by EWBrown » Thu Oct 25, 2007 8:30 am
The 470K ohms input is not all that critical, it's basic function in life is to be the grid resistor for G1 of the input tube, be it 7199, EF86, 12AX7, 12AT7, etc. The input resistor could be anything from 100K to 470K,
typically use 100K and 270K. In days past, it was the common practice to use higher input resistance, 470K and 1 megohm were pretty much "standard".
The VTA driver boards are modelled after VTL's (Vacuum Tube Logic) driver circuitry, and they use either 270K or 135K (2 x 270K in parallel) for the input / grid resistor. For home use, 270K is the better choice, 135K is intended for studio / professional usage, as it matches the rest of the studio equipment.
The output of the Dynaco PAS series preamps is what is considered as being "high impedance" ,and should work with just about any tube amp.
Most SS amps, however, tend to have much lower (10K to 50K) input impedances, so the mis-match could cause problems.
For a volume control / attenuator, I normally use 100K, anything from the Radio Shack stereo pot to the "Hong Kong" 100K stereo stepped (ladder) attenuators found on E-bay..
These attenuators are built in two form factors, I call them "long" and "radial".
The "long" ones are about inches overall length, and the resistors are laid down against the switch body, which keeps the overal diameter to around 1.5 inches.
The "radial" ones use a shorter length switch, and the resistors extend radially outwards, and the "free" ends are all grounded to a ring of heavy buss wire. Their overall diameter is around 3 to 3 1/2 inches.
The actual circuit and function are identical, just the form factors are different.
I've also made a passive pre with a 100K pot in a box, and another using two of the "sweetest whispers" 15K, 11-step attenuators from Doc Bottlehead (these were leftovers from a "foreplay III" linestage, in which I used one of the stepped attenuators, instead). it works well with an SS source (normally a cheap CD player) into the 15K side, and then the step / "wiper" feeds the amp under test. 15K would be too low for most tube sources, unless ther final stage has a cathode follower (as does the "Foreplay" series). Eleven steps may seem to be too "coarse" but it works out very well, and I've never felt the need for any smaller "in-between" steps.
The Hong Kong lader attenuators are vrey well made, using Vishay / Dale 1% resistors, and a decent quality 4 pole, 23 or 24 position rotary switch.
They may not quite be "Goldpoints" but then they cost a fraction of the price, and I've always had good results with them. Good quality part,s and relatively low cost labor from Asia.
/ed B in NH
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