I'm planning to put a 5F6A, JTM45, and Super Lead Plexi in one box. I have a few questions.

[psychepool]

Hello.
I built a one-channel Deluxe Reverb a long time ago.
https://ampgarage.com/forum/viewtopic.php?f=6&t=37895
It's the Fender-style cabinet amp near the bottom of this thread.

I built it to be a platform for effects pedals, with a clear, standard clean tone. But, I rarely use effects pedals these days.
So, I'm losing interest in this amp, and I'm thinking of modifying it into a Bassman instead.

The Blackface is not bad when cranked up, but the Bassman is much closer to the Holy Grail in this regard.

While this is a natural thought for DIY enthusiasts, I'm starting to think, "Since I'm building a Bassman, why not make it usable with a JTM45? And with a few more features, it could also be used with an SLP." So, I have a few questions.

First, let me clarify: This amp is built with a Hammond 1760H (Deluxe Reverb replacement) and a 6V6 pair, diode-rectified, and the power transformer is also designed for that purpose.
I'll be using them as is.

1. Looking at the preamp alone, the 5F6A and JTM45 appear to be identical except for the 500pF treble cap on the 270K mix resistor of Bright channel.
However, listening to various sound samples, the two amps sound quite different.
Seeing that the 500pF cap is rarely mentioned in discussions of the differences between the two circuits online, it seems like the difference isn't that significant.
So, what is the crucial sound difference between these two amps?
The V1 tube? The output tubes? The OT? The speaker cabinet? The NFB?
I'll be using the same power amp and the cabinet. Then, will the V1 tube and 500pF treble cap be enough to fully express the character of the two amplifiers?

2. The 5F6A uses a 12AY7 (or 5751?) in V1, while the JTM45 uses a 12AX7 in V1.
This seems like a significant difference, but unfortunately, there's no way to switch between tubes, so I'll have to use a specific tube permanently installed.
Is there a way to achieve the nuances of a 12AY7 while using a 12AX7? Adjusting the cathode resistor value? Local feedback?
Or, if I just use a 12AX7 in V1, will it still sound like the 5F6A without any major issues?

3. I have a question about switching to Superlead Plexi mode. In terms of the preamp, the most noticeable differences between the JTM/Bassman and the SLP seem to be the V1's split cathode, the Bright channel coupling cap (22n to 2.2n), the mix resistor (270K to 470K), and the tone stack (56K, 250pF / 33K, 470pF).
3-1. The V1 cathode will likely be implemented using the same design as Ceriatone's All Access series.
https://ceriatone.com/wp-content/upload ... e-2023.pdf
The implementation itself doesn't seem to be a problem, but will it cause problems? I'm concerned because the cathode is completely separated from the contact during switching. I'm willing to tolerate some popping noise, but I'm concerned that it could cause more serious problems.
3-2. The mix resistors for both channels are set at 270kΩ for the 5F6A/JTM45 (the JTM45 has a 470pF cap on the bright channel), and 470kΩ for the SLP.
While this change will undoubtedly have an impact, I'm curious as to whether it will make that much of a difference.
I'm hoping to minimize the switching portion as much as possible.

4. I have a question about the global NFB resistor.
These three amplifiers have different NFB resistor values ​​and different OT taps for signal extraction.
Ultimately, I'll need to select one tap to draw the signal, and depending on which tap, I'll likely need to apply a resistor value different from the one shown in the original schematic.
Are there any reference materials?

5. Since I have a reverb tank and reverb transformer, I'm planning to add a tube driven reverb. https://el34world.com/Forum/index.php?topic=17354.0
Would connecting a suitable circuit like the one on this page with a 100K mixing resistor be safe?
I previously built a Dumble and it worked fine with a 100K resistor, but I'm not sure if this amp will be okay since it's a non-master volume amp.


That's all.
Actually, since it is a simple amplifier, it seems like the basic physical aspects such as the transformer and output tubes will be important, but since I am starting this project by using a 6V6 and 20W transformer, such aspects are basically ignored, so I am not sure if it is very meaningful to consider these various aspects. However, if you could answer, it would be very helpful for me when building it.

[Stevem]

If your not listening to sound clips of both amps be played thru the same speaker(s) then that is a very flawed way of judging the sound of each amp, and of course if it's not you playing the amps then the feel of each amp is lost .

[martin manning]

JTM 45 copied the Bassman’s negative feedback network but it was connected to an 8 ohm OT secondary. Consequently the feedback level in the JTM 45 was 2x higher. You could cut the NFB resistor in half to simulate that.

The lower gain aspect of a 12AY7 vs. 12AX7 could be achieved by going to a split load on the plate, something like two 51k in series, and taking the output from the junction for 12AY7 mode.

[Snicksound]

The biggest differences between these amps are in the power amp and the cabinet they are played through. A JTM45 is mostly a 5F6A using a closed back 412 instead of an open 410, that and the different NFB (they copied so closely they kept the 27k NFB resistor even though they were using a 16ohm output instead of a 4ohm, meaning twice the voltage swing)

The change from 12AY7 to 12AX7 mostly means it overdrives faster, but sound isn't that different. Don't know where you saw a 500pF cap on the mixing resistor, that's not typical of an original JTM45 but they did make revisions along the years

You can definitely switch NFB levels, you need to put a cap in line so it doesn't pop. Problem is it really impacts the volume so it's not a subtle shift (less NFB = more power amp gain)

Superlead goes to lower NFB (100k resistor from 8 ohm tap), but it also uses diode rectification and higher filtering. The split cathode is easy to implement, I've done it. You do have to choose between the 270k or the 470k mixing resistors, but the switch CAN engage the bright cap over the related one in Superlead mode.

My opinion? I've done this too, called it the "Ultimate Plexi" and it did all the variations. It's a fun project, but it's a complex one, it requires a lot of time. So many switches to wire, so much to go wrong or get unpredictable squeals and what not.

You play around with the switches at first, thinking this versatility is the bee's knees and why doesn't everybody do this? But after a while... I realized the good old Superlead specs was it for me. Hardwired to that and didn't look back.

Here's the schematic I could find (it went through many iterations, this was probably not the final one). You'll notice it also allowed cascaded mode and has a master volume and an FX Loop. The tube rectifier was not switchable, but I eventually took it out and replace with diodes and upped the filtering (sounded better for me). Oh, it was cathode biased with 6V6 so I could actually run it all out on stage without killing small animals.

[Roe]

There are too many compromises with all circuits in one box. the transformers, voltages, and filtering differ greatly and cannot be switched. Get 10+ plexis like me instead

[Snicksound]

Oh right. Tone stack is also different on Superlead (500p/33k), which mostly means more mids.

It's possible to put this on a switch, again I've done it, but there will be high voltage on that switch (you can always put a massive cap ahead of it). Nowadays I'd rather just use a midway slope resistor (39-47k) as it doesn't make that much of a difference (nothing you can't compensate), however treble cap matters. You can put a 470pF, then a 2nd one in series that a switch can bypass (similar to Dumble mid boost switch), that gives you both values without dealing with switching DC.

[Snicksound]

The Ceriatone JTM46 all access might interest you as well. It does almost everything (only the mixing resistors are not switchable)

https://ceriatone.com/wp-content/upload ... e-2023.pdf

[Helmholtz]

JTM 45 copied the Bassman’s negative feedback network but it was connected to an 8 ohm OT secondary. Consequently the feedback level in the JTM 45 was 2x higher. You could cut the NFB resistor in half to simulate that.

The JTM45 had the NFB connected to the 16 Ohm scecondary, so the difference is sqrt (16/2) = 2.83.

[martin manning]

You’re correct. After posting that I looked up an early schematic, started to edit, but got distracted :^)

[Roe]

marshall used 100k @ 4 ohms and 27k @ 16 ohms. To cover all, use the latter but add a 220k pot in series,

[statorvane]

Having built a 5F6-A and simply replacing the 12AY7 with a 12AX7, I found the difference to be huge. I eventually morphed the 5F6-A into a JTM-45, then a 1987X. Also changed power tubes, negative feedback and output transformer along the way.

I do miss the 5F6-A ...

[psychepool]

If your not listening to sound clips of both amps be played thru the same speaker(s) then that is a very flawed way of judging the sound of each amp, and of course if it's not you playing the amps then the feel of each amp is lost .

I've never tried swapping out actual cabinets on the fly, but when I tried switching cabinet simulations with Torpedo, I noticed a completely different sound, even from the same amplifier. I believe the same thing applies in real life. When analyzing specific amplifiers, I usually look at the circuitry of the amplifier itself to compare its structure and component values. However, these circuits are so similar that it's difficult to understand the differences based on the circuitry alone.

JTM 45 copied the Bassman’s negative feedback network but it was connected to an 8 ohm OT secondary. Consequently the feedback level in the JTM 45 was 2x higher. You could cut the NFB resistor in half to simulate that.

The lower gain aspect of a 12AY7 vs. 12AX7 could be achieved by going to a split load on the plate, something like two 51k in series, and taking the output from the junction for 12AY7 mode.

My output transformer doesn't have a 2-ohm resistor, so I'm planning to set it to 8 ohms. Would a Bassman lower than 27kΩ be acceptable? Is there a formula for calculating the relationship between output impedance and resistor value?
Also, I'll likely be using small toggles for signal lines. Would switching high-voltage components like plate resistors be safe?
Or, instead of switching the plate resistor, what about switching the cathode resistor?
I'll probably just use a 12AX7 in V1 and run it without any modifications.

The biggest differences between these amps are in the power amp and the cabinet they are played through. A JTM45 is mostly a 5F6A using a closed back 412 instead of an open 410, that and the different NFB (they copied so closely they kept the 27k NFB resistor even though they were using a 16ohm output instead of a 4ohm, meaning twice the voltage swing)
The change from 12AY7 to 12AX7 mostly means it overdrives faster, but sound isn't that different. Don't know where you saw a 500pF cap on the mixing resistor, that's not typical of an original JTM45 but they did make revisions along the years
You can definitely switch NFB levels, you need to put a cap in line so it doesn't pop. Problem is it really impacts the volume so it's not a subtle shift (less NFB = more power amp gain)
Superlead goes to lower NFB (100k resistor from 8 ohm tap), but it also uses diode rectification and higher filtering. The split cathode is easy to implement, I've done it. You do have to choose between the 270k or the 470k mixing resistors, but the switch CAN engage the bright cap over the related one in Superlead mode.
My opinion? I've done this too, called it the "Ultimate Plexi" and it did all the variations. It's a fun project, but it's a complex one, it requires a lot of time. So many switches to wire, so much to go wrong or get unpredictable squeals and what not.
You play around with the switches at first, thinking this versatility is the bee's knees and why doesn't everybody do this? But after a while... I realized the good old Superlead specs was it for me. Hardwired to that and didn't look back.
Here's the schematic I could find (it went through many iterations, this was probably not the final one). You'll notice it also allowed cascaded mode and has a master volume and an FX Loop. The tube rectifier was not switchable, but I eventually took it out and replace with diodes and upped the filtering (sounded better for me). Oh, it was cathode biased with 6V6 so I could actually run it all out on stage without killing small animals.

Most of the JTM45 circuits and layouts I referenced had a 500pF resistor in that location. The mixing resistor refers to the 270K resistor (470K in SLP) connected from pin 2 of each channel's volume pot to the V2 grid.
I experienced the effect of adjusting the NFB with a pot while building the Bogner Ecstasy. It significantly changed the overall character of the amplifier. Higher resistance (lower NFB) results in a harsher, more aggressive sound with a duller presence pot. Conversely, the opposite results in a much smoother, more compressed sound with a much more sensitive presence pot.
This time, I don't plan on adding a separate pot. Instead, I plan to adjust the values ​​for the three amplifiers with a toggle switch. For now, I'm planning to set a fixed center value and connect resistors in parallel on the left and right sides.

I also tend to add a lot of options whenever I build something. After building and using it for a long time, you can distinguish between what you use frequently and what you don't. However, at the time of building, it's not easy to determine and select which of the numerous possibilities will be most useful to you.
The ideal result would be to apply a mode that closely mirrors the circuitry of the three amplifiers, either through a rotary or toggle three-way switch. However, the many points of differences between the SLP and the JTM/5F6A make this challenging.

Thank you for sharing the circuit. I'll definitely refer to it. It looks like a lot of useful information.

There are too many compromises with all circuits in one box. the transformers, voltages, and filtering differ greatly and cannot be switched. Get 10+ plexis like me instead

That's a natural thing to say, and I'd love to do it too. However, for many reasons, making that choice would be difficult for most peoples.

Oh right. Tone stack is also different on Superlead (500p/33k), which mostly means more mids.
It's possible to put this on a switch, again I've done it, but there will be high voltage on that switch (you can always put a massive cap ahead of it). Nowadays I'd rather just use a midway slope resistor (39-47k) as it doesn't make that much of a difference (nothing you can't compensate), however treble cap matters. You can put a 470pF, then a 2nd one in series that a switch can bypass (similar to Dumble mid boost switch), that gives you both values without dealing with switching DC.

I've used the Bassman/SLP EQ switch a lot in the past. The Bassman EQ is smooth and clean with a scooped feel, while the SLP's EQ produces a rather aggressive tone, with the mid-pots leaning toward the high-mids. I don't want to compromise with the mixed value, so I'm planning to use both. This will make this amp's simple mod switching impossible.

The Ceriatone JTM46 all access might interest you as well. It does almost everything (only the mixing resistors are not switchable)

https://ceriatone.com/wp-content/upload ... e-2023.pdf

The Ceriatone All Access series has been a great reference. However, I don't want to add so that many features.
If ignore the popping noise, the mixing resistor could simply be switched with a DPDT toggle. But, I'm wondering if it's really necessary to go that far.

The JTM45 had the NFB connected to the 16 Ohm scecondary, so the difference is sqrt (16/2) = 2.83.

Thank you. I will keep that in mind.

marshall used 100k @ 4 ohms and 27k @ 16 ohms. To cover all, use the latter but add a 220k pot in series,

Due to space matter, I'm planning to use a switch rather than a pot. Thank you.

Having built a 5F6-A and simply replacing the 12AY7 with a 12AX7, I found the difference to be huge. I eventually morphed the 5F6-A into a JTM-45, then a 1987X. Also changed power tubes, negative feedback and output transformer along the way.

I do miss the 5F6-A ...

Even when we've clearly made better choices, it seems inevitable that we'll always miss the options we don't currently have. However, if we haphazardly add switches to gain more options, we'll end up wondering, "What have I really created?" This seems to be the inevitable dilemma for DIY amp builders.

[Stevem]

The current drive difference between the two preamp tubes must be factored in here also if you ask me,

The Y7 drives the tone stack a good amount better, so it reacts differently.

A good empirical example is to replace a AT7 with a AX7 in a Fenders reverb tank drive circuit.

Also as I have posted here along with others, the differences between speakers is a big one!

Just look at the frequency response chart here between the Jensen P10R and the Celestion G12-15.

To these differences you need to know that the Alnico magnet used in the build of the Jensen compress much sooner then the ceramic magnet in the Celestion once driven hard enough.

The Bassman sings much better when cranked up then the Marshall does due to this compression factor.

Then a mentioned by others you got the cabinet differences.

[cdemike]

I understand having the "first draft" of an amp be an all-access type of build that eventually gets simplified with time. I built a 4-hole Marshall circuit that way and eventually settled on something similar to a Bass-spec amp. I think I'm reading that your goal is similar. If that's the case, then having each of the circuit changes available discretely, as opposed to having all changes controlled via a "master" 3-position switch, is that you can identify what aspects of each circuit you like individually. As the amp evolves, then it's a lot easier to just hard-wire the spec you like, but identifying your preferred values of individual components is basically impossible if you're switching several things at once.

One thing that may simplify your build considerably is using a workalike "tube rectifier" where you can introduce sag resistor between a your rectifier diodes and reservoir capacitor. Most report a difference of ~20V when switching a JTM45 to diode rectification, and a JTM45 should idle consuming somewhere around 80-100mA. That would mean a resistor measuring between 250 and 200 ohms would approximate the sag you'd expect from a GZ34. For the "Super Lead" setting you could just use a switch shorting that sag resistor.

Roe already mentioned filtering, and I think this is one of the most important differences between a Bassman and JTM45 vs a later-spec plexi. Very different transient response. I experimented with switchable filtering by placing parallel capacitors at a given node. One of the capacitors had its anode connected to the B+ line directly as a normal filter cap would, and the other had a series 220K resistor which could be shorted via a switch. That wouldn't necessarily account for the difference in topology between the preamp filtering where the later plexi spec has a separate B+ node for V1 as opposed to having V1 and V2 share a node, which by itself does make a difference in the transient response aside from the actual value of the filter cap there (two separate RC time constants at play when considering the voltage drop and recovery times impacting V1 under heavy sag makes that relationship nonlinear).