Concerned with current dual 5U4's...

[chops]

You may have seen my other thread about my modified Baldwin tube amp. It's an old mono electronic organ amp that was converted over to stereo operation by a gentleman named Will Vincent. He sells many modded Baldwin amps as well as modded Dynaco ST-70 amps.

At any rate, this amp uses two 5U4GB rectifier tubes. I assume it was designed this way to provide ample current and output on demand without "sagging" when full organ was required. It proves to be a very solid design as this amp effortlessly powers my inefficient NHT 2.9's (86dB @ 1w/1m) with ease. Bass attack is extremely tight and punchy. Even my old Technics SU-8099 amp with its 115w per channel didn't provide this kind of controlled bass.

Anyway, I've been doing some brief reading around the forum about the 5U4 tubes and rumor has it they heat up very quick and throw B+ to the output tubes way before they are heated up properly to receive that voltage. Apparently that can cause damage to the output tubes and I'm sure the driver tubes as well.

I also read that the 5U4's are known for sparking or "spark out" as TubeLab calls it. What's curious about his statement ( Rectifier tube 5AR4 vs. 5U4GB? ) in this link is that these 5U4's did this with 47uF caps which is what I'm using in my amp, FOUR of them in fact, all rated at 450v. That really concerns me!

Now I haven't noticed any sparking of any kind when I power up this amp and maybe it doesn't because there's twice as much draw being there's two 5U4's there. However I do notice that the filaments in the 6L6's are glowing within a second or two of flipping the switch.

Should I possibly be looking at replacing these 5U4's with other tubes such as 5AR4's or 5V4G or something like that if this amp allows it?

Should I consider installing a CL150 thermistor?

This amp no longer sees any kind of high demands like it did as an organ amp, but I still require it to provide the strong, tight and punchy deep bass that it has now. I just want it to be as safe as possible and be gentler on all the tubes. And unfortunately, I do not know what the different voltages are in this amp. I guess I'd have to speak with my father and see where I need to probe with my old table top HP DMM to take measurements.

Any info will be greatly appreciated!

[kevinkr]

No, if you aren't having a problem with your 5U4s I would not replace them with something else. In fact all of the arcing problems I have encountered have been in 5AR4, and I have run 5U4 to 800Vdc out without any problems at all.

There shouldn't be a problem with allowing the B+ to come up before all of the other tubes are fully warmed up, cathode stripping with standard receiving tube families is largely a myth. It is a valid concern with high perveance, low mu triodes like the 6AS7G, 6080, 6336 etc, but not with the sorts of tubes found in your organ amp which was designed to be long term reliable with fairly minimal maintenance.

[chops]

So as far as the amp in its current state, it's good to go and the 5U4's won't self destruct or cause damage to the other tubes? That's good news!

I do have to replace the 6L6's though as one of them produces noise.

[tubelab.com]

In fact all of the arcing problems I have encountered have been in 5AR4, and I have run 5U4 to 800Vdc out without any problems at all.

I second that. I have seen a 5U4 spark out. In fact it sparked out almost every time the amp was switched on but is was an old worn out tube. The owner liked the distortion and sustain that the old dead tube gave him in a guitar amp. It lasted like that for about a year. We went through my stash of used TV tubes for some more like it which we found but none of them sparked out.

The 5AR4 is the spark out king. Many of the new production tubes have poor build quality so some will spark out the first time they are used. Most of them are dead the first time they spark.

The 5R4 is another tube prone to arcing. Many people stick them in place of a 5AR4 to get some distortion and sustain, but they can't handle the surge current.

I agree, if your amp works and you like it, done mess with it. The 5AR4 will increase your B+ by 20 volts or more. Todays line voltage is higher than it was when your amp was built so the B+ voltage is already higher than it was meant to be.

[chops]

Originally Posted by tubelab.com

I agree, if your amp works and you like it, done mess with it. The 5AR4 will increase your B+ by 20 volts or more. Todays line voltage is higher than it was when your amp was built so the B+ voltage is already higher than it was meant to be.

Thanks for the great advice.

So answer this for me please... Is it normal for 5U4's to have a little rattle to them when they're running?

The reason I ask is because even these old radio 5U4's that I'm borrowing from my father also have a slight rattle to them. They're nowhere near as bad as the originals that came with the amp, but they still rattle. You have to have your ear within about a foot of the amp to hear them. The original ones you can hear from about 4-5 feet away.


Oh, and what vintage 5U4's would you suggest I go after? They can't be the coke bottle type as I don't think they'll fit. The sockets are pretty close together.

[Eli Duttman]

NOS GE 5U4GBs remain affordable and they are quite good. Only 1 current production 5U4 is (IMO) worth a damn. That variety is the ElectroHarmonix (EH) 5U4GB.

[kevinkr]

I'd look for 5U4GTBs which should fit the limited space available, the older GB are much more common but might not fit. Get extras as they seem noisier to to me than the older GB not all of which are quiet.

The noise you are hearing is most likely due to varying electrostatic stress in the tube - I use these in the regulated power supplies of my stereo 300B SE amplifier and had to select one that was quiet out of a pile on hand here. Internal construction plays a role here as some sang quite loudly, but one I am using is silent even at six inches, it is a very ancient GB, now with a lot of hours as it has been in the amp psu for 6yrs, and this amp gets a lot of use...

[chops]

Do these 5U4's have to be matched pairs? The two I'm using at the moment aren't and the amp sounds just fine.

Also, I've been looking a little on eBay and there seems to be a lot more of the RCA black plates than any GE's. I guess any of the vintage ones are fine?

Sorry for all the rudimentary questions, but I just want to make sure before buying anything.

[kevinkr]

Originally Posted by chops

Do these 5U4's have to be matched pairs? The two I'm using at the moment aren't and the amp sounds just fine.

Also, I've been looking a little on eBay and there seems to be a lot more of the RCA black plates than any GE's. I guess any of the vintage ones are fine?

Sorry for all the rudimentary questions, but I just want to make sure before buying anything.

Unmatched but new should be fine.. Depending on where you are located you might try tube hunting at a local hamfest or two, tubes particularly these types are often very cheap, and most sellers are honest enough to tell you whether or not they are new or used if you cannot tell by looking. Hamfests can be a lot of fun, and a good source of parts, tools, test equipment, and tubes.

[chops]

What's funny is that I shut down the amp an hour ago and just pulled my father's 5U4's back out to put my originals back in. Guess what they are... Old GE 5U4GB's! And I'm pretty sure they're black plates as well. Go figure.

Also, I was looking at the socket spacing and they are the same exact spacing as for the 6L6's, so maybe a pair of coke bottles will fit. My brother has a pair of coke bottles that's not being used at the moment, so I might borrow them to see if they fit.

I went ahead and put the RCA 5751's back in as well as I think they sound a bit more refined over the no-name 12AX7's.