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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.

12/19/2003 2:28 PM
Sacha	Hey Bob! Nice tweaks! How do you like the V2-5751 and V3-12AT7... interesting but I'm afraid it might lower the gain too much for my tastes. By "NF switch in off position", you mean completely bypass the NF resistor ? I should try it on my C50... I also saw you modified the REV circuit... I don't know how the reverb sounds in a C30, but in the C50, is quite "boingy", and really lacks some depth. Do you think your mod would help for this ? Thanks in advance! Sacha

12/19/2003 2:28 PM

Sacha

Hey Bob! Nice tweaks! How do you like the V2-5751 and V3-12AT7... interesting but I'm afraid it might lower the gain too much for my tastes.

By "NF switch in off position", you mean completely bypass the NF resistor ? I should try it on my C50...

I also saw you modified the REV circuit... I don't know how the reverb sounds in a C30, but in the C50, is quite "boingy", and really lacks some depth. Do you think your mod would help for this ?

Thanks in advance!
Sacha

12/19/2003 2:41 PM
Sacha	Also, I also found the PRE pot to be effective only past 6... unusable below that setting. I can only see a 2.2 Meg resistor (R9) on the C30 schematic. Is it the one you removed ? Click here for schematic : http://www.blueguitar.org/new/schem/peavey/c30schem.gif So in the C50, I guess it is the R67 1 Meg resistor ? See link : http://www.blueguitar.org/new/schem/peavey/c50schem.pdf Thanks again! Sacha

12/19/2003 2:41 PM

Sacha

Also, I also found the PRE pot to be effective only past 6... unusable below that setting.

I can only see a 2.2 Meg resistor (R9) on the C30 schematic. Is it the one you removed ? Click here for schematic :

http://www.blueguitar.org/new/schem/peavey/c30schem.gif

So in the C50, I guess it is the R67 1 Meg resistor ? See link :

http://www.blueguitar.org/new/schem/peavey/c50schem.pdf

Thanks again!
Sacha

12/19/2003 3:21 PM
dale	I found... the cathode bias mod to really liven up the amp, and was the mod that seemed to make the biggest differnce to my ear. I was suprised to hear it did so little for yours. Where did you get the mod from, or did you just use your own ideas on how to do it? I've done some of the things you have, but not all, so i may try a few.

12/19/2003 5:53 PM
Sacha	Cathode Bias? Where can find some info on this mod ? I was going for the adjustable bias... but never though of the cathode bias...

12/19/2003 10:41 PM
Dale	It's at blueguitar.org It may be a bit of an ordeal finding it, but look at the ftp link and go thru the folders. There is one by steve and one "update" of it by i think enzo who recommends a 100 ohm resistor instead of the 60 ohm steve recommended. I found the 100 to be more correct.

12/20/2003 4:04 AM
Bob G	The 5751 at V2 lowers the gain but it was an immediate warmer tone than the stock 12AX7. Please note that only a 12AX7 seems to work at V1. I tried the 5751 in this location which sounds great for clean but then creates problems for the OD circuit (not enough gain or something). By neg feedback sw in OFF position I mean it opens the circuit - removing the neg feedback completely. Replacing the resistor mentioned in the Rev circuit probably contributed some to the noise of the Rev because now the gain is 100 instead of ~20 (my notes are not handy right now). DON'T change the other 2k resistor to 1k like I initially tried (since it is easy to just tack on another in parallel) because this seems to really create a lot of noise. I think I would say there is more depth to the reverb now, not just volume, it is very dramatic. I think you're right about the 2.2M value at the grid of V2A, in any case removing this gets your 1M PRE pot back to 1M and not 680k. And don't put any resistor across C4 either or you just end up tying the OD PRE pot across the clean channel volume resistance (again reducing the PRE pot resistance). Most of the schematics I reviewed just use a 1M pot to the grid of the OD tube and I found this to be exactly what the doctor ordered. Now the ratio of the signal is perfect across a good percentage of the PRE control and not just the upper half. I was/am a bit dissappointed with the cathode bias difference. Maybe I expected too much? I really don't hear the compression that I had read about. I can A/B the circuits pretty easily. I'm using a 130 Ohm resistor (for just TWO tubes mind you) - similar to an AC15 or a DC30 in half power mode (which have comparable plate voltages). Also, I've switched from a 100uF cap to a 22uF to increase the f3db point closer to 60 Hz and reduce the boominess. The biggest contributor to this mod is in removing the neg feedback. When you A/B the stock fixed bias amp (with neg feedback) and the cathode bias without neg feedback the difference is certainly more noticeable. The 100 Ohm with four tubes will certainly get the plate current closer to the stock 8W dissipation. The 130 Ohm with two tubes puts it around 11W. It is for this reason that I only run the cathode bias mode with two tubes instead of all four (I was concerned that the OPT would not be able to handle the increased power with a 60 ohm resistor for the cathode bias. To be honest, you really cannot discern a noticeable difference in the amp running at half power - this should be about a 3dB change wich is barely perceptible volume wise. Bob G.

12/20/2003 4:04 AM

Bob G

The 5751 at V2 lowers the gain but it was an immediate warmer tone than the stock 12AX7. Please note that only a 12AX7 seems to work at V1. I tried the 5751 in this location which sounds great for clean but then creates problems for the OD circuit (not enough gain or something).

By neg feedback sw in OFF position I mean it opens the circuit - removing the neg feedback completely.

Replacing the resistor mentioned in the Rev circuit probably contributed some to the noise of the Rev because now the gain is 100 instead of ~20 (my notes are not handy right now). DON'T change the other 2k resistor to 1k like I initially tried (since it is easy to just tack on another in parallel) because this seems to really create a lot of noise. I think I would say there is more depth to the reverb now, not just volume, it is very dramatic.

I think you're right about the 2.2M value at the grid of V2A, in any case removing this gets your 1M PRE pot back to 1M and not 680k. And don't put any resistor across C4 either or you just end up tying the OD PRE pot across the clean channel volume resistance (again reducing the PRE pot resistance). Most of the schematics I reviewed just use a 1M pot to the grid of the OD tube and I found this to be exactly what the doctor ordered. Now the ratio of the signal is perfect across a good percentage of the PRE control and not just the upper half.

I was/am a bit dissappointed with the cathode bias difference. Maybe I expected too much? I really don't hear the compression that I had read about. I can A/B the circuits pretty easily. I'm using a 130 Ohm resistor (for just TWO tubes mind you) - similar to an AC15 or a DC30 in half power mode (which have comparable plate voltages). Also, I've switched from a 100uF cap to a 22uF to increase the f3db point closer to 60 Hz and reduce the boominess. The biggest contributor to this mod is in removing the neg feedback. When you A/B the stock fixed bias amp (with neg feedback) and the cathode bias without neg feedback the difference is certainly more noticeable.

The 100 Ohm with four tubes will certainly get the plate current closer to the stock 8W dissipation. The 130 Ohm with two tubes puts it around 11W. It is for this reason that I only run the cathode bias mode with two tubes instead of all four (I was concerned that the OPT would not be able to handle the increased power with a 60 ohm resistor for the cathode bias. To be honest, you really cannot discern a noticeable difference in the amp running at half power - this should be about a 3dB change wich is barely perceptible volume wise.

Bob G.

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