Possible Mod For VOX AC30CC1

[zozoe]

Hi all~ I have this very cool AC30 1x12 combo from the late 90's or early 2000's, & it has these switchable bias options, 50 or 82, where the 82 offers a slightly lower output, 22w before distortion, and I'm curious to know if I could have many tech make it higher than the 82 setting & expect even earlier breakup? I'd need to find a schematic, but is this even possible & safe with the proper value changes?
Many thanks in advance.

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[roberto]

This is the schematic of your amp:
http://schems.com/bmampscom/vox/Vox_AC3 ... )%20SM.pdf

[roberto]

What you can try to do is:

- pull out a pair of EL84s (one per side)
- switch to 22 uF
- switch to 82 Ohm (it's like having 41 Ohm on a quad)
- switch to 8 Ohm and plug a 16 Ohm speaker (this will give you twice the Raa, so correct for a pair of EL84s)

Tubes will run very hot, so the only mod you could do is change R12 (page 7) from 82 to 150R.
This will transform that switch from hot/warm to pair/quartet of output tubes, biased warm.

[Roe]

larmar works better. the 50R cathode resistor is essential for most of the classic ac30 sounds

[roberto]

Yes, but IIRC there's no space for in on that combo.

[Stevem]

The problem with yanking out 2 output tubes is now you have a power supply that can produce 30 watts now making only 15 watts or less!

Also without those tubes in the amps heater supply is less loaded and all the other tubes in the amp, in particular the preamp tubes making more gain and changing the tonal and play nature of the amp even more!

[roberto]

without those tubes in the amps heater supply is less loaded and the preamp tubes making more gain

are you serious?

[mhuss]

Well, halving the output stage consumption will result in a higher B+, which will increase the preamp gain a little.

[roberto]

Do you really think to hear any gain increase by removing two output tubes from the sockets?
By heaters or even by plate voltage variation? Really?

Can we expect 60-70 ma less by removing a pair of EL84s?
OT primary DCR is 300 Ohm per side? 150 Ohm total?
PT secondry resistance is how much? 100 Ohm?

So will it be 15V more?
15V on 350V is 4% more. I doubt anyone has a so stable power supply at home.

[EL504]

What you can try to do is:

- pull out a pair of EL84s (one per side)
- switch to 22 uF
- switch to 82 Ohm (it's like having 41 Ohm on a quad)
- switch to 8 Ohm and plug a 16 Ohm speaker (this will give you twice the Raa, so correct for a pair of EL84s)

Tubes will run very hot, so the only mod you could do is change R12 (page 7) from 82 to 150R.
This will transform that switch from hot/warm to pair/quartet of output tubes, biased warm.

I don't think a pair of EL84's at Vak = 300 V and Vg2k = about 310 V, and with a cathode bias resistor of only 82 Ohm, will survive very long.

[ChopSauce]

What you can try to do is:

- pull out a pair of EL84s (one per side)
- switch to 22 uF
- switch to 82 Ohm (it's like having 41 Ohm on a quad)
- switch to 8 Ohm and plug a 16 Ohm speaker (this will give you twice the Raa, so correct for a pair of EL84s)

Tubes will run very hot, so the only mod you could do is change R12 (page 7) from 82 to 150R.
This will transform that switch from hot/warm to pair/quartet of output tubes, biased warm.

I don't think a pair of EL84's at Vak = 300 V and Vg2k = about 310 V, and with a cathode bias resistor of only 82 Ohm, will survive very long.

I think that's why Roberto suggest changing for a 150R resistor. Dunno for the rest.

[pompeiisneaks]

Vox used to run a quad of EL84's at 50ohms (like on the drawing here selectable between 50 and 82.) The higher the resistance, the cooler they'll run, so why do you say a pair at 82 is going to do worse? Or is it more the implication that if it were a pair it should really be at 100 or 164 ohms at that point?

~Phil

[sluckey]

Vox used to run a quad of EL84's at 50ohms (like on the drawing here selectable between 50 and 82.) The higher the resistance, the cooler they'll run, so why do you say a pair at 82 is going to do worse? Or is it more the implication that if it were a pair it should really be at 100 or 164 ohms at that point?

Most 2 x EL84 amps I've seen run with 130Ω or 150Ω and that's already hot. If you decrease down to 82Ω they will run dangerously hot.

[roberto]

I don't think a pair of EL84's at Vak = 300 V and Vg2k = about 310 V, and with a cathode bias resistor of only 82 Ohm, will survive very long.

I think that's why Roberto suggest changing for a 150R resistor. Dunno for the rest.

Exactly.

[roberto]

why do you say a pair at 82 is going to do worse?

Because 82 for a pair is like 41 for a quad, so less than the already hot/low 50 Ohm.