Bogner XTC Red channel mod question

[Setneck]

Hi!

I think about modding my XTC amp to open it up a bit, by changing anode and cathode resistors in dirty input stage. Currently Rp = 100K and Rk = 820R. How about increasing Rp to let's say 220K (or 150K) and RK to 1.5K? These are the input values in ie. Splawn Quickrod, Marshalll 6100, etc. Would the tone become more open only by modyfying those resistors?

I'd like to discuss that matter theoretically before opening the amp and messing with it

Schematics included. Note that I have already changed Structure cap from 1.5uF tantalum to 1uF Wima MKS2.

Thanks in advance for Your help

Andy

[soundguruman]

Well I don't know about "open" but it will decrease the gain of the amplifier by about 50%. If that's what you mean by "open."
Rp and Rk adjust the gain of the input.
Why would you want to modify an amp that is already modified at the factory? Oh I know, you are a gain crazed maniac. I know your type.
I think the mod you want to install is the "jet" mod. But you should probably get the directions from Bogner for doing that first.

[Setneck]

No, I am not a gain maniac . I removed 2004 factory gain mod and decreased value of a Structure cap to 1 uF (which also decrased gain and bass response of second gain stage).

I only try to slightly adjust the tone of the amp. And by "open" I mean a bit less compressed and more "in-front" and cutting. I know that those resistors set the gain of input stage and values I wrote about was inspired by Splawn Quickrod values. I know - totally different animal but I only consider the tweaks.

PS.
What is that "jet" mod You mentioned?

Regards, Andy

[defaced]

And by "open" I mean a bit less compressed and more "in-front" and cutting.

When someone says open, I think of the SLO preamp - I think the schem is on Schematic Heaven. I suspect you'll need to adjust values throughout the preamp to get what you want though.

Also, the XTC has a plate fed tone stack doesn't it?

[soundguruman]

You cant avoid the compression because the whole design is aimed to that end. You would have to rip it out all the way along and replace it.
The values you install for the input stage, just cause it worked in another design amp, won't usually replicate the sound of another amp...
The jet is an overdrive, with what I would say is even more overdrive than the (unbelievable already) Bogner Overdrive.
You are not obsessed with gain, you are trying to move the opposite direction. Sounds like a Marshall Super Lead is what you want. Much less compression, but still can't be ignored.

[Gregg]

I'd like to discuss that matter theoretically before opening the amp and messing with it

Well theory is one thing, practice another so my advice to you is to grab the soldering iron and make the mods using small switches so you can instantly make A/B tests. Then use your years to hear what's going on.

Also, the XTC has a plate fed tone stack doesn't it?

The XTC has a CF driven tone stack.

[Setneck]

You cant avoid the compression because the whole design is aimed to that end. You would have to rip it out all the way along and replace it.

Well, in general I like that Bogner flavour, that is why I bought that amp. So I accept its rather compressed nature. I just try to tweak the tone a little bit, make it a bit brighter and cutting thru in the mix. I simply want to add a little bit more classic Marshall crunch, not loosing Bogner character of course .

I must admit that the changes I've done so far in Structure circuit (1uF bypass cap and 2.2K resistor, which when engaged gives me 1.5K overall resistance in 2nd gain stage) are leading me to what I'd like to achieve . There's still a place for trials IMO here, although the tone is much better than before.


I was also advised to remove or lower 2nd stage plate bypass capacitor. There is 1000pF cap there now. What do You think about that? I am afraid of the squealing (I use hot pickups). But would that brighten the tone? I ask about that before I'll do anything with it because it is not so easy to remove Wima cap out of the XTC's pcb .

Well theory is one thing, practice another so my advice to you is to grab the soldering iron and make the mods using small switches so you can instantly make A/B tests. Then use your years to hear what's going on.

I try to do so every time it's possible, but I'm a little bit afraid of messing such way in the plates, where the voltages are much higher than in cathode AFAIK. So, could I for instance install safely a dpdt switch with 220pF and 470pF caps in a place of plate bypass cap? Would it be dangerous to touch the switch while the amp is working?


Thanks for the help and patience guys and sorry for the questions which may sound silly and naive for the skilled technicians who I am not obviously .

Regards, Andy

[Gregg]

I try to do so every time it's possible, but I'm a little bit afraid of messing such way in the plates, where the voltages are much higher than in cathode AFAIK. So, could I for instance install safely a dpdt switch with 220pF and 470pF caps in a place of plate bypass cap? Would it be dangerous to touch the switch while the amp is working?

It's not a problem provided the switch is not bad. I've done this many times. If you're still concerned remember the old rule when working with tube amps: Use only one hand and keep your feet in the air.

[soundguruman]

Putting a switch on the bypass caps is just asking for trouble and a lot of noise.
If this were such a great thing to install, manufacturers would be doing it. And yes it can shock the crap out of you, or kill you.

[Setneck]

Putting a switch on the bypass caps is just asking for trouble and a lot of noise.
If this were such a great thing to install, manufacturers would be doing it. And yes it can shock the crap out of you, or kill you.

I wanted to install it only for testing purposes, to choose the right cap value. But if it's dangerous I won't do that.

What cap value would You recommend then? 470pF? 220pF? Delete the cap?

Regards, Andy

[Gregg]

Using a switch for testing different cap values is something basic but if you don't feel comfortable working with high voltages present in a tube amp better don't mess around with them.

[Setneck]

Using a switch for testing different cap values is something basic but if you don't feel comfortable working with high voltages present in a tube amp better don't mess around with them.

Well, I feel totally fine with testing caps in cathode using switches where the voltages are minimal. I was playing with it some time and now I have a switch installed with two caps in Structure circuit (have 1uF and stock 1.5uF, prefer 1). But I'm awared that in a plates the voltages are much higher and I wondered if installing switches here is safe. If not, I will test the caps soldering an desoldering one after another. Of course, have them soldered to the switch would be more practical, but I still have some gigs to play in my life .

I still consider lowering 2nd gain stage plate bypass cap value, or even deleting it. Consider 470pF or 220pF. Wonder if the difference between 470 and 1000 will be noticeable and if I choose 200pF the amp will not squeal. Is it possible to predict/calculate it or doing the tests is the only way to find out?

Regards, Andy

[mozwell]

various bogner schems

[Setneck]

Thanks Mate, especially for the XTC schem

[Gregg]

But I'm awared that in a plates the voltages are much higher and I wondered if installing switches here is safe.

I haven't seen a toggle switch that has a rating more than 250VAC but they are regularly used as Standby switches and obviously can pass up to 500VDC without any problem. So if you're still concerned get a 250V/10A toggle switch.

The XTC schematic has at least couple of things wrong but since you have the amp you can verify them for yourself.