DC Mods - Revisited...........Sort of.............

[JustPassingThrough]

Phase 2 gain mod

Changing the 150k to a 220k does a little, but it doesn't do enough. After playing my rectifier it left me wanting more from the DC10. Granted, there's less gain stages in the DC amps but they still sound a little lacking to my jaded ears. I had a look at the Dual Rec schematic and made a single change to my DC10.

V3A. The plate resistor is 100k. I made it 180k. A Dual Rec would typically use 220k.

And it lives here, by the big white resistor and the big orange cap. I've left it raised in case I wanted to reduce the value or add a cap in parallel to cut down any high end futz (seems stable enough at this point in time).

 

[Fei Fong Wong]

Phase 2 gain mod

Changing the 150k to a 220k does a little, but it doesn't do enough. After playing my rectifier it left me wanting more from the DC10. Granted, there's less gain stages in the DC amps but they still sound a little lacking to my jaded ears. I had a look at the Dual Rec schematic and made a single change to my DC10.

V3A. The plate resistor is 100k. I made it 180k. A Dual Rec would typically use 220k.

View attachment 5375

And it lives here, by the big white resistor and the big orange cap. I've left it raised in case I wanted to reduce the value or add a cap in parallel to cut down any high end futz (seems stable enough at this point in time).

View attachment 5374

Thank you so much for these great updated posts. Have you done the reverb mods? If so, what were your results? Can this amp sound like a fender reverb?

 

[JustPassingThrough]

Thank you so much for these great updated posts. Have you done the reverb mods? If so, what were your results? Can this amp sound like a fender reverb?

I did yeah. I changed the 22k resistor to a 7.5k resistor. I made no difference and the reverb is still hardly audible.

Since I modded the loop though I use a source audio ventris for reverb, which is another thing to carry and power and plug in, but it's fine.

Another thing I've done since this post isn't really a mod, but I replaced all the filter caps in the power supply. That made more of a difference than anything else and the amp sounds so much better. Definitely recommended for any amp over 15 years old.

 

[Fei Fong Wong]

I did yeah. I changed the 22k resistor to a 7.5k resistor. I made no difference and the reverb is still hardly audible.

Since I modded the loop though I use a source audio ventris for reverb, which is another thing to carry and power and plug in, but it's fine.

Another thing I've done since this post isn't really a mod, but I replaced all the filter caps in the power supply. That made more of a difference than anything else and the amp sounds so much better. Definitely recommended for any amp over 15 years old.

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Rats. I really hate the reverb on mine and I can hear that it could be awesome, if you could actually hear it. If you have any ideas on how to make the reverb more present, please share. How would you describe the difference the filter caps made? I would think it would be very little since their responsibility is just to filter noise.

When you replaced the filter caps, did you remove the board or just j-hook them in? I’m inclined to just j-hook mine when that time comes. Your work looks excellent.

 

[JustPassingThrough]

Rats. I really hate the reverb on mine and I can hear that it could be awesome, if you could actually hear it. If you have any ideas on how to make the reverb more present, please share. How would you describe the difference the filter caps made? I would think it would be very little since their responsibility is just to filter noise.

When you replaced the filter caps, did you remove the board or just j-hook them in? I’m inclined to just j-hook mine when that time comes. Your work looks excellent.

No ideas beyond changing the tank, the driver tube (v5), maybe the transformer itself. However, one thing I have done recently and haven't tested yet (amp is still out of the enclosure) is play through it after cleaning the internal connections of the tank. I doubt it'll change anything but I opened up the tank and cleaned all the contact points and resoldered the joins on the plugs. I'm yet to see if this made a difference.

So, no, you don't need to take out the board. 90% of the components on Mesa amps are workable topside, the components that aren't are generally the relays, everything else is accessible.

What I do is clip the leg half way off, so there's abough left in the board to grab with small pliers. Then apply some fresh solder to the join on the board and pull gently with the pliers to remove the leg. Then clean up the hole on the board with a solder sucker. Then you're free to easily install new parts.

One thing that you will find - Mesa use a silicone sealant to stop vibrations of these caps. It's a white rubbery film. You can score through this with a scalpel but a better way is to twist the part out. Don't pull straight up as the whole board will flex. I used some big wide mouth pliers, grabbed the cap around the middle and twisted it back and forth gently to release it. It works fine. Since the caps are being replaced I wasn't too concerned with plier marks on the sides.

Its honestly a very straight forward job to replace these caps and the results are immediate and obvious.

 

[Fei Fong Wong]

No ideas beyond changing the tank, the driver tube (v5), maybe the transformer itself. However, one thing I have done recently and haven't tested yet (amp is still out of the enclosure) is play through it after cleaning the internal connections of the tank. I doubt it'll change anything but I opened up the tank and cleaned all the contact points and resoldered the joins on the plugs. I'm yet to see if this made a difference.

So, no, you don't need to take out the board. 90% of the components on Mesa amps are workable topside, the components that aren't are generally the relays, everything else is accessible.

What I do is clip the leg half way off, so there's abough left in the board to grab with small pliers. Then apply some fresh solder to the join on the board and pull gently with the pliers to remove the leg. Then clean up the hole on the board with a solder sucker. Then you're free to easily install new parts.

One thing that you will find - Mesa use a silicone sealant to stop vibrations of these caps. It's a white rubbery film. You can score through this with a scalpel but a better way is to twist the part out. Don't pull straight up as the whole board will flex. I used some big wide mouth pliers, grabbed the cap around the middle and twisted it back and forth gently to release it. It works fine. Since the caps are being replaced I wasn't too concerned with plier marks on the sides.

Its honestly a very straight forward job to replace these caps and the results are immediate and obvious.

Thanks again. It is truly bizarre that some say the reverb is great in these amps. Or it’s great in the adjacent models. I’d give anything to make the reverb better in this amp.

I have compared the schematics for the A and B revisions of the DC-5. There are two changes in the B version and I wonder if these changes would help. Pics below.

(Edit: looks like yours is the B revision and if you’re saying the reverb is nearly non-existent, then I doubt a change in the A circuit would do very much. I also see that monstas mod on page one just puts the B circuit back to the A reverb circuit. Oh well.)

When I compare the reverb circuit to a fender super reverb, I see that the DC has one 12ax7 acting as both the send and recovery for the reverb circuit, whereas a super reverb has 3 triodes (a 12at7 as the send and half of a 12ax7 as the recovery) which makes me think the DC-5 simply does not have enough amplification factors to make the reverb circuit pronounced enough. Not sure.

 

[JustPassingThrough]

I suspect those 2 little caps would be to roll off the high end in the reverb signal so it sits in the mix better, under the guitar rather than on it.

Thing is, the topology of the DC reverb isn't really different from other Mesa amps with reverb so I'm not sure where the difference is coming from... The transformer itself maybe?

The heartbreaker reverb circuit is the same with a couple of resistor changes on the return tube (100k grid, 6k8 cathode) which would make it less gain than the DC... So basically, I've no idea except for the tank and transformer not doing enough.

Lemme know if the change of tank yields any improvement.

 

[Fei Fong Wong]

I suspect those 2 little caps would be to roll off the high end in the reverb signal so it sits in the mix better, under the guitar rather than on it.

Thing is, the topology of the DC reverb isn't really different from other Mesa amps with reverb so I'm not sure where the difference is coming from... The transformer itself maybe?

The heartbreaker reverb circuit is the same with a couple of resistor changes on the return tube (100k grid, 6k8 cathode) which would make it less gain than the DC... So basically, I've no idea except for the tank and transformer not doing enough.

Lemme know if the change of tank yields any improvement.

Hi there,

Swapping the tank did nothing. The posts in the thread I read were incorrect and a Princeton tank will not work in this amp. I have ordered a MOD version of the stock tank (9AB2A1B) and I’ll report back.

In the meantime, I’ve been studying the schematic and thinking about what you said about the little effect the caps in the reverb circuit would have if they were changed and your report of how little effect the mods listed in this thread have had.

Rather, I wonder if the preamp is the issue. When looking at the schematic, I see that the two channels vary quite a lot in terms of how they send the signal to the reverb circuit. I wonder if changing any of the resistors in the pics of the schematic below would be the ticket to send more of the signal to the reverb circuit. I am not a tech but enjoy learning how these things work, so I can only theorize. I have no one I can ask about this stuff.

In the pic of the rhythm channel, it looks like the signal is split at the 680k resistor, and I theorize this resistor’s value determines how much the signal is split. I wonder if increasing the resistance would send more of the signal to the tank and wonder if a similar effect would be achieved if the 220k resistor was increased in the lead channel. Since the value is lower in the lead channel, might this explain why the reverb is so much quieter in the lead channel? Or am I way off track?

Given that you seem to have dispelled the credibility of the mods listed on this page, I wonder if this would be a place to look given how much more reverb the rhythm channels have in these amps than the lead channels do.

Thanks for your time.

 

[JustPassingThrough]

Hi there,

Swapping the tank did nothing. The posts in the thread I read were incorrect and a Princeton tank will not work in this amp. I have ordered a MOD version of the stock tank (9AB2A1B) and I’ll report back.

In the meantime, I’ve been studying the schematic and thinking about what you said about the little effect the caps in the reverb circuit would have if they were changed and your report of how little effect the mods listed in this thread have had.

Rather, I wonder if the preamp is the issue. When looking at the schematic, I see that the two channels vary quite a lot in terms of how they send the signal to the reverb circuit. I wonder if changing any of the resistors in the pics of the schematic below would be the ticket to send more of the signal to the reverb circuit. I am not a tech but enjoy learning how these things work, so I can only theorize. I have no one I can ask about this stuff.

In the pic of the rhythm channel, it looks like the signal is split at the 680k resistor, and I theorize this resistor’s value determines how much the signal is split. I wonder if increasing the resistance would send more of the signal to the tank and wonder if a similar effect would be achieved if the 220k resistor was increased in the lead channel. Since the value is lower in the lead channel, might this explain why the reverb is so much quieter in the lead channel? Or am I way off track?

Given that you seem to have dispelled the credibility of the mods listed on this page, I wonder if this would be a place to look given how much more reverb the rhythm channels have in these amps than the lead channels do.

Thanks for your time.

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View attachment 5495


View attachment 5496

Not sure I dispelled anything, merely added my own thoughts to the pile

That's an interesting theory about the 680k/220k. Since it's in series it'll drop the signal as well so increasing it will make the amp quieter (it's still a 100w amp so still insanely loud).

A quick method to test this theory would be to clip another resistor in parallel with the 680k to lower the value and see what effect that has on the reverb signal. If the reverb signal drops when the resistor is in (so lower total resistance) you'll know that a lower resistance means lower reverb level. That would save removing the 220k and experimenting there.

 

[Fei Fong Wong]

Not sure I dispelled anything, merely added my own thoughts to the pile

That's an interesting theory about the 680k/220k. Since it's in series it'll drop the signal as well so increasing it will make the amp quieter (it's still a 100w amp so still insanely loud).

A quick method to test this theory would be to clip another resistor in parallel with the 680k to lower the value and see what effect that has on the reverb signal. If the reverb signal drops when the resistor is in (so lower total resistance) you'll know that a lower resistance means lower reverb level. That would save removing the 220k and experimenting there.

Hi again. After more research I’m pretty convinced that the two resistors (680k in clean and 220k in lead) are the reverb mixing resistors. According to this thread, they are used to attenuate the dry signal so that the weak wet signal can be heard. Since most fenders use a huge 3.3 meg resistor, and the DC uses a paltry amount of resistance for its channels’ reverb mixer, if bet that raising the resistance from 680k to 1 or 1.5 meg in the clean channel would be the method to increase the wet signal mix.

https://el34world.com/Forum/index.php?topic=28872.0

This site explains how a reverb circuit functions.

https://robrobinette.com/How_Spring_Reverb_Works.htm

I will try to clip a resistor in parallel to see if the wet mix is attenuated on the clean channel this weekend I think. If you still have your amp apart and feel inclined to experiment if you think this analysis is sound, let me know your findings.

Really my biggest trepidation lies in dealing with the solder pads on the board of the DC5. I had one of the pads fall out years back and had to run a terminal strip to deal with a bad resistor. If you know of techniques to deal with the pads, I’m also all ears. I try to research as much as possible, then go for it, trembling the entire time.

Thanks for your time. This amp is really awesome. It is just so annoying to have two knobs staring at you that do something you want them to do, but they just won’t just cuz. My Dr Z MAZ was the same way (weak reverb) but the Mkii mods to that amp made the reverb just glorious.

If that amp can, why can’t this one?

 

[JustPassingThrough]

Hi again. After more research I’m pretty convinced that the two resistors (680k in clean and 220k in lead) are the reverb mixing resistors. According to this thread, they are used to attenuate the dry signal so that the weak wet signal can be heard. Since most fenders use a huge 3.3 meg resistor, and the DC uses a paltry amount of resistance for its channels’ reverb mixer, if bet that raising the resistance from 680k to 1 or 1.5 meg in the clean channel would be the method to increase the wet signal mix.

https://el34world.com/Forum/index.php?topic=28872.0

This site explains how a reverb circuit functions.

https://robrobinette.com/How_Spring_Reverb_Works.htm

I will try to clip a resistor in parallel to see if the wet mix is attenuated on the clean channel this weekend I think. If you still have your amp apart and feel inclined to experiment if you this analysis is sound, let me know your findings.

Really my biggest trepidation lies in dealing with the solder pads on the board of the DC5. I had one of the pads fall out years back and had to run a terminal strip to deal with a bad resistor. If you know of techniques to deal with the pads, I’m also all ears. I try to research as much as possible, then go for it, trembling the entire time.

well that's some good research right there. I boxed the amp up a few days ago after having changed the filter caps, but if this makes a difference to the reverb I'll totally open it up again.

I've never had an issue with the solder pads on any Mesa. just hit them with fresh solder quickly and do what you need to do. Pumping heat into a dry joint is gonna wreck any PCB. I see where I'm going, then apply solder to the iron and then right away get it on the joint, then pull the part out with some pliers while its wet. Then repeat the process with more fresh solder and suck it out with a solder sucker. So far, so good. The wet solder is imperative.

 

[Fei Fong Wong]

well that's some good research right there. I boxed the amp up a few days ago after having changed the filter caps, but if this makes a difference to the reverb I'll totally open it up again.

I've never had an issue with the solder pads on any Mesa. just hit them with fresh solder quickly and do what you need to do. Pumping heat into a dry joint is gonna wreck any PCB. I see where I'm going, then apply solder to the iron and then right away get it on the joint, then pull the part out with some pliers while its wet. Then repeat the process with more fresh solder and suck it out with a solder sucker. So far, so good. The wet solder is imperative.

Ok hi again,

So I took the amp apart and found the “680k” resistor. Unfortunately, it is not a 680k but rather a 330k like in your DC5B model. I really really didn’t want to do any digging so I left the 330k and looked at the 1.5 meg and 3.3 meg resistors. I experimented with different value resistors in parallel with both of these resistors. Each value (100k, 47k, and 22k) definitely allowed more signal to hit the reverb circuit, but the result was a more distorted reverb sound. The one I liked best was the 100k in parallel with the 1.5 meg.

In the meantime the new MOD 9AB2A1B reverb tank arrived today. I decided to just leave the circuit alone and try the new tank. I really couldn’t believe the difference the new tank has made. I am shocked. I am now extremely happy with how the reverb sounds in this amp. The lead channel’s reverb is still very faint, but I have decided for now that it is what it is and that a delay pedal in the loop for the lead channel is just fine and what little reverb is present is just fine for this type of gain.

Pics below of the location of the 1.5 meg ohm and 3.3 meg resistors and you can see the 330k hiding below the big orang drop and squeezed up against the orange relay (barely poking out next to the black multimeter probe). YouTube vid attached for sound samples. I switch the reverb from 10 to 5 to 0 and back to 10 in the video. Hopefully someone finds this helpful in the future.