Testing tubes

[pacomus]

I have a ST-35 european version (220V) tube rectified for about a year. My version includes a remote control with a digital circuit feed by its own switched power supply.
You can see some photographs at

http://roble.pntic.mec.es/fmor0026/pages/st35_1.php

(Use next link called "Siguiente" to go through the pages, sorry it´s not in English)

My ST-35 that I have enjoyed for about a year, has operated without problems. But some days ago, when I was deligted with Radiohead, I heard a scratch and the fuse (1A) blows up.
I have change the fuse many times with the same result: 30-50 seconds after I swith it on, a scratch and the fuse again blows up.
Some people say that in a tube amp, many times the problems come from the tubes but, could be this case?
I would like to know how could I test the tubes and if it's possible to switch on the amp without all the tubes.

[EWBrown]

First thing, with the power off and unplugged, and the capacitors all totally discharged, try the resistance theck as described in the assembly instructions, this will show if any of teh electrolytic or coupling caps have gone bad, look for unusually low readings.

If all that looks good then:

Two simple tests, and then a couple more:

First, pull out the rectifier tube, and check if the fuse still blows. This isolates everything in the B+ stream, but still lights the six tubes' filaments. If the fuse pops then most likely the power transformer may have a problem - the tube filaments draw a combined 22 VA @ 6.3VAC / 3.64 A. which is only about 1/10 if the 1A fuse's capacity.

The rectifier tube takes another 10 VA (5V / 2A) approximately. Total of 32VA filament consumption with all tubes inserted.

If the fuse survives, then replace the rectifier tube and pull out the other six tubes, and see if the fuse blows. If it does, then check the PSU's electrolytic capacitors, look for low resistance measurements, and bloated or leaking caps.

Third, replace the 12AX7 and 12AU7 (ECC83 and ECC82) and one channel's 6BQ5 / EL84s , aand check to see if the fuse survives.

If it does, then Last, replace the other set of 6BQ5s. and see what happens.

NB: If one or more of the cathode bypass caps (470 uF / 35V) is bad / shorted, it would draw the cathode voltage down towards zero volts, which would increase the plate and cathode current rather dramatically, and this could pop the fuse.

Last but not least

Possibly the 1 amp fuse is too close to the "edge" and perhaps a slow-blow 1A or a 1.25 to 1.5A fast blow could be substituted.

HTH

/ed B in NH

[erichayes]

If you're using a 5AR4/GZ34 rectifier (as it appears you are), you should keep a spare on hand just on general principle. The odds of it failing are much greater than those of the other tubes. My high school music teacher back in the '60s would replace the 5AR4 in his Stereo 70 every six months, even if it was still working, and give the "duds" to me. All Amperex. If I'd known then what I know now . . .

[pacomus]

Ok, mothing better to do at the weekend.

the only doulbt I have is how can I discharge the capacitors in a safety way.

[Shannon Parks]

Ok, mothing better to do at the weekend.

the only doulbt I have is how can I discharge the capacitors in a safety way.

There is a bleeder resistor in the Rev D. I think I sized it to bleed it off within 5 minutes, a UL spec I think. You can just check from the power resistors to a ground lug with a DMM for a safety check.

Indeed, sounds like that rectifier is 'crapping out' once a working current is established.

[pacomus]

EWBrown wrote:
First thing, with the power off and unplugged, and the capacitors all totally discharged, try the resistance theck as described in the assembly instructions, this will show if any of teh electrolytic or coupling caps have gone bad, look for unusually low readings.



I don't know if all my readings are correct, because I have found some diferences with the assembly instructions:

J3 pin 3: 20.8K
J4 pin 3: 21.6K
j6 pin 1: 1.4M
j7 pin 1: 1.4M

[Shannon Parks]

I don't know if all my readings are correct, because I have found some diferences with the assembly instructions:

J3 pin 3: 20.8K
J4 pin 3: 21.6K
j6 pin 1: 1.4M
j7 pin 1: 1.4M

These numbers are normal (assuming you are using an 8 ohm tap for the feedback). You can continue with the tests Ed outlined and report back.

[DerekVa]

Ok, mothing better to do at the weekend.

the only doulbt I have is how can I discharge the capacitors in a safety way.

There is a bleeder resistor in the Rev D. I think I sized it to bleed it off within 5 minutes, a UL spec I think. You can just check from the power resistors to a ground lug with a DMM for a safety check.

Would that be R29 (300k @ 1W)?

-D

[pacomus]

I have recovered this old post because my problems with the ST-35 have come back from the dark side.

I completed all the tests EWBrown posted and I found no problem at all. i suspect now that the problem could be caused by a loose in the socket pins.

Another year past and now the problem come back again. After a 15-30 seconds, a scratch in the GZ34 and the fuse pops.

I have repeated EWBrown tests and those are the results:

-I pull out the rectifier tube (six tubes inserted) -> Ok, no problem
-Insert GZ34 and pull out the six other tubes -> Ok, no problem
-Insert GZ34, 12AX7, 12AU7 and 6BQ5's left channel -> Ok, no problem
-Insert GZ34, 12AX7, 12AU7 and 6BQ5's right channel -> pops!

The problem seems to be in the 6BQ5's rigth channel, sure?

I bought 4 matched 6BQ5 but I need to adjust bias.

Any idea?

[EWBrown]

The GZ34 / 5AR4 in an ST35 runs about half the current as in an ST-70 so the tube should last a good long time, though I would still avoid the "Sino" brand 5AR4s, they are marginal even at half-duty loading... The SOcvtek, J/J and of course, NOS rectifiers will be quite happy in the ST35.

Since it has been running well for a while, I would presume that all connections are properly and correctly made. Look for any suspicious looking solder connections around the tube sockets, and reflow the solder as needed. To repeat Doc Bottlehead's main bit of advice to amp builders, "about 90% of most tube amp problems are caused by bad solder joints".

If the problem persists:

It could be that your bias current is set too high - it should be around 35 mA for each 6BQ5. The VA and PI tubes consume another 10-15 mA so the total B+ idling (no signal input) current should be about 150 mA and no more than 160 mA.

Going back to square one, and prior to powering it up again, check the total resistance from each EL84 cathode (pin 3) to the power "ground" on the PC board, and preset / adjust them to 400 ohms each, if needed. Actually anywhere between 390 and 430 ohms is OK, just set them all to the same resistance. This will get you "close enough" to 35 mA, for most practical purposes. The current will drift somewhat after a few hours' use, anyway, and will need further re-adjustment until they settle down and stay constant.


If the fuze still blows, then:

A further test would be to swap the known good 6BQ5/EL84 pair from the left ato right channel, (leave the left channel "empty" for this test). tubes can, and do go bad...

This should prove or disprove that the right channel has a problem.

Check the wiring connections from the right channel OPT to the terminal connections, you may have a couple of the OPT primary leads swapped (I've dome that myself, twice). In one case the amp ran OK but sounded a bit weird, the other case the fuse blew after a few seconds aftrer warmup.

HTH

/ed B