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I am considering changing my diode rectified power supply for a tube rectified one (EZ 81 using a centre tapped transformer). Before I do major surgery, is it really worth it? Will it be a cleaner smoother sound?? The circuit is no problem to me, but your views welcomeon what it'll hopefully bring!
Cheers Paul
Follow Ups:
Hi Paul
I would not recommend to switch from ss diodes to a tube rectifier.Tube rectifiers have a relatively high internal resistance and a "soft start" when the AC changes from minus to plus. This avoids sharp current peaks, saturation of the transformer core and RF generation. This all may be responsible for a smoother sound with tube rectifiers. On the other hand you have a considerable voltage loss across the tube rectifier. So just switching from ss to tube will result in a lower + rail voltage if you do not planto change the power transformer also.
In my experience the ss diode disadvantages can be easily overcome by a choke. Fitting a good choke (around 5 Henry) directly behind the rectifier kills the current peaks and you have very low ripple. Place 10nF ceramic caps parallel to the diodes to shortcut RF, but place the big (electrolyte) caps behind the choke only.
In conclusion: A choke implanted will be a better solution than major surgery with extirpation of your diodes in my opinion.
Greetings and good luck
KlausB
Thanks for your help, it seems ss diodes win from your views!
The preamp of my (heavily tweaked)Tube Technology setup had a regulated HT power supply, which (luckily!) shorted somehow. Just to still be able to use it until getting a replacement, I put a dropper resistor in to get the same line voltage, and was surprised how much 'faster' the music was, without the IC regulator in between, taking a good share of the current as it ran very hot as designed.
I may still try the tube diode one day, as a few years back I had (as my first tube amp) the ROGERS Cadet III (really basic stuff! Not worth tweaking really as the design is rather odd, but a good learning tool) and found the tube diode sounded really smooth.
Cheers Paul
Dear Paul: Solid state diodes give a little better voltage regulation than tubes, but rectifiers like that EZ81 have a slower warmup time.. This keeps your amps tube cathodes from "striping" when the B+ voltage comes on before the tubes are up to operating tempature. Please let me know how it works out. Happy New Year!! The "beam Tube"
Hello KlausB,If my tube amp is already using ss diode rectifiers and choke filtering, will the use of FRED bring any improvement in sound? One advice I got was this will make the sound overly bright. I do not know the reason why as I thought the use of FRED is generally recommended. Any idea before I attempt the upgrade?
Thanks,
Stanley
Hi Stanley,I have to admit, that English is not my native language and I sometimes have trouble with these abbreviations in Technical English. Can you explain to me what F.R.E.D. stands for ?
Best wishes
KlausB
Hi KlausB,Happy New Year to you! I understand FRED stands for Fast Recovery Epitaxial Diode.
Cheers,
Stanley
Hi Stan,I would not have guessed that from the abbreviation. I am sorry to tell you that I have no experience with these fast recovering diodes. So a called a friend who is a high-end dealer over here in Dresden and also an experienced tweaker. He told me, that he saw these type of diodes in very expensive solid state gear but so far not in tube amps. So in conclusion I have no sufficient information for you.
Best wishesKlausB
Addendum to my previous post: And by the way, if instead of a pi filter you are talking about adding a choke to form a "choke-loaded" supply (where the first reactive element after rectification is a choke), will this not cause a much more dramatic drop in available DC voltage than does use of a tube rectifier vs ss rectifiers? Also, not all tube rectifiers drop a lot of voltage; the mercury vapor types (e.g., type 83, etc) drop relatively little voltage and the voltage drop tends to be constant regardless of current draw.
Hi Lew,you are right. A supply with a choke being the first active element will cause high voltage drop when loaded with current and the drop will be much more than that of a tube rectifier (The EZ 81 has an Rt between 200 and 350 Ohm according to my data sheets).
The EZ 81 and other tube rectifiers such as the GZ 32 and GZ 34 are vacuum tubes and they are frequently found in the old schematics. I do not know whether gas filled tube rectifiers were a standard in former audio equipment. As far as I know they were mainly used in high current applications.
From the data sheets it is obvious that they have very low internal resistance and hence it is interesting to hear, that these tubes work perfectly in your gear.
However, in my 1955 handbook I also read that they can oscillate as Jimmy mentioned.Again best wishes and a Happy New Year
KlausB
Mercury type rectifiers can oscillate causing noise in audio amps. Reference: 1950 Radio Amateurs Handbook.
I guess the operative word in your statement is "can", as in "mercury vapor tubes can oscillate". Not having read the publication you cite, I built my first-ever homebrew amps using type 83s as rectifiers (one per channel on monoblock amps). Man, do they sound great, and they are dead quiet on a 100db sensitive spkr! Also, the type 83s in my two Hickok tubetesters don't oscillate, where the tubes in question may each be 20 or more years old in circuit. I think audio, like science and medicine, is full of these little superstitions and unscientifically substantiated "facts", like the idea that mercury vapor tubes oscillate. I'm sure that they CAN oscillate, but really how often is this a problem?
Please define your use of the word "behind", as in fitting a choke "behind" the diodes. Are you talking about using a pi filter downstream from (=behind) the ss diodes, as opposed to a simple cap filter? If so, that requires no further explanation. If not, it sounds novel to me, so please explain further.
Hi Lew,you are right. What I meant is a pi filter. Talking about the "big" caps downstream (is behind an incorrect term?) the choke I forgot to mention the small cap in front of the choke.
Now you might ask: what is small ? In my opinion the cap in front of the choke should be as small as possible to avoid high rectifier current peaks. On the other hand it has to be big enough to avoid major voltage loss across the choke. In order to find the optimum value for this first cap you can run a simulation program, for instance PSU Designer II (download from www.duncanamps.com).By the way: real LC filters for power source units without caps in front of the chokes were also used in the good old days of professional tube power amps.
Best wishes and a Happy New Year
KlausB
Hi.
If your power amplifier is push-pull not a SET, I'd like to recommend not to change the type of rectifier. How about upgrade choke coil and by-passing cap ?
I think very high voltage film cap can help ss-diode supersede to tube rectifier.
Generally DIYers used to change tube diode to ss-diode. But ss-doide to tube diode is not an easy one. Because changing to tube rectifiers needs large space geometically and additional power supply.
You have said "surgery", but I'd like to say "re-creation"
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