Sunn Musical Equipment > Q & A

6AN8A Substitute/Replacement

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D.M.N.:
While 6AN8/6AN8A valves can still be found pretty cheap at the moment, there are no current tube companies manufacturing new ones. Though they can be pretty long lived tubes, I have been thinking about what will happen once the supply starts to dry up, as they are used in a few other guitar amps as well Hi-Fi stereos as well. Has anyone gone about rewiring or converting their early Sunns to use a different phase inverter with any success?

Isaac:
The original Model T uses a 12AX7 as a phase inverter. I have toyed with the idea of converting one of my older amps to that configuration, but I haven't done it.

GrannyGremlin:
It can be done, but personally I love 6AN8s.  It is half pentode half triode vs dual triode like a 12A_7 so it would be a significant mod.

There are no drop-in substitutes, but there are workalikes: 7258 (same pinout but diff heater current 13.5V vs 6.3 so nonstarter I'd think) and the 6AV8 (diff pinout - not a hard change).

I coulda swore the 6JW8/ECF802 was a drop in compatible but looks like it isn't.  The pinout is very different at least.

Anyway, as the OP said, they are not too expensive and there is still supply.  Not hugely in demand.  I just buy them up whenever I come across them (have a few things besides my Sunn amps that use them) - last year I came across a NOS Rogers 5 pack (rebranded RCAs) in a local radio repair shop and snapped it up.

Soundmasterg:
The 7199 is similar enough that with some socket rewiring it would work in the circuit without major changes. However they are very expensive these days and the new manufacture ones aren't very good.

Any change to a twin triode like a 12AX7 would change the gain structure of the amp considerably, especially if you stayed with the gain stage + cathodyne inverter setup like the Sunn has. If you went with a long tail pair arrangement or a floating paraphase inverter arrangement then some of the gain could be brought back but I still think there would be a net loss as the pentode in the 6AN8 supplies a lot of gain. That would also be a major change to the amp and would require some troubleshooting and voicing time to dial it in sufficiently.

There are probably some other triode pentodes that could be used, but I don't know of any offhand. The 6AN8 supply is still good for NOS stuff and will likely be for quite awhile. I wouldn't worry about it that much myself. Its not a tube that is used in a ton of stuff so the supply won't dry up super fast.

Greg

Ryan Phelps:
Personally, I would not go to the work of modifying the amp for a substitute tube. As the OP pointed out, NOS 6AN8's are readily available and inexpensive. I've squirreled away about 15 of them in my tube caddy, so my Sunn amps are assured of a ready supply! Pick up a sleeve of 5 of them on eBay and you are good-to-go!

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