Vanishing Voltage

[Lectroid]

I recently built a Plexi-lite 9W amp using 6BM8s but then I decided to rewire the old chassis into an 18W Marshall-ish amp with minimal surgery.  Schematic below.

When I tried to measure B+ today, it won't stabilize--the voltage comes up, then immediately drops away from its high point, falling steadily at about 10V/second.  When it fell below 300V I shut it off because I didn't and don't understand why it's happening.

Can anyone enlighten me?  I'm baffled, like I'm forgetting something really obvious.

Update:
I just tried changing out the 5R4 for a new one, and suddenly the amp voltages are stable and well-behaved.     I'm getting 325V for the EL84 plates and 317V for the screens.  Can a bad rectifier have caused this?

Anyway, I'm back in a ballpark I recognize now and I'm proceeding with testing.  I'm sure I will have another question or two.

[shooter]

tube amps are high voltage, low current, anything that's a short, partial short, will suck the volts down, the faster they go, the closer to a short you have.

[Lectroid]

tube amps are high voltage, low current, anything that's a short, partial short, will suck the volts down, the faster they go, the closer to a short you have.

Thanks.  All of that makes perfect sense.  I was stumped, but then remembered how many here have often said to "change the tubes first" when debugging weird problems.  I'm glad I listened for a change.   

[sluckey]

tube amps are high voltage, low current, anything that's a short, partial short, will suck the volts down, the faster they go, the closer to a short you have.

Not always a voltage sucking short. Sometimes the rectifier tube just rolls over and quits puttin' out.

[PRR]

Everything is a voltage divider. You can make the top go open or the bottom go short.

[Lectroid]

Everything is a voltage divider. You can make the top go open or the bottom go short.

I know that must impart some amp wisdom just because you said it.  Pithy. It reminds me of a Zen koan.  At the moment I don't get it.  It'll probably keep me awake at night until I figure it out.   

[shooter]

It'll probably keep me awake

before the internet, some idea or thought would get hung up, 3am I'd search the encyclopedia's, dictionaries, school books, trying to "find the answer".
when I couldn't, I'd be sitting outside the public library waiting for the doors to open 

[Lectroid]

It'll probably keep me awake

before the internet, some idea or thought would get hung up, 3am I'd search the encyclopedia's, dictionaries, school books, trying to "find the answer".
when I couldn't, I'd be sitting outside the public library waiting for the doors to open 

I understand that feeling.  Ignorance gnaws at me until I find an answer.   Woke up at 4:30 this morning thinking about voltages. This was supposed to be a fun hobby...     

[sluckey]

Everything is a voltage divider. You can make the top go open or the bottom go short.

I know that must impart some amp wisdom just because you said it.  Pithy. It reminds me of a Zen koan.  At the moment I don't get it.  It'll probably keep me awake at night until I figure it out.   

In your particular example, the 5R4 is the top part of the voltage divider and a filter cap is the bottom part of the divider. So, if the tube (top part) goes dead (open) you will lose the B+. Or if the filter cap (bottom part) shorts you will lose B+. Makes perfect sense, in an odd kind of way.   

[Lectroid]

sluckey,

Thanks for explaining where that particular voltage divider was.  I can apply Ohm's law once I know what we're talking about.   

Everything is a voltage divider.

Now I get this.  It was pithy.  I have much to think about.

[HotBluePlates]

Everything is a voltage divider. You can make the top go open or the bottom go short.

I know that must impart some amp wisdom just because you said it.  Pithy. It reminds me of a Zen koan.  At the moment I don't get it.  It'll probably keep me awake at night until I figure it out.   

I over-explain.  Some listeners are offended by this, because they feel the explanation insults their intelligence by covering things they already knew.

PRR pays you the compliment of assuming you grasp the all implications of what he says.

[PRR]

> the compliment of assuming you grasp the all implications

No, that "you CAN grasp the all implications, in time."

[HotBluePlates]

> the compliment of assuming you grasp the all implications

No, that "you CAN grasp the all implications, in time."

Funny, because I took an embarrassingly-long time to grasp some implications that later seemed like they should have been "obvious."

[Lectroid]

PRR pays you the compliment of assuming you grasp the all implications of what he says.

Yes, I take it as a compliment. (Unwarranted in this case    ) And I enjoy the puzzles I don't understand because I always learn something from them.  There is method in his method.

I can see the tension between not answering the question at all, and explaining it  in enough depth to someone you've never met.  It's a fine line not to say too much sometimes. Personally, I like a lot of detail.

[Dave]

I consider myself pretty good at figuring this stuff out, despite the fact that I have zero training. I proudly measure my abilities by the fact that people bring amps to me because I can almost always fix them after others, with proper training, have given up on them, or "fixed" them only to find out that they were not "fixed".


Lengthy explanations give me a headache, but I enjoy learning theory to back up the senses that I rely too heavily upon.


Dave

[shooter]

 
I had to sit in all the classes, pass all the tests, everyone else is on chapter 2, i'm hung up on the 1st paragraph of chapter 1.
once we got released into the wild, I was fixing problem #3 while the smart kids were arguing over which book for problem #1, I'm like "book?, they gave us sweet ask test equipment, just poke it here, there, everywhere:-)"

[Lectroid]

Thanks to everyone who helped get this amp off life support.  Once I got the amp working, I swapped in different rectifier tubes, trying to get the B+ down below 350.  The 5Y3 dropped the B+ down into good range, e.g.: 327 on the power tube plates.  This seems right in a happy spot for 6BQ5s without pushing them to the limit.  I set the bias to 93% of max dissipation.

Since this my first amp with 6BQ5/EL84s, I have a few questions.  I've uploaded a new schematic with voltages for plates and cathodes.

1.  I can't find it now but am sure I read that a cathode-biased p-p amp can be biased at 100%.  Was I dreaming this or is it correct?

2.  I think the 5Y3 can supply this amp without undue strain.  Is that correct?

3.  The screens are only 2-3 volts less than the plates.  Should I keep the choke between A and B, or replace it with a dropping resistor to get a larger spread between plate and screen voltages?   

4.  The reverb driver plate voltage is ~325V.  Is that high enough?  I know some Fenders run that tube up around 420V.

5.  Finally, can someone please tell me which resistor(s) to measure in order to calc to the cathode voltage on the  LTP?

And thanks again for all the help!

[sluckey]

1. correct
2. correct
3.Don't worry. Be happy.
4. Yes
5. Just measure it.

[Lectroid]

sluckey,
Thank you.  Just what I needed. 

3.Don't worry. Be happy.

   

Everyone, thanks again for all the help.