another Express type build

[norburybrook]

I made an express a couple of years ago that went to a friend who is playing with Eric Clapton. He's about to do crossroads in Dallas next month and called me to ask if I'd make another for him as a spare....I was reluctant at first but decided I'd do it. I've had to try and remember what i did with his amp as it ended up quite different from the standard express.

So I've made a start

express 1.jpg

express 2.jpg

M

[andresound]

Pulling up a chair . Of all the amps I have built so far (not too many)... my Express has the best CLEAN tones!

[martin manning]

Well, there is a thread here documenting your Express: https://ampgarage.com/forum/viewtopic.php?f=1&t=30578

[norburybrook]

Well, there is a thread here documenting your Express: https://ampgarage.com/forum/viewtopic.php?f=1&t=30578

Martin , yes I'v er scoured that thread but unfortunately i didn't take notes on all the updates i did afterwards.

main thing is the master volume. I know I used the Type 2 with the concentric/stereo pot and removing the resistors on the PI but I don't know what value it was.

I have a photo graph if you can help . The amp is on it's way to Dallas as we speak so i can't measure anything.

IMG_6354.JPG

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[martin manning]

Hard to see anything there re the value of the pot. I used 250kA on my Express' PPIMV, with 2M2 safety resistors. https://ampgarage.com/forum/viewtopic.p ... 15#p205948

[norburybrook]

Hard to see anything there re the value of the pot. I used 250kA on my Express' PPIMV, with 2M2 safety resistors. https://ampgarage.com/forum/viewtopic.p ... 15#p205948

thanks Martin. What do the 2M2 resitors do ? what do i have on my first build? they look like 100k no?


M

[martin manning]

Thy look like 100k to me too. Those are there to protect against loss of bias voltage in the event of a wiper failure on the pot. That’s the Lar-Mar improvement on the KF type 2 MV. 2M2//250k is 220k, so the MV is effectively equal to the 2x 220k resistors when it’s all the way up. If you reduce the value of those that will load down the PI at high volume settings and reduce its headroom. Maybe you followed exactly the KF Type 2 schematic from the Trainwreck pages?

[norburybrook]

Thy look like 100k to me too. Those are there to protect against loss of bias voltage in the event of a wiper failure on the pot. That’s the Lar-Mar improvement on the KF type 2 MV. 2M2//250k is 220k, so the MV is effectively equal to the 2x 220k resistors when it’s all the way up. If you reduce the value of those that will load down the PI at high volume settings and reduce its headroom. Maybe you followed exactly the KF Type 2 schematic from the Trainwreck pages?

thanks, yes i probably did. So am i safe with 2x 100k resitors then with a 250k pot?

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[martin manning]

On closer examination those resistors in your photo look like 1M8... gray band between the brown and black. If so that’s fine. If the one you just built has 100k, they could be swapped for higher value, or clipped out. KF Type 2 as shown in TW pages has 100kB dual pot with no safety resistors.

[norburybrook]

On closer examination those resistors in your photo look like 1M8... gray band between the brown and black. If so that’s fine. If the one you just built has 100k, they could be swapped for higher value, or clipped out. KF Type 2 as shown in TW pages has 100kB dual pot with no safety resistors.

you are correct. I've just read about the Type 2 and 1M8 with a 250K pot is what it is I don't have 1M8 resistors......bugger...I always forget something. Ordering 2 resistors is not cost effective.


anyway here's today's work. I'm slightly worried bout the Cap stack, on my first build I managed to get a tie wrap around them but on this build there's no room because of the transformer mounting. I've 'glued' them to the chassis with RTV silicone. Will this be OK? Also where do we stand regarding grid stoppers? on my first amp I used one on V1a and I used the 820R on the V1b back to the volume. I'm wondering if I should be bold and go for it without

Express day 2 .jpg

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[martin manning]

Got any 2M2 resistors? 224.5k vs. 220k... Close enough I think.

Small grid stoppers are a good idea. I’d like to see something mechanically securing that bundle of caps to the chassis.

[norburybrook]

Got any 2M2 resistors? Close enough I think.

Nope, not a common value for me 22M yes and 4.7 M I suppose I could parallel two 4,7M to make 2.3M...close enough?



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[martin manning]

2M35//250k = 226k. That’s within 3%. Still good enough.

Move this thread to the discussion sub-forum? Phil?

[norburybrook]

2M35//250k = 226k. That’s within 3%. Still good enough.

Move this thread to the discussion sub-forum? Phil?

oops just noticed this...


Sorry Phil


M

[pompeiisneaks]

2M35//250k = 226k. That’s within 3%. Still good enough.

Move this thread to the discussion sub-forum? Phil?

oops just noticed this...


Sorry Phil


M

Done, no worries, takes like 3 seconds to move a thread

~Phil