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  Ghost Sound LondonPower StandardPreamp
Posted by: Strelok - 01-13-2021, 08:03 AM - Forum: Preamp Mods & Design - Replies (3)

Hi all!

We all know the song "Ghost Town" by "The Specials".
I have a problem with a ghost sound, but it is not coming from that town.

I built a Standard Preamp a long time ago, using circuit card and the
layout from the kit.
The gain channel has an extra sound that is a shrill and thin version of the gain channel.
It does not listen to any tone controls, neither on the clean nor on the
gain channel.

When I turn up the volume of the gain channel the sound remains the
same, same volume, but the normal sound comes up like it is supposed to.
So of course you cannot hear it anymore but it is still there.
I can tell because annoying buzzing is much worse in that preamp, so
those frequences get amplified more.
I have to turn op the volume (Pump Up the Volume) way too high to swamp
out the ghost.

A while ago I built another Standard Preamp and it has the same thing.

I don't have this issue with single channel preamps I have built, like
the Vintage and High Gain variations and the single channel "Y".
They are also on circuit card.

What can this be?

I remember somehting about electro-magnetic coupling, but I'm not sure
where I read that.
That would occur near tone controls or channel switches.

Or maybe I just dreamed that.


Kind regards,

Strelok

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  Mods to Preamp with 7025/LND150 Cascode Input Stage & LND150 TS Driver
Posted by: NGW - 01-01-2021, 03:37 AM - Forum: Preamp Mods & Design - Replies (2)

Hi Guys,

It sure is quiet here. Where is everyone?

I need some advice on how to calculate the output and input impedances of an LND150 Depletion Mode MOSFET and the ouput impedance of one in a Cascode with a 7025.

I have done some Mods to an Albion TCT 35 (designed & manufactured by Steve Grindrod; former engineer for Marshall and Vox).
Attached is my schematic of the Preamps with my Mods.

The owner didn't like the Drive sound very much, so he used the Clean channel with pedals instead.
The idea was to do minor mods to the A channel (Clean) to sweeten the tone and change the B channel (Drive) to emulate the Overdrive sound of a Mesa-Boogie Mark 2C.
I have done the mods and it sounds good.

The interesting part of this amp is the 7025/LND150 Cascode Input stage (shared by both channels) and the LND150 Driver for the B channel Tone Stack.
I am not up to speed with the LND150, so I was surprised to see a voltage gain of nearly 140 from the Input stage.
100mV Input to 13.8V output, which required the gain pot to be kept very low to get anything like a clean sound (dirty Clean).

The original circuit had a 470pF coupling cap (C51) to the B channel, followed by 100k (R81) - 470k (R80)/bypassed with 2n2 - 1M Gain pot with a 100pF Bright cap.
I changed C51 to 1n, which suited the Mods better than the 470pF and more capacitance produced too much Bass. 
I added the 2M2 resistor and removed the 2n2 bypass and the Bright cap, to attenuate the signal into the Gain pot; which is workable now.

I don't know how to calculate the output impedance of the Input Cascode and therefore the load on the cascode output, coupled with the following circuits and how much effect this has on the Voltage Gain. It will be good to know this, for future reference; and I need to increase my Solid State knowledge.
I am also wanting to know how to calculate the input impedance of the LND150 Tone Stack Driver to ascertain how much load it and the Tone Stack apply to the V2B Valve Stage.
It must be quite a heavy load because this stage is only showing a Voltage Gain of approximately 12, or there is something else affecting the gain.
A 7025/12AX7 with a 270k Ra and an unbypassed 3k3 Rk should produce a Voltage Gain of about 34/30dB into a 1M load.

I forgot to add the Tone Stack circuit.
It is the same as the A channel, but with the following changes in values: Slope R= 33k. Treble Cap= 470pF. Bass & Mid Caps= 22nF. Bass Pot Dropping Resistor= 10k.

All info and help much appreciated.
Happy New Year to you all and I hope you guys aren't freezing in Canada and the Northern Countries; we could use a bit of your cool here in hot, steamy and rainy Gold Coast Oz.

Cheers, Noel


.pdf   NGW Mark 2D - December 202001012021.pdf (Size: 491.27 KB / Downloads: 4)

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  Greetings!
Posted by: DCarrer - 12-13-2020, 05:03 PM - Forum: New Member Introductions - Replies (1)

Greetings everyone!

After a brief ten year sabbatical, I am returning to Canada and this crazy amplifier business we all love.

I look forward to sharing and learning with you all.

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  Question about bobbin materials in Output Transformers
Posted by: makinrose - 12-10-2020, 01:42 PM - Forum: TUT Q&A - Replies (2)

There seem to be a lot of conjecture about whether modern plastic bobbins are as good sounding as traditional paper bobbins.  I've seen a two different opinion pieces on the subject but  the articles were both published by transformer companies with apposing views on the subject and who each adopt a corresponding different manufacturing  technique. No data or evidence was cited in either article.    I haven't seen anything scientific or logically convincing on why one manner of construction would sound different from another in the same design.   Is there a technical reason to think the two manners of construction should sound different?  Are there other reasons to choose one type over the other?

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  Hammond Chassis covers
Posted by: Sam - 10-04-2020, 10:31 PM - Forum: Personal Projects - Replies (3)

Hi all,

Looking for advice from anyone who has used a Hammond perforated chassis cover and installed carry handles on it.
Having never used one of the perforated covers before, i'm not sure how much strength there is, or how much flex (if any) to expect.

The attached image shows my intent. I'm thinking 1mm thick plates sandwiching the cover to spread the load.
The chassis is 16" x 8". Matching cover is 20ga (0.91mm) thick. Amp is 5W single ended, so not a lot of weight in the transformers.

Cheers

   

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  Old Electrolytic
Posted by: George A - 09-16-2020, 09:58 PM - Forum: Power Supplies - Replies (2)

Hi all - I have many, probably hundreds of "NOS" electrolytic caps left over from Polytone's inventory, probably dating from the mid to late 1980's. (They made an interesting tube amp called the Fusion to compete with Mesa Boogie - didn't really pan out for them).

They're nice looking caps - Mepco/Electa 310uf/300V - a couple in series would make a nice reservoir/smoothing leg.

My question - Is there any hope for using these, or am I asking for disaster if I do?  I've read about "reforming" capacitors - does anyone have thoughts or experiences related to this?

Thank you,

George

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  Hello Everyone
Posted by: Amptroll - 09-10-2020, 08:58 PM - Forum: New Member Introductions - Replies (1)

Hello fellow Forum Members,
    My name is Rob.  I love designing and building valve amplifiers as I assume that most everyone here does.
I have a small company that caters to a small group of musicians in NY, NJ, PA, and MD.  I have a degree in electrical 
engineering but my valve knowledge and experience does not come from my education but from investigating and using valve circuits since approximately 1970 as a HAM Radio operator and of course reading TUT (I own all of them) and for the past 18 months correspondence with KOC. 
     I worked in aerospace industry in the United States for 3 different DOD 
contractors, mainly embedded work for the last 20 years of my career, writing millions of lines of code, mainly in C and 
C++.  Before the embedded stuff, I was involved mainly in the early development of hybrids. Before I obtained my degree
I was an electronics tech so I have a fair amount of diagnosis and repair experience.  
  
   With all that said; this is a deep subject, which is not just tech but also has an element of an art form.  I look forward to
learning from everyone here and hopefully, I can contribute to other members knowledge and help out with projects, theory, 
construction etc.  One last thing, I was also a machinist for about 15 years, mainly building engines, but also jobber stuff.
One might say, how can you have done all this, well the simple answer is, I am in my 7th decade on this planet (and maybe
more on another).  Humor, is also a very important part of life, never take it to serious, as you won't enjoy it.

    One thing I noticed reading the forum before I joined was the civility practiced here.  I have left others because of the lack of
this aspect, so I thank everyone before hand for being considerate of other members, realizing we all have more in common that not.
Take care and peace,
Rob

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  Hi
Posted by: Sam - 08-22-2020, 09:15 PM - Forum: New Member Introductions - Replies (2)

Hello TUT members,

I'm Sam from Australia. Designer of gravity processing plants for the sand mining industry by day, guitar amp entusiast by night.
I stumbled across the TUT books years ago whilst trying to find out how to mod my Marshall TSL60 online.
After reading some reviews, and having a favourable exchange rate AUD/CAD at the time, i ordered the full set of books.

After getting into TUT i got stuck into the TSL. Swapping the power tubes for 6L6's and taking the speakers out of the open back combo cabnet and making some detuned cabs for them. This was a marked improvement over the original tone, but rather than trying to mess around with the amp any further, i decided i had to build one from scratch. There were plenty of options from TUT3 & 5. I probably should have started with a Champ, or something from TOT, but went straight for the Doppelsonde.
It was a huge learning curve, but the amp turned out just fine. Since then it's had an SB PS kit, SUS kit, and RBX added. The front end now has a dual channel 'Plexi' with a BFX loop.
Awesome amp.

After a few more projects, and lots of tweaking, i got sidetracked with other things in life and haven't built anything new for a long time. Just enjoying the gear i had already made.
2020 saw me stuck at home for a while with time on my hands. Thought i'd start working on a new amp build. And now i discover there's a TUT forum.

Hope to have fun shooting the breeze with you guys, and learning something new, or remebering stuff i forgot.

Cheers,

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  fx loops
Posted by: Adam - 08-19-2020, 11:15 AM - Forum: Preamp Mods & Design - Replies (2)

Hello.
no one seems to have posted about this so I thought id ask a few questions!

Firstly if I add a ‘Best all-tube loop’ (BATL) in TUT (fig 7-16) if I want to bypass it do I do it a) Completely with relays or b) can i just mute send/return portion?
Secondly, If I have two preamps with their own loops, each channel can have their own send but share a recovery ( like the example in TUT 4 fig. 6-45)

Where would the two signals mix? I’m guessing at the “0” virtual earth point in the BATL? I can mute each loop when each channel is switched in/out?

Thanks
Adam

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  Hi all!
Posted by: Champ81 - 08-11-2020, 11:01 PM - Forum: New Member Introductions - Replies (2)

Hi everyone.  I am from Ontario Canada and just found and joined this forum and glad I found it. Just wish it was sooner.  I have been a big fan of Kevin's stuff for years and used his power scaling and many other amp improvement kits to great success. I am still relatively a real novice at this but managed to build two "ultimate" amps through the kits.  Hats off to you Kevin for the books and which i reference quite often for my builds as well as sharing of important knowledge.

Where is everyone from?

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Come in where it's warm!
A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor's The Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
Please remember these three principles: respect, sharing, community.
Not familiar with The Ultimate Tone book series? See discussion topics, or click here to visit London Power/Power Press Publishing.

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