Heater Issue Solved?

[surfsup]

All Tubes In
Everything Maxxed except Vol
Vol turned past 2pm a squeal starts

Preamp tube 2 removed, tube 1 in, power tube in
hum is negligible, amp is super silent everything cranked

Preamp tube 1 removed, tube two inserted, power tube in
hum is negligible, with everything cranked there is a very slight hiss

Based on this, what do you guys think?

Changes:
-Redid two shielded runs: from input to Pre1 and from Pre2 to board
-Added a 5k grid stopper to pin7 Pre1 on a terminal strip
-redid the heaters to both preamp tubes coming in sideways to Pre2
-tightened the twist to/from standby
-moved some leads around to give better 3D space around the parallel power sockets

Here's the full monty:
[img2000]http://chicagocadcam.com/ChrisHahn/FULLSHOT.jpg[/img]

[ampdoc1]

Need a schematic or a layout to tell! You've got a very clean build and the circuit looks simple. Maybe something in the feedback loop?

ampdoc1

[xtian]

Does it squeal if the tone knobs are NOT maxed?

[Cliff Schecht]

I see a lot of long parallel wire runs and parts clumped together, a lot of potential for bad solder joints and unwanted feedback. Also I'm not a big fan of your grounding scheme, it looks sorta like a TW in that sense but that's not the best method either. It's nice that you put the caps right on the stages but overall the layout needs tweaking.

[paddy]

Get it just to the point at which squealing starts and then
use a chopstick to move wires around. This may be a little hard
given that a few of your wires are tight.

Cheers
Paddy

[martin manning]

I'm thinking your 2nd pre-amp socket wiring was the source of the hum problem from the start, and I'll bet you could go back to elevating the heaters to the power tube cathode potential and get rid of all the extra stuff.

Re the squeal, I see that you have the output transformer's secondary leads running directly under the power tube's cathode resistor and bypass cap, which could cause positive feedback. Notice in the AX84 layout those wires are located further to the right, lined up with the right-most speaker jack. Try getting both of the purple wires as far away from the speaker wires as you can and see if that helps. It wasn't squealing before, so maybe when you spread things out over there you pushed it over the edge.

[surfsup]

I'll check if it squeals without the G1/G2 maxxed.

Thanks for looking. So if there's minimal (barely audible) hum with those settings, the heaters are good?

The top view pic looks like stuff is clumped by the parallel octal/noval output sockets but those leads are all separated pretty well in 3D space.

I checked the solders and they have all (I mean ALL) been reflowed several times.

I'll try and set it to minimum squeal and move stuff around but yea, the leads are all pretty minimal.

Here are the schematics for the Pre/Out and the PS:

[img391]http://chicagocadcam.com/ChrisHahn/PRE_HO.jpg[/img]

[img431]http://chicagocadcam.com/ChrisHahn/PS_HO_elevation.jpg[/img]

[surfsup]

Re the squeal, I see that you have the output transformer's secondary leads running directly under the power tube's cathode resistor and bypass cap, which could cause positive feedback. Notice in the AX84 layout those wires are located further to the right, lined up with the right-most speaker jack. Try getting both of the purple wires as far away from the speaker wires as you can and see if that helps. It wasn't squealing before, so maybe when you spread things out over there you pushed it over the edge.

The hole for the leads to come into the chassis is basically right under the cap/5Wresistor. There's no way to move the leads. Every build I've seen pics of have them there. I don't know what I can do to move them short of redoing everything with a whole new layout/chassis unless I'm not understanding something. The only lead I moved, really was the green one that runs from ground to the octal. The purple wires are probably 3/4-1" high up above the secondary OT leads too.

I remember it doing a squeal before but because of the hum, I never cranked it because it was LOUD, or it was only for a VERY short period and thought it was feedback because I had a guitar around my neck standing in front of the amp/speaker. So I think the squeal has always been there.

[martin manning]

The chassis layout drawing shows two holes, one under the Rk/Ck and the other closer to the OT... does yours only have one?

Also, there are some specific warnings about squeals using the parallel-power-tube-sockets option, with the suggestion that you should get it working (hum and squeal-free) without those extra leads as a first step. Maybe it's time to remove those wires?

[passfan]

If you have the chassis sitting directly on a speaker cabinet with no bottom plate on it your could get noises out of it that you won't get when it has a bottom plate installed and it's sitting in a cabinet with feet. Some of the squealing could also be caused by microphonics setting it off in one or more tubes. This is a high gain design and when you max everything your really pushing a ton of gain. You may try just setting everything normal and just play it and see how it is.

[martin manning]

Some of the squealing could also be caused by microphonics setting it off in one or more tubes.

Good point passfan; especially the first pre-amp tube. Turn the volume up and try tapping on the chassis near that socket with a chopstick... If you get a distinct sound of tapping-on-a-chassis-with-a-chopstick coming through the speaker, then maybe try to find a tube that is less microphonic.

[Jana]

According to the schematic you have the heaters elevated with the 100 ohm resistors to the B+ voltage reference AND with a reference to the cathode of the EL84. Pick one or the other, not both.

[surfsup]

The chassis layout drawing shows two holes, one under the Rk/Ck and the other closer to the OT... does yours only have one?

*EDITED: There are two, I'm already using the one next to the OT. I did screw the plastic clip into the hole next to the octal and moved the wires over, but now they are closer to the heaters. See next comment:

squeals using the parallel-power-tube-sockets option.......Maybe it's time to remove those wires?

**EDITED: I tried it with no guitar and I get no squeal with everything at max! But I get squeal if I have a guitar plugged in (single coil Gretsch). The hum is also a touch worse than last night but its only too much with MV=10 with G1 and G2 both at like 2pm ish so its very workable. Also it seems like if I touch the guitar, the amp pops/crackles slightly.

If you have the chassis sitting directly on a speaker cabinet with no bottom plate on it your could get noises out of it that you won't get when it has a bottom plate installed

***EDITED: I put the amp on top of a cookie sheet. Still squeal with guitar but no squeal w/o guitar.

Turn the volume up and try tapping on the chassis near that socket with a chopstick... If you get a distinct sound of tapping-on-a-chassis-with-a-chopstick coming through the speaker

****EDITED: Tapping with my fingernail it sounds like a stack of plates in the kitchen rattling. I tried a couple tubes in there (swapped between three 12ax7s) and they all did that.

According to the schematic you have the heaters elevated with the 100 ohm resistors to the B+ voltage reference AND with a reference to the cathode of the EL84. Pick one or the other, not both.

Oops, I don't have the pins 3-4 on the EL84 jumpered anymore, the {A} in the box should be taken off the schematic as the amp currently sits...I just have the two 100R going to the elevation circuit.

Also, quick Q, should I change the grid stopper to the 22k instead of the 5k?

I still have the 5k grid stop resistor. No squeal with no guitar. Squeal with guitar.

[David Root]

Yes change the 5K grid to 22K or 33K.

[surfsup]

I put the 22k CC in there. No noticeable difference. I shortened the ground lead from the input jack to the chassis to the shortest possible length.

With all tubes in and maxxed, no guitar, I get a slight hum and hiss with a slight motorboating sound (flapping).

With only a guitar cord, (no guitar), the hum is outta control

With guitar plugged in, minimal hum, but I get a loud hissing sound and squeal still.

When I tap on Pre1, the noise is much louder than when I tap on the side of the second preamp tube. Not sure if that is because stages 1&2 are amplified by the next tube? I swapped three tubes in there including an ecc83S and ecc802s and ecc803s. the EL34 gets very hot, I don't recall it ever getting this hot too hot to touch.