2 - Decoupling Caps
This addition to the original design is used for creating a reservoir of "fast delivery" power for the tubes to use, the supply of the power will be more independant of the PSU, hence the name "DE-coupling"...
The PSU has "large output caps" and is working on "constant load" mode. This is one design that is very musical and effective. It brings/improves silences, air, dynamic, clarity ...
To improve this PSU, we have decided to add to the original design a "decoupling" cap near each power tube. The current feeding the headphones will be provided by the PSU and/or the decoupling cap, depending on the respective impedence at this time.
The impedence will be both resistive (ESR) and inductive (ESL) and will depend on the technology of the cap, its value and the frequency of the signal. And the higher the impedance, the more difficulty the PSU will have to provide the current, resulting in ... well ... less air, less dynamic, less clarity ...
Electrolytics caps can have huge values in a compact package, the average ones have high (bad) ESR and ESL. Their impedance rise significantly from the 10Khz onwards, even for the rather small value ones used in Tube amps.
Of course, there are better Lytics ... FRS type (Philips BC 56-57 series), TFRS type (Philips BC 154-155 series) or low impedance ones (Philips BC 135-136 series) but they are hard to find and mostly offered in packs of 1000, which makes the few we need a bit costly.
Good film caps (MKP or PIO) on the contrary have virtually no ESR ( +/- 1milliOhm) and can have a significantly lower impedance on a large frequency band (few thousands Hz and above). Therefore, they will provide most of the "fast" energy/current in this FB. And this will improve significantly the sound.
Good Decoupling caps :
- (stellar) Foiled/Metalized polypropylene (FKP/MKP) for pulse circuit or with huge current capacity ( +/- 100 Irms). See what Coinmaster has stuffed his extrabox with ...
- All other FKP/MKP caps
- The "Paper In Oil" (PIO) will do well
The value/capacity will be the prime factor here, no need to go "boutique" here, WIMA DC-LINK, Mundorf EVO or SCR/Solen will do fine.
For those "on a budget", the motor-run MKP caps used in DC electrical motor are cheap and can be found in high values. And they are very good (and very large)
Where to put those Decoupling caps ?
Each centimeter/inch of wire will bring an extra impedance that will have an impact on the speed of the current needed.
So those caps need to be put as close to their associated tube as possible.
Ideally, they will be soldered directly on the leads of the Anode resistor (+200V side) and the Cathode resistor (Ground side).
Don't underestimate the difficulties that this constraint will bring, having to stick large caps in a small space is not that easy. The extra-box option will seem more appealing then. If you really don't want the extraBox, talk to Maxx, he's been through this and survived ... mostly.
From the top :
A closer view, look for the red vertical wires soldered to the blue ("original" cathode) cap and to the black ("original" anode) resistors.
The resistors may not be black in yours tho.
It's not worth putting anything lower than 10µF, the impedence at high freq. will be too high.
The higher the value, the better. Between 47µF and 100µF should be fine.
From what I remember from Maxx experience, 47µF is the largest he was able to put in an amp without extra-box.
Voltage (*) :
Basically, we have the same requirements than for the Coupling caps.
Min : 250Vdc
Good : 600Vdc
Stellar : The sky's the limit
Same as above. MKP caps should be given for Max T° : 85°, it may be less for PIOs. So you have to be careful and adapt the choices you make with the Max Temp° in your amp.
(*) we have been said that choosing a cap with a higher "rated voltage" could be beneficial to the "max. pulse rise time" (V/msec), giving faster current to the system. WIMA is documenting this in its datasheets, but only for pulses equal to the rated voltage.
As there is no negative impact in increasing this "Rated Voltage", we tend to always choose the highest ones, if size, price, avaibility ... are similar.
Before (none, this is an addition to the design) :
A quad of "Clarity Caps SA 33µF/630V" ... and a secret ingredient
Or the compact version (white Mundorf Mcap 47µF) :
Or some industrial "over the top / because I'm worth it" AVX FFVI 100µF/1100V monsters ! The 4 in a row.
More to come in the "how to connect those beauties" chapter...
Or the very inspiring Decoupling caps of a Belgium MK2 ... a pair of white Mundorf Evo 82µF
About soldering points for the Driver Stage decoupling caps :
You can take the board out : the easiest is to remove the small round cap and the red rectangular WIMA near it (both are connected together in parallel) and put/solder the decoupling one there
The 'round cap', one at each side of the amp.
Same location, from the other side of the board :
You can't take the board out : it's more tricky, you'll have to solder at the Driver's Anode/Cathode ends (you leave the round caps alone). This is what I did, I'll put some pics in the 1st page post.
Anode : From Maxx's pic, it's the (+) at the top, from the other side of the board, it's the leads of the 215K/220K 2W resistors (the side the 2 leads have "0" Ohm resistance). Easy to find, easy to solder. The "decoupling" wire is the yellow twisted one, obviously !
Cathode : the tricky part ! From the group of 3 resistors you have both ways near the pot, the CathoDe are the caps nearer the pot (the 2 other ones are Grid resistors for each triode of one tube).
In the pic below, they are the soldering points (on the left) just above the "Dot" writing and (on the right) just above the " sword_yang"
Edited by Redge78 - 10/18/15 at 4:54am