Guidelines for optimizing sound qualities of tube amplifiers

These guidelines are based on my research and experiences with DHT tube amplifiers and may be used prior designing an amplifier or for upgrading purposes. I'm fully aware that some of these recommendations are "food for thought" and will be spoken off, not to say made fun of by lots of "technicians".... Remember: "The proof of the pudding is in the eating of the pudding"! The choice in applying is yours!

Chassis.
Due to unavoidable resonances within metal, best material to be used for the chassis is wood. This is natural wood, no chipwood, MDF, HDF etc. Practical is the use of hard wood (ie. oak or some tropical woods) to avoid possible damage during manufacturing or use. However, softer woods like (Oregon) Pine or Spruce will do best. Metal shielding is necessary for eliminating EMI resulting in hum, so power supply transformers and/or -chokes should be potted when using wood chassis.

Soldering.
In my humble opinion solder having a (mere) percentage silver is not necessarily needed. I see more benefit in good solder joints... So: put effort in each solder joint and be sure to use a clean soldering tool all the time!

Wiring.
Solid core is to be used throughout the whole amplifier. For about 80% soundquality is determined by the optimal diameter of the core and not the choice in material. Some minor profit can be obtained when shifting to "better" material maintaining the same diameter (ie. copper to silver). Insulation material has a major influence on sound quality: cotton or and/or (rice)paper insulated solid core wire is the best choice, when this is not available magnetic wire is second best. The use of synthetic materials for insulation purposes will affect the natural timbre in a negative way.

Recommendations:
PowerSupply	AC-mains up to tube's plate	Magnetic wire 0.8mm diam.
PowerSupply	Ground rail	Copper diam. 1.8 up to 2.5mm diam.
PowerSupply	DHT filaments	Magnetic wire 1.5mm diam.
Signal	Except speakercabling	Paper/cotton ins. 0.4mm diam. (or magn.wire)
Signal	Speakercabling	9 individual paper/cotton ins. 0.4mm wires, giving a total of 18 for + and -.

Grounding
When using a wooden chassis a so called ground-bus/star-grounding can not be used. One ground-rail of 1.8 upto 2.5mm diam. is used instead. The ground-rail will be guided along, where possible, with the powersupply cabling (parallelling) creating a genuine rail. This means: from AC-inlet to powersupply voltage centertap to 1st powersupply capacitor, 2nd capacitor and so on. The housings of (all) potted transformers are connected to this ground-rail thus creating a surrogate-chassis. For each amplifying stage a separate star-ground is created and is to be connected to the negative end of the corresponding power supply capacitor. This way for each amplifying stage a separate "circle" is created and located at the logical place within the ground-rail topology thus eliminating cross-loop currents that eventually will cause hum.

Power supply.
Best results are obtained when using a C-L-C-L-C configuration (where C=capacitor and L=choke). All transformers must have sufficient "headroom" with respect to operating currents. Transformer core material should be 0.05mm laminated or thinner for power supply chokes, and 0.3mm laminated for powertransformers (not necessarily thinner). For each DH filament a separate transformer is to be used, for rectifiers a filament winding on the power supply voltage transformer is sufficient. IdH filaments may share the same (separate) transformer (where DH= Directly Heated and IdH=Indirectly Heated).

Capacitor choice.
The powersupplies' 1st capacitor must be a low value (2uF max.) paper-in-oil type, followed by BlackGate WKZ or Elna Cerafine or, even better, BlackGate NH in super E-cap. Cathode bypass should be BlackGate, Elna or, even better, BlackGate N(x) in Super E-cap. A minimum value in uF is to be used except when using Super E-cap which has no negative effect on the tonal quality up to twice the value of a WKZ/Cerafine. Coupling capacitors should be Jupiter Beeswax, second choice UltraTone/VTV. For low-voltage coupling (up to 50V/DC) BlackGate Nx in super E-cap can be used.
A special note at Super E-cap: best results will be obtained when Super E-cap is used on all places i.s.o. at "one place only"!

Resistor choice.
Power supply (ie. for a necessary voltagedrop for the driver); Allen Bradley, sufficiently rated, will do best especially when combined with Super E-cap. Second best is Mills, wirewound 12Watt.
Cathode circuitry; in preamplifiers Audio Note 2W Tantalum (not 1W!) are best, in power amplifiers at the driver and powertube Mills, wirewound 12Watt are best, may be parallelled.
All grid resistors and stopping-resistors should be 1/2W RikenOhm.

Filament circuitry.
When using DH-tubes: On low signal applications a SLA-battery may be the only choice (SLA=Sealed Lead Acid). Good alternative (more practical) is C-L-C-L-C configuration using Schottky diodes and Super E-cap where the L may not be substituted by a resistor.. Also a double CS (current source) will do fine. Voltage stabilisers (ie. 78S05 etc) are killing for a good timbre! However these will do fine at IdH circuits. When "feeding" a driver, best resuts are obtained using Schottky diodes and a single capacitor (high value). Voltage drop will be realised through a resistor at the AC side of the diodes and residual hum can be adjusted by a hum-pot at the cathode. This also applies for powertubes although, when possible, AC-heating prevails!

Output transformers.
The choice in output transformers, interstages and choke-loads should primarily concern lamination thickness. A minimum of 0.05mm is needed to provide for the neccesary speed and detail in music. Best choice is a dedicated wounded transformer meaning that it has only taps, both primary and secondary, which are actually needed. All others are a compromise in winding technique and can act as an aerial!

Connectors.
When connectors and binding posts are used a choice for "less material" should be made. Most RCA connectors/receptables have a relative big "minus" (metal jacket) which will affect grounding in a negative way. Best choice are Lemo connectors, XLR-connectors or Eichmann Bullet plugs (or equivalents). It is better to use plain/bare copper or silver connectors than (badly) gold-plated connectors. Badly gold-plated connectors can act as a "capacitor": between original metal and "gold" is still some residue material that can act as an insulator.

Volume control.
Best volume control is no volume control! Good alternative is a (well designed!) inductive volume control, second alternative is a resistor based series- or ladder attenuator. When using an attenuator best place is between two transformers, this way the attenuation will not have any negative affect on the preceding or following eletrical circuit. If this is not feasible place the attenuator directly following the "source", this way only affecting the following circuit.

 

!!This guideline is primarily meant for upgrading tonal qualities!!

 

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