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My essential Classic 30 mods

12/19/2003 6:34 AM
Bob G	My essential Classic 30 mods I've spent the last 6 months tweaking my Classic 30 and trying out different mods (thanks to the information in "The Blue Guitar," this site and the Aiken Amp page. Here's my list in order of ease and benefit: 1. tube change - JJ Tesla EL84's, a 5751 for V2 and a 12AT7 for V3 (easy). 2. speaker change - to a Celestion V30 (from Carvin) easy and nice change. This very efficient speaker makes the amp loud. 3. Negative Feedback switch - even though I added a three position (center off) switch for three levels of feedback only two are really necessary, stock and off. The off position increases volume and distortion, great for blues, while the stock position is good for tighter Marshall-like character. 4. Fender-like input circuit - I added a DPDT switch to A/B between the two Fender type high and low impedance styles of input. I originally wanted to A/B with the Fender High impedance input and the stock circuit but I didn't see any real benefit in the stock circuit so I simply wired in a low impedance circuit that cuts the input in half to use with a high output pedal board into the input. 5. Volume box - while this isn't really a mod to the amp (the only reason it's not #1), I found the benefits to be enormous. It allows the preamp to be cranked and the power amp to be quite enough to not wake the baby (or wife). I modded a Whirlwind IMP-2 to pass the high impedance IN/OUT through a 1M pot (this goes into the C30's FX loop). I use the IMP-2's balanced out to go to a mixing board. Along with the volume control this box "REMOVES ALL NOISE" from the amp surprisingly well. The silence is amazing and I can only guess it's because of the filter created with the pot. 6. Bias adjust - very important for saving your tubes but the tone benefit is hard to pin down because I just set it to protect the tubes from max dissipation. 7. Standby switch - I added an ON/OFF/ON switch wired for standby just like the Classic 50. This is another means to protect and extend the life of tubes but not a tone benefit. 8. Cap and resistor mods to the tone stack, OD and Reverb circuit. I did increase C4 from 470p to 940p (but only by paralleling 2 caps and decreased the slope resistor in the tone stack from 68k to ~40k (by the same method) and found these to be good but subtle changes. I found that removing the 2 Meg resistor at the grid of V2 to be very beneficial in increasing the control range of the PRE pot. This change allows you to go from a very subtle clean-like break up at the low positions to very high gain. Also, changing the gain resistor of the Rev circuit (R29?) from 47k to 249k increased the reverb dramatically to get a Fender-like, Dick Dale reverb going. 9. Cathode/Fixed bias switch - while this is cool in principle the tone benefits have been very subtle to my ears. I managed to incorporate this using just one DPDT center off switch. 10. Half Power switch - I wanted to drive the output tubes into distortion at lower volumes and this was my means. I initially used a single DPDT switch to disconnect two tubes in both Fixed and Cathode bias modes but then became concerned with running all four in cathode biased mode with the current I was measuring (easily exceeding the normal design of the amp. I ended up using a single DPDT to control the Fixed bias mode for full and half power and the cathode bias mode for either V4/V7 or V5/V6 (it took some time to figure out the circuit using just the two switches but I'm really happy with its simplicity). 11. Changed the Boost circuit to control a relay that increases the distortion of the OD channel only. This was a major mod to the amp but well worth it as I now have three switchable "channels" in the amp. The mod required changes to the PRE pot circuit as well as the POST pot circuit (incorporating a mid-cut switch and/or high gain boost) along with the addition of a relay circuit board using two sets of relay contacts to switch in the higher gain at the PRE pot, feed a mid cut circuit (tying the mid cap to gnd), and changing the level at the POST pot. 12. Adding a lighted footswitch to go with the new "channel" configurations. I modded a Vox dual switch pedal to include an LED for each switch that has both LED's OFF for Clean, LED 1 ON for OD and LED 1 and 2 on for BOOST/Higher Gain. This was another clever circuit for me that really complements the channel control. Also, the Vox pedal works with the stock amp and is very sturdy. However, the cable is short and fixed to the box so I removed it and added a stereo jack so that I could use a longer cable. 13. Star grounding the amp - while there may be some reduction in noise I still cannot remove the Reverb hum (at least not without the volume box connected). This was a major change to the amp with little benefit, the amp was relatively quiet before and with the volume box eliminating all noise it became superfluous. I must admit I was determined to quiet the amp without the use of the volume box but was unable to get it to 100%. Bob G.

12/19/2003 6:34 AM

Bob G

My essential Classic 30 mods

I've spent the last 6 months tweaking my Classic 30 and trying out different mods (thanks to the information in "The Blue Guitar," this site and the Aiken Amp page. Here's my list in order of ease and benefit:

1. tube change - JJ Tesla EL84's, a 5751 for V2 and a 12AT7 for V3 (easy).
2. speaker change - to a Celestion V30 (from Carvin) easy and nice change. This very efficient speaker makes the amp loud.
3. Negative Feedback switch - even though I added a three position (center off) switch for three levels of feedback only two are really necessary, stock and off. The off position increases volume and distortion, great for blues, while the stock position is good for tighter Marshall-like character.
4. Fender-like input circuit - I added a DPDT switch to A/B between the two Fender type high and low impedance styles of input. I originally wanted to A/B with the Fender High impedance input and the stock circuit but I didn't see any real benefit in the stock circuit so I simply wired in a low impedance circuit that cuts the input in half to use with a high output pedal board into the input.
5. Volume box - while this isn't really a mod to the amp (the only reason it's not #1), I found the benefits to be enormous. It allows the preamp to be cranked and the power amp to be quite enough to not wake the baby (or wife). I modded a Whirlwind IMP-2 to pass the high impedance IN/OUT through a 1M pot (this goes into the C30's FX loop). I use the IMP-2's balanced out to go to a mixing board. Along with the volume control this box "REMOVES ALL NOISE" from the amp surprisingly well. The silence is amazing and I can only guess it's because of the filter created with the pot.
6. Bias adjust - very important for saving your tubes but the tone benefit is hard to pin down because I just set it to protect the tubes from max dissipation.
7. Standby switch - I added an ON/OFF/ON switch wired for standby just like the Classic 50. This is another means to protect and extend the life of tubes but not a tone benefit.
8. Cap and resistor mods to the tone stack, OD and Reverb circuit. I did increase C4 from 470p to 940p (but only by paralleling 2 caps and decreased the slope resistor in the tone stack from 68k to ~40k (by the same method) and found these to be good but subtle changes. I found that removing the 2 Meg resistor at the grid of V2 to be very beneficial in increasing the control range of the PRE pot. This change allows you to go from a very subtle clean-like break up at the low positions to very high gain. Also, changing the gain resistor of the Rev circuit (R29?) from 47k to 249k increased the reverb dramatically to get a Fender-like, Dick Dale reverb going.
9. Cathode/Fixed bias switch - while this is cool in principle the tone benefits have been very subtle to my ears. I managed to incorporate this using just one DPDT center off switch.
10. Half Power switch - I wanted to drive the output tubes into distortion at lower volumes and this was my means. I initially used a single DPDT switch to disconnect two tubes in both Fixed and Cathode bias modes but then became concerned with running all four in cathode biased mode with the current I was measuring (easily exceeding the normal design of the amp. I ended up using a single DPDT to control the Fixed bias mode for full and half power and the cathode bias mode for either V4/V7 or V5/V6 (it took some time to figure out the circuit using just the two switches but I'm really happy with its simplicity).
11. Changed the Boost circuit to control a relay that increases the distortion of the OD channel only. This was a major mod to the amp but well worth it as I now have three switchable "channels" in the amp. The mod required changes to the PRE pot circuit as well as the POST pot circuit (incorporating a mid-cut switch and/or high gain boost) along with the addition of a relay circuit board using two sets of relay contacts to switch in the higher gain at the PRE pot, feed a mid cut circuit (tying the mid cap to gnd), and changing the level at the POST pot.
12. Adding a lighted footswitch to go with the new "channel" configurations. I modded a Vox dual switch pedal to include an LED for each switch that has both LED's OFF for Clean, LED 1 ON for OD and LED 1 and 2 on for BOOST/Higher Gain. This was another clever circuit for me that really complements the channel control. Also, the Vox pedal works with the stock amp and is very sturdy. However, the cable is short and fixed to the box so I removed it and added a stereo jack so that I could use a longer cable.
13. Star grounding the amp - while there may be some reduction in noise I still cannot remove the Reverb hum (at least not without the volume box connected). This was a major change to the amp with little benefit, the amp was relatively quiet before and with the volume box eliminating all noise it became superfluous. I must admit I was determined to quiet the amp without the use of the volume box but was unable to get it to 100%.

Bob G.

Replies:

Sacha Hey Bob! Nice tweaks! How do you li... -- 12/19/2003 2:28 PM
dale I found... -- 12/19/2003 3:21 PM