Fender Twin UL > AB763 (re) build.

[Sea Chief]

Howdy folks- hope all UK guys enjoying our fab weather! like hawaii here y'day.. but w'out lava in the garden.

I've rebuilt my UL TR (runaway bias issue/ conductive board diagnosed- redo needed) going the AB763 route inc a choke etc, as advised by Rift. All went well, everything checked 3x (as its a biggie) > comprehensive tests all going well..

..up to final checks: 1stly I wasn't getting anything from the spkr: no hum, hiss, zilch but dead silence. Then, once told to turn vol pot up (ch1) after treb & mid maxed too.. the (3A) fuse popped.

The fuse started crackling (for 1 sec b4 failing) -concurrently- with my nudging the vol pot up a mite, suggesting to the layman here of a clear cause/ result, ie the two linked (it -felt- like I caused it via the pot nudge). But told not so (by Rift, going thru tests on phone at time) only a coincidence the low amps @ vol pot just couldn't be the cause.

Anyway.. I'm back checking no4 time as advised. Thoughts if you would please. Thanks, SC

[pdf64]

I suggest that it's a bad idea to subject those expensive components to fault current, and to avoid that, you build and use a light bulb limiter before you do anything else.

[Sea Chief]

Hi pdf64,

I used a CL- I wouldn't have dreamt of doing without one tbh, esp such a comprehensive & fairly complicated re-build.

[pdf64]

...I used a CL...

What exactly is meant by 'CL' (current limiter?)?
Whatever, the fuse blowing indicates that a light bulb limiter would be better.

[J M Fahey]

1stly I wasn't getting anything from the spkr: no hum, hiss, zilch but dead silence. Then, once told to turn vol pot up (ch1) after treb & mid maxed too.. the (3A) fuse popped.

This hints at shorted speaker out or terrible OT miswiring.

[Sea Chief]

pdf.. I used a light bulb Current Limiter.

[pdf64]

Yikes, what wattage bulb is in it?

[Sea Chief]

Hi Pdf (site went down for a while there..)

60w. No the fuse didn't pop with the CL.. by this time I'd done the checks enough to plug straight into mains.

So godamn frustrating.. I bought the amp nearing a year ago now, been at fixers for 9 months, ccame back with chassis ruined in transit so I had to strip then hammer/ bend for a wknd/ then me rebuilding for last month.. all finially rebuilt, tests all going good, then @ last leg of testing. pop. urghhh.

[Sea Chief]

Hi out there- can anyone help me out here? Fuse issue seems under control (scrubbed 9-pin sockets, odd board resolder spot.. may have been it). Not one blown since.

I tested v1-6, & a pair of 6L6's in v7 / v10 (not yet 4 together). They started to redplate, not cherries/ more subtly up a side of part of the tube plate. The bias pot is in the midway position. On for 5 mins max when I realised- hope tubes ok.

Chris has suggested I might 'just run them colder, maybe'.. presumably that means just turn the bias pot down? But are there any drawbacks to turning it to min- doesn't that in turn lead to stress somewhere else? may have got that wrong.

Got gtr sound for 1st time, during 6L6 test! woohoo ~(2 tubes/ one 8ohm spkr). Bad reverb buzz thing- but cross that bridge later.

Thanks. SC

[pdf64]

If your overall bias is adjustable then adjust it to suit the tubes that you've got in there, ie to idle at about 30mA plate or cathode current each.
There's no point in having it adjustable if it's just left in the middle
Care needs to be taken regarding terminology, eg does 'turning the bias down' mean that the control grid voltage gets more or less negative?
I suggest to think of things in terms of results, ie adjust bias to achieve hotter or colder idle point.
If the plates are even mildly red then they are way too hot.

[Sea Chief]

Understood pdf. When I say turn bias down I mean less negative voltage/ a lower numerical figure/ cooler tubes.

Ok I just plucked up courage to check current (at my 1.5r resistor, V7), in an attempt to turn down this pot/ cooler tubes as Chris alluded to. I switched on & saw 85mA! so quickley dialed down the pot to get me to 35mA. Great > played a bit (gtr in for test, trem ON) & heard good tones.. & trem worked ~ok. 1st time I'ver got to hear amp, properly, since I bought almost a year ago!

But hang on.. noticed the current creeping up, very slowly, to 45mA.. then I switched off.

[pdf64]

Understood pdf. When I say turn bias down I mean less negative voltage/ a lower numerical figure/ cooler tubes...

Less negative voltage (ie a lower magnitude) at the control grids would cause the power tubes to run hotter (ie more cathode current).

...Ok I just plucked up courage to check current (at my 1.5r resistor, V7), in an attempt to turn down this pot/ cooler tubes as Chris alluded to. I switched on & saw 85mA! so quickley dialed down the pot to get me to 35mA. Great > played a bit (gtr in for test, trem ON) & heard good tones.. & trem worked ~ok. 1st time I'ver got to hear amp, properly, since I bought almost a year ago!

But hang on.. noticed the current creeping up, very slowly, to 45mA.. then I switched off.

1.5 ohms is a weird value? 1 ohm is more usual. A 1 ohm resistor will tend to measure ~1.5 ohms unless a special low ohm meter is used (they use 4 test leads).

Regarding the upwards creeping current, it would be useful to use 2 meters, one for the cathode current, the other for the bias.
Are the 1k5 power tube control grid stoppers new?

[Sea Chief]

Ok sure its a 1r then.. whatever Chris put in there. V7-V10 completely new sockets & components, again by Rift, including the bias 0.1 caps on the board.

Ive been reading that the current isnt best read asap after amp switch on/ cold tubes.. is that right? I read the 85mA fig only a few secs after amp on > then dialed down the bias pot a few sec later: so I read 35mA prolly only 20 secs after tubes stone cold.

Alas only 1 meter pdf. thx SC

[Sea Chief]

Can anyone help me out here?

I tested current at the next 6L6, v8.. & get the same thing happen/ steady @ 35mA for a few mins, then creeps up. More slowly though than v7.. say 10 mins to get to 45mA, rather than 5 mins for v7.

Does this suggest my innitial problem, ie it had "runaway bias" (which I rebuilt the whole amp to fix).. hasn't been fixed/ remains??

[Sea Chief]

I just tested vdc at pin 3 of v7 to ground.. & it went up to, & flew around at between 680v- 750v.

WTF?