Tremoverb LDR Problem?

[cscotto]

HI,
The problem I am having is located only in the orange channel when switching from clean to vintage. Should there be a volume bump when switching from clean to vintage? When I throw the toggle switch, the distortion increases, but the volume actually seems to decrease, as if some of the signal was being shunted to ground. Just for comparison, in the red channel switching from blues to modern produces a substantial volume jump.

Everything else in both modes of the orange channel work. That is, all the tone controls affect the signal, the master and output both increase volume, etc. I thought it might be one of the LDRs that are lifted when switching to vintage mode from clean, such as LDR3a, LDR3b, or LDR 13, which are listed as cathode cap LDRs on the schematic. From what I can tell, they either put a resistor in series with cathode cap for the vintage mode or bypass the resistor for the clean mode. Wouldn't one of these affect the volume when switching to vintage, if it did not lift?
The other LDR I thought it could be was 4 or 5, the V1a output pad, or 19 the master 82k bypass. Any insights would be greatly appreciated. And if it is not asking to much, how would I test an LDR? I know with a multimeter (from reading other threads), but exactly what would the reading be for a good vs bad LDR? I am experienced working inside an amp, but LDRs are a new skill.

Thanks a million in advance.

All the best.

 

[94Tremoverb]

I'm almost certain that LDRs only ever fail open-circuit, not closed - so a faulty one would be lifted all the time, rather than failing to. I've definitely never come across one that failed closed-circuit, and knowing how they work I'm not even sure it's theoretically possible.

You can test them by using a multi-meter reading resistance, across the 'cell' side (marked on the LDR, all of the ones in each stack are the same way round if I remember correctly - which also from memory is usually or always the side nearest the back of the amp) - you should see a large change in resistance from extremely high to near-zero when operating the switching function that controls it. You should have the amp set to standby when doing this, it's a lot safer! The switching circuitry operates the same whether the amp is on or off standby.

It's interesting that you should mention that switching Red from Blues to Modern produces a volume bump though - on mine, with the settings I normally use, it's actually the other way round - Blues is louder. I do get a volume boost when switching Orange from Clean to Vintage (not a decrease) but I wonder if it's just a settings thing? There are *some* settings on Red where the volume change is the other way round, for me - I think when the MV is higher, but I'd have to check.

 

[cscotto]

I'm almost certain that LDRs only ever fail open-circuit, not closed - so a faulty one would be lifted all the time, rather than failing to. I've definitely never come across one that failed closed-circuit, and knowing how they work I'm not even sure it's theoretically possible.

You can test them by using a multi-meter reading resistance, across the 'cell' side (marked on the LDR, all of the ones in each stack are the same way round if I remember correctly - which also from memory is usually or always the side nearest the back of the amp) - you should see a large change in resistance from extremely high to near-zero when operating the switching function that controls it. You should have the amp set to standby when doing this, it's a lot safer! The switching circuitry operates the same whether the amp is on or off standby.

It's interesting that you should mention that switching Red from Blues to Modern produces a volume bump though - on mine, with the settings I normally use, it's actually the other way round - Blues is louder. I do get a volume boost when switching Orange from Clean to Vintage (not a decrease) but I wonder if it's just a settings thing? There are *some* settings on Red where the volume change is the other way round, for me - I think when the MV is higher, but I'd have to check.

Hi 94Tremoverb,
Thanks for answering; I knew you'd have the scoop. I did some research on LDR/LEDs, so I have a better understanding of how they work. But I have to to still straighten out some terminology. When the LED is dark, the resistance increases, and when the LED emits, the resistance of the LDR increases. I am assuming when LDRs fail, the connection from one side to the other is broken. A failed LDR should then read OL on a meter (I have a Sears digital meter). Is this correct?

I found all 25. You are right about the leads being difficult to get at on some of them. I also found the 'cell' side. I tested them with the amp on standby as you suggested. Some of them operate in the range on the spec sheet (I downloaded it from the manufacturer). On LDR 2 the switch that engages the orange channel, for example, with the LDR lifted (the amp switched to the red channel), the resistance was 10 M ohms, and with the orange channel engaged the resistance drops to around 200 ohms. On some LDRs, however, there is a change, but maybe not as drastic, for example, from 90 ohms to around 500K. A couple of LDRs, however, went from low readings to OL on the meter. Now I am not sure I just did not have a hold on the lead.

Just to make sure things were working, I lived a little dangerously. I had my son play the guitar while I ran a jumper across an LDR. For example, LDR 13 is a V3a cathode that is lifted in the clean mode but engaged in the vintage mode. This was one of the LDRs that registered a change but not as drastic as LDR 2. I put the amp in the clean mode and my son played while I ran a jumper across the LDR. When I did this, the clean mode definitely became louder, which is what adding that resister to V3a should do. So, in spite of the reading, LDR 13 appears to be working fine.

One last question, LDR 3 has three leads instead of two because it controls the cathode cap on V1a and V2a. I had some trouble getting the leads in there but should the reading be taken from the center lead to the outside leads or outside/outside and inside/outside?

Thanks for all the help!

 

[cscotto]

It's interesting that you should mention that switching Red from Blues to Modern produces a volume bump though - on mine, with the settings I normally use, it's actually the other way round - Blues is louder. I do get a volume boost when switching Orange from Clean to Vintage (not a decrease) but I wonder if it's just a settings thing? There are *some* settings on Red where the volume change is the other way round, for me - I think when the MV is higher, but I'd have to check.

Based on the schematic of the switching in the Tremoverb, I made a chart of what LDRs are active/not active in each channel and mode. I retested all of them. This time I was able to get my probes securely on the leads, so I am confident on the readings. Also, LDR 3 with three leads test from center leads to the outside leads. Every one of them tested fine. From my chart I see that if LDRs fail to the open position, then most of the failing LDRs would effect the clean channel, not vintage. The only ones that change the sound of the vintage channel would be 3 and 13. All the rest would effect only the clean channel (19, 23, 24, 25) or interrupt the signal in both modes. I do hear a volume difference when switching from clean to vintage; it just was not as great as I remember. That is where your idea on settings comes in. Here is a passage from the manual:

"NOTE: it is normal to hear a volume level difference when switching from CLEAN to VARIABLE HIGH GAIN. This difference will get larger as the MASTER volume is increased and the amp is played loud. The MODERN RED channel will display a similar volume difference when switching modes from BLUES to MODERN HIGH GAIN."

I think I may have had the master set differently the last time I used the amp, which would account for hearing not as great a volume difference. I am glad nothing is wrong, and I picked up some new skills.

 

[94Tremoverb]

Major kudos for teaching yourself how to do this on as complicated an amp as the Tremoverb, too!

I have also noticed differences in the on and off resistances of LDRs, but I think this is just normal variation. If they switch from under 1K to at least a few hundred K it shouldn't make much difference to the operation. If it matters, all the failures I've ever seen have been of the LDR side not the LED too - which is maybe not what you would expect.

 

[cscotto]

Just to be sure about this volume thing, I went GC where they have a used Tremoverb for sale. It behaved exactly like mine. I now think it is normal when switching from clean to vintage for the volume to drop a bit, and in the red channel switching from blues to modern a big volume bump. The interesting thing is cloning the channels switches the behavior: Red cloned to vintage, volume drops going from blues to modern; vintage cloned to modern, volume jumps going from clean to vintage.

Before I put the chassis back, I decided to just check everything. I found something new! In between two of the power tubes, there is a large orange cap rated .047. It is mounted vertically with one lead soldered to the chassis and the other end connected to two wires that lead to the PCB board. I really can't located it on the schematic. I would have thought this might be the cap that is between the the driver and the power tubes, but there is only one of them.

In any event the lead that connects to the two wires leading to the pcb board had a broken solder joint. I re-soldered the connection. Wow, what a difference. The amp seemed to have more growl, volume, and punch. It now really sounds like I imagined this amp should sound. Could one cap make such a difference? And, if anybody has any insight into the function of this cap, I would appreciate knowing why it made such a difference.

All the best,

 

[94Tremoverb]

The only one I can see that would correspond to that is the .047/600V cap on the 'out' side of the standby switch, connected to the 'A' point (main feed to the OT) in the power supply chain, and the 'in' side of the choke. (Page 7 of the standard Tremoverb schematic.) It's a while since I've been inside the amp but that would also figure from where it is physically - as it will be quite a large cap due to its high voltage rating. Its purpose is most likely to suppress voltage surges and pops when the standby switch is operated, but it may also filter high-frequency noise from the power supply that the main filter caps - which like all electrolytics have high Equivalent Series Resistance (ESR) - can't deal with as well, since it's in parallel with the main filters when the standby switch is closed. It *could* then also provide a more responsive supply of voltage to the power stage, on transients and high frequencies, which again the main filter caps can't because of their ESR. If so, it could potentially affect the tone of the amp.

Another thing to check next time I'm working on something and I remember! For reference though, most hi-fi and solid-state amp designs do parallel their main filter caps with smaller non-electrolytic types exactly to deal with the ESR problem.

 

[cscotto]

The only one I can see that would correspond to that is the .047/600V cap on the 'out' side of the standby switch, connected to the 'A' point (main feed to the OT) in the power supply chain, and the 'in' side of the choke. (Page 7 of the standard Tremoverb schematic.) It's a while since I've been inside the amp but that would also figure from where it is physically - as it will be quite a large cap due to its high voltage rating. Its purpose is most likely to suppress voltage surges and pops when the standby switch is operated, but it may also filter high-frequency noise from the power supply that the main filter caps - which like all electrolytics have high Equivalent Series Resistance (ESR) - can't deal with as well, since it's in parallel with the main filters when the standby switch is closed. It *could* then also provide a more responsive supply of voltage to the power stage, on transients and high frequencies, which again the main filter caps can't because of their ESR. If so, it could potentially affect the tone of the amp.

Another thing to check next time I'm working on something and I remember! For reference though, most hi-fi and solid-state amp designs do parallel their main filter caps with smaller non-electrolytic types exactly to deal with the ESR problem.

D'oh! I didn't think to look at the power supply page of the schematic, but I think you are right. I think your idea of "a more responsive supply of voltage to the power stage" explains the change I am noticing, especially the transients. Palm muting, for example, seems much tighter, and the amp reacts faster. Thanks for the explanation!