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Re: new build 36lite with vvr

Posted: Mon 01/16/23 5:46 pm
by 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...

Re: new build 36lite with vvr

Posted: Mon 01/16/23 6:43 pm
by JMPGuitars
chaliapin wrote:
Mon 01/16/23 2:53 pm
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.

Re: new build 36lite with vvr

Posted: Tue 01/17/23 4:50 am
by 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.

Re: new build 36lite with vvr

Posted: Tue 01/17/23 6:22 am
by 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.

Re: new build 36lite with vvr

Posted: Tue 01/17/23 6:34 am
by 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...

Re: new build 36lite with vvr

Posted: Tue 01/17/23 6:49 am
by 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?

Re: new build 36lite with vvr

Posted: Tue 01/17/23 6:51 am
by Bieworm
chaliapin wrote:
Tue 01/17/23 6:34 am
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...

Re: new build 36lite with vvr

Posted: Tue 01/17/23 7:03 am
by Bieworm
chaliapin wrote:
Tue 01/17/23 6:49 am
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

Re: new build 36lite with vvr

Posted: Tue 01/17/23 8:12 am
by 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.

Re: new build 36lite with vvr

Posted: Tue 01/17/23 8:47 am
by Bieworm
chaliapin wrote:
Tue 01/17/23 8:12 am
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.

Re: new build 36lite with vvr

Posted: Tue 01/17/23 12:36 pm
by 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?

Re: new build 36lite with vvr

Posted: Tue 01/17/23 1:07 pm
by 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.

Re: new build 36lite with vvr

Posted: Tue 01/17/23 4:40 pm
by chaliapin
Im guessing I should get this amp working 100 percent before messing with adding the MV?

Re: new build 36lite with vvr

Posted: Tue 01/17/23 5:14 pm
by Bieworm
chaliapin wrote:
Tue 01/17/23 4:40 pm
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…

Re: new build 36lite with vvr

Posted: Tue 01/17/23 6:55 pm
by 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?

Re: new build 36lite with vvr

Posted: Tue 01/17/23 10:48 pm
by Bieworm
Better begin with the 1M pot type. It’s easier and you can practice.

Re: new build 36lite with vvr

Posted: Wed 01/18/23 6:50 am
by chaliapin
Yeah, I'm clearly not ready to move up to level 2, simple stupid is where I'm at...

Re: new build 36lite with vvr

Posted: Wed 01/18/23 9:08 am
by Bieworm
It’s a process.. we’ve all been there 😉

Re: new build 36lite with vvr

Posted: Thu 01/19/23 3:30 pm
by chaliapin
So ive been working on my cab waiting for bits to arrive.

Can anyone explain to me WHY exactly my b plus voltage and dissipation are too high? Are these two issues linked? My wall voltage seems normal and the rectified voltage at the standby switch before switching on is significantly lower than when the amp is running. I'll really do my best to understand any answers provided to me...

Thanks all.

Re: new build 36lite with vvr

Posted: Thu 01/19/23 4:29 pm
by JMPGuitars
chaliapin wrote:
Thu 01/19/23 3:30 pm
So ive been working on my cab waiting for bits to arrive.

Can anyone explain to me WHY exactly my b plus voltage and dissipation are too high? Are these two issues linked? My wall voltage seems normal and the rectified voltage at the standby switch before switching on is significantly lower than when the amp is running. I'll really do my best to understand any answers provided to me...

Thanks all.
They are related. My guess is you're using a classic style 290-0-290 transformer. Except that those classic specs were expecting lower wall voltage. So the result is that you're somewhere between 300-0-300 to 310-0-310 depending on how high your wall voltage is.

To combat that higher voltage, we use a higher impedance cathode resistor, and Zener dropping diodes for B+. That gets you into better specs for safety and better tone.

Thanks,
Josh