Need your help: Change from el34 to 6550 ends in a desaster

[Blitz]

Hi,

I am running for some years now the ul40s2 from Menno with the autobias:

Autobios improved

Everything is nice and quiet. I am running this amp in ultralinear.

I have just pulled the el34 and plugged 6550 from svetlana in withou changing anything else.

i got a huge hummmm...music could be played, but the humm was a lot. I plugged in a matched quad of kt88 of the treasury series...same behavior.

This is issue no 1. No idea why this can happen.

Worse, I plugged in the el34, and zhe amp was not quiet anymore. slight hum, but the channel as well is now not as loud as the other anymore (i did this experiment only on one channel). I did not change anything in the autobias module or anything.

What happened here ?

Thanks for your support

[Loren42]

This is like calling a doctor on the phone and saying it hurts here.

You need to get a voltmeter and see what exactly is happening to the bias voltage and what your other voltages are at those tubes.

Be quick and shut the thing down before tubes go into thermal runaway. My guess is that the bias voltage is wrong due to a failed part in the circuit, but don't overlook other possible causes.

You can always look for burned parts, too.

However, fixing these kinds of things requires good detective skills, knowledge of the circuit, and at least a VOM.

[M Gregg]

What?

What makes you think that you can change from one tube to another without a rebias?

It is reliant on some preset value to set the bias on the tube. So my guess is you have possibly damaged the op Tx, or something has been damaged in the cathode circuit..

Ie unless I am missing something there should be a way to set the bias current reference. Has this changed or is it set to high? It would be different for different tubes. So yes it will self adjust for tube ageing however there should be a preset for the circuit to adjust the value for a given tube. Not one value fits all tubes

So in a nutshell how does the circuit know what value to bias the tube at if its set for EL34? can it read the lable on the side of the tube..or is there something I'm missing here?

I have no Idea whats in the circuit so I'll have a guess along similar lines..

I'll give an example..if you change from EL34 to 6550 you will probably need more negative bias voltage to shut the tube down, so looking at your circuit I assume the tubes get a variable negative supply via R13/R20 220k and this is derived via a cathode shunt that is fed to some sort of comparator, can the circuit supply enough neg bias for a 6550..is the preset for the comparator correct it was set for EL34?

Here is another guess..

I see some jumpers that say 0-90V is this the amount of bias available or a setting for the cathode shunt? is it set via the jumpers for getting a more accurate setting of the autobias..ie its easier to set 0.1v on 0-10V than 0-90V...

Regards
M. Gregg

[DUG]

Data sheets here:

Tube DataSheets

[Blitz]

Ok, I found that one of the auto bias circuits for one tube did not work anymore, so one tube got a neg. grid voltage of -80v, while the rest got -35v. the 6550 needs about -45v, so well in the set range of -90v.

the auto bias was set for the el34 to 50ma. sure, the 6550 could get more like 80ma, but besides this, should it not work with 50ma as well ? Not following your point.

The key question though: Why did this all happen at all ? And if I now plug in the next 6550...what will happen than ?

[rmyauck]

Do you have a schem. for the autobias board? It may have a failed part or two.

50mA should be good for the 6550 but the total dissipation really depends on the B+ also, so if you have too high a B+ for 6550's that will cause problems even if the Bias is set lower.

Randy

[M Gregg]

I have seen,

Output tubes flashover and blow cathode resistors apart..if this was to happen, what will happen to the cathode shunt and anything connected to it, if the shunt survives and the board fails. It is also quite common to see Op amps go faulty with the correct voltage out with the wrong polarity..

As mentioned running 6550 at lower idle is OK...however can the board output enough neg bias (Ok we know it should but can it?)to control the tube..is the bias supply on before the B+?


Without a schematic its hard to say what has happened...what protection is there for run away with this circuit? ie what would happen if you lost the supply to the bias board?(fusing for the Output Tx)or does it fail in a safe mode with high bias voltage and shut the tube down?

It would be interesting to see what has failed on the board..

Regards
M. Gregg

[tinitus]

regarding grid connections, are those tubes the same.... 'internally' ?
I think Mr. Menno chose EL34 because of the 'internal difference' (for his new cathode feedback designs)



apart from that, wouldn't EL34 also be able to run on lower trafo impedance load ?
if so, maybe it does, maybe it doesn't

[tinitus]

well, I checked, and 6550 have internal grid connection...EL34 doesn't

[Blitz]

Well...I just changed the svetlana el34 to an canada fuller el34...same phenomen: I get a strong hum...but this time i can go back to the svetlana el34 without an issue. But why do I get the humm ? The svetlana has one pin not even there...but the should a el34 not be a el34 ?