new build 36lite with vvr

[Bieworm]

Better begin with the 1M pot type. It’s easier and you can practice.

[chaliapin]

Yeah, I'm clearly not ready to move up to level 2, simple stupid is where I'm at...

[Bieworm]

It’s a process.. we’ve all been there

[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.

[JMPGuitars]

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

[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.

[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?

[JMPGuitars]

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.

[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.

[JMPGuitars]

Don't mount a strip to the board. You don't want future mechanical issues causing electrical issues.

[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?

[chaliapin]

Also, am I being too impatient and just need to wait till I have four power tubes?

[JMPGuitars]

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.

[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...

[JMPGuitars]

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