Hi there, I bought an old transformer to built a guitar amplifier for personal use. When it arrived I took a no load test to see if it was working. The voltages on the HT reads 450v -0 -450v. The transformer is very heavy as it came out of a 100w amp running 4 x EL34s. My idea was to use it on a 50w build but I dont want to use it with that high voltages as I am planning to use solid state rectifiers. That would bring me into the 630 range and feel that is going to load the tubes too much. The circuit I have in mind is a Hiwatt clone. Would voltage dropping resistors work to bring the voltage down to a reasonable 450v dc or should I look at another transformer. Where should I put them? before or after the rectifier. Any help would be great Thanx.
Either use another transformer or use a choke filter to give you 400-ish volts. The required choke might not be inexpensive, unfortunately. And you’d still have to be able to tolerate 630 volts at start up. Even with a dropping resistor you’d have to deal with start up - and the heat in the resistor on top of that.
If you didn’t insist on an exact clone, you could just use a higher Z OPT and design for the 50 watts you want with a 600 volt supply. My guess is about 8k. EL34’s would work, but 6550/KT88 or your favorite sweep tube would likely hold up better under abuse. Adjust g2 DOWN from what it would normally be. Thats the knob you turn to match the load impedance with the available power supply voltage. Probably need less than 300V on g2.
If you didn’t insist on an exact clone, you could just use a higher Z OPT and design for the 50 watts you want with a 600 volt supply. My guess is about 8k. EL34’s would work, but 6550/KT88 or your favorite sweep tube would likely hold up better under abuse. Adjust g2 DOWN from what it would normally be. Thats the knob you turn to match the load impedance with the available power supply voltage. Probably need less than 300V on g2.
I think the easiest way to go is to use a swinging choke input right after the diodes followed by a 100 mu electrolytic resulting in about 430 v dc. This will feed the anodes. Then either a choke or a 220 ohm resistor and another 100 mu feeding the grids. 3k4 is perfect for PP EL 34's.
At Vb= 425 v
Rg2 = 1k (common resistor)
Vg1= -38 v
Raa = 3k4
Wo = 55 w
according to Philips data book
At Vb= 425 v
Rg2 = 1k (common resistor)
Vg1= -38 v
Raa = 3k4
Wo = 55 w
according to Philips data book
Thanx Kees, a swinging choke is a new term to me. Is it a special choke that differs from a normal choke? I will have to do some research on that topic. I have a 100ma 20h choke that I planned to use but I suppose that would be too small?
Swinging choke just has variable inductance - slow (ish) controlled saturation. There is more inductance at low current so there is sufficient inductance to freewheel, and L drops off at higher current. 20 H at 100 mA may be about right - but you will need more current capacity. You won’t need as much inductance at higher current, but you don‘t want complete saturation or meltdown either.
the difference between tube rectifier dc and ss rectifiers is around 15 volts only...that is not much..
losing dc voltage can be done is many ways, like series dropping resistors on the secondary feeding rectifiers, then dropping resistors along the dc buss....
At full power the max. current is about 240 ma. and the choke must be able to handle that so your 100 ma choke can not be used as such. There are a few things I forgot to mention. The dc voltage after the swinging choke is 0,9 times the ac voltage feeding the rectifier- at full load! This means that at switch on when the tubes don't draw current the voltage across the first electrolytic can be more than 600 v.Although I mentioned 100 mu in my first reaction this can be lower because the swinging choke has a regulating effect. Even two 50 mu 350 v caps in series (each with 220k across them) may be sufficient. You don't have to worry about the high voltage under no load as the EL 34 can stand 800 v. on the screen when Ig2 = 0.
If you want to be absolutely sure nothing goes wrong add a stand by switch between the rectifiers and the choke. Switch on the mains, wait a minute and switch on the stand by knob.
Good luck with your project!
If you want to be absolutely sure nothing goes wrong add a stand by switch between the rectifiers and the choke. Switch on the mains, wait a minute and switch on the stand by knob.
Good luck with your project!
Thanx, I am well aware that el 34`s can withstand that voltage, the problem I have is .......can modern El 34s withstand that voltage? Where I am staying in South Africa, valves or tubes are quite expensive and with Mr Putin`s moves things are becoming more expensive. I have heard so many stories of modern GZ 34`s/ 5AR4`s failing on startup. At the moment there is a 18 month waiting list for 5881 tubes in our country. I think I should just look out for a more suitable transformer and count my losses.
Rectifiers failing on start up has to do with the very high H-K voltage. That won’t be an issue with the plates in the amplifier being at 630V under no bias. Peak voltages double the supply under normal operation anyway. Even cheap Chinese octal sockets can take 700V as long as you don’t let the dust bunnies get away from you. You will want silicon rectifiers with this. You just can’t use a single 450 or 500 volt electrolytic in the power supply - you will need two 400V caps in series, and probably 1kV DC blocking caps between stages (even in the preamp, unless you’re regulating it down for the front end).
Least troublesome by far, doubly so because of being in ZA and tube situation being complicated is just getting a 340+340V (or nearby) or single 340V winding for bridge rectification and call it a day.
You can build a 400V screens 600V plates amp but then your OT impedance is too low; personally I´d rather hunt (or order) a proper PT (which any regular winder can make) than a custom OT (which few can do properly, if at all) any day of the week.
Also use easier to find main caps,wider choice of rectifiers, etc.
You can build a 400V screens 600V plates amp but then your OT impedance is too low; personally I´d rather hunt (or order) a proper PT (which any regular winder can make) than a custom OT (which few can do properly, if at all) any day of the week.
Also use easier to find main caps,wider choice of rectifiers, etc.
There is also the SMPS route for high voltage. I have been using a setup involving a powerful 12V supply and and power inverter, I get 220/400/440/560 HV @220mA, using a voltage doubler, 12V for heaters, no need for big capacitors (30Khz converter frequency) etc. Not everyone's cup of tea, but I probably will never spend money on a power transformer again.
If you are willing to entertain the SMPS route, there is also the possibility of building a simple buck converter regulator. Start out with 630 volts and get what you want. One 1200 volt IGBT, one 1200 V SiC diode, a simple 100-ish-kHz LC filter and your favorite control chip. Maida-style regulator is possible too, but my guess is you’ll blow your brains out after blowing too many mosfets. It’s just too high a voltage/power NOT to do it switch mode.