Kit 6550 resistor failures and voltage irregularities

[Revoli]

Originally Posted by bob orbell

My thoughts on the first post, John suggests C11-12-13 and14 should be checked and I agree if they are paper in oil, but C15-16-17 and 18 have not been mentioned could be the problem. Bob

C15, 16, 17, 18 replaced July

[Revoli]

Originally Posted by A Stuart

Is this just a typing error saying pin 8 is no connection?
Three out of three sites I have just looked at say pin 6 is the one with no connection.
This site https://frank.pocnet.net/sheets/049/6/6550.pdf describes pin 1 as "Base Sleeve" connection.

EDIT - Sorry, on re-reading your post, ignore the above comment. My best guess is that pin 1 could be connected to 8 just to "earth" it to the cathode, so that it is not floating uncontrolled.

And, don't forget, John pointed out there is something seriously amiss with the heater voltage pins 2-7, assuming 2.6 volts is a true reading and you didn't inadvertently have a range-doubling function on your meter.

Think meter good. Agree those voltages a concern. Testing following.....

[Revoli]

Originally Posted by John Caswell

Puff of smoke from heater bridge rectifier implies that either the an ECF80 valve heater is s/c (unlikely) or the output smoothing capacitor is s/c (more likely) A KBU04 is fine
Test without valves first of all to make sure that you have roughly the correct ac and dc voltages at the right points. They will all be high because no current is being drawn.
Then fit the 5U4 rectifier valves and check that you have (high) dc HT. If all is well fit the 6550 and ECF80 power up again and measure all the necessary voltages. The 6550 voltages should be very near to that stated but the ECF 80 voltages are likely to be all over the place - see my FAQ re ECF80.
Once again any problems report back here

John

I did check valves visually and multimeter for obvious shorts but that is all. New valves now obtained.

Now with new bridge rectifier replacing diodes. No valves.

Tested at V3 pin 4 and 5: -2.95v ; 2.95v
Tested at V4 pin 4 and 5: -2.95v ; 2.95v
THIS WORRIES ME. WAS EXPECTING NEARER 6.3 OR HIGHER GIVEN JOHNS COMMENTS.

Given these readings I will pause testing and await any responses you kind soul care to provide. In particular how to safely proceed from here. I dont particularly want to destroy my nice new valves....

Glenfidich calls.

footnote: I have this morning tried replacing KBU4D bridge rectifier and C6 (4700uF 10V) with ones previously removed which still appear sound. Now tests at 3.05v coming out of bridge. What am I missing???

[Richard]

Originally Posted by Revoli

Thanks all for suggections. I will work through and record here as I address each. May not be in order....

Firstly valves. Too old possibly. I have bought new and will proceed with those once other suggested tests complete.

Tests. Without valves first:

Transformers without diodes/bridge rect (ie removed completely)
mains trans black wires: 380v; and 380v
purple wires: (v2, pin 2) 4.53v; (V2 pin 8 0v)
grey wires: 2.87v; 2.87v

These are lower than schematic but incoming mains measuring 197v which I guess would account for the difference? (mains 82% lower, if adjust numbers by same factor looks better but guess this isnt a linear relationship....?)

Please shout if any of that is of concern.

Hi, there will be a linear drop with a drop in mains so those voltages look fine.

[Richard]

Originally Posted by Revoli

I did check valves visually and multimeter for obvious shorts but that is all. New valves now obtained.

Now with new bridge rectifier replacing diodes. No valves.

Tested at V3 pin 4 and 5: -2.95v ; 2.95v
Tested at V4 pin 4 and 5: -2.95v ; 2.95v
THIS WORRIES ME. WAS EXPECTING NEARER 6.3 OR HIGHER GIVEN JOHNS COMMENTS.

Given these readings I will pause testing and await any responses you kind soul care to provide. In particular how to safely proceed from here. I dont particularly want to destroy my nice new valves....

Glenfidich calls.

footnote: I have this morning tried replacing KBU4D bridge rectifier and C6 (4700uF 10V) with ones previously removed which still appear sound. Now tests at 3.05v coming out of bridge. What am I missing???

Something not right, are you checking across pin 4 to 5, or 4 to ground and 5 to ground? Also check back at the new bridge rect for AC V into the rect and DC V out of the rect.

[Revoli]

Muppet with a multimeter. Just shows don't play with big volts when tired.

Having now tested correctly V1 pin 4 to pin 5 and V2 same 6.06Vdc

Complete set of etsts done. I now have a couple that are slightly outside 15% tolerance. Should I be concerned? They are:

LC v4 pin 6 reading 147Vdc (ok ish) should be 130Vdc
RC V3 pin 6 reading 151Vdc should be 130Vdc

and
LC V6 pin 3 560Vdc. should be 445Vdc

Other than that new valves and various bits and bobs seem to have got it working again but will not put through serious work until I get a view on these rogue voltages.

Many thanks

[Richard]

Originally Posted by Revoli

Muppet with a multimeter. Just shows don't play with big volts when tired.

Having now tested correctly V1 pin 4 to pin 5 and V2 same 6.06Vdc

Complete set of etsts done. I now have a couple that are slightly outside 15% tolerance. Should I be concerned? They are:

LC v4 pin 6 reading 147Vdc (ok ish) should be 130Vdc
RC V3 pin 6 reading 151Vdc should be 130Vdc

and
LC V6 pin 3 560Vdc. should be 445Vdc

Other than that new valves and various bits and bobs seem to have got it working again but will not put through serious work until I get a view on these rogue voltages.

Many thanks

That's mostly good news! V3/4 readings should be fine but just check what you got at their cathodes pin7. Should be around 1.75V which would indicate the amount of current flowing through the valve is correct.


V6 is more concerning. Again, check all the cathode pin8 voltages for approx 44V to indicate the current. 560V at the anode is far too high as you will have realised. Try moving the valve to another socket and see if the voltages follow with it or stay with the socket.

[bob orbell]

Been watching this thread, a lot of voltages are confusing, for example, post 14, black wire 380 black 380, grey 2.87 2.87, and now HT 560 with only 197 mains ? Bob

[Richard]

To expand a bit, the high anode voltage will be due to no current being drawn by that valve.

If the voltage moves with the valve suspect a bad valve. If it stays with the socket check for a dry joint in the cathode circuit.

(And don't be fooled if the amp still plays music as it will still sound fine at low levels with just one valve in the channel.)

[Revoli]

Originally Posted by Richard

That's mostly good news! V3/4 readings should be fine but just check what you got at their cathodes pin7. Should be around 1.75V which would indicate the amount of current flowing through the valve is correct.


V6 is more concerning. Again, check all the cathode pin8 voltages for approx 44V to indicate the current. 560V at the anode is far too high as you will have realised. Try moving the valve to another socket and see if the voltages follow with it or stay with the socket.

I have just spotted the speaker cable was loose on the left channel. Tightened and now my voltages look much better. Am I safe to assume this was my dry joint?

Cant get format right in this table. Test result is last figure, expected result first figure.

Location Expected Test result
02/02/2022
V1 and V2 across
pins 4 and 5 (no valves) 8.4Vdc
LC - V4 pin 1 to earth 125Vdc 124.5Vdc
RC - V3 pin 1 to earth 125Vdc 129.5Vdc
LC - V4 pin 2 to earth 0Vdc 0Vdc
RC - V3 pin 2 to earth 0Vdc 0Vdc
LC - V4 pin 3 to earth 95Vdc 87.1Vdc
RC - V3 pin 3 to earth 95Vdc 93.6Vdc
LC - V4 pin 6 to earth 130Vdc 147Vdc
RC - V3 pin 6 to earth 130Vdc 151Vdc
LC - V4 pin 7 to earth 1.6Vdc 1.6Vdc
RC - V3 pin 7 to earth 1.6Vdc 1.6Vdc
LC - V4 pin 8 to earth 6.5Vdc 6.6Vdc
RC - V3 pin 8 to earth 6.5Vdc 6.5Vdc
LC - V6 pin 8 to earth 42Vdc 43Vdc
RC - V5 pin 8 to earth 42Vdc 41Vdc
LC - V8 pin 8 to earth 42Vdc 41.8Vdc
RC - V7 pin 8 to earth 42Vdc 42.2Vdc
LC -V6 pin 3 to earth 445Vdc 466Vdc
LC - V8 pin 3 to earth 445Vdc 466Vdc
RC - V5 pin 3 to earth 445Vdc 472Vdc
RC - V7 pin 3 to earth 445Vdc 467Vdc
LC - V6 pin 4 to earth 435Vdc 454Vdc
LC - V8 pin 4 to earth 435Vdc 455Vdc
RC - V5 pin 4 to earth 435Vdc 472Vdc
RC - V7 pin 4 to earth 435Vdc 456Vdc
V1 and V2 across pins
4 and 5 (with valves) 6.3Vdc 6.06Vdc
V5 - pin 2 to pin 7 6.3Vac 5.7Vac
V6 - pin 2 to pin 7 6.3Vac 5.7Vac
V7 - pin 2 to pin 7 6.3Vac 5.7Vac
V8 - pin 2 to pin 7 6.3Vac 5.7Vac
Choke 1 pin 1 HT 450Vdc 477Vdc
Choke 2 pin 2 465Vdc 485Vdc
Positive terminal of C5 360Vdc 382Vdc