First TW Express Build Issues

[bal704]

I'm in the middle of building my first TW Express. I'm using the schematic from here:

https://tubeamparchive.com/files/tw_exp ... 08_334.pdf

and the Ceriatone Layout:

http://ceriatone.com/images/layoutPic/t ... iatone.jpg.

I'm not shooting for an exact knock-off, so I also added a Lar-Mar PPIMV.
Also, there are a few minor differences between the Ceriatone layout and the schematic, such as a 2k2 resistor on V1 pin 7 and an 820 resistor on V1 pin 2.

I've got a B+ of 380V, and a bias of -33.1V. The amp sounds fine, with no major issues so far.

A few questions:

1) I've got a 1 ohm resistor from each power tube cathode to ground. I'm measuring 32.5 mA on one tube, and 35.0 mA on the other. Is this difference significant, and if not, how do I adjust it?

2) The Treble and Mid knobs induce a squeal when the level goes above 7-8.

3) Playing my Les Paul, when I adjust my bridge volume knob from 10 to 0, I get a squeal from about 9 to 7. Sounds fine everywhere else. Adjusting the tone knob just moves the squeal around and changes the duration (for example the squeal happens from 6 to 5 instead of 9 to 7). The neck pickup does the same thing. What's causing this?

[passfan]

Well congrats, it sounds like you have a normal Express. B+ seems ok , bias seems alright, imbalance between power tubes is negligible, and man these things can squeal with a lot of treble. You might do some tube tasting through V1 as some are more microphonic than others. What year is the les paul and are your pickups unpotted or without the covers ?

[bal704]

I'm using an LP I built from a Precison Guitar Kits kit. It has a Lover in the neck, and a Zhangbucker in the bridge, both with covers. I also tried my PRS Mira, and it does the same thing.

[ampgeek]

Is it squealing with or without the bright switch engaged in either postion?

Cheers,
Dave O.

[bal704]

That's an error on my part. I mistakenly put an On-On switch in rather than an On-Off-On switch. I'll fix it and report back.

[ampgeek]

Got it!

Make sure to put a healthy dab of silicone between the two treble caps if you haven't done that already.

I had similar issues with my first Express build with either bright mode selected until I did that. Not sure exactly why...but it did the trick for me!

Good luck,
Dave O.

[bal704]

I swapped the switch. In the middle position, it sounds OK. In either cap position, it squeals.

When in the middle position, I get serious squealing when either pickup is solo'd. For example, with the Mira, it has one volume, one tone and a 3 way. When both pickups are engaged, it sounds great. As soon as I move the 3 way to bridge (or neck) only, it squeals. bringing it back to the middle (both pickups) the squeal stops.

For as much gain is on tap with this amp, it is suprisingly quiet. Then a squeal pops up out of nowhere. very touch amp.

[LOUDthud]

The problem I've run into with Gibson Humbuckers is that the covers actually vibrate. While the squealing is occuring, touch (finger or pick) the cover near it's center. I'm told the cure is a bit of parafin wax between the cover and coils.

I noticed that the tail resistor (R11) is 10K on the schematic and 22K on the layout. The other two resistors probably help suppress oscillations. I noticed that grid stoppers do increase noise.

[bal704]

I'm getting the squeal on my Mira as well. It has uncovered pickups, so I don't think that's the issue.

At the risk of asking an ignorant question, how does the tail resistor impact the circuit?

[bal704]

I've taken out the LARMAR, and changed the tail resistor to 10k. Now when I strum hard, the amp sounds like it's distorting heavily. Sounds fine when I genly pick the strings, but any hard playing makes it distort.

Any ideas?

[LOUDthud]

The tail resistor affects many things. It plays a role in phase inverter balance and headroom. These things in turn affect the character of the overdrive tone because they determine how hard the output tubes are driven. As such, it's impossible to say how changes in the tail resistor value will affect tone (for better or worse).

From your discription in the above post I'm not convinced your amp has a problem. Might be just a microphonic tube. Have you compared your voltages to those in the Express build guide? Do you have an oscilloscope? The first thing I would do is look at the output and measure the output power at clipping.

My build oscillated like crazy, mostly caused by bad placement of the OT on a Traynor chassis and the Stancor OT's 250 and 500 ohm output taps. It oscillated around 25kHz that would stop if I moved my hand within a couple of inches of the pots.

[bal704]

I was using 2 brand new JJ's in the V1 and V2 slots. I swapped them out with a Chinese tube and another JJ I had laying around. That seemed to get rid of the squeal best I can tell since I have this other problem. I am also not getting any more squeal when I dime the Mid and Treb knob, so maybe I'm past that. I also disconnected the brite caps from the volume knob.

I'm getting a wierd behavior with my volume knob. When I sweep from 0-10, the amp sounds OK at about 1(low volume and low gain). As I continue sweeping, at about 2 I get this loud hum, which disappears when I get to about 3, but the sound is thin and nasaly. At 4 I get almost no sound then at 5 I get blasted with a loud distorted tone. I thought I might have a bad volume pot, so I swapped it with another new one I had, and I get the same effect.

My voltages (vs Expected):
V1-1 214 (210)
V1-3 1.86 (1.9)
V1-6 189 (182)
V1-8 1.41 (1.5)

V2-1 269 (267)
V2-3 2.7 (2.8

V3-1 203(190)
V3-2 25 (28
V3-3 36(42)
V3-6 207 (193)
V3-7 25 (27)
V3-8 36 (42)

V4 - EL34
3 - 400 (415)
4-392 (397)
5 - -33 (-33)

V5- EL34
3 - 399 (415)
4-392 (397)
5 -33 (-33)

Heaters 6.6 VAC
B+1 402 (420)
B+2 395 (405)
B+3 317 (317)
B+4 300 (298)
B+5 284 (283)

[LOUDthud]

Those voltages look good.

Some but not all DMMs can detect an ultrasonic oscillation. Set the meter to AC Volts and connect across the speaker. Noise, hiss, hum etc might measure 100mV but a full on oscillation will be 5 to 10 volts or more. Try all control settings with no guitar connected. Mine would oscillate when the Treble control was between about 4 and 8 with the Presence above 5. Could be just about any combination of control settings though. Another thing to look at is the B+2 voltage. It will fall 10 to 20V when the amp starts delivering power. At fill power it drops about 80V.

[bal704]

The loud osciallation that I have when the volume is at 1-2 goes away when the presence control is at 9-10. You can dial in the oscillation and simply turn the presence control most of the way up and it disappears.

I tried this when plugging in a guitar. The oscillation is gone, but the amp sounds like ass. Even when the volume is at 4-5, the output is thin and weak.

Turning the volume control kind of reminds me of an old AM radio. Sweeping the knob brings a variety of pops, osciallations, squeals and silence.

[martin manning]

Post a few pictures. Hi-res, in-focus, and well-lit. Flat, diffuse lighting is best.