JEI - Jennings AC40

GordonB

I have a 1973 AC40, which has 25 Watt Fane speakers dated 1974, it's only guesswork but I think probably the origional speakers were blown and the owner didn't want to pay for new Silver Bells.

Amp has only had 1 owner from new and was used exclusively for keyboards, which the Fane Pop25's probably suited better.

Anyhow I have found a pair of 1964 Vox Blues, arriving next week.

But I have a dilema, they are 8 ohm, and my amp needs to see 8 ohms.

As I see it I have 2 options, I have a pair of 200Watt 8 ohm resistors, so I could put 1 in series with each speaker, then connect in parallel as would be correct with 16 ohm speakers.

The other way would be to put the speakers in series and the resistors also in series and wire that lot in parallel.

Either way would load the amp output correctly.

There is also another way with each speaker having a parallel resistor making 4 ohms, then put that setup in series.

Aaarrrgg my brain hurts!!! LOL

Has anyone ever had a similar situation? What do people think would be the better option?

ICBM

I think the speakers are likely to be original.



https://www.voxac100.org.uk/jennings_ac40.htm

I've never seen an AC40, but it wouldn't surprise me if Jennings was using Fane Pops, he was always a cheapskate.

If you want to use the Celestions, I think it's very likely the amp has a 16-ohm tap on the output transformer as well, like an AC30. Have a look at the connector block on the chassis - if there are three wires, it does.

If there isn't, it will be better to run the amp mismatched and accept the slight loss of power output than mess about with resistors which will just waste even more power.

GordonB

So let me be certain I'm understanding you correctly, if there is a 16 ohm tap, I just wire the 2 speakers in series, which presents the correct load to the amp.

Not surprised you've never seen an AC40, they are like the proverbial "Rocking Horse Shit", extremely rare, to all intents and purposes it is an AC30 Top Boost.

From everything (not much) I can find the standard speakers were T1096 (Silver Bells).

While searching for those I came accross the "Blue Bells" which I've bought, one is a Vox labeled one from 1964, the other is a 90's Celestion UK made one, which from all reports is indistinguishable from the origonal, which to be honest I doubt as age definitely plays a part in sound reproduction, but close enough for me I expect.

Thanks for your reply BTW that is an option that hadn't occured to me and I agree about avoiding resistors.

ICBM

GordonB said:

So let me be certain I'm understanding you correctly, if there is a 16 ohm tap, I just wire the 2 speakers in series, which presents the correct load to the amp.

Yes, that's correct - that's how AC30s are wired as stock. If not, wire the speakers in series anyway and connect to the 8-ohm tap. Although valve amps very often prefer a low mismatch to a high one, I'm fairly sure Voxes are mostly the other way round so it will be better to run the amp at 8 ohms into 16 than into 4.

Regardless of whether they're original, using the Fanes may not be a great idea anyway - back in the old days they were commonly known as "Pops, because if you put anything like the rated power into them, they pop!" (It actually stood for Popular.)

The rather optimistic power ratings of Fanes, Goodmans and other non-Celestion speakers is why they always had a reputation for being inferior to Celestion, although in fact many of them sound very nice as long as you're careful with them. The power ratings are more like those of a hi-fi speaker, whereas Celestion seemed to understand what musicians would do to them, and rated them for 'a cranked amp of that power' - basically about double the clean rating.

GordonB

Cheers your input is very much appreciated.

GordonB

Update to this, I have took the amp out of the cabinet and it looks like there are 2 taps on the transformer, but no markings. The unit is a; Hoffman Amp H-30WO 0110

I  can't find anything specific about that but this is what it looks like;

ICBM

GordonB said:

Update to this, I have took the amp out of the cabinet and it looks like there are 2 taps on the transformer, but no markings. The unit is a; Hoffman Amp H-30WO 0110

If there are three wires and it's the same as an AC30 transformer, there should be black, green and blue - which are ground, 8 ohm, and 16 ohm respectively.

GordonB

Don't know why my picture kept vanishing and now even the link I posted has vanished.

Thanks again for your reply, if the pictures had stayed intact, you would have seen the wires are Yellow, Black and Brown, currently the Black (Ground) and Yellow wires are in use, which I know is definitely 8 ohms, as I don't expect there was ever a 4 ohm tap, then the Brown wire would be 16 ohms.

Would you agree that to be reasonably logical?

Thanks again.

ICBM

GordonB said:

Thanks again for your reply, if the pictures had stayed intact, you would have seen the wires are Yellow, Black and Brown, currently the Black (Ground) and Yellow wires are in use, which I know is definitely 8 ohms, as I don't expect there was ever a 4 ohm tap, then the Brown wire would be 16 ohms.

Would you agree that to be reasonably logical?

Yes. Do you have a multimeter? If so, measure from ground to the other two wires. Although the resistances will be very low, usually under 1 ohm, you should be able to see one is higher than the other - the higher is the 16-ohm.

GordonB

Cheers! that matches perfectly, the one in use which should be 8 has a resistance of .85 ohms and the unused 1 ohm, so exactly what I wanted.

The new (old) speakers are being delivered tomorrow, so the pair in series on the 16 ohm tap and all is well.

Totally unrelated but have you any idea why my images and links would appear in my message for just moments and then vanish, even an url link did the same, also as I'm typing this between the text box and Leave a Comment, there is script saying, "Body is 5583 characters too long, just changed to 5671?

Quite strange.

ICBM

GordonB said:

Totally unrelated but have you any idea why my images and links would appear in my message for just moments and then vanish, even an url link did the same, also as I'm typing this between the text box and Leave a Comment, there is script saying, "Body is 5583 characters too long, just changed to 5671?

Quite strange.

Sorry, I may apparently be known for my understanding of amps, but software is mostly a foreign language to me .

easyrazors

I have a late AC40 which the previous owner believed was a prototype (almost identical to the strange one on the AC100 site here: https://www.voxac100.org.uk/jennings_v30.htm
other than mine has AC30 style chicken head knobs and Celestions dating to 1976 which appear original). The one in the link has Fanes, though, so your Fanes may well have been how it left the factory.

Pigknows

Great amps. In my view, they feature a couple of genuine improvements on the stock AC30 design, namely the deletion of the (useless to me) cut control, and more widely spaced el84s.  For what it's worth, I tried mine with Blue alnicos, but thought it sounded best with either two G-12Hs or one G-12H and one Blue. I only wish that Jennings had gone just a little bit further and got rid of the AC30 Vibrato sound, and changed the three position Tremolo controls to standard pots. Good luck!

ICBM

Pigknows said:

Great amps. In my view, they feature a couple of genuine improvements on the stock AC30 design, namely the deletion of the (useless to me) cut control, and more widely spaced el84s.  For what it's worth, I tried mine with Blue alnicos, but thought it sounded best with either two G-12Hs or one G-12H and one Blue. I only wish that Jennings had gone just a little bit further and got rid of the AC30 Vibrato sound, and changed the three position Tremolo controls to standard pots.

I think the Cut control and the Vib-Trem channel are both great and one of the things that are good about vintage and (proper) reissue AC30s, but missing from most modern versions. I agree about the Vib-Trem speed control - it's actually puzzling that it was a switch, since they're slightly more expensive (both the component and needing more work to wire up) than a pot.

I'm not sure why spacing the EL84s more widely is useful though!

GordonB

ICBM said:

Pigknows said:

Great amps. In my view, they feature a couple of genuine improvements on the stock AC30 design, namely the deletion of the (useless to me) cut control, and more widely spaced el84s.  For what it's worth, I tried mine with Blue alnicos, but thought it sounded best with either two G-12Hs or one G-12H and one Blue. I only wish that Jennings had gone just a little bit further and got rid of the AC30 Vibrato sound, and changed the three position Tremolo controls to standard pots.

I think the Cut control and the Vib-Trem channel are both great and one of the things that are good about vintage and (proper) reissue AC30s, but missing from most modern versions. I agree about the Vib-Trem speed control - it's actually puzzling that it was a switch, since they're slightly more expensive (both the component and needing more work to wire up) than a pot.

I'm not sure why spacing the EL84s more widely is useful though!

The theory was that the wider spacing aided cooling and allowed the tubes to develop more power, not knowing the relevant voltages etc used relative to an AC30, it's difficult to be certain.

There's a guy in Notingham who specialised in vintage amps he mentions the AC40 on his website and he has doubts that it would develop 40 Watts.

I'll try adding a link and hope it doesn't vanish as my others have.

http://www.chambonino.com/work/vox/vox12.html

I have lots of very good clear photos which some may find interesting, but seems I can't add them here.

Mine also has the "chicken head" knobs and it is in superb condition.

I've just taken the Fanes out as the Vox Blues are due today, I had the date of the Fane speakers wrong, they are week 43 1972, which is 23-29 October 1972, they appear to have Pulsonic cones both origional, I'm debating with myself wether or not to sell these.

Pigknows

I had understood that the wider spacing of the el84s plus better ventilation in the later AC30s/AC40s helped with cooling (some of the older Voxes I've got really hot). I'm happy to admit that it may be irrelevant though.
Personally, I've never found the cut control useful on any amp I've owned (vintage Vox, TBX, Tophat - it just seems to make the sound duller. Likewise, I much prefer the Magnatone vibrato and Fender tremolos over the AC30 versions. I think they got the '60s AC10 right - sounds great, lighweight, no superflous effects, Fender style trem controls. That said, I do like the versatility of the three differently voiced channels of the vintage and TBX AC30s. Horses for courses I suppose! Tele player btw. 

ICBM

Pigknows said:

I had understood that the wider spacing of the el84s plus better ventilation in the later AC30s/AC40s helped with cooling (some of the older Voxes I've got really hot). I'm happy to admit that it may be irrelevant though.
Personally, I've never found the cut control useful on any amp I've owned (vintage Vox, TBX, Tophat - it just seems to make the sound duller. Likewise, I much prefer the Magnatone vibrato and Fender tremolos over the AC30 versions. I think they got the '60s AC10 right - sounds great, lighweight, no superflous effects, Fender style trem controls. That said, I do like the versatility of the three differently voiced channels of the vintage and TBX AC30s. Horses for courses I suppose!

Very much - I don't like the AC10 at all, it sounds weedy to me - it's like an oversized AC4 instead of a small AC30.

I don't think the spacing of the EL84s makes any difference to how hot they run, the key is the ventilation in the chassis and the top of the amp. The maximum power output is limited by them being run far too hot *electrically* - ie well above the plate dissipation rating - not the bottle temperature. I very much doubt the 'AC40' actually puts out more power than an AC30 either - about 33W.

GordonB

WWWow, well from what ICBM said and other things I've read, I'm pretty convinced that the Fanes are the origonal speakers.

But and a big BUT, they should not have been.

Just got the Vox Blue alnico's fitted and wired and the difference is amazing.

I now have an absolutely awesome Amp Combo.

Only tried it so far with my Frampton Sig Les Paul Custom and the warmth of the humbuckers is beautiful, then push the thing with the middle and bridge pickups together and it sounds like Rockin the Filmore, yes I know Humble Pie used Marshal and Hi-Watt, using the Brilliant channel full volume, bass in the middle, treble right up, using wrong impedance input which tames the top a little, I can pretend I'm Pete Frampton as I did late 60's through 70's, a girl at school told me I looked like him, that made me look to see if he looked like me, saw him on Top of the Pops with the Herd and bugger me he did, but better still I liked the music and so I've followed his ups and downs and back up again.

So I am extemely happy.

I'll not be able to use the amp properly until the weekend when I have the house to myself, then I have a Strat and a 335 Pro to try with it.

Thanks all for your input to this topic.

ICBM

GordonB said:

WWWow, well from what ICBM said and other things I've read, I'm pretty convinced that the Fanes are the origonal speakers.

But and a big BUT, they should not have been.

Totally agree. They are just not a very good speaker.

It's actually another indication that the amp doesn't put out a true 40W - that they haven't blown. There's no way I'd want to put a proper 20W amp through one, the realistic power rating is more like 15W at most.

GordonB

Totally agree regarding the power output of the amp, probably Jennings and Dick Denny didn't want to clash with the new owners of Vox but wanted a name as close as they dare to AC30, so they optimistacally named it AC40, Personally I think they should have called it an AC33.

I'm not knowledgeable regarding the overdriven behaviour of valves, other than they produce even order harmonics, which the human ear likes.

It seems to me that the relationship between speaker power handling/rating and amplifier output, is totally different from transistors.

With transistors it’s better to have more power than the speaker theoretically needs, it is then very unlikely that the speaker will be damaged, quite simply before that occurs it would be so loud it would probably be turned down.

The opposite way about, that is with an underpowered transistor amplifier, trying to get high volume is very likely to damage the speaker, as an overdriven tranny amp clips the top of the waveform, this produces DC spikes both + and -, 2 things then are likely to occur, the cone is driven hard against it’s stops, very likely to decentralise the coil, the other likelihood is the DC heats the speech coil and it could seize and or again decentralise. 

GordonB

Speakers in their new home.