OT Primary Doubt

[Bombacaototal]

I’ve been studying the Fender DP by Zinky (schematic attached) and have some questions about the primary of the OT.

The OT is Schumacher #026478 with 2.35K primary and speaker out is 8 ohms

The 2x12” speakers are 8ohms each but wired in parallel at 4ohm

Given the unmatched OT (8ohms) and Speakers (4ohms) the power tubes are seing half of the primary = 1.18K.

My understanding is that:
25W - 2x6V6 = 8k primary
50W - 2x6L6 = 4.2K primary
75W - 2x6550 = 3.5K primary
100W - 4x6L6 = 2.1K primary
150W - 4x6550 = 1.75K primary

Now, the DP is running 100W, 4x6L6 but is it seeing a lower primary (1.18k) due to pentode PI/power tubes and hence higher voltage?

And how can the reverb 6V6 not redplate at 1.18K primary?

[pompeiisneaks]

The tubes don't have to be at an exact impedance, they just reflect the impedance shown, 1.18k vs 2.1k isn't a massive difference, about half, and people do that level of impedance mismatching all the time, but it does alter the tone. You could use that and instead get 1 8 ohm speaker or two 16 ohms in this setup so you get the 2.35k instead. Or is the question more of 'why did the Zinky amp do this? they may have loved the tone of the impedance at that setup? As a guess only. Others may know if there's more to it than that.

~Phil

[Bombacaototal]

Thanks Phil, the main question I am trying to answer is, would the reverb circuit with the 6V6 work at other Impedances for example 50W amp at 4.2K or 100W amp at 2.1K without major changes or if I ever decide to incorporate this circuit elsewhere keeping the same OT and mismatch at 1.18K would be key

Also curious to understand Zinky approach and how he made this work

Rafael

[Ten Over]

I don't see anything on that schematic that would lead me to believe they used two 8 Ohm speakers. Vintage 30's come in 8 or 16 Ohms and I'm guessing they used 16.

What does the reverb have to do with the output arrangement?

[Bombacaototal]

The amp is indeed 8ohm OT and 4 ohm speakers (2x 8ohm in parallel)

The reverb uses a 6V6 and why the output arrangement is relevant

[Charlie Wilson]

Amp was designed to run off the 8 ohm tap into a 4 ohm load. Dumble had no problem mismatching impedance. Puts out about 65W. The 6V6 drives the reverb like a reverb tank on the front end. Has no bearing on the output.
CW

[Bombacaototal]

Amp was designed to run off the 8 ohm tap into a 4 ohm load. Dumble had no problem mismatching impedance. Puts out about 65W. The 6V6 drives the reverb like a reverb tank on the front end. Has no bearing on the output.
CW

Thanks Charlie, I appreciate the input. What would be the reasoning to designing an amp mismatched like this?

So the primary OT impedance does not matter at all for the 6V6 tube at the reverb? And hence choice of OT dpes not matter

[martin manning]

So the primary OT impedance does not matter at all for the 6V6 tube at the reverb? And hence choice of OT dpes not matter

This is an odd question. What makes you think it would?

[sluckey]

So the primary OT impedance does not matter at all for the 6V6 tube at the reverb? And hence choice of OT dpes not matter

The 6V6 reverb tube has it's own OT. That OT must match the impedance of the reverb tank input transducer to the plate of the 6V6. Nothing in this reverb circuit has any bearing on the big OT that is connected to the speakers. And likewise, the quad 6L6s, big OT, and speakers have nothing to do with the reverb circuit.

[Stevem]

He likley uses a Fender type reverb tank that has a 8 ohm impedance, in which case that 6v6 driving that little OT is at most just a 4 watt SE amp.
You could hook up a 8 ohm speaker to it if you unplug the tank and have a bedroom amp!

[Bombacaototal]

So the primary OT impedance does not matter at all for the 6V6 tube at the reverb? And hence choice of OT dpes not matter

The 6V6 reverb tube has it's own OT. That OT must match the impedance of the reverb tank input transducer to the plate of the 6V6. Nothing in this reverb circuit has any bearing on the big OT that is connected to the speakers. And likewise, the quad 6L6s, big OT, and speakers have nothing to do with the reverb circuit.

That is perfect Sluckey. Understood! Sorry for my confusion

[pdf64]

He likley uses a Fender type reverb tank that has a 8 ohm impedance, in which case that 6v6 driving that little OT is at most just a 4 watt SE amp.
You could hook up a 8 ohm speaker to it if you unplug the tank and have a bedroom amp!

The tone would be rather thin, due to the C3-R5 HPF, corner freq ~600Hz

[didit]

Looks similar to Vibro-King/repro-'63 6V6 'verb, which has 5.5K primary and is "10 Watts".

Best .. Ian

[Bombacaototal]

Just one more question about this amp. Is the screen grid (Y) 69uF (47uF and 22uF in parallel)?

As far as the reverb transformer which also uses the screen grids (Y) would there be much/any tonal/feeling difference between having it at 22uF or 69uF?

I know this will make a big difference in the tonestack, Phase Inverter etc, but not really sure of the impact on the reverb transformer.

[Bombacaototal]

About the Footswitch, the DP uses a relay, but if going for a traditional Fender style I was checking for reference the 6G15 reverb unit which places it right after the reverb pan return. Now my question is where would be the best placement for the Ftswtch, before or after the 6.8K to ground (illustration below)