KiT 6550 ECF 80 Pin 6 Voltage

[TimN]

Thanks John,

That reinforces my suspicions. I find working with shielded cabling it a bit fiddly around those areas.

[TimN]

Originally Posted by bob orbell

Tim, I had an issue with my KAT6550 years ago, it was standard trim and I never had any thoughts of changing any capacitors, I still do not subscibe to these fancy capacitors as do a few on here, but, I had a small hum from the word go and joined the forum for help, I was told that the value of C5 (120uF 500volt) had been reduced to 82uF, I did this and the hum was no longer presant, not saying this is your problem, but . As for burning in . Bob

Bob,

If you don't subscribe to fancy capacitors, how about other fancy components?

Do you think spending out on fancy valves makes a difference to the sound? I see there are some very expensive ones.

Or how about fancy resistors? Do they make a difference?

[Richard]

A few easy checks you can make Tim,

With the amp not powered up, put a meter probe on valve pin 6 and the other probe on one end of anode resistors R15/16 to check for 100K (one end will show 0 and the other end the resistor value).

With a meter probe on valve pin 7 and the other probe on signal ground check the value of cathode resistors R45/46 is 560R.

These 2 resistors affect the current through the pentode section of the valve and hence what voltage is dropped by the anode resistor to give the voltage at pin 6.

With the amp powered up, you can check voltages at pins 6 and 7 when idling, then change the valves L to R to see if the voltages follow the valves (faulty valves) or stay with the channel (faulty amp).

With it powered up also check the voltages of both channels at pin 2 to signal ground. With no signal input this should be 0V. (A voltage here will change the flow of current through the valve.)

If you are in doubt about your ECF80s I can check them for you, or I have many NOS Mullard ones and can supply a matched pair which pass similar current on the AVO test settings, send me a PM.

[bob orbell]

A little off topic but yes, I have more natural and detailed sound from fancy resistors for the likes of Charcroft Z foil, Dale WWA26 wire wound resistors than any fancy capacitor, my Phono III has 10 Charcroft and 4 Dale resistors, plus two fuly regulated PSU's for both Pre and Phono, the Charctofts alone were £160.00. Sadly, not enough DIY's pay much attention to the most important part of their hi-fi, the power supply. Bob

[TimN]

Originally Posted by Richard

A few easy checks you can make Tim,

With the amp not powered up, put a meter probe on valve pin 6 and the other probe on one end of anode resistors R15/16 to check for 100K (one end will show 0 and the other end the resistor value).

With a meter probe on valve pin 7 and the other probe on signal ground check the value of cathode resistors R45/46 is 560R.

These 2 resistors affect the current through the pentode section of the valve and hence what voltage is dropped by the anode resistor to give the voltage at pin 6.

With the amp powered up, you can check voltages at pins 6 and 7 when idling, then change the valves L to R to see if the voltages follow the valves (faulty valves) or stay with the channel (faulty amp).

With it powered up also check the voltages of both channels at pin 2 to signal ground. With no signal input this should be 0V. (A voltage here will change the flow of current through the valve.)

If you are in doubt about your ECF80s I can check them for you, or I have many NOS Mullard ones and can supply a matched pair which pass similar current on the AVO test settings, send me a PM.

Thanks Richard,

When I have it back together again I will see what's what.

[TimN]

Originally Posted by Richard

A few easy checks you can make Tim,

With the amp not powered up, put a meter probe on valve pin 6 and the other probe on one end of anode resistors R15/16 to check for 100K (one end will show 0 and the other end the resistor value).

With a meter probe on valve pin 7 and the other probe on signal ground check the value of cathode resistors R45/46 is 560R.

These 2 resistors affect the current through the pentode section of the valve and hence what voltage is dropped by the anode resistor to give the voltage at pin 6.

With the amp powered up, you can check voltages at pins 6 and 7 when idling, then change the valves L to R to see if the voltages follow the valves (faulty valves) or stay with the channel (faulty amp).

With it powered up also check the voltages of both channels at pin 2 to signal ground. With no signal input this should be 0V. (A voltage here will change the flow of current through the valve.)

If you are in doubt about your ECF80s I can check them for you, or I have many NOS Mullard ones and can supply a matched pair which pass similar current on the AVO test settings, send me a PM.

Hi Richard

After having to rebuild the power supply tag board after watching the rectifier go up in smoke, I have finally got it back together again and been able to do some testing.

I have checked the resistance values of R15/16 and R45/46 as you suggested and all is good.

However, the voltage imbalance is still present at pin 6 and also at pin 1 to a lesser extent. As you suggested, I tried swapping the valves around and the imbalance followed, so faulty valves.

I also have the original set of ECF80s which I rebuilt the amp with. Strangely, the same fault manifests itself with those, with the fault following when I swap them over.

You offered to supply a matched set of NOS Mullards. Can I take you up on your offer, please? I will PM you.

[Richard]

Hi Tim, ah, well it seems you may be making progress.
As John has said some ECF80 do show various voltages so it may be that although yours are out of the nominal 10 or 15% tolerance they make be working fine.
That said, your voltages follow the valves so I'm happy to check and send you 2 which draw similar current on the AVO and have sent a PM if you want to go ahead

[Richard]

Got the PM thanks Tim and will sort the valves

For the interest of anyone following, if we look back at Tim's first post in this thread it is the voltages at pins 6, the ECF80 pentode sections, which are out of tolerance.

I checked 10 NOS Mullards and they varied considerably. Characteristic value for the pentode section is 7mV and these varied from 10mA to 5mA. The triode sections and all the slopes were very close to spec. So a variation in pin 6 anode voltages might be expected if the pentode sections are drawing different currents. Given a few valves to try without a valve tester, it should be possible to compare them in the amp at idle by checking their cathode voltages as John has detailed elsewhere.

John's post in the faq though (see posts by Mark and Greg) suggests this should not be a problem in practice and the voltages may make little differerence to the performance.

[Mark]

Originally Posted by Richard

Got the PM thanks Tim and will sort the valves

For the interest of anyone following, if we look back at Tim's first post in this thread it is the voltages at pins 6, the ECF80 pentode sections, which are out of tolerance.

I checked 10 NOS Mullards and they varied considerably. Characteristic value for the pentode section is 7mV and these varied from 10mA to 5mA. The triode sections and all the slopes were very close to spec. So a variation in pin 6 anode voltages might be expected if the pentode sections are drawing different currents. Given a few valves to try without a valve tester, it should be possible to compare them in the amp at idle by checking their cathode voltages as John has detailed elsewhere.

John's post in the faq though (see posts by Mark and Greg) suggests this should not be a problem in practice and the voltages may make little differerence to the performance.

Richard,

I have attached my measurements for EI, Edicron and Mullard ECF80s from 2007 for my KEL84 build. IIRC the Mullards came from you - thanks again. Pin 6 voltages were as much as 50% over the target voltage. Some of the voltages on the other pins were just as bad. However, as already mentioned John Caswell checked the performance and it was fine.

Mark

[TimN]

Thanks very much for your help.

I changed out R13/14 as R13 was open circuit.

I changed out R19/20 as R19 measured 180 Ohms rather than 100.

I changed out R13/14 while I was there.

I put the valves back in and started the testing again. Everything right in the middle of the tolerance band for all ECF80 valve pairs I have except the Mullard gold pin pair (eBay) I had been using. One was consistently low on Pin 6.

All other voltages comfortably in tolerance.

I plugged some speakers in for a switch on, gleeful at the prospect of having my valve amp back for the first time since April, only to find a significant hum coming from both channels. It gets louder with the volume control cranked right up, particularly the right channel.

I am now about ready to throw it out of the window.

One of the choke terminals fell off during my recent tinkerings. I have soldered it back on as best I can within the plastic moulding it has broken inside of. The choke still measures 2.5H dead, but could this be the source of the humming when under power?

I can only think to go through it to tidy up the cabling and earth connections, but can anybody point me towards a testing process to try to focus in on the source of the problem?

After that, the only thing left will be to remove and renovate the signal boards to better access what is behind them, which I would rather avoid if possible. I have already rebuilt the power supply board as the terminals on the tag board had started to detach and the board delaminate.

I have attached a couple of photos for your amusement. Any help will be much appreciated.

Thanks,

Tim