What would account for the low-gain input having vastly lower volume than the high-gain input on a JCM 800?

[Delicieuxz]

On a JCM 800 2203 I've just bought, the low-gain input is incredibly quiet compared to the high-gain input. With the master control at 10 and using the preamp control as a volume control, to keep the amp nice and clean, I can play pretty comfortably with the preamp control at 1 - 5. And this contrasts with what seems to be people's normal experience with the low-gain input. If I plug into the high-gain input, with the master at 10 and the preamp at 0.5, the sound is devastatingly loud. To get the same volume from the low-gain input, with the master still at 10, I need to turn up the preamp control to 8 - and then the amp is no longer clear-sounding, but is thick and dark-sounding, being coloured by the preamp control, which I don't want.


Here's a summary from another thread of people's reports about their own JCM 800s:

On my 2203, with the master volume at 10, the high gain input with the preamp volume at 0.8 is comparable to the low gain input with the preamp volume 8. So, I guess the high-gain input is about 10x louder than the low-gain input. And at 8 on the preamp control, through the low-gain input, the preamp section is colouring the sound a lot and making it thick and not crystal clean anymore.

Because it only has one stage of preamp gain as opposed to high having been cascaded through two.

Is there something that could affect the output level of the low-gain input, while not affecting the high-gain input?


I'm wondering what could account for the massive difference in volume between the low-gain and high-gain inputs of my amp, when my friend says that 0.5 on the preamp control and 10 on the master, through the low-gain input, is ear-piercingly loud on his 2203, while mine is very far from being that, and while all these other posters also describe a very different experience:

My 2203 is modded a bit, but the low input is pretty much as loud as the high input.......unlike my stock 2204 where the low input is much quieter than the high input?!?

You should be able to take the amp to maximum volume with the master cranked and the preamp volume turned up all the way, in both inputs. The difference would be in how much distortion/drive you get. Not in how loud it gets.

I have a 2204 from 1987 that sounds fantastic. But the low input is about half as loud as the high input channel. I use a Power Station attenuator and run the high input channel Master and Pre-amp at around 7-8 each. That is sweet spot for the amp where it sounds full and glorious. The low input, which I rarely ever use, stays mostly clean all the way up on both volumes and doesn't put out anywhere near the same power.

 

[Dr.Twang!]

They’re ALL like that unless modded.
Plug into low jack; turn preamp volume to 10.
Master volume to whatever loudness you need.
Thats how 2203\4 owners have been doing the “Low” gain input pedal trick since 1976.
The high input is cascaded gain from 2 halves of V1 preamp tube; low is only 1 half.
The high gain has exponentially more gain and louder signal going to the phase inverter and power tubes.

 

[Tatzmann]

Why are you opening another thread for this?

Its been clearly stated by more than one owners
of Marshall 2203/2204 that this is normal.

I too have a couple of those and tell you now that it is normal.

There is one less pre-amp (half)12AX7 in play if you use the low input, thats why.

 

[Delicieuxz]

Why are you opening another thread for this?

Its been clearly stated by more than one owners
of Marshall 2203/2204 that this is normal.

I too have a couple of those and tell you now that it is normal.

There is one less pre-amp (half)12AX7 in play if you use the low input, thats why.

After asking a general 'how does your amp sound' question in the general amp discussion forum, I thought I'd ask a specific tech question ('what causes an amp to sound like this') in the Workbench forum, where the tech minds would see it.

It looks to me that all the respondents in the other thread who commented on their personal experience with the 2203s they own said their experience doesn't match what mine is, and that many more people replied to say it isn't normal than those who suggested that, in theory, it is.


Would a 2203 with 6550s or KT88s get more output volume from the low-gain input than one with EL34s?

 

[Delicieuxz]

They’re ALL like that unless modded.
Plug into low jack; turn preamp volume to 10.
Master volume to whatever loudness you need.
Thats how 2203\4 owners have been doing the “Low” gain input pedal trick since 1976.
The high input is cascaded gain from 2 halves of V1 preamp tube; low is only 1 half.
The high gain has exponentially more gain and louder signal going to the phase inverter and power tubes.

Preamp volume to 10, with master volume to whatever is needed for loudness, is a completely different sound than setting the master volume to 10 and using the preamp knob as a volume control. It sounds much nicer as a clean and pedal amp with the master at 10, and the preamp set low. The higher the preamp is set, the murkier the sound becomes - and I think it's pretty unusable past 7. Also, setting the master to 10 and using the preamp knob as a volume control, playing into the low-gain input, is how Billy Corgan of Smashing Pumpkins used his on their earlier albums, like Siamese Dream. His clean tone is of the master-at-10 variety and not preamp-at-10.

What mod can be done to the low-gain input, to give it more volume while keeping it clean?

 

[mickeydg5]

On a JCM 800 2203 I've just bought, the low-gain input is incredibly quiet compared to the high-gain input. With the master control at 10 and using the preamp control as a volume control, to keep the amp nice and clean, I can play pretty comfortably with the preamp control at 1 - 5. And this contrasts with what seems to be people's normal experience with the low-gain input. If I plug into the high-gain input, with the master at 10 and the preamp at 0.5, the sound is devastatingly loud. To get the same volume from the low-gain input, with the master still at 10, I need to turn up the preamp control to 8 - and then the amp is no longer clear-sounding, but is thick and dark-sounding, being coloured by the preamp control, which I don't want.


Here's a summary from another thread of people's reports about their own JCM 800s:

Swap V1 to see what happens to start but.......................
Your preamp amplifier tube has two sides. The gain of each side can be different, sometimes vastly different. The lower gain side could be on your low-gain INPUT side. It is possible

For example your preamp tube could have measurements of 95% on one triode and 75% on the other. If that lower measurement correlates with your amplifier low-gain side then guess what?

 

[Dr Frost]

None of the powertubes has any affect on the preamp, pointing out the obvious maybe.
But they are not involved until the end of the show.

And no matter what whom did on any album, all answers here are right, they are supposed to be just like this, it is the way they were designed.

You seem to be a case of wrong owner of the wrong amp, to be frank.

 

[Jon Snell]

Low gain input means what it says on the label; Low Gain.
High Gain is the opposite ... I have a feeling you should already know that.

 

[Pete Farrington]

The problem may be due to not having a conceptual grasp of what ‘gain’ actually is?

 

[aikiguy]

In my mind….. Gain is the amplification of the signal from the pickup, where volume is the overall “how loud I’m going to turn up this amplified waveform from the pickup”.

The low gain input has one less gain stage apparently (the half of a 12ax7 mentioned above) and therefore the resulting wave is less than if the same waveform were to see the final gain stage (which would amplify it even more).

Because the low gain waveform is smaller (quieter), the master can only turn it up so much. Putting the master to 10 would make this smaller waveform as loud as it can be.

if you put the high gain waveform into the master at 10, the high gain waveform is immediately louder because of the extra gain stage causing the waveform to be bigger. If you looked at on a scope, you would be able to see the difference.

I have an en electrical background, so this makes total sense to me. I apologize if it doesn’t help you out, but I hope that it does….

Gain = input volume
Master= output volume

Lower gain has less input volume.

If each input had their own master volume, you could match them more easily.

Uncle Doug on YouTube has some really great explanations on how gain stages work.

Cheers.

 

[Pete Farrington]

With regard to the amp’s front panel controls, ‘gain’ and ‘volume’ are just labels, both controls are basic pots (variable potential divider) that are used to adjust the signal level that passes through to the next chunk of the amp’s signal path.
That’s another way of saying they’re both gain controls.
Note that many Peavey amp front panels labelled them ‘pre gain’ and ‘post gain’, which may help people to get their head around this.

https://assets.peavey.com/literature/manuals/80300095.pdf

 

[Gunner64]

What would account for the low- gain input having vastly lower volume than the high- gain input on a JCM 800?...long story short that's the way it was designed.

The inputs are actually labeled high and low sensitivity.

 

[paul-e-mann]

On a JCM 800 2203 I've just bought, the low-gain input is incredibly quiet compared to the high-gain input. With the master control at 10 and using the preamp control as a volume control, to keep the amp nice and clean, I can play pretty comfortably with the preamp control at 1 - 5. And this contrasts with what seems to be people's normal experience with the low-gain input. If I plug into the high-gain input, with the master at 10 and the preamp at 0.5, the sound is devastatingly loud. To get the same volume from the low-gain input, with the master still at 10, I need to turn up the preamp control to 8 - and then the amp is no longer clear-sounding, but is thick and dark-sounding, being coloured by the preamp control, which I don't want.


Here's a summary from another thread of people's reports about their own JCM 800s:

It doesn't matter, its not a channel switcher, you dial in your tone and volume whichever input youre in.

 

[Delicieuxz]

You seem to be a case of wrong owner of the wrong amp, to be frank.

Definitely not. I own many amps and the JCM 800 sound is exactly what I'm wanting here, and I'm loving it. I'm just questioning the volume I'm getting it at, at a certain setting - after other JCM 800 owners, including my friend who uses their JCM 800 the same way I use mine, told me there should be a lot more volume at the setting I'm using.

The problem may be due to not having a conceptual grasp of what ‘gain’ actually is?

While not being a tech or having deep electrical understanding, I do understand the concept of gain, and so also get what some people are saying when they suggest that, while there differences in volume between the channels is expected, the volume between the low and high gain channels shouldn't change as much as the cleanness / distortion of them.

It doesn't matter, its not a channel switcher, you dial in your tone and volume whichever input youre in.

The amount to which the preamp control is dialed up greatly shapes the tone. So, those two goals (tone and volume) can be at odds with each other. Still, I think I can get a lot of volume from the amp. Just that, to get it to a screaming level, the preamp colours the sound more than I was expecting to need to let it.


Thanks for all the feedback, it's helping to know how the amp isn't performing outside of theoretical expectations.

 

[paul-e-mann]

Definitely not. I own many amps and the JCM 800 sound is exactly what I'm wanting here, and I'm loving it. I'm just questioning the volume I'm getting it at, at a certain setting - after other JCM 800 owners, including my friend who uses their JCM 800 the same way I use mine, told me there should be a lot more volume at the setting I'm using.


While not being a tech or having deep electrical understanding, I do understand the concept of gain, and so also get what some people are saying when they suggest that, while there differences in volume between the channels is expected, the volume between the low and high gain channels shouldn't change as much as the cleanness / distortion of them.


The amount to which the preamp control is dialed up greatly shapes the tone. So, those two goals (tone and volume) can be at odds with each other. Still, I think I can get a lot of volume from the amp. Just that, to get it to a screaming level, the preamp colours the sound more than I was expecting to need to let it.


Thanks for all the feedback, it's helping to know how the amp isn't performing outside of theoretical expectations.

Have you taken it to a good tech just to be sure there isnt anything wrong?

 

[myersbw]

Ok, to the OP's original question... there's no single reason for the vast difference you hear. But, there are two I can readily see. Mickydg5 relayed the first possibility...i.e. that stage two is significantly weaker that the first stage. As suggested...change it for a fresh tube and note the difference. After all, the gain level is fixed from stage one and it's just amplifying the very small guitar signal. Stage two is getting slammed continually from stage one with a now much larger signal, pending where the preamp volume is set.

Now, the second possibility is not so common, but still a possibility. Some folks that do home-brew amp maintenance use DeOxit on their pots and actually know to squirt it into the opening where the three lugs enter (or the backside hole if not easily accessible). However, some think,"Ok, I'll give the jacks a squirt to clean those, too!" That can be a problem. If you squirt it directly on the visible contacts of the Cliff jacks and use DeOxit D5, you might get away with it. If you use the concentration of D100, it's not all going to dry up completely.

The switch contact levers rise up when a plug is inserted. And, if enough excessive DeOxit solution is around the shorting contact area from overspray, it can actually bead up and keep a bit of conductivity in the switching circuit that should now be open. That means you still have a resistive path to ground that's sucking the input down when plugged into the LOW jack.

To test and see if this (or the jack) is at fault, plug your guitar into the LOW jack and set it to a low volume setting. Then, changing nothing else, insert a dummy jack (or one end of a patch cable) into the HIGH jack. If the volume increases at all from only that...investigate and properly clean or replace the LOW jack. That's a fast check for an unusual circumstance. I'm not saying you or anyone did that...I'm just giving you a possible "why" reason like you asked for.

If that test proves nothing, like mickeydg5 said...replace V1. Like others have said, that stage is ALWAYS much lower gain than the first because they're cascaded stages and the first stage (HIGH) is disconnected and you're bypassing that to insert the guitar directly into the second stage...AND...you fight with the voltage divider/volume control sucking your pickup signal down.

To avoid the first issue...never ever spray DeOxit directly into the jacks. Now, if you want to do a partial clean of jacks and not comfortable with pulling the amp chassis, spray DeOxit on a 1/4 plug and wipe off any excess...plug in a few times and slightly twist it. Might solve any jack tip/sleeve contact issues.

I've been inside a fair amount of 800's 2203 and 2204 and have never been impressed with the LOW input results for my style of playing...all depends on "how" you choose to use it. It's a one channel amp with a not so great LOW option for most that gravitate to those amps. YMMV!

 

[mickeydg5]

Ok, to the OP's original question... there's no single reason for the vast difference you hear. But, there are two I can readily see. Mickydg5 relayed the first possibility...i.e. that stage two is significantly weaker that the first stage. As suggested...change it for a fresh tube and note the difference. After all, the gain level is fixed from stage one and it's just amplifying the very small guitar signal. Stage two is getting slammed continually from stage one with a now much larger signal, pending where the preamp volume is set.

Now, the second possibility is not so common, but still a possibility. Some folks that do home-brew amp maintenance use DeOxit on their pots and actually know to squirt it into the opening where the three lugs enter (or the backside hole if not easily accessible). However, some think,"Ok, I'll give the jacks a squirt to clean those, too!" That can be a problem. If you squirt it directly on the visible contacts of the Cliff jacks and use DeOxit D5, you might get away with it. If you use the concentration of D100, it's not all going to dry up completely.

The switch contact levers rise up when a plug is inserted. And, if enough excessive DeOxit solution is around the shorting contact area from overspray, it can actually bead up and keep a bit of conductivity in the switching circuit that should now be open. That means you still have a resistive path to ground that's sucking the input down when plugged into the LOW jack.

To test and see if this (or the jack) is at fault, plug your guitar into the LOW jack and set it to a low volume setting. Then, changing nothing else, insert a dummy jack (or one end of a patch cable) into the HIGH jack. If the volume increases at all from only that...investigate and properly clean or replace the LOW jack. That's a fast check for an unusual circumstance. I'm not saying you or anyone did that...I'm just giving you a possible "why" reason like you asked for.

If that test proves nothing, like mickeydg5 said...replace V1. Like others have said, that stage is ALWAYS much lower gain than the first because they're cascaded stages and the first stage (HIGH) is disconnected and you're bypassing that to insert the guitar directly into the second stage...AND...you fight with the voltage divider/volume control sucking your pickup signal down.

To avoid the first issue...never ever spray DeOxit directly into the jacks. Now, if you want to do a partial clean of jacks and not comfortable with pulling the amp chassis, spray DeOxit on a 1/4 plug and wipe off any excess...plug in a few times and slightly twist it. Might solve any jack tip/sleeve contact issues.

I've been inside a fair amount of 800's 2203 and 2204 and have never been impressed with the LOW input results for my style of playing...all depends on "how" you choose to use it. It's a one channel amp with a not so great LOW option for most that gravitate to those amps. YMMV!

Checking and cleaning was next on the agenda.
That goes for tube pins and socket prongs as well.

 

[Delicieuxz]

Have you taken it to a good tech just to be sure there isnt anything wrong?

No. I've ordered new filter cans, bias caps, and some resistors to convert the EL34 amp to 6550. I've also ordered some spare jacks and pots. When all of those things arrive, I'll install the filter cans, bias caps, resistors, and then do the recommended testing with the jacks. If I still am not feeling right with the result, I'll perhaps install new jacks. If I still don't figure it out after that, or am not convinced it's just normal the way it is, then I'll likely take it to a tech.

Ok, to the OP's original question... there's no single reason for the vast difference you hear. But, there are two I can readily see. Mickydg5 relayed the first possibility...i.e. that stage two is significantly weaker that the first stage. As suggested...change it for a fresh tube and note the difference. After all, the gain level is fixed from stage one and it's just amplifying the very small guitar signal. Stage two is getting slammed continually from stage one with a now much larger signal, pending where the preamp volume is set.

Now, the second possibility is not so common, but still a possibility. Some folks that do home-brew amp maintenance use DeOxit on their pots and actually know to squirt it into the opening where the three lugs enter (or the backside hole if not easily accessible). However, some think,"Ok, I'll give the jacks a squirt to clean those, too!" That can be a problem. If you squirt it directly on the visible contacts of the Cliff jacks and use DeOxit D5, you might get away with it. If you use the concentration of D100, it's not all going to dry up completely.

The switch contact levers rise up when a plug is inserted. And, if enough excessive DeOxit solution is around the shorting contact area from overspray, it can actually bead up and keep a bit of conductivity in the switching circuit that should now be open. That means you still have a resistive path to ground that's sucking the input down when plugged into the LOW jack.

To test and see if this (or the jack) is at fault, plug your guitar into the LOW jack and set it to a low volume setting. Then, changing nothing else, insert a dummy jack (or one end of a patch cable) into the HIGH jack. If the volume increases at all from only that...investigate and properly clean or replace the LOW jack. That's a fast check for an unusual circumstance. I'm not saying you or anyone did that...I'm just giving you a possible "why" reason like you asked for.

If that test proves nothing, like mickeydg5 said...replace V1. Like others have said, that stage is ALWAYS much lower gain than the first because they're cascaded stages and the first stage (HIGH) is disconnected and you're bypassing that to insert the guitar directly into the second stage...AND...you fight with the voltage divider/volume control sucking your pickup signal down.

To avoid the first issue...never ever spray DeOxit directly into the jacks. Now, if you want to do a partial clean of jacks and not comfortable with pulling the amp chassis, spray DeOxit on a 1/4 plug and wipe off any excess...plug in a few times and slightly twist it. Might solve any jack tip/sleeve contact issues.

I've been inside a fair amount of 800's 2203 and 2204 and have never been impressed with the LOW input results for my style of playing...all depends on "how" you choose to use it. It's a one channel amp with a not so great LOW option for most that gravitate to those amps. YMMV!

Thanks for that information! I look forward to trying that out.

I just had delivered to me some Doxit D5 that I was going to try in the jacks. Maybe I'll do the twisting-plug thing.

Swap V1 to see what happens to start but.......................
Your preamp amplifier tube has two sides. The gain of each side can be different, sometimes vastly different. The lower gain side could be on your low-gain INPUT side. It is possible

For example your preamp tube could have measurements of 95% on one triode and 75% on the other. If that lower measurement correlates with your amplifier low-gain side then guess what?

Thanks for that information, too. I tried that, but didn't hear a difference.

 

[mickeydg5]

You would have to know the strength of each diode in the preamp tube to know if you are using an equal, higher or lower value in the low input side.
That requires a decent transconductance tester providing values for each side.

 

[myersbw]

Checking and cleaning was next on the agenda.
That goes for tube pins and socket prongs as well.

Dang it! I forgot the pins & sockets. HOWEVER, if he doesn't already know this...it ought to go to a tech! (I just like to get it all over with in one post, if possible!)