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Silvertone 1474 amp
Silvertone 1474 amp
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Old 26th November 2017, 02:10 AM   #1
DeathRex is offline DeathRex  United States
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Default Silvertone 1474 amp

So dude brings a Sivertone 1474 to fix. It's all stock. It does kinda work except for 2 burnt resistors and the reverb. It has a piezo reverb that they don't make any more. I've been researching and researching, and everyone just says to match your old reverb with a new one. Since I can't, I decided to rewire the circuit for either a 6CG7 or 6N1P driving a 8FB2C1B since it had the highest impedance (1925Ohms in). The Ampeg Gemini 1 used a 6CG7 to drive a reverb tank, but they didn't say what the impedance was.
I don't want to add another tube, just remove the 2nd 12AX7 and put in a 6CG7 or 6N1P, both can get down to 6.5KOhms internal resistance, with the 6N1P giving me a lot more gain.

Will the MOD 8FB2C1B that I already have mate up with a 6CG7?

Top picture is the Ampeg, bottom is the Silvertone.

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Old 27th November 2017, 01:50 AM   #2
PRR is offline PRR  United States
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Quote:
Originally Posted by DeathRex View Post
....I don't want to add another tube...
SilverTone did the piezo because it saved a tube. Both Gain and Drive. I don't think you will get there, well, without a tube or equivalent amplifier.
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Old 27th November 2017, 02:49 AM   #3
DeathRex is offline DeathRex  United States
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A 6N23P seems to be a closer impedance match. With it I could get a gain of 31 for the gain stage and 27 for the drive stage. With the original 12AX7, Sears got a gain of 64 for the drive stage and 31 for the gain stage, so I do believe my signal is just going to be halved.
I don't know how much is lost in the reverb, and I don't know if there is a difference in loss between the transducer and piezo reverbs.
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Old 27th November 2017, 03:45 AM   #4
Enzo is offline Enzo  United States
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If you want a tube drive for a high-Z pan input check out the Ampeg circuits. Like the VT-40.

That piezo reverb is utter junk. Even when working right, it has all the tone of a screen door spring.

I have in the past successfully repaired those by harvesting a piezo disc from a cheap tweeter, and modified the little clamps to suit the disc. (One side versus two sides connections) But for all my efforts I just wind up with the same junky reverb.
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Old 7th December 2017, 11:46 PM   #5
DeathRex is offline DeathRex  United States
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Well I got the resistors and caps replaced. I replaced the reverb driver and gain with a 6N23P. The 6N23P has a Ri of about 3K at 9.34ma. It all does work, except I'm getting a small bandwidth through the reverb. From 1khz to 2.3khz I'm getting about 4 times more signal out than at 600hz or 6000hz. In that band I'm getting more signal than through the non reverb channel. Why is that?
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Old 9th December 2017, 01:23 AM   #6
jjman is offline jjman  United States
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"Small bandwidth thru the reverb" = "tone of a screen door spring"
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Old 9th December 2017, 03:54 AM   #7
DeathRex is offline DeathRex  United States
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I got the reverb replaced with a MOD 8FB2C1B. Going by the Ampeg schematic I used a 0.47uf after the 6N23P driver, and am wondering if it's too small. I've been testing it to day and I found a 0.22uf cap on the input or output of the reverb does wonders, it increases 600hz while lowering 1Khz. But it also lowers 2.3Khz a lot. I've tried putting a pot in series with the 0.22uf cap and adjusting it, it helps, but very little. I've also tried a 6N1P, 6CG7, 6DT8, and 6BZ8, but it doesn't change the bandwidth.
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