terminating unused wiring bundle question

[Helmholtz]

I read and re-read all the posts that talk about reflected impedance and all that, and got lost. Some gear page article said they changed from 6.6k then "upgraded to 8k". No idea what that means. More k's better?

There was a lenghty forum discussion some months ago on this.
Early JTM 45s with RS OTs mainly used 6.6k. Later ones with Drake OTs only used 8k. 6.6k is closer to the KT66 manufacturer's recommendation of 5.0k. 6.6k gives more output power than 8k.
8k may clip smoother, sound more compresssed. Try both options and tell us.
As the KT66 and 6L6GC charts are very close, impedances down to 4k can be used, increasing output to around 45W with a plate supply voltage of 400V at full power.

I have seen reports claiming that some early Bluesbreaker combos with RS OT used the 8k primary and hardwired 16 Ohm secondary with the 8 Ohm speaker load (two early 16 Ohm/20W Celestions in parallel). This wiring results in a primary load of 4k, increasing output by around 15W. I wonder how long the weak speakers survived!

[mikepukmel]

Thanks Helmholtz! Will re-read. And, I'll wire up the 6.6k, but keep the 8k handy and try that as well. Ohhh, can't wait to get sound out of this thing!

[mikepukmel]

Ran out to MicroCenter in Cambridge yesterday and got a bunch more heat shrink tubing. For the unused wiring, I put a piece of shield I salvaged from old serial cable, (the wiring has neat trimmed ends and heat shrink over that), then ran a long piece of heat shrink over that. Decided not to heat the heat shrink, so it can be removed if necessary.

The green wires are 6.6k, will wire up the amp first with 6.6k. Yellow are 8k. I left the 8k wiring out of the bundle since Id like to try Helmholtz' suggestion and see which sounds better, at some point.

hard to see in the photo, but the shielding is sticking out 1/2" or so from the end of the heat shrink, so I can attach it to something.



Nothing routed under the preamp board. I can do a neater job and route the yellow (8k tap) wires close into the corner of the chassis and put a cable clamp on the whole thing.

This way, when my great grandchildren get this amp and decide to go ultra-linear, all they need to do is slide the heat shrink and shielding off, switch the wires, and they're good to go.

[Chuck H]

"I" think that'll work.

[mikepukmel]

Awesome, thanks Chuck.

[Helmholtz]

hard to see in the photo, but the shielding is sticking out 1/2" or so from the end of the heat shrink, so I can attach it to something.

Please do ground the shielding. An non-grounded electrical shield is no shield. It just acts a floating capacitor with a large radiating surface.

[mikepukmel]

Please do ground the shielding. An non-grounded electrical shield is no shield. It just acts a floating capacitor with a large radiating surface.

Thanks for the reminder Helmholtz, will do.