Tube rolling with MC could be a lot of fun and could lead to a sound that you will not only like, but love. The flexibility is just incredible. This post will focus only on preamp tubes. Power tubes are out-of-scope 🙂
There are 7 preamp tubes in reissue MC 275, V1 (12ax7) – phase-splitter, V2, V5 (12ax7) – input amplifiers. V3, V6 (12at7) – output tube drivers, V4, V7 (12at7) – Cathode followers.
First of all, if you are using balanced inputs, you can forget about V1. It’s not used by the amp. However, keep it in. It’s in the heater chain with the other tubes (since MC uses 24v for heaters), so removing it will remove a part of the heater circuit. But place there something cheap. It doesn’t even have to be 12ax7 – you can place GE 12au7 there with the same results and invest your money in the other tubes. On the other hand, if you use unbalanced input – V1 is one of the most interesting tubes to play with and produces some very unexpected and in some cases phenomenal results due to the circuit design.
First, let’s get the 12at7 tubes out of the way. Use good tubes there. Grey plates work better than the majority of black plates (I can’t believe I just said that) – because they are more transparent. Try to use old stock – they have sweeter HF. You need a tube with a lot of transparency and great sound-stage. Telefunkens would work, but expensive. Sylvania 12at7 Gold Brand is much cheaper and works as well. (see my previous post on 12at7s) One interesting result I was able to achieve is with 12av7 and 7062 tubes. Sound stage was just out of the is world! Try to stay with the same type in the chain, but no harm in going with different types for Driver & Cathode Follower. I have Sylvania JHS 12at7 as a driver and Amperex 7062 as Cathode Follower and love the results – your results might vary. So again, save your money and focus on 12ax7 tubes.
About V1. The design of the phase-splitter is quite unconventional since the amp is fully balanced from balanced inputs through the power tubes, but uses V1 to invert the signal before passing it on into the balanced chain. However, it uses 1/2 of the dual triode for each channel for the negative phase and feeds positive phase as is into the input stage. This could create time delays – depending on the tube. So rolling different tubes in V1 could result not only in different tone, but also in a time delay – which is quite an experience when part of the signal is unchanged and the other part, negative phase, is delayed – opens up a new dimension into the music!!!
V2 & V5 are a more conventional design of the input stage of voltage amplifier and thus could be very efficiently used to roll tubes with fantastic or horrific results.There are a lot of reviews on this site that talk about results of the rolling – read up! 🙂
About tube matching… There is not much need to match input tubes. V1 is completely independent of the other tubes – and although matching triodes could lead to a potential benefit, it’s very likely to be unnoticeable. V2 & V5 don’t have to be matched either because of the deep negative feedback loop that would eliminate the majority of variances and could leave you with 1-3% of difference that is practically impossible to hear. So don’t waist your money on matching. The independent feedback control inside of each phase pipe would also dramatically reduce the triode differences. The amp is very forgiving.
Roll on!!! Post your experiences on this site.