Fender 'The Twin' Red Knob Version Mods and Repairs

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xenrelic
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Fender 'The Twin' Red Knob Version Mods and Repairs

Post by xenrelic »

Thought I'd share some excellent sounding mods for the Fender 'The Twin' Red Knob Combo. The amp stands up to high dollar boutique amps after the following mods.

-

Power Supply Section:

Bias supply filter
C303 = 100uF 100v F&T
C304 = 100uF 100v F&T
C305 = 100uF 100v F&T
C202 = 100uf 100v F&T

Series Mains filter
C302 = 220uF 300v F&T
C301 = 220uF 300v F&T

Series Power tube Screen Filter
C306 = 100uF 300v F&T
C307 = 100uF 300v F&T

Preamp node filters Nodes A-C
C110 = 30uF 500v F&T
C111 = 30uF 500v F&T
C112 = 30uF 500v F&T

R131 = 12k 3W Metal Film Preamp Node C Voltage Adjust

6.3VAC Heater Supply for Tube Filaments Hum Reduction Mod
Add DC heater elevator with 1M 1W CF attached from C306 B+ node to 100k 1W CF, with other end of 100k attached to ground. Add 47uF 100v electrolytic cap in parallel to the 100k resistor. Make sure 100k GND point is where R302 and R301 used to be grounded.
R302 = 100R 1/2W CF Lift ground side and attach to DC elevator circuit between 100k and 1M
R303 = 100R 1/2W CF Lift ground side and attach to DC elevator circuit between 100k and 1M

Install new isolated twisted heater wire for all tubes to further reduce AC hum/noise. I prefer to elevate this wiring above the sockets to isolate it. Use solid core or stranded silver plated teflon coated 18AWG up until last power tube and then switch to solid core or stranded silver plated teflon coated 20AWG. Make sure wires are tightly twisted with as many turns you can get without damaging the wire, because this will further reduce hum and vibration noise.

--

Output Section
R124 = 820R 1W CF
R119 = 4.7k 1W CF
R121 = 390R 1W CF
C106 = 22nF 600v CDE 6PS
C107 = 100nF 600v CDE 6PS
C108 = 100nF 600v CDE 716P
C109 = 100nF 600v CDE 716P

---

Clean Channel
C1 = 4.7uF 63v Vishay BC 138 AML
R7 = 3.3k 1W CF
R17 = 3.3k 1W CF
C8 = 4.7uF 63v Vishay BC 138 AML
C3 = 2nF 600v CDE 6PS
C2 = 250pF 1kv ceramic
R8 = Move side connected to R10 to GND
R13 = Install 47pF 1kv ceramic from input to wiper.
C4 = 100nF 600v CDE 6PS
C5 = 50nF 600v CDE 6PS
C6 = 68pF 1kv Ceramic
R5 = 180k 1W CF
R16 = 180k 1W CF
R14 = 110k 1W CF
R15 = 110k 1W CF
C7 = 22nF 600v CDE 6PS

----

Overdrive channel
C10 = 4.7uf 63v Vishay BC 138 AML
R24 = 1.8k 1W CF
C12 = 470pF 1kv ceramic
C11 = 470pF 1kv ceramic
R25 = 47k 1W CF
C14 = 22nF 600v CDE 6PS
R30 = Install 47pF 1kv ceramic from input to wiper.
C15 = 68pF 1kv ceramic
R23 = 220k 1W CF
C17 = 4.7uF 63v Vishay BC 138 AML
C22 = 4.7uf 63v Vishay BC 138 AML
C18 = 4.7nF 600v CDE 6PS
C24 = 22nF 600v CDE 6PS
R42 = Add 1000pF 1kv ceramic in parallel to this resistor to tighten drive channel dynamics and reduce noise.
C19 = 120pF 1kv ceramic
R41 = 2.7k 1W CF
C25 = 4.7uf 63v Vishay BC 138 AML
R45 = 1k 1W CF
C27 = 22nF 600v CDE 6PS

-----

Reverb
Recovery Stage
C105 = 22uF 63v Vishay BC 138 AML
Mixer Stage
C102 = 22uF 63v Vishay BC 138 AML

------

Channel Swtiching
IC101 = NTE834 Socketed

-------

Wire stock combo speakers for 4ohm & Switch Impedance switch to 4ohm setting. Sounds fuller than 16ohm wiring. If using extension cabinets, keep 16ohm wiring.

--------

TUBES

Pramp Tubes
V7 = Sino 7025 or JJ ECC83S (CH1 Stages 1&2)
V1 = Sino 7025 or JJ ECC83S (CH2 Stages 1&2)
V2 = Sino 12AX7B (CH2 Stages 3&4)
V8 = Sino 12AT7 or JJ ECC81 (Reverb)

V4 = N.O.S. 12AT7WC or JJ ECC81 (Do not use Sino 12AT7 for the Phase Inverter on this amp)
V3 = Sino 12AX7B (FX Loop)
V9 = Sino 12AX7B (Reverb Recovery & Mix)

Power Tubes
V5, V6, V10, V11 = Sino 6L6WGS (Black Plate RCA Style) Apex Matched Quad with Bias set to factory spec 40mVDC Between TP202 & TP203 0VDC Between TP201 & TP202 All controls to zero with no input signal.

---------

General Maintenance

Tighten & Clean Tube Socket pin holders.

Tighten, clean or replace bad jacks as needed. Use DeOxit D5 Contact Cleaner (These amps are notorious for switching related issues from bad jacks)

Clean pots with DeOxit Fader Lube. Be sparing with fluid or use D100 Fader Luber to maintain motion control grease.

Touch up any bad solder joints on PCBs. especially near the big voltage drop resistors where a lot of heat is dissipated.

If experiencing channel bleed, microphonic noise or excess reverb hum, swap preamp tubes with tested good tubes and it will clear these issues up.

Replace all low voltage signal wires to preamp tube input grids on pins 2 and 7 of each preamp tube (except the phase inverter) with Canare GS-4 shielded. This will remove microphonics and noise related issues with the crappy PVC coated signal wires picking up EMI and creating oscillations.

Replace the Phase Inverter input grid wires with Teflon Coated 22 awg to reduce noise from the poor quality PVC coated wire.

Remove the Orange and Yellow reverb pot wires, cut the traces where pot pins are and attach shielded wires to the pot instead. This will get rid of reverb noise and oscillation issues related to turning the reverb up.

Check the reverb tank and hard wire the wires to the reverb transducer pins with teflon coated stranded wire of the same gauge instead of the PVC coated stuff, this will further reduce reverb noise issues.

----------

Final Touches

Flexible vibration dampening and securing glue applied to all new filter caps and new film caps, for structural integrity against vibration. I like to use Goop or Shoe-Goo, because it holds up on the road very well at all temperatures. (keep in well ventilated area until glue is cured after 24hrs)

New Zip Ties applied to existing wire networks after heater wire replacement.

FIN
Last edited by xenrelic on Mon Apr 01, 2019 7:13 am, edited 4 times in total.
Mark
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by Mark »

Thanks for sharing your mods, do you have any clips of the amp after it was modded or a clip comparing the amp to a stock red knob Twin?
Yours Sincerely

Mark Abbott
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by Stratscratcher »

Thanks for posting the Mods, it saved me a ton of time and trial and error head aches. I'm working thru most of your recommendations at this time. I have all my heater wires replaced and twisted up neatly above the tubes..

I have a a question about your heater elevator mod. I'm a little confused you indicate:
"6.3VAC Heater Supply for Tube Filaments Hum Reduction Mod
Add DC heater elevator with 1M 1W CF attached from C305 B+ node to 100k 1W CF"

C305 is a 100uf/100v cap on the -75 not the B+ .... Is C305 a typo, and you meant R305?

Thanks in advance,
Andrew
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drew
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by drew »

^ He has two different caps listed as C305. First one is under "Bias supply filter." The second one, under "Series Power tube Screen Filter," should I think be C306 , and I think that's the one he means in his heater elevation discussion.

Schematic I'm looking at: http://el34world.com/Forum/index.php?to ... #msg226075
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by Stratscratcher »

Thanks Drew, that clears it up for me. Now I just have to figure the best way to place the extra resistors and cap on a the power board so it will be relatively neat and reliable.

Thanks again.
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xenrelic
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by xenrelic »

Yep, that was a typo. C305 for the series screen supply filter was supposed to be C306. It was difficult to read from the schematic I was using. I fixed the O.P. above to reflect that change.
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by Stevem »

Your using the 4 ohm OT tap and getting better sound is odd as all OTs will sound better and dam well last longer when driven hard when running on its highest impeadance tap since that full winding couples better with the primary winding.

If you are truly getting a tone improvement it's due to the change in voice coil inductance from the parallel wiring!
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by Roe »

Is R101 8.2M instead of the usual 3.3M?!
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by Stratscratcher »

R101 is 8.2 meg on both Red knobs I have, as it shows in the schematic. C117 may not be installed on your amp, my older amp had it,the newer one didn't.. I wound up adding it to the newer amp that didn't have one installed.
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by xenrelic »

Here's a different version of the mod that was applied to a similar Fender Red Knob Dual Showman Reverb Head. It's a bit less involved and for people that don't want to mod their clean channel, because they're pretty happy with it. I'd say this mod is more for people who like the stock clean channel tone for indie/surf stuff and are looking for a better Drive channel and overall response.

Drive Channel Mods:

C11 .002uF CDE 6PS Increases gain and shapes dynamics more toward lead and rock tones with more depth and warmth in the coloration

C12 Remove

R26 Remove and Install a 500pF SM 500v+ rated in its place and add 4.7M .5WMF resistor from C11/R26 junction to ground at the closest ground jumper. This will allow you to switch from a milder plexi kind of crunch to a high gain lead tone. For cleaner OD response go with a 390pF SM 500v+ or even smoother BF coloration go with a 330pF 2kv Ceramic.

C15 68pF SM Changes bright cap value to 115pF in Treble boost mode.

R30 Add 47pF to input and wiper of 2nd part of the pot furthest from the front panel. Left and middle lugs if looking from the front of the amp at the preamp circuit board. This keeps the bright cap on the Overdrive gain at 47pF in non Treble Boost mode, which I find to be the best value for the non Treble Boost mode.

C10 4.7uF Adjusts first stage gain and dynamic response. Reduces muddiness and tightens up the drive.

R34 1.8k 1WCF Adjusts the gain and dynamic response of the second gain stage slightly hotter. You can keep this at 2.7k for more of a plexi vibe. Cuts through the mix with more modern edge to it with 1.8k and 1.5k values. You can also add a 250k trim pot in series with a 1.8k resistor to dial R34's value in by ear with parallel resistance and the lowest the cathode resistor value can reach with the trim and 1.8k resistor is 820R when using a 1.5k value for R34.

C16 10uF Adjusts the gain and dynamic response of the second gain stage slightly hotter with more bass.

C17 10uF Adjusts the gain and dynamic response of the second gain stage in Bass Boost mode to match the stock Bass Boost value when the C16 mod is employed. Keep the stock 22uF value for even more Bass Boost at 32uF when engaged.

C18 .022uF SoZo Mustard Allows fatter bass response after 2nd gain stage. Though the stock .0068uF Mallory PVC cap is decent too, I just find it a bit nasally for my tastes.

-----------------------------------------------------------

Optional mods for the Drive Channel (not required):

C13 .1uF CDE 250P or 6PS enhances tone stack warmth response slightly, but more transparent with stock capacitor

C14 .047uF CDE 250P or 6PS enhances tone stack warmth response slightly, but more transparent with stock capacitor

C22 10uF Adjusts the gain and dynamic response of the third gain stage in Bass Boost mode with less bass and slightly less gain. This is optional if you want a slightly brighter bass boost mode. I like 22uF if I'm playing stuff that requires more boosted bass frequencies and slightly higher gain, though it sounds good stock with C17 as 10uF for a middle ground between both caps as 10uF and both caps as 22uF.

C24 .05uF CDE 6PS Adds warmth to the dynamic response after third gain stage. Makes the amp more vintage organic sounding. Only change this if you intend to do the clean channel mods as well. Without it the drive channel will match dynamics closer to the stock clean channel.

C23 .02uF CDE 6PS Adds warmth to the dynamic response after 4th gain stage. Makes the amp more vintage organic sounding. Only change this if you intend to do the clean channel mods as well. Without it the drive channel will match dynamics closer to the stock clean channel.

-----------------------------------------------------------

Clean Channel optional mods if you're so inclined:

C7 .022uF SoZo Mustard Adds more depth and warmth in the coloration after the second gain stage.

C101 .002uF CDE 6PS Adds more depth and warmth in the coloration after the second gain stage.

C201 .68uF CDE DMT Adds more depth and warmth in the coloration after the third gain stage, but not as drastically needed here, we chose a more transparent smooth warmth with the DMT series capacitor.

R13 Add 47pF SM to input and wiper lug of the volume pot. This keeps the bright cap on the clean Volume at 47pF in non Treble Boost mode, which I find to be the best value for the non Treble Boost mode. Though no bright cap is also really good sounding with C6 at stock value.

C6 68pF SM Makes Treble Boost 115pF which is a nice balance in Treble Boost mode when used with the 47pF mod above in non Treble Boost mode.

C3 330pF SM 500v+ Adds a slightly brighter and smoother push. 280pF Ceramic 2kv+ sounds really good too and is my personal favorite for more BF style tone. Alternatively you can go with the same tonestack mod here as the drive channel has and turn the mid cut into a mid boost instead by copying that C11/R26 circuit that the drive channel employs which will give you a really nice cut going on that will get you great clean and crunch tones.

C4 .1uF CDE 250P or 6PS enhances tone stack response warmth slightly, but more transparent with stock capacitor

C5 .047uF CDE 250P or 6PS enhances tone stack response warmth slightly, but more transparent with stock capacitor
-----------------------------------------------------------

Phase Inverter and Power Section Mods for increased sustain and dynamic response of the Power Section:

R121 390R 1WCF Adjust phase inverter presence

R119 5k 1WCF Adjusts negative feedback response hotter (you can move the NFB to the 8ohm tap for even more response. This is optional and sounds great too)

C108 and C109 .033uF CDE 6PS Adds more depth and warmth in the coloration after the PI. Slightly increased bass response overall for both channels. This can be modified to .05uF or even .1uF, though the higher in value you go, the more bass response you get, but it can start to be too much any higher than .1uF. I'd say stick around .033uF for Rock and Indie tones and .047uF to .1uF if playing fuller and cleaner Jazz stuff where extended bass frequencies are needed.

R67, R68, R62 and R63 3.3k 1WCF increases noise rejection at the swamp resistors when increasing gain in the preamp. You don't want too high a value here, but you can go up to 5.6k. I find 3.3k to be a nice improvement with good tone.

-----------------------------------------------------------

Optional Phase Inverter coupling cap mods for smoother depth and warmth overall in the coloration of the PI and output.

C106 .022uF CDE DMT or CDE 6PS for even more coloration

C107 .1uF CDE DMT or CDE 6PS for even more coloration

-----------------------------------------------------------

Voila! Enjoy your more interesting and complex tone!!!

If you'd like to enhance the amp a little more, update the electrolytic caps.

3x Epcos 22uF 500v axial Part number: B43698-S6226-Q1 (fits the best and has great specs)

2x Vishay BC 47uf 350v axial Part number: MAL204315479E3 (fits better than F&T or JJ and has great specs)

2x F&T 220uF 300v axial Part number: C-ET220-300-FT

4x Vishay BC 100uf 100v axial Part number: MAL211819101E3 (great long lasting cap for bias supply and switching circuit)

For the remaining 22uF 16v electrolytic capacitors use either Vishay BC 118AHT series or 138AML series with 25v to 40v values, 63V rated is okay too but a bit of a tight fit for the 4.7uF and 22uF 118AHT series caps. Vishay BC 030 AS series caps will be a good substitute if 138AML better or 118AHT best are out of stock.
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by Mark »

Has anyone tried these mods in a red knob Twin.

It would be good to get some feedback.

Bias supply filter
C303 = 100uF 100v F&T
C304 = 100uF 100v F&T
C305 = 100uF 100v F&T
C202 = 100uf 100v F&T


Series Mains filter
C302 = 220uF 300v F&T
C301 = 220uF 300v F&T


These are stock values.

Series Power tube Screen Filter
C306 = 100uF 300v F&T
C307 = 100uF 300v F&T


The stock value is 47uF, this may stop sag from the screens conducting hard.

Preamp node filters Nodes A-C
C110 = 30uF 500v F&T
C111 = 30uF 500v F&T
C112 = 30uF 500v F&T


Increased from 22uF to 30uF, given the tolerance of the filter cap, this may not be noticed at all.

Clean Channel
C1 = 4.7uF 63v Vishay BC 138 AML
R7 = 3.3k 1W CF
R17 = 3.3k 1W CF
C8 = 4.7uF 63v Vishay BC 138 AML
C3 = 2nF 600v CDE 6PS
C2 = 250pF 1kv ceramic
R8 = Move side connected to R10 to GND
R13 = Install 47pF 1kv ceramic from input to wiper.
C4 = 100nF 600v CDE 6PS
C5 = 50nF 600v CDE 6PS
C6 = 68pF 1kv Ceramic
R5 = 180k 1W CF
R16 = 180k 1W CF
R14 = 110k 1W CF
R15 = 110k 1W CF
C7 = 22nF 600v CDE 6PS


C1 and C8 is a cathode cap on a first and tone control recovery stage, it was originally 22uF instead of the modded value of 4.7uF. Reduces bass response and a Dumbleque value.

R7 and R17 are cathode resistor on the first and second stages. The original value is 1.5 K, they are replaced with 3.3K. Plate resistors R5 and R16 are changed to 180K, so this is a bit of a Dumble mod. I don’t know about this one.

C2 is the 250pF treble cap which is typically replaced with silver mica, but in this case ceramic @ 1KV. I’m not sure this would be any better than the stock cap.

C3 doesn’t seem to do much of anything, the circuits original intent was to reduce the treble cap to 100pF, in this case the 250pF is reduced to 222pF. I’d give this mod a miss.

R8 is a 10M resistor which removes clicks from the cap C3 charging. Unless you are encountering clicks I’d forget this mod.

R13 cap from top to wiper is the Deluxe Reverb mod to add a bit of brightness. Maybe, not a big deal really.

C4 and C5 replaces stock values in the bass and mid caps with Orange Drop 6PS caps. I don’t know the quality of the caps in the amp. It may make a difference.

C6 changes the bright cap from 120pF to 68pF. But when you consider the 47pF hard wired bright cap, the 68pF allows the whole 120pF when the brightness switch is pulled. R13 and R6 must be changed together,if you’re not after a bit more sparkle at low volume, forget about it.

R14 & R15 are there for the channel switching, increasing the two resistors would effect the signal being shunted to ground and is more or less an attempt at a Dumble mod but without the 500pF cap. I wouldn’t do this one either.

C7 stock value to a Orange Drop 6PS value. Maybe it makes a difference?

Overall, I don’t think I’d do any of these mods to the clean channel.

Overdrive channel
C10 = 4.7uf 63v Vishay BC 138 AML
R24 = 1.8k 1W CF
C12 = 470pF 1kv ceramic
C11 = 470pF 1kv ceramic
R25 = 47k 1W CF
C14 = 22nF 600v CDE 6PS
R30 = Install 47pF 1kv ceramic from input to wiper.

C15 = 68pF 1kv ceramic
R23 = 220k 1W CF
C17 = 4.7uF 63v Vishay BC 138 AML
C22 = 4.7uf 63v Vishay BC 138 AML
C18 = 4.7nF 600v CDE 6PS
C24 = 22nF 600v CDE 6PS
R42 = Add 1000pF 1kv ceramic in parallel to this resistor to tighten drive channel dynamics and reduce noise.
C19 = 120pF 1kv ceramic
R41 = 2.7k 1W CF
C25 = 4.7uf 63v Vishay BC 138 AML

R45 = 1k 1W CF
C27 = 22nF 600v CDE 6PS


C10 22uF to 4.7uF on the first stage of the overdrive channel is a Dumble like value and R24 from 1.5K to 1.8K is much the same again. The plate resistor is bumped up to 220K. A bit more gain with slight reduced bass.

C11 and C12 change the treble control from 250pF to 470pF, the mid boost is changed from 750pF to 470pF, the intent is changing the mid boost cap from 750pF to 470oF is to maintain the 1000pF treble cap when mid boost is pulled. The channel will sound fatter with a 470pF treble cap. You have to try this for yourself to see if you like it.

R25 changes the slope resistor from 100K to 47K, this will make the channel capable of more bass and less gain. I don’t think I’d want to do this one.

C14 changes the mid cap from 0.047uF to 0.022uF which is the Brownface Fender value.

R30 and C15 is the Deluxe bright cap and 120pF on the pull bright switch instead of 50pF. Try out for yourself?

C17, C22, C18 are about replacing the 22uF caps in the pull bass circuit for the 4.7uF Dumble value. This will reduce the bass boost, but it might stop the amp sounding muddy.

C24 changes a coupling cap from 0.047uF to 0.022uF, I’m not sure you’d hear the change in value.

R42 is a 100K plate resistor, putting a 1000pF cap across will attenuate highs and not tighten the sound of the amp up. Only if the amp is too bright, this is a tweaking mod and depends on personal taste.

C19 changed from 250pF to 125pF. This backs the highs boost down after the bright switch across the volume mod was changed from 50pF to 120pF.

R41 changes a cathode resistor from 1.5K to 2.7K. This is to get a Marshall like sound from the stage.

C25 changes 22uF to 4.7uF. A Dumble value which rolls off bass.

R45 changes cathode resistor from 3.3K to 1K. The gain is increased before the notch filter. I suppose it counters all the lowered cathode resistor which dropped the overall gain of the channel.

C27 changes the stock coupling cap to a Orange Drop 6PS type.

I certainly wouldn’t do this mod all in one hit. I would carefully experiment and listen to each change that is made. Definitely do before and after recordings to verify the change to timbre.
Yours Sincerely

Mark Abbott
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Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by Charmless »

xenrelic wrote: Mon Mar 20, 2017 8:22 am Thought I'd share some excellent sounding mods for the Fender 'The Twin' Red Knob Combo. The amp stands up to high dollar boutique amps after the following mods.

-

Power Supply Section:

Bias supply filter
C303 = 100uF 100v F&T
C304 = 100uF 100v F&T
C305 = 100uF 100v F&T
C202 = 100uf 100v F&T

Series Mains filter
C302 = 220uF 300v F&T
C301 = 220uF 300v F&T

Series Power tube Screen Filter
C306 = 100uF 300v F&T
C307 = 100uF 300v F&T

Preamp node filters Nodes A-C
C110 = 30uF 500v F&T
C111 = 30uF 500v F&T
C112 = 30uF 500v F&T

R131 = 12k 3W Metal Film Preamp Node C Voltage Adjust

6.3VAC Heater Supply for Tube Filaments Hum Reduction Mod
Add DC heater elevator with 1M 1W CF attached from C306 B+ node to 100k 1W CF, with other end of 100k attached to ground. Add 47uF 100v electrolytic cap in parallel to the 100k resistor. Make sure 100k GND point is where R302 and R301 used to be grounded.
R302 = 100R 1/2W CF Lift ground side and attach to DC elevator circuit between 100k and 1M
R303 = 100R 1/2W CF Lift ground side and attach to DC elevator circuit between 100k and 1M

Install new isolated twisted heater wire for all tubes to further reduce AC hum/noise. I prefer to elevate this wiring above the sockets to isolate it. Use solid core or stranded silver plated teflon coated 18AWG up until last power tube and then switch to solid core or stranded silver plated teflon coated 20AWG. Make sure wires are tightly twisted with as many turns you can get without damaging the wire, because this will further reduce hum and vibration noise.

--

Output Section
R124 = 820R 1W CF
R119 = 4.7k 1W CF
R121 = 390R 1W CF
C106 = 22nF 600v CDE 6PS
C107 = 100nF 600v CDE 6PS
C108 = 100nF 600v CDE 716P
C109 = 100nF 600v CDE 716P

---

Clean Channel
C1 = 4.7uF 63v Vishay BC 138 AML
R7 = 3.3k 1W CF
R17 = 3.3k 1W CF
C8 = 4.7uF 63v Vishay BC 138 AML
C3 = 2nF 600v CDE 6PS
C2 = 250pF 1kv ceramic
R8 = Move side connected to R10 to GND
R13 = Install 47pF 1kv ceramic from input to wiper.
C4 = 100nF 600v CDE 6PS
C5 = 50nF 600v CDE 6PS
C6 = 68pF 1kv Ceramic
R5 = 180k 1W CF
R16 = 180k 1W CF
R14 = 110k 1W CF
R15 = 110k 1W CF
C7 = 22nF 600v CDE 6PS

----

Overdrive channel
C10 = 4.7uf 63v Vishay BC 138 AML
R24 = 1.8k 1W CF
C12 = 470pF 1kv ceramic
C11 = 470pF 1kv ceramic
R25 = 47k 1W CF
C14 = 22nF 600v CDE 6PS
R30 = Install 47pF 1kv ceramic from input to wiper.
C15 = 68pF 1kv ceramic
R23 = 220k 1W CF
C17 = 4.7uF 63v Vishay BC 138 AML
C22 = 4.7uf 63v Vishay BC 138 AML
C18 = 4.7nF 600v CDE 6PS
C24 = 22nF 600v CDE 6PS
R42 = Add 1000pF 1kv ceramic in parallel to this resistor to tighten drive channel dynamics and reduce noise.
C19 = 120pF 1kv ceramic
R41 = 2.7k 1W CF
C25 = 4.7uf 63v Vishay BC 138 AML
R45 = 1k 1W CF
C27 = 22nF 600v CDE 6PS

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Reverb
Recovery Stage
C105 = 22uF 63v Vishay BC 138 AML
Mixer Stage
C102 = 22uF 63v Vishay BC 138 AML

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Channel Swtiching
IC101 = NTE834 Socketed

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Wire stock combo speakers for 4ohm & Switch Impedance switch to 4ohm setting. Sounds fuller than 16ohm wiring. If using extension cabinets, keep 16ohm wiring.

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TUBES

Pramp Tubes
V7 = Sino 7025 or JJ ECC83S (CH1 Stages 1&2)
V1 = Sino 7025 or JJ ECC83S (CH2 Stages 1&2)
V2 = Sino 12AX7B (CH2 Stages 3&4)
V8 = Sino 12AT7 or JJ ECC81 (Reverb)

V4 = N.O.S. 12AT7WC or JJ ECC81 (Do not use Sino 12AT7 for the Phase Inverter on this amp)
V3 = Sino 12AX7B (FX Loop)
V9 = Sino 12AX7B (Reverb Recovery & Mix)

Power Tubes
V5, V6, V10, V11 = Sino 6L6WGS (Black Plate RCA Style) Apex Matched Quad with Bias set to factory spec 40mVDC Between TP202 & TP203 0VDC Between TP201 & TP202 All controls to zero with no input signal.

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General Maintenance

Tighten & Clean Tube Socket pin holders.

Tighten, clean or replace bad jacks as needed. Use DeOxit D5 Contact Cleaner (These amps are notorious for switching related issues from bad jacks)

Clean pots with DeOxit Fader Lube. Be sparing with fluid or use D100 Fader Luber to maintain motion control grease.

Touch up any bad solder joints on PCBs. especially near the big voltage drop resistors where a lot of heat is dissipated.

If experiencing channel bleed, microphonic noise or excess reverb hum, swap preamp tubes with tested good tubes and it will clear these issues up.

Replace all low voltage signal wires to preamp tube input grids on pins 2 and 7 of each preamp tube (except the phase inverter) with Canare GS-4 shielded. This will remove microphonics and noise related issues with the crappy PVC coated signal wires picking up EMI and creating oscillations.

Replace the Phase Inverter input grid wires with Teflon Coated 22 awg to reduce noise from the poor quality PVC coated wire.

Remove the Orange and Yellow reverb pot wires, cut the traces where pot pins are and attach shielded wires to the pot instead. This will get rid of reverb noise and oscillation issues related to turning the reverb up.

Check the reverb tank and hard wire the wires to the reverb transducer pins with teflon coated stranded wire of the same gauge instead of the PVC coated stuff, this will further reduce reverb noise issues.

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Final Touches

Flexible vibration dampening and securing glue applied to all new filter caps and new film caps, for structural integrity against vibration. I like to use Goop or Shoe-Goo, because it holds up on the road very well at all temperatures. (keep in well ventilated area until glue is cured after 24hrs)

New Zip Ties applied to existing wire networks after heater wire replacement.

FIN
These sound super cool! How would you describe the amplifier’s tone after the modifications?
miniqtone
Posts: 2
Joined: Tue Nov 30, 2021 4:05 pm

Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by miniqtone »

With apologies for resurrecting an ancient thread.....!

Has anyone at this outstanding forum performed these mods and if so...can anyone share the results?
I know that describing tone via the written word is akin to "dancing about architecture" (!) but Ive recently obtained a red knob The Twin for next to nothing in great condition, and so I am curious about these mods...and any others that someone may be able to share.

Many thanks in advance!
Best wishes,
Terry McInturff
jabguit
Posts: 154
Joined: Tue Aug 18, 2020 11:53 am
Location: NC
Contact:

Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by jabguit »

miniqtone wrote: Mon Dec 06, 2021 6:44 pm With apologies for resurrecting an ancient thread.....!

Has anyone at this outstanding forum performed these mods and if so...can anyone share the results?
I know that describing tone via the written word is akin to "dancing about architecture" (!) but Ive recently obtained a red knob The Twin for next to nothing in great condition, and so I am curious about these mods...and any others that someone may be able to share.

Many thanks in advance!
Best wishes,
Terry McInturff
Howdy, Amigo!

cheers,
Jack Briggs
Briggs Guitars
miniqtone
Posts: 2
Joined: Tue Nov 30, 2021 4:05 pm

Re: Fender 'The Twin' Red Knob Version Mods and Repairs

Post by miniqtone »

Yay! Hello Jack!
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