Picked up a 1969 Laney Supergroup 100w. Some questions.

[Jon Snell]

There is no fixed relation between tube idle dissipation and operation class. Fixed bias class AB amps typically run between 50% and 80% plate dissipation. Cathode biased class AB amps are often biased at 100% or more (e.g. Vox AC30).

The Mullard EL34 datasheet specifies an anode idle current of 25mA with 800V anode voltage, corresponding to Pa= 20W or 80% dissipation.
So I think Iao = 25mA should be safe with Va = 570V, meaning a dissipation of 57%.

Much lower idle current than that while being even safer but will result in class B and crossover distortion..

I doubt anyone would get crossover distortion above 10%, this is a valve guitar amplifier, not intended for HiFi. 80% will damage the output valves over time and just waste power.

[Jazz P Bass]

Quote: " The bass channel is unusable really, just like a superlead's."
On V1 cathode you will most probably find a huge value bypass capacitor, (ie: 330uf)
Knock it down to something usefull and the bass channel will be useable.
I have used a 1 uf in that spot in the past.

[greengriff]

Quote: " The bass channel is unusable really, just like a superlead's."
On V1 cathode you will most probably find a huge value bypass capacitor, (ie: 330uf)
Knock it down to something usefull and the bass channel will be useable.
I have used a 1 uf in that spot in the past.

Thanks, I can live with it as is (i.e. not using it) - I don't want to start modifying such a clean and unmolested amp. Interestingly though, some demo videos I've seen of Laney's reissue of this amp show it being used with a booster through the low input of the bass channel, and it sounds good. That's the last place I'd think of plugging in my guitar, so it just goes to show!

[Chuck H]

I doubt anyone would get crossover distortion above 10%, this is a valve guitar amplifier, not intended for HiFi. 80% will damage the output valves over time and just waste power.

Well,.. Remember that these amps are often used (if not actually intended) when clipping hard. Pretty sure the additional grid conduction under these conditions would cause heavy crossover distortion biased at 10%. Like you said, these are not HiFi amps. So the operational criteria as well as considerations of tube life are different.

[greengriff]

Chuck. This amp also has the 'scratchy' presence circuit, as discussed in y other thread regarding my Marshall. However in this case the 'scratchiness' when turning the pot is quite tame - nothing like the extremely loud noise on the Marshall. So given this, and the loud 'pop' when switching the Marshall to standby, maybe there is something 'wrong' with the Marshall after all, something that contributes to its excellent tone.

[Chuck H]

Chuck. This amp also has the 'scratchy' presence circuit, as discussed in y other thread regarding my Marshall. However in this case the 'scratchiness' when turning the pot is quite tame - nothing like the extremely loud noise on the Marshall. So given this, and the loud 'pop' when switching the Marshall to standby, maybe there is something 'wrong' with the Marshall after all, something that contributes to its excellent tone.

Yes. I suspect this and alluded to it before. If the amp sounds great and isn't blowing up components I guess we can't say there's something "wrong" with it Did you try the 22pf cap across the FB resistor in that amp? If it's a phase issue it may help. If it's a grounding difference then it won't.

I always use that presence circuit when I include one in my builds because it's more of an "instant gratification" in it's range and allows me to turn the presence all the way off if I want to.

[greengriff]

Did you try the 22pf cap across the FB resistor in that amp? .

Not yet. I'm umming and aahing about whether I should do anything else to it. I feel a bit sorry for that amp - it's been stripped more times than a 20 dollar hooker!