Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mesa3077boogie on February 15, 2019, 08:43:43 am
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I am trying to determine how feasible it would be to add a tube rectifier to a solid-state rectifier amplifier. There are so many great amplifier choices out there right now however none of them are tube rectified and I really like that characteristic based on current and previously owned amplifiers.
If we assume that I can power a tube rectifiers filaments how feasible would it be to convert a solid-state rectified amp to a tube rectifier?
There is a very simple mod where you can convert a tube rectifier to a solid-state rectifier using two diodes soldered inside an octal tube housing. It's basically Plug and Play.
What is to stop me from merely reversing that schematic removing the existing diodes and putting a tube rectifier into an amp of my choice? Again let's take powering the filaments out of the equation. What other circuitry or factors might have to be perceived in order to add a tube rectifier to a solid-state rectified amp?
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Google for SAG Resistor mod
(Read about at Tone Lizard - Modifications)
Franco
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I know all about it.. I want a tube!
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There are so many great amplifier choices out there right now however none of them are tube rectified
Hmm, I disagree with that. I see lots of great amplifiers with tube rectifiers.
Since filaments is out of the equation, there is no reason why you can't use a tube instead of ss rectifiers. But usually only in amps under 60 watts. Once you get over that power rating you really should look to ss rectifiers or be prepared to use multiple tubes in parallel.
And you should match the current rating of the rectifier to the current requirements of the amp. That is, don't try to use an EZ81 to power two EL34s.
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So I am looking 20 watt or similar amplifies. Things like the orange OR15, origin 20, vox AC 15, etc. to add a tube rectifier on.
I have owned the Vox AC30 hand wired and loved it. Also have the mesa stiletto ace, mesa lonestar and love the tube rectifier!
So picking the correct tube, ✔
What else circuitry wise other than reversing the tube to SS schematic?
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What else circuitry wise
when ur spec'n, look at 1st node, cap or inductor, there is usually a max spec for amount of cap at the 1st tap
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I agree with sluckey & kagliostro.
If you must have a tube: you need room for physical layout, tube socket, etc. and wiring. Drill hole for tube socket without harming anything. The rectifier tube must be able to handle the voltage & current of your amp. If more than one tube is suitable, pick one with the voltage drop you want. You need to supply the tube with proper voltage & current. This might require a supplemental filament tranny with its own layout and wiring requirements.
If your amp draws a lot of current you may need dual rectifier tubes.
The value of the 1st filter cap off the rectifier should match the tube's specs.
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Changing to a tube recto in a 2xEL84 is easy because you can use an EZ81 tube and run the filaments from the existing 6.3 filament winding (assuming the existing winding can provide an extra amp of current. DON'T EXCEED 50µF INPUT FILTER CAP.
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One thing to remember is that generally valve rectified (2 valve diodes with a common cathode) circuits require a center tapped secondary winding on the power transformer HT supply. Where as most solid state "4 diode" circuits do not.
G34 has a common cathode so is not capable of even running a pair instead of 4 SS diodes. Theoretically you could use either 4 single valve rectifiers or one GZ34 style dual rectifier as normal for the + side and 2 more for the negative side. Bit complicated whereas a HD metal clad wire-wound resistor of 25 - 50 watts would be more practical/reliable and give you "close" to the same effect.
If you really must have valve rectifier you can always run it after the SS as kind of a pass through. Here you would disconnect the output of the SS rectifier common up the anodes to the output of the + SS rectifier and rejoin to the + going to the A+ capacitor. The current draw thru the valve rectifier would then create a "similar" effect.
Second thing is when comparing class A/B or B to class-A amps, the latter draw a constant current and typically less when overdriven so the "Sag" people are always going on about might actually be the inverse of what they think! What you may be hearing is the Class-A amp power output sagging when clipping and then output growing as the signal decays back into unclipped Class-A operation. Class- B draws lower idle current so yes at low volume the rectifier can supply a steady voltage but at higher output power levels it will "Sag" which is why a Class A/B Boogie will sound sweeter with valve rectifiers in the power chain. I've always swapped the valve rectifiers in AC-30's for solid state for the extra voltage, power/clean headroom. Of course that's usually what blows your pretty alnico blues to smithereens but hey they make Gold's now too...
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Tube rectifiers require a certain degree of protecting resistance in series with the rectifier plates; usually it is provided by the HT winding resistance, but PTs intended for silicon rectification may be designed to run cooler by minimising HT winding resistance.
Point being that a tube rectifier may require additional plate resistors.
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You can do also this way if the PT has no CT
http://www.valvewizard.co.uk/bridge.html (http://www.valvewizard.co.uk/bridge.html)
(https://i.imgur.com/cbZpdoB.png)
one other way is to place te Vacuum rectifier after a Full Wave Bridge
(the effect on B+ will be the same as using a Vacuum Tube, well, there is a benefit, the max voltage allowable (due to the kind of tube) is increased for the presence of the diode bridge)
(https://i.imgur.com/Ex5FMDw.jpg)
One other option is to use a single Vacuum Diode (same effects and advantage of the previous solutions)
(https://i.imgur.com/0XuzdFq.jpg)
K
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K, better flip that tube around in your last pic. :wink:
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OOOPPPSSS
Image corrected
Franco