new build 36lite with vvr

[chaliapin]

Another highlighter test done. No lightbulb moments. Haha. Sob.

Things maybe worth mentioning.

The resistor from a-b is not 1k but 1.5k.

V2 is grounded with the pre amp and inputs, not the power amp.

The screen from the mains side of the PT is grounded through a fuse to the star ground. The ct for 275v supply to the rectifier is also grounded there.

I honestly can't think of anything else im unsure of or haven't checked. I know I know, there must BE something...

Thanks for all your patience, mine is wearing a little thin with myself...

[JMPGuitars]

Without the bulb limiter the b plus starts to rise and the humming begins.

What does this mean? You need to be specific so we can help you. The B+ rises? To what? Does it stabilize? Or does it count to infinity (and beyond)?

Ignore the hum for a moment and get useful information.

[chaliapin]

Sorry, I get nervous when it starts humming and heating like it's going to blow up in my face...

Weirdly, it's behaving a little different this morning.

With standby off I have 230vac and 310vdc rectified at the standby switch.

Standby on

B+ 354
P3 11.6
P7 350
P9 325

Settling to

B+ 352
P3 12.2
P7 345
P9 322

That's after about twenty seconds when it all feels quite hot and hummy so I switch off. Volume is at 0. It could be normal hot but I don't know.

Sorry if this is very tedious for you guys, I really appreciate the help.

[Bieworm]

Those numbers mean your power tubes are biased at 106.7%. That's not a disaster, but that is also why we generally put a 180R cathode resistor in there. That will drop the bias. We aim for 85%, give or take... 100% is ok, but 85 is better for tube life. It's a cathode biased amp and it's generally accepted that 100% is no problem.
can you make a recording of the process from silent to excessive hum? The hum will decrease when it's biased to 80-90% for sure, but the hum you have may have another cause...making you chase your tail at the power section.

[chaliapin]

Thanks bieworm, I'll try to get something and post it, will the numbers look better when I have four tubes?

Can I ask which Ukrainian tube guy you used and I'll order a bunch. There's actually a guy in Ireland selling NOS reflektor military grade but 4 is going to be over a hundred euro and then I'd probably blow one up...

[chaliapin]

Also I only have am 8 ohm speaker and no 4 ohm output option until I finish the cab the 8 ohm speaker is in the 8 ohm output, is that affecting things/ voltages? I could hook up two loose 8 ohm speakers to give the correct impedance but would they get damaged just lying on the table?

[Bieworm]

Thanks bieworm, I'll try to get something and post it, will the numbers look better when I have four tubes?

Can I ask which Ukrainian tube guy you used and I'll order a bunch. There's actually a guy in Ireland selling NOS reflektor military grade but 4 is going to be over a hundred euro and then I'd probably blow one up...

I doubt the numbers will change significantly with 4 tubes, but they will to some degree.
I bought mine at Etsy (Radiotubeshop)

Regarding hum inducers... I notice you have the B+ (DC) in parallel with a bunch of twisted AC wires (heaters, PT primary,..)
Somebody correct me if I'm wrong, but I avoid AC and DC wires in parallel. I don't know if that is a problem, since this is not a signal wire...

[Bieworm]

Also I only have am 8 ohm speaker and no 4 ohm output option until I finish the cab the 8 ohm speaker is in the 8 ohm output, is that affecting things/ voltages? I could hook up two loose 8 ohm speakers to give the correct impedance but would they get damaged just lying on the table?

no damaging there, but it will sound like crap when not in a cab

[chaliapin]

So wall voltage up 6v to 236vac this afternoon. Here's the latest with two 8 ohm speakers in series into the 8 ohm socket. Hum seemed to come later and quieter, unaffected by turning volume up full with no input. Voltages now

B plus 357
P3 12.35
P7 354
P9 323

I'll try to get the cab finished to I can get the speakers into it.

[Bieworm]

So wall voltage up 6v to 236vac this afternoon. Here's the latest with two 8 ohm speakers in series into the 8 ohm socket. Hum seemed to come later and quieter, unaffected by turning volume up full with no input. Voltages now

B plus 357
P3 12.35
P7 354
P9 323

I'll try to get the cab finished to I can get the speakers into it.

That means you have the bias at 110.8%
You will have to get some extra resistors for the cathode biasing. 180R, 220R,... prepare for that. The amp will get more quiet when the bias is set properly. But I think that regards more to the hiss instead of the hum.
You should also get some 5V 5W zener diodes to be able to knock down the B+ to 345V. But I would do these adjustments when you have the 4 power tubes in there.

[chaliapin]

Ok I've ordered and resistors. Can I play it as it is now when ive the 16 ohm speaker load sorted or is something gonna blow?

[Bieworm]

If it was 100% I’d say go for it. But at 111% I’m advising against it. Don’t you have an extra 5W resistor in the range of 33 to 82 ohm? You could put them in series and be safe.
I know vox amps are often biased over 100% dissipation, but they are also notorious on catching fire and blown transformers.

[chaliapin]

Im guessing I should get this amp working 100 percent before messing with adding the MV?

[Bieworm]

Im guessing I should get this amp working 100 percent before messing with adding the MV?

There’s no harm in doing the ‘simple stupid master volume mod’ at this point…

[chaliapin]

You suggested the type 2MV to me before pretty sure I have the components or is it a bit in depth at this point?