-to-cathode.  Given this tube choice, and given this biasing choice, and given the choice of a 600 volt plate voltage: it follows (from the tube characteristics) that about 2.5 mA of average grid current is required to obtain the desired ~50 watt output power level (at carrier). Since Johnson decided to use only self-biasing, the required grid resistor is 70 volts/2.5 mA = 28,000 ohms

Stu

[WD8BIL]

I didn't sift thru all the thread Tom, and maybe it's been said but;

Why not put the SS amp somewhere else? Put it near your operating table and run shielded cable from it to the bassackwards transformer. Sounds like an induced problem to me. Relocating the amp might be the simple thing. But, there again, I'm just a buddly.

[K1JJ]

OK Stu -

Good summary on class C.   I've pretty much settled on 5K grid leak which gives me the recoomended 40 ma grid current at -315 total bias. They recommend -200V, so it's well into class C.  I see no plate color at all at full power.

Yes, Buddley, moving the amp was the final solution. I blew it out right after moving it and found it worked FB -then blew it by wiring the output into a short. I was tired and screwed up. Stu sent me another amp and it works fine now. Both RF and audio keying is pretty clean. I still have a tiny pulse on unkey, but it appears related to simply picking up the audio transients on the low level audio line, since I can disconnect the input audio and it goes away.  No more arc gap sparking like before.

Should be on with it over the next few days for testing. I'll be fishing for "opinion" audio reports, mainly related to EQing, audio quality and bandwidth.  If ya hear me, please give me a call.


T

[w1vtp]

Tom

If I catch you on this weekend (including Friday), I can make a recording with my Flex 5000 and send it to you.  There are others too like Bill IFR who can do the same thing

GL, Al VTP

[WA1GFZ]

Tom,
I will also listen for you. I'm home today XYL under the WX but I will be in the shack later with the RX on 85. I'm doing some metal work on my rig. I used this hard 1/8 inch aluminum for the chassis rear deck and need to file .030 off each side so the vacuum variable holes end up in the right spot. Too thin a cut for a saw.

[N3DRB The Derb]

Tom,

you was strapping pretty large here, audio to me sounded super clean but it also sounded kinda flat, meaning it didnt have a lot of
color to it. Matter of eqing and getting some peaks and dips in the right places. I am using a big hi fi speaker with the yeasu, a mid 70's 3 way with a 12" woofer, 6" midrange, and 3"tweetie. It's good to go 20hz to 20K.

 

[N2DTS]

I use the same type of speaker, 12 inch 3 way with the Marantz audio amp.
My son found the pair of speakers in the trash, some new meterial on the covers and they looked like new.
Big, heavy old school construction...

The Flex 5000 into the Marantz, into the big speaker sounds fantastic, the best cleanest hi fiest receiver I ever heard.
I often leave the system running as I do things, or watch TV in the den (keep wife company) and I had rumbles from someones voice the other nite, they were using a flex and had the low end boosted I guess, it sounded good, and rumbled as well....

I cant understand how the Marantz still works, its old as the hills, and besides some dirty switches (fixed with deoxit) it has been trouble free for the last 30 years of constant use....they dont make stuff like that anymore...

Brett

[w1vtp]

Well, while we are at it.  Here's my RX / AF system  -  My Flex 5000 into a KLH R3100 100 / ch receiver and then to MTX TP112 speaker.  You HIFI AM guys really drive this RX / AF system FB

Looking forward to working you, Tom

Al

[W1RKW]

I cant understand how the Marantz still works, its old as the hills, and besides some dirty switches (fixed with deoxit) it has been trouble free for the last 30 years of constant use....they dont make stuff like that anymore...

Brett

I have a Marantz 2275 receiver.  It's a 70's vintage SS receiver.  I picked it out of the trash back in the 80's.  It looks like new still. Had to do a few repairs over the years but it works OK fine.  Have the original wooden cabinet. It's a nice piece of furniture. And it's a heavy mother.  Works like a charm today.  It's part of the home audio system and  loves the old LP's.

T,
Hope to hear the 4x1 in the next few days. Maybe the 2 of us can critique and adjust the audio of our rigs under full quieting mode since we're so close.
B

[K1JJ]

OK guys, thanks for the offers.

Looks like we will have some good, diversified gear and listening ears...

My objective is to take my average voice and try to get the "Big Boss" AM sound in the low end with the highs rolled off starting at about 5KC. So it will be an effort to get on the edge before the low end starts to get muffled - while the on-air signal  bandwidth stays within reason on the highs.

I have a variable cut filter that is good at rolling off the highs, but I am more concerned about generated IMD garbage. Hopefully the signal will tune and end with wisps and not harsh splatter sounds.

Getting the EQ balance right does take some time. But I know there is a certain combination with my voice where most everyone likes the sound and so do I.  Some guys have great voices and need little tailoring. But mine has mid-bass that needs to be cut on the EQ.

I'll be running the rig at low tap for now until I can make sure all the bugs are tamed.

I'm also looking for just a touch of blower noise, cuz it is there, but not much. It's a challenge to keep it low with the compression running. It is faint with the compressor off, but then there is no audio limiting for transients or mistakes.

So, that's what I'm trying to achieve so you'll all know when you hear it.

Thanks.

T

[K1JJ]

Hola!

Found out something new.

I swept the rig again tonight and found the highs were rolling off and getting distorted above 4 kc. At 6kc they looked terrible. I didn't take the time to look closely before.

The bottom line is I took a hint from the Tron's old saying: "Use the WHOLE transformer winding."  For the 8 ohm input I was using about 1/4 of the winding = 20 ohms. This time I went into the whole winding = 265 ohms. It made a huge difference. The highs are not perfect, but pretty clean up to 8kc or so now. I'm still using the whole 7K ohm secondary as before.

Logic would tell you to use the biggest step-up in this case, but was wrong. Now the amplifier uses less power to drive and the unkey pulse is even less, two good things.

Last night during operation, I couldn't understand why the rig would not do over 50% modulation on the S's.  Now I can easy over-modulate the highs. I located the culprit (driver transformer) by putting a tone through and methodically looking at the waveform in each component, from the low level stuff all the way to the mod transformer. The driver xfmr stuck out like a sore thumb.

Eventually I'd like to try an 807 tube driver or a MOSFET driver, but this SS driver and xfmr will suit me fine in the meantime.

Glad I found this problem or the reports wuda been disappointing, I'm sure.  Derb was right when he said my audio "lacked color" last night.  Poor/no highs will do it.

T

[Gito]

Hi.

Yes logically we needs to used the highest turn ratio for the interstage transformer.

 But I think its not always like that,when You used the 20 ohm winding and 7000 seconder ,it's impedance ratio of 350,and the turn ratio is 18.7

If the  the grid to grid  of the Modulator tubes has a 500 ohm impedance (for instance) than your SS amplifier looks into a 1.4 Ohm load,and if it needs a 200 v peak to peak grid voltage  it needs a 11 volt swing in the SS output, it seem not right to me,but maybe I'm wrong.

I think there must be a compromise between the turns ratio  to have a good drive for the Modulator tube.
and ,I think we needs the right turn ratio to have a good signal/power for driving the Modulator Tubes.
It depends on the grid impedance of the Modulator tubes and the impedance of the amplifier,of course SS amplifier can have different load and still gives a good sound.
But I think there's a limit to it.

Gito

[AB2EZ]

Tom

In this new configuration... the step of ratio is lower... so I am surprised that you have enough voltage to fully drive the grids of the 4-1000's.

The high frequency rolloff you observed suggests that the transformer has too much leakage inductance, particularly when used with only a portion of the low impedance winding. The leakage inductance appears as an inductor in series with the winding.

I would suggest that you purchase one of the Antek toroidal ferrite power transformers. They work as almost ideal audio transformers (in the range 30Hz - 10kHz)...  as long as you run them in Heising mode. I'm using a small (25VA rating) Antek transformer to externally plate modulate my Ranger in Heising mode. I'm using a pair of large (800 VA) Antek transformers (in series to get the step up ratio I wanted) to plate modulate my legal limit homebrew amplifier (a pair of GS-35b's in parallel, running at 1700 volts B+)

Pick one with a pair of identical primary windings, and a pair of identical secondary windings ... so that you can place the windings in series of parallel to obtain a step up ratio of X, 2X, or 4X.... where 4X is the step up ratio when input windings are in parallel and the output windings are in series.

Pick one with a VA rating of at least 100VA

For example: the transformer has a 200 VA rating, and it will give you a step up ratio of 230/15 = 15.3, or 115/15 = 7.66, or 115/30 = 3.83

http://cgi.ebay.com/15V-15V-30VCT-200VA-POWER-AMP-AUDIO-TRANSFORMER_W0QQitemZ370192779213QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item56313587cd

Stu

[Gito]

Hi

One  thing not to forget ,a 100 watt ampli design for an 8 Ohm load ,if i'm not wrong needs  a 3.5 A current (I X I X R), so it is design with a power supply with maybe 4 A rating.

When it is used for an 2 Ohm load it needs a 7 A current, Can the power supply gives this current,can the final  power Transistor pass this current?

 In my opinion used the lowest turn ratio that still can supply the peak voltage needed but with enough inductance in the interstage Transformer and again maybe I can be wrong.

Gito

[N3DRB The Derb]

Tom,

I was comparing it with audio I've heard you on with before from memory, not the best test, but "the JJ sound" is pretty firmly fixed in there. There's one more observation: it lacked a quality I've heard in the past for lack of a better term; it didn't have any 'projection' quality. It was very flat sounding, just like a good BC transmitter would sound with a maladjusted audio rack that has'nt been set up yet. Smooth as vanilla puddin, but there was just no bark to it.

I hope that my overuse of highly technical terms does not offend, OM hi hi fb. 

[WD5JKO]

Tom,

   I agree with the Tron to use the whole winding, and with Gito's concern about the driver current budget. Now I know I'm paddling upstream with my suggestion, but the series cap between the driver AMP and the interstage transformer will still be helpful. Remember what Bruce said, a series cap shunted by a resistor (3-4 ohms)? I suggested you use the dual 1600uf/450v caps in series (++ together). Here is why, your voice does not speak in sine waves, and likely has a DC component to it, or asymetry. This DC component will see the DC load (primary of driver transformer), and 1 volt into 0.1 ohm is 10 amps of real current wasted. The caps/resistor eliminate the issue.

Good luck with things!
Jim
WD5JKO

"its better to have a bottle in front of me than to have a frontal lobatomy"

[N2DTS]

I think you just need some oxygen free cables between the audio amp and the transformer.

The correct driver transformer would likely fix you right up, 8 ohm to 5k or above, 20-20khz response, whatever wattage you need.

Does Hammond make one?

Brett

[WD5JKO]

I think you just need some oxygen free cables between the audio amp and the transformer.
Brett

  Brett,

   A direct coupled SS amp looking into a DC short is a bad thing. In order for that to work, the AMP must have zero DC offset, and the audio waveform must have ZERO asymetry. Those two things are tough to achieve. I don't put the series cap idea in the catagory of oxygen free cables. Sorry.

Jim

[WA1GFZ]

Inductance too high hurts highs with excessive leakage inductance.
Inductance too low hurts lows. Sometimes you can shift things with series caps on primary or even a series resistor. also a grid to grid load.

[K1JJ]

Jim,  I will try the non-polarized caps again with the 4 ohm resistor across them.  Your DC asymetry point makes sense.

Yes, the highs were getting killed with the bigger xfmr step up. Even NFB didn't help much. The lows were not affected at all and still looked good. But now things look quite acceptable as an interim fix until I come up with another driver.

Stu, I'll look into the ferrite core idea. Also, I'm waiting for Bruce to post the redrawn 807 schematic and Steve to post the MOSFET one to get additonal ideas for the next step.

Derb - that's FB - that's what I'm looking for OM. You were right.  It's hard to hear the problem in the monitor, but when I fixed the highs, it was so obvious that they were lacking. Now I can hear more hi-freq blower noise and my breathing. Before it was muted.

More tests today into the dummy load and hope to get on for the weekend. Talked with Joe, PJP late last night but skip was long.

T

[K1JJ]

Inductance too high hurts highs with excessive leakage inductance.
Inductance too low hurts lows. Sometimes you can shift things with series caps on primary or even a series resistor. also a grid to grid load.

Interesting point Frank.

I wonder if I should put some more loading at the 4X1 grids.. like 10K to ground or across the winding to CT, etc?  Also 100 ohms in each grid lead for stability?  I have parasitic chokes in each plate lead.

Right now I have a 10K, 10w resistor across the xfmr secondary for safety. Also a 500 pf doorknob at each modulator grid pin to ground, that's all.  The 10K is wirewound - maybe this shud be carbon?

I do see a slight instability on the modulators when I unkey shown as some hash on the scope when watching the grid pins.

 Any suggestions for values, etc?
T

[W2XR]

Tom,

The subject of using driver transformers for driving class B grids is a complicated one.

I have an article from the 1936 Proceedings of the IRE that goes into this subject at length from an detailed engineering standpoint. Unfortunately, the quality of the copy is poor and not suitable for scanning, etc., but there is a wealth of information on this very subject. It was entitled, "The Minimization of Distortion in Class B Modulators", or something to that effect.

There are two modes of thought with regard to loading the secondary of the driver xfrmr. In some cases it can improve the regulation of the driver, but at the expense of available voltage swing at the modulator grids. Others believe that the less loading on the driver tubes (i.e. no resistive loading), the better the driver regulation. I think it has to be determined expermentally, in each specific driver amplifier circuit.

As you are using a solid-state driver amplifier with a large amount of internal negative feedback, the regulation (or output impedance) of the driver is probably excellent to begin with, so the loading on the driver transformer secondary becomes less critical.

I think you need at least some resistance across the secondary of the driver xfmr, to minimize any ringing inherent in the xfmr. I shunt a 15K, 25 watt resistor across the secondary of my driver xfmr for this purpose, and it clearly makes an improvement. In the case of a vacuum tube driver, you also want to use the highest impedance available on the primary side (plate-to-plate impedance of the driver tubes) and the lowest available impedance on the secondary side, so the step-down ratio presented to the class B modulator grids by the driver xfmr is as high a possible. This improves the driver regulation and reduces the bias shift as the modulator grids are driven positive, but at the expense of available modulator grid swing. Obviously, this all has to be taken into consideration when designing the driver/modulator circuit.

I question how well conventional high-quality audio output xfmrs connnected backwards work as class B driver xfmrs. I did this for years and was generally satisfied with the results, but it may not be the optimum solution, particularly when a low impedance output tube amplifier is being used as the driver, due to the cascading of the two xfmrs and the attendent phase shift issues, etc.  Driver transformers designed for class B service must have a very low value of leakage reactance and a low DCR in the secondary winding, to minimize the effects of bias shift as the class B modulator tube grids are driven into grid current. Designing such as transformer is very difficult and costly as a result. And as I had stated in a previous post, when using some high-power solid-state direct-coupled audio amplifiers as drivers with backward-connected output transformers, the amplifier must be properly loaded with a high value capacitor and suitable resistor to maintain stability of the amplifier when it is looking into an extremely low impedance or low value of DCR such as the xfmr primary.

The best solution is to use a driver transformer designed to and suitable for the task at hand, but they tend to be expensive and quite scarce nowadays. I was lucky; the driver xfmr in my rig came out of a Gates BC-1F, and was designed to match a pair of class A push-pull 845s to a pair of class B 833As. But Gates stopped using that xfmr in the mid-1950s, so aside from a Peter Dahl replacement unit, they are essentially unobtainium.

That said, I think the optimum solution is to use the cathode follower or FET driver and be done with it, as it eliminates the troublesome driver xfmr issue once and for all. This is especially true if you want to apply some aggressive audio processing to that rig; this becomes very difficult to achieve in those plate modulated  rigs with an abundance of cascaded audio coupling transformers, due to ringing and overshoot issues. I have an Orban Optimod 9100A, and this processor is basically unusable with my rig for this very issue; it is difficult to have consistent control of the negative-going peaks of modulation. These later generation processors are suited and designed for use in transmitters (class E comes to mind) where there are no complex impedances anywhere in the audio signal path.

Just my 2 cents!

73,

Bruce

[WA1GFZ]

Tom,
those 500 pf caps could be rolling off your highs. If you really need them I would add some shunt R to raise the time constant. Resistors could go back to the CT as long as you have a heavy bypass for RF and audio at the CT. Make sure the 500 pf caps are right at the tube sockets.

[W2XR]

Tom,
those 500 pf caps could be rolling off your highs. If you really need them I would add some shunt R to raise the time constant. Resistors could go back to the CT as long as you have a heavy bypass for RF and audio at the CT. Make sure the 500 pf caps are right at the tube sockets.

I agree with Frank on this one.

Personally, I do not see the need for bypass capacitors at that point in the circuit, nor am I familiar with any broadcast or other rig that used them at the modulator grids-to-ground either. I think they are superfluous. At that impedance point and value of capacitance, I'm sure they will have some negative impact to the high-frequency response, and that capacitance could be reflected back through the driver transformer and cause some potential ringing and overshoot issues.

I can also see a potentially negative effect of putting series resistors between the each secondary leg of the driver xfmr and modulator grid, in an attempt to control potential audio parasitic issues. You want to keep the resistance as low as possible at that point in the circuit; as the grids are driven positive in class B service, a voltage will be developed across these high-vlaue resistors and the modulator grid bias point will shift accordingly. The load the driver looks into, as well as the grid bias supply, must be as low an impedance as practical, including (as I had discussed above) a driver xfmr with leakage reactance as low as possible.

If you are concerned about audio parasitics in the modulator stage, the place to put resistive suppressors would be right at the plate connection of each modulator tube.

73,

Bruce

[WD8BIL]

The reactance at 10Kc in a 500pf cap is >30K. I don't think it's a problem.