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Hi Guys,
I recently purchased a 500C as a second amp to my Jolida 502, the Fisher has quickly become my primary unit and the Jolida now sits idle. With the Jolida I had experimented with quite a few 12ax7, 5751,6550, and also 12at7 tubes to find a few combos that worked well. Changing the preamp tube pairs of course gave the most flavor difference, while 12at7 and 6550 tubes also change different and more subtle things about the sound.
In the 500C the preamp tubes function in series with each tube carrying both channels. Until last night I had been rolling pairs of the same tube into these positions, like I had with the Jolida, RCA's, Telefunken, Mullard, and some new production pairs all rolled in at some point. The Tele sounds right in this amp and is the original, I find myself coming back to it for reference after listening to others. The RCA BP longplate has less energy in the HF but this tube has flesh....meat...the mids and lower mids are exceptional, bass is deep and prominent, but a bit flabby though. If only I could combine the Tele and the RCA....wait a minute, they are in series, I can! Last night I rolled in a Tele Smooth plate into V14 and the RCA into V15. The combo is just what you'd expect, the strength of the Tele for imaging, HF, and punch are there while the Rca's smoothness and flesh help with the sometimes thin Tele sound, the Tele helps tighten and define the bass while lending clarity to the top end. It's close to perfect.
Does anyone else mix tubes in this way with their 500C? It seems to be a nice way to narrow down and customize the sound of the 500C, makes me feel like a chef trying different ingredients that will compliment each other.....bon appetit!
Follow Ups:
Yes, anyone who is seriously into Fisher receivers rolls the small signal tubes. Many also experiment with coupling cap combinations.FWIW, if you're not using the phono stage, leave those sockets empty. That will significantly lighten the load on the bias supply circuit that feeds their heaters.
The audio output on the MPX-65 and the 6HA5 in the FM front end are other good places to roll cheaply on that particular model. But don't play with the IFs unless you're realigning it.
Do you use the phono section? If not, put crappy Sovtek 12AX7WXAs in those slots as place holders. If the phono section is in use, put a pair of Sovtek 12AX7LPS tubes in that have been carefully selected for LOW noise. AA sponsor Jim McShane is a good source for the 'LPS.
Eli D.
I tried the Phono stage with a few different tubes including the LPS, I had high hopes for it but find my little Bellari unit sounds better than the Fisher stage. I'm rarely using my TT now as it needs some work so for now the phono stage gets to burn up some cheap Shuguangs.
What kind of combo 12AX7's are any of you guys using in preamp V14-V15 positions? I have quite a few 5751's lying around....SylevaniaGB, RCA's, GE...etc. I wonder if 12ax7 and 5751 combo might be interesting. I've tried pairs of 5751's in those positions but it sounded strange....just not right, others would agree in general that 5751 are no so good in 500C, but maybe the right 12ax7 flavor might be interesting in series with a 5751. Hmmm...GB and a Mullard perhaps? Recipes anyone?
...V12,13 (phase inverter) using JAN 5751's (tighter),
V14,15 using 5814a's,
V17 retained telefunken 12ax7.
"Upgraded" coupling caps with metalized box type (edgy speed, not for everybody).
Used what I had on hand; haven't tried mullards, etc.
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