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Posted by: NGW - 09-11-2019, 12:45 AM - Forum: Preamp Mods & Design - Replies (10)

Hi Ultimate Tonesters,

I have had to shelve amp work for most of this year, due to other pressing issues, but I am now getting back into it.
The TUT Forum is looking a bit quiet; where are you all?

I have a query regarding bypassing a Gain Stage, which is a bit foreign to how I would arrange a Drive stage, so this is not something I have tried before.

The attached circuit shows a 4 stage preamp that uses 3 for Clean and adds the 2nd stage (V2B) for Drive.
The cct. is shown in Drive mode and when switched to Clean, Relay 1B enables a full bypass of the 2nd stage. 

In Clean mode; would you advise grounding the output from V2B, as shown with Relay 2, to remove any possible signal or noise from V2B entering the grid of V1B; or will the Clean Bypass be sufficient to ensure uncorrupted Clean sound?
Modifying the PCB traces and installing wiring to enable Relay 2 to ground V2B output is not a fun job but, if it is advisable, I will do it rather than have the clean sound compromised.
The owner loves the Clean sound and doesn't want to lose it.

Thanks for any advice,


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  Looking for some Insight in Dumble Steel String Singer Phase Inverter
Posted by: makinrose - 08-26-2019, 08:04 PM - Forum: TUT Q&A - Replies (5)

After reading the Dumble chapter in TUT6 (if you don't have it get it!)  I got really interested in the different Dumble circuits. Most are simply variations of what is in TUT6  but the Steel String Singer caught my attention since it's a little different. Basically what I'm seeing is the Dumble clean channel but with a reverb configured a bit like standalone reverb unit and switchable filter.  That makes sense to me.  

However the topology of the phase inverter does not. It looks like a schmitt but has a cathode follower for each drive line.  Is there any benefit to this approach over the standard plate driven configuration? Has anyone here played an amp with this topology? I've attached a schematic!

Hmmm....I've having trouble  attaching  the PDF.  Hopefully you guys can still envision what I'm talking about....

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  SOMA 84
Posted by: sinusoids - 08-21-2019, 12:05 AM - Forum: TUT Q&A - Replies (12)

Why is the input jack not insulated from the chassis? 

What is the 270R from the OPT 16 Ohm tap to ground on the schematic, is it on the wiring layout?  Gathering parts for this.

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  Fixed to Cathode Bias switching problems...
Posted by: liquidair - 08-16-2019, 12:37 AM - Forum: TUT Q&A - Replies (14)

So I built an amp with a fixed/cathode bias switch like that for the Standard project in TUT5. The only difference was that I used a relay in place of the switch and a cap across the cathode resistor, but I've had some problems.

While testing without tubes, I found that in cathode bias mode, 1/2 of whatever the bias voltage was set to showed up at pin 5 of the tubes...fixed bias was fine. Simulations confirmed this behavior, and I found that changing the 2x 30k1 resistors around Q10 to 331k (c-b) and 2.21k (b-sw) ensured the bias didn't leak through anymore. Is that normal?

The second thing is the relay keeps sticking after a couple of switches. I assume this is the contacts welding from the cap discharging when the relay shorts it. Oddly, when I unsolder it and activate it out of circuit with a battery, it no longer remains stuck and works again. This relay looks heftier than the PA66-CR ones, so I expected it to be fine. Is there a way to stop this from happening like soldering a cap across the contacts to take the initial discharge or something?

My solution for now was to just get rid of the cap, but the problem is testing with a sine wave was putting 10W into that resistor (470R 7W). The ones in TUT5 are all 5W, and I realize that a constant sine wave is not like actual playing, but is this a big enough resistor? I'm going to assume that with the cap, all we need to worry about is more or less the idle power...any AC will go through the cap?

Thank you all!

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  Schematic management
Posted by: K O'Connor - 08-06-2019, 07:50 PM - Forum: PCB Design - Replies (4)

Hi Guys


When we draw a schematic by hand we are limited by the paper size as to how much circuitry we can comfortably fit onto the page. Symbols and text have to be large enough to read clearly, preferably without the need for a magnifying glass. Hand rendering usually invokes our sense of proportion and we try to keep similar symbols the same size as each other, and try to have room for what needs to be shown. We try to organise the circuit flow so it easy to follow and often use connection symbols such as GND and V+ to show things that are tied to each other without having to pull a line all the way across the page.

With computer software, and PCB design programs like Eagle, schematic entry is much the same as hand rendering except that we go to a library within the program to find symbols, then 'drop' them in place. Then we connect them with 'nets'. As we add more components we run out of space as far as the scale of our view goes, because Eagle defaults to a certain effective page size. We can zoom out and add more parts then use the 'fit' function to resize the circuit to the screen size. Things are smaller now, but likely with modern monitor sizes we can still read component names and values. However, it is very easy to get to a point where you have to begin zooming in to read text, and that becomes a problem if you ever want to print out the schematic.

In drafting, there are standard size "sheets", or paper sizes. 'A' is the same size as a North American letter size, 8.5" x 11". The orientation can be long-side vertical (portrait) or long-side horizontal (landscape). Size 'B' is twice the area; 'C' is four times; 'D' is eight times and 'E' is sixteen times. Obviously, for hobbyists and home users, printing anything other than 'A' size is impossible  Yes, there is 'legal' size paper and in Europe size A4 is slightly larger than letter size.

Eagle provides a facility to have multiple sheets for the schematic. This allows you to break the circuit into smaller portions that can be printed more easily and more legibly. When we need to make a connection tie from one sheet to another, we use "ports" or "wire links". These are DEVICES in the Eagle library that let you have these page breaks yet on the board the same-labelled ports are tied together by a trace.

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  New kid
Posted by: FormyxAmps - 05-21-2019, 05:26 PM - Forum: New Member Introductions - No Replies

Hi all,

I am a 40-yr-old youngster who builds tube amps and repairs/renovates instrument and Hi-Fi amps. Started reading the TUT series and building amps as a hobby, aside from guitar playing with our band, at around 2007-8.

After messing around, building, repairing and reading trying to figure out how stuff work, I decided that life is not worth it if you don't do what you love, so last year I decided to give it a go and launched my small company in Stockholm, SE in the hope that someday I won't have to be employed at my boring day job any more...

Very glad to be here in the forum with you all! Hope to contribute however I can! Smile


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  Question about Applying SUS-2 to See-saw Inverter
Posted by: makinrose - 05-05-2019, 01:19 AM - Forum: Sag & Sustain - Replies (6)

I'm working on designing a new amp.  I've used SUS kits before and really like them but have question: When applying the SUS-2 kit to cathode bias amp with the "Old Style" See-saw Inverter is it okay to lift the grid-leak connections to ground even though one of the triodes is sending signal to the other?  Will this affect the operation of the phase inverter?  Here's link to style of inverter:  Huh

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Video Londonpower PSU-20L, BFX, L-Pre, Z-Pre, Line-Out Pre amplifier
Posted by: peteotto - 04-26-2019, 09:50 PM - Forum: Personal Projects - No Replies

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  Heat management with Power Scaling??
Posted by: makinrose - 04-02-2019, 08:06 PM - Forum: Power Scaling - Replies (1)

With Power Scaling when is additional heat sinking and/or a fan necessary? Is there a certain temperature at which one should worry? I'm guessing chassis up in the cabinet designs are more problematic that the typical Marshall configuration.

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  New PSU kits +/-12V
Posted by: K O'Connor - 03-15-2019, 05:19 PM - Forum: London Power Kits & Mods - No Replies

Hi Guys

We have two new power supply kits to support low-voltage circuitry, such as for switching and opamps. The kits provide symmetric voltage outputs aka "split rails" of +/-12Vdc at two different power levels.

PM12-6 provides +/-12V at 175mA

PM12-12 provides +/-12V at 350mA.

PM12 has universal mains fusing, and uses Hammond semi-toroidal PTs, metal-film resistrs, and 10,000-hour caps.

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A warm welcome to those interested in tube and hi-fi audio! Fans 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.
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