Bogner XTC Red channel mod question

[soundguruman]

There are 600 volt rated industrial switches, but I don't see what you are trying to accomplish.

[Setneck]

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.

Thanks for the tip Gregg, I'll think about that. All I am afraid is a shock hazard, but maybe I'll find the switch which could stand plate voltage. DO You think that additional isolation of a knob with rubber or sth would be also useful?

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

Yeah I know that there's no XTC schem around which would be free of mistakes . I got plenty of them and I've done some reasearches inside my amp and basically the mistakes aren't major. Just a couple of small bugs, ie. if there is a 470pF treble peaker on 500K grid resistor before last gain stage or not. It is one one schems and the others doesn't show it. But using those schems it is possible to trace the circuit and where to tweak the sound .


Thanks and regards, Andy

[Setneck]

There are 600 volt rated industrial switches, but I don't see what you are trying to accomplish.

As I've said before, I'd like to check if deleting or lowering value of 2nd gain stage's snubber cap wouldn't brighten my overall tone.


Regards, Andy

[Gregg]

There are 600 volt rated industrial switches, but I don't see what you are trying to accomplish.

I've seen industrial switches that can handle kilovolts but you can't find those in a mass production tube amp.
The best quality switches I've seen in a tube amp so far have ratings of 250VAC/15A or 400VAC/10A. Most of the time the manufacturers wouldn't even list DC ratings in their datasheets (if such are available at all). Since most of the time you see lower quality switches that are not known to fail every day that means they can handle the job pretty well. I've seen amps that have cheap unknown brand chinese toggle switches that work for years without any problems.
What I'm trying to accomplish is to say that such switches can be used for cap testing and will handle the voltages without problem.
If you are concerned about safety you can ground the metal parts to make sure you don't get shocked through them provided you take appropriate measures not to get shocked in other ways. Or you can use plastic toggle and rocker switches without metal parts.
If you're tweaking a tube amp you should know high voltages that are present inside can be lethal in the first place.

[soundguruman]

And so, taking out or lowering the value of the snubber will for sure allow more highs to pass. But Bogner selected the cap to prevent the amp from oscillating and ringing.
And so, assuming that an experimenter can out-design Bogner is a bit of a reach. If Bogner put the cap in there, it's for a damn good reason.
There are high voltage cap substitution boxes, with switches, for this lab purpose. But the length of the wires, even short wires, can screw up your evaluation. It's squirrely.
And Bogner does make a really nice amp, that's for certain. One of the best around.

[Setneck]

And so, taking out or lowering the value of the snubber will for sure allow more highs to pass. But Bogner selected the cap to prevent the amp from oscillating and ringing.
And so, assuming that an experimenter can out-design Bogner is a bit of a reach. If Bogner put the cap in there, it's for a damn good reason.
And Bogner does make a really nice amp, that's for certain. One of the best around.

No doubt about that, that is why I bought that amp. And the quality and design of Bogner amps are beyond any question.

I don't even try to be wiser than guys at Bogner. As You may see, I don't try to redesign the circuit, I just try to tweak a little bit, because live my amp is a little bit hidden in the mix for its low-mid voicing.

And from gig to gig the things get better and better. Removing factory gain mod was the first step, than I lowered Structure cap to 1uF changing it also to Wima PET, and then I lowered overall 2nd gain stage resistance to 1.5K, changing R43 Structure resistor to 2.2K.

All those mods have brighten the sound in a positive way (no harshness and stiffness) and Bogner flavour wasn't lost at all. I feel like being almost there, the only one thing that left is to try lowering 2nd gain stage snubber cap value. If the amp will oscillate and squeal after that I will return to 1N. But maybe 470pF would be a good compromise here.

And that's the whole story

Regards, Andy

[soundguruman]

You need a variable capacitor, vacuum grade. And put them on all the stages, so you can tweak to your hearts delight.

[Setneck]

OK, so I've decided to tweak the amp in 3 places:

- 470pF//499k before 3rd stage -> remove the cap (btw, that was done to 20th Ann XTC)
- 2nd stage plate bypass - lower the cap to 470pF
- 1st stage input cathode - here I have a dillema: to go 1k5//1uF or to 2k2//.68uF. Hipass is nearly the same (100Hz), I only wonder which one will sound better

As to 2nd gain stage, would You advise to stay with 1k5//1uF I currently have here or bring it back to stock 2k35//1.5uF?

Thanks and regards

Andy

[GainFreak]

Instead of wondering what will happen just go for it and even better record some A/B samples so we can hear it as well.

[Steve Conner]

You need a variable capacitor, vacuum grade. And put them on all the stages, so you can tweak to your hearts delight.

You called?


The small, cheap toggle switches should be fine. Just make sure the toggles are grounded, put the capacitor between the B+ rail and the switch (instead of between switch and plate) and nobody will get hurt.

Oh and expect a big pop when flipping the switch, unless you put a 10M resistor across the contacts.

[Setneck]

Hi!

Did some mods. 1.8k//1uF at first stage, 2.8k//.68uF at second. The tone has been improved the way I wanted. Then I lowered 2nd stage snubber cap to 470pF and the amp started to squeal a bit, even at low volumes. But the tone was great... :/

Went back to 2.35k at 2nd stage, the squeal was a bit lower, but I liked 2.8k better. Tried 680pF snubber cap but the difference was subtle. Looks like You were right soundguruman - Reinhold put 1000pF 2nd stage snubber cap not accidentally . It seems that this cap rolls off those exactly squealing highs :/. I use external boost for solos, having stock config the amp is just about to squeal when I kick it. So now it will squeal for sure (haven't tried the amp with booster though).

Still the tone could be a bit more open, but I think I'll have to go back to 1000pF. Or try 470pF and go down with the cathode... :/... I don't know...

The one thing I could still do is to remove 470pF cap from low-pass filter before 3rd stage, but I wonder if it's worth to do it... the amp isn't easy to service...