I feel the Attenuators may stop many of us attempting to realise an historically authentic build, I'm sure alternative options will be discussed. Perhaps this project may spawn a sister thread on the creation of DIY 600 stepped Attenuators ?
Can someone shed more light on the Verniers used in the originals. Are these indicated as R1 and R30 300 ohm on the 176 schematic? and, are they a Vernier reduction dial attached to a standard pot or something else entirely.
Thank you Rainton for another brilliant project.
Great idea - maybe we could check the forum - someone else might have done DIY 600/600 attenuators before.
Alternatively - as mentioned before one could use something like Mallory 600 Ohm T-pads which are not stepped and actually pots, but would work in this project for sure.
R1 & R30 are just plain 300 Ohm linear pots place between the input attenuator and input transformer and between output transformer & output attenuator respectively to fine adjust the levels, since the stepped attenuators were only adjustable in 2db steps. I'm not sure wether they used 300 Ohm pots indeed or just any other linear pot with resistor in parallel. (e.g. a 500Ohm pot with a 750Ohm resistor in parallel - or a 10K pot with a 310 Ohm resistor in parallel)
That pretty much answers the following question:
Also, does the taper need to be "audio" or can it be linear? I'm not sure what was used on the original.
The attenuators can control the signal from 0db down to -40db in 2db steps -> linear.
Actually all pots found across the 175b & 176 circuit are linear pots.
Hi Martin, I wonder will a "ladder" attenuator work instead of a "T" - does it matter? I'm not clear if this will compromise the operation of the unit.
Yes a ladder attenuator does hold the impedance.
As far as I found out - by default the 175 & 176 were equipped with 600/600 Daven LA-353-G attenuators:
According to the spec sheet these were unbalanced LADDER attenuators with 600/600 in/out impedance:
As you can see there were other attenuator models that went from 0db to -infinite, which means - as stated before, with the knob full CCW the one model would attenuate the signal by 40db while the other one would turn off the signal completely.
I tried to explain that in more detail in an earlier post.
Anyway - the default config for these units was the LADDER attenuator. BUT as a "built-to-order" configuration both the 176 as well as the 175 were available with T-pad attenuators on either input, output or both - which would add either one "T" at the end of the model number ( for a T-pad on either input or output) or "TT" (for a T-pad on both).
So it would have been a "UA 176-TT" for example.
Each T-pad would increase the level by 6db compared to using the regular LADDER attenuator.
I've seen a 176 that had a regular Daven LADDER attenuator on input and a Langevin T-PAD attenuator on output.
Since I now have a working unit at hand I can say that this configuration can make sense indeed:
I have Daven on input & output at the moment and I while on input I feel it gives me a perfect range to control the gain reduction (which as you most likely all know is the same principle as in an 1176, driving the input signal against a predefined threshold) where I think the 6db increase of a T-pad in this position would make the unit compress much earlier on the scale and thus make it a little more difficult to control. (Also I really love how the "VERNIER" pot let's me fine tune the compression here)
On the output on the other hand I mostly have to turn up the attenuator to somewhere around the 3 o' clock position to make up for the gain reduction. Which is not a huge problem, since astonishingly the 176 I've built here has an extremely low noise floor - I haven't exactly measured it yet, but even with the attenuator fully CW I can't hear any noise in a real world situation.
But still - I think here the 6db increase of a T-PAD would come in handy. So I might just try to use a Mallory T-PAD in this spot...
...anyway this should give all of you many more options
June 17, 2015, 04:42:16 AM
