Homebrew Attenuator

ecc83

clarkefan said:

I looked into this a couple of months back and eventually thought I'd end up spending like 30 quid and maybe damaging something expensive, or end up with a crap sound. Changed my mind and bought a Palmer PDI06 from a fellow fretboarder which works brilliantly with my DSL 50w. They're about 120ish on the bay, works way better for me, safety first

It is actually quite difficult to damage any 1/2 decent valve amp with a load box. The main thing is you MUST own a test meter and once you have built the thing do 'cold' check for resistances. So long as the load on the amp is around 50% of the setting ON the amp you will be ok.

Dave.

ICBM

ecc83 said:

clarkefan said:

DSL 50w

It is actually quite difficult to damage any 1/2 decent valve amp with a load box.

Hmmm...



@clarkefan - what impedance is the load? If it's 16 ohms, no problem. If it's 8 ohms, don't crank the amp through it unless you've had the switch in the 16-ohm jack bridged - the current to the 8/4-ohm jacks goes through it. This contact point is a cause of blown output transformers on these amps.

It's also worth checking the screen resistors and making sure the points of their standoff legs are well clear of the filament traces - this is a notorious spot for arcing when the amps are pushed hard. Bend the tips of the legs up and away from the board with a pair of fine pliers if necessary.

clarkefan

Thanks for the advice @ICBM It's an 8 ohm job bit, which matches my 2x12. I will get on the 16 ohm jack switch asap, you recommended it before and I haven't done it yet. Will look at screen resistors as well. Now you mention it I've been getting crackling sounds recently, I might have been pushing something I should look at... I have to say, the worry about attenuators and treble loss has turned out to be a non-issue with the Palmer. I'm using a GE-7 to push the front end and I need to pull back the treble frequencies with it, never mind boost them.

ICBM

clarkefan said:

Thanks for the advice @ICBM It's an 8 ohm job bit, which matches my 2x12. I will get on the 16 ohm jack switch asap, you recommended it before and I haven't done it yet.

I also forgot, parallel the two halves of the 8/4 impedance switch as well - only one half is used. Both these things increase the chance of an open circuit. What's so stupid is that it must have been a deliberate design choice by Marshall to do it the way they did, since it's *more* complicated than doing it the better way - although since it's just a difference in PCB traces, it wouldn't actually have cost them anything.

It should look like this when done. (One link wire at the lower left, three at the right.)

ecc83

ICBM said:

ecc83 said:

clarkefan said:

DSL 50w

It is actually quite difficult to damage any 1/2 decent valve amp with a load box.

Hmmm...



@clarkefan - what impedance is the load? If it's 16 ohms, no problem. If it's 8 ohms, don't crank the amp through it unless you've had the switch in the 16-ohm jack bridged - the current to the 8/4-ohm jacks goes through it. This contact point is a cause of blown output transformers on these amps.

It's also worth checking the screen resistors and making sure the points of their standoff legs are well clear of the filament traces - this is a notorious spot for arcing when the amps are pushed hard. Bend the tips of the legs up and away from the board with a pair of fine pliers if necessary.

Hmmm?...Well I do base that on the fact that I have driven the absolute ***t of levels and over long periods of time through a dozen amps . This was done a t max mains (253V for UK) and although into 'nominally correct' loads, both purely resistive and horribly reactive I had very few failures save a valve and sometime a fuse, often one then the other.

I look upon this as an 'accelerated life test' ? If they can stand that sort of punishment for 4 hours continuous at a time then even a load that is a 'bit wrong' will be survived for a decent few minutes?

I cannot of course speak for ALL makes of amplifier!  BTW, never had an OPT blow until about my last month when we were testing a different supplier (no, sorry cannot recall the name) 20 mins in an HT Stage 60, Pop!

Dave.

clarkefan

@ecc83 , I believe ICBM was coughing politely about your reference to decent valve amps, and my reference to a JCM2000. It's difficult to be sure but, reading between the lines, I'm not convinced he's a fan

clarkefan

Thanks for the additional advice @ICBM , I'll get this done this week. Apologies for the tangent all

ecc83

clarkefan said:

@ecc83 , I believe ICBM was coughing politely about your reference to decent valve amps, and my reference to a JCM2000. It's difficult to be sure but, reading between the lines, I'm not convinced he's a fan

Well I did preface my point with "HALF DECENT! valve amplifier". 

Dave.