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Help for a novice!

79.4.157.15
Posted on October 30, 2009 at 07:59:41
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007



Hi all. I'm a novice in tube amp DIY.
I did replace a couple of E182CC (V3a and V3b) in my DIY mono tube amps (V2 = 2A3, V4 and V5 = 845).
I did not find any E182CC around over the Internet and so I bought a pair of 5687, being the electrical parameters the same.
Some time later I did notice a faint noise coming from the speaker resembling something sizzling in the background...
I opened the amp and the voltage divider (R8, R16) just before the 5687 cathodes was fried (even if the measured resistance was still ok).
Moreover in one channel (after replacing the divider) I heard a distortion (with volume knob at noon). Any suggestions? The diagram is attached. How many volt should I expect on the V3 cathodes to be ok? Thanks in advance.

RE: Help for a novice!, posted on November 2, 2009 at 10:01:08
Michael Koster
Industry Professional

Posts: 585
Location: Bridgeville, CA
Joined: October 24, 2007
 How have you handled elevating the filament voltage for V3 so as not to exceed the +/- 90 volts 5687 max filament to cathode voltage? I would think one would want a separate filament transformer and have it swing with the cathode.

edit:

V2 and V3 make a SRPP circuit. The center point where the output is taken can be around 1/2 B+ which I guess in your amp is about 1100V so 650V is in range. The circuit works by developing drive voltage for the top element (the 5687) across R8 and R17 in proportion to the output current which in your amp is minimal. The 5687 is really functioning more as an active load for the 2A3 in this case.

Michael

PS be thankful for that shock. It was a wake-up call

RE: Help for a novice!, posted on November 1, 2009 at 21:19:06
casouza
Audiophile

Posts: 485
Joined: March 18, 2008
 Over-simplifying, this seems like a parallel single ended amplifier driven by two complicated 5687 cathode followers with a DHT tube as a variable cathode load and and upper tube grid driver...kinda different SRPP????.
Anyway, if R8 and R17 were fried, they must be replaced by resistors with a higher power rating.
Mills 12W is my favorite resistor for cathode folower and plate positions, specially with 5687, since those tubes draw lots of power.
After you install the new parts, measure all the DC voltages under no-signal condition (cathodes, grids and plates), then keep the chart for future reference in case the amp fails again.
I do this every time I buid something new or tweak an existing circuit. It is much better than shooting in the dark if something goes wrong.
Good luck
RE: Help for a novice!, posted on November 2, 2009 at 00:26:10
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007
 Thanks everybody!
Now, after a lot of handling, I got the problem.
Notwithstanding being the fried resistors replaced, a premature clipping problem persisted on the right channel.
So I measured V3 cathodes voltages:

V3 cathodes (left channel) +680V
V3 cathodes (right channel) +118V (!!!)

I checked again and again the circuit (above all around V3) and everything appeared to be ok (sohb..)
The 845 bias voltage were correct (-145V) and the V2 anode also (+52V) on both channels.
Moreover I did notice that while on the working channel the capacitors discharges after 10/15 minutes on the right channel I got a shock and after that I measured the voltage on the V3 cathodes ---> about 650V !!!!!
Using the tester that voltage was rapidly dropping so I discharged the capacitors using the tester itself and started again working...
I cannot understand what's happening!!!!
Capacitors seem ok, resistances also.
The only thing I did not check again were filaments...
Could be a wrong filament voltage?? Maybe 2A3 filament???
HELP!
RE: Help for a novice!, posted on November 2, 2009 at 09:07:21
casouza
Audiophile

Posts: 485
Joined: March 18, 2008
 Hi
Premature clipping is usually a driver problem, most likely insufficient B+, a weak tube or a bias problem.
Your amp's driver is the 2A3/5687 stacked combo.

Since the wrong voltage is common to both resistors in the bad channel, I suspect the 2A3, it sets the grid and cathode bias for the 5687.

I believe you will find what's wrong by comparing left channel to right channel methodically, from B+ all the way down to the 2A3 cathode, as follows:

- DCV from GND on top of R11 (junction with C4)
- DCV from GND on bottom of R11

- DCV from GND on bottom of R13 (2a3 plate)
- DCV from GND on top of R8 and R17 (both cathodes of V3, pins 3 and 6 )

- DCV from GND on top of the 2A3 bias resistor, R16
- 2A3 DC or AC filamente voltage end-to-end (pins 1 and 4);

We need all measurements for both channels (12 readings) in order to help.

I suggest that you Copy my post and Paste your readings, this will make it easier for the smart fellows here to understand the circuit and point what's wrong.
Good luck

RE: Help for a novice!, posted on November 5, 2009 at 01:18:33
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007
 And here are measurements:

- DCV from GND on top of R11 (junction with C4) +862V (L) +44V (R)
- DCV from GND on bottom of R11 +690V (L) +45V (R)

- DCV from GND on bottom of R13 (2a3 plate) +325V (L) +53V (R)
- DCV from GND on top of R8 and R17 (both cathodes of V3, pins 3 and 6 )
+755V (L) +122V (R)
+755V (L) +120V (R)
- DCV from GND on top of the 2A3 bias resistor, R16
+58V (L) +9V (R)
- 2A3 DC or AC filamente voltage end-to-end (pins 1 and 4);
+2.7V (L) +2.7V (R)

Moreover:

- 845 grids -148V, -153V (L)
-148V, -148V (R)
- 845 filaments +9.7V (L) +9.8V (R)


I tested +Vdc on the bad channel and is > 1000V (my tester is out of range, max accepted is 1000V).
How can be +Vdc > +1000V and voltage from GND on top of R11 (junction with C4) only +45V???????????
Could be a problem with the bypass capacitor C10?
When amp is turned on the V3 cathodes voltages are around +120V.
When I switch off the amp I measure about +700V (!) rapidly discharging with the tester impedance...
I do not understand. Connections seem ok...
Thanks for help.

RE: Help for a novice!, posted on November 5, 2009 at 08:21:15
casouza
Audiophile

Posts: 485
Joined: March 18, 2008
 Hi
I suggest that you look how the +VDC or B+ connection is made to both channels. There is a chance of a bad solder joint.
Since your DMM does not measure above 1000 VDC, I suggest that you connect the DMM common lead to the left channel on top of R10 and the positive lead to right channel on top of R10 and measure DCV at the maximum range. It should read zero volts.
A DCV difference means trouble, either a bad connection from + VDC to the driver stage or a bad +Vdc supply, if there is one separate supply for each channel.

If everything is OK, I suggest that you power down the amp, wait 10 minutes, remove the tubes and measure all the resistors from top to bottom in the driver circuit, R10 all the way down to R16 and compare channels.
Then compare to the parts list.

I believe one or two resistors are bad or wrong value in the bad channel, because R11 has the same voltage on top and bottom (~44V), meaning that the driver circuit (including the 2A3) is not dropping voltage, which under Ohm's law means it is not passing current (not-operational).

Also, the 2A3 filament voltage is slightly high. I believe it will be OK when the right channel 2A3 starts to conduct. Otherwise, you will need to install a series resistor to drop it to 2.5 V. But that is something for later....
Let us know
RE: Help for a novice!, posted on November 6, 2009 at 01:16:59
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007
 Et voila! R10 was burned out! Resistance > 1.3Mohm!
Yesterday night (CET) I replaced it and here are new measurements:

- DCV from GND on top of R11 (junction with C4) +862V (L) +868V (R)
- DCV from GND on bottom of R11 +690V (L) +698V (R)

- DCV from GND on bottom of R13 (2a3 plate) +325V (L) +332V (R)
- DCV from GND on top of R8 and R17 (both cathodes of V3, pins 3 and 6 )
+755V (L) +762V (R)
+755V (L) +762V (R)
- DCV from GND on top of the 2A3 bias resistor, R16
+58V (L) +58V (R)
- 2A3 DC or AC filamente voltage end-to-end (pins 1 and 4);
+2.7V (L) +2.7V (R)


Everything seems ok now BUT:

I was wondering if tubes may have suffered during this wrong biasing situation (I do not know how long it was lasting because the problem was audible only at loud volume).
After measurement I closed the amps and after few seconds from switching on, I heard a dreadful noise (not audible before) coming up from the right channel (the one not working before), something mechanical, a sort of vibration during about 2 secs, perhaps from tubes.
I took away tubes and swapped them and everything seems ok now.
As the original schematic forsees the usage of a E182CC (7119) instead of a 5687, may the 5687 damage other tubes?
Where to find over the Internet a couple of good paired E182CC, seeing that the ones I already bought (on e-bay) went bad after a while?
I thank all of you! Your suggestions drove me on the right way...

RE: Help for a novice!, posted on November 6, 2009 at 08:35:07
casouza
Audiophile

Posts: 485
Joined: March 18, 2008
 AHA, I suspected R10!
Do not worry about the tubes, they were actually under-stressed at such low voltages.

I have recently purchased the tubesyou need at www.tubesandmore.com, an AA sponsor.
They have a good brand in stock and may match the tubes for a fee. I know for sure that they match power tubes.
Enjoy your amp!

RE: Help for a novice!, posted on November 2, 2009 at 09:22:59
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007
 Ok. Thank you again.
It will take a couple of days to get all measurements, most likely I'll reply on thursday...
RE: Help for a novice!, posted on October 30, 2009 at 09:08:54
Palustris
Audiophile

Posts: 251
Location: Cape Cod
Joined: September 12, 2008
 voltage divider (R8, R16)

These aren't a voltage divider. They are the cathode resistors of the 2A3 and the half the 5687. No doubt R16 is going to pass a lot of current and R8 will pass half of the current of R16. You give no values to these resistors but I am guessing that R11 and R13 are just a voltage divider to bias the 5687s and pass little current. If you intended to say R8 and R17 they, they are the cathode resistors and they have acted as fuses probably as a result of the tubes passing too much current. Anyway, what are the values of the resistors and what is the wattage rating? Maybe they are just not the correct wattage. Voltages readings at various points would help too.

This seems like a very ambitious undertaking for a beginner with very dangerous voltages on the output tubes, no doubt. I wouldn't want to get anywhere near the prototype with the power supply energized.

RE: Help for a novice!, posted on October 30, 2009 at 09:30:05
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007



Here is the list of the components...
I know it is possibly very dangerous. And I pay much attention every time I work on it!
Yes, I was intending R8 and R17!

RE: Help for a novice!, posted on October 30, 2009 at 11:06:23
Palustris
Audiophile

Posts: 251
Location: Cape Cod
Joined: September 12, 2008
 This is not really something that one can diagnose from afar. If this were on my bench, I would check all the voltages looking for any that seemed incorrect. Any voltages that are incorrect will point to the component that is not functioning properly.

What is the voltage across R16? That, using ohms law will give us the current passing through the 2A3 and the two 5687s. Then measure the voltages across R8 and R17. Using ohms law will give us the current through each half of the 5687.

The current through the 2A3 should be about the same as the current passing through both halves of the 5687. The voltages dropped by R8 and R17 should be the same indicating that they are passing the same amount of current.

That's where I would start.

RE: Help for a novice!, posted on November 2, 2009 at 02:55:49
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007
 Thanks everybody!
Now, after a lot of handling, I got the problem.
Notwithstanding being the fried resistors replaced, a premature clipping problem persisted on the right channel.
So I measured V3 cathodes voltages:

V3 cathodes (left channel) +680V
V3 cathodes (right channel) +118V (!!!)

I checked again and again the circuit (above all around V3) and everything appeared to be ok (sohb..)
The 845 bias voltage were correct (-145V) and the V2 anode also (+52V) on both channels.
Moreover I did notice that while on the working channel the capacitors discharges after 10/15 minutes on the right channel I got a shock and after that I measured the voltage on the V3 cathodes ---> about 650V !!!!!
Using the tester that voltage was rapidly dropping so I discharged the capacitors using the tester itself and started again working...
I cannot understand what's happening!!!!
Capacitors seem ok, resistances also.
The only thing I did not check again were filaments...
Could be a wrong filament voltage?? Maybe 2A3 filament???
HELP!

RE: Help for a novice!, posted on November 2, 2009 at 08:49:26
Palustris
Audiophile

Posts: 251
Location: Cape Cod
Joined: September 12, 2008
 I suggested that you check the voltages ACROSS the resistors R8 and R17. That will tell you what the current is passing through each tube half. they should be the same.

When you say right channel and left channel are you talking about V3a and V3b or do you actually have two prototypes built?

RE: Help for a novice!, posted on November 2, 2009 at 08:54:30
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007
 I do currently have two monoblocks.
I will check voltages through R8 and R17 (R16 also, as you suggested) and let you know. Thanks again...

RE: Help for a novice!, posted on October 30, 2009 at 09:50:17
FenderLover
Audiophile

Posts: 3309
Joined: July 31, 2007
Contributor
  Since: May 17, 2009
 Data I could find seems that the 5687 draws more heater current than the E182CC (900mA versus 650mA at 6.3VAC). This shouldn't make a difference has long as the tranny isn't borderline, heater current draw. Sounds more like the 5687 went bad (shorted out) on you. You have access to a tube tester?

RE: Help for a novice!, posted on October 30, 2009 at 10:12:17
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007
 nope. I haven't one. But I already swapped the tubes from one channel to the other and nothing happened (the problem remains in the right channel only). I believe it is not a tube problem...
Moreover I hear a distortion in the right channel when using a 1KHz tone at 0Db (volume knob at 1/3). In the other channel seems ok.
What about trying to follow the signal from the input to see where it goes bad?

RE: Help for a novice!, posted on October 30, 2009 at 13:11:00
Triode_Kingdom
Audiophile

Posts: 623
Location: Travelin' Man
Joined: September 24, 2006
 I would compare voltages between the two channels. That should find the culprit quickly.
RE: Help for a novice!, posted on October 30, 2009 at 10:27:35
FenderLover
Audiophile

Posts: 3309
Joined: July 31, 2007
Contributor
  Since: May 17, 2009
 Have you tried subbing out other tubes (besides the E182CC/5687), in the right channel? Yeah, looks like you will need to go from section to section (input to output or vice-versa) on that bad channel to narrow down the problem. May have popped another resistor or cap when the cathode-tied ones to the 5687 went bad. Something shorted out those resistors or caused an excess current draw which caused them to pop the first time.

Hmm ..., posted on October 30, 2009 at 08:28:51
mikeyb
Audiophile

Posts: 1284
Location: Minnesota
Joined: November 8, 2002
 From where did this schematic come? I have never seen a configuration like this. V1B is kind of like a constant current source, but I can't calaculate the grid to cathode voltage and therefore, cannot calculate the current through the tube.

RE: Hmm ..., posted on October 30, 2009 at 08:39:53
845cmax
Audiophile

Posts: 81
Joined: August 7, 2007



It is an original project by Fulvio Chiappetta from Italy (he is the director of "Costruire HIFI" DIY magazine).
A project similar to this was published on the magazine and then reviewed by Fulvio later on (adding the 2A3 driver).
Here is my prototypal version...

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