Bugera 1960 Swap out 5881s for 6L6GC Tubes - Necessary Mods?

[Jecarroll7]

This is a question that might better have been asked earlier but what mods (if any) are required to swap out the Bugera 5881 tubes for JJ 6L6GC tubes?

Had the JJs in coming up to temp, started to bias the amp and popped the T3.15 main fuse, which I assume is a 3.15A fuse.

No schematics for this amp, but thought the 6L6 could handle it and it did, but the amp apparently didn't.

Also,any special tricks to biasing this amp? I started to adjust up to value with the amp quiet only to watch it come back down on the meter.

Again, no visability into the circuit so help is much appreciated.

[MWJB]

The only modification required is rebiasing.

refit the 5881 and bias as cold as they will go. Now sub in the JJ's...what idle current do you get? I suspect the JJ's pull considerably more current than the 5881s.

"I started to adjust up to value with the amp quiet only to watch it come back down on the meter." I can't work out what you mean here, you set the isle bias current with no signal going into the amp, not quiet, but silent.

[Jecarroll7]

With the amp idle, I'd adjust into the 40+mA range and it would float back down into single digits in a few quick seconds. yet the fuse blew. I did notice bias pot seemed very sensitive.

Could be something else with the amp, but was playing fine with the 5881s before the swap so trying to rule out the obvious before doing a deep dive.

How close is this amp to the Marshall JCM900 dual reverb?

[g1]

Sounds like the bias pot is giving you trouble. Can you give it a shot of pot cleaner? While you were turning it it may have hit a glitch and you may have lost bias temporarily and blown the fuse. But if that were the case you should have got a high reading on the meter.
Could you describe the test points/meter hook up?

[MWJB]

Why are you adjusting into the 40mA range, if you mean 40mA per tube that's pretty high....if it's 40mA for 2 tubes then that's low side of normal.

What is the lowest current you get with the 588s1 installed? See if you can set them to a handful of mA, say 5-8ma per tube? If you set them to 0mAthe amp will make no sound.

I trust you are giving the amp time to warm up & settle when setting bias, the readings fluctuate before everything is settled & drawing proper current.

Your fuse blew because there was too much current being drawn accross it....not too little.

I have no idea how closely your amp resembles the JCM900, it's not relevant to your problem.

[Jecarroll7]

If you're familiar with the 1960 amp it has the bias pot and an RCA connector to attach the meter to measure bias current, both easily accessible next to the power tubes. As there is only one test point I assume the measurement is for all four tubes.

I asked about the JCM900 dual reverb as I've read the 1960 is basically a reversed engineered clone.

[Jazz P Bass]

The Bugera 1960 bias test point is measuring the -dc voltage at the tube grids.
It is not a 1 ohm resistor on the cathode setup.
Therefore, in my opinion, it is next to useless, unless you purchase Bugera's hand picked tubes. (as far as adjusting but not monitoring current draw)
If you have a set of tubes that are 'hot', the test point will not tell you that.
So the op was setting the bias to -40 Vdc .

[Jecarroll7]

Hey Jazz,

My first tango with Bugera, appreciate the help.

This 1960 doesn't have a 6L6/EL34 switch, in fact each socket is labeled 5881, no mention of 6L6 as an option, this is the reason I asked if mods were needed.

Looks like this amp is an '09 model, made in China if that helps.

Thanks for your attachment, At this point I'm wondering if it would be helpful to get bias instructions for this specific amp?

This amp is a bit different from others, will a bias probe that inserts the one ohm resistor work properly with this amp?

[Jazz P Bass]

Yeah well, that was a 'generic' file that Bugera put out.
Still, you do need to monitor the actual current draw.
If you have a bias probe with the one ohm resistor, that should do you fine.
You have 4 6l6's huffing away.
I would shoot for the low end of biasing.
25-28 ma's.