Ina born this weekend

[DerStever]

Hey there,

Another Express clone born this weekend named after my wife's real name, Ina. Pix and stuff to follow sometime.

While the amp sounds really great - my THD attenuator is a necessity. I don't have a cab for it yet but it seems I can't play the amp sitting on a foam pad on top of my speaker cabinet. I tried sitting it on top of a metal plate and she really didn't like that either. I get lots of noise and or some sort of feedback so that there is no volume and terrible distortion.

So how do you all get around the incredible gain and stuff this amp puts out? I used Nicks boards and parts with Mojo trannies and a spec made polished aluminum chassis. Layout is similar to Nicks except I grounded the pots using braid to a chassis mount near the volume control pot. A PT lug became ground for the AC line and power supply ground while the filter caps are grounded to a chassis ground near them like the layout.

Any advice or help would be great. Is there any documentation other than posts which lists common issues and fixes for them?? WHat would be the best method of shielding the amp?

Thanks - Steve

[Allynmey]

DerStever, how do we get around the gain??? It's why we build it!!!

Allynmey

[DerStever]

Allynmey, I know that and thank you for answering!!!!! I should have said how do you use the gain to your best advantage and tame the strays. As Firesign Theatre once said "Weve been waiting for this for hundreds of years!". I have two customized 18 Watt clones that I built and I had added a very similar Presence Control setup to them which made them sound more like mini plexi's than 18 watters but this is a whole new level!

I'm a believer!!

My issue was some serious oscillations and other assorted funny noizes when the amp was in close proximity to the speakers (and also in some cases depending on control settings). I did find that by changing my output jack to a self grounding type and putting the heater dress close to the chassis - not up in the air ala Fender - I cut a lot of noizes down that way.

My next trick is to get the amp to play comfortably on top of my speaker cabinet. So is there any specific guidelines to shielding or do we just put a screen or foil bottom on the chassis and call it a day?

Also a point of curiosity - does anyone have any idea of the average tube life of the EL34s ?

Thanks again!

[Allynmey]

Der Stever, try this...no kidding. Turn the amp on its side while on the cabinet, if a lot of the noise goes away, install a metal shield to the bottom of the cabinet. It works.

Allynmey

[DerStever]

I was able to tame the noise level down with the speaker / amp oscillation with a aluminum foil board under the chassis - just for experimentation - I'll make something more permanent when I get the cab built.

I still have some other noises - 120hz hum which comes and goes on its own. We traced it thru the presence control - break that and no hum.... pull the PI tube and no hum. I remember once having a similar type hum (similar symptoms too) with a Silvertone I was fixing and the end result was to go to a shorted input jack - I'm beginning to think even though I have one on this amp that there may still be something going on there so I may try different ground points for it.

Are these amps generally noisey anyway??? Or is it me?

[Allynmey]

Der Stever, you will get Hiss...that is normal but, loud hum is not. Check the 10K tail resistor on your PI to Presence pot. Make sure you have it properly grounded there.

Allynmey

[DerStever]

I FIXED THE HUM !!!!! Spent the better part of the morning moving wires around and redoing the presence circuit. Once I got past testing possible wiring issues I decided to tackle the tubes cause lots of the problems are there - probably should have reversed the method - so I looked again at tha bias on the EL34s and noticed it was pretty off for a "matched set" - I also know that unbalanced tubes can cause a hum so I fished around for some 6V6GTs since I didn't have any more EL34s. I found some mongrels that I'd bought or taken out of amps I worked on and was able to match a set really close even though one is a RCA and the other Sylvania - they look different so they aren't the same but they match really good with the Allessandro bias meter. Set them at 20 ma each and both were within .5ma of each other - it don't get much better than that.

I put them in and fired it up and lo, NO FREAKING HUM - nadda - the entire amp quieted down - of course I don't have as much sparkle as the EL34s but its a fairly decent drop off in volume and a warmer creamier tone - I found some old Mullard 12AX7s I use in the 18 Watt clones to get that authentic Marshall tone and put them in as a counter balance to the more "Fendery Sounding Output Tubes" and basically have come up with a really nice sounding amp that works great with my guitars and while still fairly loud - won't kill everyone in the room unless I run flat out on the attenuator - theres a noticeable difference in the volume.

So again thanks Allenmey for your comments and help - another happy 'Wreck is alive and living and I can't wait for my guitar friends to hear this thing. I'm going in the studio end of January and plan to use this amp as much as possible.

Cheers! Steve

[DerStever]

AHHHHHH!!!! I did not fix the hum - I fooled myself with that attenuator - and the 6V6s. Switching back to EL34s (Winged C this time) I had to run the amp louder than usual at practice last nite for the first time and it definately still hums. I am using Mullard 12AX7s everywhere else and I have swapped them out so they are NOT the cause. Here are the symptoms ... please help.

Hum is continual and doesn't decrease or increase with amp volume.

Comes on when stand by is shut off. ie when there is HV on the plates.

Hum stops when either the PI tube is pulled or the presence control connection to either chassis or pot is broken.

Hum is present at the main filter cap for the OT primary. But not on the other filter caps.

Hum is present when guitar is plugged in or not. I am using a shorting input jack.

Enclosed are a couple of pix. Please let me know if you can thing of anything and or some debugging options - Ive done everything I can think of.

Also - Allynmey - on your previous post you mention grounding the tail resistor 10K at the presence pot? It does not show that done either on A1b or the Ceriatone layout - is this something new??
THanks

[HiGain]

It looks like you might be using desolder braid for your ground bus. I'd look there first. Also, I know the rule of thumb is to attach the bus to the back of the pots. However, I minimized hum considerably on my build by using a heavy guage solid core wire floating off the pots for my ground bus. I attached the bus wire to the chassis at the mid pot (you might try moving the ground point closer to the input jack).


It also looks like the foil of the shielded cable for your output tube grids could be touching the chassis, making a ground loop with your ground connection at the other end.


I'm not sure if it's been said above, but make sure you have no cold solder joints for any of your ground connections.

Last, it looks like you did not twist your secondary output tube wires.


Good luck,
Jake

[DerStever]

Thanks for your help !! - I'll check those items out tonight - one other thing I saw was 4VAC on the two 40uf caps which are in parallel - I'm not sure I should see ANY ac on those caps should I?

Other schematics - A0 & A1a & A1b (older version in PDF file) - show a much higher value of cap setup here - including the ones where they use the 160ufs in series with the balance resistors - is this a more preferred method? I know this increases the voltage protection alot and was standard issue in most Fender amps after 1965.

[DerStever]

OK - I redid the ground buss as in the new pix and tried both grounding it to the chassis in one spot and ended up with it on the back of the volume pot. I also swapped out the filter caps where I saw the AC (its actually only 1.5vac) and neither had any effect on the hummmmmmm.

I have 3 major ground points - one is the filter supply (with the bleed resistors in pic 3), another is under the PS board and contains the center taps and AC earthground, and the third is the buss. There is also the diode train off the 2 plates which are grounded at the tube socket mounting screws.

I believe the power supply filter caps are done the same way as the Ceriatone drawing says to - that was my guide for the build.

Thanks again for all your help gentlemen. I really love this amp and want to get it right.

[castor]

Perhaps it's not the 'solution', but try to disconnect the long bar (connected to the pots) from the chassie... I think it makes a big loop connected in this way!
This amp is not silent... but noise is low respect to the volume you hear while playing.

[DerStever]

Thanks for your help

The buss is only connected to the volume pot back - thats how it gets to ground - the pots that are connected to the buss are only connected from the tabs to buss if required - they are not connected to the buss on their backs (excepting the volume as previously mentioned) - if I disconnect the buss from the volume pot back it won't see ground anywhere. Ive tested this with my meter already.

This isn't a low level hum - its quite prominent.

[castor]

sorry, it seemed to me that the bus went over the jack and was connected to the chassie in the left of the photo.

When I did this amp (see LARA) I had some problems and learned something new. The bigger problem was a very thin sound and a very low distorsion --> I tryed (replacing every cap) and found that the 0.002 was not properly working.
The second fact was that I checked all pot backs before connecting to the bar... all backs were just connected to the ground, so it seemed to me a ground loop to connect them to the bar, but I did it anyway and, even if i was especting high hum, everything was (and is) right!
some noise problems can be generated by tubes.. try changing one by one (12ax7 tubes). I use tungsol (before I used JJ and they made more noise and less ditorsion than this ones)

[DerStever]

Thanks Castor,

Heres what Ive done so far to reduce (or eliminate) the hum:

Swapped out all of the tubes with replacements I know are good both NOS and new type. I get a slightly less hum with 6V6s but then they are also less gain.

Tested each cap for AC voltage. Most found was 1.5 on the filter.

Tested all grounds with meter.

Isolated the hum as originating either in or after the PI.

Shortened all grids or shielded them - using braided shield solid core copper cable or signal type wire. Grounds on 1 side only.

Made sure unshielded grids cross any other wires at 90 degrees as much as possible.

Swapped out the PS Filter caps for the OT with double the value caps.

Changed the ground buss from braid to copper wire.

Thanks again.