Flash in power tube

[htrboy]

Hello,

I have a scratch built bass amp based on the ampeg B15N circuit with some modifications - the major ones are: I only have one input channel, and I'm using a 5U4GB power tube instead of the 5AR4. My chassis was scavenged from a wurlitzer organ amp, and I'm using it's PT which was meant for a 5U4GB and 6L6GC power tubes. During the build I worked through the numbers from the datasheets for the different power tubes and concluded that the 5U4GB should work fine in my circuit, and I think it has been - I've been using the amp for several weeks now.

The other day, I was playing through the amp, facing so that the amp was in my peripheral vision. All of a sudden there was a bright flash from the amp's direction, accompanied by a very small 'tick' from the speaker. I jumped over to the amp and shut it off, and proceeded to talk myself out of hyperventilating . I'm 90% sure, in hindsight, that the flash came from the power tube.

Today I got up the nerve to put it on the bench and check things out. All the voltages seem to be fine, nothing seems to be smoked or getting too warm, and it's quiet. I plugged in the bass and played a little bit, and it seems normal...

So my question is: What the hell happened??

Thanks for any insights!

[ronman]

sounds like you have a bad power tube .Try a new one and see what happens

[passfan]

You could have too much capacitance on your rectifier tube.. I cooked the life out of one putting too big a cap on it. It worked fine for a little while then went off like a fireworks show. Check the data sheet for the max allowable.

[htrboy]

The filter might be the culprit...

When I researched it, the datasheet I found said 40uF was typical operating value and didn't list a maximum. Now, upon digging a little harder, I see that it looks like 40uF is the max. My filter is a stacked pair of 100uF caps for 50uF (in case anyone's wondering, I am using balancing resistors ).

I have another 100uF cap I can put in the series, or I guess I could buckle down and get a 5AR4 - though that will be a while.

[Firestorm]

A 5U4GB will be just fine with 50uF. The capacitance "limits" we're used to seeing come from "typical operation" figures that keep getting recycled and sometimes mislabelled.

There isn't a "maximum capacitance" figure for any rectifier tube (though there are guidelines). But there is a peak plate current rating and that is the rating you don't want to exceed. Typical operation values will never exceed the maximum, but that doesn't mean they are the limit.

A good "guideline" is 60uF for 5U4GB and GZ34 (some say 80uF for the 5U4GB) and 40uF for 5Y3GT (some say 60uF).

There are a couple of ways to exceed the capacitance "limit" if you should ever need to.

[passfan]

The filter might be the culprit...

When I researched it, the datasheet I found said 40uF was typical operating value and didn't list a maximum. Now, upon digging a little harder, I see that it looks like 40uF is the max. My filter is a stacked pair of 100uF caps for 50uF (in case anyone's wondering, I am using balancing resistors ).

.

When you say stacked you mean in parallel right , and not series. I know it's a dumb question but never assume...

[David Root]

+1 on Firestorm's rectifier cap ratings. I have seen circuits by reputable manufacturers that exceed the RCA book ratings considerably. eg a 5Y3GT with 40uF on it. I've used 33 uF on a 5R4GB which is only rated for 20uF.

So you can cheat a bit, but DON'T go nuts and put 100uF on 'em. Too much peak current.

I also recently saw an audiophile thread, I think it was on diyaudio.com, where one guy recommended using two 5Y3 GTs rather than a higher power rectifier. Said the 5Y3 tone is wayyy better than 5U4GB, 5AR4, etc. This may be an example of what I call the "EJ Effect", but, can anyone else offer anything on that?

[htrboy]

Okay,

They ARE in series - for half the capacitance. So if the tube is fine with 50uF, then I'm thinking that Ronman is correct and the tube is bad...? It's a brand new JJ, which I know doesn't really mean anything (and could be part of the problem in some peoples book).

[paddy]

I agree that you probably have a bad power tube.
Unfortunately, just because the tube is new does not
mean that it is good. I repaired an amp for someone
last week and installed new EL34's. It only lasted a few
days. When they brought it back, there was a magnificent
fireworks show inside one of the tubes. A new pair was installed
and all is well. This happened to me about 6 months ago
with another amp. Unfortunately, they just don't seem to make
'em like they used to!

cheers
Paddy

[passfan]

Okay,

They ARE in series - for half the capacitance. So if the tube is fine with 50uF, then I'm thinking that Ronman is correct and the tube is bad...? It's a brand new JJ, which I know doesn't really mean anything (and could be part of the problem in some peoples book).

You're right, I got it backwards.

[htrboy]

You're right, I got it backwards.

I had to triple check myself

[allende]

htrboy--just came across this thread, and it caught my attention because I'm in the process of building a 5E7-based harp amp using parts salvaged from an old Wurlitzer organ, including the PT and OT. Like yours, the tubes are 5U4GB and 6l6GCs (RCA black plates!). Since I have some 100uf/350v Sprague Atoms on hand, I'm planning on using 2 of these in series (paralleled with 220K resistors) as my reservoir capacitance--it sounds like this is essentially what you did. I know that 40uf is usually given as the max recommended capacitance for the first filter cap with a 5U4GB, but I've also read that this is a conservative rating and 50uf should work ok. How has yours been holding up? Also, mine isn't to the point yet where I can check voltages, I'm curious what sort of b+ voltage I can expect with the Wurlitzer iron and 5U4GB.