Remove Ultraliner OT taps - quick glance

peteb

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Still, I think @Jon Snell 's post #3 for Pentode mode gives you the full power of the tubes with all of the desirable power tube distortion characteristics. Maybe not less power or stress on the power section but certainly making better use of tone for guitar application

why pentode?

why triode?

why not beam tetrode, 6L6?


why not wire the screens like the pre UL Twin reverb?
 

GlideOn-Designs

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You could take the Ground switch out and replace it with a UL/Triode switch.
View attachment 1014798

Damn, you're good.

Drawing and everything!

why pentode?

why triode?

why not beam tetrode, 6L6?


why not wire the screens like the pre UL Twin reverb?

Why not indeed?

Being said, I'm not necessarily of any mindset that pre-UL Silverface or Blackface Fenders were superior, in some ways they were not. It's all trade-offs.

I'm more of the mindset of "what's been improved in the decades since and how can this best benefit from the lessons learned?"

I'm actually reading more and more that Triode mode is somewhat weak and anemic sounding when implemented, many preferred Pentode despite the inconvenience of being louder.
 

Ten Over

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Damn, you're good.
Not that good. You asked for Pentode/Triode and I gave you UL/Triode.
Fender Twin 135W Pentode Triode Sw Layout.png
 

GlideOn-Designs

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Don't be bashful, your drawing alone over the stock layout deserves praise! As someone who spent a fair amount of time and resources formerly pursuing graphic editing as a career, it has certainly not gone unnoticed and greatly appreciated.

Seems like a free unlocked feature to me and not too much wiring to do so. I call it genius from the point of view of the player who will now realistically use the same amp for practice and gigging
 

Jon Snell

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Whereas my schematic has a different overall appearance, the circuit contained therein is exactly the same as the Fender schematic.

The 135W Twin Reverb was remarkable in the consistency from unit to unit and in the adherence to the published schematic.

I suppose so since the most that anything can charge up to would be 250V and since the entire load would have to go through the bottom 100k balancing resistor. It would be short-lived, however, because the bottom reservoir capacitor would be charged with a reverse polarity. Ka-Boom

_________________________________________________________________________

Here is how the reservoir capacitors are grounded in the Fender schematic exactly like they are grounded in my schematic:
View attachment 1014731
Oh the joys of being dyslexic. I always miss out details that seem obvious to me on my next read of what I have written. Of course the centre tap point must be moved from the halfway point of the smoothing capacitors to the physical chassis ground. Appologies for any confusion.
DOH!

Edit; More data added.
 
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Lynxtrap

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I'm actually reading more and more that Triode mode is somewhat weak and anemic sounding when implemented, many preferred Pentode despite the inconvenience of being louder.

I guess you are about to find out, but personally I have never liked it in the amps I have heard.
 

NTC

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Oh the joys of being dyslexic. I always miss out details that seem obvious to me on my next read of what I have written. Of course the centre tap point must be moved from the halfway point of the smoothing capacitors to the physical chassis ground. Appologies for any confusion.
DOH!

Edit; More data added.


After disconnecting the two diodes in the bridge that goes to ground as Mr. Snell pointed out in a previous post.

However, if one does that, the power supply voltage will be about half of what it had been. That is quite a drop off.
 

GlideOn-Designs

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Been a minute, but I tried the Pentode wiring scheme and capped off the UL wire taps in case I needed to revert back.

I'm glad I did.

At 3-4 on the LarMar Master Volume after several minutes, V7 starts going into redplate. Swapped the power tubes and it is the tube itself which is an old Groove Tubes that looked worse for wear. The rest are Sovtek, so perhaps due for a change anyways. It has a slight low 60hz-ish hum despite brand new F&T filter section.

Anyways, I could detect zero change in tone, perhaps decreased reliability but then again I haven't changed the Grid Stoppers from the 470 values even though they are now ceramic 3w.

So I can either revert back to the UL or bump up the Grid Stoppers to these 1k 5w wirerounds I have to avoid any future reliability issues.

At the same time, it does have me thinking about overall stress on the power tubes. Did this change to Pentode stress the amp more? Was it because I failed to compensate the Screen Grids to more fail-safe values? I still want to soften or warm the tone of the amp a bit more.

Appears either way this amp pushes the power tubes to their limit, maybe a more aggressive approach like raising the filter section Dropping Resistors a few K from 1.2k to 2.2k and 4.7k to 6.8k may work across the board and desirable soften the tone throughout?
 

GlideOn-Designs

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It is fixed bias but has a balancing pot. I'd expect it to be behaving as designed considering all of the power supply components were either cleared or replaced.

I hear of lower power or 50w/x2 6L6 Deluxe models lowering their 2nd and 3rd filtering stages Dropping Resistors to 1.2k region for increased voltage and headroom with no ill effect. I'm curious as to why the opposite wouldn't be possible for a much higher powered amp that can afford to shave 30-40v and soften things a bit.
 
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Lynxtrap

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The balance pot may or may not fix the hum. But if it was me, I'd want to check the bias just to make sure it's in the ballpark.
Personally, I never expect old amps to behave as designed 😉
 




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