old Laney 610v B+ what tubes?

[Alex R]

Hi all. OK I have another little problem. An early Laney head, two-tube push-pull maybe mid to late 60s, turret board. On the plates is 610 volts!

It has EL34s in it and it isn't happy; however much I throttle back on the bias after a bit it want to pull too much current and blows fuses. Screen resistors are ok, but I wonder if the screens are happy with the voltage. OT primaries measure 80 ohms and 130 ohms so it look like the OT has suffered a bit, maybe.

Is this perhaps designed to be a KT88 amp I wonder?

Well I think it needs a new OT anyhow, so what do you think - could new-manufacture EL34s be happy at these voltages? How about KT88s?

The amp is coming back in later and when it does I will work out the turns ratio on that OT to try to guess at what it was designed to do, but in the meantime any comments about operating new-manufacture EL34s or KT88s at 610v B+?

[Enzo]

I think you are assuming a lot.

The OT primaries for example. If you double wind a transformer, then both halves of the turns have the same wire length and thus the same resistance. But the easier cheaper route is one winding. Wrap half the number of turns areound the core, attach the center tap, then wind the remaining number of turns. The turns will be right, but the second batch of wire will be much longer than the first, since it is wrapped outside the first, and so will have amuch higher resistance. That tranny could be fine.

In my experience, it is current, dissipation really, rather than voltage which kills tubes. This wouldn't be the first amp that needed some resistor tweaking to get the bias in line with its needs.

Look up the data on the EL34, 600v is well below its ratings. Maybe an old one was better suited to the 800 volts or 700 volts, but 600v should not be a challenge.

FInd out why the dissipation is too high, or why the tubes are not stable.

Have you tried a known good set of power tubes?

When the tubes start to pull current, what is happening at the control grid? is the bias level stable, or is it drooping away?

What voltage is on those screens then?

[Gtr_tech]

I think you are assuming a lot.

The OT primaries for example. If you double wind a transformer,

The term for this is "bifilar wound". Better quality (and more expensive) output transformers are bifilar wound. Not that it makes an improvement in sound quality....the mismatch via primary likely contributes to what we expect to hear from a tube guitar amp. Better on paper does not always equal better performance for the end user.

[Gtr_tech]

Hi all. OK I have another little problem. An early Laney head, two-tube push-pull maybe mid to late 60s, turret board. On the plates is 610 volts!

It has EL34s in it and it isn't happy; however much I throttle back on the bias after a bit it want to pull too much current and blows fuses. Screen resistors are ok, but I wonder if the screens are happy with the voltage. OT primaries measure 80 ohms and 130 ohms so it look like the OT has suffered a bit, maybe.

Is this perhaps designed to be a KT88 amp I wonder?

Could be, who knows. Find the schematic and see. At any rate....my Traynor YBA3 puts upwards of 700v on the plates and it runs just fine with *good quality* EL34s (it was intended to use 6CA7s). I like higher plate supply amps. There's *nothing* spongy about this beast.....kick you in the chest attack and a roar that eats up Mar$halls at will. Even has the original filter caps and they's hangin' tough. Very fun to play.

[cbarrow7625]

Same thing with the old hybrid music man amps. They had 700+ volts on the EL34's as well. I think they were biased in Class B, if I'm not mistaken.

Like Enzo said, it's not the heat, it's the humidity......or something like that.

[Alex R]

Thanks for these replies. I will stop assuming and check the grids. The amp arrived back yesterday and I will get to it in a week or so.

Yes MM amps are high voltage, I remember. I'll take a look at the circuits.

This is a really ancient amp, hand cut alu chassis (with the name 'Spud' crudely engraved on it under the board with a burr!), no schematics exist afaik? We suspect it's the amp known as a Laney 70 - anyone know anything about these? Esp. power amp and power tubes...