Microphonics driving me nuts!

[pdf64]

The way the plate and power supply voltages can be way high makes me think there might be a bad connection in a cathode or ground return path.
The dropper resistors in the power supply are conservatively rated to put it mildly - the 10k between the screen node and phase splitter dissipates 0.8 watts (according to the schematic) but a 25 watt part is specified (safety critical!), the next dissipates 0.009 watts but a 4watt part is specified.
Strange but nothing to do with the problem. Peter.

[Satamax]

The way the plate and power supply voltages can be way high makes me think there might be a bad connection in a cathode or ground return path.
The dropper resistors in the power supply are conservatively rated to put it mildly - the 10k between the screen node and phase splitter dissipates 0.8 watts (according to the schematic) but a 25 watt part is specified (safety critical!), the next dissipates 0.009 watts but a 4watt part is specified.
Strange but nothing to do with the problem. Peter.

Same thinking here. I don't know any other cause of voltages raising, if not B+ rail problem. There's a weird thing which isn't drawn on teh schemo, a surface mount FL4 iirc marked 000 in parallel with the R1, which, when the jack socket is closed makes teh R1 10K go down to 0. I don't see the point in this! Since the cathode res would be fully bypassed by the 2µ2 and increase the gain, isn't it? I'll change the cap for peace of mind. And see what the cathode resistor says. Well, otrherwise i'll do a quick mod. Something fenderish jsut to see what happens. Dai, don't worry. You've helped me great times soo often. Do you remember any discusion about raising voltages in preamp tubes over here. I seem to remember one, did a litle search, but to no avail.

Thanks a lot guys.

Max.

[dai h.]

yeah how about the 2u2 (cathode bypass) ground side which is switched on through the jack. Maybe make sure the jack switching contact is clean and also that the 10k (R1) connections are good (I remember once where I forgot to ground a cathode bypass cap and I got "motorboating"--the putt putt sound). Also, maybe it's fine but from my understanding of grounding if the 2u2 ground is on the same side of the cathode R as R5 (1Meg) it's incorrect and should be on the other side (between the 1M and the cathode there should be no current flowing but if you put the cathode bypass cap ground there the output voltage/very tiny current will go through that ground line portion).

The "000" just sounds like a jumper (zero ohm resistor).

Is this Laney some pretty recent amp with surface mount parts?

[Satamax]

Hi Dai!

Well, yep it has some smc on the underneath of the circuit board. The rest is normal components. I'd say, thoses amps, for modern ones are not too badly built imho. Ill check thoses details you've pointed.

Thanks a lot.

Max.

[dai h.]

also the "raising voltage thread" nothing really comes to mind especially without more details. Maybe you can think of someone who participated and try searching their posts using the advanced search?

[Satamax]

Arghhhhh!!!

Well, solved the high voltage problem, that was a bad trace between R3 and P3But i still have the microphonics between R4 and P8, that's mad, i touch the ebony chopstic onto the resistor pin and i can hear a craclke, then if i tap it lightly, i hear a loud pop! I have HF oscilation as well as audible one. At some point i had 625hz, then some KHZ, then MHZ etc! My tekmeter doesn't seem too good for troubleshooting this. The most flagrant place i have this is on V2B grid wire.

Well, i gonna go to bed, i'm feeling ill, cos it's cold in my workshop. And this amp is depressing me.

Thanks a lot guys.

Max.

[Satamax]

Hi Guys!

Well, sorry to come back to bother you, but i'm realy stumped.

Well, rechecking voltages, i've found 314vac at the first cap, and 129 vac at the plate of V1????? This being for 414 VDC at first cap, and 194VDC at V1 plate. 160vac at C2, for i don't know how many VDC. But there's something buggey in this B+ rail. I've removed the snubbing caps, and changed the 1N4007 for brand new ones. Is my meter too fussy or is this the amp acting? Power caps dead? Or may be the choke has gone astray?

I'll go back to the tube amp debuging page, iirc there's something about B+ rail and caps.

Thanks a lot and best regards.

Max.

[Satamax]

That much AC should be the diodes no?

[Alf]

Max , I didn't read that you tried other good quality tubes in that position.

I once had a similar problem not as severe as you have in a Vox Ac30.

I thought it were bad tubesockets but in the end it turned to be tubes after all.

Alf

[Satamax]

Hi Alf

Thanks a lot.

Well i've swapped the tubes around and with other ones. No effect.

[dai h.]

max, after you put in the new tubes (preferably some cheap chinese or russian ones) you have to push it off of a very steep cliff to complete the process



just kidding man, hope you can figure it out! Some things just drive you towards insanity don't they?

[Satamax]

max, after you put in the new tubes (preferably some cheap chinese or russian ones) you have to push it off of a very steep cliff to complete the process



just kidding man, hope you can figure it out! Some things just drive you towards insanity don't they?

Well, some things make you reconsider your limits, and now i feel verry dumb. I know i've read about a problem verry similar in the years of reading ampage and plexi palace forums. Frankly i suspect a problem of either a cap being flakey, but i wouldn't have 100+ volts AC of riple, even with the bridge straight to the DMM i wouldn't have this. Or may be i have HF AC being amplified somewhere, but i'm not verry sure of this either. A 1N4007 being dead, may be, but i've put brand new ones in, and i've taken out snubbing caps in case one is gone. Or i have leaking between HT ac supply and DC side of B+ But i have to take the DMM back to it and measure frequency, and other things. I don't have a frequency meter on my tekmeter IIRC.

Well, i'll try to get back to it this week end.

Bye.

Thanks a lot.

Max.

[Satamax]

Hi everybody!

Well, sorted, kinda!

C11 and C11 being still microphonic! P7, P8, R4 too! God knows why! Even the solder are microphonic, on a PCB and with good solders!

But the culprir for the horrendous rumble was a shieldied wire, P31! Laney uses shite shielded wire! And the 174vac on the B+ rail must be from my meter, since the scope side of it isn't detecting any AC ! Or it's over 5mhz! I think if i had 174vac at over 5mhz, the amp would melt down! Owner has his amp back!

Thanks a lot for the help guys.

Bye.

Max.

[Alf]

Glad you found it Max , so once again it proves that any part in an amp can contribute to problems , and even parts you never suspected.

Interesting to learn what it was.

I think you were happy to get rid of this amp.


Regards , Alf