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Old 11th October 2003, 04:10 PM   #1
ed is offline ed  Singapore
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Default Help needed on Mullard EL34PP

Hi,

I have DIYed 2 x EL34PP and both Monoblocs are having problems...

Monoblock 1 - Everything is ok except for sound being soft when power up. All voltage are consistent except for one going to output trans. The plate voltage to one of the optx is around 380v. Whereas the rest of the AN2, G1, G2 and CT are all around 430v. What could be the cause? Tried swopping in other tubes but its still the same. Have almost resoldered all joints but still cannot catch the problem.

Monoblock 2 - Getting a "zzzzzz" sound when power on. This is not the wrong phase hum as I have tried swopping the grids and plate from the power tube to the optx. All voltages look ok as in consistent. I can perpetually feel the power tranny producing the "zzzzz" sound and I am pretty sure thats the same sound I hear on the speaker. Have changed the wires to the power tranny but still the same. Also resoldered all joint. Could this be a case of the power tranny being faulty? If not then what else could be wrong. My filtering caps is 32uf+32uf 450v. When power on I am get a voltage of around 500v from the rectifier, could I have punctured the filtering cap?

Schematics is very much the same as Mullard 5-20

Any helps would be appreciated.

rgds,

ed
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Old 11th October 2003, 04:17 PM   #2
Zen Mod is offline Zen Mod  Serbia
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Default Re: Help needed on Mullard EL34PP

Quote:
Originally posted by ed
Hi,

I have DIYed 2 x EL34PP and both Monoblocs are having problems...

Monoblock 1 - Everything is ok except for sound being soft when power up. All voltage are consistent except for one going to output trans. The plate voltage to one of the optx is around 380v. Whereas the rest of the AN2, G1, G2 and CT are all around 430v. What could be the cause? Tried swopping in other tubes but its still the same. Have almost resoldered all joints but still cannot catch the problem.

Monoblock 2 - Getting a "zzzzzz" sound when power on. This is not the wrong phase hum as I have tried swopping the grids and plate from the power tube to the optx. All voltages look ok as in consistent. I can perpetually feel the power tranny producing the "zzzzz" sound and I am pretty sure thats the same sound I hear on the speaker. Have changed the wires to the power tranny but still the same. Also resoldered all joint. Could this be a case of the power tranny being faulty? If not then what else could be wrong. My filtering caps is 32uf+32uf 450v. When power on I am get a voltage of around 500v from the rectifier, could I have punctured the filtering cap?

Schematics is very much the same as Mullard 5-20

Any helps would be appreciated.

rgds,

ed
Monoblock 1-pull tubes out and measure resistances from CT to both anode taps-they must be in 5% max;
if that is not case-you have problem with OPT
Monoblock 2-did you measure current drawn from power xformer?
looks that you have somewhere excessive current draw,or somewhere bad electrolytic.
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Old 11th October 2003, 05:09 PM   #3
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what do mean by
Whereas the rest of the AN2, G1, G2 and CT are all around 430v. What could be the cause? Tried swopping in other tubes but its still the same. Have almost resoldered all joints but still cannot catch the problem.
what is AN2,G1,G2 and CT
your cap is not design for the higher voltage. It is not right to run the cap close to its maximum surge rating of 500Volts and plus the el34 as one of the slowest turn on rate. Means the B+ is going to be higher than normal design for along time.
The caps could be making the zzz sound try to listen carefully i know mine made that sound even though they were rated very conservatily. But my design did not have the zzz sound from the speakers. you could check out my design at www.geocities.com/nickchua81
how long does this zzz sound last for? Could be your cap running in overvoltage when starts up and then go away after stabilizing.
Or it could be oscillation. Grid resistor to power tubes are good to stablize it or in the worse case use a small cap from grid to ground say 100pf or less to form a RC rolloff with the grid resistor
Your caps could be faulty aready as suggested by the other member and the other cap could possibly go then thus bring both voltage to opt to 380V
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Old 11th October 2003, 06:51 PM   #4
ed is offline ed  Singapore
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Default Re: Re: Help needed on Mullard EL34PP

Monoblock 1 - CT to both anodes measure 104.5 ohms. Does this mean nothing wrong with optx? Then where else?? Scratch head...

Monoblock 2 - Will measure the current drawn. I suspect it the PS cap that is bad.

thks...
ed


Quote:
Originally posted by choky


Monoblock 1-pull tubes out and measure resistances from CT to both anode taps-they must be in 5% max;
if that is not case-you have problem with OPT
Monoblock 2-did you measure current drawn from power xformer?
looks that you have somewhere excessive current draw,or somewhere bad electrolytic.
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Old 11th October 2003, 06:58 PM   #5
ed is offline ed  Singapore
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Hi,

AN1, G1 is the Plate and Grid of the first EL34 and AN2 and G2 is the plate and grid of the 2nd EL34. CT is center tap where the B+ goes to.

I ever read some where that the Power Tranny having a "zzzzz" mean it working very hard. Which I guess there may be leakage some where. Is leakage likely caused by faulty caps?

Will try to get my hands on the filter caps that can handle higher voltages. My current filter caps are rates 450v but my supply is giving out 500v. I suspect that is the likely cause. Wlll change and post results.

thks,

ed


Quote:
Originally posted by NickC
what do mean by
Whereas the rest of the AN2, G1, G2 and CT are all around 430v. What could be the cause? Tried swopping in other tubes but its still the same. Have almost resoldered all joints but still cannot catch the problem.
what is AN2,G1,G2 and CT
your cap is not design for the higher voltage. It is not right to run the cap close to its maximum surge rating of 500Volts and plus the el34 as one of the slowest turn on rate. Means the B+ is going to be higher than normal design for along time.
The caps could be making the zzz sound try to listen carefully i know mine made that sound even though they were rated very conservatily. But my design did not have the zzz sound from the speakers. you could check out my design at www.geocities.com/nickchua81
how long does this zzz sound last for? Could be your cap running in overvoltage when starts up and then go away after stabilizing.
Or it could be oscillation. Grid resistor to power tubes are good to stablize it or in the worse case use a small cap from grid to ground say 100pf or less to form a RC rolloff with the grid resistor
Your caps could be faulty aready as suggested by the other member and the other cap could possibly go then thus bring both voltage to opt to 380V
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Old 12th October 2003, 12:44 AM   #6
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Monoblock 1 - CT to both anodes measure 104.5 ohms. Does this mean nothing wrong with optx? Then where else?? Scratch head...

Thats are reasonable value, opts are fine.

COuld it be rectifier noise zzzz that you are hearing being reflected back into the mains transformer?
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Old 12th October 2003, 03:00 AM   #7
DSummer is offline DSummer  United States
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#1: Check the pass cap to tube 1, also make sure correct connections (components to the socket pins), also correct values.

#2: Change the filter cap (or just add another one in series with it, also add 2 shunt resistors, it's the fastest way to find out is it the problem with filter cap)

PS, for #1, did you check the cathode voltage on AN1? Is it much higher than AN2? If so, 100% sure defective coupling cap! If much lower, than check the cathode resistor/electrolytic cap...
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Old 12th October 2003, 04:21 AM   #8
SY is offline SY  United States
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Disconnect the circuitry from the power supply and see what the power supply rails look like: DC voltage, ripple. If that's OK, start tracing your wiring around the output stage.

Don't run excessively high voltages across supply electrolytics. This can be physically dangerous. They can fail explosively. If you've really got 500V running across 450V caps, STOP and replace them with something with at least 525V ratings before doing even one more step.
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Old 12th October 2003, 06:56 AM   #9
ed is offline ed  Singapore
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Rectifier noise? Not sure of that. Ok when I touch the power tranny I can almost feel that the vibration is the same as the "zzzzzz" that I am hearing from the speakers.

Quote:
Originally posted by NickC
Monoblock 1 - CT to both anodes measure 104.5 ohms. Does this mean nothing wrong with optx? Then where else?? Scratch head...

Thats are reasonable value, opts are fine.

COuld it be rectifier noise zzzz that you are hearing being reflected back into the mains transformer?
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Old 12th October 2003, 07:03 AM   #10
ed is offline ed  Singapore
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#1: Check connection with schematic and its correct. Swop in another coupling cap and still the same. Cathode voltage both at 33v. If no choice I will swop in the optx from #2.

#2: I will change the filter cap. But itst Sunday over here so got to wait till tomorrow. Or may swop in the filter cap from #1.

Advantage of have 2 of the same kind. I would be happy if at least one of the two is working.

Quote:
Originally posted by DSummer
#1: Check the pass cap to tube 1, also make sure correct connections (components to the socket pins), also correct values.

#2: Change the filter cap (or just add another one in series with it, also add 2 shunt resistors, it's the fastest way to find out is it the problem with filter cap)

PS, for #1, did you check the cathode voltage on AN1? Is it much higher than AN2? If so, 100% sure defective coupling cap! If much lower, than check the cathode resistor/electrolytic cap...
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