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Dr. Bob
Moderator
Australia
6593 Posts |
 Posted - 01/16/2008 : 10:44:42
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quote:
Originally posted by Goran
This is a great document for anybody going to mod Valve Jr:
http://www.s2amps.com/docs/vj_kit_inst.pdf
Hi Goran
That is a very extensive & well written document.
I'm still trying to speed read it.
Is this the mod that you did to yours?
Regards Dr. Bob |
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Goran
Double Platinum Member
Sweden
2203 Posts |
Posted - 01/16/2008 : 11:29:32
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I did some of the mods described, but far from all. I didn�t change the output transformer or speaker.
Most I did came from a Swede who has gone deeply into VJr, nothing expensive but a 100uF 450 V capacitor in the power supply.
I�m not ready yet, and this is the fun of it. The last thing I did was to change the preamp tube from J/J Tesla ECC83S (12AX7) to a NOS Tungsram 12AX7 with good result. The J/J sounded good but was quite microphonic.
I have learned a lot about tube amplification during the process.
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 01/16/2008 : 14:29:50
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quote:
Originally posted by Goran
I did some of the mods described, but far from all. I didn�t change the output transformer or speaker.
Most I did came from a Swede who has gone deeply into VJr, nothing expensive but a 100uF 450 V capacitor in the power supply.
I�m not ready yet, and this is the fun of it. The last thing I did was to change the preamp tube from J/J Tesla ECC83S (12AX7) to a NOS Tungsram 12AX7 with good result. The J/J sounded good but was quite microphonic.
I have learned a lot about tube amplification during the process.
Hi Goran
Did you change the Cathode resistor, to 820 ohms, like is in the article?
And have you experimented with other tubes, like the 12AU or AY or AT valves.?
I would be interested in knowing if it mellow out the amp, as these other values have a lower (gM) gain.
Regards Dr. Bob |
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Goran
Double Platinum Member
Sweden
2203 Posts |
Posted - 01/16/2008 : 15:12:50
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I have not yet tested it with 12AT7 or 12AU7, but I have a couple of those at home and will test some day.
With all the mods I�ve done the amp has much more headroom and are more �Fenderish�, and above all sounds much much better. I like lots of headroom, that way I can use all the od�s I have collected over the years.
Here are the mods I�ve done so far:
R1=1M
R2=22K
R3/R4=tried 150K but returned to 100K
R5=150K
R6/R7=100K + 1000pF parallell to R6
R8/R9=1,5K
R10=330R
R13=1,5K
R14=270R
R15=10K
C3=took it away
C4=2,2uF
C6=100uF 450V
Tungsram 12AX7
J/J Tesla EL84
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 01/16/2008 : 15:27:25
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Hi Goran
Is your Jr, from the first batch or from a later batch?
I have read that on the first batch, they ran the 12VAC for the heaters, very close to the low level audio signal lines on the PCB, & this induced a large enough AC field, to cause hum on the pre-amp tube (12AX7)
I also read that on the later batch's they fixed this problem, & they also very slightly changed the front panel metal work.
I think they moved the text at the top of the panel, to be closer to the center.
Regards Dr. Bob |
Edited by - Dr. Bob on 01/16/2008 15:28:41 |
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Goran
Double Platinum Member
Sweden
2203 Posts |
Posted - 01/17/2008 : 07:04:07
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There are three batches of VJr:
1. With lots of hum, not very good Output transformer
2. Hum reduced, including new PCD-design and DC heated filaments
3. Better output transformer, black PCB (and probably more)
I have version 2.
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Epiphone Valve Junior
