Output Transformers: Jet City 100H vs 100HDM vs 100LTD

MentalSharps

It seems as if the 100w variants of the Soldano designed Jet City have different output transformers. Wondering what differences it gives to the amps as a result?

The 100 H -

YW OT-JCA100H
1.4K - 0 - 1.4K

FastRedPonyCar who did some demos on YouTube, had swapped to KT88s but was advised by doug/mike soldano that it wouldn't be good for the transformer in the long run, so to use KT66s instead.

The 100 HDM -

YW OT-JCA100HDM
0.7K - 0 - 0.7K

Apparently it the 100HDM has a different feel and is less loud than the 100 H. This is a whole new rabbit hole to me trying to understand how amps work when I still haven't properly figured out how to sit comfortably with a Les Paul (seriously how). But is it the because of less output power due to the impedence change? ie 6L6's with the 100H will output 100w of power from the 16Ohm output, but 6L6s biased the same with the 100HDM transformer will output less than 100w at the 16Ohm output.

And if so, does this mean that the amp can safely use EL34s or KT88s at the 16 Ohm output? (and achieve 100w of output power with a warmer bias?)

Or what reason is there to re-spec the transformer like that for the 100 HDM when there are not circuit changes, the only differences are the depth control and the 50w switch?

Could it be to cover the OPT better for warranty purposes? By avoiding transformer failure due to ignorance / user error of players sticking in EL34s and KT88s themselves without knowing what they're doing. Similar to how the amps supposedly come biased from the factory really cold at around 11ma, and Jet City's standard advice when people ask is just "you don't have to rebias just stick in new tubes and go".

Also, the 100H is often described as being "loose" on the low end, or being slower in terms of attack / saggy. Whereas the 100 HDM people seem to say is tighter / stiffer. Is this the different output transformers? And if so... would this difference in looseness / stiffness change by using different type of power tubes with different plate(?) impedence...

My 100 HDM does not have "design by Soldano" on it, like the 100 H did. So I wonder if this was something which reflects Soldano's feeling that it had now deviated from the Hot Rod 100+ enough for him to not put his name on it.

The 100 LTD - Don't have specs for this, I'm not actually sure if it's a different output transformer, but I am guessing it must be because it comes stock with EL34s. I'm guessing that this one maybe was consulted by Martin Kidd as he was doing the Custom 22 and Amelia at the time for Jet City?

ecc83

Having actually swapped out transformers from different makers (so you don't have all you eggs did da) checked the ratios and done live A/B tests (built a pretty lethal traff switcher) The players, experts all, could never tell the difference.

Don't want to sart any kind of cow but I suspect, so long as it is close to rating and ratio a traff swap it going to have less impact than an OP valve change and many say the latter is often bloody subtle!

Dave.

jpfamps

The 100H (3k8) has twice the loading of 100HDM (1k4). 1k4 is a bit on the low side for 4 x EL34s or 6L6s, but probably works OK with KT88s.

3k8 is more appropriate loading for a 50W amp running at the usual HT for amps using EL34s, which would be around 450 VDC. You would need a higher HT to get 100W with this loading.

Any deviation from ideal loading with reduce output power; neither transformer looks ideal for a 100W amp.

Alternatively the 100W output power could simply be a guess......

ICBM

A lot of transformer ratios seem to be less than ideal, perhaps because they were chosen for some other reason that no longer applies to the amp it's in, or because someone else used that ratio in another amp, or possibly just pure guesswork!

The Fender Deluxe Reverb springs to mind... my best guess is that the amp was probably originally intended to use 6L6s and Fender decided to substitute 6V6s to cut costs, but the ratio isn't really right even then.

MentalSharps

jpfamps said:

The 100H (3k8) has twice the loading of 100HDM (1k4). 1k4 is a bit on the low side for 4 x EL34s or 6L6s, but probably works OK with KT88s.

3k8 is more appropriate loading for a 50W amp running at the usual HT for amps using EL34s, which would be around 450 VDC. You would need a higher HT to get 100W with this loading.

Any deviation from ideal loading with reduce output power; neither transformer looks ideal for a 100W amp.

Alternatively the 100W output power could simply be a guess......

My 100HDM came stock with Chinese 6L6WGB, and I think the Sovtek 5881 are, or were preferred by Mike Soldano for his amps, so might that be it? I think the Jet City amps are 500V.

Cool though, glad to hear KT88s could be an option, as I'm planning to bring it to you for a bunch of mods soon as I get some cash together 

ecc83

ICBM said:

A lot of transformer ratios seem to be less than ideal, perhaps because they were chosen for some other reason that no longer applies to the amp it's in, or because someone else used that ratio in another amp, or possibly just pure guesswork!

The Fender Deluxe Reverb springs to mind... my best guess is that the amp was probably originally intended to use 6L6s and Fender decided to substitute 6V6s to cut costs, but the ratio isn't really right even then.

I do know IC that at least one amplifier's loading was chosen by "seat of pants" i.e. various resistive loads were made up in series (by YKW!) and the loading chosen for max power tempered with a good sound.

No names, no pack drill but it was a very wee amp that you quite like IC!

Dave.

jpfamps

ICBM said:

A lot of transformer ratios seem to be less than ideal, perhaps because they were chosen for some other reason that no longer applies to the amp it's in, or because someone else used that ratio in another amp, or possibly just pure guesswork!

The Fender Deluxe Reverb springs to mind... my best guess is that the amp was probably originally intended to use 6L6s and Fender decided to substitute 6V6s to cut costs, but the ratio isn't really right even then.

Well I think a lot of loadings are chosen either because they are using an off-the-shelf unit, or because the designer has simply copied another design, or even misread the data sheet.

The canonical Marshall 3.4 k loading for 50W (and hence 1.7k for 100W) amps, is really too low for 2 EL34s, and is expect due to misreading the datasheet. And of course loads of companies copy this design.

With new designs, I like to plot loading versus power using as it is my opinion that valve amps sound better when the loading is optimal for power ouput (actually there is a plateau of maximum power and I prefer using a loading towards the higher end of this).

Thus the EL34 and KT66 loaded heads use 5k8 loading, which is not a readily available off-the-shelf-unit.

The Deluxe Reverb is an interesting case. Fender really did know what they were doing, so I think it's unlikely it was an error (although possible).

Another possibility is that by increasing the plate voltage and hence the power output, a higher wire gauge was specified and it might not have been possible to get enough turns onto the bobbin to get the "correct" loading.

Modulus_Amps

ecc83 said:

Having actually swapped out transformers from different makers (so you don't have all you eggs did da) checked the ratios and done live A/B tests (built a pretty lethal traff switcher) The players, experts all, could never tell the difference.

Don't want to sart any kind of cow but I suspect, so long as it is close to rating and ratio a traff swap it going to have less impact than an OP valve change and many say the latter is often bloody subtle!

Dave.

Agreed in that changing one M style 50w ot to another M style 50w ot will sound pretty much the same, but change the winding pattern or the lamination type and you can get some big differences, mostly in feel and response.

Saying that with these modern 100w amps I guess the manufacturer are not expecting them to be pushed anywhere near where the OT really starts to make a big difference.

@MentalSharps if you want the amp to be tighter etc it would be easier to change this in the pre-amp and resonance circuits than hoping to get the best results of an OT change, you may swap in an OT with a bigger stack to more power to handle the bass, but you might end up with high end harshness you don't like or with the note attack being too immediate (in your face)