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Old 22nd July 2012, 08:59 PM   #1
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Default Fender Twin Troubleshooting Help Please - Jedi Master Needed!

Calling all tube amp jedi masters! I've hit the wall for my limited troubleshooting knowledge and am hoping someone out there might have wisdom to share. I'm getting a significant amount of hiss noise that I can resolve. I am just a player/hobbiest but have some basic electrical knowledge. I'm forever grateful for any tips or suggestions you can provide...

Amp: This is a '74 silverface fender twin reverb (sn: A66103), 100W version with push/pull master volume (not 135W ultralinear). I was told it had been sitting in the previous owners closet for the last 20yrs. I've owned it a few months.

Previous Owner Mods: After opening it, I found the previous owner had converted the bias balance pot to an actual bias adjustment design using this circuit mod from EL34 World. He had also disconnected the vibrato speed control wire (green), disabling vibrato. Last, it had mismatched brand (and bias) power tubes installed. I reversed all mods (for the time being), returning it to a bias balance system and reconnecting the vibrato.

Servicing: I personally did a full electrolytic cap job (F&T power filter caps, Sprag Atom bias caps), installed a new full set of JJ power and preamp tubes, cleaned all pots and jacks. I took special care in making good/clean solder joints. The power tubes have been balanced and the cathode bias (1ohm, 1W resistor test method) are measuring ~23mA. (I understand this is cold and should be ~30-35mA... but I'm leaving it alone for now.)
My Current Problem: I'm getting a considerable amount of noise that is tracking with the channel and master volumes (off when volumes are off... raises when either channel volumes and master volumes raise). I don't believe the noise was present after my initial cap/tube job but seemed to have gradually increased after a few weeks of playing. I also have a 100W '72 fender super six (very similar head design, same tube set). At same volume levels, I'd say the twin is about 3-5 times noisier. The noise doesn't change with or without an instrument. Reverb or vibrato on/off doesn't seem to matter either.

Troubleshooting So Far:
- Tube Swap: I swapped tubes between by two amps (same type/set) and it didn't have an impact (twin had same noise with new tubes and the twin tubes didn't make noise in my super six).
- Power Filter Component Tests: I removed all the new power filter caps and old (reused) carbon comp resistors to test values (multimeter). All values are in spec an no sign of damage.
- Speakers: I've test both heads with opposite speaker stacks. The noise is consistent with the twin head independent of speakers used.
- Power Tube Screen Resistors: I tested all 470 ohm 1W power tube screen resistors. V10 had increased to ~725 ohms and was replaced with a new carbon comp 470 ohm. The other three were in spec. No change in noise.
- Preamp Tube Plate Resistors: I tested all 100k ohm 1W carbon comp plate resistors and all resistor values are within spec.
- Different Power Source: I've plugged it into a variety of different power sources (plugs from different circuits) with no effect on noise. Also, the Super Six shows no noise from same power circuit the Twin is operating.
- Hiss recording: I posted this recording to youtube (here) so you can hear it and how it tracks with volume levels. It’s much louder in person… about 3-5x louder than my Super Six at like volume levels.
- Operational Pin Voltages: With amp fully on, zero volume and no instrument plugged in, Pin voltage are the following (Heaters all @ 3.2Vac)...
Power Tubes: Control Grid @-51Vdc, Plate and Screen @450Vdc.
12ax7 V1a: Plate @234Vdc, Grid @NA(Chnl 1 input), Cathode @1.7Vdc
12ax7 V1b: Plate @244Vdc, Grid @NA(Chn 1 vol), Cathode @1.7Vdc,
12ax7 V2a: Plate @240Vdc, Grid @NA(Chnl 2 input), Cathode @1.5Vdc
12ax7 V2b: Plate @246Vdc, Grid @NA(Chnl 2 vol), Cathode @1.7Vdc
12at7 V3a: Plate @ 428Vdc, Grid @0.26Vdc, Cathode @5.6Vdc
12at7 V3b: Plate @ 428Vdc, Grid @0.28Vdc, Cathode @5.6Vdc
12ax7 V4a: Plate @143Vdc, Grid @0Vdc, Cathode 1.8Vdc
12ax7 V4b: Plate @235Vdc, Grid @0.17Vdc, Cathode 1.8Vdc
12ax7 V5a: Plate @438Vdc, Grid @-38Vdc, Cathode @~0Vdc
12ax7 V5b: Plate @392Vdc, Grid @-38Vdc, Cathode @~0Vdc
12at7 V6a: Plate @311Vdc, Grid @122Vdc, Cathode 159Vdc
12at7 V6b: Plate @313Vdc, Grid @124Vdc, Cathode 160Vdc
I'm stumped. Besides the hiss, everything seems to work great. I don’t know what else to do other than take every component apart and measure values. I'm hoping someone might have had a similar experience, test recommendations or can tell some of my values are off above. PLEASE HELP ME!! ThanksJ
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Old 22nd July 2012, 11:12 PM   #2
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I have found in the past that a change to the grounding system can work wonders for reducing noise. Try running all power amp grounds to a single star point. Then set up a ground bus from an isolated tag or terminal to the input jacks. I use a copper wire of sufficient stiffness. Run all of the remaining grounds to this buss. While you are working on the amp I would replace those CC plate resistors with metal films. Can't hurt and the DAle RN65 series can be ordered from Mouser for a dollar each. If the noise is still bad, tryisolating the reverb and tremolo jacks on the back of the amp. Also, if the heaters do not have a center tap to ground, create a virtual one with two 100 ohm 2W metal oxide resistors going to ground.

Let us know how it goes.

Jp
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Old 23rd July 2012, 12:56 AM   #3
mgreene is offline mgreene  United States
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hiss in fender amps is often a burn out resistor somewhare. look for a burnt/discolored resistor and or try freeze spray. i had a similar situation on a silver champ. the problem turned out to be a loose chassis connection on the speaker output rca. cleaning and tightening this connection took care of a crackling white noise hiss that rode the signal on certain notes. cleaning and retightening the input and output connections where they ground to the chassis, is a last resort repair option that has worked for me several times in the past.

mike
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Old 23rd July 2012, 01:42 AM   #4
eyoung is offline eyoung  United States
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I use cheap bamboo skewers found at the local grocer (high dielectric properties) using the flat end to tap on various parts looking for a hard to find errant part.

Regards, El
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Old 23rd July 2012, 03:39 AM   #5
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Since it is tracking with Volume controls it is in the preamp section.
When you did the re-cap job did you also do the electrolytic cathode bypass caps on the preamp stages. If not, that is what I would attack next. These are often overlooked when guys do a re-cap job.

Cheers,
Ian
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Old 23rd July 2012, 04:18 PM   #6
kevinkr is offline kevinkr  United States
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This thread belongs in the Instruments And Amps forum so I will move it there. Please note the header at the top of the tube / valves forum which states:

Quote:
Tubes / Valves All about our sweet vacuum tubes Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum
There is a similar header in the Instruments And Amps forum relating to this topic.

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Instruments and Amps Everything that makes music, Especially including instrument amps.
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Old 23rd July 2012, 06:04 PM   #7
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Occasionally, Fender amps are affected by moisture absorbed by the eyelet board. Leave it outside (chassis out of the box) on a dry day for a couple of hours or use a hair dryer to carefully warm the board on the preamp end.
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Old 24th July 2012, 12:55 AM   #8
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Default Update!!!

Wow, thank you all so much for taking the time to share your knowledge. I had some time last night to apply a few of the suggestions (cleaning contacts, resoldering grounds, replacing some CC resistors) but unfortunately the hunt remains as the hiss is still present.

Below, I’ve added a few additional observations (in light of a few comments) as well as direct feedback/question to each post. I really appreciate all the help… I can't tell you enough!

Additional Observations:
- Hiss Noise: I thought I’d highlight that the hiss was not present just after I’d completed my electrolytic cap job (power filter, cathode bypass, power bias balance, rectifier circuit) and new tubes. It surfaced after a few weeks of playing. It was pretty darn quiet right after the cap/tube job.
- Eyelet Board Wax: Loudthud mentioned eyelet board moisture as a possible noise source. I thought I’d note that the eyelet board is in pretty rough shape. Make me wonder what the previous owner did with it. The wax/felt board appears to have gotten hot at some point because some wax can be seen on the caps and resistors making me think it got hot enough to flow (ouch). Check out the sprag atom cathode bypass picture below and note the white wax on the blue coupling cap. The wax flowed up so the amp was upside down when it happened. Weird, makes me a little concerned what was overheating it. Again, the amp was nice and quiet for a few weeks after my cap/tube job, then the darn hiss surfaced…
- Gentle Vibration: I may be just dreaming up things but I swear there is a very light vibration that can be felt from the chassis pan edge or when touching the tubes. This isn’t present in my Super Six. It is present even with just the “on” switch on, prior to actually flipping “Standby”. I’m probably just going crazy (chasing amp noises can do that to youJ) but it seems odd, especially in contrast to my other very similar amp that doesn’t vibrate at all.
- More Tests/Data: I'm happy to take picture, measure or test anything if that helps.
Sootboots: Thanks for your help! Last night, I replaced all the carbon comp 100k ohm 1W pre-amp plate resistors with 1W metal film resistors. While I was at it, I also replaced the carbon comp 470 ohm 1W power tube screen resistors (6l6gc pins 4-6) with 3W metal film resistors (pics below). No chance in the noise. Questions: Can you tell me more about your grounding recommendations... more descript and what the intent is? I'm not sure I'm picturing it right. Regarding the power amp "star", are you suggesting making a ground lead from the power tube grounds (pin 8, cathode) to one central point soldered to the chassis? Regarding the ground buss, are you suggesting soldering a ground lead from the input jack ground to the chassis (instead of just relying on the thread/nut)? Also, are you suggesting tying all the preamp circuit grounds to this new buss (instead of solder leads to the chassis)? What do you mean by isolating the RCAs... remove them from the circuit to test if they were the cause of the noise? Last, what do you mean by a "Center tap to ground"? The ~6Vac heater leads from the power transformer go to the jewel light and each rail has a 50ohm 1/2W CC resistor to ground. Is this what you're talking about? Sorry I'm such a newbie and thanks for your suggestions. I really appreciate the help! (I'm not sure if these pictures will post correctly?)
Click the image to open in full size.Click the image to open in full size.Click the image to open in full size.

Mgreene: I'd read something similar about the carbon comp resistors being a common source of noise. I'd done some visual inspection prior and then did more last night. None show sign of wear/burn. I've replaced all 1W CC resistors in the pre and power amp sections now, replacing them with metal film. I also fully cleaned (acetone) and firmly re-attached all inputs/RCAs/Output jacks. No changes yet... still hiss. Question: What do you mean by freeze spray? Thanks again for your help!

Eyoung: I'm sorry... I'm such a newbie, I'm not sure I follow your suggestion. Use the stick to move wires so I can visually inspect better? Use the stick to tap/touch parts and see if the hiss goes away (or increases)?

Gingertube: Thanks for the suggestion. I think I did a full cap job about a month ago... replacing electrolytics in the power filter section (bottom, under pan), the cathode bypass (I think I called those cathode bias by error) and the two electrolytics on the bias balance pot as well as the rectifier circuit. Question: I'm such an amateur at this... how does one know the noise source is in the pre-amp section if it tracks with the channel and master volume nobs? Thanks for your help... hopefully I can pass the favor on to another newbie someday

Kevinkr: Sorry for my error... and thanks for moving it to the correct location.

Loudthud: Thanks for your help! I've had the chassis out of the cabinet for a couple weeks now. I'll hit it with a hair drier tonight.

Looking forward to trying a few more things and figuring this out...
Thanks!
Kyle
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Old 24th July 2012, 07:35 AM   #9
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Kyle,
Here is my bit.
I interpreted what you said above to say that the hiss dissapears when you turn down the volume controls. If that is the case then the source of the hiss is prior to the volume controls in the circuit, that is, back in the preamp somewhere.

I would check the "grid stop" resistors feeding the preamp tube grids and the "grid leak" resistors which go from the ends of the grid stops resistors to 0V.

Good Luck with it.

Cheers,
Ian
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Old 24th July 2012, 10:35 AM   #10
Zoran is offline Zoran  Serbia
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First check condensers in power supply
(i would replace them ALL without checking with original parts if possible...)
.
then tale off pots - all of them
open each gently and clean with alcohol only
do not use any of sprays or quick methods...
.
Check negative biasing
eventually replace the tubes with new pieces.
.
Because it is combo unit it is expose to the vibrations
You could check soldering points, first visually
and optionally re-solder them
BUT with some good "vintage" solder
multicor for the example
do not use contemporary lead free or silver added...
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