Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on February 09, 2022, 08:37:16 am
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Will it hurt the transformer or have any other adverse effect? I know the sound will be stiffer (I'm after that). It seems dumble did this. Ex. i have a 2 6L6 transformer and am thinking of running these caps i have on hand: a reservoir cap of 165uf 630v. B+2 at 110uf 700v and B+3/4 at 50uf 900v. B+5 at 20uf 500v . Any issue with this? I'm running a FWB, modded 2204 circuit
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Tube rectifiers have a limit for one usually 20uf or lower is recommended by tube data in RCA manuals. I believe it could damage the rectifier tube if the uf value of the first filter cap is too high.
Other than that I would imagine it would give a stiffer/less sag effect to the amp
My first amp I cobbled together with parts I had around mostly and since I used a bridge rectifier I figured it would kill the Hun better so I upped the values on all the filter caps and I noticed when I turned of my amp it continued to make sound for at least 2 to 3 seconds. Normally with standard values you shut it off no more sound , but with the high cap values I could still play and hear the guitar through the speaker .
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any other adverse effect?
assuming your FWB is SS, it takes time to charge large caps so they "see" the full unloaded B+ for several seconds.
so verify your PT secondary voltages with no load and "expect" that will be the volts on the 1st cap for a couple seconds.
I build one filter network that took ~~15-20 seconds at >600vdc before the tubes were up n idling
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I believe what i have proposed is overkill. I will still go with high values likely 100uf at reservoir followed by 50uf down to to 20uf on V1 B+
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Higher capacitance means more work has to be done to fully charge the filter caps because there's more (filter cap) charging current required. But if there isn't more than a usual amount of current being drawn from the power rail, then the power supply (including the diodes* and the PT) can survive happily with more filter cap capacitance.
On-going charging current requirements would only become an issue in the (highly theoretical) situation where you had a lot of current being drawn from the power rail (resulting in a demand for more charging current), which won't happen in your typical guitar amp.
The peak demand for charging current is at startup and that only lasts for a relatively short period of time - after that, a smaller amount of charging current is required for maintaining filter cap charge at the peak of each cycle of the rectifier pulse (as the current gradually gets drawn from the power rail).
When you hit a power chord, it will cause more current demand, and if the filter caps are 'smaller' the power supply voltage may tend to sag more.
More filter capacitance will help reduce tendency to sag (if the PT and diodes can handle the current supply). So tonally, you may not like too much filter cap capacitance, depending on how you like your amps to sound. If you're a bass player, maybe you'll love bigger filter caps.
*SS diodes that is. Tube rectifier plates can only handle so much peak power before they exceed their maximum dissipation, which is why there is a reservoir capacitance maximum rating recommendation on tube rectifier data sheets.
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fwiw
this is my "stiff PS" basically stole from Marshall
I build SE amps and this works well for keeping hum nice and quiet.
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...I'm running a FWB, modded 2204 circuit
Just a pedantic point, but a 2204 has a 2 phase / centre tapped type full wave rectifier, rather than a full wave bridge (an example of which is shown in post #6) https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_2204_lead_series.pdf
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Tube rectifiers have a limit ... recommended by tube data in RCA manuals. ...
... which is why there is a reservoir capacitance maximum rating recommendation on tube rectifier data sheets.
I only recall seeing a "maximum capacitance" on the Philips GZ34 data sheet (https://frank.pocnet.net/sheets/010/g/GZ34.pdf) (see Page 3). Everyone else (including every RCA sheet I recall seeing) simply provides an "example condition" (often with much lower capacitance than we would typically use today).
The issue is Peak Plate Current in the rectifier: there is a maximum peak plate current listed, above which you may damage the tube.
The issue is "Capacitive Current (https://www.sas.upenn.edu/LabManuals/BBB251/NIA/NEUROLAB/APPENDIX/MEMBRANE/capcurr.htm)" which is drawn to cause a change of voltage across a cap in a unit of time. More µFs or More Volts or Less Time, and that charging current goes up.
So American rectifier data sheets often mention an amount of "series resistance per plate" (Rating Chart III on Page 3 of this G.E. data sheet (https://frank.pocnet.net/sheets/093/5/5AR4.pdf)). Ohm's Law tell you for a given amount of Volts, More Resistance causes Less Current.
So there's a tricky bit of math to figure out if you'll retain a peak plate current lower than the tube's limit, and that has much to do with your desired supply voltage, the µFs of capacitance, how much current the amp pulls from the supply, and how much series resistance (in transformer windings, or in external resistors) are in the amp-design.
The safe move is to look at commercial examples of amps like what you plan to build, and don't exceed what they're using (unless there are chokes/resistors in the way).