2 stroke/ Angela pse - hum but no guitar

[SWA]

Hi
I've just completed a parallel single ended 6v6 amp from scratch - similar to those in the title but solid state rectified, 2 x 12ax7's (each with its own 22uf ps cap & triodes parallel connected) for the gain stages, no voice switching but a master volume. HT is 400v without valves & 355v with - total draw 84.5ma of 160 available.

Even without valves there is a hideous hum at the speaker (the amp is mechanically silent) & installing the valves makes no difference - there is no guitar sound & turning the 3 pots makes no difference. Sounds like a dead short but I've so far drawn a blank with all continuity checks plus current & voltages looking very typical.

Any top tips for the best checking method (without a scope) would be much appreciated.

Many thanks

[tubeswell]

Hi there

Start by checking basic things in the signal path like the speaker connection and input jack connection to each grid etc. You may've missed a wire to somewhere.

Then measure the DC voltages at the B+, screen and pre-amp supply nodes in the power supply rail, then take the plate, screen and cathode DC (idle) voltages.

If the voltages are what they (ballpark) should be, then work your way backwards from the speaker back through the output tubes, grids, driver stage, and v1 stage to the input. The amp should make a clicking sound (in the speaker) if you (very carefully) touch each stage's grid connection with your insulated meter probe - working from the putput tube back toward the pre-amp tube stages. When it doesn't make a clicking sound in the speaker, you have found the dodgy stage, which will make it easier to isolate the problem.

After that we can think about the humminess. Have you got any (gutshot) pics you can post?

[MWJB]

Post some pics, pot wiring, circuit board, input & speaker jacks.

Hideous hum at the speaker, with no tubes installed points to some awry with the power transformer wiring. Check all secondary voltages (inc. B+ feed to the rectifer AC & heaters, measured AC from pin 2 to 7 on the 6V6). Did you use a 6.3VAC centre tap wire, grounded to main power amp ground, or did you use 2x 100ohm resistors as a virtual centre tap? Make sure any shield on the power transformer legen is grounded to main power amp ground. Post pics.d

Triple check output jack wiring, don't keep turning it on until you are happy this is 100%. The grounded tab on a switchcraft jack is easy to spot, you can see the metal from the grounded tab continues to the ring of the socket whe you look at the socket internally.

When you are totally sure that the output is wired OK, try "pop" testing. When you probe voltages at the power tube plates
(pin 3) & 12AX7 plates (pins 1 & 6) you will hear slight popping, getting louder as you get nearer the inputs. Any stage that doesn't pop, is not connected to the output, or inadvertantly grounded somewhere. Also plug in a CD player headphone out to the amp's input, an instrument lead will do as you only need L ot R not stereo, turn the CD player (or a signal generator) up until you see 100mV at the tip of the input jack, now probe the plates of the tubes as you work down the amp, from input to the power tube & see what AC voltages you get at each stage (check the cd player is not between tracks when probing, use long songs).

[SWA]

2 stroke/ Angela pse - hum but no guitar

Hi & thanks for the speedy replies.

I should get a chance to go through the amp as suggested tomorrow but here are a few pix & the schematic to be going on with. It's all tightly packed into a stainless steel F Deluxe chassis.

Speak to you all again soon.

[loudthud]

Even without valves there is a hideous hum at the speaker (the amp is mechanically silent) & installing the valves makes no difference

If you hear hum before the tubes warm up, the most likely cause would be coupling between the power transformer and the output transformer. Try to move one or both transformers to try to find a location or orientation where there is no hum. Things are packed in the chassis pretty tight.

[Cosworth]

Nice! I would go out on a limb and guess that the hum and no signal is due to a open ground connection. I would check that all the 750 ohm and 560 ohm resistors are solidly connected back to the main ground at the chassis lug. You can measuere all these connections to have the approiate resistance at the respective cathodes. They look OK but, I see the leftmost 750 ohm resistor (from the bottom view) is connected to a ground bus but it looks like it and its companion capacitor only connect to the top leftmost solderpost. But it does not look like that solderpost is connected to the tube socket and its respective cathode? Ha! Good luck! Should be a cool amp. Cool.

[Cosworth]

Oh, I see the yellow wire connections now. Those must be the cathode connections. Anyway I would check the grounds. Dosn't hurt to check the cathode resistances. The black wire at the top of the input jack? On the left side bottom view, looks like it might like to be reflowed. Hum is usually associated with grounding issues. The signal path also requires good grounding.

[MWJB]

You output transformer primary & secondary wires share the same entry hole into the chassis, there is a chassis hole by the power tubes, your primary (B+ & power tube plate wires) should go through this hole.

Do you have the legend for the output transformer, red for the OT secondary common wire is obviously possible...but seems unusual/counter intuative?

What are the black wires bolted to the chassis by the output jacks?

[MWJB]

The green grid wire feeding the 6V6s wants to be well away from the high voltage dc wires & the heater wires.

[Enzo]

If you get loud hum with no tubes present, then SOMETHING is causing current flow in the OUTPUT transformer. Coupling with the PT is possible, but that generally would be described more as subtle than "hideous."

Since you just built it, I worry about wiring errors. Or as much as I hate to go there early, a bad OT. What resistance to ground is there from pins 3 of the power tubes? Go over the socket wiring there again, any mistakes? How about some excess component lead that got soldered to something instead of trimmed off? Is ther any continuity between the primary and secondary of the OT?

[SWA]

Hi again

I’ve been doing some ‘probing’ this morning & here the findings:

Power Supply Voltages

B+ 372, plate node 367, screen node 356, V2 node 314 & V1 node 312

At the valve voltages

v1 v2 v3 v4

plate 196 213 361 364
screen - - 351 354
cathode 1.54 1.60 24.0 24.2

100mv input test

The signal arrives at the grid of v1 but doesn’t make it to the plate or beyond to the other 3 valves! Why?

Interestingly, if I switch the amp from run to standby the signal does appear at the plate of all 4 valves! The standby switch is ac side of the rectifier & has a 100k 2watt resistor between its legs. (I’ve tried removing the standby switch entirely but no change).

Click/Pop Test

All present at v2 v3 & v4 but dead at v1 – looks like a pattern emerging!

V1 lead continuity

All leads check OK from the valve base to the board & there is no shorting between the pins on the base itself. New 9 pin base & 3 known valves tried without change.

Ground Connections

Check OK for solidity & continuity throughout.

Output transformer

Primary – red to power node & brown to plates (2.5K connection)
Secondary – orange common, grey shielded (other end only) 4 ohm nfl to 68k carbon comp resistor on board, purple 8 ohm jack & pink 16 ohm jack.

I’ll split the primary & secondary bundles – daft to have jammed them through 1 hole when 2 are available.

Black Leads

Those close to the outputs are ground connections for the insulated Cliff jack sockets & for the nfl shield referred to above.

Green Grid Leads

This is something that I’ll have to address once I can get the signal to run through. I ended-up with much less space than I expected despite several ‘dry’ runs.

And Finally

I guess something dodgy lurks beneath the board but I’d welcome any further suggestions that might help avoid the horrid task of removal!

Thanks again & I’ll speak to you soon.

[MWJB]

The pic of your volume & tone pot wiring isn't clear. It looks a bit jumbled up there, can we have a better pic please?

You have run the hook up wire under the board, looks neat, but makes tracing a nightmare.

I can't see (due to photo) where the OT orange common lead actually connects to the black ground lead at the output jacks?

You shouldn't need to shield the NFB wire. I can't see where the NFB wire connects to the output jacks?

[SWA]

2 stroke/ Angela pse - hum but no guitar

Hi there & thanks again for your great efforts.

Resistance between pins 3 6v6's & ground is 106.7k ohms & there is no continuity 'beep' on my dmm between the OT primary & secondary windings.

I've attached more pix:

of the speaker jacks showing the the orange common to black/groound across both sockets.

of the volume & tone control wiring.

of the OT connections with nfl to 4 ohm lug.

Hope this all helps you to help me. I really am very grateful.

Bye for now.

[MWJB]

You know the break in the signal chain is between V1a & V2a. Something between these 2 points is not connected, or inadvertantly grounded (check both ends of the 0.022uf cap feeding the volume pot for resistance to ground - vol pot up full), check both ends of the 0.022uf cap feeding V2 grid for same, it only takes a loose strand of shield from one of those shielded wires to screw things up, or a blob of solder hanging out the back of a turret).

Double check pin wiring at V1.

[SWA]

Hi MWJB & thanks for your continued support.

The volume feeding cap reads a straight 1 on the plate resistor side & a fluctuating 0.4 to 2.3 on the pot side.
The cap feeding the grid of V2 reads a straight 1 on the grid side & a fluctuating 0.7 to 2.4 on the pot side.

I've re-checked the valve base, jumpers & leads to & from V1 but cannot see (with a magnifying daylight worklamp) any errant wire whiskers. I've tried hopping from turret to turret looking for continuity via ground but found nothing unexpected - all very weird. I must be missing something obvious - pity I don't have room to slide my dentist's mirror under the board.

Shall I rev-up the iron?

Thanks again.