Blowing main fuse after 40mins GZ34

[Chubsman]

I have a dual rect tube/ ss . In SS it goes on forever... in JJ tube mode... the mains fuse blows after 40 mins... weird...

[kd]

It will work but, make sure you have plenty of amperage rating on your heater taps. The 5U4GB pull 3 amps where the GZ34 pulls 1.9 amps.

Ken

[tubetek]

Howdy Chubsman-
Are you thinking 5U4 to drop a little more B+ than the 5AR4 or is it just because you have one? (NOS examples ARE a lot cheaper than 5AR4/GZ34s)
If your PT has 3 amps available on the 5 volt winding it'll work just fine. The conversion (voltage drop) factor is approximately 1.3 for the 5AR4 and 1.2 for the 5U4 begause the directly heated cathode (the filament, actually) of the 5U4 is physically farther from the plates than the indirectly heated cathode of the 5AR4. This results in more sag, but only if the current draw is near max.
Hope this helps...
tubetek

[Chubsman]

That helps alot!! The PT is a Heyboer 50W. used in a 18watt preamp, El34 power amp ( ).

The amp has SS and tube rectifier with a DPDT switch that switches the B+ and 1/2 a cap can in and out, so that when in tube rect I get 50Uf filter and when in SS I get 100Uf rect...


The amp runs forever in SS rect... but when switched to tube recto it runs for 30-40mins ... then blows the mains fuse...

It was suggested i replace the 500mA fuse on the HT CT to 2X 350mA fuses on the HT lines.

[PeterS]

If it blows the mains fuse I would say there's two possibilities:
(1) your rectifier tube has an intermittent fault
(2) adding the heater current for the tube rectifier pushes the current draw over the edge, in which case 5U4 will make it worse.

When it runs on SS, is the tube rectifier still there and still heated?

[rjgtr]

Does the PT have a separate fillament supply for the rectifier?

If it doesn't then you're pulling too much current. If it does then I'd look for a wiring issue or cap issue.

[Chubsman]

Ya it's a heyboer with seperate 5v taps.
I have the SS and recto both running all the time. This is the layout I put together. Basicaly, when in tube mode, I switch out 1/2 the cap can, because a GZ34 cannot handle more than 60uF after itself.

Diodes don't care, so I switch in the 2nd cap can in parallel when in diode.

The resistors you will see in the layout are a segestion of KOC.

[PeterS]

Have you tried changing the rectifier tube?

[PeterS]

Have you tried changing the rectifier tube?

[PeterS]

Have you tried changing the rectifier tube?

[Chubsman]

That is my next step... I'm scared I'll fry the PT HT tap....

[Merlinb]

I would temporarily disconnect the 50uF reservoir caps and see if the amp will last longer than 40 mins that way (just leave it running I mean, not actually playing it!) If it does last, then you may need some series limiting resistance to take the strain off the rectifier- they are the most fragile valve in the whole amp, after all.

[tarzanalog]

I hope this isn't too far off topic, but...

I had my original standby switch break on my Ceriatone 36W TMB EF86 head. I had an on-off-on switch handy and put it in as a replacement. Essentially no change to the circuit really, one side of the new switch was just NC.

Last weekend I decided to build a SS rectifier on the unused pins of my tube rectifier socket and then run the resulting B+ to the other side of the standby switch.

I played it at volume for the first time yesterday and switched back and forth between tube and SS recs several times in the space of 10 min. (always waiting a few seconds in standby first) and blew my mains fuse in tube rec mode.

Here's my layout:
http://www.ceriatone.com/images/layoutP ... 111207.jpg

It was the original mains fuse that came with the kit (2A) and I'm in the USA (Iowa) at about 118V AC wall voltage. Should I be using a 3A fuse instead? I have 3A slow-blow fuses on hand, can I use them in place of a fast fuse?

Edit: Do I need extra filtering for the SS rec?

[tarzanalog]

UPDATE: Replaced fuse and blew it right away again in tube rec mode... smelled like the smoke was trying to get out... removed the SS rectifier mod. Learned my lesson: Don't mess with perfection.