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Old 25th May 2006, 03:25 PM   #1
Trout is offline Trout  United States
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Question Choke Replacement Question.

Hi Guys,

Basically, I am in the process of sorting out a homebrew guitar amp.

I have a question about replacing the choke with a simple resistor. Right now, I have the unit built, But my voltages are low after the replaced part. I have no specs on what the original design used as far as a choke, Several places sell a universial unit that is 5-8H @ 200ma, 125 ohms.

What value resistor would be approximately the correct Value?

The original PS Was as Follows,

16UF->Choke->16UF->10K5W->16UF->10K1W->8UF-> to preamp
Back when this circuit was produced 1950's, I figured they needed better filtering with those caps.

I have the unit set up as follows

30UF->10K10W->30UF->10K5W->30UF->10K1W->16UF-> to preamp.

The voltage drop is way off spec, more than 100V!!
I suspect the resistor should be around 125ohm 10W?

Trout
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Old 25th May 2006, 03:55 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

I presume the high impedance section feeds the preamp.

Whilst the power amp is fed from C2.

10K as R1 as you say is far too high, ~100R sounds more like it.

/sreten.
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Old 25th May 2006, 04:33 PM   #3
Trout is offline Trout  United States
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Quote:
Originally posted by sreten
Hi,

I presume the high impedance section feeds the preamp.

Whilst the power amp is fed from C2.

10K as R1 as you say is far too high, ~100R sounds more like it.

/sreten.
Yep, I was suspicious of that, I'll swap it out and give it a try.
Thanks!!!
Trout
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Old 25th May 2006, 07:07 PM   #4
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Just a silly suggestion. I have had great success with using the secondary of a microwave oven transformer as a cheap choke (ie free). Happily handles 0.5A. Mine has 100R series resistance.
If your've got the room its worth looking at.

Shoog
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Old 25th May 2006, 09:46 PM   #5
Trout is offline Trout  United States
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OHHHHhhhhhhhh YESSSSSSSSSSSS


Swapped out the 10K-10W to a 130-10W and everything hits about perfect. 405V at the plates, and -37V bias never sounded so good!!

I finally found an opt I like, I like A LOT! Rated for 50W this thing sounds like a million bucks. Tightest bass I have ever gotten, Yet the high end sparkles. My good American Strat is at my brothers for some video soundtrack job he is recording, So I have my daughters 150.00 fender squire plugged in, Oddly, It sounds as good as a $4000 Gretsch with the clean Jazz type settings!
Only thing I never thought about was now that its this loud, I need ear muffs to get into the heavily overdriven tones lol

Perhaps a few tweaks on one of the preamp channels will bring that back a little.

Now its time to roll a few tubes, I am dying to try the JJ KT77's out lol
Trout
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Old 25th May 2006, 10:53 PM   #6
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What 50's amp did you build and I'm wondering why you'ld want to replace the choke with a resistor? My understanding is that the choke is better at filtering unwanted noise.
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Old 25th May 2006, 11:34 PM   #7
Trout is offline Trout  United States
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Quote:
Originally posted by maxwedge
What 50's amp did you build and I'm wondering why you'ld want to replace the choke with a resistor? My understanding is that the choke is better at filtering unwanted noise.

An old 1955 Fender Bandmaster 5E7 with a beefed up output stage.

I have built a few 5D4 Supers with a 5E7 front end prior to this one, but that did not use a choke at all on the original 5D4.

I got a great deal on a few 5E7 compatable power transformers and wanted to go to an adjustable fixed bias since the PT had the proper 50V tap.

I may have spoke to soon though on this fix. I just experienced the classic " To much capacitance" symptom Internal Rectifier Tube Arc . I am guessing that 30uf on the first cap is excessive when combined with only 130R and another 30UF.
Right now I am as follows
5U4->30UF->130-10W->30UF->10K5W->30UF->10K1W->16UF-> to preamp.

It behaves like it is seeing way more than 30uf so most likely I will need to lower the first cap to 16UF.

It arced once when I flipped on the standby switch, I replaced the tube(just in case)
Played about 30 minutes and got another arc while pushing the volume a bit on the extreme side.
Once again, Replaced the 5U4 to an NOS Sylvania, No arc after 20 minutes of hard playing. But I am guessing its borderline and its best to just fix the problem. a .90cent cap? or a 18.00 tube? I'll replace the cap.
The 2-5U4's tubes that arced had very low miles on them and came right out of a couple working units. ( Known good tubes)

As far as noise goes. Its not an issue. These are very quite amps, or at least mine are. The only real noise I get is from the poorly shielded guitar I am using for testing.

Trout
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Old 26th May 2006, 12:02 AM   #8
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Cool
I just finished (almost ) a black face Twin with the 6G8 twin tone circut. I use a Weber WZ68 copper cap rectifier. It's like 2 GZ34 tubes. If you go solid state you won't have to worry about too much capacitance.

I've got a thread about my amp over here
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Old 26th May 2006, 12:59 AM   #9
Trout is offline Trout  United States
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Quote:
Originally posted by maxwedge
Cool
I just finished (almost ) a black face Twin with the 6G8 twin tone circut. I use a Weber WZ68 copper cap rectifier. It's like 2 GZ34 tubes. If you go solid state you won't have to worry about too much capacitance.

I've got a thread about my amp over here

Thats a great looking job you did there!! Good Thread also.

I changed out the cap, no arcs "YET"

Interestingly though, When I compare the My Super 5D4 PS to the 5E7 I am a bit puzzled. I have been running 3X30UF on my supers and it never arcs the tube. I am a bit puzzled about why this one does.

I have the 6L6Gc's biased dead on -40V now and it seems really sweet. I plan on {hopefully} going to KT77's yet today.

I may just pop for the copper cap or someones plug in SS replacement. Nothing wrong with more POWER!! lol.

This was my first eyelet board amp. Generally I go real old school point to point, Here is the underside of one of my 5D4/537 Hybred

I'll get a picture later of the one I finished today.
BTW,It will be turrets on the next unit, My eyelets though easy to work with were a bit small and anything over 3 leads was a very tuff fit.
Trout
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Old 26th May 2006, 02:54 AM   #10
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Quote:
Originally posted by Trout



Thats a great looking job you did there!! Good Thread also.

I changed out the cap, no arcs "YET"

Interestingly though, When I compare the My Super 5D4 PS to the 5E7 I am a bit puzzled. I have been running 3X30UF on my supers and it never arcs the tube. I am a bit puzzled about why this one does.

I have the 6L6Gc's biased dead on -40V now and it seems really sweet. I plan on {hopefully} going to KT77's yet today.

I may just pop for the copper cap or someones plug in SS replacement. Nothing wrong with more POWER!! lol.

This was my first eyelet board amp. Generally I go real old school point to point, Here is the underside of one of my 5D4/537 Hybred

I'll get a picture later of the one I finished today.
BTW,It will be turrets on the next unit, My eyelets though easy to work with were a bit small and anything over 3 leads was a very tuff fit.
Trout
Now thats real PP! Ya got all them puppys hagn' in mid air, lol.

I don't know why the 5e7 would arc and the 5d4 doesn't with 30uf. Seems like it should be ok but I think someone with more knowledge of these things needs to answer this one.
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