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Question for you pro amp guys. Screen Grid Resistor

 
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vulture



Joined: 01 Mar 2008
Posts: 4503
Location: Chicago

PostPosted: Thu Jul 23, 2015 11:53 pm    Post subject: Question for you pro amp guys. Screen Grid Resistor Reply with quote

I have this Alamo Embassy amp that I finally got around to switching out some caps and decided to mess a little bit with reducing some of the extra bass response. Anyway that aside it runs on a 12ax7, 6V6, 5y3 and a 6av6 for the trem.

So as I was digging around I noticed that the screen grid resistor running off my 12ax7 (pin 2) had been removed. As it stands it goes directly into a set of (3) 47k (each to 3 jacks) and a 680pf cap to ground. There is also a 1 meg in series with this cap tied to the inputs as well. This is stock in this amp given that no one has messed with it given the age of the solder.

My voltages run a little hot, but no red plating, nothing out of the ordinary and never had a problem with RF signals or anything else, just thought I ask around.

Of course there is no schematics to be had out there for this type of amp.
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tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Fri Jul 24, 2015 6:30 am    Post subject: Reply with quote

Just to clarify a little thing that has me confused...

Pin 2 of a 12AX7 is a control grid, but there is no screen grid on a 12AX7. Am I correct in assuming this is what you meant, or were you trying to talk about the screen grid resistor on the 6V6?

Since you're talking about the input jacks, I assume you're talking about grid stopper resistors on the 12AX7 from the input, but it sounds like you already have 47k grid stoppers in place, plus the 1M resistor that sets the input impedance.

Do you have a schematic, by chance? that will be the easiest way to figure it out!
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vulture



Joined: 01 Mar 2008
Posts: 4503
Location: Chicago

PostPosted: Fri Jul 24, 2015 7:38 am    Post subject: Reply with quote

Ah, yeah, thats what i meant. It was late when i wrote this. The grid screen coming off the 6v6 is 220k. I figured that 47k was probably right for the 12ax7. No original schem for this amp to be found. Been looking for years. Currently there is a shielded cable coming off of the questionable pin 2. Clearly there was a component wired into it due to the shielded area having been cut out and residual leads and solder present. Maybe i'm overthinking this and someone just attempt to mod it at one point..
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tangelolemon



Joined: 26 Jan 2009
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Location: Brooklyn, NY

PostPosted: Fri Jul 24, 2015 8:36 am    Post subject: Reply with quote

I'd say if the amp works fine, I wouldn't worry about it!
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vulture



Joined: 01 Mar 2008
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Location: Chicago

PostPosted: Fri Jul 24, 2015 10:00 am    Post subject: Reply with quote

yeah, at this point I will. At first I was trying to restore some of the components back to there original specs based upon piecemealed information I know about what is out there on Alamo amps, but to no avail it will have to be what it is.

Someday I will get the layout of this drawn out and put it out there. More information about these amps (50's- late 60's) the better as they really are great simple point to point wired amps with a great sound.
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ajeffcote



Joined: 09 Nov 2009
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Location: Baytown,Tx

PostPosted: Sat Jul 25, 2015 6:49 am    Post subject: Reply with quote

Can you reduce the bass with smaller coupling caps? That's what I ended up doing on mine. Different amp from yours, but should be the same idea.
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vulture



Joined: 01 Mar 2008
Posts: 4503
Location: Chicago

PostPosted: Sat Jul 25, 2015 9:26 am    Post subject: Reply with quote

ajeffcote wrote:
Can you reduce the bass with smaller coupling caps? That's what I ended up doing on mine. Different amp from yours, but should be the same idea.



Yeah that's what i did at first and it reduced some of the bass. I also tweaked the cap off the tone pot as I was maxing out my tone pot all the time and this helped make it a better useable range.

But what really helped me was adding a treble bleed cap off my volume pot. I had a pre-existing hole in the chassis so I made this switchable.

The amp is very flexible in tone now, super happy.
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ajeffcote



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PostPosted: Sat Jul 25, 2015 7:11 pm    Post subject: Reply with quote

Very cool! Victory!
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