Bias help for a 1969 Laney LA60BL pleeeease!

[Chuck H]

If a new production valve does not meet with the specification of older types, the identity must be change otherwise it is fraudulent.

Preaching to the choir comrade.

[Chuck H]

Couldn't you replace the 56K resistor in the bias supply with a 100KL pot to make it adjustable?

+1000
Ideally I prefer to use something like a 50k pot and a 33k "safety" resistor (so's the bias can't be dialed to zero). But as long as the owner/user is aware then a plain ol' 100k variable resistor seems fine. Since this is a vintage amp and part of the goal is to keep it original I would probably lift one end of the 56k resistor and leave it in the amp. Then have the 100k variable actually soldered in it's place. That way the OEM part is there for a quick reversal of the mod if it ever comes up.

[Gtr0]

Do you use the 250V PT primary tap?

Indeed I am - That was the only setting that would get the heaters to set in the proper zone (6.2 VAC) - Then on the 230 tap, the B+ was well over 600. yikes!

Couldn't you replace the 56K resistor in the bias supply with a 100KL pot to make it adjustable?

Perhaps - I'll check what I've got... I have a 50k for sure.

+1000
Ideally I prefer to use something like a 50k pot and a 33k "safety" resistor (so's the bias can't be dialed to zero). But as long as the owner/user is aware then a plain ol' 100k variable resistor seems fine. Since this is a vintage amp and part of the goal is to keep it original I would probably lift one end of the 56k resistor and leave it in the amp. Then have the 100k variable actually soldered in it's place. That way the OEM part is there for a quick reversal of the mod if it ever comes up.

Sounds lovely! Thanks.

The screen grid resistors are 470 ohms and so something quite a bit less than 570V when operating and been working for 50 years. I don't see the need to change it. Some brands only spec 425V for the screens, others e.g. the Shuguang EL34B are 500V (and 30W plate dissipation instead of 25W). If you want a bit of peace of mind then change the screen grids to 1K.

My screens, at the pin (post 470R) were in the 550's... 557 or 558'ish I seem to remember - are you saying that since it ran this way for so long (as do other Supergroups apparently) that it should be fine? Again, let me stress my healthy fear for electricity in general ;-). - Thank you!

[Randall]

"Perhaps - I'll check what I've got... I have a 50k for sure."

That won't help at all, in fact it will not even get you to where you are now. You need more than the 56K that is there. The goal is to get more negative voltage to the grids than you have now if you want to lower idle current. The 56K is shunting or bleeding or dragging down some of that voltage before it gets to the grids. A higher resistance at the 56K to ground will allow more of the negative bias voltage to the grids, and less of it to be dragged to ground.

[Gtr0]

"Perhaps - I'll check what I've got... I have a 50k for sure."

That won't help at all, in fact it will not even get you to where you are now. You need more than the 56K that is there.

Sorry, I meant to say that it would be in line with another resistor. I could just leave the 56k there and add the 50k trimmer after.

[nickb]

My screens, at the pin (post 470R) were in the 550's... 557 or 558'ish I seem to remember - are you saying that since it ran this way for so long (as do other Supergroups apparently) that it should be fine? Again, let me stress my healthy fear for electricity in general ;-). - Thank you!

Yes, it should be fine. A healthy fear of electricity is not a bad thing at all. Always pull the plug from the wall and discharge ( check with meter) before touching.

Some tubes you might wish to avoid in this amp (and presumably any rebranded derivatives) IMHO would be.

EL34 TungSol Re-issue (Screen 425V 8W/ 800V Plate 25W)
EL34EH Electro-Harmonix (Screen 425V 7.5W / 800V Plate 25W)
EL34 JJ (Screen 450V 8W / 800V Plate 25W)

[Gtr0]

Thanks for the tip!

Any brand tube you would explicitly recommend in this case? I tried a TAD EL34-STR but they went into meltdown mode rather quickly - once I fix the bias issue I will try again.

[Gtr0]

BTW, I am not entirely opposed to a 5881 conversion either.. those old Soviet coin bottom 5881's could handle this screen voltage right? I have a JTM45 with 5881's and I love that amp - in terms of component choice, I think this LA60BL comes closer to that - or a superbass - than any lead type Marshall.

AND, according to TAD's data sheet https://www.tubeampdoctor.com/media/...5-30-11-18.pdf their KT88 can handle 600 on the screens.

[SoulFetish]

[QUOTE=nickb;557627]Yes, it should be fine. A healthy fear of electricity is not a bad thing at all. Always pull the plug from the wall and discharge ( check with meter) before touching.[QUOTE]

YES! do this. The last several Laney amps I've worked one do not have any resistors to bleed off the HV. In fact, it's not a bad idea to install some ballast resistors across the reservoir caps. In the schematic, it looks like they gang up a parallel combination of 2 x serially connected 33µF caps(4 in total).
I'm not sure what the manufacturer recommends according to the leakage, but it's common to see values of around 220k in parallel across each cap (in your case, 1 resistor for each pair of caps). This helps balance the voltage across the series connected caps because of the differences in tolerances in electrolytics. But they will also serve to bleed off the high voltage on those caps as well, as an additional measure of safety, in case one might forget to bleed them off manually – which happens to the best of 'em.

[Justin Thomas]

Some tubes you might wish to avoid in this amp (and presumably any rebranded derivatives) IMHO would be.

EL34 TungSol Re-issue (Screen 425V 8W/ 800V Plate 25W)
EL34EH Electro-Harmonix (Screen 425V 7.5W / 800V Plate 25W)
EL34 JJ (Screen 450V 8W / 800V Plate 25W)

Given that 2/3 of those cone out of the same factory, I'd not use ANYthing from that factory. Which is basically ALL of those "Golden Era" reissues & a few "dubious" ones to boot (Mullard 6L6GCs, anyone?)

I think most of those EL34s must have only been made to pander to the Marshall crowd. There aren't many EL34-equipped Marshalls running around with 700V on the plates. And since those guys make up 90% of the marjet anyway...

If you're open to 5881WXT conversion, go for it it. They're about as indestructible as you can get, and nobody REALLY knows their max specs for voltage. Care to contribute to science?

Justin

[nickb]

Given that 2/3 of those cone out of the same factory, I'd not use ANYthing from that factory. Which is basically ALL of those "Golden Era" reissues & a few "dubious" ones to boot (Mullard 6L6GCs, anyone?)

I think most of those EL34s must have only been made to pander to the Marshall crowd. There aren't many EL34-equipped Marshalls running around with 700V on the plates. And since those guys make up 90% of the marjet anyway...

If you're open to 5881WXT conversion, go for it it. They're about as indestructible as you can get, and nobody REALLY knows their max specs for voltage. Care to contribute to science?

Justin

The TAD El34-STR should be fine. I think you just had the bias too high. Shuguang EL34B are 500 V 8W / 800V 30W have been good to me but you do have to select to weed out the micro-phonic ones.

I'm not on board with the 5881's due to the lower plate dissipation. It means you can't bias as hot which can make it sound a tad hard. But that's just me. YMMV.

[Justin Thomas]

TADs are Chinese, right? They're like Eurotubes with JJ, correct?

Justin

[Leo_Gnardo]

TADs are Chinese, right? They're like Eurotubes with JJ, correct?

Justin

Not quite. While it's true that Shuguang is the source for - possibly - all the TAD tubes, there are some TAD's that are obviously "STR" that is specially made for them. One I've noticed is a type of 6L6 with dark black plates, and the examples I've run across seem to be built better than average. There may be others.

[Justin Thomas]

I just might have to try a few out if I ever need EL34s. & if the 6L6Gcs have reasonable prices those too.

I've had mostly bad experiences with Chinese preamp tubes, finding them only usable in "function" slots. Even supposedly tested for noise & such, they like to start ringing on their own. And the one pair of (GT-branded) EL34s I tried sounded like popcorn as soon as I put em in. Glad I didn't pay much for them.

Maybe time to give em a shot again.

Justin

[Chuck H]

I actually really like the Shuggie 12ax7's. IMHE they're high gain, full fidelity and less microphonic than most of the Sovtek offerings, and in fact it's often Shuggies that end up in the V1 slot for some of my higher gain amps. But my collection was mostly tubes from mid 90's when the Chinese preamp tubes were in all the production amps. Things may have changed. I'm out of those now so I've been plucking (stingily) from my UOS (used old stock = pulls) tubes that includes Mullards, Bugle Boy, Sylvania, RCA, etc. Almost out of those now too