Output transformer secondary wiring & feedback

[carausius]

On my amp a 6G16 Vibroverb type build, I have used various makes of speakers with my combo & external cabs without any problems, single 8ohms & 2 x 8ohms wired in parallel for 4ohms as I have a output transformer with both 4 & 8ohm taps.

But when I try to connect any of my 3 JBL D130F speakers I get howling loud feedback straight away, even changing the speaker polarity doesn't work. I'm assuming the OT secondary wiring is wrong, the blue & brown wires to the output tube plates? Can I just disconnect the negative feedback wire to the speaker output to confirm this? Or should I just go straight ahead & swap the OT secondaries?

Very weird how it has worked fine without any of these problems up until  now!!  

[Willabe]

Here, read this, it's on JBL's and guitar amps. I think you'll find it interesting.

http://www.el34world.com/Forum/index.php?topic=14406.0

Might not be your problem but you should still be aware of this with JBL's.


              Brad      

[carausius]

Cheers Willabe,

Just quickly reading that thread makes a bit of sense of what I'm experiencing.

I forgot to actually mention I also tried the JBL's with my '72 Twin Reverb (main speaker output) & it works fine.

The TR has a solid state rectifier as does the Brownface Pro 6G5 (A), Vibrasonic 6G13 which both usually  run a 15" JBL speaker, both also with SS rectifiers.

Both those Brownface amps have a 100K NFB resistor compared to my 6G16 10K.



 

[HotBluePlates]

Yes, you can unhook the feedback wire to see if it stops what's happening.

This is odd, because I wouldn't think a Fender amp has enough feedback to risk being unstable with a speaker change...

[carausius]

Works fine with the negative feedback disconnected from the speaker output with the JBL D130F.

Turned up to 10 & still no feedback, it is a little bit "ratty" so maybe I should try a different value NFB resistor?  It does sound really good though! 

[HotBluePlates]

Swap the OT wires. Should cure your feedback issue.

I don't know why you never had problems with other speakers, unless maybe you used a larger value series feedback resistor than stock (for less feedback), and the loop's phase is effectively changed by a very different amount of reactance with the JBL's.

[stratele52]

On my amp a 6G16 Vibroverb type build, I have used various makes of speakers with my combo & external cabs without any problems, single 8ohms & 2 x 8ohms wired in parallel for 4ohms as I have a output transformer with both 4 & 8ohm taps.

But when I try to connect any of my 3 JBL D130F speakers I get howling loud feedback straight away, even changing the speaker polarity doesn't work. I'm assuming the OT secondary wiring is wrong, the blue & brown wires to the output tube plates? Can I just disconnect the negative feedback wire to the speaker output to confirm this? Or should I just go straight ahead & swap the OT secondaries?

Very weird how it has worked fine without any of these problems up until  now!!  

Problem is at primary wires , reverse them.

[carausius]

Thanks Guys! I'd assumed that if the OT wires were around the wrong way you would automatically get the howling feedback, never thought that it might depend on the speaker used.  

It actually has made the amp less noisy, I can only really hear the tremelo pulse at high volume settings now. I've not even connected the 10K NFB resistor back in circuit yet, I'll do that later as I actually like it without for now.


The 6G16 has a negative feedback resistor pair of 10K / 470R, different values compared to the other Fender Brownface amps.
 
The Tremolux (2 x 10") has a 56K / 1.5K pair (as does the Deluxe & Vibrolux)

There's a 56K OR 100K along with a 25K Linear Presence Pot c/w 4.K resistor on the Super, Pro, Vibrasonic & Bassman.

It's probably also worth trying some different values for NFB in this amp. Maybe a Presence Pot?  

[HotBluePlates]

My experience with it has only been a full-power howl, not odd noises that depend on the speaker. But, I've learned to trust that if oddness stops when the feedback wire is disconnected, that you should reverse the polarity of the feedback.

[alerich]

I'd assumed that if the OT wires were around the wrong way you would automatically get the howling feedback, never thought that it might depend on the speaker used.

The flipped primary OT wiring thing can be a little deceptive for other reasons at times. On one of my home brew amps everything seemed fine and it sounded great on first power up. However, I lived in an apartment where I could not really dime it. Once I was able to take it to a space where I could air it out the howling began with the same speakers and other setup and reversing the OT primary fixed it right up.

[Geezer]

I've had the same problem (reversed polarity not being evident at first) and discovered a little trick to determine if the primary OT wires are indeed correct.......

*Disconnect the NFB
*Turn on the amp & turn it up to where you can hear some hiss (if you have a really quiet amp, plug in a guitar cord & let it hang, not plugged into a guitar, to generate some noise
*Now CAREFULLY reconnect the NFB while listening to the noise....I use an alligator clip wire connected to the hanging NFB resistor & touch it to the circuit while listening for volume change results.

Does the sound become quieter/less when NFB is reconnected? Then the OT primary is correct....NEGATIVE feedback is being applied.

OR....Does the sound actually get LOUDER?? If louder (even if just slightly), then your OT wires need to be reversed....you are applying POSITIVE feedback to the circuit.

When building a new amp, I leave the NFB resitor connection until last, and leave the OT primaries long enough so that they can be switched, if need be. Once the amp is up & running, then I do the NFB test & adjust OT wires accordingly.

G

[carausius]

I actually tried a lot of speakers before the problem suddenly appeared, Jensen C12 RI, Emi Cannabis Rex 12, Greenback, JBL E110's, Emi Copperhead, Emi 102 Alnico.!!! 

None of them were noticeably noisy apart from the JBL D130F's, an obvious difference in noise & hum with OT polarity reversed & NFB connected on or off!

Luckily I left the OT wires long enough to swap over