Odd microphonic problem

[pgussler]

I have an odd service question. The unit that I have on the bench is an AC15 type boutique amp: EF86 -> 12ax7CF -> 12ax7PI -> 2xEL84 and EZ81 Rectum-frier. It is a combo and the cabinet is the size of a standard Marshall 18 watt. ie: Plenty of room for everything. The speaker is a Weber Alnico Blue Dog.

This problem only occurs with the chassis installed in the cabinet. With the master on 10 and the player rotates the pre-amp gain towards max the amp will start to oscillate with a high pitched squeal until the pre-amp gain is turned back down. The chassis also becomes very microphonic and tapping on the chassis or cabinet can be heard amplified through the speaker. When the chassis is out of the cabinet and on the bench running through the bench speaker both controls can be dimed w/o any oscillation. Also, disconnected the internal combo speaker and connect the chassis to an external cab and there was no oscillation in that scenario as well. This is a clean and well built amp with good lead dress. I have tried the simple tube exchange hoping to get lucky but that would have been too simple.

Any thoughts would be appreciated.

[Enzo]

It may not oscillate on the bench, but is it still microphonic? Tap the tubes, they are the most likely suspects. It is not a large step from microphonic to oscillation - feedback.

Alternatively, when it is in the cab, turn it up so it oscillates, now grasp each tube one at a time firmly. use a rag or a glove if they are too hot for you. Does clamping your fingers around any tube make it stop? Replace that tube.

Tapping and vibration is conducted through the chassis. One tube gets real microphonic and tapping on ANY tube will make it react, tapping on the chassis too. But usually when you get the the bad tube, it is REALLY microphonic.

I just had a Crate BV120 head in here yesterday that was feeding back on itself. The first preamp tube was so microphonic I could tap on the end of the wooden cab and the amp would ring out. Ting, ting.

[olddawg]

Try a different EF86. From what I understand they are prone to be microphonic and you may have to try 2 or 3 before you get on that works right. I have heard this is why Dr Z uses a NOS EF86 in their EF86 equipped amps. You might want to try an O-ring damper or two. You also might ask over at 18 watt.com if you are still stumped.

[pgussler]

Thanks

I am sure that you guys are right. The EF86 is bring driven by an R/C network similar to the AC15 and that is really hot. After looking a little more closely at the load lines I'm going to back off the gain a bit and see if that brings the amp under control. I have a pretty good stock of EF86's and have tried most of them. Some are better than others, again substantiating the idea that the problem is centered on the EF86.

[pgussler]

More Data

I think that we may have been a little off base in looking at the root of this problem. It seems that the oscillation is occurring due to the proximity of the speaker voice coil to the chassis. With the chassis on the bench and a speaker sitting about 6 inches behind the chassis I can recreate the problem. As I move the speaker closer or further away from the chassis the oscillation will increase or decrease with the proximity of the speaker to the chassis. Different speakers all have the same affect. It seems that the speaker is creating a magnetic field that is probably interfering with the ground buss of the chassis.

[Enzo]

Did you do all the microphonicity tests? I mean the actual physical tests.

Bringing the speaker within 6 inches also puts the chassis in the vibration field, and that is far more likely than a magnetic field interfering with the ground buss.

[Robert M. Martinelli]

+1 on the EF86 being microphonic..... The bad news is that at a certain point in time, being exposed to mechanical vibrations, ANY EF86 you'll drop in as a replacement will probably become microphonic, even the ones that behave well in the first place, so the problem is very likely to occur again in the future.

EF86s are mechanically fragile, and known to be ( or become ) microphonic, and this issue is well known since the early '60s. This is the main reason why Dick Denney, while designing the second issue of his AC30, dropped the EF86 and switched to ECC83s.

My advice would be to use an ECC83, combining the two triodes' gain to get the same gain of a single EF86....You'll have to redesign the RC networks though, because the two triodes' Miller capacitance ( obviously absent in pentodes ), multiplied by their gain, is probably going to hinder the stage's high frequency response.

Hope this helps

Best regards

Bob

Edit....I have just seen Enzo's last post, and I agree with him, 6 inches is close enough for the speaker to "shake, rattle and roll" any EF86

[pgussler]

Well there is no vibration because there is no audio signal going to the amp. This problem does not need any input to occur. In other words, with the chassis on the bench, a speaker approx. 6" behind it, master on 10 and the pre-amp gain rotated up towards 10 an overwhelming oscillation will become present at about 7 on the gain volume. There is nothing plugged into the input.

[pgussler]

See reply to Enzo post. By the way I am in total agreement with everything said in your post.

[pgussler]

Actually the problem may be caused by the magnetic field being generated by one of the transformers interfering with the magnetic field of the speaker.

[Enzo]

I think - only my opinion of course - that you need to get away from the magnetic fields theories. The lack of an applied signal is irrelevant. Stick a mic in front of a speaker, and it will feed back - you don't need a signal, it will make its own.

I beg you, just reach in there while it is doing it and GRAB the tube. Does holding it tight in your hand alter the oscillation? if so, it is acoustic feedback, you hand has no magnetic fields.

[Robert M. Martinelli]

I'm with Enzo, I don't think the problem has nothing to do with magnetic fields...
This said, it could either come from microphonics, or from the stage oscillating.

There's a way to state if the problem is generated by the tube oscillating, try to put a cap ( in the 220-330 pF range, and with an appropriate voltage rating ) between the tube's plate and control grid. This cap forms a NFB for the higher frequencies, greatly reducing the stage's gain at those frequencies, so, if the problem comes from oscillation it should stop, if it doesn't stop then your problem is most likely the result of microphonics.

If you find the above mentioned cap cures your problem, you'll have to lower the cap's value to find the "minimum" cap value capable of stopping the oscillation, as greater values will keep the stage from oscillating, but they'll limit the stage's bandwidth on the high side as well.

Hope this helps

Best regards

Bob

[oc disorder]

I hate to go against the grain here but I will..!
Try earthing the frame of the speaker.
I leave the transmitter theory to the experts !

[Robert M. Martinelli]

Hi, oc,
No problem at all for your " going against the grain", after all one of our common goals is to help others fix their problems, so your input is more than welcome, no one holds the "absolute truth", and, being thousands of miles away, we can only offer our "educated" guesses....

As to EF86s, their tendency to be or become microphonic is well known, and being high gain pentodes, ( no Miller capacitance to "automatically" act as a NFB at high frequencies ) oscillation can be a problem as well.

Cheers

Bob

[pgussler]

After going back to Enzo's microphonic hand test I totally agree with him that the EF86 is in fact microphonic. To further compound the problem I have attempted the use of no less than 10 different EF86's most from different makers and they are all in varying degrees microphonic and unusable in this circuit. Most, if not all of these I am pretty sure would be fine in an AC30-4 circuit where the Ef86 output feeds directly into the phase inverter without the cathode follower and fender type tone stack. I tried every remedy suggested here and more from other sources in an attempt to bring the microphonics under control to no avail. I finally made the minor wiring changes to replace the EF86 with a 5879. The 5879 is a much more stable tube I believe and the R/C network can be set up so that it will produce a tone very similar to that of the EF86. Thanks again for everyone's input. I hope this thread helps the next tech out who encounters this problem.