6AN8A Substitute/Replacement

[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!

[loudthud]

AES ( www.tubesandmore.com ) sells an adapter that lets you use a 6GH8A in place of a 7199. I thought there was an adapter that lets you use a 6U8 in place of a 6AN8, but I can't find it now.

[EdBass]

I agree with Ryan, 6AN8's are easy to get at least for now, and squirreling away a little stash should hold pretty much anyone over for quite awhile. I think an adapter to use 7199's would be pretty easy to make (or maybe find one already being made as 'thud mentioned) should 6AN8's ever become unobtanium.

[GrannyGremlin]

Aren't 7199s even harder to find that 6AN8s?  Why bother modding if just to switch to another out-of-prod tube.  They are certainly more expensive (at least in the usual places I shop - like $50 vs 15).

The 6EA8/6U8 are much more readily available and have higher max voltages (more robust).  Cheaper too (like $10).

Since there is no new prod tubes that are sub-able here, I repeat that modding the actual amp is a losing proposition as the stock /availability of various tubes changes, waxes and wanes, over time (demand being a big factor) - just by one of them tube bases (like you'd use for a DIY bias probe) and a socket to cap it off and make an adapter to switch the pinouts.

[Soundmasterg]

Aren't 7199s even harder to find that 6AN8s?  Why bother modding if just to switch to another out-of-prod tube.  They are certainly more expensive (at least in the usual places I shop - like $50 vs 15).

The 6EA8/6U8 are much more readily available and have higher max voltages (more robust).  Cheaper too (like $10).

Since there is no new prod tubes that are sub-able here, I repeat that modding the actual amp is a losing proposition as the stock /availability of various tubes changes, waxes and wanes, over time (demand being a big factor) - just by one of them tube bases (like you'd use for a DIY bias probe) and a socket to cap it off and make an adapter to switch the pinouts.

I mentioned the 7199 because the lower wattage versions of Sunn amps used it and some people might think to switch to it. But I also mentioned that it would be a bad idea because of the cost and quality - NOS 7199's are super expensive due to their use in some hi-fi stuff and their rarity as a result today, and modern 7199 manufacture is lousy quality.

I think the general consensus is that the 6AN8 is cheap for NOS stuff and has a ready supply today and there isn't much of a need to worry right now about them getting scarce or hugely expensive. If someone was concerned they could stock up and get 5 or something to stash away and that would supply their Sunn amp for decades.

Greg

[Ryan Phelps]

Just an FYI......Antique Electronic Supply has NOS 6AN8A tubes on sale right now for $12! Much better price than many on eBay. Stock up!
No affiliation, just a word to my Sunn brethren.

[EdBass]

Great info; I imagine a dozen of those should take care of the lifetime needs of pretty much ANY Sunn-o-phile.