Unstable Bias, blowing fuse

[MWJB]

Pins 2 to 8 (rather than 2 or 8 to ground), set to volts AC, best to use clipsa/wire grabbers. On a large amp like this, it should take a 3A rectifier, but better safe than sorry.

[OC75]

Switched out the 5AR4 for the 5U4gb. Voltage stayed the same at 4.8. The plate voltage did correspondingly decrease to 511. That allowed the bias current to go down to a low of 39mv. I set the bias at 45.5, which is a little less than 70%.

So, I plugged in a played a bit incrementally increasing volume.With the volume on approx 5, the current went up as high as 127mv when I dug into the strings pretty hard. It did not blow a fuse, but as soon as the current got that high I switched to standby, as I am down to 3 fuses left until I make it to rat shack.

I've never played while watching the bias current before. IS that normal to climb that high?


EDIT:

So, I tried it again...
I got the volume up to 10, amp sounded great. While playing on 10, the bias current was in the 180 mv area. Then one of the kt88's ( I had my bias probe on the other one), started flashing blue and loud buzzing noise in sync with the flashes...I quickly hit the standby.

Didn't pop a fuse, nor kill the tube.

Any more Ideas??

[MWJB]

Are these KT88s new? The tube might be dying if not exactly dead? You don't have any bear trap retainers, or spring clips that could be grounding the metal band at the tube base? Your voltages are not out of line for use with sturdy 6L6/5881 WXT, either of these will reduce current draw.

"That allowed the bias current to go down to a low of 39mv. I set the bias at 45.5, which is a little less than 70%." This perplexes me a little, your plate current was 47.7mA, you wanted to try a cooler current, then you had 39mA...which you then dialled back up to 45mA - back where you started? Forget "70%"...it's a red herring, your fuses are blowing because of a big current demand, so reduce current draw.

Rectifier winding at 4.8VAC sounds low-ish, what AC voltage do you get from pin 2 to 7 on the power tubes?

[OC75]

Are these KT88s new? The tube might be dying if not exactly dead? You don't have any bear trap retainers, or spring clips that could be grounding the metal band at the tube base? Your voltages are not out of line for use with sturdy 6L6/5881 WXT, either of these will reduce current draw.

"That allowed the bias current to go down to a low of 39mv. I set the bias at 45.5, which is a little less than 70%." This perplexes me a little, your plate current was 47.7mA, you wanted to try a cooler current, then you had 39mA...which you then dialled back up to 45mA - back where you started? Forget "70%"...it's a red herring, your fuses are blowing because of a big current demand, so reduce current draw.

Rectifier winding at 4.8VAC sounds low-ish, what AC voltage do you get from pin 2 to 7 on the power tubes?

Supposedly new KT 88's. As I was told. I suppose they have been taking a beating with all of the high current?

After reading that back, it perplexes me too. SO I went back and set at lowest point of approx 38mv. After playing for awhile at all volumes, the bias has settled at about 45mv. Plate Voltage is 502.
The amp sounded better until volume 8 and above where it started a rhythmic "thumping" or "knocking " sound. Lower the volume below 8 it goes away.

Again, THANKS for all of your help. And please pardon my newbishness here. I took on this amp as a learning experience and I am learning with every reply. I can't thank you enough.

OC.

[MWJB]

If the flashing blue coincides with the thumping/knocking (sounds like "motorboating", often a layout/grounding issue) and is just the gas in tube flashing, rather than sparks/arcs, the flashing may not be the primary issue, just a side-effect? 25W dissipation shouldn't be troubling the KT88s, unless they are past it, or the cheapy low voltage Chinese tubes that GT used to rcommend for <450vdc operation.

[OC75]

The one instance was definitely sparking inside the tube.they haven't done that again. The tubes are Sovtek.

While making the "motorboat" sound, the bias shoots up very high,there is a corresponding blue flash inside one tube, and my meter maxed at 136mv. Also the plate voltage drops to around 468-470 volts while making the same sound, again this is while the volume is near full, at 9 or 10.

[MWJB]

The Sovteks should be pretty robust.

Bias will rise with motorboating, as will the voltage drop...it will whenever it is making any kind of sound, even if not music. 136mV isn't particularly high under signal with KT88.

Do you really need to run the volume at over 8? If the fuse has stopped blowing, you may have sorted the firat issue, only to find that you now have a seperate issue...if all your grounds are good, it might even be a feature of that amp? Are the tube sockets in good order, no arcs, good tension on the tube pins?

[Rhodesplyr]

The factory-standard bias setting for a Dynaco MkIII with KT88s was 70mA per tube. You tested for it by measuring 1.56V (same as a fresh battery) across a 11.2 Ohm resistor that was common to both KT88 cathodes. Sunn left this resistor out. 1.56V divided by 11.2 Ohms gave you 139.3mA for the pair. Of course, Dynaco was known for running output tubes on the hot side.

With that bias and 117VAC, B+ supply to the center tap of the output transformer is *supposed* to be 480V.

The Dynaco MkII, predecessor to the MkIII, used the same power transformer and used a 5U4, so you should be fine using one if you want. It used EL34s rather than KT88s and had a slightly different bias supply range. The only other major difference is that it lacked a 4 Ohm tap on the OPT. The 5U4 has a larger internal voltage drop that the 5AR4, so you get a lower output voltage.

[OC75]

Do you really need to run the volume at over 8? If the fuse has stopped blowing, you may have sorted the firat issue, only to find that you now have a seperate issue...if all your grounds are good, it might even be a feature of that amp? Are the tube sockets in good order, no arcs, good tension on the tube pins?

Do I need to? Probably not.More or less just trying to figure it all out, and want to make sure things are right, not just putting a bandaid on a real problem. I will go in and check the tube sockets a little closer.

The factory-standard bias setting for a Dynaco MkIII with KT88s was 70mA per tube. You tested for it by measuring 1.56V (same as a fresh battery) across a 11.2 Ohm resistor that was common to both KT88 cathodes. Sunn left this resistor out. 1.56V divided by 11.2 Ohms gave you 139.3mA for the pair. Of course, Dynaco was known for running output tubes on the hot side.

With that bias and 117VAC, B+ supply to the center tap of the output transformer is *supposed* to be 480V.

The Dynaco MkII, predecessor to the MkIII, used the same power transformer and used a 5U4, so you should be fine using one if you want. It used EL34s rather than KT88s and had a slightly different bias supply range. The only other major difference is that it lacked a 4 Ohm tap on the OPT. The 5U4 has a larger internal voltage drop that the 5AR4, so you get a lower output voltage.

Thanks for that explanation. The 5U4 seems to have made a great difference.

Would it be worthwhile to still change the bias resistor as we were talking about earlier to get a little more range on the bias pot?

Again,THANKS to all for the help. This place is GREAT.