New 4-4-0 Mini Wreck Build...up and running

[amplifiednation]

Hello All,
I have a new build I have just completed, it's a mini wreck with parralleled 6sn7 triodes for a whopping 4 watts. This is the second one of these I have built. The first had no troubles, but this one is having some issues. I was hoping for some advice.

I got through the normal startup procedures, variac/limiter, the amp is up and running, tubes are biased, heaters check out, we have pleasing sound.

No volume or tone controls are working at all. My presence is fine, but it's independent of the tone stack. I get absolutely nothing when turning any of the tone knobs. The amp sounds like it's turned up about halfway and just stuck there.

Thoughts? I immediately thought grounding, but I've checked, everything checks out (both layout and schem) so I must be missing something elsewhere? I didn't stray from the grounding scheme.

Also..voltages

Primary/Secondary/Bias all checks out...what doesn't is the DC in the amp. Preamps are 15-70 volts off, and the 6sn7's are 100 volts too low.

I added a choke to the original design. The original design has a 100uf cap for B1, I split it to a 50uf/50uf can cap with a 9H choke in series. I did not change the resistor R2 1k/5W. Is the choke dropping my voltages? Can I adjust R2 to compensate?

[dartanion]

Can you post the schematic and layout? If this is just a low power version of a standard Wreck, then I see issues with the tone stack wiring for sure. However, it is hard to tell exactly what is going on by the photos provided.

Another item is the filament wiring. Are the 2 cement resistors supposed to be a virtual center tap? If so, they need to be grounded at one end otherwise they are acting as dropping resistors as currently wired.

[Tillydog]

Take a look at the turret with the yellow lead from the volume pot and the 2k7 resistor on it in the 2nd picture - it looks like you haven't soldered the components attached near the bottom of the turret.

Andy

[amplifiednation]

Can you post the schematic and layout? If this is just a low power version of a standard Wreck, then I see issues with the tone stack wiring for sure. However, it is hard to tell exactly what is going on by the photos provided.

Another item is the filament wiring. Are the 2 cement resistors supposed to be a virtual center tap? If so, they need to be grounded at one end otherwise they are acting as dropping resistors as currently wired.

The .1R cement resistors are to drop the filament voltage from 6.9v to 6.3v. I may do the artificial CT, haven't decide yet. The filaments are elevated with about 30-40v of DC. I'll see if there is some hum.

Here is the schematic and layout (from AX84). I'm sure it's a dry joint or something. I'll reflow some of the joints.

[amplifiednation]

Take a look at the turret with the yellow lead from the volume pot and the 2k7 resistor on it in the 2nd picture - it looks like you haven't soldered the components attached near the bottom of the turret.

Andy

Andy, good eye! That is the ground bus on the bottom of the turret. I don't know if it was soldered, but I reflowed that turret and has a nice connection now. I thought for a second you were going to solve the mystery.

[dartanion]

I would follow the express layout here and wire the tone stack that way as the AX84 layout is wonky in my mind. Also make sure the ground from the mid pot is there and solid.

[amplifiednation]

I would follow the express layout here and wire the tone stack that way as the AX84 layout is wonky in my mind. Also make sure the ground from the mid pot is there and solid.

You read my mind. I just rewired the controls to match the Express. It was probably a good idea anyway, I buried some of those wires under the pots.

Still nothing on volume, nothing on tone.

Why would the volume not work? I tested the pot, it is working. Doesn't the signal have to be passing through there if it is making it to the second triode?

[M Fowler]

Because it is not getting grounded.

[dartanion]

Because it is not getting grounded.

Check all the grounds in the preamp as the grounding scheme in the layout is quite different than a standard Express/Liverpool.

[amplifiednation]

Because it is not getting grounded.

Check all the grounds in the preamp as the grounding scheme in the layout is quite different than a standard Express/Liverpool.

The grounding scheme has everything grounded on the bus bar on the board, then is supposed to ground out at the input jack with a jumper to the first turret. The only thing I don't have running back to the bus are the speaker jacks.

I moved the jumper to a lug right by the input jack and still am not getting any response from the controls. It would make sense if the bus bar was not carrying ground but I am getting continuity at all the right spots.

Do I need the OT ground to the bus? I'll keep digging.

[passfan]

Does the bright switch respond properly. Is it loud or soft right now ? Has the first section been inadvertently jumped with a wrong connection or shorted component (e.g. C18 shorted could allow bypass of the tone stack). Check your grounds for something that shouldn't be grounded instead of looking for something that should. If your cathodes are reading proper bias then in all likelyhood your grounding scheme is fine. I gave up on that scheme from AX84 during an HO build that I could never get to quiet down and went to a dumble style for all my builds.

[Tillydog]

That is the ground bus on the bottom of the turret. I don't know if it was soldered, but I reflowed that turret and has a nice connection now. I thought for a second you were going to solve the mystery.

You sure that the wire from the ground bus doesn't just circle the turret without touching it? - Can't tell if you've caught it or not from that latest photo.

Just my 0.02p

Andy

(edit to add 1000 words)

[amplifiednation]

I figured this out. After trying different tone stack wiring, and moving to a single volume control and totally removing the tone stack I still hadn't gotten anywhere.

Turns out I had my plate wires switched from V1a and V1b so essentially I had taken the entire tone stack/volume/gain stage out of the circuit.

I only figured it out when i couldn't get anything else to work!!

Anywho, the lesson to be learned is if you are tempted to do under board wiring to keep neat appearance be careful of the wires you can't see under the board!!!

Thanks for the help guys. This amp is a ripper with nice 6sn7's and is extremely loud for 4 watts.

Now on to the cabinet...I've got something special in mind. I'm doing a combo.

[Colossal]

Great to hear you got it sorted out Taylor. Which OT primary did you end up using for four triodes (2 6SN7s)?

[amplifiednation]

Great to hear you got it sorted out Taylor. Which OT primary did you end up using for four triodes (2 6SN7s)?

Dave is there more than one primary on this OT? I have the one from Chappelle amps, as you probably remember. I know he made one with dual taps, but I don't think this is it.

I used 2R resistors for the bias points to account for the paralleled tubes, and I have the NFB loop coming off the 16 ohm tap vs the 8 ohm tap. Is there anything else I can do?