Another SE amp - transformers

[EFK]

I found this schematic somewhere and had it stored for a while and can not now remember where I found it. I'd like to try this out but I'd like to use it w/ a bigger power tube, maybe a KT66 or KT77. Also, I'd like to use it with a tube rectifier. So, if I wanted to grab a pair of new Hammond transformers from say AES, can someone recommend a PT and OT model which would handle all of this (including just about any tube rectifier)? Any help much appreciated. Thanks.

[Chuck H]

Download Duncan PSU

PSUD2

And look up the tube data for any tube you want to use

NJ7P Tube Database Search

Do some simulations and then look up the specs for Hammond transformers

Hammond Mfrg. - Electronic Transformers - AUDIO Index

Duncan PSU will allow you to determine the voltage at each B+ node with any rectifier arrangement provided you plug in the appropriate curent conditions.

The tube data will tell you the approximate voltage and current requrements for your tube of choice so you know what numbers to plug into the simulator.

The Hammond transformer specs will show you how close you can get to your determination of "ideal" and tell you if a given transformer meets your criteria.

You don't even have to do any math...

A better question might be (example) 'How will a KT66 sound at 400 volts compared to 350 volts in class A?'

I personally can't answer that question. But someone here probably can, and that would get you closer to finding the tone you want from this project than having someone spoon feed transformer part numbers to you. Your much better off doing the above research for yourself and learning about these resources than you would be with one sided opinions.

Chuck

[EFK]

But mmmoooommmm, I LIKE being spoon-fed!

Seriously thanks for the links Chuck. The Duncan program looks promising but I'll have to steal my daughter's laptop again to use it (I'm on Mac). And she wonders where all of these funny programs come from...

Confused about something on the tube data sheets: KT66 shows 500V plate, 400V screens max, then for class A single tube *triode* it shows two sets of numbers max anode, 270 and 400. ??? No explanation of the two sets - ??? For class A single tube pentode, it shows 270 as plate max. So I can only run a single KT66 at 270 V or below? Not sure if I'm reading this correctly. Any advice?

[Steve Conner]

Those aren't limits, they're recommended operating conditions.

[MWJB]

Tube data sheets are written by people who make tubes, not amps. No popular guitar amps conform to the voltages recommended by tube data sheets.

Typical B+ for a SE 6L6/KT66/KT77 amp will be around the 400v mark (+/- 30vdc or so), so I'd suggest a PT with a B+ secondary rated around 330-0-330VAC, at <100mA. A BF Deluxe PT would be ideal, or a Hammond 373BX...you could play with different 2A rectifiers (5Y3, 5V4, 5AR4) to get the B+ you want, you might want a 680-800ohm cathode resistor for the power tube with the 373BX and a 5AR4.

125DSE or ESE would be fine for the OT, use the 2.5K primary.

[EFK]

HA! He spoon fed me! He told me what to use!

Just kidding. What's the deal with the primary impedance - 2.5K? Why so low? My current KT66 amp is about 8K at 16 ohms, and I had thought most 6L6 type generally use @ 5K and up. Of course they are p/p not se, but if you could explain this briefly to me it would help jsut for the sake of understanding. Or point me towards somewhere online written for dummies where I could read up on it a little?

[Chuck H]

What's the deal with the primary impedance - 2.5K? Why so low? My current KT66 amp is about 8K at 16 ohms, and I had thought most 6L6 type generally use @ 5K and up. Of course they are p/p not se, but if you could explain this briefly to me it would help jsut for the sake of understanding.

That 8k seems high to me. Around 4k or 6.6k is more typical for most guitar amps. Half that for one valve instead of two and you get 2k and 3.3k (around 2.5k) As Steve pointed out, those werent max voltages in the "typical operation" data. It's there to show you what to expect under some given conditions. I did find one example for an MO Valves KT66 SE at 440Vp. In this example they used a 4k primary. All other examples used 2.2k. It is worth noting that a higher primary is usually used with a higher plate voltage. This could be the case with your KT66 PP amp that uses an 8k primary. If you do go with a Vp of 400 you may want to go with a 3.3k primary. 2.5k will "work" but seems a little low to me. But I like the tone of power tubes at highish voltage with a highish primary. Bristley but smooth. And, of course, every tube type has it's own signature character. I have never used KT66's.

IMHE tubes operated at high voltage with a lowish primary sound very punchy, loud and dynamic, overly dynamic really, and some mid character is sacrificed.

On the other extreme, tubes operated with lowish voltages and a high primary impedance sound very even and distort very smooth. Almost too compressed.

Those are the two extremes. All points in between are implied, with a few sweet spots hiding in there.

Preamp characteristics will also effect what kind of power tube arrangement sounds best for a given amp. That's why a tweed Fender Pro can sound great with a Vp of 390, primary 6.6k (two valves, same as 3.3k for four valves), and a BF Twin Reverb can also sound great with a VP of 460, primary 2k (four valves, same as 4k for two valves). Each amp has a different mojo. But both sound great and are at either end of the voltage/primary impedance range.

So you have to consider what kind of tone you want and try what you think will work. Do that about ten times and you might even nail it...

Chuck

[MWJB]

"What's the deal with the primary impedance - 2.5K? Why so low?" You could use 5K (or just switch secondary taps), see which you prefer the sound of? 2.5K is not uncommon in British amps.

"and I had thought most 6L6 type generally use @ 5K and up." I have only seen much over 5K in budget amps that used surplus/off the shelf xformers. 2.5K to a little over 4K is the norm. In your SE project 5K would be a good compromise if you wanted to switch between K66/6L6 and 6V6 (assuming the 6V6 would take the voltages & current present).

What is your KT66 amp with 8K primary out of interest?

[EFK]

I should make clear that I am talking 8K in a current PP amp w. KT66, not a SE amp. Gibson GA42RVT which is a 6L6GC amp which I use with KT66. I measured the OT via the method using a small DC power source and doing the math; from memory, had @ 8.5 K at 16 ohm which is the tap I use w. (2) 8 ohm speakers in series. Please keep in mind I don't necessarily understand all of this but I did follow the directions I found somewhere online when I measured it. Imag would not give me the OT specs as they stated it was a proprietary OT made only for Gibson. It's pretty big, bigger than a JTM45/Bassman OT *I think.*

BTW, this is cathode bias and right now has 493 V on the plates, @ 491 on screens (121 wall) and I have it idling at about 90% or so w. a common 400 ohm cathode resistor.

Chuck those are some REALLY interesting OT impedance comments.