Author Topic: Pickup amps/volts  (Read 13894 times)

0 Members and 1 Guest are viewing this topic.

Offline misterg

  • Stadium Superstar
  • ******
  • Posts: 2437
  • Good Vibes 125
  • Wales, UK
Re: Pickup amps/volts
« Reply #25 on: December 18, 2012, 06:27:57 pm »
Ch 1 Sensitivity: 1mV for clipping
Ch 2 Sensitivity: 1mV for clipping
Ch 3 Sensitivity: 16mV for full output

...

I have come to the conclusion that this means if you crank the gain knob up to 10, a signal as low as 1mV can be made to clip.

Those look sensible to me - my TW Express style amp will give full (clean) output for ~3mV input. This is with volume on max and tone pots centred. With the guitar and amp volume up full, it's practically impossible to get a clean sound out of it (but that's kind of the point :) ) - the sustain, though.... ;D. While it's pretty high gain, it isn't into metal territory, so I can believe those numbers are ~3x more sensitive.

Quote
... what is the hottest signal I can shove in the front end that WILL NOT clip (i.e., gain on 0).

The input stage in valve amps is very often a cathode biased 12AX7/ECC83 with a voltage gain of about 60 (~35dB). For the values of anode(plate) and cathode resistors used, this stage is pretty much impossible to overdrive with a guitar signal - it needs several volts, and even then, it just compresses the signal a little rather than clips (with common cathode resistor values, anyway). You can overdrive it with... "Overdrive" pedals :)

The max signal you can put in usually depends on what happens after this - if it goes straight to a gain pot, then you can turn the gain down and can lose as much signal as you like.

The input arrangement in Scooters' post is incredibly common:
R1 is 1M ohm grid leak resistor which sets the input impedance and provides a DC ground reference to the grid on pin 7;
R2 is a 10k grid stopper that (together with stray capacitances inside the valve/tube) sets the roll-off of gain at high frequencies and prevents this stage oscillating or picking up radio signals (tailor value to suit - 68k is common). The 'Low' jack in Fender style 'High' / 'Low' input arrangement usually has an input impedance of ~136 k ohms, made by switching 68k grid stopper resistors through the jack contacts.
R4 is the cathode bias resistor (another common value is 820 ohm);
R3 is the anode/plate resistor (occasionally I see 220k used for higher gain);
C1 is the cathode bypass cap that sets the low frequency roll-off of the gain for this stage.

If you're designing amps, effects, etc. you need to cater for peak values, not RMS, btw.

There is all sorts of excellent information on the Valve Wizard site, especially the introduction to triodes which is chapter one of his new book (pdf file)

I do quite a bit with this stuff if anyone wants to dig deeper.

Offline TB-AV

  • Honorable Ex-Mod
  • All Time Legend
  • *****
  • Posts: 12698
  • Good Vibes 331
Re: Pickup amps/volts
« Reply #26 on: December 18, 2012, 07:12:21 pm »
That sounds right. the controls have to be set to full on or they would be always be a problem for your  specs. I suppose the 16mV on the clean channel is the most it can take before it begins to clip.


==========
Sidebar
2MG - do you know if ....oh,,, I can't remember his name... chantell amps, chris c... ??? Anyway the guy that used to sell TW kit with Heybeor<sp> trans and chassis, etc. Last time I checked he stopped selling them. I had boght some tubes adn stuff for the 440 TW 2W and have thought about buying th erest and building it. anyway... have you heard anything on those kits lately. this was basically from AX84.com


Gone

Offline misterg

  • Stadium Superstar
  • ******
  • Posts: 2437
  • Good Vibes 125
  • Wales, UK
Re: Pickup amps/volts
« Reply #27 on: December 18, 2012, 07:53:12 pm »
chantell amps, chris c... ???

Chappell Amps? & Cliff Chappell?

If so, I'm not sure what he's doing. I recall he posted asking for interest in 4-4-0 transformers: http://www.ax84.com/bbs/dm.php?thread=455473

That who you were thinking of?



Offline TB-AV

  • Honorable Ex-Mod
  • All Time Legend
  • *****
  • Posts: 12698
  • Good Vibes 331
Re: Pickup amps/volts
« Reply #28 on: December 18, 2012, 11:00:46 pm »
Yes, yes, I couldn't remember his name. the last time I was there which was a pretty long time ago his web site looked different and I had a hard time even finding 440 info much less a kit. Before that... he had a nice kit of various completeness and then soon offered transformer choices. Then... later though... I couldn't find anything and figured he must have stopped doing that stuff.

I'll check his link. So did you buy your kit from him?

ETA: Ok, yeah that site looks like the stripped down version. He used to have a 440 kit of various levels. You could pick board, chassis undrilled, drilled, tubes, no tubes, Heyboer or Hammond, etc.

I might have have to email him. Maybe he has some leftover stuff and can make me an offer I can't refuse. He never said what the specs/model number were on the Heyboer. I could source all the rest and of course have the layouts from AX84.

some of my projects begin their shopping phase in one year and get built..... well .... not that year...
Gone

Offline misterg

  • Stadium Superstar
  • ******
  • Posts: 2437
  • Good Vibes 125
  • Wales, UK
Re: Pickup amps/volts
« Reply #29 on: December 18, 2012, 11:10:49 pm »
No, I didn't buy from him - I'm in the UK, and the shipping costs & VAT make it uneconomic. I sourced all the parts here. I used a Hammond 125E OT and wasn't impressed. I'm sure the Heyboer (sp?) would be better. I gave up in the end, bought a new OT and converted it to an Express with 6V6s. With voltage scaling, I can get it to home levels quite successfully.

I'm not even sure if he does the kits any more - his kit page still shows up as an old web page: http://www.chappellamps.com/amplifiers/4-4-0/4-4-0_kits.html


Offline TB-AV

  • Honorable Ex-Mod
  • All Time Legend
  • *****
  • Posts: 12698
  • Good Vibes 331
Re: Pickup amps/volts
« Reply #30 on: December 18, 2012, 11:14:36 pm »
Oh right I forgot you were UK. Yes, that looks like the old page and it seems like maybe he made one run of that and sold out and then drop off the scene.

That page is dated 2008 which is probably about right. I loose track of time.
Gone

Offline Scooter Trash

  • Stadium Superstar
  • ******
  • Posts: 3025
  • Good Vibes 83
Re: Pickup amps/volts
« Reply #31 on: December 18, 2012, 11:31:16 pm »
Thanks for that great information  MrG  :)
I dream of a better tomorrow where chickens can cross roads without their motives being questioned.

Offline misterg

  • Stadium Superstar
  • ******
  • Posts: 2437
  • Good Vibes 125
  • Wales, UK
Re: Pickup amps/volts
« Reply #32 on: December 18, 2012, 11:36:52 pm »
You're welcome!  :)

 

Get The Forum As A Mobile App