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Old 22nd June 2008, 08:51 AM   #1
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Default help with OT for 15 watt tube amp

Can anyone help me figure out what theOT's input impedance of this amp based on this schematic below? I just dont know what specific OT to get for an upgrade. It is a 15 watt 2x el84 amp.

http://www.blueguitar.org/new/schem/..._palomino.pdf.
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Old 22nd June 2008, 09:12 AM   #2
Tweeker is offline Tweeker  United States
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404 error, file not found.

Going to a higher quality OT does not necessarily improve things in guitar, unlike hifi.
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Old 22nd June 2008, 10:49 AM   #3
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This is the good URL:
http://www.blueguitar.org/new/schem/...6_palomino.pdf

What are those, EL84? It's most likely an 8K A-A load, though I've used as low as 6.6K.

Cheers!
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Old 22nd June 2008, 12:02 PM   #4
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6.6 KOhms is what Fender used in the "Deluxe".

Deep bass extension is not a concern in a guitar amp. Edcor's GXPP15-8-8K looks like it will do the job.
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Old 22nd June 2008, 06:20 PM   #5
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I am a newbee when it comes to tube amps. So it will be an edcor 15watt 8k primary then since it is the lowest priced i can find. Thank you so much.

It is going to be a replacement OT. I got the amp used for a cheap $75 and the original owner said it was intermittently loosing volume. When i got it it was not intermittent anymore but less volume and less fuller sound than my 5 watt el84 amp.

I already replaced all tubes and reheated almost all solder joints but the problem still exist. I tested the OT with an ohmeter in the primary and it reads 120ohms on the right side of the center tap and 145ohms on the left side. Isn't it supposed to be equal since it is a center tap?

I aslo googled and found out about the AC test to figure the pri/sec ratio but i only have a DC power supply.
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Old 22nd June 2008, 08:51 PM   #6
PRNDL is offline PRNDL  United States
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Quote:
I tested the OT with an ohmeter in the primary and it reads 120ohms on the right side of the center tap and 145ohms on the left side. Isn't it supposed to be equal since it is a center tap?
That's close enough (about 15%), especially when it's measured in the circuit.

Have you checked the electrolytics?
It's the most common cause, after bad tubes.
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Old 22nd June 2008, 09:43 PM   #7
JandG is offline JandG  United States
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Definately re-cap PS caps at least. I have done many a guitar amp with just PS lytic change & that is all it took to make em sing. It is highly doughtfull your OT is bad. What amp did you get if I may ask. My favorite of all my amps is a lowely Airline of 6v6 SE that does no wrong is is truely special. I paid 20.00 for it & recapped it with extremely generic lytics & bam she came alive..& does things my Soldano can't. Good luck on venture.
J & G
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Old 23rd June 2008, 12:21 AM   #8
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Output transformers (and power transformers...) often have different DC resistance on the halves of the center-tapped winding. The second half is wound on the outside of the first half, so it's LONGER for the same number of turns. More wire: more resistance.
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Old 23rd June 2008, 03:24 AM   #9
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I did not suspect the electrolytics at first because the original owner purchased it new only 3 years ago. It is a Crate palomino v16 the one that comes in peach tolex with gold trimming.

I might as well replace all electrolytics since it doesnt cost too much to do.

JandG, re-cap PS meaning power supply caps? By the way, a 20.00 tube amp thats such a good find!
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Old 23rd June 2008, 06:46 AM   #10
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Eli, i did not notice earlier that the edcor you suggest is for a 8ohm speaker at 8k input impedance.

I forgot to mention that this amp comes with a 16ohm speaker so should i get a 4k primary impedance OT or is 8k primary for 16ohm load is ok. What efect will it have?
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