Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ACHIEVEIT on March 19, 2012, 06:08:11 am
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Hi guys,
I am in the process of retrofitting my drri with a hoffman ab763 board. I read about how removing the tremolo circuit slightly raises preamp gain. In the weber layout for instance, a 50k switching pot is used. In the hoffman layout though, a 250k is used as the intensity pot, so i know that the tremolo implementation is different (hoffman uses bias vary, right?). So, i would like to ask, is it possible to use the same approach (switching pot) in the hoffman layout to increase preamp gain and if yes where can i find a 250k switching pot? Thanks a lot!
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is it possible to use the same approach (switching pot) in the hoffman layout to increase preamp gain
No. Hoffman uses a different circuit from the true AB763. See link below...
http://home.comcast.net/~seluckey/amps/misc/Hoffman_AB763.pdf (http://home.comcast.net/~seluckey/amps/misc/Hoffman_AB763.pdf)
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Thanks a lot for that sluckey!
Instead of creating a new thread, i modified the title to include my nfb question. After researching quite a bit, i decided to add a spdt on-off-on switch to switch between 820, 1500 and no negative feedback. Do you think i should try another value as well? Thanks
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Those resistor values will be fine. Some people actually put a pot with switch on the NFB loop.
About your first question... I looked again and there is a way to provide the boost switch on the Hoffman board. I had forgotten that Doug put a 47K resistor in the circuit to simulate the removed INTENSITY pot from the orig. AB763. If you break the ground connection to that 47K, you will achieve the same results as the Weber circuit. You will have a noticable gain boost in the VIB channel. The attached pic shows a very easy way to do this mod. Sorry for the mis-information in my first post.
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That's great, thanks again sluckey! By the way, do you know where i can find a pot with a switch that on 0 is open and when rotated is closed? I can't understand if this one works like that:
http://www.banzaimusic.com/CTS-270-SW-250k-LOG.html (http://www.banzaimusic.com/CTS-270-SW-250k-LOG.html)
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> a pot with a switch that on 0 is open and when rotated is closed?
Standard radio power/volume part. "0" the switch is open, radio dead. Turn past click, radio is on, "1" to "10" control volume (or whatever pot-controlled parameter you want).
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If ytu are looking for a pot to replace that 47K resistor in that circuit, I sell it for just that purpose.
Use the 50Ka pot with a switch
When the pot is turned all the way to the left, the switch contacts are open
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Thanks for the reply!
Wouldn't that require another hole in the chassis though? I mean, in the layout the intensity pot is 250k, so if i use a switched pot in place of the 47k, wouldn't that mean that i need another hole in the chassis? Also, what would the 50k pot control? The intensity and speed are already controlled by the other pots
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Mouser has a 250K Alpha pot with the SPST switch like you want. Less than $4. Personally, I'd rather have a push/pull switch on the pot. Mouser has plenty of those too.
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Hmm, i just figured out that in the Hoffman layout the 250kL (intensity) is actually a linear pot and not a log. That makes it easier, as i have found quite a few 250k linear switching pots. I still don't get how i can use a 50k switching pot without utilizing a separate chassis hole and what would this pot control. Thanks
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I still don't get how i can use a 50k switching pot without utilizing a separate chassis hole and what would this pot control.
You can't. Not on your Hoffman hybrid AB763.
But, that 50K pot with SPST switch is a popular mod on a purebred AB763 that uses an opticoupler trem with a 50K intensity pot. Just replace the INT pot in this case.
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That was exactly the answer i needed - i am not paranoid after all :smiley: So, i will be using a 250k switching linear pot as the intensity pot, switching the ground of the 47k you indicated, case closed!
Thanks a lot sluckey and everyone who contributed.