Output transformer observation/question/thought process

[gui_tarzan]

I have mentioned before that I have a bunch of organ tubes & transformers to use for projects and while looking at something I want to build I came up with some thoughts that may or may not be accurate so I wanted to throw them out there for feedback.

I understand the primary impedance ratings (4k, 6.6k, etc) and how they relate to the value of the speaker(s) used based on the windings as well as how to figure out which tubes you should use with them. But is it true that it also depends on the plate and grid voltages you use? For example, looking at the 6L6GC datasheet along with an old chart showing what primary values can be used with what power tubes (usually a generalization) it shows different transformer values for the same tube types. The chart below is very old but it shows the 6L6 being usable with different primary value OTs. The datasheet captures also show different values for primaries that correlate with the different voltages OR bias type OR single ended or PP (which I knew).

So my questions are: given the comments here over the years about tube amps not being real picky about component values within reason, is it plausible to use a transformer that's not quite to a certain spec? For example, if I have a pair of 6L6s and only have a 4k OT, as long as I have the plate and grid voltages in the right area/ratio (per the datasheet) it shouldn't have a detrimental effect on either the tubes or the OT?

[Helmholtz]

A 4k OT of sufficient size/wattage will work fine with a pair of 6L6s in a typical PP amp with B+ up to 400V at full power.
Fender OTs typically are 4.2k.

Datasheet examples specify primary impedance for least distortion.

Of course SE class A would be different.

[gui_tarzan]

Of course. My question is more along the lines of what is acceptable variance and how would I go about figuring that out? Diagnosing/building amps with like/known part values is simple enough, but taking things that I have on-hand so I don't need to buy something is where the guesswork starts. If that makes sense.

[Helmholtz]

Please help me to understand and maybe be more specific.

Generally the optimum plate load for a given circuit and tube is found from a loadline construction/calculation.

[gui_tarzan]

For example: one of the OTs I have is from a Hammond organ and it handled 4-6V6GTs. I marked down the tube info on the OTs when I salvaged the parts but I haven't checked the actual measurements on the transformer itself. I really don't want to run 4 6V6s for a home amp, I don't need that kind of volume but if I drop two of the tubes it throws off the balance unless I change the speaker. I only have 8ohm speakers, no 4 or 16ohm so I'm trying to figure out which alternative tube configurations I can use safely based on their load values. I know I can look at datasheets until I find something that will work, but it would be nice to have a chart that shows the OT specs and what tubes I can use with that particular OT. If that's not really possibly due to the variables I mentioned above I will just have to figure out another way to figure it out.

[Helmholtz]

Two 6V6s will want a primary impedance betwee 6.6k and 10k. Will also work with two EL84s.

The 4x6V6 OT might have a primary impedance around 4k and might work with 2x6L6s as well.

Always best to measure an unknown OT.

[gui_tarzan]

Agreed, which I will do. Thanks for the suggestions, I think you see what I'm trying to figure out and it's appreciated.

[eschertron]

It hasn't been mentioned, But Nick B. has a loadline calculator on his website.
It can take some of the guesswork out of what you are trying to accomplish.

[g1]

Sometimes you can get a bit of insight from manufacturers if part #'s for the transformers can be found. For example, Marshall uses same OT regardless for their amps that can be shipped with either "5881" or EL34 types. But they also used same transformers for 4 x EL84 and 2 x EL34 and 2 x 5881, for example (Drake 292 772).
Fender may have similar things going on, where you use the same OT across various configurations.
But 'in the ballpark' seems to be the norm.

[loudthud]

Gui_tarzan, you have cut off some of the most important data from your RCA data sheets above. Some on the data is for single ended class A1 operation. Some of it applies to 6L6 {nothing}, 6L6G, 6L6GA and 6L6GB which is a lower spec tube than the 6L6GC. Look under where it says Maximum Ratings.

The 6L6GC can generally be used in any of the circuits designed for the older 6L6 {nothing}, 6L6G, 6L6GA or 6L6GB, but those tubes won't last long in circuits designed around the 6L6GC's higher plate and screen dissipation specs.

Look carefully through old Fender tweed schematics. If it says 6L6 {nothing}, 6L6G, 6L6GA or 6L6GB, it likely has a higher primary impedance output transformer and only produces 20 to 25 Watts.

[gui_tarzan]

Yeah, I know I did because I was focused on a specific thing but I know what you're referring to. And you hit on part of what I was asking at the same time. The changes over time in both tube specs and transformer ratings vary quite a bit which led in part to my asking.

[gui_tarzan]

It hasn't been mentioned, But Nick B. has a loadline calculator on his website.
It can take some of the guesswork out of what you are trying to accomplish.

I forgot about that site. Thanks!

[kg]

gui, first question is: musical instrument amp, or hifi amp?

that will guide your design goals/criteria.