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Post subject: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 1:07 am
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I have a '65 Super Reverb Reissue I bought used last May from Guitar Center. I've made only a couple alterations: switched the factory Jensens to Webers, and tried JJ and other preamp tubes. Everything has been great.

A week or so ago I noticed that, when I was playing chords on the low end of the E or A strings, the sound was distorted to the point of getting garbled if the volume was above 3. If I turned the volume up higher or hit the strings harder, I could get harsh distortion across all the strings.

I tried new power tubes, and biased them. I swapped out different preamp tubes, reverb tubes, vibrato tubes, etc. I used a new tube (12AX7 or 12AT7 as needed) to replace an existing one and, if no change, put the existing one back and moved on down the line.

I tested the speakers by plugging them into my Peavey Classic 30. No problems with them.

I tried three different guitars. I used different cables. I used different channels, and different preamp tubes for each channel. I think I covered every variable there can be.

I got new capacitors for the filter board and installed those. I bought new resistors, too, but the size didn't seem right. I tested the resistance of the existing ones and they were spot on.

I cleaned the tube sockets and tightened the pin connectors.

One thing I noticed, and I don't know if this means anything, is that when I drained down the capacitors after installing and testing the new ones, the 2nd of the 350v 100uf capacitors had about half the voltage of the other 350v 100uf and the three 500v 22uf. I tried this a couple of times, and each time that same resistor was half the voltage of the other four.

I live in an area where there's no Fender authorized service techs, and I really would rather not take it to one if I can figure out what the problem is. My next step is just to go through the chassis with a pencil and knock on connections to see if they're solid while playing an open E & A string chord. I may also try freon on some components as well.

Does this problem sound familiar to anyone? Any ideas?

Thanks for any replies.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 1:15 am
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Does the symptom manifest on both channels?

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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 4:24 am
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Check for defective output tube , broken printed circuit ( ? ) , defective resistor in phase Inverter circuit . Check resistor at the 6L6 tubes circuit.

Read power supply voltage and report .

Any link for this schematic , I found only old vintage SR


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 8:42 am
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Quote:
Does the symptom manifest on both channels?


Yes.

I checked all of the tubes. By "output tube", I assume you mean power tube, which I checked, double-checked and triple-checked. Actually, I checked all tubes several times.

I can check the resistors for the 6L6 and the phase inverter. How I'd check for a broken printed circuit board I don't know. Take it out, I suppose. There's no outward signs of breakage.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 8:57 am
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One thing I noticed, and I don't know if this means anything, is that when I drained down the capacitors after installing and testing the new ones, the 2nd of the 350v 100uf capacitors had about half the voltage of the other 350v 100uf and the three 500v 22uf. I tried this a couple of times, and each time that same resistor was half the voltage of the other four."

If you mean that the voltage of the first 100mfd is half of the second 100mfd cap, while the amp is on, standby engaged --- this is normal. As the B+ is divided equally between the two cap, in situ. But, if the voltage is half when the amp is off... and caps are draining... you have a problem.

If you have issues while draining the caps, then the two voltage distributing resistors of the first filter stage (that balance volts and amps across the two 100mfd/350VDC cap) --- are bad. One cap is getting no voltage off the rectifiers. The resistors are R65 and R66 (220k-ohm, 2-watt), parallel under the two 100mfd caps. These two 100mfd caps should be soldered with (+) in opposite polarity on the board under the doghouse.

Pull the hood off the doghouse. Lift one leg of each 100mfd cap. Check connections and drift in those 220k-ohm resistors. Replace them if they are beyond 5% tolerance match between each other or look damaged.

Replace them with GOOD quality metal oxide 2-watt, flame-proof CC (like Kiwames), or 5-watt WW Mills. I like the Mills. They last forever, very precise under all conditions. Low inductance.

http://support.fender.com/schematics/gu ... ematic.pdf

Good luck. Keep us posted. :D


Last edited by BMW2002Ti on Sun Jan 19, 2014 9:11 am, edited 1 time in total.

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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 9:10 am
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Thanks for the reply. The 1/2 voltage measurement was on the new replacement capacitors. I never measured the old ones. Measurement was taken while the amp was off, and draining. Even as I drained down the caps, the measurement stayed the same. When the others were at 200, it was at 100. When the others were at 50, it was at 25.

Those two capacitors measured correctly, but I'll put in the replacements I bought just to see. The spec sheet said 1/2 watt for those, not 2 watts.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 9:17 am
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No, do not use 1/2-watt resistors in those positions. They should be 2-watts or more. These resistors take the brunt of voltage surges when you turn the standby switch of the output tubes off-to-on. If they start to fail, you will never get even distribution of electricity across those two 100mfd caps. And the resultant filtering will NOT be to 700VDC levels.

The two 100mfd/350VDC caps in series = 50mfd/700VDC final rating, only if the B+ is distributed equally across the two caps. Current levels at this point can reach an amp or more, esp during transients or heavy bass runs (like playing an open E-minor chord).

Check this schematic, again. Looks like Fender has 1-watt resistors in this location. This is really borderline, when you consider what kinda current and voltages these two resistors are seeing. And the important job they must do... to get adequate filtering for the entire PSU.

BTW... it's called output tubes, because they are in the output stage of the amp (or finals stage). Power tubes, literally, can be anywhere in a unit. The 12AT7 that drives the reverb tank is really a power tube of this portion of the circuit.

____________

Addendum: What kinda rectifier are you using? Is it a Russian or Chinese made 5AR4? These tubes have a reputation of failing early. Esp those not selected carefully by the manufacturer of amps. QA tend to vary all over the place.

Anyhow, you have a good, strong, preferably US 5AR4 or Mullard GZ34 you can drop in and test the amp? Failing rectifiers often have poor bass and garbled harmonics as early symptoms, prior to complete failure or short-outs.

If you want to keep a Russian or Chinese 5AR4/GZ34 --- I recommend putting two UF4007 in series with the rectifier. These solid-state diodes help relieve the stresses placed on the tube rectifier. I can post a diagram of how-to-do, if you wish.

Good luck! :mrgreen:


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 9:50 am
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The Fender spec sheet for the amp lists 220K 1/2 watt for the R66 and R65 resistors for the 350v 100uf capacitors. I have two new resistors I can put in place just to see.

The rectifier tube is a Fender-marked Chinese 5AR4. I can see if Guitar Center here has them in stock, but it's doubtful.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 10:05 am
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Another example of Fender schematic-spec sheet boo-boo. ooops! :oops:

They've been 1-watt rated resistors from the days of the Holy AB763.

http://www.webphix.com/schematic%20heav ... _schem.pdf

You need a better rectifier option than a run-of-the-mill Shuguang 5AR4. Do you have a good 5U4GB (hopefully US made) that you can tryout in the amp? This rectifier will work fine for the few minutes you need to test the amp. These are good rectifiers to keep as backup. And good NOS USA-made one are relatively cheap. Any brand will do. RCA, GE, Sylvania, whatever...

http://www.ebay.com/bhp/rca-5u4gb-tube


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 11:42 am
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OK, here's something. I don't know if it means anything. I'm poking around inside the chassis with a wooden dowel. No noises, except that the V1(6), V1(7), V1(8), V2(6), V2(7)), V2(7), V3(6), V3(7), and V3(8) leads to the sockets are all microphonic. I can tap them with the dowel and hear the tapping. It gets louder as the volume knobs increase. The V3 leads aren't as loud as the V1 or V2, I'm sure because that's only a partial preamp tube.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 11:49 am
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Try removing the metal shields from the tubes and retap. Or loosen the socket screws that hold them to the chassis.

Also, I have heard of this in other amps with thin PCB circuit board. Try putting a little weight, some ppl use non-conducting putty onto the board. And then tap the leads.

I really doubt that this microphonic issue is what is causing the distortion in the amp. But, yeah, you need to rule-it-out.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 4:47 pm
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The heat shields weren't on. I held the wires down and tapped. They still made a noise. I don't think that's it, though.

I replaced the two 220K 1/2 watt resistors. No change. I got another rectifier tube. No change.

I'll have to figure out where the other resistors are.

On Ebay there's complete used boards for Super Reverbs for $75. That's tempting.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 5:41 pm
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stratele52 wrote:
Read power supply voltage and report.


This is one thing I have not seen addressed anywhere in this thread.

Whenever I am troubleshooting an amp, the very first thing I do is check all of the DC voltages and heater voltages. This includes the power supply (first with all tubes out of the amp, then adding tubes one at a time to see if there are any big drops in voltage). This includes all voltages at the power supply, the bias supply, plate, screen grid, and cathode voltages.

In my opinion, swapping tubes, checking microphonics, or anything else is a waste of time if the quiescent voltages are not known.

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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 5:56 pm
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Monkeyleg wrote:
On Ebay there's complete used boards for Super Reverbs for $75. That's tempting.


To what end?

There's no guarantee that those boards aren't just as problem-laden as your own.

Don't be a sap -- fix what you got.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 7:21 pm
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Yeah, I know the boards could be junk. This is just getting frustrating.

I have no idea where to measure screen voltage, bias supply, plate, screen grid, and cathode voltages. I'd hoped there was a simpler solution. (Edit to add: but I'll find out what those are and do them anyway).


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