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Post subject: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 7:01 pm
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I recently purchased a used SRRI (circa 2001) and discoverd some interesting repair work while inspecting the amp chassis. The R27 resistor location has a burn spot on the PCB, and there are two resistors in that location that are twist-soldered together. Also the nearest cap is also a double-cap twist-solder install. Following the schematic, it appears to be part of the V3 reverb send. Has anyone seen such a repair, should I be concerned, and what would be the proper replacements to bring it back to original.

The reverb on the amp does work fine, and the amp itself sounds really sweet. There is a constant background hiss when the amp is on, without any input in the vibrato channel (volume about 6 - 7) - not noticable at stage volume playing. I run it with the V1 tube out and have a 12AX7 in V6/PI. V2 and V4 are new Tung Sol 12AX7s, the rest of the tubes are stock GT's at factory values - soon to be replaced.

Thanks.


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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 7:02 pm
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Pics and Schematics:

Image
Image
Image
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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 7:14 pm
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I have no idea what purpose the double resistor and double cap mods serve.

But the higher-mu 12AX7 as a phase inverter may be responsible for the abnormally-audible hiss at idle. You might try a nice, balanced 12AT7 there and see if it quiets the amp down.

Arjay

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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 7:27 pm
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Thanks for the reply. It makes the hiss at idle with a 12AT7 as well... with the stock one, anyway.


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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 7:35 pm
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Hi prigi3,

+1 on the AT in the PI position.

The doubled up components were likely used because that was all that the repairer had on hand that would match the values, and the value of both 4.7K ohm resistors in parallel give approximately the correct value of 2.2K ohms. I can't see the two caps clearly, but in order to get close to the correct value of 22 µF, they would have to be something close to two 10 µF caps in parallel, or two 50 µF in series.

The repair is a little rough looking, but probably perfectly sound, although I would likely redo the repair with the correct components. The cap and resistor might have burnt because of a faulty reverb driver tube previously in V3, which should be a 12AT7.

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Last edited by shimmilou on Wed Dec 18, 2013 8:09 pm, edited 1 time in total.

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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 7:47 pm
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Here's a link to the SRRI schematics - the pics were a little on the blurry side:

http://www.studiosoundelectronics.com/schematics/65_super_reverb.pdf


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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 7:57 pm
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shimmilou wrote:
The repair is a little rough looking, but probably perfectly sound, although I would likely redo the repair with the correct components. The cap and resistor might have burnt because of a faulty reverb driver tube previously in V3, which should be a 12AT7.


+1

When I bought my '64 Pro Amp I discovered the 470Ω/1W screen resistors had each been replaced with a paralleled pair of 1KΩ metal oxide types. The amp sounded and played just fine but in the interest of "purity" I supplanted them with vintage-correct Allen-Bradley carbon-comps of the proper value.

Arjay

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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 8:01 pm
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Hey Rock Star - appreciate you checking this out. I'm with you on the redo repair. Thanks for the values. I have plans for a Mullard CCV4204 for the PI...

Any ideas on the hiss issue?


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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 8:10 pm
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shimmilou wrote:
The cap and resistor might have burnt because of a faulty reverb driver tube previously in V3, which should be a 12AT7.


When I was checking/tightening up the tube sockets during this initial maintenance, I pulled the V3 reverb tube and discovered it was a 12AX7. I swapped it with the 12AT7 from the PI for now while I shop for new tubes all around.


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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 8:18 pm
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prigi3 wrote:
...Any ideas on the hiss issue?


Maybe normal, as many tube amps have some hiss. Lower gain tubes in strategic positions might help reduce the hiss. :?:

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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 8:32 pm
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Retroverbial wrote:
shimmilou wrote:
The repair is a little rough looking, but probably perfectly sound, although I would likely redo the repair with the correct components. The cap and resistor might have burnt because of a faulty reverb driver tube previously in V3, which should be a 12AT7.


+1

When I bought my '64 Pro Amp I discovered the 470Ω/1W screen resistors had each been replaced with a paralleled pair of 1KΩ metal oxide types. The amp sounded and played just fine but in the interest of "purity" I supplanted them with vintage-correct Allen-Bradley carbon-comps of the proper value.

Arjay


Although it can't be seen when the amp is assembled, and even if there is no difference in sound, that kind of thing still really bugs me and I feel like I just have to "fix" it.

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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 8:46 pm
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...any recommended sources for replacement components?


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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Wed Dec 18, 2013 10:29 pm
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The tubesandmore.com website likely has everything you need. I score a lot of vintage-spec parts on Ebay.

But I'd put the priority into getting a proper phase inverter into V6.

Arjay

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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Thu Dec 19, 2013 9:09 am
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prigi3,

The resistor (R27) & cap (C15) are the cathode bias resistor and bypass cap on the 12AT7 reverb driver tube. The resistor should be a 1-watt one. But, taking two 1/2-watt ones at twice the resistance is one way to double the wattage rating (you need to double the resistance of each parallel resistors to get the final proper value).

I'd change this to ONE correct wattage resistor (2.2 k-ohms, 1 watt). Use a metal oxide resistor.

The caps are paralleled to double the voltage rating to around 22mfd/63VDC. Try to replace with ONE good quality 25mfd/75 VDC or 100VDC cap. Like a Nichicon MUSE axial cap.

http://www.mouser.com/new/nichicon/Nichicon-Audio-Caps/

The reason why there are more heavy duty rated resistor and bypass cap off this 12AT7, is this tube is being pushed to full current flow rating. It is really the output tube of a small single-end amp --- driving the reverb tank.

Notice how the two sections of the duo-triode are tied together (at the anodes)? This doubles the current passing capacity of the tube. Nearly ten times the current passing capacity of a 12AX7.

Good 12AT7's are needed in this position. As the reverb circuit is hard on this tube. Often they get prematurely noisy or have early failure issue. I like mil spec Sylvania or Philips 12AT7WA, -WB, or -WC, US or Philips (Holland) made 6201 in this position.


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Post subject: Re: Super Reverb Reissue - Burnt R27 Resistor Repair
Posted: Sat Sep 16, 2017 1:46 pm
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Good day,
I smoked the r27 last night. Only 3 hours of running after being down for a long time while I replaced the r54 and r55 with 1/2 watt carbon film.

I see the c15 is bulged. The v3 is the GT 12AT7

So WHY would it smoke? Cap failure? I don't use the reverb. The pot was at 1
Input into the vibrato channel 1 from a rp1000 digitech.
It sounded fine. Took a break. Left the B+ (standby) on. After 30 minutes I returned and the speakers started to emit a loud buzzing. Turning the volume down from 3 to 1 did nothing. I shut it off but could smell the burnt board. The front panel felt hot to me. But it is a tube amp running in the tropics.

So I will replace the resistor, replace the cap with one with a higher voltage rating.
No way nowadays to check tubes, so maybe I should change them all? $$$
Inspection stickers indicate this was built in 2001. These are original tubes.
It is a model PR469

By the way, WHY would the r55 and r54 fail? Is everything designed on the edge of their ratings?

Voltages from the B+ are pretty high. For example: tp2, tp3, tp4 are 465, 454, 415. Schematic shows 433, 420, 385

Line voltage here is generally 128vac

Looking for your advice / suggestions.


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