Rectifier tube/Power tubes causing fuse to blow?

[MattB930]

My friend's "River City" amp is having some issues.

He said the rectifier tube 'blew' and so he put in an equivalent.
He says the amp was super loud and worked fine and sounded good after that.

He then purchased the correct rectifier tube for the amp and it immediately blew a fuse.

He then went back to the previous rectifier tube that worked and continued using it for what I assume months and yesterday the amp stopped working. Another fuse blew apparently.

So ever since the rectifier tube quit, attempts at installing another rect. tube and fuse haven't solved the problem.

I haven't looked at it yet but this almost leads me to believe its the power tubes. Wouldn't that be the most likely culprit at this point?

Thanks

[Gtr_tech]

Possibly output tubes, but the failed recto tube may have damaged the filter caps if it shoved AC to them....

[lowell]

What kind of rectifier tube and what is the value of the first filter cap that it feeds? It wouldn't be the first time someone designed a tube rectified power supply and used filter caps that place too-big of a load on the rectifier. Just a thought.

[MWJB]

If the amp worked with any of the tube rectifiers, then filter cap size is unlikely to be an issue 5Y3/5U4/5V4/5AR4 all take 100uf.

Prime suspect is power tube idle current (bias current), this needs checking, as do the power tubes.

[lowell]

MWJB it's perfectly reasonable that the amp may work... for a while. The possibility should not be discounted. It sounds to me that it COULD be eating rectifier tubes.

[Enzo]

If it is "eating rectifier tubes" the most likely reason is that a power tube has failed. Bad caps can certainly stress a rectifier, but so can bad tubes and they are many times more common than failed caps.

[MattB930]

Alright. Here are some of my calcs when I got the amp last night..

This amp is a River City Little Boy Blues. Great sounding amp really.

The rectifier is now a 5y3 like it needs to be. Someone had installed a 5AR4 and the plate voltage read 443 V on a 6k6 tube plate!

this amp states that it can take 6k6's in place of the stock config of 6v6's

2 x power tubes, 1 5y3 rect., 2 x 12ax7's

These first measurements were taken with the 5ar4...

Plate voltage= 443v
Cathode voltage= 25.5v
cathode resistor= 253 ohms

25.5v/253= 0.10079
0.10079/2= 0.0504 A or 50.4 mA per tube!!

(50.4/1000) * 417.5v= 21w

The plates were glowing red as one can expect.

I had a 5y3 rectifier laying around, installed it, and measured.

plate voltage= 395v
cathode= 22.4v

22.4/253= 0.08854
0.08854/2= 0.0443

44.3mA per tube
still way too hot.

Installed a cathode resistor equal to 493 ohms after this and got
plate voltage= 422v
cathode= 31.2v
resistor= 493 ohms

31.2v/493= 0.06328

0.06328/2= 0.0316 A or 31.6 mA per tube

Still very hot.

Isn't the "max" plate voltage around 300v? The current is pushing it to I suspect but is certainly better than 50mA like before.

What are your recommendations?

The PT is a Mercury Magnetics upgrade from the factory btw.

[MattB930]

they are 6k6 GT power tubes also.

[lowell]

I've read on this forum that it's generally not the plate voltage that kills the tube, unless it's WAY over spec (200-300v). It's more the current that will do it. Also being cathode biased it's my understanding that current is fairly constant so biasing hotter is generally ok. If you can get the bias down to 29ma that's 90% dissipation at 422 plate volts and should be fine for 6v6. 6K6 GT has almost half that Pa rating at 8.5watts on the Duncan Amps page. That being the case it looks like you may need more of a 20ma setting.