AC30s and hum

[Colossal]

Hum can be an issue with AC30s due to low filtering, 130% bias, grounding, and other factors. Power tubes need to be reasonably matched as they can also be a source of hum. The four EL84s all share a 50Ω cathode resistor that is bypassed by 220uF. In this arrangement, if one tube fails, the other three are forced to pick up the slack and at 130% of max dissipation, they are already working very hard.

Ignoring the small added cost of parts, from a performance perspective, any thoughts on individually biasing each tube or biasing in pairs, like what was done with the Liverpool? Individually biasing and even adding a series trimmer to dial in current for each tube would be very convenient, and you could use unmatched tubes, matching and balancing as desired (a little mismatch is nice for added second harmonic content). If one goes up, then the other three can manage. In a paired bias scenario, the other and inner pairs share a cathode resistor. If a pair goes out, the other pair keep working, albeit into half the expected impedance (Matchless mode ).

[Stevem]

That's how I would build one or mod one up if it's going out on tour for night after night reliability, and yes I would build it with standby switch just for slower thermal expansion rates and the longer mechanical tube life that it provides!

You could also fuse the plate with a flame proof whatever wattage resistor like Ampeg didin the SVT so that if a output shorts the amp keeps on playing .

[Mark]

Have a look at Greg Fryer’s AC-30 circuits. He has pairs of EL-84’s fused.

https://fryerguitars.com/ac30s/

[ViperDoc]

Is there an error on V1B of the penned schematic here or is it just me?

[Colossal]

No, that's how it's written. My take on this was, this is just Greg Fryer's note pad and this is one of several iterations of amps he's built for BM and also the WWRY musical tour. I know it is not *the* exact Brian May amp, because there are many. That 220k/2k7 voicing with the variable gain dump on the back end was used for one of the WWRY amps and also can act like a Rangemaster boost when placed in front of a 220k/1k5 Voxy Normal channel input stage. This was a mod for the Mike Ryde amps; one hot input, one Normal Channel input. Maybe the channel in that drawing was just grounded, having been previously voiced that way, and Greg documented it so he wouldn't forget what he had done in what amp. I dunno.

[Roe]

mine is very quiet except for some hiss, but I am using CC resistors. Try merlin style multi star grounding and a CLC filter

[pdf64]

If there’s mains frequency hum / buzz, bear in mind that the original 60s JMIs had a single ended heater circuit. Balancing and elevating it, even just to the output valve cathodes, can help to pretty much eliminate that.

Yes, for reliability, individual cathode bias resistors (and bypass caps) would be beneficial, and also most valves then seem to ‘self balance’, to idle currents that are reasonably close to each other. Other than to accommodate outliers, or to introduce imbalance, there’s no real need for trimmers.

… I would build it with standby switch just for slower thermal expansion rates and the longer mechanical tube life that it provides!
…

Is that a hypothesis, or is there anything to back it up?

Is there an error on V1B of the penned schematic here or is it just me?

Yes, as drawn, it doesn’t seem to be doing anything.

[Mark]

I would like to elevate the heaters to 75vdc. Apparently that is a Kevin O’Conner recommendation. I checked the spec sheet and both EL-84’s and 12AX7’s are good for 100vdc.

[Mark]

mine is very quiet except for some hiss, but I am using CC resistors. Try merlin style multi star grounding and a CLC filter

Shere Sound sell suitable chokes for the Pi (CLC) filter prior to the output stage.

“The chokes are still available and can be ordered by e-mailing Valerio Capodagli on alfa758@libero.it.

The OT and screengrid feeds are common in the "DP" model AC30, as in the JMI/Vox AC15 on which it's based.”