ATTENTION: The more expensive Electro Harmonix "Gold" Version is sold out. The SOVTEK is an ideal replacement. Personally I think the Sovtek is even a MUCH better tube as the Electro Harmonix was suffering from vacuum often. My expectation is, the SOVTEK are probably NOS anyway, since I had lots with 1972 datacodes before, what else shall I say.
The 6C45 tube
A most amazing tube from the days of the cold war. This is a frame grid tube, meaning the grid wires are so extreme thin, you cannot see them with the eye. If you remove the grid from a broken tube, it looks like a light absorbing, transparent foil. (same effect as sunglasses). This is how thin the grid wires are. You can't see them, but still they absorb some light of course. For making a grid like this, first a frame is made of hard metal. Around this frame, the grid is wound, and then frame and grid together go inside the tube. Like this, very precise tubes of high gain can be constructed. I really want to put upfront this most amazing technology.
Characteristic for frame grid tubes is the nice uniformity of transconductance. The 6C45 has a gain of 50x and it has enough plate dissipation to build a small SE output stage with it. Originally this tube was not made for SE stages, but it's a secret tip and it works very nice.
The problem with this tube:
This is a difficult to make tube, since parameters are combined in this tube that bite each other. Meaning, when you still do so, uniformity of production data will not be achieved. Yet it is an amazing tube, but you have to accept the large spread of bias data, and transconductance, which results in different gain of the amplifier stage. When selecting, we see basically high Gm tubes have a lower bias current and vice versa. This is nothing bad, but it simply the anode distance is variable. Then, by looking at the tube construction, we already see what causes this, the anode fine adjustment as with ECC88 is not possible here. So this is something NORMAL we have here, and no quality issue. It is just, the only way to deal with this, is parametric selection both for Gm and Bias DC current at the same time.
It can make you really unhappy, when most sellers ship any tube whatsoever. Also the ones that I think are questionable. For this reason, we recommend to buy only the selected versions that we sell, tested with the AMPLITREX computer tester. Another problem these tubes can have, is vacuum defects, but this is a general problem with all Electro Harmonix 9 pin tubes. Specially for 6C45 which gets extremely hot, the smallest sign of something not 100% with the vacuum, is a reason to reject the tube. The practical situation proves we are right by this. If you store such tubes which are initially "ok" but perhaps the vacuum not exactly perfect, then after some years you find them on the shelf with a white getter, and all vacuum lost. So the ones we sell, are selected for perfect vacuum, and high transconductance. Actually this process we have, seems to reject the weak ones very good.
So we do not want you to be afraid of this tube, but you should be afraid of 6C45 that you buy from any random sellers in the internet, which only want to sell by price, and not by quality. That doesn't pay off with 6C45.
Select the right transformer:
This tube is ideal for use with Lundahl LL1660 driver transformers. You can wire the transformer for instance in a 4.5 : 1 step down scheme. This will still leave you with a total gain of 50/4.5= 11x which is a good number. The good part is now, the output impedance of this tube is reduced with a factor (4.5)² = 20. So the original Ri of the tube, of apr. 1100 Ohms will appear as 55 Ohms (!) at the output of the LL1660. This 55 Ohms is very nice for head phone amplifiers, or pre-amp outputs. So a line output stage with this tube, using the LL1660/25 would have these specifications:
Design example for line output stage:
This tube is ideal for use with Lundahl LL1660 driver transformers. You can wire the transformer for instance in a 4.5 : 1 step down scheme. This will still leave you with a total gain of 50/4.5= 11x which is a good number. The good part is now, the output impedance of this tube is reduced with a factor (4.5)² = 20. So the original Ri of the tube, of apr. 1100 Ohms will appear as 55 Ohms (!) at the output of the LL1660. This 55 Ohms is very nice for head phone amplifiers, or pre-amp outputs. So a line output stage with this tube, using the LL1660/25 would have these specifications.
Transformer LL1660/25 with connection scheme: Alt Q (see this website for Lundahl transformers)
Note, when designing with LL1660: First let's say here the normal LL1660/25 works very nice. For superior noise isolation with line driver amps, or for user increased safety with head-phone applications, the "S" version (shielded) can be used. The shield is in between primary and secondary and must be grounded. Both the standard or the "S" version are available with amorphous core.
datasheet of LL1660