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  #1  
Old 26th July 2013, 02:54 PM
robaustralia robaustralia is offline
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Location: Noosa QLD Australia
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Default KEL84 rebuild low HT voltages

I'm just having a bit of a dilemma. As per the new circuit I upgraded the Diodes to UF5408 from BYV96E.

C1,2,3 were upgraded from 100uF to 220uF a couple of years ago to add weight to bass and this was very effective with my 'flat' response speakers.

However I just noticed that the EL84's melted their sockets so I have replaced the sockets with ceramics again and found a couple of dodgy joints which I have rectified.

I checked all voltages today (with two meters) and the HT voltages are 20% to 30% low across the board. Before the upgrade, they were all very close to specs. Heater voltages are OK. The Power Transformer is outputting 250 volts to the HT rectifier bridge diodes.

The only components that are different are the diodes. The voltage at the choke is only 267 volts instead of 320 volts.

R38 is 242 volts and bias on pin 2 is zero.

Do you think it is possible that the upgraded diodes are causing this? If so I will try changing them back to the originals or is there another fix.

Thanks.
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  #2  
Old 27th July 2013, 12:57 AM
Richard Richard is offline
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Default Re: KEL84 rebuild low HT voltages

Hi Rob, The diode swap should make almost no difference to the HT voltage and if a diode had gone open circuit I'd expect the voltage to be lower still. That leaves us with the possibility of an excess load dragging the supply down. You say bias at pin 2 (EL84?) is zero which is correct so hopefully no coupling caps leaking there. Check the cathode voltages of the EL84 pins 3 to chassis to see if any are much in excess of 11V. Are any of them glowing red and are they old valves?
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  #3  
Old 27th July 2013, 01:40 AM
robaustralia robaustralia is offline
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Default Re: KEL84 rebuild low HT voltages

Thanks Richard.

El84's have been in about two years and used for few hours per week.

All the EL84's are all showing some burning on the corners of the top element support. (see pic)

The cathode voltages are all around 8.3 volts which is way down.

I am using a 5 ohm 5 watt dummy load on speaker posts.

Are there any voltages I can check with valves removed that might indicate something?

Thanks,
Cheers, Rob
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  #4  
Old 27th July 2013, 09:50 AM
John Caswell John Caswell is offline
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Default Re: KEL84 rebuild low HT voltages

Hi Rob
Following on from Richard's comments I would suspect that there is a wiring error somewhere in your replacement diodes or maybe a track break on the PCB. It is almost as if there is no connection between the diodes bridge output and the smoothing capacitor(s) for the HT to be that low.
The fact that all the voltages on the EL84s are very similar tends to rule out a faulty valve, as, as Richard says, the suspect valve would most likely be glowing red.

John
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  #5  
Old 27th July 2013, 10:56 AM
Richard Richard is offline
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Default Re: KEL84 rebuild low HT voltages

Quote:
Originally Posted by John Caswell View Post
Hi Rob
Following on from Richard's comments I would suspect that there is a wiring error somewhere in your replacement diodes or maybe a track break on the PCB. It is almost as if there is no connection between the diodes bridge output and the smoothing capacitor(s) for the HT to be that low.
The fact that all the voltages on the EL84s are very similar tends to rule out a faulty valve, as, as Richard says, the suspect valve would most likely be glowing red.

John
Agreed, 8.3V would be in line with expectations with the lower HT, suggesting the circ is working fine, so go over the new diode installation with a magnifying glass and meter checking for breaks, or maybe install a new set of diodes if you feel any may have been damaged or tracks broken when removing the old ones.
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  #6  
Old 27th July 2013, 01:11 PM
robaustralia robaustralia is offline
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Default Re: KEL84 rebuild low HT voltages

Thanks Richard and John.

The diodes were a tight fit in the new PCB so I'll start focusing on that area.

I've taken the valves out and checked that components on each side are close in value with power off. They all seem close.

Cheers,
Rob
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  #7  
Old 28th July 2013, 10:07 AM
robaustralia robaustralia is offline
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Default Re: KEL84 rebuild low HT voltages

I have checked diodes and made sure the soldering flowed nicely and can't find any damage to the tracks.

I am using the latest PCB which is much improved on the original one. All components were replaced one by one to make sure I didn't put anything in the wrong place.

I just did a series of tests with all valves removed and R38 had 345 volts without a load and other HT voltages were in proportion. So I assume the power supply is ok.

I have tested the EL84m valves for shorts and efficiency. They were all good but HT voltages dropped again when valves were put back in again, so it seems that something is breaking down under load.

I also tested a new set of JJ EL84's valves which I were only used for 10 minutes because one had a hotspot. There is no hotspot now, so I assume there may have been a bad joint that affected the bias before I did the rebuild. HT voltages are even lower with these valves.

Do you have any suggestions as to what might cause an excessive load on the power supply?

Thanks again.

Cheers,
Rob
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Kel84 Amplifier
Transcendent Sound OTL 'Son of Beast' Amplifier
TD160 with Ortophon 2M Blue
Australian VAF DCX Gen IV 'time aligned' tower speakers
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  #8  
Old 28th July 2013, 03:59 PM
John Caswell John Caswell is offline
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Default Re: KEL84 rebuild low HT voltages

Hi Rob,
With the amp turned off measure the DC resistance of the choke and note down.
Then turn the amp on and measure the voltage drop across the choke ie from C1+ve to C2+ve then using ohms law calculate the current, or just put your DMM, set to say 2 Amp range, in series with the choke to give you a direct reading.
If the amp is working OK I would expect a reading in the region of 160mA ie 4X the current drawn by the EL84s at approx 40mA each plus a few mA for the ECF80. You should also measure the voltage drop across R1, 2K2 and calculate the current drawn here by the ECF80 should be about 11mA)
If it is significantly lower than this (I suspect so) then the HT is low due to some problem with the HT circuitry. If significantly higher then there is a fault on the HT line or excessive current drawn by something.
I am suspicious that you may have fitted a diode in the wrong way around or the track to C1 is broken or something similar.
As it was working OK before you fitted the diodes then it must be something that you have or haven't done.

John
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  #9  
Old 29th July 2013, 12:44 AM
robaustralia robaustralia is offline
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Default Re: KEL84 rebuild low HT voltages

Thank you John.

I really appreciate your and Richard's advice which is helping me understand how my amplifier works. The picture is starting to take shape.

The problem I'm having is educating me to the real magic of valve amps which I had never explored.

Cheers,
Rob
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Kel84 Amplifier
Transcendent Sound OTL 'Son of Beast' Amplifier
TD160 with Ortophon 2M Blue
Australian VAF DCX Gen IV 'time aligned' tower speakers
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  #10  
Old 29th July 2013, 01:58 AM
robaustralia robaustralia is offline
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Default Re: KEL84 rebuild low HT voltages

Hi John.

Yes the current appears to be way down. So I will pull our the PCB again and also check components around the ECF80's.

CHOKE
27 ohms
Voltage drop 265v - 261v = 4 volts
Current .148 A

R1
2.2k
Voltage drop 261-239 = 22 volts
Current .0100 A

I have tested ECF80's on valve tester and they are in good health.

Cheers,
Rob
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Kel84 Amplifier
Transcendent Sound OTL 'Son of Beast' Amplifier
TD160 with Ortophon 2M Blue
Australian VAF DCX Gen IV 'time aligned' tower speakers
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