Is this a good transformer for both 6V6 & 6L6

[plumcrazyfx]

I'm converting a Hammond AO-29 into a modified Fender Bassman 20.  I want to upgrade the output transformer to mostly use 6V6 but have the ability to use 6L6s for a little cleaner tone at times and also have a bigger sound with the 6V6s.  I'm leaning toward the Hammond 1760JB with a 6K primary.  I'm figuring between 375 to 400v on the plates.  I know it is made for 6L6 but since the BF Deluxe has a 6.6K primary I figure I'm good with both.  Does my thought process make sense?

[tubenit]

From Kevin O'Connor's FAQ from his London Power website:

Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and also a ridiculous suggestion. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout our TUT book series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

My understanding is that Dumble purposely mismatched OT and speakers in some of the Dumble amps he built.

[plumcrazyfx]

I've read this.  I think I'm safe here but I'm unsure as to where fly back begins to be an issue.  I have a Fender Tonemaster and it doesn't spell out impedance matching but instead talks about tone and or vol with various speaker impedance combos.  No mention of mismatch or damaging the output transformer.  I know there is not a definitive answer to my first question but I wanted to be sure I was in the ballpark.

[vampwizzard]

Right now im working through completion on an AO-29 to 5F6A tweed bassman conversion. I used a 1760J for my output transformer with dual 6L6s

You may run into a major hurdle with serious overvoltage on the AO-29 depending on a couple of factors. First, the AO-29 power transformer can run a bajillion tubes and has a massive amount of current available to it. When I first converted my 29 to a deluxe this was a major problem. Second, you may have even more current available if you have an AO-29 that powered a field coil speaker. In that case, youll almost certainly need a 700 or 1K, 30W resistor stuck between the red-yellow mixed PT center tap and ground.

Oversizing the output transformer does not seem to be a problem electrically. It will change the way the amp sounds however.

[plumcrazyfx]

I was considering a 5R4 rectifier or sticking with the 6L6 if need be.

[vampwizzard]

With a resistor on the CT to ground (cant remember what I put in there), 3 12ax7's and two 6L6's I had a B+ voltage around 470VDC with a 5Y3. You may want to estimate voltages in that ballpark and design the power supply accordingly.

[sluckey]

A 5Y3 is pretty light weight for a pair of 6L6s.

[vampwizzard]

Sluckey-> I use a 5U4 now.

[ALBATROS1234]

It's a subjective things from what I have learned from experimenting. Some tube guys might want to burn me at the stake if they knew some of the mismatched I have done but I go with what sounds best to me. I even used a 277v to 24v control transformer for an ot because it sounded better than the 8k to 8ohm. I didn't even measure the impedance I just was not 100%  satisfied so I experimented with stuff I had on hand. I have also hooked up some transformers that made the amp sound horribly thin like a toroid I tryed. Anyway use the rule book as a general ballpark and built what sou d's good to you

[plumcrazyfx]

470V - Yikes.  Just for the record the I'm using on of the later (1963?) models with the smaller black transformers rather than the larger silver transformers.  Definitely didn't have the field coil speaker although I have one of those to work with in the future.

[vampwizzard]

470V - Yikes.  Just for the record the I'm using on of the later (1963?) models with the smaller black transformers rather than the larger silver transformers.  Definitely didn't have the field coil speaker although I have one of those to work with in the future.

Same. And yes. Its still going to be a hurdle.. Kley De Jong did a video series on his conversion and found the same thing. Went straight to 6L6s. At one time I correlated his thread on another forum to his thread on this forum but its been awhile. Its good iron. Super good iron actually. Youll get it figured out just be prepared.