Bogner Ecstasy Blue channel to JCM800 Mod

[psychepool]

It may be a bit funny,
I want to put a mode that can use the blue channel of my DIY Bogner Ecstasy as JCM800.
Usually, I've seen a lot of cases where the JCM800 is modded with other Hot Rod amps, but I don't think I've seen many attempts to rollback a Hot Rod amp to the JCM800.

In fact, I already made a mode like that. As you know, both amplifiers have almost identical circuitry.

This is the full circuit diagram of my amp,

01_whole_schematic.jpg

Here, if only the blue channel preamplifier circuit is separated and organized, it is like this.

02_blue_schematic.jpg

And this is the JCM800 2204 preamplifier circuit.

03_2204.jpg

Bogner Ecstasy has a switch called Structure.
This mode adjusts the amount and texture of distortion by adjusting the cathode value of the second stage of the blue channel.
Originally, it was possible to select two modes like this,

04_structure_origin.jpg

I configured it like this so that 3 modes can be selected.

05_structure_mine.jpg

As you can see by comparing the circuit of the JCM800, the 10K cathode resistance mode in the center is the mode configured to produce the same sound as the JCM800.
However, the powerful distortion unique to the JCM800 does not come out.
Of course, it is true that it takes less distortion than the value of the original blue channel, but it is a weak and weak sound that is difficult to see as a JCM800.

I thought it was almost the same as the JCM800 in terms of circuit, but it is still judged that there are differences due to some other parts.
What part is causing the decisive difference?


In the JCM800 circuit diagram below, I wrote the value of Bonger Blue in blue in the part where there is a difference compared to the Bogner Ecstasy Blue channel.

06_compare to xtc.jpg

Crucially, what part is causing the gain to be much weaker than expected?
I'm trying to check each one, so please advise.

1. Coupling capacitor values of the 1st and 2nd stages
Like other modern high-gain amps, the Bogner seems to have drastically lowered the value of the coupling capacitors for low-end tightness.
I had previous experience making a Matchillis DC30 EF86 channel, and it had the ability to adjust the low end by changing the coupling cap via a 6-position rotary switch.
As the value is increased, the low frequency also increases, but as the signal being cut out decreases, the amount of distortion applied varies considerably.
Would it make a big difference if I changed these 4.7n and 2.2n capacitors to 22n of JCM800 respectively?

2. Gain Pot Capacity
JCM800 uses a 1M pot and Ecstasy uses a 500K pot.
I don't know exactly what the relationship between pot value and sound is, but considering the concept of band pass, I expect that the signal will become brighter and louder as the capacity increases. What do you think?

3. The order of gain pots and filters in the 1st stage
The JCM800 has a 470K/470pF filter in front of the gain pot and the Ecstasy is placed behind the gain pot.
I don't think this will make any special difference. What do you think?

4. Cathode value of the 3rd stage
This seems to be difficult to see as a difference because the value of the JCM800 is also found to be slightly different for each circuit, but I still want to ask what you think.

5. Capacitor in parallel with the plate resistor of the second stage
At first glance, I heard that it plays a role in properly reducing noise, but is there a possibility that this will make a decisive sound difference?


I guess it will be roughly like this.
One difference that hasn't been mentioned is that the power tube is a 6L6, but will this make a big difference?
Among the factors mentioned, the gain pot is actually a difficult part to apply as a mod point.
Personally, it is expected that the value of the coupling capacitor will have a considerable effect.

Many opinions please.

[Lynxtrap]

The additional 500K/500K voltage divider right after the 500K gain pot is probably the main reason your amp has less gain than a Marshall.

[maxkracht]

2. Gain Pot Capacity
JCM800 uses a 1M pot and Ecstasy uses a 500K pot.
I don't know exactly what the relationship between pot value and sound is, but considering the concept of band pass, I expect that the signal will become brighter and louder as the capacity increases. What do you think?

Think of volume controls like grid leak resistors. The bigger the value, the less signal goes to ground at minimum rotation.

As Lynxtrap says, that voltage divider is also shunting signal to ground.

Everything else you mentioned will change how things sound. Plate bypass cap will reduce noise, but it is reducing noise by cutting highs, so it might be really subtle with 500pf, but it will get pretty noticeable as that cap gets bigger.

[psychepool]

I changed the 1st and 2nd coupling capacitors to 22n through the toggle switch.
Many changes have taken place. As expected, there was an increase in the low end and an increase in gain.
Although the amount of gain itself is a very slight increase, the increase in the low end dramatically improves the existing thin and meager sound.
It sounded pretty vintage Marshall-like, grungy and howling.
In fact, I'm quite satisfied with this alone, to the point where I feel like I don't need any additional work.
The lack of gain could be supplemented with the OD pedal.
At Bogner's small coupling capacitor value, excessive boosting by the OD pedal made the sound muddy, but the 22n capacitor secured a wide headroom, enabling boosting with the OD pedal without munging.
In the meantime, it was difficult to understand pedals like the MXR Dist+, but I think it would sound very cool when used here.

The additional 500K/500K voltage divider right after the 500K gain pot is probably the main reason your amp has less gain than a Marshall.

However, the amount of gain still feels a bit lacking. Although the JCM800 is originally an amp with significantly less gain than modern amps, nevertheless it's definitely less.
I'm interested in your answer because I think it would be perfect if the gain amount was increased just a little bit more.

Can you elaborate on the 500K/500K pot you are talking about?
Is this about where it is mounted? Or is it about value?

Since moving the location is a structural change, it is difficult to change through a switch, so I will not to try it.
If the value is to be changed, I am thinking about responding through an additional switch.
Basically, this amplifier is Bogner Ecstasy, not the JCM800, so I hope it will be an attempt in a line that does not harm the structure of Ecstasy.

Please explain more about the 500K/500K Voltage divider.

[psychepool]

Think of volume controls like grid leak resistors. The bigger the value, the less signal goes to ground at minimum rotation.

As Lynxtrap says, that voltage divider is also shunting signal to ground.

Everything else you mentioned will change how things sound. Plate bypass cap will reduce noise, but it is reducing noise by cutting highs, so it might be really subtle with 500pf, but it will get pretty noticeable as that cap gets bigger.

I understand what you mean by increasing the value of the gain pot has a definite effect on the amount of gain.
Just my guess, but I suspect the high frequencies will also increase.

Unfortunately, changing pots to switching is difficult in many ways, so it's hard to try.

Instead, can we expect the same effect as increasing the gain pot value by increasing the grid leak value of the 2nd stage?
Looking at the circuit from another point of view, it seems that the chain of 500K grid leak and gain pot largely constitutes one grid leak resistor.
Therefore, as a result, I wondered if similar results could be obtained by simply configuring a 1M grid leak at the maximum gain pot setting like the JCM800, so I asked a question.

[Lynxtrap]

Can you elaborate on the 500K/500K pot you are talking about?
Is this about where it is mounted? Or is it about value?

It's not a pot, it is in your Bogner schematic right after the gain pot. You have a 500K resistor in series with the signal, followed by a 500K resistor to ground. The two resistors form a voltage divider dumping 50% of the signal to ground.

[psychepool]

It's not a pot, it is in your Bogner schematic right after the gain pot. You have a 500K resistor in series with the signal, followed by a 500K resistor to ground. The two resistors form a voltage divider dumping 50% of the signal to ground.

Sorry. "pot" is just a typo.

So, how can improving this part help get closer to the JCM800? Of course, it would be impossible to make it the same as the JCM800, but since I got satisfactory voicing through the current 22n coupling cap mode, it would be nice to add a little gain and high frequency compensation. Change the 500K grid leak to the ground to 1M Will there be any significant improvement?

[maxkracht]

Unfortunately, changing pots to switching is difficult in many ways, so it's hard to try.

Take a 1m pot, add a 1m resistor across it, you essentially have a 500k pot. Probably more useful to just have one or the other so the taper doesn't get weird, but you can play around. Increasing gain can also uncover more problems, so might not be worth the trouble if you are already happy with the amp.

[Lynxtrap]

Sorry. "pot" is just a typo.

So, how can improving this part help get closer to the JCM800? Of course, it would be impossible to make it the same as the JCM800, but since I got satisfactory voicing through the current 22n coupling cap mode, it would be nice to add a little gain and high frequency compensation. Change the 500K grid leak to the ground to 1M Will there be any significant improvement?

You could approach the same gain levels as the Marshall with 500K/1M (or 250K/500K) if you use a 1M gain pot.
Or just wire it as the Marshall if possible. Third option is to just leave out the two resistors and use the 500K gain pot.

[psychepool]

As shown in the figure below, the grid leak of the 2nd stage was changed to 500K and 1M through the switch.
However, there was no change in sound at all.
I thought there would be a switch pop noise or something, but there wasn't even that.
It's probably because the resistor value is already connected to ground through the pot.

gridleak.jpg

I continued to compare the circuits and realized that there were some differences between the two amplifiers that I had missed.
I just found the treble peak of 470pF located between the second and third stages.
It is present in the JCM800 but omitted in the Bogner.

treblepeak.jpg

When I installed this with the switch, there was a significant difference.
Like the coupling caps, the increase in gain wasn't huge, but the sound was quite different.
It's not that the tone shape is vastly different, but the nuance has changed a lot.Until now, I felt that there was no noisy sparkling that is unique to the JCM800, so it was quiet and dry, but the noise and vitality unique to the vintage hard rock amp came to life.


As a result of testing by changing some parts so far, I realized the obvious fact that there is no way to increase the amount of gain other than modifying the bias of the tube itself.
The tone is reduced due to filter or attenuator, etc., affecting the amount of gain, but it seems to be a very subtle part.


Anyway, I was able to get the sound of the desired direction through coupling cap switching and treble peak switching.
I don't currently own the JCM800, so I don't know how close it is to it, but at least I think I got the feel I was aiming for in a satisfying enough way.
Thanks to everyone who responded.


In addition, I tried to change the configuration of the treble bleed cap (bright cap) of the gain pot.
Previously, it was a configuration of 470pF - off - 4.7n,
The configuration was changed to 100pF - 1n - 4.7n through the on on on switch.

The existing configuration lacked the sparkle of low gain in off mode, resulting in a frustrating sound, and 470pF sounded unsatisfactory in both low and high gain.
The current configuration feels quite appropriate to me.
The 100pF felt like having a JTM45, and the 1n gives the sparkle at low gain, and less excessive bites at high gain than the 4.7n, providing another option with the 4.7n.

[maxkracht]

As a result of testing by changing some parts so far, I realized the obvious fact that there is no way to increase the amount of gain other than modifying the bias of the tube itself.

What do you mean by "gain"? Yes, to change the voltage gain from a tube, you must change the power supply and/or how it is biased, but there is a lot you can do to change the amount of signal that gets to the next stage and how much of what frequencies are allowed through. If you mean gain as more signal getting through to the next stage, your gain knob adds gain and decreasing the amount of signal going to ground by changing the value of the gain control and/or voltage divider after it will add gain. You can make a clean sounding amp with many gain stages that are biased pretty hot if you cut enough signal between each stage.

[Lynxtrap]

You still need to change the 500K gain pot to 1M (with the grid leak at 1M) if you want it to have about the same amount of available gain as the Marshall.

Are you familiar with the concept of voltage dividers?

[psychepool]

What do you mean by "gain"? Yes, to change the voltage gain from a tube, you must change the power supply and/or how it is biased, but there is a lot you can do to change the amount of signal that gets to the next stage and how much of what frequencies are allowed through. If you mean gain as more signal getting through to the next stage, your gain knob adds gain and decreasing the amount of signal going to ground by changing the value of the gain control and/or voltage divider after it will add gain. You can make a clean sounding amp with many gain stages that are biased pretty hot if you cut enough signal between each stage.

I'm not an engineer, I've never majored in electricity, and I'm just doing DIY as a hobby, so there may be some immaturity in using the correct conceptual words. I ask for your understanding.
You can think of me as used the word "Gain" to mean the role that "Gain Knob" plays.
I talked about increasing the signal and the amount of clipping (or distortion) it causes.

The signal is connected to the next stage while changing the tone shape with a coupling capacitor or RC filter. By changing this, there can be a concept of how much the signal is "dropped less", but even if the amount of signal passing increases, the size of the signal I said that the amount of distortion doesn't seem to increase dramatically.
Of course, this may also be expressed incorrectly due to lack of knowledge and misunderstanding.

[psychepool]

You still need to change the 500K gain pot to 1M (with the grid leak at 1M) if you want it to have about the same amount of available gain as the Marshall.

Are you familiar with the concept of voltage dividers?

I only know very superficially about voltage dividers. I have never looked deeply into fundamental knowledge and tried to understand it. It is only a level of simple modification with the knowledge that has been produced through open and verified circuits and vaguely known.

Replacing the gainpot is usually easy to work, but there are two reasons I still haven't tried it.

First, because I built this amp into an extremely small chassis, I had to layer the parts in order to fit all the features I wanted.
This means that in order to remove a deeply placed component, all of the components around it must also be removed.
The condition around the gain pot of this amp is like this

01_gainpot.jpg

In fact, I had a lot of trouble removing the bright switch next to the gain pot and reinstalling it with a new combination.
However, the bright switch is very small compared to the gain pot.
It's not easy to decide to remove the 24mm long shaft's gain pot and replace it with a new one in this space.

Second, this amp was originally a Bogner Ecstasy, not a JCM800.
Bogner Ecstasy is marked to use a 500K pot on the circuit.
Expecting the sound of the JCM800 from this amp is a kind of 'mode', because this amp doesn't want the sound of the JCM800 to be the focus.
So it is reluctant to change the pot, which is not easy to switch through a switch.
Of course, a 1M pot might suit the Bogner Ecstasy better than you think. It is expected to brighten slightly and increase the gain.

If the pod could be easily replaced, of course I would have tried it. I am also very curious what the outcome will be.


One curious thing is, the thread above said that I tried doing something like this and nothing happened.

02_grid leak.jpg

In fact, there was an option where the grid leak was completely removed because it was an on-off-on toggle rather than an on-on toggle, but this also did not change anything.
But in fact, in the Bogner circuit, a resistor and a cap that play the same role are connected to a switch called "boost" from the grid to the ground at exactly this location.
In fact, it is a concept of "Unboost" as the signal decreases when the connection is established.

03_boost.jpg

This "Unboost" function is working normally, but I wonder why the 500K Grid Leak at the same location did not change the signal even after changing or removing the value.
I checked the switch but it was normal.
Why was there no change at all when the Grid Leak value was changed?


Anyway, let's get back to the main topic. Could you please explain the concept of the voltage divider you mentioned?

[Lynxtrap]

This "Unboost" function is working normally, but I wonder why the 500K Grid Leak at the same location did not change the signal even after changing or removing the value.
I checked the switch but it was normal.
Why was there no change at all when the Grid Leak value was changed?

Anyway, let's get back to the main topic. Could you please explain the concept of the voltage divider you mentioned?

If changing the grid leak did not affect the signal/gain, something is not right, either in the schematic or in the circuit. It should have the same effect as turning up the gain pot.

In fact, a (volume) potentiometer is a variable voltage divider. It's quite important to understand voltage dividers when designing and modding amps. Instead of me explaining it, you might just as well google it.

The math is straightforward, but there are online calculators. https://www.allaboutcircuits.com/tools/ ... alculator/