Big brain question on matching OT Impedance

[midnightlaundry]

I'm doing a Deliverance build but I have Transformers from a JCA 50 that are for 2-6L6's. I want the Output Transformer to have a good match for the Power Tubes..

Since the Deliverance uses KT88's, I think I need a KT type tube to get the intended frequency response without going with a new OT and KT88's..

I hear tell that KT66's are real close to 6L6 spec and might be the ticket. I also have a pair of KT77's that I could use but I'm not sure about the Impedance match to a 6L6 OT..

I know about Heater Current. There's a little Hammond Filament Tranny for 40 bux I wouldn't mind getting to keep the smoke away. But I don't want to spend a ton on this heap..

So what say you? KT66's, KT77's or go all the way and get an OT for KT88's?

 

[glpg80]

Id suggest just running KT66. KT88 tubes and a new OT sounds expensive.

 

[midnightlaundry]

Probably a good choice.
I'll probably get it running with 6L6's and it will sound way to scooped. I'll then put in the 77's and it will sound better, but I'll always wonder what 66's will be like and then I'll finally lay out the cash for a set. LOL

 

[Erock]

I only have recent experience with KT66, but they remind me way more of KT88 than 6L6.

 

[SpiderWars]

I think Jeremy @RedPlated at Ground Zero amps went to KT66 by choice. He did clips when he made the switch so you might be able to find them.

 

[midnightlaundry]

Kinda' relearning this, but part of the response of a Tube is having the right impedance OT. The differences between tubes within the same OT is the effect of impedance mismatch.. I remember SMF talking about this.

So a lot of the real differences between tubes are in the feel is everything else is optimized..

Chime in Big Brains..

 

[glpg80]

Kinda' relearning this, but part of the response of a Tube is having the right impedance OT. The differences between tubes within the same OT is the effect of impedance mismatch.. I remember SMF talking about this.

So a lot of the real differences between tubes are in the feel is everything else is optimized..

Chime in Big Brains..

EL34s and 6550s have similar plate impedances and heater current requirements, 6L6 variants are different enough that they cannot be hot swapped since the heaters draw less current and the bias range is also different.

Impedance matching from tubes in parallel to the primary of the OT is needed for maximum power transfer so that fluctuations in the B field in the OT as you’re playing guitar don’t induce reflections back to the tubes which can lead to shortening of tube life or even cause failure. When power is not transferred effectively it does not dissolve or dissipate into nothing - it is reflected back at the tubes. What’s worse is that when primary impedances are not matched, the power reflected back can be much higher voltage than B+ and therefore exceed the absolute maximum ratings of the power tubes since there’s massive amounts of reactive energy stored in the B field of the OT that acts as a charge pump. The effect is even worse if you start dabbling in mismatched secondary impedances. The B field is what allows the OT to pump massive current into the secondary winding to drive speakers with AC. People forget you’re modulating the B field of the OT as you play guitar.

If you really know what you’re doing you can miss-match primary impedances but you must change secondary impedances to compensate. If you don’t you’re begging for catastrophic tube failure especially with the shit quality of tubes these days. It’s a practice no one should try or dabble in as it’s counterproductive and bad practice in general.