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Old 25th September 2012, 09:02 AM   #11
TV7 is offline TV7  Australia
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Success! the riaa was the culprit me thinks, an oscillation between the first tube and the second. A 10K resistor inserted to the grid of the CF and a .001uF polyester from the grid to ground removed the parasitic oscillation. Seeing as the first stage has gain to burn this cap serves to not remove noticeable audio quality while removing that pesky 1.8M signal. Now the KT88s run cool and fine. Currently listening to Ray Charles greatest hits and the audio is magnificent. I am using ecc83 siemens tubes into ecc827 (silver plate siemens) instead of the original 12AT7 CF, but those sounded fine also, just my personal preference in sound.

I can post a final schematic if anyone is interested in building this fine, cheap and simple little preamp.

Many thanks to Hans and all the others that supplied info on fault finding, it was much appreciated.
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Old 25th September 2012, 09:13 AM   #12
PET-240 is online now PET-240  Australia
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Hey TV7,

I have a TV-7 D/U on the way over freshly calibrated, any relation?

I am interested in the schematic etc, do you know the input sensitivity? And here's a can of worms, I want to run my cart balanced........

Any thoughts?

Cheers,

Drew.
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Old 25th September 2012, 09:46 AM   #13
TV7 is offline TV7  Australia
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Originally Posted by PET-240 View Post
Hey TV7,

I have a TV-7 D/U on the way over freshly calibrated, any relation?

I am interested in the schematic etc, do you know the input sensitivity? And here's a can of worms, I want to run my cart balanced........

Any thoughts?

Cheers,

Drew.
Hey Drew, yeah the TV7 is my favorite tube tester hence the user name.... I own 3 TV-7D/U units manufactured by ecco, forway and bruno. I also have a TV-2B/U.

The Bruno needs a couple of parts replaced, i.e the bias pot and a couple of ceramic switches which I'm currently engaged in when I have the time. Then it might go on eBay or something because its like new.

I also repair TV7 meters, these are hard to find by any standard.

When you get the TV7 make sure you put diodes across the meter to prevent accidental burn out. Just remember 2 in series one way and another 2 in series the other way, a lot of self proclaimed experts use only one, this starts to conduct around the 100% area of the meter movement and causes incorrect readings.....
Use signal diodes, I can find out exactly which ones if you like. I have a bag of them in front of me, but they are worthless and easily available......

I hope Dan Nelson calibrated your tester, he is on this forum as user TV7GUY, and is a wealth of knowledge and spare parts . If someone else did it then don't hold your breath, I took me a while to get the exact 1kohm loading for voltage measurements. Once this was done the units are amazingly accurate. Even across my units the only slight difference is the printing around the bias knob causing the slightest reading change when testing because of tiny differences in bias voltages....

Input sensitivity? think 12AX7 and around 20Kohms.....Balanced cartridge? hmm get back to you on that. SY wrote a great article on phono preamps that might be right up your alley, although more work than this little gem.

His Master's Noise: A Thoroughly Modern Tube Phono Preamp

I'll post the schematic once I build the power supply, although there are plenty of good ones on here.

I'll leave you with a pis of one of my TV7 units. This ecco manufactured unit was refurbished by myself including the lettering. Hard to find a nicer one anywhere...

Cheers.Click the image to open in full size.
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Old 25th September 2012, 10:31 AM   #14
PET-240 is online now PET-240  Australia
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tv7

yup just done by Dan, had a tube supplier put me onto him, he did the diode thing, shall ask if he used two or not, tis an ecco, now has a new gauge(used methinks),bias pot, rectifier tubes and spares, calibration tubes etc etc, bit of paint missing from the knob areas, wasn't a cheap repair, so shall see how it goes, really easy to deal with he is and really quick turnaround for me considering it is a hobby for him.
Now to learn how to drive it......
it is in good nick!!!
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Old 25th September 2012, 10:43 AM   #15
PET-240 is online now PET-240  Australia
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This is where I'm looking to go/build.....Atma-Sphere music systems, inc.

shall be a mission.......calculating the RIAA stage is the hard bit I figure.....
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Old 25th September 2012, 10:56 AM   #16
TV7 is offline TV7  Australia
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Originally Posted by PET-240 View Post
tv7

yup just done by Dan, had a tube supplier put me onto him, he did the diode thing, shall ask if he used two or not, tis an ecco, now has a new gauge(used methinks),bias pot, rectifier tubes and spares, calibration tubes etc etc, bit of paint missing from the knob areas, wasn't a cheap repair, so shall see how it goes, really easy to deal with he is and really quick turnaround for me considering it is a hobby for him.
Now to learn how to drive it......
it is in good nick!!!
Yeah Dan would have used two diodes, If I remember correctly it was him who put me on to using 2 in the first place. Glad to hear it is coming from him, at least you know the job is done right. Surprised about the amount of work done, bias, rectifiers, meter, paint?? wow by the end of that you will have a new tester indeed! I would put money on the meter being one I supplied to him.....

About the amps, they look scary expensive. Hope they sound as good as they look. Perhaps you should build one instead?
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Old 25th September 2012, 11:14 AM   #17
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Hi TV7

Glad you stopped the unit from oscillating.

ECC83 = 12AX7 and ECC827 = 12AU7 i think, never heard of ECC827 though.

Putting in a 10K with 1000pF to ground is a little drastic, that puts a pole at 16kHz and will put a significant error in the feedback loop. It probably does not matter out at 16kHz, but the quality of that capacitor has to be good. COG ceramic or polycarbonate or polypropylene capacitors would be best.

Greetings

Hans J Weedon
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Old 25th September 2012, 11:23 AM   #18
PET-240 is online now PET-240  Australia
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Heya TV7,

Waaaaay outta my range to buy, so yes looking to build one, the tubes not a major issue, the RIAA in balanced could be, the values would I imagine be much the same but what they will be for a balanced rig is my unknown, that said, Ralph is one of the most helpful manufacturers I have ever had the pleasure to deal with, if you can think of the question, he does what he can to answer.
you seem to have a stock of NOS valves aboout up there, perhaps we should chat, my email is in that pm.
Catch you soon! Glad the preamp is sorted too! I am always thankful that people smarter than me are willing to share their know how to help others. Sorta restores some of the faith in Humankind.
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Old 25th September 2012, 11:54 AM   #19
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Hi again TV7

After further thought I believe that putting a 0.001uF cap inside the RIAA feedback loop is a very bad idea. This is how good designs gets ruined and propagate into the world of circuits. Please find a proper way to stop the 1.8MHz oscillation, my bet is on the power-rails not being bypassed properly. By loading the loop-gain with a 1000pF there will be nothing left to track the RIAA components.

Hans J Weedon
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Old 25th September 2012, 11:56 AM   #20
TV7 is offline TV7  Australia
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Quote:
Originally Posted by HJWeedon View Post
Hi TV7

Glad you stopped the unit from oscillating.

ECC83 = 12AX7 and ECC827 = 12AU7 i think, never heard of ECC827 though.

Putting in a 10K with 1000pF to ground is a little drastic, that puts a pole at 16kHz and will put a significant error in the feedback loop. It probably does not matter out at 16kHz, but the quality of that capacitor has to be good. COG ceramic or polycarbonate or polypropylene capacitors would be best.

Greetings

Hans J Weedon
Yeah it was a little drastic. I'll re-think the problem over the next few days and post if I get a better solution. Maybe I could run the input through a cap and bias up the CF grids with a voltage divider, a nice phase inversion. But this could introduce power supply noise ect ect..I'm still concerned about the effect to the RIAA curve.

Indeed I have not heard of a ECC827 either, that was a typo it was meant to be ecc82/12au7.
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