Here is the Sony C800G PSU schematic

[mad.ax]

Hello everybody, this is my first thread.

I’ve been lurking this forum for some month with growing interest. There’s such an amazing amount of knowledge around here, and being an autodidact, it fits quite well my way of learning.
I joined the board to participate, and hopefully share the little that I know or have in order to contribute to the community.

So I have a friend who owns a Sony C800G tube mic… As it was randomly making “noises”, I took the psu to the bench in order to check if everything was ok.
And it was, we finally found out that the 6AU6 in the mic was guilty…

Anyway, as I noticed that there was no schematic available for the power supply, and I had one on the bench, I drew it down…
This one is definitely not your regular tube mic psu…

As everybody saw it on the pic, there are 2 tubes inside… but those tube are not rectifiers, 6X4 as one could expect… No, those are 6AU6! All the grids are tied to the plate, so it works as a pseudo diode.

Now I’m wondering why did Sony choose that config? Is there any advantage to use a pentode wired like that, rather than a valve rectifier? May be the pentode is a bit “faster”, something like the schotky diodes compared to regular ones?
Or is it just that it was easier (and cheapier!) at the time for Sony to stock only one type of tube for this product? They may have select the best 6AU6 for the mike, and used the too noisy ones for the power supply?

Here are the schematics;
The HV section
http://twin-x.com/groupdiy/displayimage.php?pos=-1325

The twin cap is a very big double electrolytic, with no marking at all.
The pcb traces suggest that the 6AU6 could be replaced by 6X4.
All voltages where measured unloaded.

Heater section
http://twin-x.com/groupdiy/displayimage.php?pos=-1312

The twin cap is a very big double electrolytic, with no marking at all.
The 2 X 1 farad output cap is not a typo. Those are two super caps of one farad/5.5V wired in series. May be a bit overkill? At least all those big caps provide a bit of ramping…

Pelletier section
http://twin-x.com/groupdiy/displayimage.php?pos=-1323

The twin cap is a very big double electrolytic, with no marking at all.
The PNP transistor is a 2SA1220A.
The internal switch cut the ramping off. Ramping time is about 2 minutes unloaded.
I didn’t had a way to mesure the output self, but I doubt that anybody’s planning to built a Pelletier clone!

Has anybody tried to use triodes or pentodes as rectifiers and noticed improvements?

axel

 

[mcs]

[quote author="mad.ax"]Has anybody tried to use triodes or pentodes as rectifiers and noticed improvements?[/quote]
In old amps (pre 1940) triodes were often used as rectifiers. When the tubes in the amp sections needed to be replaced, they were moved to the rectifier positions. I don't think it was done to get improvements though...

Best regards,

Mikkel C. Simonsen

 

[Gus]

interesting

Thanks for the schematics

Are the peletier and heater schematics are labled correct?

5.4V something requlated for the heater makes more sense than 7V something unregulated.

Take the orignal tube and build a small circuit 100V or so B+ a 100K 1meg etc grid to ground a 100K plate and a cathode R for -1V to -1.8V bias. Run the heaters DC or AC at 6.3 to 6.5 or so volts for a few hours then try the tube again.

 

[mad.ax]

Thank you for your comments...

Mikkel
I didn't knew that... Do you mean the tubes and pinout where ready for that switch, from the factory? Or was it a common trick used by techs?

Gus

I double checked for the same reason... but I only had the psu on the bench so maybe the pin number are not the same at both side of the cable?
However, the overkill filtered and unregulated circuit make sense for an heater supply, no? And the 7,3V was mesured unloaded, so it must get lower when loaded, plus the loss in the cable...
I must go to the studio next week to apply some thermal past in the pelletier element, so I may check the voltage into the mic when loaded...

What is the purpose of drawing current trough this circuit? Reduce the tube noise?

 

[guy_4]

Hello,
I spent some times two years ago on a C-800G of a friend, and will share here my experience
As usual, it was noisy. classic problem on this microphone.
Sony says the tube lasts 100 hours !
If that's true, I would say the design is bad.
How about Neumann U47's with VF14 working well for the last 50 years with the original tube ? !
I can't understand you have to pay 250 euros every 100 hours( that's what my friend paid at Sony in France one year before... ) for a microphone that he paid 5000 euros brand new ! ! !
Don't know where the problem(s) is ( are) but for me it is ridiculous.

I also had a look at the power supply, and discover the unusual design with the 2 6AU6 used as rectifiers.
As my friend is the ( lucky!) owner of two C-800G, I did a test.
On one power supply I replaced both tubes with two silicium diodes I had on my bench ( 1N5402) and offer to my " golden ears" friend to compare one C-800G with the two different power supplies. I did not tell him what the mod was, and he did not know what power supply had the mod.
He did tests for several months, and never could find a difference between the original " tube" power supply, and the modded " silicium" power supply.
I kept the two 6AUA 's from the power supply as spares, and used one to replace the noisy tube of the C-800G !

 

[mcs]

[quote author="mad.ax"]I didn't knew that... Do you mean the tubes and pinout where ready for that switch, from the factory? Or was it a common trick used by techs?[/quote]
It was standard - they came with triodes as rectifiers from the factory. Have a look at these two schematics for instance:

http://stiftsbogtrykkeriet.dk/~mcs/we42a.jpg
http://stiftsbogtrykkeriet.dk/~mcs/we43a.jpg

The WE tube symbols take a bit of getting used to :wink:

Both amps were part of a cinema sound system.

Best regards,

Mikkel C. Simonsen

 

[Gus]

mad.ax

I built a microphone using the sony setup of the gain stage.

http://www.groupdiy.com/index.php?topic=9

factory schematic is in that thread

That schematic seems to show two supercaps inside the microphone in the heater circuit and 5.7VDC .

I tried a number of 6au6s. The way I could get the russian 6au6 type to be low noise was to run the heaters for a few hours at 6.3VDC to 6.5VDC in a burn in circuit before using them in the microphone

I read about heater voltage cycling and noise at
http://members.aol.com/sbench101/
had a writeup about tests



Plate, G3 and G2 to a 100K plate resistor

G1 to a grid to ground resistor

cathode resistor selected for -1 to -1.8VDC bias

Heater at 6.3 or more DC or AC

This "cooks" the oxides. Sometimes the tube noise is very low when installed in the C800G circuit microphone. From the small sample size of 6au6 tubes I tried . The russian 6au6 tubes seem to work the best in that circuit. I tried mullards, GEs etc

When you underheat a tube in a microphone circuit sometimes they get noisy from what I have read it is a function of the oxides and cathode alloy

 

[mad.ax]

Guy_4
Thanks for sharing your experience...
I can't believe that Sony advertize that their own 250 euros tube is lasting only 100 hours!

Gus
At the end of your thread you said:
I powered up the microphone after not using it for some time.
I am not that happy with it now. I changed the tube again and a cap.
I believe this design has some issues as built from the sony schematic.
This microphone build did not age well even with different capsules installed still missing some bass when used with voice.

Could it be that the 6AU6 finally died? With all the intensive testing that you made, you should have reach the 100 hours limit a few times!


Guy_4
Tube psu have one real avantage upon silicium ones: the rectifier valve provide a slow ramp B+...
With sand state the high voltage hit the tube very hard and that may shorten a bit more the allredy short life of those expensive Sony tubes... It is even more critical in the case of the C800G where the filament voltage ramp slowly because of all those supercaps... So the mic tube is cold when the HV hit it!

Mikkel
Thanks for those schematics but in those, the triode are used as rectifiers from the start, so my question remains...
We know it works, but does it works beter? the same? or is it just a way to find an utility for second grade tubes?

Gus
I had the factory schematic (downloaded from a previous reading of your thread about the C800) ans I confirm that there are 2 (relatively) small super caps inside the mic, 2 bigger ones (1 farad) in the psu, plus the big dual cap in the psu. This is the most overkill heater psu commercial design that I know off!

I had a look at http://members.aol.com/sbench101/ but I couldn't find the information about heater voltage cycling...
Anyway, I will definitely build your circuit and give it a try...

As I understand, it is necessary to "pre cook" when you underheat. In other words, it would be ineffective with a regular heated filament...

Are those russian 6AU6 sold by Sony really beter than NOS tubes? That would be a very rare case of a new build being superior to a good NOS...
My friend did some test with different NOS tubes and the Sony sounded the worst. But maybe the Sony was fried...

I plan to make deeper testing and I'll keep you informed.

 

[Gus]

mad.ax

The sony tube looks like the russian 6au6 type with the bigger bulb.

I have built/moddded/tested a few more microphones after I built the c800g type one. I have changed a few things in the microphone and it sounds better than that last post of mine.

If I was to build another already made and sold 6AU6 tube microphone I would maybe try a CV60 schoeps circuit. I will try to find the link to a web page with that schematic.

For me the heater supply needs to be well made to get rid of the current spikes from the diodes when I added the super caps to the heater supply it helped with getting rid of the low level ticking I could hear in a quiet room with headphone

 

[mad.ax]

Gus

did you notice that fast wearing effect of the 6AU6 in the C800 circuit, or is it just a legend to sell more tube?

I totally agree with the need to have a very well made psu...
What do you think of trying Pb gel battery at least for the heater?
The battery used in alarm security system fit the bill... They are sealed, (no problem with the gas), compact, low maintenance, easy to charge, and very reliable, no matter how bad you use them....

 

[guy_4]

Mad.ax

The price charged by Sony several years ago was really 250.00 euros for replacing the tube.
The 100 hours life of tube was what the tech at Sony said it lasted...

About the mod I did : you're absolutely right about the heating of the AU6A of the mic, now it receives HT before heating, quite bad
However it works like that since 2 years :shock:
I'll have to look again on tthe PS, and fit a delayed relay on the HT.

Also something I remember : I had some 6AU6A that I wanted to use during my trials, but they were a small bit too large on diameter and could not fit in the mic... those from the PS fitted well.

 

[mad.ax]

Guy_4

If it works like that since 2 years, that's the proof that the 6AU6 can last more than 100 hours, even the ones sold by Sony!

You can fit a delayed relay but the HV would still not be ramped...
Another option would be to use a transistor or a mosfet driven by a zener and a cap to provide a progressive turn on time...
But I think the best bet is to use tube to rectify the HV... You don't need 25O euros Sony valve here... There's no peletier element in the psu so the dimension of the valve are not an issue, and it looks like the quality of the valve is not crucial either, so you could use any second choice, noisy, or even worned out tube in there...
You could also use diferent type, triode or pentode, as long as the pinout is compatible...
For the price of a good delayed relay, you can get a full box of such tubes...

 

[mad.ax]

I've been a bit overswamped with work this past week, so I didn't manage to finish the litle circuit as advised by Gus...
Anyway, I went to the studio yesterday, and I took some mesurements with the mic loading the psu. The wall voltage was 225V.
The heater reads 5,52V
The HV 218V
And the peltier slowly ramp up to 3,96V

This answer the question about the filament voltage. Althought it reads 7,3 unloaded, the 6AU6 is definitely underheated.

As I was there, I took some pics of the guts of the mic. Here is the porn!

inside:
http://twin-x.com/groupdiy/displayimage.php?pos=-1389
http://twin-x.com/groupdiy/displayimage.php?pos=-1391
http://twin-x.com/groupdiy/displayimage.php?pos=-1387

transformer
http://twin-x.com/groupdiy/displayimage.php?pos=-1388

peltier element
http://twin-x.com/groupdiy/displayimage.php?pos=-1390

the mesh
http://twin-x.com/groupdiy/displayimage.php?pos=-1384

front capsule
http://twin-x.com/groupdiy/displayimage.php?pos=-1385

and back
http://twin-x.com/groupdiy/displayimage.php?pos=-1386

The capsule is very dirty, so a cleaning is on my todo list, but if you look closely at the last pic, you will see that the back capsule is off axis... On a tube mic of this price range, it's rather disapointing, isn't it?

 

[jackinthebox]

i'm guessing the schematics were removed due to it being a currently produce
d product?

 

[Deadly Mix]

This is an old thread but it popped up in my search for part numbers to make sure I'm buying the right tube and I figured I should point out a few things I see that could use explanation.

I obviously wasn't there during the design but 1 thing can be said for certain with this mic, NOTHING was done any certain way to cut costs. There are cheaper ways to do things, as the knockoff designers certainly saw, but this mic is designed exactly as I would if I were the designer. That is to say that every choice was made because it sounded best when the mic is working as it should.

The short tube life is not necessarily untrue but it's not entirely true either.
As my Sony guy explained to me (and is confirmed in the manual) this mic was designed to be placed above the vocalist with the "fin" tilted upward (capsule back when upside down and capsule leaning forward when right side up). This, as it was explained to me, has to do with heat rising and moving away toward the back of the fin.

The C800-G is FAR from a noisy mic. In fact, it's incredibly quiet. The noise people report is due to 1 of 2 reasons.
1: Their tube is burned up and needs replacement. I haven't run into this issue yet but I've been told by other owners that the tube usually isn't anywhere near burned up in this case but rather, most of the time, it has to do with the tube running too cold. Word is that all you have to do is warm it up in something else that doesn't manage heat as well and put it back in and it'll be like new.
Like I said; I haven't run into this issue yet but I theorize, a much easier solution would be to simply tilt the fin down and run it for 30 minutes or so then tilt the fin back up.
2: They're not using the correct tube. This mic was designed for 1 tube specifically and, as all of you who've replaced a Sony tube before can attest to, it's not just any old 6AU6A. Holding it next to a 6au6 right now, they don't look to even be the same tubes.
Sony's tubes came from 1 specific factory in Ukraine that produced this tube specifically for Sony and nobody else. As if that didn't make them special enough to begin with, Sony then tests them all 1 buy 1 and reportedly only passes approx 1 out of every 20 tubes on average. Myself and many others I've spoken to all seem to think the tubes in the PSU are taken from that "rejects" pile it would explain why it has a different part number despite otherwise being the same tube. Perhaps the PSU tubes are the best of the "rejects".
I'm not sure on the PSU but the point is that the C800-G is extremely quiet when fitted with the correct tube in working order and the life of the tubes is greatly extended by tilting the fin up when in use.