Rivera Fandango 212 combo problem

[Le Basseur]

A customer came 2 weeks ago with his Rivera combo claiming that when played in "vintage mode" (pseudo-triode mode) some notes were sound "strange".Hooked up a guitar,played a few notes and voila,after a few light pencil ticks,the two EL34 winged-C Svetlanas proved to be microphonic (especially one of them,badly...though no visual signs of worn-out).
We ordered a fresh batch of JJ Tesla's selected E34L from Bob (Eurotubes) wich arrived two days ago.I installed the new tubes,biased them and they behaved normally...job done,the happy buddy takes his baby home and starts playing.After an hour,the phone rings...the same thing,microphony like we'd changed nothing.
Opened once more the amp today,put in it a pair of (my) NOS Siemens' and just after a few minutes,microphony's up and alive.Remember,all these only in the triode mode...when switched to pentode,the amp runs normally no matter what output tubes has in it.And,of course,the owner became addicted to the "vintage mode" stuff,no matter what...
Is it possible that a passive component (like,say,the big resistors in the triode/pentode circuit) make such a mess?
Have you experienced anything similar?
Any idea where should I look?
Please help!
Regards,
Le Basseur

[Rob Mercure]

Are the "big resistors" microphonic? Over the years I've seem almost any component - 'cept a transistor - become microphonic. Capacitors are the worst of the passive components but resistors - particularly carbon comp and wirewounds - have become microphonic. Of course tubes are the worst and I truly remember a 6SL7 that I could actually sing into (the fellow who owned the amp stated that it was an "improvement" in my singing voice) but I would suspect some other component that vibrates when you tap the tubes. Hell, change the resistors anyway - they're cheaper than the tubes and perhaps there are poltergeists hiding within.

Rob

[Enzo]

Can't say as I recall either way on xstrs. But I do know the hard way that little TO92s LOOK opaque, but they're not. IR goes right through them and it affects the junction.

[Le Basseur]

Thanks Rob and Enzo!
I have some further comments about what you said:

Are the "big resistors" microphonic? Over the years I've seem almost any component - 'cept a transistor - become microphonic.

Agree!...but....
Is the resistor microphony issue possible in a modern amp,manufacured a year or two ago with modern components (not even NOS ones),played only a few months?

I truly remember a 6SL7 that I could actually sing into (the fellow who owned the amp stated that it was an "improvement" in my singing voice)

Slightly offtopic...my fave is a 1943 NOS NIB Ken-Rad 6SJ7 metal-jacket pentode wich acts like a crackled microphone through a bad reverb tank!

Can't say as I recall either way on xstrs. But I do know the hard way that little TO92s LOOK opaque, but they're not. IR goes right through them and it affects the junction.

Sorry Enzo but your post is a little cryptic for me (my poor English,again...) as I don't fully understand your tech slang and shortcuts.
What do you mean by "xstrs"?Maybe "extras",if read phonetically,but what is this?
Regarding the TO92s,I presume you're talking about the ICs (yes,the Rivera combo has a few).The IR (InfraRed,if I got it right) issue you mentioned is a very valuable info as I never encountered such a problem with ICs (it'just me with my limited knowledge and experience) and this points in an unexpected direction...thank you for mentioning it!
I was thinking that some of Ampagers had already to deal with such a problem as mine (not necessarily in a Rivera...I guess there are some other "triode mode" PP output stages,even experimental ones,in use).
If yes,what did you found?
Frankly,it's the "time interval" that bothers me the most.The amp looks stable at the beginning but after a 1/4 of an hour goes whacky.What component could take such a period of time to make havoc in the output stage,and why this specific part doesn't manifest immediately,say within the first 5 minutes?
Again,thanks!
Regards,
Le Basseur

[Rob Mercure]

For some reason in American slang "trans" has been replaced with an "x" thus Enzo meant transistors - I've seen this "abbreviation/symbol" used for transformers, transmitters, transcievers, etc. - probably comes from an old military or ham slang.

Any part made at any time can be microphonic and parts with a time delay, IMHO, are parts that heat up - tubes and resistors and that occasional capacitor with sufficient leakage to get hot. Again, while probably not in this amp, carbon comp resistors are notorious for various failure modes when hot - do you have some variety of "circuit cooler" available? You use this stuff to cool a specific suspect component. Conversely you can purposely heat a suspect component and see if it will act up more quickly. Whatever is microphonic, once heated, should be readily detectible though.

Rob

[Enzo]

Agree!...but....
Is the resistor microphony issue possible in a modern amp,manufacured a year or two ago with modern components (not even NOS ones),played only a few months?

One piece of advice learned as always the hard way:

NEVER think of reasons NOT to check something.

When you allow yourself to think, "Oh it couldn't be THAT part, it's too new." or anything similar, you are asking for trouble.

I am sorry LB, I didn't mean to be obscure. Yes, X means "trans", so xstr means transistor, xmit means transmit. You will often see the send/receive switch ona short wave marked "XMIT/RCV"

My little diversion realy had nothing to do with your problem, I added it after Rob mentioned he had never seen a transistor get microphonic. I related my story because they might not get microphonic, but they can fool you in other ways.

"TO" numbers like TO92 or TO3 refer to transistor shapes. TO means "transistor outline." TO92 are the little black plastic ones that are round, about 4 or 5 mm in diameter, but with one side flat, and three little legs. You see them in stuff all over. The black plastic looks opaque, but if you shine a bright light on them, it can affect their function, and confuse test readings.

I was not referring to parts in your amp, it was just a general observation.

Yes, IR means infrared.


ANd there are a lot of components that take a while to get warmed up fully. That is why an amp should be running for a solid 20 minutes before making a final bias adjustment. Not only do components like resistors and tube heat up, but even caps can heat up, both from sitting next to something else that is hot, and from their own internal processes. But caps can also be marginally leaky, and it takes a while for the insulation or dielectric inside to start breaking down.

Are you sure it is the tubes themselves? If you tap on a microphonic tube, noise comes out the speaker, sure, but when you tap on the tube, you are also sending little shock waves through the chassis by way of the socket and what it is anchored to. In other words, rap on the chassis, bang on all the parts. Rapping the tube might be affecting something nearby. Just a thought...

[Le Basseur]

Gentlemen,
Thanks again for replying!

For some reason in American slang "trans" has been replaced with an "x" thus Enzo meant transistors - I've seen this "abbreviation/symbol" used for transformers, transmitters, transcievers, etc. - probably comes from an old military or ham slang.

Got it,thanks!
...but wait...how's the "Christmas" thing when written "Xmas"?...it's a "trans" or not?(...just kidding!)

Again, while probably not in this amp, carbon comp resistors are notorious for various failure modes when hot - do you have some variety of "circuit cooler" available?

I'm not sure what device are you referring to...if it's a hair dryer without the heating elements,yes,I have it and I'll try using it for a forced airflow on a specific (susceptible) component or PCB segment.
If you're thinking of a cryogenic installation (liquid nitrogen),no,I'm not that rich!

NEVER think of reasons NOT to check something.
When you allow yourself to think, "Oh it couldn't be THAT part, it's too new." or anything similar, you are asking for trouble.

Thanks Enzo for the cold shower,I have to admit that I deserved it!
What I meant to say is that when you spell "Rivera" you'd think on a much more stable and verified stuff than,say,a Chinese-made Fender or whatever.At least,this is the Rivera's image in my buddies' eyes.Of course,from my own point of view,I totally agree with the non-prejudiced mind principle.After all,all amps are prone to the known "SH*T HAPPENS" thing...

I am sorry LB, I didn't mean to be obscure. Yes, X means "trans", so xstr means transistor, xmit means transmit. You will often see the send/receive switch ona short wave marked "XMIT/RCV"

No problem,Enzo!
It looks like I'll have to renew the visits to my older radio-amateur friends...at least,for a freshened-up terminology!

ANd there are a lot of components that take a while to get warmed up fully. That is why an amp should be running for a solid 20 minutes before making a final bias adjustment.

I'm well aware about what you said.Really...each time I get an amp for biasing,I spend around 30 mins for warm-up and another 30 mins,sometimes more,for the biasing itself....just to be sure that the tubes are working like they're supposed to do.

Are you sure it is the tubes themselves?

Once that you pointed this,...nope,at this moment I couldn't swear about it.

If you tap on a microphonic tube, noise comes out the speaker, sure, but when you tap on the tube, you are also sending little shock waves through the chassis by way of the socket and what it is anchored to.

This is exactly what happens in the Fandango...I'll give it a run tomorrow and I'll report back as soon as I manage to find out what's all about.
Regards,
Le Basseur

[Rob Mercure]

LB,

By circuit cooler I meant a spray product that chills a specific component - I'm not sure what's on the market these days but back in the 1970s-80s we used a CFC "ozone killer" in a spray can and it would give you frost bite if you sprayed enough on your finger. I believe that there are similar products now using CO2 or some such. Or if you can get some anhydrous alcohol of some sort you can put a bit on a q-tip and swap a component - it doesn't work as well as the freons but on a hot part can evaporate quickly enough to cool it. Conversely you can use a low wattage soldering iron/gun to heat a specific component if you're careful not to damage any plastic covering (not a problem with older style carbon comp resistors).

Yeah, Rivera's are good amps but I did warranty work on them in Cinty and like any product some will fail. I don't know if he's as "available" anymore but Paul Rivera would answer the phone at his plant and he was good a describing modifications that had been made to an amp that a customer brought in that weren't reflected in the "official" schematic - nice guy as is his son.

And don't forget a bad PCB solder contact - if I remember correctly the Riveras had the tube sockets directly soldered to the board which would transmit any mechanical movement to all of the components within a certain radius.

Rob

[Enzo]

We call the stuff "freeze spray" around here.

And I know you were kidding, but the X in Xmas is the Greek letter Chi, and is the first letter of "Christ" as spelled in Greek. SO it is equivalent to writing Ch-mas. COntrary to popular opinion, it is not a way to remove Christ from Christmas.

[Le Basseur]

After several days of testing/probing/freezing/heating/changing suspect components with the chassis out of the box,I concluded that nothing's wrong.Again,put the chassis back in the combo and,voila,microphony again...
The Svetlanas are totally gone (very noisy) and the newer JJs are developing a slight "bzzz" after a certain note played on the guitar...a sign that they're gonna go south very soon.
I concluded that this is happening because the VERY tight contact of the output tubes' sockets to the chassis.Simple as that....when I put some kind of dampening on the tubes (when tubes aren't very hot,in the first 5-10 minutes of working),the noise dissapears,but this is only a temporary solution as I don't see what could survive thermally in that place.
Obviously,this is a very embarassing situation (Rivera makes great stuff but I think they overlooked this aspect when they decided to make the PCB/tube sockets stuff soooo tight...output tubes wouldn't survive too much in that shaky environment,no matter what brand they are).Currently,I'm looking for an alternative solution/mod in order to isolate the sockets from the chassis as much as possible by slightly lifting the PCB on some kind of spacers (this means a socket replacement since the existing ones have short legs) or maybe....I'm really stuck at this moment.The main issue is that there's relatively a small clearance between the PCB and the "Vintage/Modern" switch placed on the rear panel,right above the PCB.This is gonna be tricky to solve....
How do you guys solve (radically and efficient) a microphonic construction?
Any input greatly appreciated!
Regards,
Le Basseur

[Le Basseur]

Hello again!
After analyzing the situation from all angles,there's no way for a light physical modification on that PCB.Sure,it could be done but on the expense of some major metal work on the chassis (enlarge the sockets' holes and acoustically decoupling them from the chassis) wich I'm not willing to do...and neither the amp's owner.
There are two other solutions,though:
1.Instead of a pair of EL34 I thought using a pair of 6L6/5881s (a slight loss of power doesn't bother).The later seems much less prone to microphonics (a stiffer internal construction) as I never had any such problems with them.The question is,how's a pair of 6L6s fitting with a EL34-made OPT in terms of primary impedance?From the top of my head,I recall that a PP EL34 needs a lower primary than a pair of 6L6s.Is a good and doable idea the swapping?
(of course,I know that I'll have to check the pin connections and to adapt the PCB for a 6L6 compatibility).

2.If I can't cure the cause,I'd might be able to minimise the effects...by the mean of using some dampers.
Here,I have two possible solutions:

a.To use some tube dampers/coolers like Pearl Tube Coolers:
http://www.pearl-hifi.com/
In the past,I had great results with them about taming some nasty 5687s...but I never used them on power tubes.

b.To make some dampers using some kind of impreganted kork (I got such stuff from a friend who works for the local power plant-they use this material as electrical and thermal isolators).I'd cut some little square pieces (1/2-by-1/2 inches each)from this material and I'd put 3 of them radially on the glass bulb using a steel spring to keep them in place.In a few minutes of testing,I'd be able to determine the place where they are most efficient as microphony suppressors.

Any comment or suggestion heartly welcome!
Thanks,
Le Basseur