Volume Drop on TM 18 Combo

Hello .. recently I noticed a volume drop on the clean channel of my amp. I pulled each tube/valve and swapped them out to see if that was the problem, and that wasn't the issue. When I look at the TSC on the back, the lights are off when the amp is on. When I put a pick in the slot, it blinks 10 times and then the light go off. Anyone have a similar issue and know what the solution(s) may be? Thx!

Are you observing the same behavior at all power levels, i.e. 1, 5 and 18W?

Since the lead channel is not having this issue (you mentioned the issue is with the clean channel) the tubes seems to be working fine.

Does the volume drop after some time or is the volume always low? Does it change when you turn the gain and master knobs?

Strat007 wrote:Are you observing the same behavior at all power levels, i.e. 1, 5 and 18W?

Since the lead channel is not having this issue (you mentioned the issue is with the clean channel) the tubes seems to be working fine.

Does the volume drop after some time or is the volume always low? Does it change when you turn the gain and master knobs?

The issue is consistent across all 3 wattage levels. You don't hear the volume drop, meaning it isn't one level for a bit then it drops off. I just noticed that it seemed much quieter one day and when I set the same levels for the lead channel, the lead was/is much louder. The clean channel will change when the levels are set higher, it's just the overall volume seems to have dropped in general. Hopefully that makes sense .. thx!

Yerfy wrote:

Strat007 wrote:Are you observing the same behavior at all power levels, i.e. 1, 5 and 18W?

Since the lead channel is not having this issue (you mentioned the issue is with the clean channel) the tubes seems to be working fine.

Does the volume drop after some time or is the volume always low? Does it change when you turn the gain and master knobs?

The issue is consistent across all 3 wattage levels.  You don't hear the volume drop, meaning it isn't one level for a bit then it drops off.  I just noticed that it seemed much quieter one day and when I set the same levels for the lead channel, the lead was/is much louder.  The clean channel will change when the levels are set higher, it's just the overall volume seems to have dropped in general. Hopefully that makes sense .. thx!

Apologies for the delay as I was not following the forums for some time.

If you are in the warranty period, best approach would be to check with your retailer or tech support. Even if it out of warranty they may still guide you.

I had excellent support from H&K local support.

Are the Boost and Lead LEDs working as they should? We are looking for an area which is exclusive to teh Clean channel and just about the only one is the switching for gain and tone when the Clean cjhannel is selected. This is mostly done with discrete logic driving FETs. The logic control for a couple of the FETs is controlled by the Lead and Boost switches and their signals pass through the footswitch socket and on to the LEDs and the control circuit. If the switch leaves on th eFootswitch socket get dirty and corroded it can affect the working of that part of the circuit. We have had a case similar to this before and it did show up as odd behaviour in the LEDs.

I would clean the socket anyway as soon as you can as a matter of course, or maybe even just plug a jack in and out a few times to see if it affects it.

Just reread the thread and I would also say a preamp valve could be the culprit.  The Clean channel only uses T1A driving T2A and bypasses T1B completely.  The Lead channel brings T1B into the chain as well so T1A drives T1B drives T2A.  With that amount of extra gain it is possible you are just not noticing a difference in the Lead channel which is obvious in the Clean.  When you said you swapped out the valves did you mean you replaced them with a different set or did you just move them around?


EDIT:  Just noticed something else re swapping the valves.  If you just moved them by swapping with each other then you exchange T1A and T2A which are both of the Clean channel triodes.  You also replace T1B with T2B the Lead triode and the Phase Splitter (this amp only uses one triode to do this totem pole style).  Basically that would mean that if there was a problem in either T1A or T2A but T1B and T2B were still good the overall Clean gain would be low but would stay the same, T1A X T2A = T2A X T1A. That is to say, you could have a problem with an "A" valve which doesn't show up when swapped around.

bordonbert wrote:Just reread the thread and I would also say a preamp valve could be the culprit.  The Clean channel only uses T1A driving T2A and bypasses T1B completely.  The Lead channel brings T1B into the chain as well so T1A drives T1B drives T2A.  With that amount of extra gain it is possible you are just not noticing a difference in the Lead channel which is obvious in the Clean.  When you said you swapped out the valves did you mean you replaced them with a different set or did you just move them around?


EDIT:  Just noticed something else re swapping the valves.  If you just moved them by swapping with each other then you exchange T1A and T2A which are both of the Clean channel triodes.  You also replace T1B with T2B the Lead triode and the Phase Splitter (this amp only uses one triode to do this totem pole style).  Basically that would mean that if there was a problem in either T1A or T2A but T1B and T2B were still good the overall Clean gain would be low but would stay the same, T1A X T2A = T2A X T1A. That is to say, you could have a problem with an "A" valve which doesn't show up when swapped around.

I had replaced the factory valves months ago, and I still have them. I pulled each one of the replacement valves out one by one, dropping in the original and testing as I went to see if I could isolate the issue. The issue remained, so I don't think it's a valve issue. Would a bad fuse cause this? I don't think it's a jack issue either, they are clean and have pulled out the cable multiple times to no avail. I talked the H&K's support shop in NY, and basically I'd have to ship the amp across the country for what could turn out to be a small issue, so I'm not really sure what to do.

Check my post No5. Are the LEDs working as they should?

bordonbert wrote:Check my post No5. Are the LEDs working as they should?

When you say LED, wasn't sure which lights you are referring to. The green light on the clean setting lights up, and the red light on the lead light ups as they should. Both lights on the footpedal work. On the Tube Status Monitor, both light are off when the amp is on and not in standby, but they are red when in standby mode. When I stick a plectrum into the TSM slot, both lights blink red for 10-12 times, then stop.

Hope that helps, if you are asking something that I'm not answering please let me know. Thx for your help!

Sorry, I didn't realise it would be confusing but you're right, it is.     You'll have to forgive me, I've never actually used a TM18, mine is a TM36 which is very different, but I know the TM18 schematic very well.

I can see that the TM18 has the Channel Select switch with a blue LED inside it which comes on when Lead is selected, and it has a panel red or green LED, one for each channel, then it has the Boost switch internal red LED.  Those are the ones I meant as those LEDs are mixed in with the action of the selector switches which turns on and off FETs to select various voicing options in the amplifying stages.  If they don't switch fully that can affect levels and the LEDs not working correctly is sometimes an indicator of them not doing their job properly.

I would guess these are ok now though as you would have pointed any unusual visual clues out before.

I'm running out of simple ideas to try here which you will be able to perform there.  Without having the amp on the bench in front of me to chase through signal levels it's very difficult to come up with things you could do which would help.  I will have to put in a word for the guitar techs out there and say this is not an easy amp for them to work on, many will just refuse I'm sure.  The functionality and the build are both extremely complicated for fault finding, they are really intended to just swap out boards as the cheapest option, (even though that may be expensive).  It's worlds apart from the average guitar valve amp, even the top fliers are not usually as complex as H&Ks.  Then they will plain have to have access to the H&K Service Documentation, and I believe H&K have a list of approved techs who they will only release them to.  H&K is that sort of gear.  When you buy it you have to accept that in the event of it going wrong you will have this level of inconvenience in getting it fixed.

Is there anyone else out there who can come up with any ideas or has had this experience, I'm running out of places to go?

bordonbert wrote:Sorry, I didn't realise it would be confusing but you're right, it is.     You'll have to forgive me, I've never actually used a TM18, mine is a TM36 which is very different, but I know the TM18 schematic very well.

I can see that the TM18 has the Channel Select switch with a blue LED inside it which comes on when Lead is selected, and it has a panel red or green LED, one for each channel, then it has the Boost switch internal red LED.  Those are the ones I meant as those LEDs are mixed in with the action of the selector switches which turns on and off FETs to select various voicing options in the amplifying stages.  If they don't switch fully that can affect levels and the LEDs not working correctly is sometimes an indicator of them not doing their job properly.

I would guess these are ok now though as you would have pointed any unusual visual clues out before.

I'm running out of simple ideas to try here which you will be able to perform there.  Without having the amp on the bench in front of me to chase through signal levels it's very difficult to come up with things you could do which would help.  I will have to put in a word for the guitar techs out there and say this is not an easy amp for them to work on, many will just refuse I'm sure.  The functionality and the build are both extremely complicated for fault finding, they are really intended to just swap out boards as the cheapest option, (even though that may be expensive).  It's worlds apart from the average guitar valve amp, even the top fliers are not usually as complex as H&Ks.  Then they will plain have to have access to the H&K Service Documentation, and I believe H&K have a list of approved techs who they will only release them to.  H&K is that sort of gear.  When you buy it you have to accept that in the event of it going wrong you will have this level of inconvenience in getting it fixed.

Is there anyone else out there who can come up with any ideas or has had this experience, I'm running out of places to go?

Ha, no problem .. I'm not much of an engineer, so I need pretty specific instructions to keep up! I have a good amp guy in my town that I can bring it to, but based on your comment above, I hope he can diagnose this issue without too much work.

How many dB's would you say the difference is?

The typical person can hear around 3dB of difference in volume changes. Us guitarist hear better than the average person since we are trained to listen closely.

Can you post a recording or a video somewhere for us to hear the volume drop? Are any effects active in the preset? Have you compared four presets that have zero effects added?

Like BB, I only have a GM36 and I have never seen a TM18 before.

HwyStar wrote:How many dB's would you say the difference is?

The typical person can hear around 3dB of difference in volume changes. Us guitarist hear better than the average person since we are trained to listen closely.

Can you post a recording or a video somewhere for us to hear the volume drop? Are any effects active in the preset? Have you compared four presets that have zero effects added?

Like BB, I only have a GM36 and I have never seen a TM18 before.

Not sure if I know how to approximate the drop in db's. The drop is present with and without effects. When I have time, I'll try and make a short video and post it here. thx!

HwyStar wrote:How many dB's would you say the difference is?

The typical person can hear around 3dB of difference in volume changes. Us guitarist hear better than the average person since we are trained to listen closely.

Can you post a recording or a video somewhere for us to hear the volume drop? Are any effects active in the preset? Have you compared four presets that have zero effects added?

Like BB, I only have a GM36 and I have never seen a TM18 before.

You can use a phone to measure the sound level, there are many apps available. Although not calibrated, it can still show the relative changes.

PS: Many guitarists actually can't hear the nuances as their ears are knackered after playing next to all those loud amps

Strat007 wrote:

HwyStar wrote:How many dB's would you say the difference is?

The typical person can hear around 3dB of difference in volume changes. Us guitarist hear better than the average person since we are trained to listen closely.

Can you post a recording or a video somewhere for us to hear the volume drop? Are any effects active in the preset? Have you compared four presets that have zero effects added?

Like BB, I only have a GM36 and I have never seen a TM18 before.

You can use a phone to measure the sound level, there are many apps available. Although not calibrated, it can still show the relative changes.

PS: Many guitarists actually can't hear the nuances as their ears are knackered after playing next to all those loud amps

ah, good idea! I have Tinnitus, so that does apply to me. My hearing is fine, but I have a constant buzz in my ears. I will look into the app suggestions and post the results.

Yerfy wrote:

Strat007 wrote:

HwyStar wrote:How many dB's would you say the difference is?

The typical person can hear around 3dB of difference in volume changes. Us guitarist hear better than the average person since we are trained to listen closely.

Can you post a recording or a video somewhere for us to hear the volume drop? Are any effects active in the preset? Have you compared four presets that have zero effects added?

Like BB, I only have a GM36 and I have never seen a TM18 before.

You can use a phone to measure the sound level, there are many apps available. Although not calibrated, it can still show the relative changes.

PS: Many guitarists actually can't hear the nuances as their ears are knackered after playing next to all those loud amps

ah, good idea! I have Tinnitus, so that does apply to me. My hearing is fine, but I have a constant buzz in my ears. I will look into the app suggestions and post the results.

I used Decibel 10th on my phone, and while not exactly perfect, the approximate Db difference from channel to channel was approximately 15-20 Dbs. I set the gain and master at identical settings for both channels, and that was the result. I took 2 samples, one at lower volume and one at mid volume.

Okay...  10dB is way to much of a volume drop and that should not be happening.

I'm at a loss why. I think BordonBert covered everything from an engineering standpoint. My guess would have been a preamp tube too. If anyone can answer the technical issues BB is always up for the challenge.

I think the only way to resolve this is to have an authorized H & K tech have a look at it. Sorry we aren't being any more help. I got nothing to add. Sorry Mate!

HwyStar wrote:Okay...  10dB is way to much of a volume drop and that should not be happening.

I'm at a loss why. I think BordonBert covered everything from an engineering standpoint. My guess would have been a preamp tube too. If anyone can answer the technical issues BB is always up for the challenge.

I think the only way to resolve this is to have an authorized H & K tech have a look at it. Sorry we aren't being any more help. I got nothing to add. Sorry Mate!

I live near San Diego, CA .. is there an authorized shop anywhere in this area?

The best thing to do Yerfy is to click on the "Contact" support button on H & K's website so that they can find somebody down in San Diego who can help you. http://hughes-and-kettner.com/support/

They should be able to help you find someone.

Sorry we can't be of more help.