Dr. Tube is also specialized in the modification of guitar amps and tube amps. By modification we mean altering and improving the device or its circuit(s) so that the sound/tone is better. “Better” is ofcourse a very subjective matter so we always want to discuss with the customer what the problems and/or wishes are.

Common guitar amp modifications at Dr.Tube are:

Bias mod

When the power tubes are replaced, almost always the bias needs readjusting. Biassing is very important to obtain the right tone and for a reasonable lifetime expectancy for the power tubes. There is no bias pot in (almost) all Mesa Boogie amps, most Peavey amps and many older Fender amps, whilst these amps really do need a bias pot. A notable exception to the bias pot requirement are cathode biasssed amps like the Vox AC15s and AC30s, the Fender Champ, Fender tweed Deluxe, etc. These amps do not need rebiassing when replacing the power tube(s), allthough more often than not the cathode bias setting is very hot in these amps reducing the lifetime expectancy of the power tube(s). We often increase the cathode resistor(s) to reduce the bias somewhat to increase tube life.

The bias mod is installing a bias pot, so that the bias can be adjusted and set correctly, and often installing one or more bias measurement resistors to actually measure the bias current.

JCM2000 Stable Bias Mod

The early Marshall JCM2000 amps often are troubled by a not stable bias current (drifting bias). On the JCM2000 Stable Bias Mod page it is explained what the root cause is of this problem and, more importantly, what can be done to remedy this problem.

Master Volume mod

The older “classic” guitar amps (e.g. the Marshall 1987/19569 lead amps) will only “crunch” or “overdrive” when the volume is set to 10. In practice this isn’t always desirable. An added Master Volume can help keeping the volume down. This way the preamp can distort without having the power amp running at huricane levels.

Mercury Magnetics mod

See the Mercury Magnetics page for more information on modifying guitar amps with these excellent transformers and chokes.

2 Channel mod

The older “classic” guitar amps aren’t exactly “hotrods”. Some models (e.g. Marshall 1987, 1959, 2203 & 2204) are suited for the popular Dr.Tube 2 Channel Mod. This mod will turn your crunchy amp into a hotrodded 2 channel rock amp.

Mesa Boogie power transformer mod

There are quite a few guitar players, outside of the USofA, that buy a (domestic 115V) Mesa Boogie amp on eBay/Reverb in the USofA because this much cheaper. The problem is that these domestic amps cannot be rewired for 230V use, like their export brothers. Now Dr. Tube can supply and fit suitable 230V power transformers for most Mesa Boogie amps. Do you have such a domestic 115V Mesa Boogie amp and you want to have this amp converted to 230V, then tell us the exact amp model and the number(s) on the power transformer and we’ll check if your amp can be converted to 230V operation.