Page 12 of 21

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

Re: new build 36lite with vvr

Posted: Mon 01/23/23 12:00 pm
by chaliapin
Thanks Josh,

So tackle b plus first, get below 350v, any particular advice beyond what you have linked in your signature?

Then look at dissipation/ bias/ cathode resistor value?

I have 5w 5 and 10v zeners coming and 220 ohm resistors, ordered 180 as well but out of stock and coming later.

Re: new build 36lite with vvr

Posted: Mon 01/23/23 4:52 pm
by chaliapin
My pt is supposed to supply 275-0-275 ... I'll check what's on the rectifier pins. My circuit has one big 47u cap instead of two smaller, should I fit it after that cap and after the pt connection or use double zeners and fit straight after the standby?

Re: new build 36lite with vvr

Posted: Mon 01/23/23 6:16 pm
by JMPGuitars
chaliapin wrote:
Mon 01/23/23 4:52 pm
My pt is supposed to supply 275-0-275 ... I'll check what's on the rectifier pins. My circuit has one big 47u cap instead of two smaller, should I fit it after that cap and after the pt connection or use double zeners and fit straight after the standby?
Oh, weird. When I build 36W, I still use a cap can, but it's 50uF/50uF instead of 32uF/32uF.

Anyway, the best placement is after the first filter cap, in your case that's still after the 47uF. If space is an issue, then yes, you can go after the standby switch, but you'll have to double the total Zener dropping value as you mentioned.

You should see what the DC voltage is coming from the rectifier going to the standby switch. That's the value you need to drop under 350VDC.

Re: new build 36lite with vvr

Posted: Tue 01/24/23 1:21 pm
by chaliapin
Can I work on the b plus voltage reading I get with the standby off? I was thinking I could add a solder strip near the end of the board to mount the zeners, so go

Standby to big cap, to solder strip with zeners, wires to pt, power tubes, and on to the next cap on the board.

Annoyingly, I've plenty of space on the turret board at that end but no spare turret. I could probably bolt the strip to the board? See photo, the red wires with the zip tie are because I still have only two power tubes in.

Re: new build 36lite with vvr

Posted: Tue 01/24/23 8:24 pm
by JMPGuitars
Don't mount a strip to the board. You don't want future mechanical issues causing electrical issues.

Re: new build 36lite with vvr

Posted: Wed 01/25/23 5:23 am
by chaliapin
So with standby off I have 292v at the rectifier supply and 317vdc at the standby switch. Turning standby on and the b plus quickly goes up to 357. This is still with only 2 power tubes and a 16 ohm load into the 8 ohm output Jack.

Im thinking of installing 2 10v zeners in series on a solder strip fastened to the chassis between standby and turret board to save making any more messy changes on my board. This should give a 10v drop as the zeners are only roughly half as effective on the unsmoothed supply?

The change in voltage when I turn on the standby isn't telling me something is wrong is it?

Re: new build 36lite with vvr

Posted: Wed 01/25/23 5:47 am
by chaliapin
Also, am I being too impatient and just need to wait till I have four power tubes?

Re: new build 36lite with vvr

Posted: Wed 01/25/23 10:31 am
by JMPGuitars
chaliapin wrote:
Wed 01/25/23 5:47 am
Also, am I being too impatient and just need to wait till I have four power tubes?
Yes.

357 isn't that high. Since your goal is a 36W, you should tweak your specs for a 36W, not an 18W. See what your voltages are when you have what you actually want.

Re: new build 36lite with vvr

Posted: Thu 02/02/23 5:09 pm
by chaliapin
Oooooook. Got my valves from Ukraine. Fitted them. All good with bulb limiter. Without bulb limiter after a minute or two getting toasty and hummy funny noises...

AC supply 237v
B plus 339

Only got v7 before flicking off
Pin 3 11v
Pin 7 334
Pin 9 306

An 8 ohm speaker into what im pretty sure is the 8 ohm socket.

Plate voltage is too low as im understanding it. Does changing the cathode bias resistor affect that? Off to read over stuff again on rob's site...

Re: new build 36lite with vvr

Posted: Thu 02/02/23 5:30 pm
by JMPGuitars
chaliapin wrote:
Thu 02/02/23 5:09 pm
Oooooook. Got my valves from Ukraine. Fitted them. All good with bulb limiter. Without bulb limiter after a minute or two getting toasty and hummy funny noises...

AC supply 237v
B plus 339

Only got v7 before flicking off
Pin 3 11v
Pin 7 334
Pin 9 306

An 8 ohm speaker into what im pretty sure is the 8 ohm socket.

Plate voltage is too low as im understanding it. Does changing the cathode bias resistor affect that? Off to read over stuff again on rob's site...
You're doing a lot of guessing instead of testing. Turn all the knobs on the amp to zero. Get all your voltages. Try this voltage chart, and use both rows to cover all 4 power tubes: files/JMPGuitars_18_Watt_Lite_2b_Voltage_Test_Chart.pdf

A lot of my amps target 340VDC at the EL84 plates. It's fine. Your screen voltage is also lower, which is good. Get your voltages quickly, and carefully, and then post them all, including your cathode resistor value.

What does toasty mean? Did you ever increase the cathode resistor value?

Thanks,
Josh

Re: new build 36lite with vvr

Posted: Thu 02/02/23 5:52 pm
by chaliapin
Cathode resistors still at 150 ohms. I have 220 ohm I can try. I meant SCREENS are low sorry, ie, pin 9.

Toasty as in, I can feel warmth on my face leaning over the amp and can smell heat. No sparks or smoke. I don't know what a normal amp feels like. Should there be an increase in heat maybe thirty after flicking the amp off standby? I feel like if I patiently take all the pin voltages something bad might have happened by the time ive finished...

I'd been sort of ignoring the preamp side as ive never had any off measurements there but I will add them as ive never taken them with a full set of power tubes.

I did take ABC
A 339
B 312
C 269

Re: new build 36lite with vvr

Posted: Thu 02/02/23 6:00 pm
by Bieworm
About the toast.. EL84 tubes are really hot by nature. There’s no comparison with 12ax7 operational temperatures.

Re: new build 36lite with vvr

Posted: Thu 02/02/23 6:15 pm
by chaliapin
Maybe toast is actually ok. Took all these turning off the standby between each one.

They all look right to me. Everyone asleep now, I'll give it a blast in the morning, maybe a bit of chopstick on the lead dress is all that's needed?

Re: new build 36lite with vvr

Posted: Thu 02/02/23 7:29 pm
by JMPGuitars
Those voltages are fine. With your 150 ohm cathode resistors, you're about 90% dissipation, which is also fine. I think 220 would be too high, and we usually shoot for 85%, so you're close enough.

Play it in the morning and see how it sounds. If there are noise issues, deal with them. If not, record a demo and share it!

Re: new build 36lite with vvr

Posted: Fri 02/03/23 5:35 am
by chaliapin
Something nasty/ cracky/ trashy going on in the sound. Gets worse after a few minutes, much worse higher volume. Definitely accompanied by increased heat and a smell of hot electrical stuff but maybe this is irrelevant. Eurrrgh. Anyone link me to a good trouble shooting guide to work through?

Im assuming as my voltages are good then everything is wired up in the right order at least?

Am I looking for a bad solder joint?

Tried different pre amp and pi tubes, same deal. Chopstick produced no crackling sounds of any sort with volume at 2 or 3, lead in, but no guitar connected. Some change in hum/tone moving wire to pin 2 V2 but nothing dramatic.