octal rectifiers in 18watt

[okstrat]

Ok, I built my 18watt with an octal rectifier socket because my power tranformer had a 5v tap and I have a full shoebox of good used and NOS octal rectifier tubes. I've done some research, and the 5Y3GT rectifier is shown to not be good over 20uf or so of filtering. I have a 50/50 for my first can cap... will this just kill the 5Y3GT quicker, damage my amp or create an impressive fireworks display?

I put in a GZ34 and a 5U4GB with ok results, but I would like to sag it up a bit - and resorting to the resistor trick kinda defeats the purpose of using a rectifier tube. Any advice for other octal rectifier tubes to play with? FYI, a few stats on my amp:

50/50/22 filtering
355v at the EL84 plates w/the 5U4GB.

Pete

[daschnoz]

The filter cap size issue is due to the inrush current when the standby switch is flipped to "go". If the filter cap is too large, the rectifier will arc internally.

You can give the 50uF cap a try with the 5Y3. Worst case, the tube arcs a bit when you flip the switch. It may also have a shorter life due to the arcing. I don't think the rest of the amp will know the difference.

[s2]

Is this less critical in an amp without a standby switch? I see a lot of old Fenders recapped with 22uF or 33uF and it doesn't seem to be a problem for them.

[EBE]

I have 4 amps out being used extensively with 5V4s on GDS/Heybour PTs and no problem reports.

I haven't tried a 5Y3GT but I have one in my parts bin. I might try it in my current build I am working on.

Norm

[okstrat]

Thanks for the replies guys. I have a bunch of 5Y3GTs, so if I smoke one or two I'm not going to cry about it. I'd rather run the amp with one because it will cut down more on my B+ and also isn't as stout, so I'd figure on maybe getting a little more sagalicious tone with the amp cranked. The Mullard GZ34 I tried in there was pretty stiff.

Pete

[EBE]

I finished another tremolo combo this week so when I was running some voltage checks I compared the 5Y3GT to the russian 5V4 which I have been using. I have a NOS RCA 5Y3GT and it dropped the voltage another 15 volts or so and worked fine. BUT it did give a small blue flash when you come off of standby. The 5V4 does not flash. Also, I thought I would try a different rectifier in my 36 watter so I put in a Ruby 5U4G that I have had laying around for years. The amp immediately began to crackle. I put a 5V4 back in and the crackling went away.

[novemberrain]

I have a NOS RCA 5Y3GT and it dropped the voltage another 15 volts or so and worked fine. BUT it did give a small blue flash when you come off of standby. The 5V4 does not flash. ... Ruby 5U4G began to crackle.

Regarding the 5Y3: Referring to my good old RCA RC19 and RC30 tube manuals... they specify to keep the first filter cap down to 10 microfarads in the RC19, the stats have been updated in the RC30 to 20 microfarads for low voltage applications. Which doesn't sound like a lot...

Apparently the larger filter cap drops the load impedance on the rectifier to the point that it arcs on startup, thus the blue flash.

I took a quick look at some vintage stuff I have around, one phonograph amp has a 40/30/20 can (stock) with the first filter being the 40 microfarad section, no standby switch. As a rule of thumb if I see something that was common practice in 1958 I expect we can get away with it in 2005, all else being equal. Also note that late Vibro Champs have something like a 40/30/20 Mallory can (NOT what is shown on the schematic!) in there, no standby and they seem to work o.k..

Ruby tubes are another re-label venture, I've had misfortune with some of 'em. Apparently they test fine back at Ruby's shop although they may not function well in all applications. NOS 5U4s are by no means a rare tube, IMHO there's no reason NOT to use NOS here!