Vox AC30 that has a 15W - 30W Output Power switch

[wittyjeff]

I'm in the planning stages of building a Vox AC30 clone of sorts and basing the power amp side somewhat on Kevin O'Conner's recommendations in his The Ultimate Tone Vol 3 page 10-10.  I like the idea that he puts forward in that with his design (splitting the cathode resistance) one can pull a pair of the EL34s and have something akin to an AC15 (less output power/volume anyway) making it more useable for practicing at home and small venues.  I noticed on the current production Vox AC30 Hand-Wired (AC30HW2) it has a switch designated 15W and 30W (see: https://voxamps.com/product/ac30-hand-wired/ ).  I can't find a schematic for that new production amp and I'm wondering how they might have wired that.  Is it as simple as interrupting the signal input to one pair of the EL34s (in order to achieve half the output)?  I've attached the schematic I've been contemplating (again based somewhat on O'Conner's TUT3).  I'd appreciate any thoughts and suggestions.

[PRR]

........interrupting the signal input to one pair of the EL34s ...

Vox AC30? EL84, I assume you mean.

Tube Compliment
3 x ECC83 | 12AX7 (preamp)
4 x EL84 (power amp)
1 x GZ34 (rectifier)

That drawing's "Full Tone" switch takes two power tubes out of play. The output is not *exactly* half (unless you do more tricks). But it is of course "less", which is usually what people want.

[sluckey]

Here's one way to do it...

http://www.ceriatone.com/ceriatone/wp-content/uploads/2016/08/36WNewCeriatone.jpg

[wittyjeff]

Yes, EL84 (yikes!).  Thanks for the tips.  The ceriatone idea looks like it should work well.  Thanks!

[Leevi]

If you use an ac30 OT (~4K primary) and if you switchi to half power you will need 8K  primary impedance from the OT. You can for instance compensate this by plugging the 8 Ohm speaker to the 4 Ohm output.


/Leevi

[wittyjeff]

Leevi, That's good to know (impedance issue).  I wonder why Kevin O'Conner claims you can just pull out a pair of the power tubes and be good to go.  Does this impedance issue only occur if you install the switch suggested by Steve in the Ceriatone 36W example (i.e. the switch just eliminating the cathode resistance to ground in one pair... or am I not understanding this mechanism)?  If so, I don't see any provision in the Ceriatone example to account for that (i.e. change speaker impedance) either.

[Leevi]

Normally 2xEL84 has an 8K Zout  and 4xEL84 4K.


http://tdsl.duncanamps.com/show.php?des=EL84


If you pull out a pair of EL84s the impedance is no longer optimal for 2xEL84.


This will not break the amp but it will most probably affect the sound.


/Leevi

[PRR]

> I wonder why

This is not Government Work. Nobody checks to see if we get Exactly HALF power or something else.

It may "affect the sound" but don't get hung-up on that until you try it.

[wittyjeff]

Thanks for the replies.  Sounds like pulling (or switching off) a pair of the power tubes will reduce the output power but since the entirely of the amp circuit (the OT most importantly) is designed for the 4 tubes, the sound (as compared to an amp/OT designed for a single pair) will not be what one might expect for a "true" AC15.  I can live with that as long as it won't fry the OT... and sounds like it won't (especially being cognizant of the issue and adjusting the speaker impedance selector to compensate).  ...just thought it would be cool to have the option to reduce the output a bit when not needed.  As with most of this stuff one sometimes must... "just build it and find out."  ...which can be expensive and painful sometimes.
Again, my thanks.

[HotBluePlates]

I'm in the planning stages of building a Vox AC30 clone ... one can pull a pair of the EL34s and have something akin to an AC15 (less output power/volume anyway) making it more useable for practicing at home and small venues.  ...

I've got a couple AC30s, one of which is a "Super Twin" head (though not with reverb or Top Boost, as the amp at the link has).

The AC30 has a mammoth power transformer, and even the head is heavy.  They are also LOUD unless you don't intend to turn them up until they distort.  They easily keep up with a Super Reverb or a 50w Marshall.

I also have an AC10.  Volume may be limiting if playing onstage un-miked, but at home it is also quite loud before distortion.

I bring this up to get you thinking about how much power you really need, because there's a stout weight-penalty for the bigger amp.  In an apartment, my 1/3w amp might still annoy neighbors.  If you won't need all the loudness, you can save some expense & weight by simply building a lower-power version.

[Leevi]

They are also LOUD

That's fully true. The AC15 is loud as well and if you are planning to play in an appartment a master volume or VVR is a must. PPIMV master works well.


/Leevi

[d95err]

They are also LOUD

That's fully true. The AC15 is loud as well and if you are planning to play in an appartment a master volume or VVR is a must. PPIMV master works well.

I read somewhere on Greg Fryer’s site (he builds AC30s for Brian may and the We Will Rock You shows) that during the shows, the guitar player often didn’t even notice when an amp blew a tube and went into (mismatched) 15W mode...

EDIT: found a link to it:
https://fryerguitars.wordpress.com/el84-cathode-fusing-on-brian-may-ac30s/

[Leevi]

I have built numerous AC15 and AC30 based amps.
You can barely hear the difference when you switch from 30W to 15W mode.
Let's say that 30W has a higher headroom but it's not much louder.
/Leevi

[ac427v]

It looks like you are using a "safety resistor" R98 to protect the amp when the speaker is accidentally unplugged. I have copied other designs and used a 250 ohm at 5 watts for this purpose. That is much smaller and cheaper than an 8 ohm at 100 watts. Are your values coming from an established design? Sorry for the hijack but I'm curious and suspect you are too.

[d95err]

I know Kevin O’Connor recommends using a safety resistor of something like 250 ohms. It’s not intended as a dummy load, it’s just there to protect the output transformer in case of intermittent load (e.g. accidental unplugging while playing or a glitchy jack or speaker cable).

According to O’Connor, intermittent load is the most common cause of OT failure.

I use a safety resistor on all builds. It’s cheap and easy insurance.

[wittyjeff]

Great info on the safety resistor vs dummy load.  I just used a "dummy load" since I'm basically a dummy myself.  I figured if that if that amp is running and for whatever reason that speaker plug is not in that jack or gets yanked out... no load at all is really bad, shunting straight to ground (as most circuit designs do) is still bad, it's better to at least have an 8 ohm resistor that can keep from overheating until they figure out that the jack is not where it should be.  I just did that because I knew about dummy loads.  I didn't read anything about the safety resistor in TUT3 which is the only O'Conner that I've read so far.  Your suggestion seems much more practical for sure.  I'll switch over to the 250 Ohm 5 watt instead.  Thanks for the tip!

[wittyjeff]

d95err:  Thanks for the Brian May link and your suggestions!

[sluckey]

The circuit in the original post only provides protection if the cable is unplugged at the amp. No protection is provided if the cable is unplugged from the speaker cab. Permanently connecting a 250Ω to the highest impedance tap on the OT will provide some protection against an open speaker load regardless of where it occurs. The small amount of power lost in the 250Ω resistor is not noticeable.

[wittyjeff]

Steve, Is what I drew in the attached correct (i.e. permanent connection to highest impedance tap)?  What do you think I should do with the shunt?  Is there anything beneficial by leaving a resistor (250 Ohm 5W or 8 Ohm high watt) just for added protection?

[sluckey]

I would connect the shunt to the sleeve. R99 is not needed.

[wittyjeff]

Got it, will do.  Thanks!

[wittyjeff]

Steve (or others), have you ever tried the bypassing of the cathode caps on the power tubes like O'Conner suggested?  He suggested in TUT3 on page 10-8 "We can also lift the cathode bypass cap for a rounder sound and less power gain."  I thought that sounded interesting and wanted to try it, but I don't know if I've implemented what he was suggesting correctly. (see attached Power Amp schematic in comments above).

[wittyjeff]

Here's an updated schematic...

[sluckey]

Look at Da Geezer’s Bypass Cap attenuator on page 6...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[wittyjeff]

Thanks again!