TubeMeister 18 effect loop

Hi everyone,
I am the proud owner of a brand new Tubemeister 18 that i bought 1 month ago.
One of the reasons i choose this amp was the effect loop, which is not so common on a small full tube and low wattage amp...

Before buying this tubemeister head, i was playing a blackstar HT-5RH, that was too 'metal metal sounding' for me...
My rig consists of using a BOSS RC-3 loop pedal in the effect loop. to record a loop in clean channel and then switch to the lead channel to work on my solos...
This is where my issue is... When i record a loop on the clean channel (clean chords for example) and then set the playback volume of the recorded loop at almost the same level on the clean channel everything is ok, but when i want to switch for soloing on the lead channel, the volume of the playback loop is boosted at the same time i switch the channel. This is a slight boost, but a very annoying one... ! It seems that the amp rises the volume of the effect loop at the same time i switch the channel. That makes the playback clean loop too loud to correctly overdub on it...
With exactly the same setup, with my previous blackstar HT-5 head (that i sold) the loop level never changed when switching between the channels of the amp...
Do you think my tubemeister has an issue and needs a repair, or is it a known issue of the tubemeister series ???


Do you think that a device as the Morley effect loop corrector will work ?

Thanx for your answers

Hi,

Do you have any answers on this?
I am using a Boss loop channel in exactly the same way with my GM 36, and what I see is that after switching for soloing some of the effects are added to the loop.
Another user said this is because the built in fx are post-loop.
To me this is really weird; WHY would the fx be post loop? That's just the idea isn't it, to have a signal bypass the fx??

Hey man I have the same issue.  After recording 1st loop and then switching channels for a lead/solo the volume of the prerecorded loop is boosted.  I bought this amp specifically because it had the FX loop and it wouldnt change any of my loop sounds.  What did you end up doing?

In the TM18 the signal feeds from the preamp valve stages into the tone/master controls and on to the buffer for the Fx Send.  It passes through the loop and back and is picked up by the reverb unit as its input.  It passes on to a mixer where the reverb is mixed back in and the signal is buffered for the power amp stage.  Then of course it is on to the PI and so on.

I'm not sure about how you are using your Looper in the Fx Loop, I'm not that sort of player myself so I have no experience of doing that.  If the looper is doing all of its work inside the loop and does not feed anything back to earlier in the amp (which I would imagine is the case) it shouldn't be affected by the other controls and selections you make.  However...   I think I can see a route where the buffer/mixer just before the PI may have its gain affected by selection of the Lead channel.  I will ask H&K about this and report back to you with any reply they come up with.

The question about the post-loop fx is asked from time to time and the answer is simply because H&K think that that is the best way to do things.  It isn't a mistake it is a choice on the designer's part and a few users will not prefer that choice.  I'm afraid there is nothing that can be done about that.


EDIT: The reason given for the post-loop fx is that the amp offers its own Modulation effects and, if you are using an overdrive/distortion pedal in the loop, (oh yes they do! ), you normally want the modulation to come after those pedals. And as they point it out, you can't have it both ways and "common knowledge" tells you to do it that way.

I've messaged H&K about the loop volume issue. They generally get back to me at some point though it may take a few days if they are flitting about at some show or other. I'll keep you up to date with any reply but I do think that it is normal circuit action with the post loop buffer/reverb mixer gain being affected slightly by the channel selection.

Sorry it took so long to get back to you on this one but the H&K response will show why.  You have to hand it to them, they are pretty open in discussing things like this.  Many manufacturers wouldn't even grace you with a reply to a technical question which goes into their circuit design.

H&K FB Support wrote:Hi BB,

First off, apologies for the massive delay in a reply – August is a month where lots of the H&K team are off on vacation, so it took way longer than usual to get a reply cobbled together. But this is what our Meister expert has just written back:

-----
“One of the secrets of the TubeMeister line to sound great in “both” worlds, clean and overdrive, is the adjustment of the current feedback in the power amp for each channel. This means: the power amp in clean mode works differently than it does in overdrive mode. The preamp and power amp are always matched, so if you use both, it always results in great sound with even levels. If you use the power amp only, it will result in a different sound and level depending on the selected channel. This might seem strange, because all other amp manufacturers don’t “touch” the power amp when channels are switched, but we do.

So, using a clean channel with a looper recorded signal that is sent to the power amp when the preamp is set to lead will sound different, and you will have to adjust the level with the looper.

For the GrandMeister amps, we explained this in the manual when it comes to the “4-cable method”, but back in the days of the TubeMeister 18 we did not. That time, we thought not to tell the “secret”. Now we do

So it will always be a compromise, but the benefit of having uncompromised sound on any channel weighs more to us than the disadvantage of having to adjust levels in combination with loopers.

Hope this helps…”

That makes sense now if you get a look at the schematics and as a compromise it is one they had to take.  They chose tone in both modes over balance.  It looks as though the level of the preamp signal is changed between the channels and the gain of the power amp is rejigged to match, so preamp level goes up and power amp gain goes down to compensate and vice versa.  If you separate the two then you get the effect of one without the other.  That is just what is happening in your looper setup.  The live guitar is using both preamp and power amp in the matched way while the looper signal is only going through the power amp and is therefore missing the preamp adjustment so has an unbalanced level which is higher.

For what it is worth there is a similar problem in my Marshall JVM205H as well.  This has 2 channels with 3 voicings for each, 6 in total.  Each one involves a rejig of the circuit to the extent of adding in additional gain stages.  In this way it duplicates the setup of each Marshall amp topology through the years.  The problem comes when you try to swap between them while playing as it is not always possible to match up the levels and it suffers in the same way as your looper.  Marshall say they have tried to keep the levels of each amp type relative to each other the same as the originals but most users would have preferred to have them balanced to a single common level.  It's something you have to either live with or find a tech solution to.

GM36 Manual wrote:Heads Up: You can bypass the GrandMeister’s preamp by routing amp models into the FX Return. However, when you switch channels its power amp is re-voiced to deliver the best tone for that channel. This means every channel sounds different, even if you decide to use only the power amp! You probably want the amp model’s sound and volume to remain consistent with your original programming. If so, you’ll have to remember which channel you used for programming and select it when you activate the amp model. However, the better option is to program a preset to do this for you.

I've not really ever taken that in before as I don't use the Fx Loop.  He's absolutely right, they do advise that the power amp is reconfigured between channels in an unusual way.  I would think it is inevitable that the volume would be altered as a consequence of that.

Not sure what can be done about that.  I'll have a think and see if anything jumps to mind.

Not sure what difference in level we are talking about here but how about reducing the level into the looper while it is in the recording stage? That would take only a couple of resistors which could even be put into the jack plug between the Fx Send and the Looper Input. That would record at a slightly lower level but it would be out of circuit during playback as it is only affecting the input/record side. if you preferred you could even make up a custom cable with a level pot fitted in it.

Just a newbie here, but I wonder what would be involved for H&K to "mod" their amps such that, if something was plugged into the "return" jack, the power amp would simply configure itself for straight "hi-fi" mode.

My 2-cents worth.
Artie

I can tell you that would take quite a lot of complex changes to circuitry Artie. It's a great idea but involves a fair bit of work. The power amp in at least some models is preceded by a SS buffer after the loop, that's not just a simple single stage loop buffer as you commonly see, this is a specifically designed unit responding to feedback and switching to alter levels and responses for the differing channels. It's part of the overall voicing of the amp and its channels. I have often said that I feel the H&K way of voicing, spreading it over so many stages and so many different areas, is too complex to be easily moddable but they are producing an amp which they don't want modded of course.

Personally I like your idea though it would take away the concept of blending another amp's preamp tone with the H&K power amp tone though that would be a small loss for my money.

Yeah, as I was typing that realized that modding a production circuit would probably be cost prohibitive. But I also was assuming, (bad, I know), that the switching might be a simple data line that each channel sends to the an amp microprocessor. A simple microswitch on the return jack could then send that same signal to the amp locking it into the clean channel mode and retaining that modes character.

I'm probably grossly over simplifying it.