I never looked for replacements, are there alternatives available? I made some experiments with 12AX7 but honestly, the Lamm Design is very stable in sonics.
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The 5651s are not expensive. About $8 each. I bought some because I had a problem with one amp where the 5651 went dark. When I looked inside, the internal fuse had blown.
Replaced the 5651 and the internal fuse and it worked for 20mins and the fuse blew again. I ran in mono for a while, thinking I had to send the amps to Lamm - which was a bit of a logistics issue. But finally I contacted Erina and she told me what to do.
The problem was that the 6C33C voltage regulator was old and not doing its job. I had a spare set of 4 x 6C33Cs from Lamm (all correct numbered parts for the amps) sitting in the cupboard. I replaced the whole set - they were all quite used up, one significantly so. These were the original valves, except for 1 - so they had done some miles (I bought the amps second hand).
There was actually nothing wrong with the 5651 voltage reference tube so I put it back in.
I've replaced the 6N6Ps once and will do so again in January. I use Pvane 12AX7s.
In terms of those 6c33c. I have brought a set from Thetubestore and worked fine. I read that they need to be carefully matched but lucky not so in my case. It seems as long as it is within range for correct bias to be set, it is fine. I know the regulation tube actually goes sooner than the power tube. Cheers.
I replaced my 6AK5's last Saturday. Although I checked them on a Hickok tV7 last December and they tested fine, I recently aqured a Mighty Mite IV tester and the life test/emssions showed they were poor. They had been installed for several years without issue, but there was a substantial improvement after the replacement (i.e. after 45 hours burn in). I have found the V2 6C33C to fail most frequently, followed by the 5651, V1 6C33C and finally 6N6p. 6AK5 is at the end of list as I have rolled the 12AX7 position several times to have any meaningful life expectancy judgement. The good thing about a failed V2 6C33C is that it can be used in the V1 position and vice versa, as the amps only use half the tube. So when I lose a V2 6C33C I just put it in a box for when a V1 goes sometime down the road.