I've been trying to come up to speed with the Express. I've bought a set of TS iron and am trying to create a more accurate BOM than I've found. The lists I've seen appear to have extra parts and substitutions.
Is the Kelly bmp schematic definitive, with the exception of the bias circuit?
It looks like I can source all the electronic components from mouser (most), allied electronics (Mallory caps) and WestLabs (PEC pots).
I've seen some references to 2k resistors. Are those substitiutes for the 1k5 octal grid resistors?
jaysg,
I'm doin' the BOM dance as well. I didn't see any 2K's on the Kelly 90 layout or the schem. And there were some missing 1M's.
Here is my list of resistors:
MooreCowbelle wrote:
Here is my list of resistors:
Where do 3W 100k's go?
The two 100k 3W resistors are bleeder resistors. The are the ones in the Franchesca pics standing upright looking like bumble bees next to the filter cap bank.
The two 100k 3W resistors are bleeder resistors. The are the ones in the Franchesca pics standing upright looking like bumble bees next to the filter cap bank.
Dana, I suspect you could use a single 220K (ish) 3W in place of the two 100K/3W, correct? Or is that resistor pair part of the mojo?
jaysg,
the 470R is in the cathode leg of the PI. Not sure if it needs to be 1W.
the 1K5 1/2W is on the cathode of V1. The 1K5/2W are on the grids of the power tubes.
10K in the PI - not sure if that needs to be 1W either.
FYI: I usually use 1W carbon film resistors by default.
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gearhead wrote:The layout in this thread has been -extremely- helpful to me. The PS caps are listed as 22/47 and should be 20/40, but most other things are accurate.
MooreCowbelle wrote:The 1K5/2W are on the grids of the power tubes. (clip) FYI: I usually use 1W carbon film resistors by default.
Okay, so the bleeder [470k vs. 200k] really just has to be something reasonable and...I've never heard of using 2W resistors for grid inputs...maybe it's what he had on hand or maybe he thought it sounded different. It's making more sense now.
jaysg wrote:Okay, so the bleeder [470k vs. 200k] really just has to be something reasonable
I usually use between 220K and 330K for bleeder resistors. I'd guess that they are more vital in this circuit than in most because it looks like the standby switch comes AFTER the first pair of filter caps, so you need the resistors to load the caps. (As a side note, I believe that this standby arrangement has the benefit of less onrush-current surge when you start the amp.)
However, the exact values are moot, as long as they are somewhere in the range you stated.
The value of the Bleeder resistors isn't critical. Like Moose said anything in the range he stated will work just fine. The only difference is how long it will take to "Bleed" off the voltage from the caps. If you go with a 200k resistor and we use the value of the first two caps in parallel for a total of say 80 uf then the time it takes for the resistor to bleed off 99% of the voltage (say 400v) from the caps is 73.6 seconds. If we use the 470K then the time goes up to 173 seconds. Basically 1 minute for the 200K and 3 minutes for the 470k. Since the voltage in the Express is a little higher than a Liverpool or Rocket, Ken used a 200k resistor in the Express and a 470k in the Rocket/Liverpool. The other thing to note is that the current flowing through the bleeder resistor(s) for a 470k at 400v is .85ma and the 200k is 2ma. So if you calculate the Wattage you get .00085 x 400 = .34 watts and .002 x 400 = .8 watts. The amount of current flowing through the bleeder resistor is constant while the amp is on and takes away from the total amount of current available for the amp. It would appear that Ken doubled or trippled the wattage value of the resistor needed. So IMO if you got a resistor in the 2 - 3 watt range that is between 200k and 470k then go for it. (Hey a dollor is a dollar) If your trying to achieve an exact copy then go with the bumble bees.
Last edited by UR12 on Thu May 11, 2006 3:17 am, edited 1 time in total.
MooreCowbelle wrote:jaysg has a good question: any spot in here where it might make sense to put a carbon comp instead of a carbon film?
I'm not a cork-sniffer, but any little mojo helps when I can get it.
I can't see where the bleeder resistor's composition would affect the sound of the amp. Ken used Carbon Comp
The 1.5ks were carbon film 1/2 w
I use a 5w 1k for the screens just because I think the look cool. It looks like Ken used a ceramic 2W resistor for his 1Ks but I can't tell if it is carbon or wirewound.
The 470 and 10K resistors in the PI tail were Carbon film 1 Watt in Franchesca. I have used 1/2 w resistors in this location before with no problems. Since these resistors are in the signal path of the PI the changing them to Carbon Comp may affect the sound. Might just have to try it. Do you have your corkscrew ready?
Fwiw, carbon comp are better for power surges than an equivalently rated carbon film.
I've been contemplating slight modifications/experiments. I plan on using UF4007's instead of 1N4007's. Rick Erickson (Ampage) had some ideas about why they would be better in the output tube spike suppression.
I will try V2b instead of V2a. The B triode is supposed to have lower noise specs, though maybe only the first stage matters? (Kelly schematic shows V1b as the input.)
Is there mojo in the lack of an input grid resistor? After reading Aiken's blurb on resistors (again), I'm going to buy a bunch of metal film 1M's and swap out all the CC and CF input shunts in all my babies. I will ponder 1W's instead of 1/2W's.
I had read somewhere that Ken didn't like Metal Film Resistors so take that for what it's worth. When you say "input grid resistor" are you talking about something like the 68k in series with the input jack and 1st tube grid? If that was what you meant then most amps have something there to cut down on noise like CB radio and and other RF interference. Ken didn't use one on the Express but he did on the Rocket (33K). Any resistor you put in series with your guitar signal will drop some of the signal before it gets to the tube. Not much but some.
