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Post subject: Re: 1970 Bronco
Posted: Mon Jul 31, 2017 3:31 pm
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Location: Coastal Bend, Tx.
Well I subbed a good 5v4st into the amp today. It raised the voltage's across the board by 20vdc. It is/was a better tube than the previous 5y3gt as far as voltage drop is concerned.
The 5v4 gave me .045ma and the 5y3 gave .055ma.
I kept the 5v4st in the amp and put it through about 30 minutes of playing, it sounded better than the 5y3 to me. It gave a little more clean headroom and when it broke up the breakup seemed more controlled, less mixing of the spectrum.
I removed the 1k screen grid resistor from the tube socket I previously installed in an attempt to lower the screen grid voltage to below the plate voltage. Currently the screen grid is slightly(+4-5vdc) above the plate voltage, although there is no red plating while playing.
This is a screaming little amp with a great clean sound in its current configuration.
Waiting on some new C-cup washers before final assembly.


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Post subject: Re: 1970 Bronco
Posted: Mon Jul 31, 2017 6:17 pm
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sfceric64 wrote:
I removed the 1k screen grid resistor from the tube socket I previously installed in an attempt to lower the screen grid voltage to below the plate voltage. Currently the screen grid is slightly(+4-5vdc) above the plate voltage, although there is no red plating while playing.


This shouldn't be. The screen is attached to the PSU, after the 1K-ohm resistor on the circuit board's edge (unless you remove & jumped across this resistor, too). This is after the B+ tap for the anode plates. Should be around 5-10 DC volts less than the plates.

http://www.electronicstudio.net/schemat ... _schem.pdf

Resistor is next to green arrow.

Image


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Post subject: Re: 1970 Bronco
Posted: Tue Aug 01, 2017 6:36 am
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Location: Coastal Bend, Tx.
Quote:
This shouldn't be.
I agree completely. I'm wired just like the pic on page 3.
Initially I added a 470ohm screen grid resistor to the socket to fix it. The 470ohm didn't drop the voltage, so I went with a 1k on the socket. It didn't drop the screen voltage below the plate either.
I guess I need to look at it again, but it plays well.
I suppose I could put a 2.2k-4.7k on the PSU rail to drop the screen voltage. Whereas you suggested the Hammond choke.


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Post subject: Re: 1970 Bronco
Posted: Tue Aug 01, 2017 7:07 am
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Try a 2.2k-ohm, 1 or 2-watt metal oxide resistor. To replace the 1k-ohm one.

Good luck!


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Post subject: Re: 1970 Bronco
Posted: Tue Aug 01, 2017 8:27 am
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Location: Coastal Bend, Tx.
Checked my on-hand supplies and the lowest I have are 3k3w then 3.6k in MO, may have a 2wattCC in the 2k range will have to check those later. I have a Dr's appt. so it will have to wait until this afternoon.


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Post subject: Re: 1970 Bronco
Posted: Tue Aug 01, 2017 9:18 am
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3.3K or 3.6K-ohm should be a good try. You are simply doing some empiric trials. :lol:


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Post subject: Re: 1970 Bronco
Posted: Tue Aug 01, 2017 10:50 am
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Location: Coastal Bend, Tx.
Well, all ok @ the Doc's in and out in under 30min's.

Pulled the 1k and soldered in a 3k 3w.
120vac input
B+394 w/ 5v4st
V3 P3 383
V3 P4 375
V3 P8 21
.038ma @ 13.75watts/98%

Looks successful on paper!


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Post subject: Re: 1970 Bronco
Posted: Tue Aug 01, 2017 11:43 am
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Yes! Better to have the screen voltage below the anode plate.

Good job!


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Post subject: Re: 1970 Bronco
Posted: Tue Aug 01, 2017 1:06 pm
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Joined: Tue Nov 27, 2012 6:56 am
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Location: Coastal Bend, Tx.
Thanks, it sounds really good to me!
Now I can rest easy knowing the screens won't melt down when playing.
My #6 C-cup washers just came in the mail also, so I can wrap it up today as a finished project and move on to the Super.

DOH, I guess Fender used #8 C-cup washers. Oh-well, it ain't goin nowhere.
The last three pics:
Front
Image
Back, notice the big bottle on the left
Image
New cover
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