How does a Vibrasonic Vibrato work?

[Voxbox]

Hi All,
I have an early 60s brown Vibrasonic in for overhaul, and vibrato is producing low frequency chuffing. I have to turn the intensity almost to max to get the vibrato effect, but it seems that the low frequency vibrato signal is getting through to the speaker rather than only causing a variation in volume.
Its a wee bit alarming to see that JBL cone pumping in and out…..

I've had a look on the web but haven't found anything really useful so far.

Cheers, Voxbox

[sluckey]

Look at page 9 of this scrapbook pdf. Close enough to explain how it works...

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[HotBluePlates]

See also "The Wurlitzer Vibrato" by Dorf.

It looks like Fender's version attempted to save a few resistors and caps. I'd need to take a moment to completely re-draw it to understand how much of a change there is. I don't know if the Wurlitzer version was patented, with Fender needed to avoid infringement.

Pumping should be avoidable if the output 7025 has balanced outputs (and mixing of those outputs would have been helped with a shared plate load resistor). That's because the two oscillator signals would be 180 degrees apart, and equal signals applied to a mixer tube with equal gain per section would yield cancelling outputs when mixed by the 470kΩ resistors.

[Voxbox]

Thanks guys, the Wurlitzer diagram looks more like the Fender (5G13) circuit. I'll try a balanced triode tonight, but I kinda suspect I have a fault to deal with, especially given the age of the amp.

Cheers, VB

[Voxbox]

HBP, you were right about it being a balancing issue, except it was to do with the 470k mixing resistors on the output of the vibrato mix valve.
All of the amps resistors had been changed by another tech, and whilst one was correct at 470k, the other was only 4k7.
The chuffing has gone, which was causing a large cone excursion, and the output hum has been greatly reduced too.
It might explain why the cone was ripped too, which leads to another question.

The cone ripped in the usual place, at the rim, and has been repaired by placing a line of silicone over it. It seems to be holding up.
Would it be preferable to get a recone?
Thanks a million!

[HotBluePlates]

The cone ripped in the usual place, at the rim, and has been repaired by placing a line of silicone over it. It seems to be holding up.
Would it be preferable to get a recone?

Only the customer can answer that.

Speaker tears range from almost-unnoticeable to catastrophic shredding of the cone. Some smaller tears can be very easily mended to be invisible with some tissue paper glued to the front and back surfaces of the tear.

Some huge tears, or very poorly-executed repairs, impede the cone's motion or cause deformation with an applied signal. These beg for re-coning.

Some customers are biased to believe that any repair or recone is necessarily detrimental to the speaker's sound; others don't mind well-executed repairs or recones, or have enough experience to determine if the sound has suffered.

Repair, recone or replace is really up to the customer, their biases, and the extent of the previous damage. You're in the best position to judge these factors.

[Voxbox]

Yep, I'd agree, its really down to the customer. Personally I'd leave it as it is. I've tested it by running an oscillator through a big, and therefore clean, tranny power amp to see if the cone is rubbing at low and high volumes (sometimes cone rub is masked at higher volumes) and dropped the frequency right down to 1 to 5 Hz. There is some cone rub when the cone pushes out which is only noticeable at these very low frequencies, so in normal use it's probably fine. Sounds ok to me, but then I've never had the opportunity to play through a 53 year old JBL D130F.
The tube chart date code is JA, making it a January 1960 amp.

If it were possible, I'd source another identical speaker and compare, but these things are like hen's teeth, especially in the UK.
Thanks for your help so far, more Vibrasonic questions to come!!!
cheers, VB

[Willabe]

Sounds ok to me, but then I've never had the opportunity to play through a 53 year old JBL D130F.

On old speakers the glue that holds the cone and voice coil together dries out and you will drop the voice coil sooner or later playing through them. Probable sooner.

A new speaker and amp will sound better, less harsh/bright, as you play through it. Give it 50, 100 hours to break in.

Some guys take a small 6.3 or 12.6 volt transformer and run the speaker on the PT's secondary for, say ~4 to maybe ~12 hours to help speed up the speakers break in time. 


                  Brad        

[Voxbox]

That's a great idea about breaking in the speaker.
Sounds a bit like that device that clamps to a guitar's strings and vibrates it 50Hz to loosen the top up (on an acoustic guitar)
I think the cone is just going to die, like you say, sooner rather than later as it is going to be used professionally.

Another question!
Would it be worthwhile getting the speaker remagnetised?

Cheers, VB

[HotBluePlates]

I think the cone is just going to die, like you say, sooner rather than later as it is going to be used professionally.

Hard to evaluate without seeing it.

I've got a JBL D-130 (not "F") with an original cone that was probably made in late-50's early 60's. I have zero reservations using it with a pair of 807's.

The D-130F was a D-130 rethought for the musical instrument market, with the biggest difference being extra "goop" on the speaker surround to help it hold up in outdoor use. These were intended to be battered by the Vibrasonic and the Showman amps (and popped up in other Fenders too).

The Jensen speakers used in Fender were, by comparison, junk. Fender used Jensen's cheaper speakers in amps, and the JBL was always more expensive and much better made than the Jensen line.

I wouldn't worry about blowing the voice coil of this speaker, unless you see something that makes you think the speaker is compromised in general.

Would it be worthwhile getting the speaker remagnetised?

I don't see why. They'e permanent alnico magnets, and again... unless you're seeing something implying obvious damage or degradation, I'd leave it alone.

If you're used to Jensen speakers, and 12's in general, this speaker should surprise you with deep low end and quite a bit more treble extension that you're used to hearing; more so if it has the aluminum dust cover.

[Willabe]

Sounds a bit like that device that clamps to a guitar's strings and vibrates it 50Hz to loosen the top up (on an acoustic guitar)

Sounds like the same thing alright.


          Brad     

[Voxbox]

I've had a close look at the rest of the cone and there are two more splits in it at the edge and a couple of small radial tears at the outer edge as well.
So its looking like a recone will be in order.

Who does decent recone kits? (Either in the USA or UK)
How close are they to the originals?
Can any Brits recommend any UK-based re-coners? I  know of of a couple but not used them myself yet.
Thanks again!

Cheers, VB

[SoundmasterG]

I've had a close look at the rest of the cone and there are two more splits in it at the edge and a couple of small radial tears at the outer edge as well.
So its looking like a recone will be in order.

Who does decent recone kits? (Either in the USA or UK)
How close are they to the originals?
Can any Brits recommend any UK-based re-coners? I  know of of a couple but not used them myself yet.
Thanks again!

Cheers, VB

The D series JBL cones have been gone forever. You can get E series still but they don't sound correct though they still sound good. Generic kits don't sound right at all. Weber probably has the most accurate vintage cones for the D series JBL's today, and they do a great job, but shipping a speaker from the UK and back, especially one as heavy as a JBL would not be worth it. I would just order a recone kit from Weber and have someone good in the UK do it for you.

Greg