Amp startup issues - light bulb glowing

[jordan86]

So I just reworked a single ended amp into a cathode biased push pull amp. I used the same PT and everything...added a PI socket and extra power tube socket and wired everything up as best I could. I just sort of spit balled a layout since I've never done this before and had to work with the space provided. 

I don't have a proper schematic, as it's thrown together from scratch. But it is basically a Marshal plexi preamp (one input, channels jumpered at tube socket) with cathode follower into a typical Vox/Matchless valued PI (NO NFB) with cathode biased 6V6 power tubes (150 ohm resistor). Sort of Marshall values on power tubes... 5.6K grid stoppers, 100K grid leaks (for 6v6s), 1K/5W screens.

With the light build limiter 100W (EDIT: 72W halogen bulb), it glows very dim on standby (same as my working 18w amps). ONLY WITH THE POWER TUBES IN does the bulb glow brighter.  When turning standby on it flashes bright for a split second, goes dim, and then slowly brightens to about 1/2 the bulbs max brightness over 3-4 second. No where near full brightness but definitely seems something is up. I tried it without the light bulb limiter very briefly. It does not blow the fuse or smoke at all. I've double checked my layout and everything seems right.  Included my voltages below running the light bulb limiter.

I'm wondering if I need to pull it from the LBL and quickly take measurements to see if that gives me more accurate data?

6V6's
1: tied to cathode
2: 3.0v ac
3: 221v
4: 221v
5: 0
6: 225v (before screen resistor)
7: 3.4v ac
8: 9v

PI
1: 158v
2: 9v
3: 27v
4/5: 3.5v ac
6: 157v
7: 9v
8: 27v
9: 3.1 ac

I can post the snapshot elements of the different stages I used for a schematic, or more close up pictures if that helps.  It's QUITE noisy and buzzy.

[sluckey]

cathode biased 6V6 power tubes (150 ohm resistor)

That resistor is only giving you 9V bias. Replace with a 250Ω and your light build limiter will be much happier.

[shaun]

You say "So I just reworked a single ended amp into a cathode biased push pull amp. I used the same PT and everything"

I didn't know that was possible. A PP OT primary usually has a center tap connected to the B+ and two leads going to the plates of the output tubes. A single-ended OT has no center tap. I guess you could be trying some sort of unusual experiment, but the typical designs we see are common because they give good results.

Anyway, without more info, I'm at a loss.

[jordan86]

You say "So I just reworked a single ended amp into a cathode biased push pull amp. I used the same PT and everything" I didn't know that was possible. A PP OT primary usually has a center tap connected to the B+ and two leads going to the plates of the output tubes.

Whoops. That's my mistake. I did replace the OT with a push pull, Hammond 1760H :)

[jordan86]

The light bulb is happier after about 500+ ohms on the cathode (I wired that 5W wire wound pot back in series with the cathode resistor). But amp still sounds buzzy and broken, sort of oscillating now.

[shooter]

time for pencil n paper 


draw it as you built it, so follow wire to......... don't draw as you "envision".  you'll probably find "Hmmm, this doesn't seem right...."

[sluckey]

Wish I could be of more help but my crystal ball has cataracts.

[jordan86]

Alright, alright! Fair enough. I know...I should've posted it to begin with. No one can read minds, etc. I just thought more experienced fellows could at least clue me in on typical issues to look for based on that data. I'll get going on a proper schematic and trace all my work and then check in.

[sluckey]

Hey I was able to give you one idea about the cathode resistor. But with a hodge podge amp such as this I just need to see an accurate schematic to go any farther. Little things can bite you. Things such as forgetting a coupling cap on the PI input. (Just an example, not saying you would ever do that). And sometimes when you just piece stuff together you end up with an amp that sounds bad. Has happened to me. I try not to talk about those! 

Get rid of the light bulb limiter and reevaluate. Then take another set of voltage readings and post along with your schematic. I'm especially interested in your PI voltages. I did not like your first posted readings, but that is probably due to the light bulb.

[jordan86]

Yes, thanks Sluckey. I did have to get my cathode resistance quite high to get the light bulb happy. So for these, I just set the cathode resistance to 250 ohm and measured OFF the LBL.  I see a few whacky things.

1) Heaters...Whacky imbalance. I think I need to add and artificial CT to the heaters. Is it worth doing that on the 6v6 cathodes for a ~10-20v elevation of the heaters? Or just create one to ground?

2) Screen voltage. The screens are higher than the plates on the 6v6. I assume this is because the resistance on the OT primaries is greater than the choke. Should I replace the screen resistors with 5K? I don't mind loosing some output power.

Anything else?

[sluckey]

Heaters need an artificial CT connected to ground. Screen resistors are fine. I don't see any issues with your schematic or voltages. I would remove R12, R13, and D3 and wire the CF same as Marshall. All I got for now.

[PRR]

....Whacky imbalance. I think I need to add and artificial CT to the heaters. ....

Why hesitate? This is basic. Costs 25 cents. Not worth asking about.

[jordan86]

Yeah I did the artificial CT this evening. Voltages are much better. Still got some crazy loud hum and buzz. I’ll chase it down. Is it likely that my lead dress and layout is the issue?

[jordan86]

So I was able to plug in and make some more noise this morning since everyone is awake. Below are some additional symptoms. I’d anyone has tips on what to check I’d be grateful:

- The amp has a terrible 60hz hum without plugging anything in to the input jack. Gets louder by turning up the volume control
- it passes signal even with volume controls fully off.
- As soon as I turn up the volume control the slightest amount it is quite loud. Like as loud as my other 18w amps on 6.
- probing the grids of the PI kills all hum and audio.

[sluckey]

Probably a missing or poor ground connection. Pull V1. Hum gone? Pull V2. Hum gone?

[jordan86]

Hum goes away with V2 or V3 pulled. Will remove the extra components around the CF to get to marshal spec and then do more digging.