Unloaded b+ low

[mike-p]

Hi all. I've been battling with my first scratch built amp for nearly a year now. 36w version of marshall 18 watt. Anyway, I think they're sick of me over on 18watt forum... so I've come to bother you good people here! I installed switched ss/ gz34 rectification. I noticed while doing my usual hunt for problems that the b+ with the amp on standby is lower than with the full amp powered. 240vac, ss recto, pt giving almost 300vac, b plus at standby switch 300v rising to 340 with the standby off.

Is this normal/ concerning? From what I've read the b plus should drop with the full load when is switched off.

Any ideas?

Thanks

[Stevem]

Can you draw up and post a schematic of the amps power supply please.

[mike-p]

Using this schematic with Rob rob's switched ss recto.

https://robrobinette.com/Generic_Tube_A ... ier_Switch

[sluckey]

...pt giving almost 300vac, b plus at standby switch 300v rising to 340 with the standby off.

Is this normal/ concerning? From what I've read the b plus should drop with the full load when is switched off.

This is normal (no concerns) for the schematic you posted. The B+ at the rectifier will be low with the standby switch open because there are no caps connected. If you move that 47µF and 220K to the left side of the standby switch your B+ at the rectifier will always be full value.

[mike-p]

Great thanks. I don't understand why the voltage is lower when only load is the heaters? On other amps the b plus drops when the standby is off, ie in play mode.

Shouldn't the unloaded be plus be roughly 300x1.4 meaning over 400v ? Not questioning your wisdom just trying to work on my meagre understanding...

[martin manning]

https://www.valvewizard.co.uk/smoothing.html
You DC reading is an average. Without a reservoir you have pulsating DC in "Standby" (light blue trace in that link above). With the reservoir the dips to 0V are filled in and the average goes up, more than the drop due to the amp being in "Play."

[mike-p]

Brilliant thankyou. But I kind of wish it was a problem as there is a problem somewhere...

[martin manning]

What change in voltage do you see for vacuum vs. SS rectifier?

[sluckey]

Shouldn't the unloaded be plus be roughly 300x1.4 meaning over 400v ? Not questioning your wisdom just trying to work on my meagre understanding...

It will be about 424V if you move that filter cap as I suggested.

[sluckey]

there is a problem somewhere...

Describe your problem. Most problems with a first scratch built amp are due to wiring errors, wrong component values, poor layout, bad soldering, and poor wire routing. And guessing based on your first questions, you don't have a good understanding of electronic circuits.

Posting some hi-rez pics of your amp may reveal some more clues.

[mike-p]

What change in voltage do you see for vacuum vs. SS rectifier?

The above voltages were using the ss without the gz34 installed at all. With it back in they both give 310v unloaded ( also reads about around 80vac which I guess is the ripple?) Then with standby off ss around 360v, tube around 330v.

[mike-p]

there is a problem somewhere...

Describe your peoblem. Most problems with a first scratch built amp are due to wiring errors, wrong component values, poor layout, bad soldering, and poor wire routing. And guessing based on your first questions, you don't have a good understanding of electronic circuits.

Posting some hi-rez pics of your amp may reveal some more clues.

Thankyou. I will annotate the schematic to where my circuit has got to so far... it looked a lot better before I prodded and chopsticked and reflowed everything... surprised I've got any intact tubes left after all the tapping for microphonics they've recieved... I'm pretty sure you're going to say I need a known good set of power tubes which I don't have...

Should I start a new thread in the marshall department?

Sounds good at low vvr (whole amp) and full power clean but gets very angry at high volume

https://vimeo.com/866797512

[martin manning]

What change in voltage do you see for vacuum vs. SS rectifier?

The above voltages were using the ss without the gz34 installed at all. With it back in they both give 310v unloaded ( also reads about around 80vac which I guess is the ripple?) Then with standby off ss around 360v, tube around 330v.

All that sounds correct.

[martin manning]

You have VVR too? So many bells and whistles!

[mike-p]

I tried to sketch in the mods and desperate additions to the schematic linked above but it looks like a dogs dinner so I'll list them here.

Normal input added 47k grid stopper
Parallel input added 56k grid stopper

V1 is rewired for 6n2p Russian type 12ax7

There is a 68k resistor after the volume and tone controls directly after the coupling cap so I can add a passive fx loop later.

Ruby mod added in parallel with grid leak resistors
One grid leak replaced by pp-se mod, mv just for one side of the power section to give pseudo single ended tone. Pot and resistor measuring 446k.

Power tubes are Russian 6p14p.

V3 and v6 share cathode resistor

V4 and v5 share cathode resistor

Cathode caps increased to 100uf

Vvr added to full b plus after standby switch
https://www.ampmaker.com/shop/vcb-vvr-v ... ntrol-kit/

Safety diodes added to gr34 and ss recto switch as per rob rob
https://robrobinette.com/Generic_Tube_A ... ier_Switch

LED indicator light is on the heater circuit.

I think that is everything!