Mike express build

[Miket]

Hey guys, I have recently finished my express build and I'm currently just trying to iron out some of the bumps.

Fist of all the amp - express build mostly with parts from rj, marstran tw transformers and Hall vvr3 installed (set to scale the whole amp).

I have been playing it mostly with an attenuator, taking it down to a bedroom level, and it sounds quite good. I prefer the sound of my L-pad attenuator over the vvr. I find the vvr loses a lot of the highs and upper mids, where as the L-pad doesn't. (some more tweaking may be required in that area).

There are a couple of issues that have got me stuck at the moment however, that have me seeking some outside assistance. I have done a lot of reading and searching on the forums and Google, but have yet to find any solutions.

Problem 1 - when a note or chord is sustaining, it starts to develop a phaser type effect. And the sound becomes a bit thin and fizzy. This happens with both of my les pauls. With and without using the attenuator (vvr set to not lower any voltage). I have changed cabs, tubes, bias settings, control knob settings. And the problem remains. The bottom cover is attached, and the amp is on my table, around three feet from the cab. No neon nights, no soldering irons, no fans are on. And I haven't had the problem in other amps I have build, and play in the same setting.

Problem 2 - oscillation - only present when the amp is running without an attenuator. Bass, treble, mid, presence at 12 o'clock. If I take my hand off the guitar strings and slowly turn the volume/gain up, once I reach 9 o'clock a mild high pitched oscillation starts, and increases in volume and intensity as I raise it up to 12 o'clock and gets worse the more I turn it up.
I have tried everything I can think of so far, the ot primaries are in the correct spot, the other way around it screeches so bad I almost blew my ear drums. Tubes don't make a difference. I have tried nos 12ax7 telefunken, mallard, tungsram, raytheon etc. I have tried nos rft and svetlana el34, and new production JJ el34.
I have had the bottom off, brought it right to the edge of oscillation with the control knobs, and tried to induce it by poking and prodding with a chopstick, but to no avail. I have tried to change any aspect of the oscillation by use of the chopstick, moving wires and tapping components while its oscillating, but no use.

I have installed a 33k grid stopper on v1a, and a 1.5k grid stopper on v1b. Sheilded cable from input jack and from volume pot to v1b grid.
It also isn't effected by the bright switch, it happens in all settings.
The ot primaries and coiled together and stuck low under the ps board (as per glens recommendation in the build guide).
The bottom cover is attached and the amp is not on the cab.

That's all I can think too add at the moment, I've tried to include as much info as possible to help diagnosis. I will attach pics and voltage levels shortly.

Thanks guys
Regards.
Mike

[Miket]

additional pics

[hans-jörg]

Hello,
a very, very nice build.

I wonder that you don´t use a swiched input jack. In this case your amp is allways on (signal, not power).

Your oscilation is with guitar in or without?
Without guitar in and no signal to ground I can imagine a squeel.

Isn´t it possible that the tip of tis Jack touches the ground connection. Its not easy to see. But put a cable in and proof the distance.

BTW you use the switched on at the output but the normal one at the input. You should swap them.
Probably somewhere there is the problem.

On the other hand I´v no clue.

But again: very beautyful.

Hans-Jörg

Edit: could you try to put the green lead wich goes to G2 (1k) of the EL34 far away from the output (Jack and swich).

[Miket]

Thanks for your kind words Hans. The tip of the input jack has plenty of clearance from any earth's.
As far as using a switching jack for the input, it wouldn't matter when the guitar is plugged in would it? I have found the oscillations present with guitar plugged in and without. Much much louder when it's plugged in though.

Update: I spent some time comparing my tone stack with Francesca's and have since shortened approx an inch from the two wires that go from c3 and c4 to the bass pot. And an inch off the wire that goes from c2 to the treble pot.
So three inches less wire has helped. Now with the bright switch off, I can max the treble without any oscillation, and can turn the gain up to 3 o'clock before oscillation starts.
With the bright switch engaged on either of the two caps, the amp hisses much louder and I get very high frequency oscillations on treble pot after 3 o'clock, and it goes out of the aural spectrum when it's almost reached max. Also with bright switch engaged the oscillations start on the volume pot around 9 o'clock again. (all this testing was without guitar plugged in)

Could this be caused or influenced by the 1m resistor and cap on the volume pot for the vvr? Would these be more suited on the tube socket?

None of the above has effected the phaser sound I'm experiencing on sustaining notes :/
Also, the presence pot seems to not really do much. I originally had it connected to the 4 ohm tab of the ot, and it seemed ok. I realised my mistake and connected it to the 8 ohm tab, no it seems to have little to no effect on the sound. Just thought I would mention that.

Thanks guys

[shaunf]

Hi Mike,

Look, I'll state upfront that I'm pretty much a beginner when it comes to building amps. My Express was the very first amp that I scratch built.

That said, given the fact that you've tried a bunch of tubes, and a bit of chop sticking without any effect, the areas I'd look at if I were you are as follows:

1) bright cap/switch section. From all that I read before completing my build, this is a very sensitive part of the amp. I followed the Francesca pictures to the letter as far as arrangement of these caps and switch goes. Also don't forget to stick the caps together and then anchor them to the switch.

2) tone stack wiring. Again I see that you haven't stuck strictly to the proven lead dress. Personally I'd rewire those pots up copying lead dress from Francesca, but ensuring the connections are according to the layout diagram. There is also some good info in the build guide that recommends how some of the more important wires are routed.

3) blocking caps and 1meg resistors. Not sure what effect it had with them installed on the jack and pot as opposed to directly on the tube. I did mine the same way I saw Richie do them, on the tube, and I have no issues.

4) grid resistors, I think the 33k goes on v1b (input jack side), not v1a ( volume pot side). Not sure what effect this has. I never installed grid resistors at first on my amp, but I recently tried them and I felt they improved the tone. I'm using a 33k on v1b and 820r on v1a.

5) only other thing that made me wonder was the position of the VVR relative to the presence pot. This is another are that I believe is very sensitive, hence I positioned my VVR way over next to the standby switch, but I'm using multicaps, not the stack cap, so not sure this in an option for you.

Good luck troubleshooting. Hope you get it right soon, and hopefully some more experienced guys chime in soon.

Cheers,
Shaun

[geetarpicker]

1) I'm not sure what the extra cap and resistor are doing on the volume pot, so I question that in regards to stabilty however perhaps that is tied to the VVR which I don't have experience with... If it is VVR related perhaps the lead dress isn't proper for that setup. That said, you could try the amp with the VVR temporarily removed just to see if you can isolate the issues first.

2) I've seen several folks cause more harm then good by shielding anything other than the input lead so I'd pull the 2nd shielded cable.

3) Having no grounding jack on the front will definately cause problems when the guitar is unplugged and the amp turned up, but it shouldn't cause issues when plugged in. I'd suggest adding it just cause.

4) At your PI circuit I see you have one of the blue wires wrapping around the yellow and blue that go off to the EL34s. On my originals the shorter blue wire is routed above and off to the side of the yellow/blue pair not wrapped under it.

If all is well you should be able to dime all the knobs without any stability issues, though you might have to shuffle through a bunch of preamp tubes to find the good ones.

Do the oscilation issues occur even with the guitar turned off?

Hope you can get it all sorted out soon!

GK

[amplifiednation]

I'm with Glen.

I would use a shorting jack. That will never been a fun experience letting the amp run idle.

What is the capacitor on the input jack?

[ChrisM]

You need to put the 1M grid leak after the 0.1uF blocking cap on the input. This may be causing your "phaser" issue.

Why have you added the cap and resistor to the volume pot? You don't need them, remove them.

[hans-jörg]

You need to put the 1M grid leak after the 0.1uF blocking cap on the input. This may be causing your "phaser" issue.

Why have you added the cap and resistor to the volume pot? You don't need them, remove them.

Hi,

Yes,there is your reconstruction aerea! Input an Vol.
The second leg of the 1M at the input has to go to tip!
And put a switched one there otherwise the amp acts like antena when no guitar in.
And try it first without VVR as mentioned above. When everything works proper you can connect it back.

Best.

Hans-Jörg

[hans-jörg]

What is the capacitor on the input jack?

Hi,
this capacitor can work as insurance against DC on his way to the guitar strings (worst case scenario).
The Rocket which plays my daughter has it there too (100n)

Hans-Jörg

[Miket]

Thanks for all your suggestions guys. Lots of things for me to start testing and changing on my next day off work.
I'll keep you updated.

Regards
Mike

[Miket]

Hey guys, I've had some time to work on the amp. I connected the shoring jack to the input, removed the blocking cap on the input and the blocking cap and 1meg pull down resistor after v1 (these are mods suggested by Dana to prevent dc making its way to the guitars and amps volume pots, causing a scratchy sound when they are turned - at low level vvr settings)

After those changes the tone stack related oscillations have almost completely gone, the only way I can make it oscillate now is if I dime the tone stack and presence, then turn gain to about 9 o'clock.

Now the next problem that's persisting is feedback.
When the gain is turned up, I get high pitched feedback if I take my hands off the strings of my guitar (when the guitar vol is turned up).
What I have noticed with this feedback is that it's pitch will change with the tone pot, and and that if I'm using a single 12 inch speaker, the feedback won't start until gain hits around 11 o'clock. However, if I'm plugged into my Marshall 4x12, the feedback starts at around 9 o'clock.
I'm running my least microphonic tube in v1 (nos bugle boy), I'm facing the guitar away from the speakers and amp. The amp is on a table across from the speakers, with bottom plate on.

That's all the info I can think to give at the moment.
Thanks again guys

Mike

[jjman]

You have to turn the guitar all the way down to judge amp feedback problems. Otherwise you are addressing guitar/pickup/cable microphony.

[M Fowler]

I've always found the presence circuit to be the cause of oscillation in the Express.

[geetarpicker]

Now the next problem that's persisting is feedback. When the gain is turned up, I get high pitched feedback if I take my hands off the strings of my guitar (when the guitar vol is turned up). What I have noticed with this feedback is that it's pitch will change with the tone pot, and and that if I'm using a single 12 inch speaker, the feedback won't start until gain hits around 11 o'clock. However, if I'm plugged into my Marshall 4x12, the feedback starts at around 9 o'clock.

It almost sounds like the feedback you are getting is natural gain/volume enduced feedback. You say when you have the guitar UP and your fingers off the strings that is when this occurs? IF you ground yourself to the tailpiece or brdige does the feedback stop? If any grounding effect (touching the taipiece or bridge) makes a difference you do have amp problems. Still, if it's just resonant feedback from the strings being left unmuted then that is THE WAY IT IS with these high gain loud amps! You then just need to learn how to control this tendency by muting the unwanted strings and using this enhanced feedback to your advantage. With my Express cranked up, as soon as I take my fingers off the strings (with the guitar up too) you can surely bet it will take off into out of control feedback within a sec or two at the most. The trick is to mute the strings you don't want but let your preferred notes sustain, in a controlled musical way.

Still, the amp should not oscillate AT ALL even all the knobs on 10 and the guitar plugged in but with it turned off.

Hope that helps!
GK