Squeal in JMP 1987 build - help needed

[sniffin]

I'm building a JMP 1987 model using a ceriatone board and MM trannies, and KT66's so I'm following both the JTM45 and JMP1987 wiring diagrams shown on the ceriatone site:

JTM45 - http://www.ceriatone.com/images/layoutPic/marshallLayout/JTM45ceriatone.jpg

JMP1987 - http://www.ceriatone.com/images/layoutPic/marshallLayout/Plexi50LeadCeriatone.jpg

After checking voltages and tripple checking wiring I proceded to fitting in the tubes, powered up the amp and as soon as I hit the standby switch I get a nasty high pitch squeal.

I lifted up one leg of the NFB resistor, put the power tubes back in and powered up and got NO squeal, so I swapped the OT primaries around, resoldered the NFB resistor and put in the power tubes and sure enough I'm still getting the squeal.... what could it be? All the voltages off the tube pins seem fine.

Since this amp was actually a BBRI there's no voltage selector so I wired the 230v wire to the power switch and the 0v wire to the power cable plug, there's no ground wire on this PT's primary. I think the squeal may actually have to do with how the OT primaries are wired, I kept them at full length just incase I had to swap them around. I passed these 2 wires in the middle of the 2 power tube sockets and the OT secondaries are connected right to the impedance selector, is this correct? Also I used belton micalex power tube sockets, should I bend the solder lugs back or keep them straight?

Here are a couple of photos of the actual wiring:

http://www.flickr.com/photos/73835524@N00/4559972325/sizes/l/

http://www.flickr.com/photos/73835524@N00/4559972319/sizes/l/

I can't seem to get bias low enough, with B+ fluctuating between 460-470, I should bias between 36mA and 38mA, but setting the bias pot to max I can't get lower than 45mA. should I change those 5.6k resistors off pin 5 as shown in the JTM45 layout to 1.5k resistors as shown in the JMP layout? I am using KT66 tubes and I decided to change the 470ohm resistors to 1k.

thanks for any help at all

[schmidlin]

The squeal is a head-scratcher. Check to see if your presence pot is properly grounded and check resistance in the FB circuit. It could be a lot of things, and others should have some good input also.

As for the bias, the 5K6s won't effect it. Start by raising the resistor in series with your bias pot: go from 56k to 68K-100K or thereabouts. This will raise the B- to a range you can deal with.

[VacuumVoodoo]

Since swapping OT primary connections to tubes didn't help:
- check NFB resistor for correct value
- try 6L6GC tubes if you have a pair. KT66 are sensitive to lead dress and parasitic capacitances. If it doesn't oscillate with 6L6GCs then you 'll know it's in the tubes. If you have oscillations anyway, it's somewhere in wiring-
Assuming we identify KT66 to cause oscillation:
- Increase grid stopper resistors to something between 15k to 47k, yes KT66 like them high. You'll have to remove those pretty red metal films made in Soviet in 1981. Pity, they're cool.
- KT66, especially current manufacture, like to have a well filtered screen grid supply. Add a 22uF/500V electrolytic capacitor from pin 6 on KT66 socket to ground if you wired the amp as per JTM45 drawing.

[Roe]

check lead dress, esp the presence circuit and the preamp. try installing 5k6-10k grid resistors on pin 5

[sniffin]

Well I cut down the OT primaries and that got rid of the squeal so all I have to do is tidy up the wiring and swap out the 47k bias resistor, I'll try the 56k resistor first and see how that goes.

The 56k resistor did the job, getting bias down to 37mA and with another quarter turn on the bias pot left so that's fine.

thanks

[VacuumVoodoo]

Good! BTW where did you get those red ex-soviet resistors?

[sniffin]

Good! BTW where did you get those red ex-soviet resistors?

I really can't remeber where I got them from, it was a few years ago anyways, and at the time I did most of the shopping from Antique music supplies and metroamp.... still think that I should swap them out for 15k's?

[sniffin]

one last question what ampere fuses should I use? the original mains fuse is T1.6A whereas the ceriatone layout shows T2A, or should I ask MM?

thanks

[Roe]

depends on where in the workd you are and whether its fast or slo-blo

[sniffin]

I'm from Italy and the fuses are slo-blo's

[Roe]

good. the ordinary value is 2a for mains and .5a for ht. usually fast blowd are fine although sometimes you need a slo blo, at least for the ht

[sniffin]

Well I got the bias down to 36-37mA and played through it for around 4 hours yesterday evening, all in all it sounds fantastic but I did notice that one of the KT66's is redplating slightly on one of the pinched-plate sections... should I get bias down a little lower?

[Blue Strat]

A good tube shouldn't be redplating at that bias level. Check the bias again when it's redplating...maybe the bias is shifting. Also, swap sockets to find out if it's tube related or socket/circuit related.

[sniffin]

I did swap the 2 tubes around and the same tube is slightly redplating in both tube sockets so it's definitely the tube itself. Another thing I noticed is that when I use a patch cable to bridge the 2 channels and turn up the bright channel volume, get up over 3/4's the amp starts to squeal. This happens only when the 2 channels are bridged together. The original BBRI setup had a 100pf cap on each volume pot, in the ceriatone layout these caps are not present at all, but I decided to follow the original JMP1987 schematic which shows a 0.005uf cap on the bright channel volume, so I left in the 100pf cap on that pot.... could this cap be causing the squeal when the 2 channels are bridged?

I will definitely recheck the bias and see what's what.

thanks

[Roe]

marshalls with high voltage often have problems with redplating. try lowering the 82k to 68k or increasing the 10k tail to 27k or even higher

[Roe]

redplating might indicate oscillation. try installing a 47k on v3's grid (besides the grid/swamp resistors on the power tubes)

[sniffin]

I did re-bias the amp today, it was hitting 43-45mA again so I had to set the bias pot back again to max but it wouldn't go further than 39-41mA. I had already changed out the 47k bias resistor to a 56k yesterday and that seemed to get the bias voltage up a bit atleast for a while, I'll try fitting in a 68 k and see if that changes anything, if I find that bias is still unstable then I'll try fitting a new trim pot and bias caps too.

thanks

[Roe]

worst case scenario with marshalls is cold bias at idle and redplating when playing :-/
this is possible since the primary is low and voltages high

[sniffin]

well the tube redplates slightly and only at one point where plates are pinched together, and redplates even when not playing, after about 3 or 4 minutes after hitting the standby switch.

[Roe]

bias is off or you got oscillation. try installing those grid resistors! and check lead dress!