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Old 25th March 2019, 07:35 PM   #21
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by ElusiveMoose22 View Post
...2 short videos...
Nice and "tubey"! I quite like the sound with single-coil pickups, though I can hear some of the excessive midrange / bass you mentioned. That is very much a matter of personal taste, though.
Quote:
Originally Posted by ElusiveMoose22 View Post
...the values of the capacitors around the tone pots are not ideal for guitar signal.
<snip>
The bass is overwhelming everything unless I cut it out almost entirely.
I put the essentials of your treble tone control circuit into LTSpice, and simulated the frequency responses that would result.

First, the stock circuit. As you said, treble adjustments start above 1 kHz, which is suitable for Hi-Fi, but not for guitar:


-Gnobuddy

I'll say up front that tweaking guitar tone controls is something I've personally found quite frustrating, and sometimes fruitless. So I'm not sure the changes I'm about to suggest will make you happy. But perhaps they'll at least suggest a starting point, so here goes!

Since you already have 2 Meg pots, and they are in working condition, I think you might as well keep them unless you find it impossible to get the results you want. New pots may produce new problems (changes in control feel, knobs not fitting, et cetera.)

So, what can we do with the stock 2 meg treble pot? Firstly, your treble Baxandall circuit will still work the same way if you increase or decrease both caps (50 pF and .001 uF) by the same ratio. In this case, we want bigger caps, so the tone control starts to work at frequencies lower than 1 kHz, where e-guitar still has enough output to be worth tweaking.

My suggestion is to increase the 50pF cap to 120 pF (i.e. multiply its value by 2.4 times). At the same time, increase the 0.001uF to 0.0022 uF (multiply it by 2.2, the nearest standard value).

By the way, 0.0022uF is the same as 2200pF, or 2.2nF. (For some inexplicable reason, we never use the SI "nano" prefix for caps in North America, so we use the clumsier and technically improper 0.0022uF or 2200pF terminology instead.)

This will move the effective range of the treble control down by a bit more than an octave, so it starts to have an effect above roughly 400 Hz, rather than only above 1 kHz.

You can of course try the same trick with other values, remembering to try and scale the original 50 pF and 1000 pF (0.001uF) caps by roughly the same factor each time. For example, you could try 180 pF and 3300 pF, or 220 pF and 4700pF, which will move the treble controls effective range even further down towards low frequencies.

Now let's consider the other problem - the overpowering bass. This is a common issue with guitar amps derived from Hi-Fi circuits. Leo Fender made the same mistakes with his 5E3 when he lifted Hi-Fi cap values from the back of the GE tube catalog, for instance.

Since you're working on the treble tone control circuit already, let's see if we can cut some bass there. The 0.02uF cap that couples the signal into the treble control pot (from the anode of the 6FQ7) is the obvious place to cut some bass. Based on your report of overpowering bass, I suggest replacing this with a 0.001uF (same as 1 nF, or 1000 pF). This drastic reduction (twenty times smaller!) will gradually roll off deep bass below 150 Hz or so.

As before, you can, of course, tinker with this to taste. You can reduce it even more if you still have too much bass, or increase it from 1000 pF if you now have too little bass.

The attached image shows what LTSpice predicts using the starting values I suggested in this post: 120 pF and 2200 pF on the tone control pot, and a 1000 pF coupling cap.

If you try out these changes, let us know how they sound!


-Gnobuddy

Incidentally, most of the valve guitar amps we hear use some variation on the tone control "stack" Leo Fender used in some of his classic amps. That circuit produces a sizeable mid frequency scoop at typical tone control settings.

You may find your Baxandall circuit sounds too midrange-dominant no matter what you do with the treble caps - if so, what you might need is a mid-scoop. If that time comes, we can discuss some of the options you have. The fact that you have so many valves on your donor chassis really gives you a lot of possibilities.


-Gnobuddy
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File Type: png bax_002.png (52.4 KB, 70 views)
File Type: png bax_004.png (64.0 KB, 68 views)
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Old 25th March 2019, 11:44 PM   #22
ElusiveMoose22 is offline ElusiveMoose22  United States
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Gnobuddy,
Thank you again for the feedback, all your help has been an invaluable resource in this guitar amp conversion!!

I appreciate you running a mockup of the treble circuit in LTSpice. I downloaded that program a while ago but have not been able to dedicate the time necessary to understanding and utilizing it properly. I have become familiar with Duncan Amp's Tone Stack Calculator, which I think is a simplified version of LTSpice focusing on just tone stack circuits. Using their standard James TS mockup and changing the values to reflect my circuit with your suggested changes to the treble I came up with the attached results. I think this is at least somewhat accurate to what I can expect then?

Quote:
You may find your Baxandall circuit sounds too midrange-dominant no matter what you do with the treble caps - if so, what you might need is a mid-scoop. If that time comes, we can discuss some of the options you have. The fact that you have so many valves on your donor chassis really gives you a lot of possibilities.
I think this may be the case. I understand that by boosting bass and treble at the same time in a James TS you can essentially make a mid scoop, but I don't think it is going to be nearly as much of a mid scoop as I'd like to get. What suggestions do you have if any for accomplishing this? If possible I'd like to keep the bass & treble as the only tone controls as those pots exist already. Could I redo the tone circuit with bass and treble adjustments but a fixed value set for mid range like an old Fender Deluxe Reverb TS? If so that may be the way I'd like to go...
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Old 26th March 2019, 09:39 PM   #23
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by ElusiveMoose22 View Post
...LTSpice...Tone Stack Calculator...
Put it this way, Tone Stack Calculator is a fantastic tool compared to what engineers had in the tube era. So much better than writing down equations and running numbers through a slide-rule!

That said, LTSpice is on another level altogether. It has a history going back to the early 1970s, when SPICE was created by genius researchers and grad students at Berkley University. LTSpice was the tool used in-house by Linear Technology engineers to design their integrated circuits. So it's incredibly flexible, incredibly powerful, and because it has such a long history of professional development and use, we can put a good degree of trust in most of what LTSpice predicts (though no simulation will get everything right.)

I started trying to teach myself to use LTSpice a few years ago, using tutorials I found on the 'Web. By the time I got as far as simulating guitar tone control circuits, I was starting to see discrepancies between what LTSpice was telling me, and what TSC was telling me. Sometimes TSC would produce lovely symmetrical tone control EQ curves, while LTSpice showed erratic curves that clumped together at one end of the pot's rotation.

At that point I stopped using TSC altogether, because I wasn't sure I could trust it. Also, LTSpice was capable of simulating experimental or unusual tone control circuits that didn't exist in TSC.

The tone control you're looking at is one such example. In your amp, the bass and treble controls are separated by an active gain stage. So they don't load each other in the way that the TSC simulation expects. This means the TSC simulation is definitely wrong - but I don't know exactly how wrong. Ten percent? Twenty? Fifty? I have no idea.

Interestingly, there are details in the LTSpice sim I just did that are missing from the TSC predictions you posted - for instance, LTSpice predicts some of the treble curves actually cross each other. TSC doesn't. Also look at the lowest treble curve in the two simulations: they don't match!

Quote:
Originally Posted by ElusiveMoose22 View Post
I understand that by boosting bass and treble at the same time in a James TS you can essentially make a mid scoop, but I don't think it is going to be nearly as much of a mid scoop as I'd like to get.
I experimented with guitar tone control circuits for a while, and came up with a tweaked version of the Voight tone control (found in Merlin Blencowe's guitar preamp design book) that had very nice, uniformly spaced, non-interactive bass and treble curves, a lot like a good active Baxandall. And I found that even with treble and bass turned up, my amp still sounded too midrangey.

I think the trouble is that the "scoop" created this way is wider (in frequency) and shallower (in decibels) compared to the Fender / Marshall / Vox mid-scoop we've become used to. It doesn't sound the same!

Quote:
Originally Posted by ElusiveMoose22 View Post
Could I redo the tone circuit with bass and treble adjustments but a fixed value set for mid range like an old Fender Deluxe Reverb TS?
You could certainly do that - you could even use a trimpot inside the amp for the "mid" pot, and set it to suit your taste.

If you want to keep and tweak the somewhat unique (for a valve guitar amp) Baxandall tone controls you have now, you also have the possibility of inserting a notch filter somewhere in the signal chain. It could be fixed (or adjustable with a trimpot). If there is room for a front panel switch, you could even make it switchable.

The attached image shows a notch-filter circuit I originally posted in a different diyAudio thread. This could go between two of the gain stages in your amp. (R1+R3) is a 1 meg pot - could be an internal trimpot you set to taste, if you don't want an external knob. The caps can also be changed to put the notch where your ears prefer it. (I like 800 Hz, to my ears that takes away a little harshness and makes guitars sound nicer. But not every guitarist wants a guitar to sound "nice"!)


-Gnobuddy
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Old 26th March 2019, 10:20 PM   #24
ElusiveMoose22 is offline ElusiveMoose22  United States
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Gnobuddy,

Quote:
At that point I stopped using TSC altogether, because I wasn't sure I could trust it. Also, LTSpice was capable of simulating experimental or unusual tone control circuits that didn't exist in TSC.
Upon further research yesterday and today I believe I agree LTSpice is the best way to go. I started down the youtube tutorial rabbit hole and was able to recreate the bass half of my tone stack (screen shot attached below). I should be able to recreate the treble half too in order to simulate the whole TS circuit, but now my current problem is how to simulate the triode that is boosting the signal between the two pots. Is there a good way of doing this in LTSpice that you are aware of?

I think the idea of replicating the Deluxe Reverb tone stack is growing on me the more I look into it. It's a decidedly mid scooped sound but a little less so than the other blackface amps of the era since the mids are fixed with a 6.7k resistor instead of an adjustable pot. I like how this keeps things from getting too hollow sounding but still allows a mid scoop that plays nice with all the overdrive pedals out there that offer mid humps when engaged. I understand the Fender TS loads down the signal a lot more than a James TS, but I think we have plenty of gain stages with which to recover signal before it is sent on to the output tubes. If I were to do this where would you suggest placing the tone stack? I'm thinking between the 2nd EF86 and the 1st 6CG7 triode might be best?
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Old 26th March 2019, 11:28 PM   #25
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by ElusiveMoose22 View Post
...was able to recreate the bass half of my tone stack...
Wow, nice work! You got there much faster than I did.
Quote:
Originally Posted by ElusiveMoose22 View Post
how to simulate the triode that is boosting the signal between the two pots. Is there a good way of doing this in LTSpice that you are aware of?
For your current purposes, all you need is something that buffers (isolates) the bass control from the treble control. This alone is sufficient to see both the bass and treble control curves. The absolute levels (insertion loss) won't be right, but the curves will still show you what the controls do.

To make life easy, you could use one of the op-amps built into LTSpice as a buffer between bass and treble sections of the tone control. Or use a JFET, wired as a source-follower.

Optionally, you might want voltage gain as well; you can get this with an op-amp in the simulation if you want.

When you get a little deeper into LTSpice, you can actually find a vacuum tube LTSpice model, and throw it in there. Honestly, I'm not sure if that particular exercise is very useful, as math models for vacuum tubes tend to be very inaccurate in my experience, so they don't behave like the real thing anyway. More crucially, to understand the tone stack, you don't actually need a distorting valve stage in the middle of it!
Quote:
Originally Posted by ElusiveMoose22 View Post
...Fender TS loads down the signal...where would you suggest placing the tone stack? I'm thinking between the 2nd EF86 and the 1st 6CG7 triode might be best?
I would suggest inserting the entire tone stack where your treble control is now. The reason is that EF86s, being pentodes, have much higher output impedance than 6FQ7 triode gain stages.

In your circuit, the EF86 driving the bass control has a 68k anode load resistor. Pentodes have an extremely high internal anode resistance ("plate resistance"), so we can expect output impedance of the entire pentode stage to be not much less than 68k.

For comparison, a typical half-12AX7 triode stage with a 100k anode load has an internal anode resistance of maybe 70k. The output resistance of the entire stage will be 70k in parallel with the 100k external anode resistance, or roughly 40k.

Meantime, a 6FQ7 has an internal anode resistance of around 7k - 8k according to the datasheet I just looked at. This varies with operating point, but still, we can expect your 6FQ7 with 100k external anode load to have an output impedance of less than its internal anode resistance - just a few kilo ohms. It was born to drive heavy loads like Fender tone-stacks!

When you remove your bass tone control circuitry from in between the EF86 and 6FQ7, you should probably insert a pair of fixed resistors there, to cut down the signal by about the same amount as the bass control used to. Cut it down to roughly one-tenth as a starting point. Without this, you'll probably have much too much preamp gain, and lots of buzzy overdrive from the 6FQ7.

Incidentally, I've read several times that the way to get good guitar tone from valves is to run the signal through several valves, cutting the signal strength back down in between each pair of gain stages. This allows each stage to add just a little sprinkle of distortion, rather than heavy buzzy overdrive. But all the little sprinkles add up, so you get good "valvey" tone by the time you reach the preamp output. I think this is already working for you in your amp - I quite liked the clean tone you were getting with your single-coil guitar.


-Gnobuddy
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Old 27th March 2019, 12:14 AM   #26
ElusiveMoose22 is offline ElusiveMoose22  United States
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Quote:
Wow, nice work! You got there much faster than I did.
Thanks! It wasn’t easy, my brain cells are still smoking lol.

Quote:
(6FQ7) was born to drive heavy loads like Fender tone-stacks
Good catch! I hadn’t even considered looking at the data sheet to figure output impedance for the dual triodes. Sounds ideal like you say.

Quote:
When you remove the bass tone control circuitry...
I will try to draw this all up tomorrow to make sure I’m doing it correctly, but should I just cut it out at the pot and attached .001uF and .01uF caps, or do I need to also cut out the 360k resistor and 100pF cap that are just before the bass pot in the signal chain? Looks like those are there as some kind of low pass filter to tame the high end maybe?
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Old 27th March 2019, 01:40 AM   #27
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by ElusiveMoose22 View Post
...do I need to also cut out the 360k resistor and 100pF cap that are just before the bass pot in the signal chain?
That is actually a "bright cap" in guitar-amp terminology, but a rather subtle one that only kicks in above 2 kHz or so.

My guess is that it would be fairly subtle with a clean guitar, and might be unpleasantly harsh if you were to overdrive one or both of the EF86s at the start of the signal chain.

At any rate, I suggest starting out with the bare minimum: a cathode coupling cap to block DC, and a pair of resistors to divide down the voltage. The attached image shows suggested values, which I picked for reasonable impedance, attenuation, and lower cutoff frequency (bass). Let me know if that's not clear.

You can always tweak it if you don't like the result - say add a bright cap, or change the attenuation (insertion loss).


-Gnobuddy
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Old 27th March 2019, 01:49 AM   #28
ElusiveMoose22 is offline ElusiveMoose22  United States
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Quote:
Let me know if that's not clear.
Nope, you answered both my questions perfectly. Just what I need to get started, thank you once again! I’ll post the new circuit revision sometime tomorrow. My fried brain needs to rest for the evening lol.
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Old 27th March 2019, 03:40 AM   #29
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by ElusiveMoose22 View Post
My fried brain needs to rest for the evening lol.
My brain's kinda fried tonight as well. Perhaps it's time for me to chill out with some good music or guitar-related YouTube videos. Have you seen this one? (That Victory Kraken amp - the one the purple Tele's plugged into - sounds pretty good, no?) YouTube

And you're very welcome - your current project is an interesting one, and I'm glad to help. By the time you get it sounding good, I think we'll both have learned some new things.


-Gnobuddy
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Old 27th March 2019, 04:03 AM   #30
ElusiveMoose22 is offline ElusiveMoose22  United States
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Quote:
Have you seen this one? (That Victory Kraken amp - the one the purple Tele's plugged into - sounds pretty good, no?)
I have seen it! I’m subscribed to Anderton’s YouTube channel for the gear reviews and for the British Humor lol. That amp is a tone monster, but so is Danish Pete’s purple tele. Also doesn’t hurt that he has the skills to make that gear sing lol. I’m also subscribed to That Pedal Show and JHS Pedals YouTube channels for the gear and the extra nerdy content, also a good bit of humor on those channels as well.

Quote:
... we’ll both have learned some new things
Well I’ve definitely learned a ton since I started this project, so I’m glad to share any information that can be gleaned by anyone else doing a similar one in the future.

Good night and happy YouTubing :-)
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