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Hey guys im doing some research into a Lite iib build and im wondering if there is a big difference in using a 5Y3 rectifier over the normal EZ81. The reason i ask is because i have a Hammond 270EX transofrmer with a 6.3v and and 5v filament taps. Id prefer to keep the rectifier off the same tap as the rest of the tubes so I can use the 5V and a 5Y3 instead.

Any thoughts are welcome

Thanks

The voltage is going to be a lot lower with the 5Y3 under 300v. Since the Hammond's secondary voltage is 275-0-275, I would use a 5ar4 which will be much closer to a EZ81 running on a standard 18watt PT at 290-0-290 .

I would use a 5V4. That will get you very close to an EZ81. A 5Y3 will sag like crazy.

The best option though is to use solid-state diodes with a 130 Ohm sag resistor. Sounds just like an EZ81, costs a lot less, wastes less energy. That's actually what I use in my amps now. 275-0-275, UF4007 diodes, 130R/5W sag resistor.

You won't get much voltage drop if you use one of the new Russian 5Y3GT. My voltages take a big jump up when I plug one of those in. A NOS 5Y3GT will drop another 40 volts in comparison to the Russian stuff. If you really want some voltage drop and sag try one of the old 5Y3s that looks like an old time light bulb. I used 5Y3GTs in several amps but only because of the salvaged PT I was using and also had a good supply of 5Y3s laying around. The EZ81 though is more true to the 18 watt design and would be a better choice.

Freddy wrote:You won't get much voltage drop if you use one of the new Russian 5Y3GT. My voltages take a big jump up when I plug one of those in. A NOS 5Y3GT will drop another 40 volts in comparison to the Russian stuff.

Word is those are actually 5V4s.

You are probably right...I tried three of them and none of them drop much voltage like a NOS 5Y3GT tube would.

I know the mains on the 270EX are 275-0-275 but thats at a 115VAC rating so with my wall voltage of 121 wouldnt that get me pretty close to the stock voltage of 290-0-290? And if this is true should I still use a 5v4 or diodes?

Just build it with the octal rectifier and then you can try a 5v4/5ar4/5y3 or a SS adapter and see which one you like.

ncusack wrote:I know the mains on the 270EX are 275-0-275 but thats at a 115VAC rating so with my wall voltage of 121 wouldnt that get me pretty close to the stock voltage of 290-0-290? And if this is true should I still use a 5v4 or diodes?

If you want to use a tube, I'd start with the 5V4 (or Sovtek 5Y3). It's very close to an EZ81. Solid-state is my personal preference and would also work.

The new Hammonds actually come with a proper 120V tap. There's still a lot of old 115V stock out there though.

Yeah the transformer I have now is an older stock with the 115VAC. Ill grab a 5V4 and try that for this build.

I've built a few 18 watters with Hammond 270FX PT's and a 5Y3.The plate voltage ends up at 303v or close to that.That's using an NOS 5Y3.With a new production 5Y3 it's higher than that.
A 5AR4 bumps it up to 360v or more.I like octal rectifiers better because you can choose between a few different ones and all will provide a different feel and voltage drop.

phsyconoodler wrote:I've built a few 18 watters with Hammond 270FX PT's and a 5Y3.The plate voltage ends up at 303v or close to that.That's using an NOS 5Y3.With a new production 5Y3 it's higher than that.
A 5AR4 bumps it up to 360v or more.I like octal rectifiers better because you can choose between a few different ones and all will provide a different feel and voltage drop.

Excellent point about the swappability.

You might like a 5U4 in there. Little less voltage and a little more sag than the 5AR4/GZ34.

It has been about 10 years since I have done this conversion so I am doing my research from scratch. The PT p/n AO 22805-1 on my AO-35 reads 315-0-315 unloaded with 122vac. Seems like I remember some tweeks to get more gain but maybe that is for another thread. I have always stuck with the 5y3 and had good results.

Your B+ voltage has little effect on gain, but will effect clean headroom and dissipation.
303V is too low for a regular 18W, but should work well for a 13W version, using ECL86/6GW8s for both the PA and PI.
360V is also on the high end of the scale for an 18W amp. Vox and Matchless amps typically run at 360V and they eat their EL84s . So, you should aim for a sweet spot of around 340V

I personally tend to avoid using vacuum-state rectifiers,as they add to the amp cost and complexity, as well as generating unnecessary heat. I typically use UF4007 fast rectifiers,followed by a "sag resistor" of a value close to the equivalent resistance of various rectifier tubes. So you can simulate different tube rectifiers just by swapping sag resistors. The resistor will also help to reduce ripple on the B+ supply, especially if placed between th two sections of a cap can.

For example, the 5Y3 spec sheet says, "Tube drop = 60V @ 125mA" So the equivalent resistance is 60/.125 = 480 ohms
However, the EZ81 spec sheet conveniently gives the value of its "Rser" as 230 ohms.