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Old 26th May 2009, 04:41 AM   #1
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Question Volume Surges Fender "The Twin" - transformer going?

Hello all -

I just finished replacing all the caps in my Fender red knob "The Twin". I also replaced a number of resistors in the power stages and across the output tubes. This corrected a number of problems (crackles, pops, hum) that I was having but not the main problem.


The main problem is that the Amp sounds fine for about 10 minutes until it gets warmed up good and then the volume surges really loud. I can stop the surge by throwing the amp's Hi/Lo switch from whatever state it is in and then volume resets. After a short bit the volume surges again. Throw the Hi/Lo switch to the opposite setting and things quiet back down.

Here is what I have tried -
1. Have run the amp with 4 or 2 outputs tubes - just need to change the speaker impedance setting. Switched out various tubes in the pair. Does not cure the volume issue as it occurs regardless of tube combinations.
2. Remove all output tubes and install a pair of Yellow Jackets and EL84s. Volume issue still present.

I have also noticed that when setting the amp bias I have to turn the trim pot all the way to get .4 volts. Actually, I only get 3.5. I can balance the tubes no problem. I don't think the output tubes are the problem. I beginning to wonder about other things like a failing transformer, a semi-fried transistor or rectifier, or just a power diode going bad.

I am not an expert a repair but do know my way around a circuit board. Also, I only have a multimeter to trouble shoot with. Can anybody give some clues as to what might be the culprit?

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Old 27th May 2009, 06:11 AM   #2
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
You can wait for the amp to anounce its problem, or you can troubleshoot it.

Isolate the problem. Put jumper cords across any effects loop jack pair to see of the return jack cutouts are dirty.

Apply a test signal to the power amp in or effects return to see if the symtpom is caused in the power amp stage or the preamp stages. Likewise, send a preamp out signal from the effects send jack to another amp. Does the signal surge when it gets there?

I don;t have the schematic in front of me, but check the speaker out jacks for solder and and switching contacts. If there is an impeadnce switch, at least clean it, and try a different impedance.

Try different speakers plugged into the amp. Your problem could be caused by intermittent speaker in the amp.

If you can localize the problem to the power amp, then monitor the voltages at the powr tubes. Is the B+ changing dramatically? Are the screens losing voltage on any of the sockets? Does the bias voltage go sky high when it dims?

How about this: Bias supply filter cap cracked solder to board. Sometimes it loses contact and bias drops causing surge.
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Old 27th May 2009, 02:14 PM   #3
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Thanks for your input. I will check out as you suggested. One thing I have noticed is that I can't set a .4v on the bias circuit. The max setting I get when turning the trim pot all the way is .3v. I replaced the bias filter cap and resistors but still no difference.
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Old 27th May 2009, 06:35 PM   #4
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Semi nubie here also, but.....perhaps the difficulty in setting the bias voltage is trying to tell you something...

Can you monitor (with a voltmeter on the test point) the bias voltage when the volume starts running away? My wild-*** guess is that your bias voltage is going away once the amp is warmed up.

Two members here that do a lot of Fender amp work are gingertube and trout. I'm sure there are many others also, these two just came to mind. Perhaps one of them will chime in.
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Old 27th May 2009, 08:09 PM   #5
cerrem is offline cerrem  United States
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Many times these volume swells are caused by the cathode bypass electrolytics used in the pre-amp gain stages....
These bypass caps are in parallel with the cathode resistors to avoid the cathode AC current feedback that would normaly occur, thus maximizing the gain for that stage.
When these electrolytics go bad or intermittent, they cause a similair problem to what you describe....
But then again I don't know the whole story so it could be a number of things...I would also agree that signal tracing would be the ticket to solve this problem quickly...

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Old 28th May 2009, 12:57 PM   #6
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Well, the volume swells stopped. But I should really say that the amp seems to running at low power now. Everything seems to work except at a lower volume. I have lost channel switching and it will not work with either the footswitch or the panel push pull pot. The channel LEDs do not light up on the panel either. Also, I notice that the V4 tube seems very faintly on. It's a brand new tube and when I swap it with another tube I see the same result.

So I am guessing that the lower voltage at the bias circuit is reflecting the amp running at a lower voltage overall. Everything sounds clean, reverb works (weakly), both channels work, all power tubes glowing nicely.

I have looked for loose wires, remelted several solder points throughout, and looked for fried components. All looks good

Bypasssing the preamp has the same problems. So now the real problem searching begins.
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Old 28th May 2009, 05:20 PM   #7
m6tt is offline m6tt  United States
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When you say V4 is "dim", you mean that the heater is not lit up as bright? Have you checked the voltages?

Some tubes have the heater less exposed visually and they appear dimmer, this is not a big problem. However, if the heater was intermittently shorted or weakly connected at a preamp socket, it could manifest as volume swells.

What is the bias voltage on the power tubes grids? Or rather, what is the voltage between the grid pin and the cathode pin? if it only swells on signal peaks but sounds distorted and low at all other times, that could indicate that it is trying to operate in class C. It's possible that the bias measurement resistor has drifted high i suppose.

Is the speaker load attached the right one? Connected to the right jack? Is the jack and all the speaker wiring in good condition?
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Old 28th May 2009, 05:31 PM   #8
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Visually the V4 tube looks dim barely a trace of color across the grid. How do I measure the bias across the tubes as you suggest? I do not own a o-scope only a multimeter. When using it to adjust the bias pot I can not .4 volts, the best I can get is .3 volts when turning the pot max.

Yes, there is a speaker load on the proper output jack. I have two 8 ohm speakers wired in series plugged into the J6 speaker output jack and the selector switch set to 16 ohms. Everything seems to be in order.

The amp plays, but low volume and the channel switching does not work. I am no longer getting volume swells. My thought is that what I thought were volume swells was really the amp operating at full power and now something has failed or become loose or disconnected leading to volume fall off. I suspect the problem may be in the preamp stage.

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Old 28th May 2009, 06:05 PM   #9
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Originally posted by jl_lensman
How do I measure the bias across the tubes as you suggest? I do not own a o-scope only a multimeter. When using it to adjust the bias pot I can not .4 volts, the best I can get is .3 volts when turning the pot max.
Just leave the voltmeter connected to the test point for the bias pot and monitor the voltage during the volume swells. If the bias voltage is dropping when the vol surges, it could be a bias supply problem. The fact that you cannot get enough bias voltage to be within spec is suspicious, at least to me.
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Old 29th May 2009, 04:49 AM   #10
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Here is an update on the ills of my "Evil Twin".

After a few minutes playing I get increasing tube noise that eventually goes into uncontrollable feedback. It occurs on both channels of the amp - even when I have (1) channel with all pots turned to "0".

The Amp is now running both channels in parallel mode and I can not defeat it with either the footswitch or the panel channel selector pull switch. Both front panel channel LEDs should light up when in parallel mode but neither is lit. I can not get the AMP tp run in a single channel mode.

I can not set the bias to .4v as specified. The best I can get is .3v and this does not change when the AMP goes into feedback.

The preamp out signal does not feedback but the AMP still only operated in parallel mode.

Here is what I have done so far -

1. Replaced all electrolytic capacitors throughout preamp and power stages.

2. Cleaned speaker impedence switch. Checked all cabling and substituted different cables.

3. Checked for loose wires and bad solder joints. I cleaned a few things up but nothing that would make a difference.

4. Switched out preamp tubes with old ones. No difference.

5. Switched out power tubes for a pair of yellow jackets with EL84s. Feedback still the same.

So it's still an ongoing mystery to me.
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