Home James! But first, which transformer?

[taj]

Okay, granted this is a bit vague but I'll throw it out there anyway... I'm interested in hearing some thoughts.

I keep rethinking plans, but recently I've been considering building a KT88 PP amp of 40-75 watts-ish (or whatever, not picky) from scratch. I'm new to tubes, despite having built a bunch of SS amps (which I understand much better), so with that in mind...

My wife is planning a trip home to Taiwan for CNY, and I thought this might be a great opportunity to purchase some goodies and have them shipped back via Spouse Luggage Express. Specifically, I was thinking of buying some James transformers for this KT-88 PP amp project. Here's the big question: Which models would I need? 1 for power and 2 for output (yeah, not monoblocks unless really necessary).

Which models would be best suited for such a project? Your thoughts...


..Todd

[boywonder]

As a start, check out Duncan Amps TDSL page for typical operating conditions for KT88 here:

TDSL Tube data [KT88]

I'm assuming that you'll want ultralinear (UL) connection for the desired wattage, so it's 4000-5000-6000 ohms primary load on your KT88's depending on your chosen B+ and watts out.

Excellent choice by the way, can your wife pick up a couple of James transformers for me while she's there?

For output, JS-6256HS (50W, 6.6K), JS-6274HF (70W 4.5K), or JS-6275HF (70W, 5K) These all have screen grid taps which are needed for UL connection.

One other thing to keep in mind when looking at the KT88 application data.........keeping the B+ under 500V makes for a smaller, simpler power supply filtering scheme, as electrolytic cap voltage ratings top out around 500V. If you want a B+ greater than 500V, you'll need to series stack caps with some balancing R's, which halves the C value, so you'll need to buy twice as many caps twice as big as a supply under 500V. Depending on your comfort level with 500VDC, and as a first project, you may want to keep the B+ under 500V anyway.

James transformers are available in champagne or black, and you may want to consider ordering them before your wife departs, as there is usually a lead time, unless a local vendor has them in stock.

Are you planning stereo or mono blocks? Fixed bias? Desired B+? Tube or SS rectification? More definition is needed to spec the power transformer......

Edit: I just noticed that you are considering stereo....check out the JS-9613 or JS-9618 power transformers.

Are you familiar with PSUDII?

[taj]

Thanks Boywonder. All great information! Much appreciated.

I'm still trying to digest all these tube-world terms like Ultralinear mode, fixed bias, B+, etc.

I've heard about and seen screen shots here of PSUDII. But I have a lot to learn about tube power still before I could dive into that.

Yes, I am getting ahead of myself by wanting to get transformers first, but I really don't want to miss out on that shipping opportunity. So I guess versatility will be a key concern when selecting the iron.

I was thinking of buying them from THLAudio, who are located in a fairly convenient part of town. I'll send them an email to see if they have these in stock.

I haven't seen a photo of their Champagne colour yet. I'll have to google that.

..Todd

[boywonder]

If the James Transformers are within your budget, I would jump on the opportunity. The 9613 power transformer and a pair of 5K output transformers will allow a number of possible topologies, including EL34 triode if that strikes your fancy.

All of the james PP transformers will allow triode, UL, and pentode topologies. Triode and pentode use the ends of the windings, and UL requires an extra connection (usually around 43% of the windings) for the screen grids.

As a general rule of thumb, increasing the load on the output tubes a little bit (say from 4K to 5K) will reduce output power and distortion, and decreasing the load will do the opposite.

Using an output transformer that is larger than needed (say a 50W transformer used in a 30W design) is fine, as you won't run into core saturation issues at higher outputs, which can lose bass. If you go too big (ie using a 75W transformer in a 10 W design), there is a chance of losing some high freq in the over-spec'd transformer, not to mention the waste of $$.

The champagne color is a sandy silver and the black is...well,...black.

If the budget allows, you may want to pick up a james choke also, they sure are sexy........

James transformers are also available directly from James (at least shipped to the US). I recently got a quote directly from James for a JS-9613; it was around $187 US IIRC.

PSUD is easy to use, download it and start asking questions here.

[taj]

Yeah, DIY budgets are always a compromise. For me it's 100% of household income, for my wife it's 0.000%. So even a conservative compromise can handle a nice transformer every now and again.

And getting over-sized transformers when one's spouse must carry them 5000 km in her suitcase can result in further high frequency hearing losses from the ranting when she finally drags her 150 lb suitcase through the door. And again later when she opens the Visa bill.

..Todd

[DougL]

I would source the power supply iron elsewhere, if you need to compromise.
Its the outputs that are important.

Doug

[boywonder]

Originally Posted by DougL

I would source the power supply iron elsewhere, if you need to compromise.
Its the outputs that are important.

Doug

Good point.

Taj, Since you are in Canada, Hammond power iron is the obvious choice if budget is an issue. The 300 series is a little more $$ than the 200 series, but has a bias tap (handy for fixed bias topologies), dual 120/240V primaries, and are built a little better than the 200 series.

Edcor has lots of choices for very reasonable $$ also. They usually have a 3-4 week lead time.

Both of the above are lots less $$ than James Power transformers.

[taj]

Good info. Thanks!

Another question, What are some quality, well-respected KT88 circuits/projects/threads (50 watts-ish) that may be floating around?

I've found it practically impossible to search for answers to questions like this due to the overwhelming number of irrelevant (but not incorrect) hits you get.

I must say, the tube forum here is much friendlier and more informative than the solid state amp forum -- generally speaking, which I've been hanging out in for the past 5 years.


..Todd

[alexg]

Originally Posted by boywonder

Good point.

Taj, Since you are in Canada, Hammond power iron is the obvious choice if budget is an issue. The 300 series is a little more $$ than the 200 series, but has a bias tap (handy for fixed bias topologies), dual 120/240V primaries, and are built a little better than the 200 series.

Edcor has lots of choices for very reasonable $$ also. They usually have a 3-4 week lead time.

Both of the above are lots less $$ than James Power transformers.

If I were you, I would really re-consider using the James OPTs, they look good but they are not the best.

James might sound as good as the Edcor and the Edcor sells for less.

Maybe you can ask your wife to swing to Bangkok and get some Silk OPTs.

[boywonder]

Originally Posted by taj

Another question, What are some quality, well-respected KT88 circuits/projects/threads (50 watts-ish) that may be floating around?


..Todd

"Mullard 5-20 KT88 PP blocks!

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