Something along the Trainwreck lines..

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Mr. Lime
Posts: 123
Joined: Sun Mar 05, 2017 12:57 am

Something along the Trainwreck lines..

Post by Mr. Lime »

Hi,

I came across the Komet Concorde schematic and always liked the sound samples I've heard from that amps.
When I saw that there were only 3 triodes used in the preamp section, I thought about a DC-coupled cathode follower for the first stage and to archive even more gain I'd like to bootstrap it like Merlin described.
I've read that the bootstraped CF is pretty suitable for the input stage and there would be less tone stack losses. Additional I'd like to bring some diode clipping in between the 2nd and 3rd gain stage. The tone stack is also modified after Merlin's bone ray circuit.
Komet Mod.png
Has anybody experience with such a topology?
Is it worth the try or do you guys have any further suggestions?
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RockinRocket
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Re: Something along the Trainwreck lines..

Post by RockinRocket »

Seems pretty interesting!
Ive always assumed there was a reason amp builders didn't bootstrap a CF. Assumed it didn't sound good.
Also for a two knob TS the James is hard to beat.

Let us know how it goes!
strelok
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Re: Something along the Trainwreck lines..

Post by strelok »

One thing you might run into is the first stage could potentially distort the CF, making it harder to get clean sounds, especially if you're using very high output pickups. So you may have to make adjustments for that. I've considered a similar circuit in the past and that was one of the things that occurred to me.
Mr. Lime
Posts: 123
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Re: Something along the Trainwreck lines..

Post by Mr. Lime »

strelok wrote: Wed Mar 08, 2017 8:50 am One thing you might run into is the first stage could potentially distort the CF, making it harder to get clean sounds, especially if you're using very high output pickups. So you may have to make adjustments for that. I've considered a similar circuit in the past and that was one of the things that occurred to me.
Thanks, you may be right, so I will add a channel switch that removes C1 from the cathode.
I also thought about the cold biased gain stage with 10k. When the diodes are engaged the signal is already heavily clipped so it makes no sense for a low gain recovery stage.
The amp should be able to get into Lee Jackson GP-1000 Preamp territory for 80s hard rock/metal if the boosts and gain pots are all on maximum.
There are two options for me:

1) adding another 10k resistor and a cap parallel to the existing 10k when the diodes are switched in.

2) somewhat like shifting the bias of last gain stage to obtain asymmetric clipping with more gain.
For this I found two methods in the sloclone forum:
A: a combination of fixed and cathode bias
B: a partial cathode bypass

This may even makes the diodes unnecessary for high gain heavy clipping but I have no clue if this would work as I expect..
Does someone ever tried something like A or B?
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Mr. Lime
Posts: 123
Joined: Sun Mar 05, 2017 12:57 am

Re: Something along the Trainwreck lines..

Post by Mr. Lime »

Here's something I once tried in another amp with pretty good results.
It's an semi active treble control where one side acts as the common lowpass called "bright control" and the other side of the pot is a variable cathode bypass cap which boosts the upper frequencies.
In the middle position of the pot, it has a flat response that let's the cold clipping stage act as usual. Theoretically nice to compensate the treble cut if the BMP tone control let only pass the lower frequencies in the tonestack.
Komet Mod #2.png
Any other suggestions before I order all the parts?
Thanks!
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RJ Guitars
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Re: Something along the Trainwreck lines..

Post by RJ Guitars »

I'm along for the ride on this. I've got an interest in separating out the high and low signals and process them independently then bring them back together. This seems to hit that same theme and adds some good ideas.
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Mr. Lime
Posts: 123
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Re: Something along the Trainwreck lines..

Post by Mr. Lime »

Well it's been quite a while since I started the topic and I tried some of the ideas.
The bootstrapped input stage wasn't too satisfying and I had to add a stage in front of it to sound good for blues.
The tone stack wasn't bad but didn't gave me the range of control that I like and it did some wired noise when the bootstrapping was engaged.

I kept going on learning what the Trainwreck and Komet amps need to sound right and focused more on the distortion generation by the PI. At the moment I have some high frequency oszillation problems when the gain pot is turned past 4 but in the progress my current amp will follow this new schematic.
A bass pot has been added to the tone stack and the bootstrapped CF has been moved to the last stage of the preamp as the CF is part of the Komet Concorde ciruit and because the PI has to be driven hard. As the 3rd stage seems to distort hardly the bootstrap circuit should not introduce harsh clipping as reported by some other users. Then I tought about a parallel effects loop to add an EQ pedal infront of the PI to get even more tonal ranges. The power amp will have no NFB at all to get the maximum gain and drive out of the EL84 tubes and at least to have the second PI input free for the effects loop return.

..and here's my question; would the effecs loop work as intended or is there something else to consider?
Komet Mod.PNG
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mojotom
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Re: Something along the Trainwreck lines..

Post by mojotom »

I'm a bit late to the party but I just want to add that a classic CF added after the first stage is an ice addition to a Fender design. It does change the sound and the tonestack as the CF output will be more cri isn't going through the TS (less loading).
On a Fender BF design it feels less scooped and controls feel more efficient too. I like it and the possibility to push the CF too much as a first stage is not an issue per se (at least for me).

Plate output to CF input. 100k to ground. Output of CF to tonestack.
I did change the TS values afterward.

It could work nicely on this design (maybe try 25u cathode cap rather than .68).
Mr. Lime
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Re: Something along the Trainwreck lines..

Post by Mr. Lime »

The CF on the input stage was one of the first things I tried but honestly it wasn't my cup of tea. The controls seem to have less range or at least were not extended at all and the inseration loss can easily be minimized by altering the TS values. If I had to buffer an early TS again I would probably do it with a LND150 follower. The prupose of using a triode for a CF to me mostly has some soft clipping in mind but even in high gain designs I tend to prefer plate driven TS more and more.
So in my point of view It would be great to drive the CF into clipping at the very end of the preamp or at least in this situation get more gain from the previous stage while the CF simultaneously buffers the FX loop without padding the signal down and still get some PI overdrive. It's all in development for now and maybe it would be even more efficient just to turn the CF into a simply none bypassed gain stage and get away with it.
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