No need to calculate much in this case, between 7K and 8K little difference. The only circuit which might give more troubles would be ultralinear but there are no taps here. Triode is extremely tolerant. Best case it needs small adjustment of quiescent conditions. Anyway I understand from initial post that those 7K transformers were used with EL84's in the radio from which he stripped off. So if one wants avoid some tweaking, just look at the radio circuit and that's it.
The OP had an old non-functional Japanese hifi receiver, which was fair quality, so all the ingredients are proven as working together. The simplest way forward is to recreate the original, known-working , circuit, in the knowledge that it could be improved. But the open ended nature of the original request has resulted in a certain amount of axe grinding.
Well, the guys has the bulk of components to build an amp, and the title of the thread is 'EL84 tube amp, how to start?', so I suppose a reasonable starting point is whether it will be PCB or point to point, and is there a chassis already, and if not, how will it be presented?.
Then there is the music that is played, and the speakers that will be used, and hence the power output to shoot for.
That, and the OPTs available, should dictate triode-strapped, pentode or ultralinear.
So that should just leave suggestions of entry level circuits that will be understandable and rewarding.
The chap has a power transformer that is not an ideal fit for most bog standard designs, so some fettling in that direction is going to be needed if that component is to be retained.
Personally, I'd vote for a Tubelab SPP. Superb support from George, 100's of satisfied builders, and perfect for toe-wetting.
It depends on the thread Andy. This is thread where the OP has no experience. So he needs something simple, something that works without troubles. Hence triode output stage or the exact power stage of the radio. This will also avoid spending money and the debate could move to the driver/input stage and power supply where he can learn 1 or 2 things and where he has shown to have options, again without spending money.
The power supply is surely simpler and the amplifier does not require feedback necessarily in triode.
A proper power supply for a pentode requires a regulated g2 voltage...that's where one can learn something useful and see the difference. Can start from string of zeners which is already a good step forward respect to unregulated supply.
A proper power supply for a pentode requires a regulated g2 voltage...that's where one can learn something useful and see the difference. Can start from string of zeners which is already a good step forward respect to unregulated supply.
Yes, agree with the above. My suggestion of EL34 in triode was to give the guy enough output for his system needs. I would have thought that PP EL84 in triode was too wimpy. It certainly was in my experience of building EL84 amps, or at least I'd say it was borderline. Unfortunately he doesn't have UL taps so that solution isn't on the table.
The magic about an EL84 comes, when its being used as a pentode. Not a pseudo triode.
It has more (and enough) power out for small speakers, it has all that pentode feeling that is so much loved with this tube. How many guitar amps with this famous tube used it in triode mode? Fairly none.
Use it the way it was constructed and you will have all that glory.
Use it as a triode- and you will have build a Frankenstein amp. Not real triode, but wired as a triode. You will have very low output power and never in this life the sound of a real triode. Go, get one. They are fairly cheap if its not the usual hyped models.
But never go and build amps with tubes, just to punch them to be something they never have been designed for.
Just to mention, go out and buy one of the famous NOS EL84. You'll end up crying about the prices of a Mullard, Tele or others. And real NOS are vitually impossible to find anyway. What could be found is NOS fakes, put in new boxes with old logos.
It has more (and enough) power out for small speakers, it has all that pentode feeling that is so much loved with this tube. How many guitar amps with this famous tube used it in triode mode? Fairly none.
Use it the way it was constructed and you will have all that glory.
Use it as a triode- and you will have build a Frankenstein amp. Not real triode, but wired as a triode. You will have very low output power and never in this life the sound of a real triode. Go, get one. They are fairly cheap if its not the usual hyped models.
But never go and build amps with tubes, just to punch them to be something they never have been designed for.
Just to mention, go out and buy one of the famous NOS EL84. You'll end up crying about the prices of a Mullard, Tele or others. And real NOS are vitually impossible to find anyway. What could be found is NOS fakes, put in new boxes with old logos.
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Isn't the trickiest element of this project making use of that power transformer?
There is plenty of current, and heaters for 4xEL84 and supporting tubes are catered for, but 260V secondary with a centre tap is a bit awkward. The original PS used silicon rectification, and then dropped 25v in the heaters of the 12AX7's, meaning they were getting DC as well. Also it meant ground was elevated. Unfortunately I can't read the voltages, and I don't fully understand how the centre tap is being used.
Assuming around 300V B+, what sort of power supply could he build from that?
There is plenty of current, and heaters for 4xEL84 and supporting tubes are catered for, but 260V secondary with a centre tap is a bit awkward. The original PS used silicon rectification, and then dropped 25v in the heaters of the 12AX7's, meaning they were getting DC as well. Also it meant ground was elevated. Unfortunately I can't read the voltages, and I don't fully understand how the centre tap is being used.
Assuming around 300V B+, what sort of power supply could he build from that?
That does seem to be the problem - using that power trafo in a normal straightforward circuit puts the B+ just a little too high for most EL84s, especially with the load impedance on the low side. I’d just leave the center tap unconnected. The original circuit is fine, you just need to concentrate on how to deal with the high B+. The resulting 360V would work, you would just have to be gentle with it and maybe run the idle bias a bit below optimum for distortion and use lower-value grid leak resistors to prevent runaway. Perhaps buck it down a bit with two of the 6V windings and gain some safety margin? Soak up a few more volts in a larger cathode bias resistor? Just add a little RC filtering and live with a bit more sag from light to full load? You could do those without buying anything but wire and resistors. Or maybe voltage regulation to make a stiff 300 volts, or bite the bullet for a B+ choke to make the supply a happy and safe 250 volts. That would run everything conservatively. Or perhaps a tube rectifier - one of the 6V windings could power the heater with a resistor to drop it to 5v. That would put the B+ right at 300. Another option would be to buy a cheap power transformer to give right at 300 volts. There are several cheap options for doing this. The one you have is way overkill, because it was designed to run the whole radio, not just the power amp. Then you have a power trafo for the next slightly larger project.
Wimpy? I have been able to get very clean 8W out of EL84 PP triode with Sovteks EL84M. Vintage top brands also can get to that power if in good condition.
Nowadays UL is always my last choice. Pentode with cathode feedback my preferred. In fact I only go with this now and use triode when I don't have the cfb tap.
Have you tried those new replica tubes all over the place the only ones available for cheap?
Coming from russia, china and only god knows where.
Because old, original quad of NOS tubes will cost a fortune though. If were a cheapskate, it wouldn't come to my mind to build with those rare tubes.
Coming from russia, china and only god knows where.
Because old, original quad of NOS tubes will cost a fortune though. If were a cheapskate, it wouldn't come to my mind to build with those rare tubes.
Triode amp always better than simplistic pentode amp. A pentode amp with unregulated g2 supply might be defined magic only in the sense of its vintage sound (with more distortion). The PP pentode will have more distortion at low and medium levels if too simplistic which is were the amp works 99% of the time if the match to the speakers is acceptable.
I think have built a few amps to know that, including 20W class A parallel push-pull with fixed bias which most people would consider difficult/problematic because in their opinion fixed bias would not work well with 4 of these tubes in each channel.....
No need to buy expensive vintage EL84's, you can buy Sovtek's EL84M (6P14P-EB) or if you want to go even cheaper buy something similar (not electrically equivalent) like 6P15P-EB that actually has higher voltage rating.
Vintage: 11GK6 and 16GK6, 11V and 16V versions of 6GK6 an uprated version of EL84 with different pin-out.
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That's not good for pentode as well if high SPL is a target. Not much difference between 8W and 15W in listening mode.....
Agree. Either more sensitive speakers or new listening room
Well the former might be easier.
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