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  50 watt amp project
Posted by: TiCatFan - 12-06-2019, 08:43 PM - Forum: Personal Projects - Replies (4)

Hey all;

looking for some help, i have put together the power supply kit PSU TPA and the power amp PA66, 50 watt version. i had the power supply working in a very prototype way, transformer and power supply board laying on my bench , but working. decided to add a power switch, it works, output transformer, power amp and front end. Hooked up an output jack and impedance selector, kept the options to a minimum at this point, master volume and presence control ( the ones available on the PA66 board ). Stuffed it in a hammond chassis, plugged it into the power limiting safety socket and turned it on, no tubes.

But, it doesn't work. I am trying to trace through the circuit to find the problem but thought I would ask on the forum if someone might know of likely suspects I should look at.

What I do know is, when I turn the amp on the light bulbs on the safety socket come on and stay on, fairly bright. The fuses do not blow, these are the fuses supplied with the kit and I am not sure offhand the wattage or blowage?? of the fuses. The panel LED does not turn on, nor does the 12v fan. Kind of afraid to leave it on long enough to check for voltages anywhere, I have left it on for a 10 count a couple of times, nothing hums or smokes or anything.

Is it possibly a messed up MOSFET?, any ideas on what to look at and how would be greatly appreciated. I have been trying to follow the AC supply but I also wonder if a short to ground would take out the fuses. What might draw power but not blow up the amp???

Greatly appreciate any help I can get...


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  Hello from England
Posted by: The_only_Tinners - 11-29-2019, 02:38 PM - Forum: New Member Introductions - Replies (2)

Hello from across the pond. I've been fixing up old amps, and building the occasional one myself and trying to take it a little more seriously. The next build is more ambitious, great to have the wealth of knowledge available here Smile

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  Hammond chassis mounting question
Posted by: FormyxAmps - 11-27-2019, 05:57 PM - Forum: Wiring - Replies (4)

Hi all,

For mounting a usual Hammond chassis in a wooden enclosure MArshall-style, I use angled brackets mounted on the chassis using blind rivets where the whole construction is then fastened using bolts and screws, but I've always wanted to have a system that uses cage nuts instead...

How would you mount it? What hardware would you use?

P.S. Couldn't find a more relevant subdir, hope its is ok to post here. Wink

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  PSU testing
Posted by: SGillespie - 11-19-2019, 05:19 AM - Forum: Power Supplies - Replies (6)

Hey folks,

So I've built my first London Power standalone preamp kit and unsurprisingly it doesn't work. This is my first amp project; I'm used to building pedals. But I have to start somewhere. It's the M-Pre (hot rodded 800 version) with the Pre-PSU for two tubes.

So what I did was build it like I build a pedal. Basically followed the instructions, put it together and turned it on. Of course in hindsight I should hve been constantly testing as I went, but as i said, it's my first one.

So the tubes didn't come on and of course there's no sound. So what should I do first? Check the psu, I guess. So what would be the best way to go about that. I have some basic electrical knowledge (mostly safety stuff) and of course a dmm. Any advice would be greatly appreciated.



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  New kid
Posted by: SGillespie - 10-28-2019, 08:30 AM - Forum: New Member Introductions - Replies (4)

Hey All,

I'm really looking forward to getting into building my own amps. I've been a guitarist (sometimes bassist) for 30+ years, played in numerous bands (none famous) on both sides of the Atlantic; first in Canada (home country) and now in Belgium (long story Big Grin )

Don't be surprised if I end up asking some real noob questions  Blush


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  Questions about Super Scaler Project in TUT5
Posted by: makinrose - 10-20-2019, 01:15 AM - Forum: TUT Q&A - Replies (2)

I've been fascinated with the Super Scaling project in TUT 5 but have few questions:

A.  As the chapter on Super Scaling indicates would I get about a 6 fold increase in power? So andamp that was 10W would be 60W through it, right?  

B. Power possible would be limited by the output power of the Super Scaler's OT?  So how much power is possible? 

C. Does the power tube type color the sound a lot? 


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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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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.
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