Shunt regulator B+ in SE Valve Jr.
Hi all! First post here, been lurking for a while. Hoping ya'll might have some advice for a newb.
I have a Valve Jr.
that I'm converting to turret board build. Looking for best clean sound, with lots of headroom. Have a great Pedal for distortion, so not looking for lots of gain. Anyway, I've always felt that the B+ PS is the weakest point on most guitar amps. Valve Jr. uses simple CRC network.
Glassware webpage has a janus shunt regulator kit
that might be a candidate for this amp. If I use solid state rectificiers, current PS transformer (filament supply) could likely power the extra 12dw7 tube.
My question is this, can I remove any of the CRC B+ network to the other tubes? If I'm getting this right, the problem of having a lot of capacitance after a shunt regulator is that when the amp is powered up, cap network acts like a short for a short time, and could actually stress the shunt circuit, so this might not be a direct drop in. The amp will have a standby switch wired in, so filaments should be well heated before B+ is powered up. If I'm understanding this right, CRC network is meant for ripple filtration and as sort of a power/voltage/current reserve for the different gain stages. Shunt regulator should take care of ripple, but not sure if there is enough 'reserve' to not starve the down-line gain stages. Should I decrease capacitance downline, relying on large filter capacitors in shunt circuit to take up some of the burden? Or am I picking nits?
I'm reading a lot, trying to understand tube circuits, and this seems a simple build for first timer, but I'm clueless as to whether this is feasible. Can anyone help?:confused:
beware. Guitar amps rely on the sag of the power supply to provide some of the nice distortion at overload.
Firming up the supply will/may (if done right) eliminate this characteristic.
:cop: Please note that guitar amplifier threads really belong in the Instruments & Amps forum. Please see the subheaders at the top of each forum for the exact reason why.
I'll second the comments wrt shunt regulation in a guitar amplifier - not really what you want to do..
Ok, you want a clean sound, and "lots of headroom".
So you probably do want quite stable supply rails.
But maybe you already have them? The two triodes and the pentode are all operating in single ended class A, so the change in current consumption with output power is minimal compared to a push-pull amp.
If you really do want a regulator, why a shunt one? I can't say that the janus looks that good for this application either, though I'm open to persuasion, if anyone's actually bothered.
I'd suggest a solid state series regulator, have a look here:
Gearslutz high voltage regulated tube supplies
- you don't need a bias supply, so you might find gyraf's circuit appropriate
or if you really want to use valves, read Steve Bench's pages starting here:
Steve Bench on Tube Regulators
The usual CRC filter with smallish filter caps (22uf) and a 3.3 K resistor contributes a lot of the "tone" found in these little SE amps. When driven to extreme (1 or more overdrive pedals) the output tubes screen current goes up driving down the preamp voltage and gain. As the note dies out the output tube begins to recover drawing less current allowing the preamp voltage and gain to increase. This improves the amps "sustain" and is useful for some music styles.
As with any amp, what you want to build is determined by what you want it to sound like. As a generality, a regulated power supply is good for a loud clean tone, a unregulated power supply is best for a loose distorted tone with more touch sensitivity.
A shunt regulator wastes energy by shunting some of the B+ to ground. They were not really intended for this type of application. I have never tried it in a guitar amp, but who knows, you might invent a new sound. I tend toward a mosfet series regulator for a guitar amp though.
Let's just read those beautiful phrases again:
Big SE amps ... driven to extreme
When driven to extreme ... the output tubes screen current goes up driving down the preamp voltage and gain.
:wiz: .. :eguitar:
Hi all! Thanks for all the reply's!!!
First off, sorry for posting in the wrong section. Thanks to the admin for bumping it!
Next, what I'm looking for is cleaning up the grunge inherent in a simple CRC network, more background noise/hiss/hum elimination, so that the cleans get cleaner. I realize a guitar amp is probably the worst environment for doing this, with all the inline equipment and long cables. It's an experiment.
In terms of the type of regulation, I could care less whether it's SS or tube, just thought tubes were generally a little more forgiving. I do have limits on what the power transformer is capable of powering, though. Worst case scenario, could get a separate transformer for the filament supply, easing demands on the transformer and maybe allowing for a little more current/voltage on the B+ secondary.
Again, the distortion pedal in front would be used for clipping at the preamp level, so I'm looking for best cleans here. Power amp clipping I do want, but relatively 'soft', if you catch my meaning. Not looking for metal. Will be using a VVR, to turn down actual volume (SPL) while still keeping output tube dist characteristics at reasonable levels. I know it's not exactly the same, but am willing to live with the tradeoffs for my many roomates sakes.
I realize routing of wires, shielded wires to pots and jacks, and grounding are all huge deals for eliminating hum, and hope to be thorough there as well. I'm actually making a separate DC filament board and mounting it far away from the B+ for the same purpose (eliminating noise). Could use some suggestions for that as well.
I'm just trying to take down all noise floor to a min to help the cleans. Maybe it's not recommended, but this is a cheap amp and worst case scenario, I can redo it. Looking at shielding sections from each other (TI shield), shielding and routing of wires, etc. anything to bring down the noise floor.
Thanks again for the reply's, hope to learn more!!!
After reading thru the links ya'll posted, I'm hoping to ask a few questions.
1st - OK, maybe a shunt reg was a bad idea. The only thing that made me think the janus might be a candidate was this statement from his order page -
"The Janus regulator is not a DC voltage regulator, as it cannot maintain a fixed B+ voltage; instead, it works to eliminate any AC perturbations on its output voltage. In this way it is similar to a large inductor-filled power supply that bucks quick changes in output voltage, but allows slow DC voltage to vary with the wall voltage variations."
Sounds like sag characteristics? My hope is for a drop in board, but I'll entertain any ideas.
Simon B - Looked at the Gyraf circuit http://www.gyraf.dk/gy_pd/g9/g9_sch.gif , I wouldn't even know where to start in trying to extract the B+ portion. The circuit by Waveborn looks interesting, but could it be done without the Bias circuit? And would the inrush current adversely affect it?
Thanks for the link to Steve Bench article. The single tube, fixed biased pentode looks interesting. Any idea of a current production tube that could be used for it? Also, would there be a solid state equivalent?
For KevinKR, you seem to know your PS's, is there a circuit you have used/developed for this type application?
Lastly, would there be any advantage to inductor filled PS over standard CRC network? Sort of a CLCLC...etc.?
Thanks again for any reply's!!!!
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