Is a 5U4GB ok to replace a 5AR4/GZ34

[davygrvy]

My nearly 3 year old J/J GZ34 started arcing and took the fuse yesterday. Local guitar shop had only a 5U4GB. The GZ34 is indirectly heated, but the 5U4GB is directly heated. I assume the 5U4 is faster to start, but can't find any ratings that say so. Bad for the EL34s? From the charts, looks like the 5U4 might have a higher B+, but can't say for sure. I'm using the triode electronics supply transformer (PA060) which has a 4A rated 5V heater winding, so I'm OK for it drawing a bit more filament @ 3A compared to 1.9A for the GZ34.

Should I get another GZ34?

[Ty_Bower]

5U4 is faster to start. It'll be harder on the power supply caps. The 5U4 will be hot before the power tubes conduct, and the B+ will surge every startup. The 5U4 takes 3 amps of heater current, but you're already on top of that. The 5U4 will drop more voltage, resulting in a lower B+ (after the power tubes are hot).

I'd use it in a pinch if there were no other options. If I had upgraded power supply caps (like a CChong or SDS board) I'd probably use it indefinitely. If you've never replaced the stock power supply can cap, it's probably time to do that anyway.

[Geek]

The original Dynaco assembly manual mentiones a 5U4 as an OK option, but expect reduced power.

I'm actually having to do this too in my rebuild, because all the GZ34's I have are junk

The voltage drop won't be too much of a problem with the old transformer, since line voltages are quite a bit higher nowadays... they'll almost cancel each other out

Cheers!

[davygrvy]

Thanks guys. New GZ34 will get ordered. What's the quality one these days? I'm finding lots of complaints of arcing on the new productions with most recommending NOS Mullards for around $120. Ack!

[Geek]

Every GZ34 I popped in has flashed, as well as a GE 5U4. I have a Sovtek 5U4 in there now and no plasma globe

I think I'll try more Sovtek stuff...

Cheers!

[burnedfingers]

I am using a 5U4 with a damper tube in my Dynaco stereo 70 and it works fine. I have the cap board that mounts under the output transformer so I removed the original cap and added a socket there for the damper tube. Another thought might be to go with SS diodes in place of the 5AR4 and install a damper tube where the 5AR4 was. You will have to make sure the voltage rating of your cap/s is high enough. Another thought might be to add diodes before your 5AR4 to take the strain off the 5AR4. I have this mod in several Dynaco 70's I own and haven't gone thru the rectifier tube since I put them in.

Stay away from the JJ 5AR4 rectifiers as they just don't hold up. There are other rectifier tubes such as Sovtek that work great without blowing $120 on an expensive un needed tube.

[Ty_Bower]

Another thought might be to add diodes before your 5AR4 to take the strain off the 5AR4. I have this mod in several Dynaco 70's I own and haven't gone thru the rectifier tube since I put them in.

Excellent advice. I've heard more than one person recommend this easy mod, and their experience is the same as yours (no more blown rectifiers). There's a good diagram explaining how to do it in this linked thread:
http://www.diyaudio.com/forums/showthre ... ost1481237

[Geek]

Great tip, thanks!

[davygrvy]

6bg6ga, what's a damper tube? A delay start?

[Ty_Bower]

what's a damper tube? A delay start?

It's a vacuum tube rectifier (diode), usually only one in the bottle. If you wanted to build a full wave rectifier out of them, you would likely need two. Many are heated with 6.3 volt filaments. They were commonly used for TVs in the horizontal output stage. That circuit was high voltage, high current service.

Damper diodes typically have nice, long warmup times and can handle lots of current. Their problem is that you'll need two to replace a GZ34, and lots of extra 6.3 volt to feed them. The solution is to use a pair of solid state diodes to build the full wave rectifier, and stick one damper diode in series with the rectified DC (after the solid state diodes). That way you only need one damper diode, but you still get the slow warmup time.

Here's a typical damper diode:
http://tdsl.duncanamps.com/show.php?des=6ax4gt

[davygrvy]

Food for thought! I like the idea. Playing with PSUD right now. 1N4007s in front of the 5AR4/GZ34 sounds like a more simple great idea as well. I was just looking at the 6AX4 from a google search on the term 'damper tube' but I didn't understand the purpose until you explained it.

[Pyre]

A 6W4-GTA works great as well with a little less voltage drop 21V@250ma you can get them by the box load cheap.

http://tdsl.duncanamps.com/link.php?target=00256217

[burnedfingers]

I used a 6DW4 damper tube in mine.

[TerrySmith]

Here's how I fixed mine:

http://diytube.com/phpBB2/viewtopic.php?t=1433

Look at page 2.

[davygrvy]

7 seconds to 508V at the second cap (B+) with a 5U4GB from a full cold start. Ordered another J/J GZ34 and some 4007s to put in series with it. Looks like it takes 30 seconds for the EL34s to draw proper bias current. About 14 seconds for them starting to draw. 7 and 14 don't match.

I'm glad I asked about this.