Vox T60 cabinet sounds horrible

[purplehearted]

Hi there,

I bought a T60 cabinet a while ago which has an Celestion G15C + G12M speaker in it. It was installed with a crossover capacitator, which practically filters the low freqs from the 12inch speaker.
I am playing through an VOX AC50 from 1965. I noticed that the overal sound is very unbalanced. When I play a D or C note on the E string, it sounds very thin, but when I play for example an A note on the E string, it is a huge sound.
I am no tech, but my gut tells me it has to do with the moving air in the cab from one speaker interacting with the other

I tried fixing it by removing the crossover capacitor inside the cabinet and the problem is still there.

I am really lost on what could be causing this and how to solve it. Could it have to do with the fact that the G12M is louder (more modern speaker, higher efficiency)?
Could I solve the problem by buying two identical (2x15inch) speakers? If so, which specs should I look for to have a vintage 60s vox tone? JBL K140s / JBL E140s perhaps?
Or would a combination of a matching 1x15 + 1x12 speaker work with the capacitor?
Or does it not have to do with the speakers at all and may the problem lie with the lack of damping inside the cabinet for example?

Any advice or experiences are highly welcomed!

Photos are of the speakers now installed. The G15C reads 50watt, bass resonance 55hz; the G12M reads 25watt, bass resonance 75hz

 

[Redbrangus]

Without a bit of experimenting, it's hard to say exactly what is happening. The design concept is that the 15 operates full range and the 12 has a single-pole high-pass filter that rolls off the LF at a gentle rate (6dB/octave) at some unknown frequency. Inherent in that concept is that the 12 will be 90 degrees out of phase at that frequency -- specified as being where the roll-off is 3dB down. That's bound to cause some audible cancellations, but since you've removed the HPF from the circuit and are still hearing the effect, that's not it. Do be aware that the 12 is only rated for 25 watts -- half the rating of the 15, and without the HPF it will be a lot easier to blow it up playing bass through it.

It seems more likely to me that what you're hearing is a cabinet resonance. Is the cab vented or sealed?

Edit: How is the apparent loudness of the A-flat and G just below that A? Are they about as loud as the A, or are they also quieter? And just to be certain, I'm presuming you do not experience this phenomenon when playing this same bass through a different amp and cab?

Re converting it to a 215 cab: It might not be a terrible idea -- that 15+12 configuration is kinda goofy. You would need sufficient internal volume along with room on the baffle board. It might be a better course to block the hole the 12 is in and make it a 115 box. Or how about a 212 cab -- that's a much easier mod. In any case, if you're looking for a vintage Vox sound, JBLs of any sort would not be a good choice -- they'd sound WAY better. Presuming they're in a properly engineered cab, that is -- no driver sounds its best when substituted willy-nilly into a random box.

You've got a bit of rare bird in that cab -- as nearly as I can tell, it was only offered in that configuration for one year, 1963. After that, Vox returned to the 215 config and designated it the AC-100 Bass, although some refer to it as the T-100. It was intended to pair with the AC-100 head, and that was the one that Paul McCartney was known to play on stage.

 

[Redbrangus]

Still another edit to the above: using the exterior dimensions, I estimate the internal volume of the cab to be 3.2 cu. ft. at best, or probably just big enough for a single 15 in a properly ported cab.

 

[purplehearted]

Hi Redbrangus,

Thanks for replying to my message. The T60 cab is sealed. I have tried to put some foam inside, but it doesn't seem to do much.
When I play through my Foundation cab there is some difference in note loudness, but much less than with the T60; also other notes resonate.
Does that mean I can conclude that it doesn't have to do with the room resonance frequency?
I will try soon the two speakers separately to see if they perhaps affect eachother badly. And I'll try it in another room.
I tried to play a bit with a Boss bass-equalizer to get some loud frequencies out the way and it only worked a bit (to turn down the resonating notes a bit), but that also makes the thin-sounding notes seem even thinner.

The search goes on... I have to keep myself from getting too frustrated

 

[Guitalia]

Just looked at my 1965 Vox AC50 head to check the speaker impedance choices. If yours is like mine, there's a jumper plug (positioned below the speaker output jacks) that enables you to select the 8-ohm tap or the 16-ohm tap of the output transformer. Assuming that both Celestion speakers are 8 ohms, you should confirm that the speakers are wired in series and that the the jumper plug is set to 16 ohms.

Also, it might be worth taking the head and cabinet to a qualified amp tech to have them checked out, if you haven't done so. My AC50 head was beginning to sound weird, so I took it to the oldest musical instrument shop in town, where the wizened old tech replaced some bad preamp tubes and a burned resistor. (FYI, for anyone reading this who is in the Baltimore area, it was Ron Cook at Bill's Music, in Catonsville, MD; great guy, great tech.)

 

[purplehearted]

Just looked at my 1965 Vox AC50 head to check the speaker impedance choices. If yours is like mine, there's a jumper plug (positioned below the speaker output jacks) that enables you to select the 8-ohm tap or the 16-ohm tap of the output transformer. Assuming that both Celestion speakers are 8 ohms, you should confirm that the speakers are wired in series and that the the jumper plug is set to 16 ohms.

Also, it might be worth taking the head and cabinet to a qualified amp tech to have them checked out, if you haven't done so. My AC50 head was beginning to sound weird, so I took it to the oldest musical instrument shop in town, where the wizened old tech replaced some bad preamp tubes and a burned resistor. (FYI, for anyone reading this who is in the Baltimore area, it was Ron Cook at Bill's Music, in Catonsville, MD; great guy, great tech.)

Yup, mine has an 8-ohm and 16-ohm output. No jumper plug, though. Not sure if that's a problem? I think the outputs have been modified by the previous owner, because there is written 8ohm and 16ohm on them. I can make some pictures when I am in the rehearsal room again this weekend.

The amp should be working fine, I had it fully serviced a few months ago.

I will try my Foundation head for comparison too.

 

[Redbrangus]

The G15C reads 50watt, bass resonance 55hz

I will observe that the offending note, the low A, is exactly 55 Hz, the same as the inherent resonant frequency (Fs) of the 15.

I would have thought your speakers would have been wired in parallel for a 4-ohm total impedance, but maybe that's not the case. At any rate, that's not the cause of your offending resonant note -- it's most likely the poorly designed cab that's the culprit. If you want to be sure it isn't room resonance, take the cab outside and see if it still happens.

It's not too surprising that a 7-band pedal EQ didn't help any -- you'd need a filter with much narrower response to do what you think you're trying to do, but that isn't likely to help anyway. You have an acoustic problem -- you need to solve it acoustically.

As I said, if you're determined to use that cab, you could replace the 15 with a half-decent driver and cover the hole where the 12 was with a piece of plywood with an appropriately-sized port in it. Another concept would be just completely covering the hole where the 12 was and trying to find a 15 that would work in that small of a sealed enclosure.

 

[purplehearted]

Yeah, first they were wired paralel, with the capacitator connected. But I tried with my tech to put them in series without the capacitator to see if that solved the problem; it didn't.

It just makes me wonder that these cabs were used by many beat groups from the sixties. But I never heard this problem from any stories from them - neither have I heard such a thing in recordings.
I know a number of recordings from that era that were most likely played through a T60 cabinet and they are among the recordings with the best bass tone I ever heard (personal opinion of course).

I think I'll try the speakers separately to see if the problem is gone then. If so, I know it has to do with one speaker affecting the other (air movement?). Then I may be able to solve it by building a separation in the middle of the cab, although I would prefer to leave things as original as possible.

 

[Redbrangus]

Well, as I said, that weird-beard 15 + 12 design was only used that one year, in '63, and was likely abandoned for good reason. It was the so-called T-100 that was popular, although it seems likely that the earlier T-60s, with 2 x 15s were also around in those early days. As far as recordings go, there's no telling what may have been used for an amp, if anything -- what artists were playing on stage bore no correlation to what was being used in the studio.

Keep in mind that in 1963, nobody knew much of anything about building bass speaker cabs; Fender's offering of the era was 2 x 12's in a much-too-small box, for instance...and that was one of better ones.

I think I'll try the speakers separately to see if the problem is gone then. If so, I know it has to do with one speaker affecting the other (air movement?). Then I may be able to solve it by building a separation in the middle of the cab, although I would prefer to leave things as original as possible.

If you disconnect one speaker, short it's two leads together to help dampen out the unused driver's tendency to resonate at the same frequency as the working one, but in opposite polarity -- that can give you some really odd artifacts. The reality is, you need to remove the driver and temporarily block the hole with something rigid if you really want to isolate the effect of one speaker on the other.

I'll wager a pint that what you suspect -- that the undesirable resonance is a product of some sort of interaction between the two drivers -- has nothing to do with anything. But hey...one never knows.

 

[Guitalia]

Yup, mine has an 8-ohm and 16-ohm output. No jumper plug, though. Not sure if that's a problem? I think the outputs have been modified by the previous owner, because there is written 8ohm and 16ohm on them. I can make some pictures when I am in the rehearsal room again this weekend.

The amp should be working fine, I had it fully serviced a few months ago.

I will try my Foundation head for comparison too.

Here's a photo of the speaker jack plate and jumper plug I was talking about. If you were originally running two 8-ohm speakers in parallel (i.e., as a 4-ohm load) and the amp had been rewired internally to match either the 8-ohm or 16-ohm output transformer tap, it's at least possible that something was damaged as a result. I hope an amp tech will jump in here to provide an informed opinion of that likelihood.

 

[beans-on-toast]

The T60 was never a good sounding cabinet. McCartney used it for a short time. Vox followed up with the T100 which had two fifteens.

 

[Redbrangus]

The T60 was never a good sounding cabinet.

This is what I'm sayin'. Fretting about the inadequacies of this old arcane cab is fruitless. Being committed to 'keeping it original' means being accepting of the performance of the original design, such as it is. No amount of tweaking around the edges is going to fix what was an uninformed and unfortunate design.

IMHO anyway, and YMMV.

Edit: I'm not meaning for this to sound like a such a dis' -- this was State-Of-The-Art, circa 1963. As I stated earlier, nobody knew nothing about building no bass guitar speakers back then -- no manufacturer's offering was anything more than speakers in a box. Or more accurately, a stab in the dark.

 

[beans-on-toast]

I think that you well characterized the approach that VOX took. The Marshall 412 was more based on using a single sheet of plywood per cabinet.

Even back in the 50’s, speaker manufacturers provided plans for cabinets for audiophiles. It helped sell speakers. There was also golden design rules of thumb. It is true that some often designed musical instrument cabs more by the seat of their pants. This was followed by extensive listening tests and revisions. It was an important part of the process that’s still part of current design.

Altec’s Voice of the Theatre speaker was first sold in 1945. It was intended for loud reproduction. A lot of bands adopted these for PA systems. Every once in a while there were happy accidents, the B-15’s double baffle cabinet in the early 60’s was designed for the bass and still available today.

Some interesting references about the early days and the evolution of cabinets.

Loudspeaker

 

[Guitalia]

I think that you well characterized the approach that VOX took.

Even back in the 50’s, speaker manufacturers provided plans for cabinets for audiophiles. It helped sell speakers. There was also golden design rules of thumb. It is true that some often designed musical instrument cabs more by the seat of their pants. This was followed by extensive listening tests and revisions. It was an important part of the process that’s still part of current design.

I remember leafing through a British book on speaker cabinet design in the local library in the early 1970s. Wharfedale designs were apparently highly regarded at the time. As I recall, the book ended with what looked like a hastily incorporated late addition enthusiastically describing an easy-to-construct design for the home hobbyist, meant to be built in two corners of the listening room, that consisted mainly of cinder blocks and mortar.

 

[Bob McAlister]

I know nothing about the cab you have, but I used a friend of mine’s Royal Guardman back in 1968 and it sounded wonderful!!!! I used it in a 3 piece power trio playing Hendrix and Cream tunes played pretty dern loud!! The speaker box was closed...I think. It had 2 12” silver bulldogs and a small horn. The guitar player played through a Super Beatle with 4 12” silver Bulldogs. The drummer was a Ginger Baker sound alike, double bass and all. The Royal guardsman’s 2 12’s handled all I threw at it! Maybe see if you can find an unmolested one, I think you’ll love it. Good luck!

 

[purplehearted]

Here's a photo of the speaker jack plate and jumper plug I was talking about. If you were originally running two 8-ohm speakers in parallel (i.e., as a 4-ohm load) and the amp had been rewired internally to match either the 8-ohm or 16-ohm output transformer tap, it's at least possible that something was damaged as a result. I hope an amp tech will jump in here to provide an informed opinion of that likelihood.

View attachment 4188507

I could have my amp tech look at it, but the previous owner of the T60 (who has been an amplifier builder / restorer for nearly 50 years) told me that it is not 4 ohm when wired paralel, because of the crossover capacitator that changes the ohmage. So it should all be fine.

 

[Guitalia]

I could have my amp tech look at it, but the previous owner of the T60 (who has been an amplifier builder / restorer for nearly 50 years) told me that it is not 4 ohm when wired paralel, because of the crossover capacitator that changes the ohmage. So it should all be fine.

I forgot about the capacitor having been in the circuit. The problem is puzzling, but at least there are only a few unusual variables involved. I'll be watching this thread to see how this gets resolved.

 

[808State]

The G15C and G12M are pretty well known classic guitar speakers.

Even with Guitar both these speakers are rather boomy in a sealed enclosure.

Guitar fundamental is around 80hz and the speakers are too boomy.
Use a bass around 40 hz and I could only imagine.

@Redbrangus
Seems to have the right Idea.
He seems to already calculated the volume of the cab around 3 to 3.2 cubic feet.

The bad news = this would be a awful cabinet for a G12M or G15C
and especially not together.

The Good news = 3 cubic feet is a very average volume used for 1x15"
And would support a driver that works sealed in 3 cubic feet.

Basically Redbrangus already called it.
The 12" should be removed completely and the hole sealed.
Then just use it as a 1x15 for bass use.

The reason some notes " Bloom" out is because they G15C doesnt work well sealed at all.
Cabinet Q or Qtc would be close to 1 to 1.1 Qtc.
Ideal you want a Qtc around .700 to .800

Im sure your saying...what the hell is Qtc.
If I showed a Graph of the G15C Qtc and a Qtc of a more suitable driver I think it would make sense.
It causes a lowend to drop off very quickly, and their is a huge huge peak before this happens.

Whatever note is in the High Q of that peak response will bloom like crazy as you describe.

 

[808State]

Keep in mind a modeling program can show you the Qtc of a speaker in a set volume.

Unfortunately another thing working against you is the depth of the cabinet.
Modeling program wont show you the affect of a shallow cabinet.
Very shallow cabinet will raise Q

Im sure its already at a very high 1.1 and the shallow cabinets pushes it up higher to around 1.2 or 1.3 Qtc

Green line is the Greenback G15C Qtc = 1.2
Yellow line is a Legend CB158 Qtc = .625
85 liters / 3.0 cubic feet

any note you play will bloom like crazy in that peak.
It is a 3dB peak.

158 would be overkill for 60 watts, but its designed for sealed

the older K140 and EV 15L or M would work as well.
158 be cheaper easier to find.
The 12" needs to go and the hole sealed

 

[Buster53]

I know this is an older thread but I recently acquired a T60 cab - the 1x12 and 1x15 model and restored it. I’m running matched Celestion speakers wired in parallel with no crossover capacitor and using a Vox AD100VTH modeling amp as a head and the cab sounds great!
The 12 & 15 combination may be a bit unusual (I also have a few Kustom 2x12 and 1x15 rigs) but I find it quite satisfactory - likely thanks to the modern speakers.