6V6 vs EL84 OT impedance rating query

[Apexelectric]

I’m planning out a new build and looking at an 18 Plexi type with either 6V6s or EL84s. I was even thinking of having the option to switch either one option or the other since it seems that the primary impedance ratings are similar. However when looking at the specs on a Blackface Deluxe OT it shows a 6.6K instead of 8k-8.4K. I know that these ratings are more like guidelines when it comes to guitar amps.

Was the choice on the BFDR OT primary impedance mismatched on purpose for tonal reasons or is there another design factor that I am not considering that would lead one to choose a transformer that seems to be somewhat mismatched to the output tubes. It seems like an OT in the 8K to 8.4K range Would be the direction I want to go for this amp but should I want to build something more like the BFDR then why the 6.6K rating?

Any light that can be shed on this would be much appreciated.

[MFowler]

Depends on what the data sheets say for the power tube you want to run.


I use Marshall 18w PT and OT -8K for most 18w EL84 builds


For 6v6 I tend to use around 6k or 7k OT but prefer beefier transformers then what Fender Deluxe use. Use a 35w OT like Bandmaster type or custom.


Heck JTM45 with KT66 used 6k6 OT.      Trainwreck Express with pair of EL34's or 6V6-rebias used 6k6 OT but rated at 300mA.

[66Strat]

If I were building a Marshall 18 Watt type circuit, I would use the Heyboer 18 watt output transformer that Doug sells.
http://hoffmanamps.com/MyStore/catalog/images/IMG_8637.jpg

This is a dead nuts clone of an original Radiospares transformer as used in the original Watkins Dominator and Marshall 18 Watt amps. When the 18 Watt group formed about 18 or 20 years ago, they worked with Heyboer to have an original transformer reverse engineered. The original transformer had an 8K ohm primary with 15 ohm, 7.5 ohm and 3.0 ohm secondary taps. The 18 Watt group elected to have the transformer tweaked to provide for 16 ohm, 8 ohm, and 4 ohm secondary taps. Heyboer stayed with the original windings for their transformer and as a result has a slightly higher reflected primary impedance load of about 8K5 ohm.

This transformer would work very well with either 6V6 or EL84 tubes. This transformer would also be a very good choice for an AC15 type amp. IMO 6K6 is on the low side for 6V6 tubes. I can only guess that Fender used this load for the Blackface Deluxe to reduce stress on the screen grids because of the voltages.

[Apexelectric]

Depends on what the data sheets say for the power tube you want to run.


I use Marshall 18w PT and OT -8K for most 18w EL84 builds


For 6v6 I tend to use around 6k or 7k OT but prefer beefier transformers then what Fender Deluxe use. Use a 35w OT like Bandmaster type or custom.


Heck JTM45 with KT66 used 6k6 OT.      Trainwreck Express with pair of EL34's or 6V6-rebias used 6k6 OT but rated at 300mA.

Are you sticking to typical voltages for the Deluxe with the 6V6s or lower as the data sheets dictate. It seems based on a higher than recommended plate voltage, like the BF Deluxe, the 6V6 would pair better with a lowered primary impedance than the tweed version.

[sluckey]

Fender chose to use a 6.6K OT for the Deluxe Reverb in order to squeeze out a few more watts of power (22 watts). The similar Princeton Reverb uses an 8K OT and only puts out 12-15 watts.

[HotBluePlates]

... It seems based on a higher than recommended plate voltage, like the BF Deluxe, the 6V6 would pair better with a lowered primary impedance than the tweed version.

Usually the opposite is true:  If supply voltage goes up and you wish to not over-stress the tube, the load impedance goes up proportionally.

Your best tool to understanding the output transformer primary impedance used is to drop the whole notion of "matching".  We're not trying to match anything at the tube plate side, so much as provide a load that allows the desired power output to happen.

Instead, think Ohm's Law:  If I use same-voltage (Princeton Reverb and Deluxe Reverb have 420v plate supply voltage), but want more-power then I need more-current.  I get more-current if I use less-impedance, because Current = Voltage / Impedance (Ohm's Law).

Fender used a lower OT primary impedance as Sluckey says to get those extra few watts.  They also switched to a long-tail inverter to make sure plenty of drive was available to push the output tubes there (the Princeton's split-load inverter has less possible output voltage swing due to how close the plate sits to the supply voltage, and how close the cathode sits to ground).

[dude]

You could do both tubes in same chassis but wouldn't be able to switch, unless you turned off amp first. Use an 8K OT .
al


You could also add a DPDT switch for the 150 - 5 watt and 250 - 5 watt biasing R, also the 22K and 47K tail resistor, but you still would have to only have one set in at a time, the switch the above DPDT for the correct set of tubes.


EDIT: Layout gives you the option of 6V6s or EL84 in one chassis. The layout doesn't give you option to change the biasing resistor or tail for each set of tubes, this switch will. You could also change that tail resistor to 10K for 6V6s and 33K for EL84s, depending on tone you're after.

[pdf64]

Fender chose to use a 6.6K OT for the Deluxe Reverb in order to squeeze out a few more watts of power (22 watts). The similar Princeton Reverb uses an 8K OT and only puts out 12-15 watts.

I guess there's more HT sag with the Princeton, given it's Champ size PT and 1k dropper to the g2 node. Plus, as mentioned, there's the possibility that the cathodyne may not have sufficient voltage swing available to fully drive the power tube grid to Vg1-k=0V.
So that may be a bigger factor in explaining the power differential.
My finding when experimenting with an RS Deluxe OT in my DR based build, was that it put out a little bit more power with the primary at 8k rather than 6k6.

[jjasilli]

I'm thinking that tranny specs are good to within 35%; whereby the impedance spread under discussion is within the margin of error. 

[Apexelectric]

Thanks all for the input. I’ll take it all under consideration. Some of this info is slowly creeping in and starting to make sense and some of it is not so clear. Still in the absorption phase of this hobby I’ve got myself into. A lot to take in.

[92Volts]

There's a summary posted a lot which I think was originally tubeswell's? Basically, don't stress about it. Among other things, a guitar speaker's impedance isn't always equal to its rating. It's all over the place (depending on frequency) and usually higher. At most frequencies, the reflected primary impedance is higher as a result.

The info in this thread is good, though. If you are worried about the same things as a 6V6 datasheet-- limit distortion, go easy on the tubes, consistent performance despite variation in tubes-- higher voltage calls for higher impedance.