Bogner Uberschall Rev 2 KT88 MOD?

I am curious what does this mod entail on the Rev 2? Do you have to rewire or add or take away anything on the circuit board or is it just a simple tube swap and re-bias? Any idea of the tonal benefits that mod has compared to the the stock tube configuration? Thanks in advance for any help!

You have to change a resistor. Tonally it has a bigger bottom end, more scooped mids, and more pronounced highs.

Sorry to bring up an old thread, but is the mod documented anywhere? Which resistor needs to be replaced and with what? Thanks.

There are a couple of limiting resistors that work with the voltage on tap to the bias trim pot. Hard to explain without a photo but I can post one in the next day or so. One of these 2 resistors modifies the higher limit; and therefore a lower rated resistor can allow for more voltage to be controlled via the trim pot. Of the 73-74 volts on tap in total, the current Rev2 config allows for about 40-45V to the pot (if my memory serves me correct). By replacing said resistor, I upped my voltage available to 60V, thereby allowing the higher draw 6550/KT88 bottles to get enough mA delivered to them (their respective sockets). As mentioned regarding the total voltage available - 60V is still well within safe and reasonable utilization.

It's still reverse compatible to EL34/KT77s as the bias pot now just has more range on tap to facilitate both types of bottles.

I'm using 6550s and just working through some finishing touches on the preamp glass. What I'm experiencing so far is just flat out balls to the wall MOAR!!!!! It's Rev2 tone just kicked up a notch in the punch and articulation department

According to Charley at Bogner, you can also just get a very cool set of KT88s and use them without having to mod the amp.

Ventura":20b4219k said:

There are a couple of limiting resistors that work with the voltage on tap to the bias trim pot. Hard to explain without a photo but I can post one in the next day or so. One of these 2 resistors modifies the higher limit; and therefore a lower rated resistor can allow for more voltage to be controlled via the trim pot. Of the 73-74 volts on tap in total, the current Rev2 config allows for about 40-45V to the pot (if my memory serves me correct). By replacing said resistor, I upped my voltage available to 60V, thereby allowing the higher draw 6550/KT88 bottles to get enough mA delivered to them (their respective sockets). As mentioned regarding the total voltage available - 60V is still well within safe and reasonable utilization.

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Thanks for the info. If you could post a picture that would be awesome. No rush - I'm not planning on doing this immediately (but I do want to get it done eventually - just haven't decided if I want to butcher my Rev 2 or buy another one to experiment on).

Ventura":20b4219k said:

I'm using 6550s and just working through some finishing touches on the preamp glass. What I'm experiencing so far is just flat out balls to the wall MOAR!!!!! It's Rev2 tone just kicked up a notch in the punch and articulation department

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If I may offer a suggestion on the preamp tubes. After around a decade of experimentation I have arrived at
v1 = Amperex 12AX7 mC (as in mC2, mC4, mC5, mC6). It's a long plate foil strip D-getter tube. A long plate Mullard works too, especially the mC1, but I find the Amperex to be more harmonically rich. And the high notes on the bridge pickups don't sound harsh at all, even if you crank the treble.
v2-v4 = Mullard 10M 12AX7 gold pin. Really any short plate Mullard 12AX7 works for me but the 10M's - especially the gold pin - have just a touch more definition.
v5-v6 = kinda irrelevant 'cause they don't affect the sound. I have Sylvania 12AX7's in there - just because they are durable and reliable tubes.
These are my personal preferences - not claiming this to be the definitive selection by any means. Curiously, as much as I love long plate Amperex/Mullards in the v1, when I tried them in the v2, it was not enjoyable.

As far as the power section, my Uber has been outfitted with Mullard Xf2 EL34 OO getter for a decade. I loved the sound of that setup. Of course after 10 years the Xf2's got tired and needed to be replaced (although I must say the amp still sounded great). I just retubed it with Mullard Xf1's (also OO getter) and... I don't love it like I used to. I only played it once so far with the new power tubes and I'm hoping they need time to break in. I'm going to give it a month. If I'm not in love with the sound of the Xf1's I am seriously considering putting in a quad of old stock Genalex KT88's. I loved the way they sounded in my Diezel VH4... until I found out Diezel advising against using KT88's with the VH4. Retubed it with KT77's (also old stock Genalex) and... the magic is gone. I mean it's still a great sounding amp and super tight in the bass but it just doesn't have what I loved about its sound before. So my guess is I just love the sound of KT88's...

leib10":3eq84rp8 said:

According to Charley at Bogner, you can also just get a very cool set of KT88s and use them without having to mod the amp.

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What does "very cool set" entail? What would make a set of KT88's cool?

leib10":1247rkoh said:

According to Charley at Bogner, you can also just get a very cool set of KT88s and use them without having to mod the amp.

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Absolutely, but IMHO it sounds like ass

azzzy":2c8hafl5 said:

leib10":2c8hafl5 said:

According to Charley at Bogner, you can also just get a very cool set of KT88s and use them without having to mod the amp.

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What does "very cool set" entail? What would make a set of KT88's cool?

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It's how they're rated.

leib10":3eocu042 said:

azzzy":3eocu042 said:

leib10":3eocu042 said:

According to Charley at Bogner, you can also just get a very cool set of KT88s and use them without having to mod the amp.

Click to expand...

What does "very cool set" entail? What would make a set of KT88's cool?

Click to expand...

It's how they're rated.

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So what's the sufficiently cool rating to be used in a Rev 2 with no mod?

azzzy":2fh9ug4v said:

leib10":2fh9ug4v said:

According to Charley at Bogner, you can also just get a very cool set of KT88s and use them without having to mod the amp.

Click to expand...

What does "very cool set" entail? What would make a set of KT88's cool?

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Tattoos, designer jeans, hip footwear, and a lowkey disposition that gets the ladies moist

I was looking to get a "cool" rated set of Sovteks but every tube seller is saying the Sovteks suck and have problems and aren't selling them anymore.

Unfortunately I may need to just have this resistor changed so I can use some KT88's and not have to worry about anything.

I think that's what I'm going to do too. I already have two quads of old stock Genalex KT88's, and I doubt they are "cool".

Does anyone know if the mod still allows using the EL34's? How about KT66's? I have a quad of EL37's I'd love to try in my Uber.

azzzy":fg9gsi21 said:

I think that's what I'm going to do too. I already have two quads of old stock Genalex KT88's, and I doubt they are "cool".

Does anyone know if the mod still allows using the EL34's? How about KT66's? I have a quad of EL37's I'd love to try in my Uber.

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The mod can be very very simple... I'll post a pic in a minute. There is no need for a switch or toggle or additional hole or anything - unless it's that that you want - but you don't need it.

On my Rev.2 there were 73v of access power to play with at the bias trim pot level; however, this was limited to about 20v to 40v in the bias range. We replaced the top-limited resistor to allow the bias range to go from the same base 20v (uselessly cold, but whatever) up to just shy of 60v. This is still "WELL WITHIN" the 0v to 73v that was running through the board and its bias pot adjustment - 20v to 60v between 0v to 73v - no problem.

The bias pot regulates voltage which translates to current allowed through the power tubes. Not entirely sure as to how it works - this wasn't my doing - this was my tech's doing; but bottom line is we bumped up the range of the ceiling parameters with one resistor and this allowed us to control greater breadth of current going to the grids. I'll take an iPhone pic and post it in a bit.

Easy. And yes, this allows for EL34s through to KT88/6550s without any other issues - just turn the bias pot accordingly.
Mo

If my memory serves me correct it was the swap out of one of these resistors which were "matching" before.

Don't quote me on this - I've got the schematic of the "for sure" swap in my office but don't feel like drumming it up right now. However, all that aside - it's THAT simple.

Hope this helps.
Mo

Ventura":1z6j6t6c said:

And yes, this allows for EL34s through to KT88/6550s without any other issues - just turn the bias pot accordingly.
Mo

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Cool. Thanks a lot. And thanks for the pic. If it's not too much trouble please post the schematic (or a link to it if available) when you get a chance - no rush. Thanks again. I'm totally doing this shizzle!

azzzy":1o11x6mq said:

Ventura":1o11x6mq said:

And yes, this allows for EL34s through to KT88/6550s without any other issues - just turn the bias pot accordingly.
Mo

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Cool. Thanks a lot. And thanks for the pic. If it's not too much trouble please post the schematic (or a link to it if available) when you get a chance - no rush. Thanks again. I'm totally doing this shizzle!

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Just make sure you get a tech to do it - totally easy to isolate the 2 limiting resistors with a probe, then he can piggy back the values of the resistors to best manage the onboard voltage (as it changes amp to amp) and once dialled - boom - just swap it out with proper heat sink treatment and Bob's yer uncle. No likey? No wanny? Put the same value resistor back and no one's the wiser.

Ventura":3c6b3klo said:

Just make sure you get a tech to do it - totally easy to isolate the 2 limiting resistors with a probe, then he can piggy back the values of the resistors to best manage the onboard voltage (as it changes amp to amp) and once dialled - boom - just swap it out with proper heat sink treatment and Bob's yer uncle. No likey? No wanny? Put the same value resistor back and no one's the wiser.

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Of course - I am useless with a soldering iron. Now, what's that about heat sink treatment??? I thought you just needed to change a resistor. Is there more to it than just desoldering one resistor and soldering in another?

You should definitely post up the Schematic if you have it. Should be stickied on here.

Good to know about not needing a toggle switch. The one I played with the mod had a switch. I am curious of why they added one if it was not needed.

I am planning to have mine modded soon. I must have the KT88's in this amp.

azzzy":x05k4bhs said:

Of course - I am useless with a soldering iron. Now, what's that about heat sink treatment??? I thought you just needed to change a resistor. Is there more to it than just desoldering one resistor and soldering in another?

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A tech will do everything right including making sure he/she uses a heat-sink at the ends of the solder points to ensure no thermal damage around the board (or component being uninstalled/reinstalled). That's all - doing things right. Simple. It's what roach-clips are for..."heat sinks" - they sink the heat, or, hold the last bit of a joint yer toking on LOL.

maddnotez":x05k4bhs said:

You should definitely post up the Schematic if you have it. Should be stickied on here.

Good to know about not needing a toggle switch. The one I played with the mod had a switch. I am curious of why they added one if it was not needed.

I am planning to have mine modded soon. I must have the KT88's in this amp.

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I'll get that schematic up soon. But as mentioned - any tech will know EXACTLY what two resistors are the limiting resistors by flipping the amp over and following the circuit from pot-to-tubes; simple. As for "why" toggles are sometimes added?? I suppose depending on the amount of regulating voltage that's available, sometimes it requires an actual step up or step down to punch it into place. In the case of my Rev.2 - there was adequate range (0v to 73v) to get to work with both tube-types; and the bias pot while now a lot more sensitive (because "it" hasn't changed, but the amount of adjustability it's responsible for has) , had the spectrum to work from cold to hot, low to high, EL34 to 6550/KT88 (20v to 40v, 30v to 60v respectively). Don't get wrapped up in the technical aspects.... Just let the tech swap ONE resistor, test it, and call it a day.

Sometimes techs like to make holes. Sometimes techs like to add toggle switches. They can charge more and they feel like they modded an amp!!