VT 64 preamp

RCA VT-64 or #800 pre amplifier

Introduction

I have been build class A2 amplifier by using RCA VT-64 or #800 tube in the past and with 2-3 design concept that you can see in my previous post. Everytime that I check the output power they always produce only 1-2W before clipping. So I am very confusing with the result. Then oneday I talk to my friend that is very expert in diy tube amp he ask me why don't you using this tube for pre amp may be it will work fine.

I buy his idea , so I plan to using it as pre amp and in the future if I get more tube I will try to make another amp again.

The design

This is very simple pre amp design , I just using LC coupling concept rather than RC because not only to reduce the heating from plate load resister but I will have more gain to this pre amp to drive my amp that using low gain input tube.

Schematic

For power supply I using Ratheon 1641 full wave rectify that are ST shape with 2 cap look like VT-64 , the concept is normal CLC filter and for filarment supply I'm using DC with LM338 to regulate.

Under chassis before I change C coupling and DC filarment to LM338

If you take a look at the volume you will see that it is after the tube to reduce hum that a bit high for DHT transmitting tube like VT-64
Another problem that I found is C coupling , at first I'm using Jupiter 0.47uF 600V that I think that it can use with this design because I'm using plate voltage around 525V , but the real life the Jupiter is failed so I need twice votage support of C coupling , I change to Mundolf that can support to 1200V , I've only 1uF value so I use this one and it's OK now.

The sound

picture of VT-64 and 1641 tube

I'm compare to my other pre amp such as #26 , Ce also my friend 300B preamp , I found that VT-64 pre amp produce more detailed and clear sound compare to #26 and Ce , but when compare with 300B I found that the high and low of this pre amp are better than 300B also detailed is better and sound more clear and natural than 300B but 300B is more open of the sound than this pre amp and on Ce and 300B is more smooth and relax than Vt-64. But all together I think this pre amp is sound a bit better than 300B or #26 and Ce .

But very sad that this tube is very difficult to find and also too costly to use this tube for pre amp application.