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  #1  
Old 12th February 2021, 11:33 AM
MarkL MarkL is offline
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Default Smoke is never a good sign, WA 88VA

Sorry folks, I never seem to post unless there's a problem!

I had a red hot KT88 valve last year in one position in the right hand channel.

I fixed that with the forum's help - I replaced the pair of valves that side, and replaced the two 470R resistors and the two 47uf capacitors on that side.

Same valve position again, this time not glowing red hot, but smoke coming out of the amp. (But still working!)

The Mills resistor was toasted. Value intact. It was bright sunlight so I couldn't tell if the valve had glowed red hot. I replaced both valves that side, and replaced the 470R resistor.

It was 15mins before I got more smoke. The resistor is burnt, and it's value has changed.

I'm not sure where to look now for the source of the problem.

Any help would be much appreciated
many thanks
Mark
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  #2  
Old 12th February 2021, 01:36 PM
bob orbell bob orbell is offline
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Default Re: Smoke is never a good sign, WA 88VA

Hello Mark, you do not say the time factor between problem one and problem two, however my suspicion would be one of the 47uF 100 volt capacitors, have you installed one of them in backward, or is one of them a bummer ? check and remove and test, if all is good then it may be another bad valve. At the positive side of the capacitor you should have about 40 volts DC with neg connected to earth post. Bob
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  #3  
Old 12th February 2021, 01:44 PM
John Caswell John Caswell is offline
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Default Re: Smoke is never a good sign, WA 88VA

Hi Mark,
Well we have a choice of several things -
1) U/S valves only checked by replacement
2) Leaky coupling capacitor(s) C5 & C6 to the grid(s) of the KT88s - this would cause the grid volts to be positive, overrun the valve, drawing more current, causing the cathode voltage to rise dramatically, and frying the cathode bias resistor and capacitor. It might also damage the Screen grid g2 anti parasitic resistor R19 & R20.
3) A bit more esoteric - a dry/open joint on either of the grid g1 bias resistors R12/R13 & R14/R15 this would cause the same sort of effect as in 2) as the grid has no earth reference. Measurement from g1 to chassis should read 220K + 10K in each case.
Even though the cathode bias resistors read OK when cooked, I would automatically change them along with the bias capacitors.
What I would do is put any old cheap and nasty components back in until you are sure the fault is fixed permanently. Are the cathode bias resistors adequately rated, they need to be at least 6 Watt rating. are the cathode bypass capacitors at least 50V rating.
Check these out then let us know accordingly

John
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  #4  
Old 12th February 2021, 05:59 PM
MarkL MarkL is offline
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Default Re: Smoke is never a good sign, WA 88VA

Thanks John, thanks Bob. I will check all the values and report back. cheers, Mark
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  #5  
Old 14th February 2021, 01:27 PM
MarkL MarkL is offline
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Default Re: Smoke is never a good sign, WA 88VA

So, I have done some testing

John,
C5, C6 measuring OK, as are C7 C8 ( I will replace C8 anyway)
I have replaced the Mills resistors(5W) with Welwyn 10W which were the original components till I swapped them out in the 1st repair
R19, R20 measuring OK, and I have resoldered R12/13/14/15
R12/R13 to the chassis gives 220K, but I can't get areading off R14/R15
I have compared R to L channels and everything seems to match
Could it just be the 5W resistors?
I came by this particular amp on ebay, and it arrived with a bonus pack of some very useful spares. In amongst which were a bag of 470R's and 47uF caps. I wonder if this was a previous problem with the amp?

Bob, are there any other voltages I should check while I'm at it?

with many thanks
Mark
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  #6  
Old 14th February 2021, 02:02 PM
bob orbell bob orbell is offline
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Default Re: Smoke is never a good sign, WA 88VA

A reading though R14/15 to earth, should read 230K, try from pin 5 to pin 8 U3/4 should be 230470 ohms. Voltage at pin 3 408v DC, pin 4 415v DC, pin 8 40v DC. As John has said, R16/17 6Watt min, the 5 watt may have been your problem, 10 watt much better. Bob
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  #7  
Old 14th February 2021, 05:58 PM
MarkL MarkL is offline
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Default Re: Smoke is never a good sign, WA 88VA

Thanks Bob. R14/15 to earth is 220K, measuring 210 across all the 220K resistors + 10K from R14/15.
Pin 5 to 8 is also 220K
Same on both channels
Will report back after changing the caps and checking voltages tomorrow.
Many thanks
M.
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  #8  
Old 15th February 2021, 08:55 AM
Richard Richard is offline
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Default Re: Smoke is never a good sign, WA 88VA

Quote:
Originally Posted by John Caswell View Post
Hi Mark,

2) Leaky coupling capacitor(s) C5 & C6 to the grid(s) of the KT88s - this would cause the grid volts to be positive, overrun the valve, drawing more current, causing the cathode voltage to rise dramatically, and frying the cathode bias resistor and capacitor. It might also damage the Screen grid g2 anti parasitic resistor R19 & R20.


John

Yep that was my first thought especially if they are paper in oil type, swap in some poly plastic types
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  #9  
Old 19th February 2021, 01:15 PM
MarkL MarkL is offline
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Default Re: Smoke is never a good sign, WA 88VA

Have checked the voltages phew! Never really like doing that bit!
I got 423V, 416V and 38V
Is that just about within tolerances?

Richard, thanks for the reply. The caps are the clarity caps that it came with. They were measuring OK. I am presuming they are polyprop. I have some spares. Should I replace them anyway?

On the plus side, no smoke!

thanks guys!
Mark
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  #10  
Old 20th February 2021, 08:49 AM
Richard Richard is offline
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Default Re: Smoke is never a good sign, WA 88VA

Quote:
Originally Posted by MarkL View Post

Richard, thanks for the reply. The caps are the clarity caps that it came with. They were measuring OK. I am presuming they are polyprop. I have some spares. Should I replace them anyway?


Mark

Hi Mark, (I don't want to butt in on you working through John's suggestions checking voltages etc.) Looking at your first post again I would indeed swap out the coupling cap for another poly cap if you have one as they are blocking very high voltage and it's not impossible it is leaking electrically. Also have a look around to see if anything could be shorting/touching in that area of the circuit to cause an intermittent fault
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