Build Ideas for OT with 4.1K Primaries - Cathode biased 6L6s?

[bcmatt]

EDIT: I tested the OT and it's has a primary impedance for 4, 8, and 16 ohm speakers of 4.1K, 4.2K and 4.0K respectively.

I'm going to test it again but I just freed up this OT, that I would guess is appropriate for maybe a 30-45 Watt amp and I think has 4.1K primaries. I'm not sure how I would test wattage, but I'm going by size comparisons and it is a Ceriatone OT for something of theirs, but it was wrong for the kit that I bought at the time (15 years ago). I remember thinking it looked pretty identical in shape to my Ceriatone DC30 OT so it might be for one of those and it has 4, 8, and 16 ohm secondaries. It spent the last decade in a 5e3 therefore giving it only 2, 4 and 8 ohm secondaries in order to be sort of appropriate.

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I'm thinking of making something in the mid-gain territory with round and sparkly cleans... a little bit spongey and most importantly VVR-capable... most ideally cathode biased and non-Master-Volumed. (probably no NFB) I have an Express and Liverpool and always feel like I'm missing out on something more like the Rocket. I also feel like the original JTM45 style amp sounds like a nice-feeling amp... but also needing to be VVR capable.

Part of me is curious about something like the power section of the original 5E5 Pro amp that is cathode biased and uses a 5U4 rectifier. I feel like an amp design that is 20-35 Watts at full stock plate voltage would be great, but not likely to melt down the Mosfet on a heat-sinked simple VVR for cathode biased amps. But I would be interested in perhaps marrying that power section with the preamp of the 5F6 to get a little more JTM45 feel. I do hear that the early JTM45s actually had something more like 6K primaries though (as well as being fixed biased). I don't want something super efficient power-wise.

I have a spare pair of 6L6s from my SSS that I yanked two from. I am quite curious about how cathode biased 6L6s would perform with VVR induced lower plate voltages.

Does anyone have any thoughts or ideas?

[wpaulvogel]

I’d test it for impedance ratio. Apply 100 volts AC to the full primary. It should be the blue and brown wires. A simple DC resistance check of the primary should make it obvious which of the three primary wires are the two plates and the center tap. Apply the 100 volts and write down the voltages from the secondary.

[Stevem]

Sounds like a Wards Airline 8517 cut back to only 2 outputs might be a interesting way to go.

Dare to be different!

Someone once said that life begins at the end of your comfort zone

[bcmatt]

I’d test it for impedance ratio. Apply 100 volts AC to the full primary. It should be the blue and brown wires. A simple DC resistance check of the primary should make it obvious which of the three primary wires are the two plates and the center tap. Apply the 100 volts and write down the voltages from the secondary.

Yes, sorry, I did say I would take more measurements.
I got Z Ratios of 1024, 525 and 252, so with 4, 8 and 16 ohm loads that would be 4.1K, 4.2K and 4.03k.
So, it is averaging about 4.1K.
I also measured my Ceriatone DC30 OT and realized this guy is slightly bigger (about 1/4" each direction) than the one for the DC30.
This one didn't actually come with the one bell cover and I think was intended to lay down. I think I happened to have a cover that fit perfectly and so I had previously stood it up with some little L brackets. However, looking through their catalogue of pictures now it seems like they pretty much always lay down their PTs and stand up their OTs.
It does make me curious what amp it was intended for by Ceriatone. I found the emails when they sent it to me by accident instead of a 5e3 Power transformer... way off!

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[bcmatt]

Sounds like a Wards Airline 8517 cut back to only 2 outputs might be a interesting way to go.

That's an interesting design. It looks like each pair of output tubes went to it's own OT and speaker... but not actually a stereo vibrato effect?

[bcmatt]

I notice that the original JTM45 with 6L6s had an OT with 6.6K primaries... one of the biggest differences from the 5F6 Bassman that it looks to me had a 4.15K primary impedance.
Maybe what I am actually thinking about is trying to build a 6L6 rocket...