mycrophonic

[castor]

Hi to everyone...
after changing the OT (from Ceriatone to Heyboer - both wired at 6600 ohm) I started having problems with mycrophonic.
The problem now starts when I put the airbrake to about -6 db to 0 db...
everything I touch in the amp, from the knobs to the tubes to the chassis make a bump like a mycrophone and even with volume at 0 sometimes I hear that orrible mycrophone whistle and I have to put the amp in stanby.
What I did:
- tryed all tubes in different positions... found a good serie
- put the alu cover on the 1st tube (a bit reduced) but the mycrophonic arrives only sometime
- rewired all pre and power according to francesca pics
- changed and rewired all 6.3 V wires according to francesca.
- moving the pre wires while the amp is on does not change the situation
As I wrote above many times I have this mycrophonic wistle.. not everytime.
I don't want to change the amp... what can I do?
pre tubes are ei 12ax7.
Do you think that changing the wire between the input and the 1st tube will change something? It's the only one cable I have not changed.. (all others are not solid core wires) just because I have not better wires with ground now at home.
Do you think that the alu cover on the bottom of the amp is important? since I'm still working on it the amp is opened, not closed.

NOTE: the sound of the amp is very good with a ceriatone OT, but is fantastic with Heyboer one!!!!!! the best amp I ever played.. both for clean and for solos.

castor

[doctord02]

Have you tried reversing the OT primary leads? Since you changed the OT, you had a 50-50 chance you wired thos two leads the exact opposite of how they were originally wired...

Your decription of the sound hints strongly towards this...

[paulster]

The blue primary should be on tube connected to the 82K side of your PI and brown on the 100K side.

Setting the presence control to full is usually a good way of establishing whether you have the primaries reversed (i.e. positive feedback).

[markr14850]

I definitely had whistle issues due to an open chassis bottom.

Details here: http://www.ampgarage.com/forum/viewtopic.php?t=3601

--Mark

[philmanatee]

Having a shielded bottom on the chassis is a must. Phil

[castor]

Tnaks to everyone... this evening I'll try all these possibilities.
As I can remember, due to luck, I connected the blue and brown wires in the right position.
As soon as I'll have news, I'll post a message.
The other pre wires are now in the best position... moving them will casue more noise or mycrofonic.
I've no foam at home but as soon as possible i'll buy a few.
Before I have the Alu bottom but it is glued to the bottom of the cab.
Now i'm waiting for a new cab and anyway at 90% my amp is reversed just because i like to move/change/test mods...
This evening I'll have a new alu bottom..

castor

[gcenker]

Castor, your problem sounds VERY similar to mine. Here's what I found out after MUCH cursing ....

#1. If your Ceriatone came with a DPDT bright switch, change it to a SPDT. Just go to Radio Shack and get this one:
http://www.radioshack.com/product/index ... 2062486&cp

#2 The blue wire coming off the volume pot going to pin 2 of V1 needs to either be bent as to not touch ANYTHING or (my preference) replace it with RG-178 coax. Be sure to ONLY ground one end of the coax or you may introduce a ground loop. This stuff is awesome in the fact it's solid shielded coax:
http://store.haveinc.com/Ebus30/Parts/P ... de=CURRENT

#3 Install a 33k (or 68k if you prefer) directly on to pin 7 of V1. THis will tame internal oscillations and eliminate RF interference.

#4 Install an aluminum panel on the underside of the amp

Hope this helps,

-Greg

[tele_player]

#1. If your Ceriatone came with a DPDT bright switch, change it to a SPDT. Just go to Radio Shack and get this one:
http://www.radioshack.com/product/index ... 2062486&cp

He's in Italy, so Radio Shack might not be available to him.

I also wonder how this might have any effect, and if it does, would strapping the poles together on a DPDT also have an effect.

[castor]

This evening I have been out for playing organ.. so I made some tests only for some minutes and here it's 10:35 PM... to late to make noise.
I cheked the wire connection of OT.. ok.. connected right.
added the alu bottom (thickness 1mm) to my box... everything seems ok.. just on the edge of stability and a fantastic sound!!!
yes, I satred with a Ceriatone kit to mount by myself (it's difficult to find all parts in Italy).. but at the end the only things from ceriatone are the PT, diodes, resistors and tube bases... everything has been changed.. (I tried even the Komet pre amp after some emails with Mr Abbott - changing everything, pots included, but come back to original wreck pre some months ago)
The only shielded cable in my amp comes from the in jack.

Thank to everyone...

Castor