First build crackle n pop.. benson monarch.

[unknowntome]

Hello! I posted this query over at the benson monarch thread however I think its best suited here.

So, Id been following that thread since it pretty much started and, with much preparation, built my first amp! Thank you to everyone who contributed to that thread... incredible! I used BobL's layout with the only difference being the HV centre Tap going to ground as most have done. Overall, hum and noise is 'acceptable' and on par with my Bluesbreaker. I have two issues with the build though...

first: there's persistent pop and crackle. it shifts and is dynamic but there.. doesn't go away... chop sticking doesn't make any difference. Ive read lots of different posts/threads whereby a couple of key changes from CC to CF resistors can make all the difference.. hoping to get some ideas re this… ?

Second: the reverb sounds great up until the last 20% of the sweep where the signal is killed and replaced by a hum.

I used the 290BEX PT and the 1750H OT.. both hammond. Im in Australia so 240v mains.
I used CC resistors throughout the build.

My voltages are about 20% higher than the schematic.


I also, as I do, made it more complicated by building it using a completely different layout! More aligned with hifi tube amplifiers.. see pics... but its pretty much worked out except for the two issues above so am feeling pretty good about that for a first build! It sounds fantastic! There could be a little less hum… but as it is I can live with that.

any ideas re above?

Layout (I added the power section… incl artificial centre tap resistors)

https://i.imgur.com/qdmtx3U.jpg

Build images

https://i.imgur.com/USMBisq.jpg

https://i.imgur.com/n2wylFc.jpg

https://i.imgur.com/jEIjrSM.jpg

https://i.imgur.com/keNybKx.jpg

https://i.imgur.com/DcizkGL.jpg

Cheers!
Tom

[brewdude]

Bad solder joint?
Did you check PA bias and plate dissipation?

[unknowntome]

Bad solder joint?
Did you check PA bias and plate dissipation?

Hey! I haven’t done a thorough check re solder joints.. I was planning on resoldering all accessible joints to begin with… tube sockets mainly. On close inspection the lugs on the turret board don’t have solder all the way around them but am certain that there’s good mechanical connection with ample solder on at least the side where the heat was applied from where the solder flowed.

Not sure what you mean exactly by checking PA bias and plate dissipation… this is my first build. .. keen to know though!

[Stevem]

Your output tube cathode bypass cap needs to be moved off of the body of that 5 watt resistor.
Heat is going to eat that cap up real fast other wise!

[unknowntome]

Your outout tube cathode bypass cap needs to be moved off of the body of that 5 watt resistor.
Heat is going to eat that cap up real fast other wise!

Hey.. yeah there’s space between the two although it doesn’t quite look like it in the pic.

[Stevem]

Well first yank out your preamp tube and if it stops ( assuming its not the tube ) then you have narrowed it down to those two gain stages, or the power supply node that is feeding them.

Most pops & crap are from plate load resistors and near twice a week here at work with brand new products I deal with bad noisy plate load resistors, so don't assume because they are new that there good!

[unknowntome]

Well first yank out your preamp tube and if it stops ( assuming its not the tube ) then you have narrowed it down to those two gain stages, or the power supply node that is feeding them.

Most pops & crap are from plate load resistors and near twice a week here at work with brand new products I deal with bad noisy plate load resistors, so don't assume because they are new that there good!

Thanks.. I’ve got a couple of questions.

- can I pull out the tubes with the amp on? Or do I completely power down, remove, and then power up?
- should I methodically start with V1 moving through to V3?

[Stevem]

Yes you can remove any of the preamp tubes with the amp on but you will get popping and noises out thru the speaker, but no harm will come to the amp.

Turn the amp off or into standby to put them back in.
Start one at a time with V3 and if no change is heard pop that back in and move on to V2 and so on.

[unknowntome]

Great, thanks for that.. I’ll give that a go this evening.. is it a good idea to pull them out whilst in stand by mode then turn it on? Then turn off to reinstall?

[brewdude]

They might be HOT

[unknowntome]

They might be HOT

Haha.. yeah I know.. will have gloves ready to go if I need them

[unknowntome]

Yes you can remove any of the preamp tubes with the amp on but you will get popping and noises out thru the speaker, but no harm will come to the amp.

Turn the amp off or into standby to put them back in.
Start one at a time with V3 and if no change is heard pop that back in and move on to V2 and so on.

So I just ran the experiment.

Pulled out V3 - crackle persisted. Reinstalled.
Pulled out V2 - crackle stopped. Reinstalled.
Pulled out V1 - crackle stopped. Reinstalled.

Hmmmm…. What next??

[pdf64]

Link for schematic?
The layout image seems rather blurry, and different to the photos?
Regarding the second issue, have you scoped it for oscillation?

[Stevem]

What you reported back with is impossible if V3 is the tube that is driving the output tubes!

[unknowntome]

What you reported back with is impossible if V3 is the tube that is driving the output tubes!

I’m pretty certain V3 is the reverb tube.. V2 is connected to the output tube.. V1 to input…. Based on the layout I’m working from and linked in my initial post.