Tube Mic Royer Mod Build Thread

[funkymonksf]

Ha! Will do! Fathead? Reminds me of Rocko's Modern Life. Brilliant show

 

[mabell313]

Did you look in the meta?  There are Royer threads
http://www.groupdiy.com/index.php?topic=256

I am not surprised you have a bass issue.  You have some reading to do.
Or use the parts listed in the build

I was going to post to this thread a couple of days ago but, followed the afore mentioned recommondation.(some reading to do) Well, I have read everything I can find on the Royer mod. I am very suprized at some of the values. 150k for the plate? 750 for the cathode? That puts the tube down in/close to the non-linear operating range. Using the 6205 and 5840 data sheets and ohms law, more appropriate values would be 33k for the plate and 1.7k for the cathode. This will allow it to work with a BV8(6.5:1) transformer better. A bright red LED with a cutoff of 2V would work wonderfully too.  I was reading in an older post that the tube was ran down in that range as a current amplifier or something like that and went on to say that you can make a tube operate in a range that doesn't have that "tubey" of a sound. I am not buying that line of malarky for a minute. The non-linear range of operation IS the non-"tubey" range! Greater voltage swing will have the tube behaving in a much more predictable fashion.

I am not a microphone expert but I do have nearly 20 years of tube amp experience. Just the fact that the tube is plate loaded almost garantees a "tubey" sound. This type of circuit drives the transformer harder. The transformer ratio in the original is way too high. 12:1? What was he thinking? A ratio of 7:1 is a better fit. Plus, that ridiculous voltage tripler power supply is junk. The half wave part of the tripler DC saturates the core. Not good! A better way would be to run 2 seperate transformers. A 12V into a 7805 regulator jacked up 1.2V with 2 1N4148's. That will yield a clean 6.2V for the filament(s). The other transformer can be a cheap iso trans; 1:1, 120V:120V. Regulate that with a zener multiplier circuit with 2 47V in series for a final voltage of approx. 94V. That is damn close to the original's 95V. These 2 circuits will give near battery quiet operation. All of the circuits I have mentioned are in Merlin Blencowe's book, "Designing Power Supplies for Tube Amps". If you are at all serious about tube technology as it pertains to musical instrument circuits, please check out Merlin's book and website, Valve Wizard. It is loaded with great info and more importantly, equations to PROPERLY choose resistor and capacitor values for tube circuits. Tubes can kill you. Well, high voltage tubes! Just randomly choosing values is irrational and unsafe. Math rules!!!

I'll try to post some designs I've been working on soon. They include all of the ideas I have covered here plus a few other.

MIC MODS RULE!!!!!!!!
Do not think I am questioning Mr. Royer's intelligence. He has wonderful products. All I am saying is the the values he chose to use were most likely not derived from the datasheets. Mr. Royer is a ribbon mic master but, leave the tubes to the experts.

 

[dissonantstring]

hmmm, interesting post mabell313.
i am in the middle of a royer mod build and may start a second one soon, so your mods might be worked in if I find the time to get to another one.
looking forward to your posts.  i don't believe that mr. royer submitted his royer mod article originally as a high end perfect sonic solution.  just his version of "skinning a cat" and I must say so far it is a nice mod, but still very interested to see what ideas you will present.
best,
grant

 

[dmp]

I was going to post to this thread a couple of days ago but, followed the afore mentioned recommondation.(some reading to do) Well, I have read everything I can find on the Royer mod. I am very suprized at some of the values. 150k for the plate? 750 for the cathode? That puts the tube down in/close to the non-linear operating range. Using the 6205 and 5840 data sheets and ohms law, more appropriate values would be 33k for the plate and 1.7k for the cathode. This will allow it to work with a BV8(6.5:1) transformer better. A bright red LED with a cutoff of 2V would work wonderfully too.  I was reading in an older post that the tube was ran down in that range as a current amplifier or something like that and went on to say that you can make a tube operate in a range that doesn't have that "tubey" of a sound. I am not buying that line of malarky for a minute. The non-linear range of operation IS the non-"tubey" range! Greater voltage swing will have the tube behaving in a much more predictable fashion.

Have you plotted load lines from Neumann / AKG designs? When I've looked at this all tube mics are biased really low - so the load line is down in the corner... Maybe someone can explain why this is again.
P.s.
- I think the plate resistor is 100k in the design by Royer
- I think the 5840 datasheets usually show pentode, not triode operation, so be careful you have the right info
 

 

[mabell313]

I have plotted the load lines for several tube mics. Yes, you are correct sir, all are biased low. This is the non-linear range of tube operation. It is similar to a starved plate design. I went off of the Philips datasheet for a 6205 which is an industrial designation for a 5840. And yes, I used the triode not the pentode curves. Using a 150k or 100k resistor for the plate is wrong. There is absolutly no voltage swing. All I was trying to say was to more centrally bias the tube. It gives the greatest voltage swing. This is what a mic needs! Low plate voltages jack up the gain into the unusable/unstable range. Take the original Vox AC15/30. They(Vox) used a 470k resistor! This is because the design was directly lifted from an old tube manual and the circuit was for phono! The correct values for a VoxAC15/30 with an EF86 should have been 100k plate and 680~820 ohm cathode resistors. That would get the gain to a more suitable range. These ultra low plate voltages belong in plate starved distortion circuits not mics!

I did also make mention of using LED biasing. A bright red LED has a cutoff of 2V. This is very close to being centrally biased. It is not the bias that is the problem. Bias occurs in the cathode circuit not the plate. The values for the plate resistor is what takes the tube load line down into the non-linear range. Remember Ohm's law; I=V/R. Back to the cathode circuit. LED biasing is a great way to get full bass extention without the use of an electrolytic cap. Electrolytics DO NOT belong in audio circuits! They belong in trash cans! The standard resistor can be used but, beware of excessive voltage ratings. Electrolytics want to operate within about 80% of the rated voltage. That is where they sound/operate best. Excessive electrolytic capacitor voltage ratings in the cathode circuit cause the dreaded low frequency sluggishness. The LED solves this problem. Low frequency extention without the sluggishness! A poly type of cap in the cathode circuit would work too. Be careful with excessive values though. Too large of a value can cause blocking distortion. This does not sound good at all. Do everything in your power to prevent blocking distortion!

By no way was I implying in my last post that I am a tube expert. I was referring to Merlin Blencoe and his wonderful books and website. Please, read these books before continuing on with any tube project. His books/ideas are firmly rooted in algabraic equations. I am not trying to sell anyone anything. But please, find the right knowledge before putting out mis-designed crap!

I would love to post some designs but, everything is hand drawn and I no longer have a scanner :'( I am not very good at using some of the common schematic drawing programs or I would post now. Let me see what I can do in the meantime. Please look into Merlin's books/website(Valve Wizard). It is extremely informative. I really like this forum. Other I have been on are not so friendly. Most of the folks over here are into knowledge not arguing over petty a## s##t. Thank you guys(and gals!) for a wonderful site! Keep up the good work!

 

[dmp]

There is absolutly no voltage swing.

If there were no voltage swing on the plate resistor, there would be no output
I've built two of these mics and they are really nice.
Maybe you'd learn a lot about your question by building and comparing both designs for quality, noise, etc... ?

 

[pasarski]

I'm not an expert, not even close, so this is hearsay and speculation at it's best. I just found this topic interesting and would like to see the discussion going on, and maybe see some tube mic experts to chime in. Though lots of it is covered in the past threads, the information is really scattered.

I think I've read somewhere (doesn't that sound authoritative, like they say on some other forums) that opereting a tube mic with too high iddle current causes noise, because of the super high input impedance. Gird to ground resistor values seem to vary from 30M (ELA M) to no resistor (Royer circuit) in well known tube mic designs. And we all know that high values are used because the resistor with the capsule's capacitance forms a high pass filter.

And I think I've read somewhere (here we go again) voltage swing won't be a problem because the capsule's  output is so low, so it won't distort the tube even very close to the ends of the load line, or something in that vain (excuse my layman terminology).  I've tried a cathode resistor as low as 100 ohms in circuit very close to the Royer one, and didn't hear any significant distortion yelling to the mic from a close distance (very scientific). I think I've read somewhere that choosing the Rc in a tube mic is more about sculpting the frequency response (altering the harmonic content?). Also, maybe the non-linear behaviour is also part of why people like tube mics?

12:1 seems high. But it's really hard to read those plate characterictis graphs at these tube mic iddle current levels.  Isn't it 'safer' to have a bit too low output impedance than too high? You just loose couple of db's and may run into trouble with some transformer coupled pre's ( http://www.josephson.com/tn9.pdf ).

I don't know if there's any sense in what I've written, but as I said, I find the topic interesting and hope to hear more about it from the experts, you are welcome to correct me! 

5840 triode curves are found at least in the Sylvania data sheet BTW.

(EDIT: corrected typos and spelling..)

 

[mabell313]

There is absolutly no voltage swing.

If there were no voltage swing on the plate resistor, there would be no output
I've built two of these mics and they are really nice.
Maybe you'd learn a lot about your question by building and comparing both designs for quality, noise, etc... ?

You are right. I should have said that there would be very little voltage swing. I do not dispute that the Royer mod is wonderful. All I am saying is that at first glance the plate and cathode resistor values look way off. The voltage swing would be extremely low. I used the 6205 datasheet to draw a load line with a more appropriate current draw. This would lead to a greater voltage swing and thus greater output. I am going off of what I have learned over the years about tubes and what I have learned from Valve Wizard. I am not pulling some magical notion out of thin air. Every value I have listed is rooted directly in algebra.

You made reference to the question I asked. What question did I ask? Are you referring to the rhetoric I was blabbing? Sorry if I offened anyone. It was not my attention. If you were to build a guitar amplifier with the load line set that low, it would wildly squeal and feedback. Would not the same methodology apply to a mic? I am currently sourcing parts for a Royer SDC mod on a couple of Nady CM-90's I have. Also, I am in the process of putting together a project based on the Royer LDC mod with some undertones of a U47. Both projects will utilize the power supply topologies I made mention of earlier namely the zener multiplier circuit for the B+ voltage. The LDC mod will consist of a 6205 tube, an Advanced Audio BV8 transformer(6.5:1) and an Advanced Audio K47 capsule. This should get me into U47 land.

The question I do have is regarding the mic circuit's front end. Which front end, the original Royer or the schematic posted earlier in this thread, will work/sound better with the K47 capsule? The original Royer front end is quite a bit more simple. Less loading on the capsule?

I will indeed try several values for the plate and cathode resistors. Thanks for the pointer!

 

[mabell313]

I'm not an expert, not even close, so this is hearsay and speculation at it's best. I just found this topic interesting and would like to see the discussion going on, and maybe see some tube and mic experts to chime in. Though lots of it is covered in the past threads, the information is really scattered.

I think I've read somewhere (doesn't that sound authoritative, like they say on some other forums) that opereting a tube mic with too high iddle current causes noise, because of the super high input impedance. Gird to ground resistor values seem to vary from 30M (ELA M) to no resistor (Royer circuit) in well known tube mic designs. And we all know that high values are used because the resistor with the capsule's capacitance forms a high pass filter.

And I think I I've read somwhere (here we go again) voltage swing won't be a problem because the capsule's  output is so low, so it won't distort the tube even very close to the ends of the load line, or something in that vain (excuse my layman terminology).  I've tried a cathode resistor as low as 100 ohms in circuit very close to the Royer one, and didn't hear any significant distortion yelling to the mic from a close distance (very scientific). I think I've read somewhere that choosing the Rc in a tube mic is more about sculpting the frequency response (altering the harmonic content?). Also, maybe the non-linear behaviour is also part of why people like tube mics?

12:1 seems high. But it's really hard to read those plate characterictis graphs at these tube mic iddle current levels.  Isn't it 'safer' to have a bit too low output impedance than too high? You just loose couple of db's and may run into trouble with some transformer coupled pre's ( http://www.josephson.com/tn9.pdf ).

I don't know if there's any sense in what I've written, but as I said, I find the topic interesting and hope to hear more about it from the experts, you are welcome to correct me! 

5840 triode curves are found at least in the Sylvania data sheet BTW.

Much of what you have said makes alot of sense. If you have read what I said earlier, I am not a mic expert. Hell, I'm not a tube expert! I have experience in tube circuits and, at first glance, the Royer mod values look off. Can we please get someone in here to verify/counter what has been brought to light?

I would love to be to I am wrong in my first assumption and have my faith restored in the Royer mod again!

edit: After doing much reading last night, I take back much of what I say. I will share what I have come up with a little later. Pasarski, you are right with what you have said. I will elaborate later. Have a wonderful day.

 

[tchgtr]

Royers' notes on the circuit are themselves interesting, but confusing.
  He starts off by talking about the M7 capsule used in the M-49 and early U-47 circuit, and then talks about mating it with the U-67 type capsule that was used in the MXL 2001 mic. After that comes an explanation of the U-47 head amp and PSU.
    In the end, he compares the 5480 tube to the AC701 used in the M-49, and sure enough, the M-49 schematic shares many traits with the Royer 5840 circuit. Both the article and U-49 circuit are worth a look if anyone wants to understand this mod. There might even be some clues to adjust the Royer circuit in the M-49 schemo.
  Naturally, he wanted simple circuit that would be easy for relative amateurs to build, and in my experience, it sounds a lot better than the original (kinda) U-87 non-tube circuit that comes in the 2001.
 

 

[mabell313]

Now I am confused. AMI's website lists the BV8-style transformers as being suitable for the 6.5:1 ratio that it is. What is the approximate plate resistance for a 5840 triode strapped? This would answer some of my questions. All of my rant started with the Royer mod schematic. The copy I have is very poor quality. It looks like the plate resistor is 150k. It is not! It is 100k. With that coming to light, much of my ranting is null and void. I am sorry if I offended anyone. The cathode is biased at around -.5V which is pretty warm but not out of the ordinary. Be careful with a 100 ohm resistor, it may exceed plate disapation. As pointed out earlier, the cathode resistor has more to do with tone and harmonic content. It was the low value of plate resistor that alarmed me. It turns out that the value of 150k, which I thought it was, is wrong!

After looking closely at the M49 schematic, the Royer mod makes much more sense. I still do not see why Mr. Royer used a 12:1 transformer. That seems excessively high to me. tchgtr, you are right, the original Royer article is cryptic at best. He is all over the place in his explinations. This may be due to the limited space provided to him for said article. I just wish Mr. Royer would have elaborated on this project in a second article or something.

Once again, I was wrong. A larger plate resistor gives a greater output swing, not less! Maybe I should be a little more careful with what I say! Although, the voltage tripler design is inherently flawed. The half wave part of the rectifier causes DC magnetization of the core. Not good! I have noticed that many people do not use the tripler though. It has been replaced with a more traditional R-C style power supply for the B+ voltage. Pasarski again was right with what he said. A mic capsule has such a low output that it is unlikely to ever drive a tube into distortion. Current is the cause of noise, not voltage. This is why a mic would be quieter with a higher voltage filament with lower current demand. This is turning out to be a great discussion on the Royer mod and some of it's inner workings. Thanks guys!

 

[gary o]

Intersting info here would like to learn more about electrolitic caps in audio circuits ( Mics)

Thanks

 

[HellfireStudios]

Intersting info here would like to learn more about electrolitic caps in audio circuits ( Mics)

Thanks

I would start a new thread for this topic so as not to derail the topic at hand.

-James-

 

[mabell313]

After much reading I take back most of my defamation of the Royer mod.  First off, a tube mic needs the tube itself to be very sensative dictating the need for a larger plate resistor. Looking back on my inflamatory comments, a 150k resistor may actually improve the mic's sensativity. Second, the cathode resistor being 750 ohm biases the tube hotter. There is next to nothing in terms signal coming from the capsule so a little hot will improve signal levels.

When it comes to a tube in a mic, it is all about sensativity. If one looks closely at a tube phono stage one will see some similarities to a mic's internal amplifier. What I had originally said about the Royer mod was correct in terms of a tube guitar amp(which I have much experience with). A tube guitar amp DOES NOT need to be super sensative! It will be a noisy, feedbacky(is that a word?) mess. If one looks again at the tube phono schematic, one will realize that the Vox engineers( I use that term loosely) did nothing but copy schematics right out of an old tube manual(new manual at the time). So, what have I learned? Guitar amps do not need to be super sensative but mics do(tube that is!). And not to jump to incorrect conclusions so quickly!

I now have to rethink, more like retrain, my brain to look at tubes from all angles instead of from just a tube guitar amp prospective. I also went ahead and purchased the Royer mod PCBs for my own Royer mod. I will post results as soon as I have some.

 

[sr1200]

I did 2 of these, you're gonna love em! They sound great as overheads.

 

[mabell313]

Has anyone tried diode-biasing the Royer mod? I have a bunch of 1N4148's I'm anxious to use!