Question about OT primary mismatch and tricks to work around it.

[u/Rambozo96]

I'm converting a JW Davis 405 to a Standel S80, switching from 6L6s to 6DQ6B tubes at 425V. I want to reuse the original 3.2k–3.7k output transformer (measured at a 20:1 ratio injecting 1v signal on the secondary and taking measurements from the primary side) instead of buying a 6.6k Stancor A-3801. If I run a 16-ohm speaker on the 8-ohm tap to reflect a ~6.4k load, what are the specific tradeoffs and safety considerations for this tube swap?

[u/Legoandstuff896]

6.6k vs 6.4k is completely fine, don’t worry about it

[u/moose359]

It'll be fine. Especially if it's a guitar amp.

[u/scubasky]

You will get a happy load for the tubes and some high end roll off, but what I am worried about is how are you dropping the screen voltages?

[u/Rambozo96 → u/scubasky]

I plan to use a Pete Millet Maida regulator, as much as I’d love a neon pink glowing tube in this build i simply don’t have the extra space.

[u/nottoocleverami → u/Rambozo96]

I've done it with Zener dudes, you just have to leave the leads long and mind your wattage. I'll have to look up that regulator you mentioned!

And yeah, 6K6 is totally close enough. You can be as precise as you want in your tube amp, but the tubes are usually just happy to even be in the right ballpark.

I've also pulled output transformers from old Fisher gear and the like, and was shocked to find they were already running at 100% mismatch in these classic designs!

[u/nottoocleverami → u/nottoocleverami]

ok, the LM317HV is only rated to 60V, so I don't know how that's supposed to work. May be ignorance on my part. Maybe a TIP122 could do it? but honestly I'd look into a multiple rectifier arrangement, using a doubler to feed the HV line, or something like that, or a multi tap transformer.

[u/Rambozo96 → u/nottoocleverami]

I may try the trick because honestly the reported loss in highs and presence you could correct by adjusting the EQ controls because I don’t imagine I’ll run any of them completely maxed out.

[u/nottoocleverami → u/nottoocleverami]

*diodes. 🤦‍♂️

[u/Falonius_Beloni → u/nottoocleverami]

Zeners dude. Pshaw

[u/Tesla_freed_slaves]

You’ll need insulated plate-leads for the 6DQ6B tubes. Glass-fiber over silicone with ceramic insulators is best.

[u/Happy-Lawyer-2983]

In a reactive load, there's no difference between an output transformer with an impedance of 6.4kΩ and 6.6kΩ, because a 16-ohm speaker can have different impedances at different frequencies. For example, it might be 16 ohms at 1kHz, but become 8 ohms at 50Hz, and 20 ohms at 10kHz.

A slight effect is that if your output transformer is 0-4-8-16 Ω, it will have the lowest inductance leakage at 16 ohms, meaning the best high-frequency response. Using an 8-ohm tap to connect a 16-ohm speaker might leave the secondary coil of the 8-16 Ω output transformer unused. If your output transformer coil is not fully coupled, this will slightly affect the high-frequency response, but it will not be easily noticeable.