Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BobL on April 30, 2019, 10:39:53 am
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Trying to identify the bright cap on a MkII Imperial - I've seen some stuff about the location of the rhythm channel bright cap on earlier versions of the Imperial, but nothing on the MkII.
There is a cap where I'd expect to find the bright cap in a blackface - center to right lug on the volume pot... but the value seems wrong at 330k (measures 440pF on my cap tester?), and disconnecting it just kills all sound from the channel.
It seems in earlier Imperials there was a 50pf bright cap on the board, but I'm not seeing anything like that when I look through this one.
There is a 33pf at C28, but it's not near the connections from the Volume pot, it's by the connections from the Reverb pot... there are a couple 200pf and a 470pf caps in the area, but... I don't know.
Traces are all on the other side of the board, so I'd have to pull it to go through there, which I'd prefer not doing if someone can point me in the right direction. I'd just like to see how the amp sounds w/o it, as apparently a lot of folks prefer it w/o, and I am finding the high end on the amp problematic.
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Can you post some pictures?
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330k is a 33pF cap.
331k would be a 330pF cap.
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330k is a 33pF cap.
331k would be a 330pF cap.
Ah, right... so that does seem like it should be functioning as the bright cap for that volume control... which confuses me why I lose signal completely by removing one side of it?
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Confuses me too!
Picture time.
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The wires coming from the volume pot (just left of the switch) are attaching where you see RV2 by the tube socket.
(http://[url=http://www.boblefevremusic.com/temp/imperial_mkII_bright_cap.jpg]http://www.boblefevremusic.com/temp/imperial_mkII_bright_cap.jpg[/url])
http://www.boblefevremusic.com/temp/imperial_mkII_bright_cap.jpg (http://www.boblefevremusic.com/temp/imperial_mkII_bright_cap.jpg)
http://www.boblefevremusic.com/temp/imperial_mkII_circuitboard.jpg (http://www.boblefevremusic.com/temp/imperial_mkII_circuitboard.jpg)
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To me it looks like the bright cap is the yellow cap soldered to the volume pot
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There is a cap where I'd expect to find the bright cap in a blackface - center to right lug on the volume pot... but the value seems wrong at 330k (measures 440pF on my cap tester?), and disconnecting it just kills all sound from the channel.
That gold colored cap is the bright cap. I can't read the value. Disconnecting only that cap will not kill all sound. Maybe you inadvertently disconnected something else too?
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Don't know for positive but I read where it was on the board and like 180pf and seemed like it was hardwired not switchable off the volume pot.
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There is a cap where I'd expect to find the bright cap in a blackface - center to right lug on the volume pot... but the value seems wrong at 330k (measures 440pF on my cap tester?), and disconnecting it just kills all sound from the channel.
That gold colored cap is the bright cap. I can't read the value. Disconnecting only that cap will not kill all sound. Maybe you inadvertently disconnected something else too?
Yeah, that is what I thought... but I disconnected one leg... no sound. I reconnected it, and I got sound back. I really don't understand why, but... nothing else was disconnected. I will try doing it again and see if there was some weird flukey thing going on, I guess?
That is the 33pf one.
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Maybe your cap tester doesn't measure that small? My Fluke 87 doesn't.
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Just re-disconnected one side of it and I'm getting sound. I have literally no idea what was going on the first time. :w2:
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Bob,
The schematic shows a 33pF across the top of the pot.
What's with the tall standoffs ?? is there a shield fitted over the board ??
John
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Bob,
The schematic shows a 33pF across the top of the pot.
What's with the tall standoffs ?? is there a shield fitted over the board ??
John
Where did you find a schematic? I'd love to get a link to that for future reference.
And yes, there is a shield that mounts over the board.
(http://www.boblefevremusic.com/temp/imperial_mkII_shield.jpg)
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schematic for the 20th ann, version of imperial can be found on this thread:
https://music-electronics-forum.com/showthread.php?t=38441 (https://music-electronics-forum.com/showthread.php?t=38441)
If you still have amp open and it's not too much trouble, i would like to see picture of that ironman reactive load/attenuator
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schematic for the 20th ann, version of imperial can be found on this thread:
https://music-electronics-forum.com/showthread.php?t=38441 (https://music-electronics-forum.com/showthread.php?t=38441)
If you still have amp open and it's not too much trouble, i would like to see picture of that ironman reactive load/attenuator
I think the 20th ones are different from what I can tell...
I just closed it and loaded it up for practice, unfortunately. That vertical board in the bottom of the picture is the Ironman II, though... not that the angle is very useful.
Also partially visible here: http://www.boblefevremusic.com/temp/imperial_mkII_circuitboard.jpg (http://www.boblefevremusic.com/temp/imperial_mkII_circuitboard.jpg)
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No worries, i found a picture of Falcon amp where that thing is too