HRDX problem. grainy overdrive on all vol settings on both channels

[visa]

my friend bought a used MIA HRDX. It came with 6L6GC STR ruby power tubes, a set of ruby preamp tubes and a Vintage 30.

It sounds like a cranked solid state pratice amp on both channels even with Vol at 1 or lower. Very grainy, ugly OD. it's not clean at all not a pinch of headroom. not a single clean note. with the Vol. at 12 I noticed a major loss of volume compared to my own HRDX which btw has plenty of headroom. it should be stated that the power light flickers. again, mine doesn't.

here's what i did so far to rule out some factors.:

- swapped all tubes (power and pre seperately and the whole bunch)
- speaker swap
- different guitar
- connected an external 8ohm cab
- tried input two (which i generally use for more headroom.

all these didn't change anything.

any hints?

[NTBluesGuitar]

Sounds like you have a bad tube(s) or bad tube socket solder joints. You may also have issues with the 5W power resistors that this amp is utterly infamous for.

I'd start with some known-good tubes and see if that helps...I know you said you swapped tubes, but does that mean you swapped them out, or swapped positions of exisitng tubes?

[visa]

i swapped the tubes with my brand new TAD tubes that work perfectly in my own HRDX. same problem. it's deffinitely not the tubes as i tested its tubes in my own amp where they worked perfectly.

i also did the "temp" 5W resistor fix.....still, nothing.

i'll probably re-solder the tube sockets next....?

[NTBluesGuitar]

Tube sockets next would be my choice. I recommend de-soldering then soldering again. Soldering on top of a cold solder joint would just result in a thicker cold joint.

Let us know if this works.

[hawkeye2u]

Surely you have seen this site, but if not here it is again

http://studentweb.eku.edu/justin_holton/

Does the amp distort even when the guitar is just plugged in to the POWER AMP IN? vol should be a lot lower but with this you can separate the input from the output stage to find out where the problem is

I would defenately resolder the complete tube pcb as mine was shocking from the mex factory.

There is also the R57 82k & R58 100k resistors to the V3 tube, phase splitter, but as said earlier you need to determine in what stage the fault is before we can help you any further

The diagram is available from the above site

As ALWAYS check that ALL supply voltages are ok BEFORE doing anything else, most problems are solved by checking that all the right voltage rails are ok!

[visa]

i found something on justin's site that pretty much sums up my problem.

This results in a dramatic loss of volume, as only one power tube is amplifying, and a terrible sounding break up even when the amp is supposed to be clean (i.e. severe crossover distortion)

from: http://studentweb.eku.edu/justin_holton/plateloads.html

i'll do the fix as soon as i can assure that it's in the preamp section by plugging the guitar directly in the power amp. unfortunatly the amp is sitting 45 minutes away from here at our practce space so i wont be able to do anything today.

[Dan-yo]

it seems most common for the bad resistor to be in the phase inverter (V3), and i guess it's more part of the power amp section, so just plugging into the the power amp would still sound like you have a problem (it might be hard to hear anything at all, cuz you can't crank any gain knobs)...that's what happened to my HRDv...i'm not saying it IS that resistor, but if you look on justin's site you'll see there's a bunch you should change while you're in there either way...i'd say to try replacing that one first (i think it's an 82k?) and see if the amp is back to normal, then change the others (100k?)

[visa]

OK.

Today I got the parts to replace R4, R11, R16, R22, R57 and R58.

And guess what....it kind of worked. unfortunately i must have damaged something of forgot something because the preamp section is now completely muted. i get no sound when i plug into input 1 or 2.

i know that the fix worked because when i plug my guitar into the power amp in i get a very clean and beautiful sound. btw, it wasn't like that before i replaced all those resistors. so i know that the fix worked.

now, can anyone here tell me what i could have done wrong to mute the preamp section? i'm pretty new to this. did i put the resistors in the wrong way around? is that even possible???

i already replaced all the preamp tubes...not the problem.

[mageerc]

You may have damaged one of the solder pads on the board... they are very fragile. Use your DMM to check continuity. Hopefully you just have a bad solder joint somewhere rather than a damaged pad. The resistors don't care which way they go it, their orientation doesn't matter.

[visa]

so today i tested all my solder joints and indeed found some bad ones. i jumpered them the next joint.

the amp works....it sound beautiful.

though the bright channel has some weird crackling sounds even at lower volumes. i biased at 68mV.

what could be causing this new problem?

[mageerc]

Plate load resistors? Did you replace them?

[visa]

yes, all six of them. i put in the ones the juston recommended.

i tested them yesterday. the resistance and the current are just right.

actually it's nearly as bas as it was before the fix. plus, it's only there when the bright swtich is enabled.

what exactly does the bright swtich do anyway? i'd love to be able to fix this minor problem even though it doesn't really bother me that much as i installed a 220pF cap as a treble bypass on the clean channel and a 360pF cap on the drive channel. (Brightness Mod from justin's site). that pretty much makes the normal channel a bright channel.

[visa]

strangely enough, the bright channel problem has vannished....yay for my guitarist.