PC900 tube as a preamp...pictures

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preludefan

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Joined 2011
#1
hi everyone,

once upon a time I was given a box of old TV tubes and tonight I decided to have a dig through them,as I was in search of a unique tube to use as the back end of a jfet phono preamp im building.

I am a very novice amp builder so forgive me...

I thought id do a quick post on this, as there does not appear to be too much info on the net of people using these for audio...

http://www.r-type.org/pdfs/pc900.pdf

I like the look of this tube and especially how it had a shield inside it, so I set it up on my bench to see, roughly how it sounds, and again, without trying it out on a good system, for far it sounds good, if you look at the pic of the scope, the output is roughly 10x that of the input (both channels set to 2v)

also posted is my schematic of the actual test rig.

if anyone has any more knowledge or experience of using this valve, please let me know, as its small size, shield and good gain are quite appealing to me, also, if anyone has any for sale, please let me know...I only have one
🙁

thanks for reading 🙂



 
#2
With the values and voltages you have there, the current through the tube is trickling through weaker than the urine stream of an 80 year old man with a prostate the size of a football. If you want that rascal to amplify, get the B+ rail higher and use a somewhat smaller plate resistor (or a CCS). Start with some of the recommendations in the Operating Characteristics section of the datasheet.

Note the high specified grid current- that means the impedance at the grid has to be low at DC or the bias is going to wander. That means a choke, transformer, or a small grid leak resistor (which will load down the previous stage).
 
#3
These VHF valves might not be too linear, except with quite small signals. Their big plus is low anode-grid capacitance, which might be an advantage at a phono input but perhaps not elsewhere. The downside is that this might require bigger grid stoppers to maintain stability, and this then raises the issue of distortion due to grid current.
 
#4
Thanks to both of you for the reply, very informative.

I have changed the plate resistor to a 4.7k and increased the voltage to 300VDC.

The result is good, even better sound, due to better current flow, if I increased the volume of the source by more than 50%, it would distort, npw it no longer does and I have to switch the scope to the 10v range to see the output and therecis no distortion that is noticable there...

Time to source a pair of. These tubes and build a nice little preamp...:wave2s:
 
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