New amp… sort of… er…

kjdowd

jpfamps said:

ICBM said:

Voxman said:

Mate, I'm so sorry to hear about your breakdown culminating in the delusion that a TSL can sound good - I had no idea you were under so much pressure from work.  I'm sending the men in the white coats round, & they'll take good care of you, I promise.   

lol… trust me, it's quite different now. A friend came round and said he'd never heard one sound like that before either!

I'm puzzled as to why though - it sounds like it's got a lot more resonance at the bottom end, as if the NFB has been reduced or removed, but I don't see how combination-biasing would do that, by itself. The presence controls still work so I don't think the NFB feed has got broken in the dozen or so times I must have had the PCB out.

I did also solder up the speaker connections to get rid of the crappy push-connectors Marshall use (and so I could be really OCD and turn the speakers round so the logos are the right way up ), but I can't see that making any difference really, they were actually making contact before.

Degenerate feedback will reduce the transconductance of the output stage and hence reduce negative feedback.

Also cathode biasing increases the output impedance from the valves which will reduce damping.

While I appreciate that this is actual real technical stuff, it sounds a bit like a shampoo ad or the science bit of a Star Trek film. I think it's the word "Transconductance". Or maybe "degenerate feedback" (which is also a great name for a band, btw).

Bridgehouse

Have you guys tried reversing the polarity of the flux capacitor?



Just tryin' to help

Voxman

Bridgehouse said:

Have you guys tried reversing the polarity of the flux capacitor?



Just tryin' to help

Personally I think it's to do with polarising the output from the heisenberg compensator, which creates a tachyon discharge with a bi-polarised and multi-phased neutrino shift.  

But even that can't explain how a TSL can sound good!

ICBM

Aha! I've got it!

As jpfamps and ecc83 might have worked out from the fact that the new cathode resistors are 50-ohm and not 47, these are actually vintage resistors I had lying around in my boxes of salvaged bits and bobs. They may even have come out of old AC30s since I often increase the resistors to 62 ohms on those, which makes them a little less hard on the valves.

It's "mojo"…

ecc83

Kjdowd, "transconductance" is NOT a made up word from the telly people! It is an alternative to the older term "mutual conductance" which relates the change of anode current to change of grid voltage. "Conductance" being the reciprocal of resistance.

There. I hope that clears that up for you!

ICBM, have you measured power delivery at clipping?

Dave.

ICBM

ecc83 said:

ICBM, have you measured power delivery at clipping?

Not yet. I will, just for completeness. It's far louder than I'll ever need though, even if it's now putting out less than 100W.

midlifecrisis

Have you tried turning it off and on again?

ICBM

midlifecrisis said:

Have you tried turning it off and on again?

Yes! Lots… the hum problem on the clean channel is definitely heat-related.

Now I understand why these amps have an Output Mute button, too - it makes chasing a fault easier if you don't want to turn the standby off and you don't want loud noises coming out of the speaker .

And the end of the day it's still a rat's nest of interboard cabling, plastic jumper connectors, crimp terminals, average-quality PCBs, flimsy pots and other assorted junk which does not exactly give a warm feeling of owning something well-made and reliable...

kjdowd

ecc83 said:

Kjdowd, "transconductance" is NOT a made up word from the telly people! It is an alternative to the older term "mutual conductance" which relates the change of anode current to change of grid voltage. "Conductance" being the reciprocal of resistance.

There. I hope that clears that up for you!

ICBM, have you measured power delivery at clipping?

Dave.

Ah that's totally cleared it up for me. 

DrJazzTap

Just imagine paying £1200 for one brand new....who would do such a thing?
I was always alarmed at just how hot the top of the amp used to get, and the fact it didn't have any sort of grill to allow the hot air to escape? My mate did drop my combo onto the ground after a gig once, it still worked though!

ICBM

DrJazzTap said:

I was always alarmed at just how hot the top of the amp used to get, and the fact it didn't have any sort of grill to allow the hot air to escape? My mate did drop my combo onto the ground after a gig once, it still worked though!

The idea is that odd arrangement at the back, with the grilles top and bottom and the solid centre panel, creates a convective cooling path. I actually don't like top vents - I always call them 'beer traps' from experience fixing amps which have been killed by this…

This one is missing the middle panel but it doesn't actually get that hot, now - maybe yours had bias runaway from the factory? They always used to bias them far too hot anyway.

ThePrettyDamned

ICBM said:

DrJazzTap said:

I was always alarmed at just how hot the top of the amp used to get, and the fact it didn't have any sort of grill to allow the hot air to escape? My mate did drop my combo onto the ground after a gig once, it still worked though!

The idea is that odd arrangement at the back, with the grilles top and bottom and the solid centre panel, creates a convective cooling path. I actually don't like top vents - I always call them 'beer traps' from experience fixing amps which have been killed by this…

This one is missing the middle panel but it doesn't actually get that hot, now - maybe yours had bias runaway from the factory? They always used to bias them far too hot anyway.

6 or 7 years ago, before the big explosion of stupid boutique overdrive pedals, biasing your amp "tubes" hot was the known tgp method. I remember reading a review of an amp in guitarist that boasted the hot biasing... 

I also remember the review of the vintage modern, amp of the year, for its bulletproof build and awesome tones at any volume... Lol. 

So maybe Marshall biased hot based on the press at the time. When I got my Laney, advice online for mods was to "put a choke in it and bias the tubes hot". 

TimmyO

Skip forward a few years and they will go for a fortune of eBay as donors for "the ICBM mods"

cj73

ICBM said:

And the end of the day it's still a rat's nest of interboard cabling, plastic jumper connectors, crimp terminals, average-quality PCBs, flimsy pots and other assorted junk which does not exactly give a warm feeling of owning something well-made and reliable...

So it's the Millennium Falcon?

Bridgehouse

cj73 said:

ICBM said:

And the end of the day it's still a rat's nest of interboard cabling, plastic jumper connectors, crimp terminals, average-quality PCBs, flimsy pots and other assorted junk which does not exactly give a warm feeling of owning something well-made and reliable...

So it's the Millennium Falcon?

You played through that? You're braver than I thought!

cj73

^wis lol wis lol?

Bridgehouse

cj73 said:

^wis lol wis lol?

DrJazzTap

ICBM said:

DrJazzTap said:

I was always alarmed at just how hot the top of the amp used to get, and the fact it didn't have any sort of grill to allow the hot air to escape? My mate did drop my combo onto the ground after a gig once, it still worked though!

The idea is that odd arrangement at the back, with the grilles top and bottom and the solid centre panel, creates a convective cooling path. I actually don't like top vents - I always call them 'beer traps' from experience fixing amps which have been killed by this…

This one is missing the middle panel but it doesn't actually get that hot, now - maybe yours had bias runaway from the factory? They always used to bias them far too hot anyway.

ah that would explain it. My simpleton brain just thought "hot air rises" 

ICBM

DrJazzTap said:

ah that would explain it. My simpleton brain just thought "hot air rises" 

It does, but it becomes more effective for cooling when you let cold air in at the bottom as well.

I like your new avatar by the way .

DrJazzTap

ICBM said:

DrJazzTap said:

ah that would explain it. My simpleton brain just thought "hot air rises" 

It does, but it becomes more effective for cooling when you let cold air in at the bottom as well.

I like your new avatar by the way .

I bow to your wisdom.

Thank you, I've always thought that Rita Hayworth was far more attractive than Marilyn Monroe