Guitar Amp Repair - Mods - Jensen Speakers




Consignment Items (Amps and More)

  Andrews Amps


Guitar Amp Modifications and Tweaking


Fender Tube Amp Modifications

Also see the section below about general mods for most amps.

Modifications for Princeton Reverb (blackface and silverface)

  • Increase intensity of tremolo effect - **$15

Blues Jr. Modifications

  • Correct the bias circuit so that the output tubes no longer overheat.  Overheating tubes can cause damage to the tube sockets, circuit board and premature failure of the tubes.  This is normally done when output tubes are replaced. $25 for fixed resistor.  An adjustable resistor can be installed as part of the BillM mod (see below) or individually for about $80.

  • Correct factory problem with wiring that causes ultrasonic oscillation, bad tone and premature tube failure.  (This is not a problem on every Blues Jr.  We can scope the output to see if your amps is in need of this mod.)  **$20 

  • Replace both plastic output tube sockets with more durable ceramic type.  **$50

  • Install and 2 position power switch so you can have a standby function without modification to the chassis or board.  **$30 

  •  On the green board versions (older production), Fender installed a linear taper bass pot and an audio taper treble pot.  This caused the bass control to be too touchy in the low numbers and the treble control to not do much until it reached the high numbers.  We can update both pots to cream board (later production) specs.  **$25  A good time to do this is during the BillM mod (below)

  • We can also install your Bill M mods.  Either you can purchase the kits from or we can supply the parts for the "Basic Mod" and several others from our inventory.  Please note that the bias and oscillation mods above are inclueded in he BillM basic mods.  Some mods such as the "clean boost module" and transformer upgrades must be purchased from Bill. 
    Cream board amps - $110.  Green board amps - $130

Modifications for Blues Deluxe, Blues DeVille, Hot Rod Deluxe and Hot Rod Deville

  1. We've developed our own modifications for the Hot Rod amps that will improve the tone and controllability of the amp.   Here is what you can expect...
    1)  The clean channel will have a somewhat more classic-type tone with a little less gain. The bass will be slightly reduced and bass frequencies will be a bit tighter overall.  The clean channel volume control will not be so touchy in the low numbers so you'll have a bit more adjustability at lower volume.
    2)  When playing the overdrive channel, the Master Volume control will be more evenly adjustable instead of being so touchy in the low numbers.
    3)  The shrill treble tone of the overdrive channel will be slightly reduced for a smoother sound.
    4)  Ghost notes that are normally heard when playing single-note leads at high volume will be greatly reduced and the low strings will sound a bit tighter (less flabby).

Cost - $135 for Hot Rod Deluxe and $145 for Hot Rod Deville.
***  NOTE - a substantial portion of the price of these mods is the disassembly of the amp, removal and re-installation of the main PC board.  If your board is being removed for other work, the price of these mods will be lowered by about $65.

  • Triode/Pentode Switch (1/2 power switch)- This allows to you reduce your output power level to about 1/2 of normal.  This usually means a reduction of around 20% in output volume. There is also a slight reduction in treble.  $85 

  • Install metal Switchcraft input jacks.  These are much more rugged than the plastic ones installed in the American-made Deluxes and Devilles   Cost is $95.  If we already have the board out for other repairs or mods, we can upgrade the jacks for $35.  The Mexican versions (Reissues) of these amps have upgraded plastic jacks that don't typically break.

  • These have have large power resistors mounted on the PC board that tend to overhead and cause damage to the solder connections and copper traces on the back of the board.  This is a very common problem on these amps and causes problems with channel switching and other issues.  We can remove these resistors and replace them with super high quality chassis-mounted resistors.  This will remove the head from the board and eliminate the possibility of ever having problems in this area.  About $50

  • There are also several Fender factory reliability mods for the early versions of these amps (especially Blues Deluxe).  We can do these as well.  We'll have to look at your amp to see if these have already been done. 

  • The 4 X 10 version of the Deville had under-rated speakers from the factory and we've seen many bad ones.  For a bright crisp vintage style tone at a good price, consider having us install a set of WGS G10C or G10C/S speakers.  For a deeper, warmer, and louder tone, try the Eminence Ragin Cajun speakers.  For a similar effect but with a bit more midrange, try the Eminence Ramrod.  They're all rated at 75 watts each and carry at least a 5 year warranty so you won't have to worry about trashing them no matter how loud you play.  Other 10" speakers are also available.  Mixing two or more speaker models is another option.

Modifications for Deluxe Reverb

  • Remove the treble boosting capacitor on the reverb channel for a warmer tone.  This is a popular mod on the re-issues but rarely needed on the vintage ones.  **$10 

  • Install a solid state rectifier and 6L6 output tubes.  This will give you a bit more clean headroom and volume.  We have to make sure your power supply capacitors are in good shape before doing this mod.  In fact, I recommend adding changing some of the filter caps to higher values for this mod.  Price varies depending on the tubes you choose.

Modifications for most silverface models

  • Convert circuit to Blackface specification.  Click here for more info. Fender Blackfacing
  • Convert to a better bias adjustment circuit for improved tone and longer power tube life for your silverface Fender amp?  This is included in the "blackfacing" conversion or can be done to many other amps as a separate mod.

  • Modify to get rid of that "ticking" sound in your vintage Fender vibrato circuit?  **$15

Modifications for most models with reverb and tremolo

  • Wire for reverb and tremolo on both channels - **$30

Modifications for most models without reverb

  • We can add spring reverb to your Fender Deluxe, Bandmaster, Concert or Showman.  You'll have to give up the vibrato function and the "Normal" channel that you probably never use anyway.  The new reverb circuit will feature dwell, bass and treble controls.  Cost is around $245 to $295 depending on whether you want it to be footswitchable or not.  Note: there is not enough room in the head cabinets to add the tank without excess humming so if you want this added to a head model you'll have to find a bigger cabinet or mount the tank outboard of the head cabinet.  There is no problem adding the tank in combo models.  Not available for Champs, Broncos and Princetons.

Modifications for Custom Viobrolux Reverb

  • These amps don't have a negative feedback circuit.  That means they have excessive background hiss and limited clean headroom.  We can add a standard Fender feedback loop for about $80. 


Modifications for all Blackface amps

  • Install a grounded AC cord and correct the out of date AC wiring so that the amp will be safer and much less likely to cause nasty electrical shocks.  **$25


Marshall Tube Amp Modifications

  • Do you find the Crunch and Lead channels in your Marshall JCM2000 to be too bright and piercing?  We can tame the treble without affecting the clean channel.  Cost is about $80.

  • Treble taming mods are available for many other Marshall models as well including JVM models. 

  • Convert from 6550 or 5881 to EL34 output tubes.  (or visa versa). 

  • Mercury Magnetics Transformer upgrade and/or the addition of a choke.  This mainly applies to newer models.  The addition of the choke and upgraded output transformer will provide slightly stronger bass response and improved note defination when the amp is played at high volume.  Little improvement will be noticed at lower volume settings.

  • Also see below.

Jet City Amp Modifications

JCA-22H - We offer a substantial upgrade for this model.  It includes an additional "Crunch" channel, voiced a bit different from the stock "Crunch" channel. The new channel will be slightly more scooped in the midrange and will have it's own gain and master volume pot.  We include a new footswitch for accessing the new channel. The footswitch has 2 buttons. One button switches between the lead channel and the crunch channels. The second button switches between the original crunch channel and the new crunch channel. Also, we make the lead channel slightly fatter sounding. Finally, we add a depth control on the rear panel so that you may dial in a more beefy bottom end for the amp overall.  The cost is $495 plus shipping.

JCA-50H and JCA-22H - SLO mod, Clean Scoop mod, Clean Tone mod, Depth Control mods are available.

Ampeg SVT VR Standby Power Modification

The Ampeg SVT-VR amp has a protection circuit that will not allow the amp to go from standby mode to play mode unless the AC line voltage is above 110 - 115 volts.  There is a modification floating around the internet that defeats the standby relay circuit to resolve this problem.  Although the mod works, it disables the turn-on delay circuit and protection against loss of the low voltage power supply.  Our mod leaves the protection and delay circuit intact while lowering the low AC limit to 95 volts, thus allowing you to still use the amp in venues where the power is a little low and still maintaining the other useful parts of the protection circuit.  (Using the amp with power below 95 volts is not a great idea anyway!)  The price of the mod is about $100.


General Modifications for Most Tube Amps

  • Would you like to have previous modifications removed to put your amp back in stock condition? 

  • We can convert many solid state rectified amps to tube rectified or tube rectified to solid state rectification.  This changes the dynamics and headroom of the amp. 

  • Triode/Pentode Switch (1/2 power switch)- This allows to you reduce your output power level to about 1/2 of normal.  This usually means a reduction of around 20% in output volume and a slight loss in treble response.  Price ranges from $75-$150 depending on amp model but is $85 for most amps. 

  • Post phase inverter master volume control - PPIMV - This allows variable output volume while keeping your preamp gain turned up for a "cranked up" sound at lower volume.  Price varies depending on amp model.  $85 for most amps. 

  • High quality solid state effects loop.  This loop features a bypass switch and a level switch so will will work with nearly any outboard effect.  It's small and will fit most amps.  In my opinion, this is a better option than the tube driven loop because it is smaller, more transparent, more affordable, has true bypass, has level switching and is less noisy than tube driven loops.  Keep in mind that virtually every effect that might be used in your loop will be a solid state device so using a tube-driven loop does not preserve the pure tube signal of your signal anyway.  This loop is super transparent and quiet.  Highly recommended!  The price is about $200 for most amps.  We do offer tube loops as well.  See below. 

  • High-quality tube-driven effects loop can be added to many tube amps.  We do this installation correctly, with shielded cable for all the signal wiring!  Price to install in any amp that has room for the circuit board, tube and jacks - around $275.  Cost will be higher for special installs requiring an external box (for the circuit, jacks or for the extra tube).  

  • We now have the once-hard-to-find two-stage power switches back in stock.  These can be used to add the power standby functions to amps that have only one toggle switch for power such as Fender Blues Jr., Princeton Reverb, Many tweed amps and others.  Position one is standby and position two is fully on.  The price of this mod is $25 to $45 depending on the amp.  Minimum labor charge applies

  • Add adjustable bias.  Some amps with fixed bias circuits don't have a bias adjustment control.  We can often add adjustable bias with or without external test  points. 

These and other modifications may be available depending on amp make and model.



** $65 Minimum labor charge applies