Recommend me a PT please

[daz]

I'm about to order parts for a four 6V6 cathode biased amp with a 5AR4 rectifier and 3 AX7's. I need to know if my choice of the Hammond 270HX PT is good or if not what you'd recommend. For the record, i don't know enough to design my own PSU, and i do not intend to stray too far off from the 36 watt amp at 18watt.com in regards to the PSU because i already have my hands full as is and don't want to spend the next year asking questions. (yeah, i know....i already as too many ) So i'll be using a 32/32uf can and a couple 20uf caps for the preamp supply. That hammond seems to fulfill all requirements, but if you have a suggestion for a different model and why, i'm open to suggestions.

[mooreamps]

The Hammond 270HX PT is a fine choice.

-g

[daz]

Great, thank you. One more thing tho....like i say i'm building it cathode biased and with tube rectification, but if i should decide to try SS rectification and possibly fixed bias at some point, will that PT be ok ?

[mooreamps]

Great, thank you. One more thing tho....like i say i'm building it cathode biased and with tube rectification, but if i should decide to try SS rectification and possibly fixed bias at some point, will that PT be ok ?

I have used this same power transformer with fast diode rectifiers in a five diode full wave / full wave bridge configuration. The dc voltage, unloaded, in full wave mode was right at 375 vdc. Under a full load of the pre-amp, an EL-34 and a KT-88 running SE, it dropped down to, I believe it was around 325 on the power tube plates.

-g

[daz]

Ok, that went rigfht over my head.........whooooooosh ! But since i posted this i've found reason to stick with a tube rectifier. (don't ask....long story) So i'm just going to order it along with all the parts for the new build tomorrow hopefully. Thanks.

[mooreamps]

Sorry. That was not my intention. I build, how should I say, more complex circuits than some. I was thinking about the high voltage power supply in my 36 watt amp. It's how I do my half power/full power switch. 300 volts plate voltage for 18 watts, 600 volts plate voltage for 36 watts. ........ But, with a full wave tube rectifier, it's still a fine choice.

-g

[daz]

gary,

Things have changed and i'm considering building a EL34 amp instead. Weber has several 40 watt EL34 based amps that look like could use my heyboer 40 watt 4k OT, and i just need to know what PT i'd have to get for an amp like that. The schematic is here.. https://taweber.powweb.com/store/6m40_schem.jpg

Any Hammond model you could recommend, or at least tell me what specs i need different than the 270HX? I guess the voltage is going to be too low, right? then again it's a 40 watt amp, so maybe lower plate V is why and i wouldn't need a different one? Then again i suppose i could jus buy the weber PT for that kit if it's not to much more than the hammond.

[mooreamps]

Well, first of all, total heater current calculates to be 4.8 amps for pre-amp tubes and power amp tubes. I'm assuming you're not planning to run a separate power transformer for power tube filaments. So, you need to narrow down the choices of power transformers that supply either 5 or 6 amps for filament power.


Secondly, it's going to depend on what kind of sound your going for. Let's say, for sake of discussion, the values of plate voltages for the EL-34 can be in the range between 300 to 600 volts. So, In my opinion :} ,
the tone at 300 volts will be a little browner sounding, with easier breakup because the Vsat is lower. Vsat being the lowest level the input signal can pull the plate voltage to ground. At 600 volts, the tone is brighter, like those 550 Plate volt Marshalls..., with more clean headroom because the Vsat is much higher......Thus choose the HV winding that corresponds to what ever plate voltage you want to achieve. So, if the HV winding on the 270HX was 275 vac, which translates into about a 350 volt vdc plate voltage, then you can use this relationship as a guideline for selecting the equivelent Hammond part, or Weber part as you prefer.

-g

[daz]

So let me get this straight...as long as i have 5 or 6 amps on the heaters (the 270HX is indeed 6) and i'm fine with a lower plate voltage for a browner less bright tone that breaks up earlier, i can use the 270HX? Because i actually would love EL34's that are a bit darker and break up earlier than the typical marshall 50. So i guess i'll still go with the 270HX unless i'm missing something in your post. Oh, and i also intend to cathode bias it for the sake of simplicity and for a squishier tone. Any idea what value of cathode R i would want to start with using that PT for 2 EL34's?
Thanks very much for this help ! I'm really anxious to get the parts ordered and start this thing, but i have so many questions and it's hard to find answers in a timely manner.

[mooreamps]

Cathode bias for two EL-34's, running in push-pull. The range for this is in the range between 470 and 600 ohms, 3 watt or 5 watt, for each tube, and also bypass each resistor with a 1000uF 100 volt cap. 470 ohms for more in the range of class A, and 600 for more in the range of class a/b. Now, I've not ever built a push/pull power amp. But I can say when I bias an EL-34 for single-ended, I do use the 470 ohm 5 watt resistor. Also, the bigger bypass cap allows the power tube to breath when running with self bias.

That being said, I also run a 100 ohm plate resistor for measuring plate current. I understand the "norm" is using a 1 ohm or 10 ohm in the cathode circuit, but when running self bias then you have to run this in series with the bias resistor. I choose to measure the plate current, oddly enough, off the plate circuit, just in case I get a power tube with a leaky screen. Really well made tubes have less than the nominal 10% screen grid current. But when I find a power tube which exceeds this 10% value, I toss it.