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In Reply to: Some things to check posted by Gordon Rankin on February 03, 2004 at 07:54:42:
Gordon, I tried the 6A3 grid with the multimeter (Fluke 8040) and my old Tektroniks oscilloscope. I havent 100hz noise here (here as in a lot of part of the circuit I can find only with my 100Mhz oscilloscope 200mV/101MHZ p.e.p. from a local broadcast FM).
On the 6A3 grid I can find rms 0,65V/100hz only when I have removed the 600V from the plate (at the amplifier burn in, becouse I start HV on only after 30-40 seconds from filaments supply on). When HV is on, on the grid there is no ac voltage.
I have shorted the ECC83 grid but noise rms 270mV/100hz persist on the 6A3 plate, with 12-14mV rms in the output transformer 8ohm.
Also, even on the oscilloscope the 100hz noise seem higher at the 6A3 plate than at the 630V (really 600V)CLC output: the multimeter rms say about 270-280mV rms at 6A3 plate and around 100mV rms at the 600V CLC output.
These informations say you something?
Many thanks Gordon.
Gian Luca Cazzola
Follow Ups:
Gian,Put a 100 ohm resistor between the 6922 plate and the 6A3 grid. Make sure there is no dc voltage across the resistor.
I really hate direct coulpled amps because they are a pain in the grounding scheme. You must make sure that the cathode side of the 6922 and the 6A3 ground are connected at a star ground. Then connect that ground to the center tap of the power transformer with heavy wire like 12ga.
As far as parallel caps here is my reasoning. First just use the best cap possible for the job and one of them. Or a matched pair in L cancelling Black Gate topology. But if you use different sizes and types of caps then the complex impedance at any frequency tends to inhibit good sound in that stage. Try it... I did always sounded better with a single or L cancelling even better.
Thanks
Gordon
J. Gordon Rankin
Gordon, I tried with a 220ohm (I havent 100ohm) between the connection ECC83 plate - 6A3 grid. There isnt voltage (ac or dc) across the 220ohm resistor and at the output persist 12mV ac.
About ground:
I used a common ground for each stage: right Ecc83, right 6a3, left ecc83, left 6a3,plus separate ground from each filtering capacitor and ground from a side of each output transformer secondary (I tried to remove output transformers output connection to star ground but again noise 12 mV persist).
Each common ground connect to the star ground where also the center wire of power transformer secondary is connected. The star ground connect to the chassis connected to power company ground. I tried also to disconnect star ground from chassis and power company ground but the 12-14mV ac noise again persist.
I tried each ground for cold solder but all soldered joints seem good.
I am thinking to try with the E83CC Ultron and VT52 Hytron to see if noise will reduce: what do you think?
I have understood your opinion about cathode capacitors, but I dont know what is a "matched pair in L cancelling Black Gate topology".
About the Loftin White (or direct coupling) now I understand that it is not a simple circuit. I choiced it becouse with E83cc+VT52 I found only Loftin White circuits. Really the only circuit with cap coupling was WE310A+VT52 and I havn't a WE310A nor I doesnt know where to find them. Take note that I had here E83CCs and VT52s bought from 20 years at very low price and stored in a box since then - hi.
Thanks.
Gian Luca
Gian,I don't know if this will help but here goes. All the grounds with the same color must be made to a STAR ground at one location. Each of the 2 channel grounds must be connected to the RED ground via a heavy wire say 12ga.
Also sorry for refencing 6922 all this time. I thought I had remembered this amp to be a 6922 to VT52B but it is a 12AX7.
I would suggest changing the 330K to a 250K in series with a 80K resistor to isolate the L+R channel. At the 250K to 80K juction place a 1uF solen or good cap to their respected channel ground.
L cancelling pairs is when you take 2 non polarized caps of the same value and put them in opposite directions (top to botton, botton to top). This way the inductance in them is cancelled out and they work to higher freqeuncies. This is best done with Black Gate BGN NX series capacitors.
Otherwise you could easily make the 2 3A 6.3V AC heater circuits to dc using a rectifier bridge (10A or better) and a large cap say 10,000uF at 10V or so. You may have to put a small 0.22-0.5 ohm resistor in series with the AC side of the rectifier to get the correct 6.3VDC output.
Good luck...
Gordon
J. Gordon Rankin
Hi Gordon, again many thanks for your qualified help. It is a lot interesting for me to improve my circuit with the help of your big know how and experience.
I will order the components and in next days I will try your modifications.
I will say you more about my VT52/E83cc audio performance with my Tannoy Edimburghs soon. I havent again tried again audio tests becouse I had a flu with otitis, so my ears have more noise problems than my amplifier :,) and I need again some days to return normal.
Take note that last year I have seen also your Bugle 45 circuit but I had doubt that the power was too low for my 94dB/1W loudspeakers.
Anyway if one day I will try Tannoy Supertweeters to my speakers I think that I will try also Bugle 45 to drive them.
I need also another opinion from you: for driving the VT52 do you think that WE310A are good-better than E83CC? I ask so becouse on e-bay I see at acceptable prices these tubes and I have found a circuit WE310A-VT52 capacitor coupled.
Again many thanks dear friend.
Gian Luca Cazzola
Gian,First your Tannoys are far from a good match for the VT52/6A3. As the xover in them really eats up the power. You would be much better off with a 300B amplifier than a smaller 6A3/VT52.
The 310 is ok but not great. Look for the 6.3V filament version I think is a 247 but I could be wrong on that.
You could easily convert the bugle into a VT52 amp but for that you would proabably need DC heaters as with the small increase in noise because of the choke input supply & AC 6.3V heaters would be a bit too high. Also change the grid resistor for the output tube to 300K and the coupling cap to 0.1uF and it will work great.
Have fun,
Gordon
J. Gordon Rankin
Gordon again thank for the new infos.
But please take note of the results that I had today changing the Sovtek 6A3 with my nos Hytron VT52:
noise at 8 ohm transformers output with VT52 is reduced to 5-6mV from the 6A3 12-14mV.
I think this is a strong confirmation that US nos tubes can be a lot better than new productions from Russia.
I will say you more after listening tests and after you mods introduction (today I have ordered other Kiwame resistors and Audyn caps 1uF-630V).
About amp power for Tannoy Edinburgh: I remember an your old suggestion, so I used 3kohm output transformers and not 5 kohm to have a bit more power. I think the amp is 4 - 4,5w rms output.
I listen to jazz, vocalists and chamber music and not many simphonic works, all at not very high volume levels, and I hope power will be adeguate.
Anyway if power will be too low I think I will try to use my old but good Quad 303 for LF (below 1khz) and VT52 amp for HF (higher than 1khz).
I used VT52 and not 300B becouse I have from more than 20 years (bought when I was 20-22 years old for about 15$) eight Hytron (6) and WE (2) VT52: so the choice was obliged!
Thanks my friend.
Gian Luca
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