I did run through my box of preamp tubes to find what works and what does not in the TC-100. When I did this with the TC-50 I did not notice any notable change to the amp. Perhaps that is good as I love the way it sounds. Sure I thought it was bright when I first got it but it has mellowed out since and sounds ideal. However the TC-100 brings more power to the speakers, change in iron on the OT (nearly looks identical to the one on the RA100 but I did not pull the chassis to check numbers on the RA, power transformer is definitely different than the RA so no point in comparing that to the TC-100). After reviewing the manual, did not realize the presence is not pulled from the speaker like it is with most amps. It is basically another tone control probably situated on the return triode section just before the phase inverter or it could be part of the phase inverter circuit (similar to the negative feedback circuit of what a presence control is but it is not a feedback circuit). In essence it is a low pass filter designed to pass the entire guitar frequency and to roll off higher frequencies created by the non linear distortion effects of the triode tube circuits. As expected, the TC-50 is the same.
Tube roll, one the clean channel V1 and V2 did not seem to change much with different tubes. the stock tubes kept the clean channel on the warm side in normal mode with enough chime to be satisfying. Drive it becomes aggressive with a higher gain setting. I actually love it but wanted to experiment. I did that part last as I was more interested in curing the ping and low frequency mud I was getting on CH3. So what I did use were in the Chinese variety. The latest gen of Chinese tubes (those that only have two mica spacers, upper and lower, and do not use the metal clips on the plates) that are generally labeled as 7025 (preferred series) same as the Gold Lion ECC83 or the Ruby 12AX7AC7HG+ or the TAD 7025-s preamp tube. Tried one in V1 and then in both V1 and V2 and the clean channel remained about the same but not quite as warm as the Stock tubes. Much more phonic chime and crisp bold bottom end reminded me of my old Fender Deluxe reverb. That actually sounded good. I also compared that to the old Chinese Mesa tube with the square foil getter (same tube you can get from Doug's tubes indicated as the Beijing 12ax7, actually Doug has it listed as NOS-Chinese 12AX7. I bought some a while back and found they perform the same as the old Mesa tubes I have in my inventory from the Mark III days. If you want more of a Fender chime on the clean channel the Chinese tubes will give you that. Look for the 7025 tube which may be easier to find than the other as those are out of production.
Curing the ping and drying up the mud on CH3: Since both V3 and V4 are related to CH2 and CH3 it was critical to find something that will provide good performance, eliminate the ping and fix the mud. Actually this seemed to be a sudden thing but I did change V2 early on with another Mesa tube as I thought the original was going microphonic. It was not. the ping noise will happen with almost every 12AX7 tube I used in V3 except for one type. I thought the SPAX7 was the fix but actually made the mud on CH3 more of an issue. With some tubes, after the amp has warmed up the ping noise with no signal may eventually go away. After first warm up it is there. The trick to cure the mud on CH3 was to install a brighter tube in V3. Think about it. The gain chain for CH3 is as follows: V3A -> V4A -> V3B -> tone stack -> V4B. that is three gain stages before the tone stack. Mesa branded JJ tubes are quite warm and may over burden the bottom end. Placing a brighter tube or one with more or less balance in V3 will enhance the grind on CH3 and not really effect CH2 too much. V4 sounded best with the stock Mesa tube. Tung Sol may work well in V3 but I found the NOS Chinese 12AX7 or the old Mesa 12AX7A tube to both cure the ping and dampen the bottom end enough that it still remains aggressive and more defined on CH3. Bonus was a bit dryer tone on the Lo gain CH2 so palm muting sounded very much like the TC-50. Open string or unmuted strings it sounds very much the same so it did not thin out CH2 by any means. All I had to change was one tube, V3 and done. Sure I wanted to expore and see what change in PI and a different 12AT7 would sound like.
PI tube swap. I did have one tube to try which was the Sovtech LPS. One would think there is not much to gain from chaning the PI tube. The signal has to pass though it and it does have its associated impedances so yes it will effect tone to some degree but may not be noticable. What will change is volume. the balance Sovteck worked ok. Not really much of a difference. I actually prefered the Mesa tube in the PI position so I kept what was original.
Last tube: FX loop V5. Yes this tube position has an impact on tone more than you would think. Sure the send is derived from a cathode follower circuit and the return needs to be boosted. Either way you look at it, both are gain stages, one is less than unity (hence it reduces signal levels) and the other is like typical common cathode circuit used to boost signal levels. The Mesa (Chinese) 12AT7 tubes are generally very bright. They do the job in V5 quite well and may not be something to consider. I found out with the RA100 a different tube in the FX loop will change the tone enough to be noted. That amp uses the same topography as the TC-50 and TC-100. Change in 12AT7 tube from one brand to another will have different results as each tube has their own set of tone response curves. Chines are considered very bright as well as a few others.
https://www.tubedepot.com/tube-comparison-tool (there was another chart comparing the NOS tubes but I cannot find it) First up was the JAN/Phillips 12AT7 whcih offers a rich warm sound with good clarity. That did make the amp sound a bit warmer in overall tone. I did like it but also tried something else sort of middle of the road. NOS RFT ECC81-12AT7, similar warmth but with a bit more edge like the Mesa tube. Now that was an improvment. For the time being, I just pulled antoher new Mesa 12AT7 from its carton and stuffed it in and called it a day. Definately considered the RFT though. Perhaps later on in life I will change.
Power tubes? not yet. I have not had a need or desire to change from the stock tubes. Sure the 6V6 is lurking in the background poking at me but I only have a pair of them. I will order more tubes soon enough. At the moment I am digging the stock power tubes.