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  Power Supply For Tube Circuit Development
Posted by: ZeusMC - 08-03-2021, 05:43 AM - Forum: Test & Measurement - Replies (1)

Hi all,
Are there any alternatives to the Heathkit Ip-17 regulated high voltage dc power supply?
Thanks.
Peter.

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  Suggestions for BJT Tremolo Oscillator
Posted by: makinrose - 08-02-2021, 03:45 PM - Forum: Preamp Mods & Design - Replies (4)

Hey everyone I'm looking for a good BJT oscillator circuit to drive a LED LDR or Jftet. I'd prefer the LDR.... I've used the London Power Wide Range Tremolo in a  tube amp and like it but since all my stuff uses eyelet boards I'd like to find a BJT circuit to do something similar. I find discrete components easier to wire up.  There seems like a multitude of circuits out there. Does anyone know of a good design to start off of? I figure I can derive the voltage from the high voltage supply and use a Zener to get the right voltages.  Thanks for the help!

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  Wanted Merlin Blencowes Book
Posted by: ZeusMC - 07-16-2021, 03:41 PM - Forum: Gear for Sale by Forum Members - Replies (2)

Designing Power Supplies for Valve Amplifiers.
Thanks.

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  Hello From England UK
Posted by: ZeusMC - 07-16-2021, 03:37 PM - Forum: New Member Introductions - No Replies

Hi everybody,
Just bought Kevins, TUT Deal - All Six Volumes also RSG - Ready Set Go.
I have two Merlin Blencowes books, Radiotron Designers Handbook, books by several other authors.

I'm Vegan, lifelong teetotal, green issues, animal welfare. I hope to learn a lot from Kevins books and also....... when I know.... what I'm talking about, interact with other forum members and contribute to the forum.

I play original Chicago style slide guitar, pedal steel, ( found it in a 2nd hand junk shop, in a black area, guys were coming in and out of the store trying to sell car CD,radios, with the wiring looms still hanging out )

Also play virtual synths, e-drum kit. Also getting started in guitar making, am already into woodwork, joinery. Have woodshaper, jointer, making stained glass doors, woodcarving.

All the best out thereSmile
Peter.

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  SV1, SV2, SV-TT update
Posted by: K O'Connor - 07-12-2021, 02:20 PM - Forum: Power Scaling - No Replies

Hi Guys

Back when I designed SV1, SV2 and SV-TT it seemed like "a good thing" to add some tiny caps to reduce the bandwidth of the otherwise wide-bandwidth regulator sections. The idea was that noise on the supply line that might be impressed across the Power Scale control could be amplified through the regulators and be heard through the amp. Of course, C1 across the pot output would clean this up all on its own.

In most installations these caps do not present a problem and everything works fine. However, some installations will show some oscillation and removing the small feedback cap 56-68pF will eliminate the oscillation. In SV1 and SV-TT, remove the 270pF from the bias regulator. Do this in TBS if there are issues. The bias regulator cap can cause a secondary oscillation.

Over the life of these kits, the 330k-1W feeding the Power Scale pot was split into 150k + 180k and a filter cap to ground was added at their junction. This cap helps reduce hum, ripple or other noise getting to the pot. Some installers found that C1 could be removed without any audible effect, although the main purpose for C1 is to keep DC-related pot-scratchy sounds from happening.

Techs report that he removal of these caps makes the amp sound better. It should, since these are very wide-bandwidth regulators which present a low-impedance voltage source to the amp allowing any frequency restrictions to be only those imposed by the amp itself.

I annotated the kit notes but if you have old notes or ones without the update, at least the information is accessible here on the forum.

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  London Power TBS Tracking Bias Supply
Posted by: K O'Connor - 07-08-2021, 01:18 PM - Forum: London Power Kits & Mods - No Replies

Hi Guys

TBS Tracking Bias Supply is a slight misnomer inasmuch as it is not the supply itself; rather, TBS is a regulator for the bias supply that tracks the screen supply (Vs)  or plate supply (Va,) as appropriate.

TBS requires a high-voltage input equal to about 16-20% of absolute Vs. For example, if there is 500Vs, then -Vb should be 0.16 x 500V = 80V, up to 0.2 x 500V = 100V.

The raw bias voltage should be generated to present a low- to medium-ompedance voltage source. We do this by using a separate winding with full bridge rectification. Filtering should be high-value capacitors with low-value resistors feeding them. A typical supply would use 100R feeding 47u to 100u, feeding a second identical RC section.

Remember to tie a 100k across the bridge output to protect it, and secondarily to drain the raw bias filter caps at power turn-off.

If the winding voltage is too low to generate the required voltage, a voltage doubler can be used. This simply places two cas across two of the diodes in the bridge. These extra caps perform the doubling function and the extra RCs are still needed.

Any kind of capacitively-coupled raw voltage source should be avoided. Same goes for deriving the raw bias supply from the end of a centre-tapped plate winding.

Once TBS is assembled and installed TEST WITHOUT TUBES.

Make sure that negative voltage of an appropriate range is present at the control grid pin of the power tube sockets.

TBS has a trimpot that can be used to set the maximum nagative output suitable to control the tubes to be used. This adjustment should turn the tubes off and is a set-and-forget adjustment. The trimpot is a convenience only. Ideally, R5 is increased to the next standard value as needed and the three holes for the trimpot are jumpered.

A panel-style pot is provided to make day-to-day bias adjustments, or when you change tubes. This pot should be mounted on the tube plane of the chassis near the tube it controls. This may be tricky to install but once in place is quite convenient. The bias pot is then as protected and as accessible as the tubes are, as TUT3 The Ultimate Tone vol.3 illustrates.

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  Wiring and PCBs
Posted by: K O'Connor - 07-08-2021, 01:00 PM - Forum: Wiring - No Replies

Hi Guys

I just posted the text below in a different thread but thought it should be here for more general viewing.

In the modern age of computerised PCB layout and overall design, it is very easy to eliminate all the wiring or very nearly so, but this means the whole amp or product is on one board. The chassis must have one panel that can be removed if one is to install or remove the card. It all looks good on the monitor and the assembly is quick, plus the board is complete so it can be tested prior to mounting in the chassis. Even though I am aware of the trap and know better, I have done this myself.

Wiring is the repair person's salvation provided it is done carefully.PCBs on the front panel, PCBs on the rear panel, maybe other PCBs for the PSU etc, wired to each other with generous lead lengths - service loops as they are historically known - allow smaller portions of circuitry to be dealt with more easily. It may end up that to replace a panel pot, say, one has to free all the front panel pots, but if this means you do not also have to free up the rear panel items or other things, then it is one step closer to being a breeze to service. Breaking the control and jack boards into yet smaller pieces makes it quicker still.

The other thing having some wiring does for you is to improve the reliability with regard to mechanical strain due to flexure of the chassis. Typically the pots, jacks and switches secure the PCB to the panel and there can be strain on the solder connections depending on how much overall support there is for the card. A further benefit to having some wiring and smaller PCBs is that each is mechanically only related to a small part of the chassis and therefore measurement and placement is less critical and tolerances of the chassis bends and dimensions is less critical. The reality is that everything is designed at once in the same software, so mechanical alignment will be as good as the chassis fabrication allows.

In the case of the PA66, the PCB is supported by metal standoffs, four around each power tube and three on the front-end. The tube socket leads are bent over for mechanical integrity prior to soldering. The preamp PCBs are similar with enough mounting positions over the length of the board. This is actually quite robust provided one takes the normal amount of care during tube insertion and extraction.

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  Power Resistors in kits
Posted by: Strelok - 06-02-2021, 04:53 AM - Forum: Preamp Mods & Design - Replies (5)

Hi all Tuberionatians!


I noticed that in the kits there are no Power Resistors anymore for local decoupling.


Or so it seems.
Maybe they are rated 1 Watt and I just don't see it.


In earlier times it was advised to raise them above the PCB or circuit card, but I don't see that notice anymore.
Have the resistors become so much cooler?


Also, it was always advised to make a shape with the legs instead of just keeping them straight.
I always wondered what that is about.
I can think of only one scenario:
The Power Resistor gets very, I mean extremely hot in case of some failure.
It melts, no it boils the solder and threatens to do a "melt down": sink down the solder holes onto the board.
There it could make a short if it touches another component on the way down.
Therefore the legs are bent, so that it can not lower itself onto the board.


Kind regards,

Eric-Jan

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  Question about bias scheme on Wards/Danelectro/Airline Amp
Posted by: makinrose - 05-07-2021, 10:55 PM - Forum: Personal Projects - Replies (2)

I have a customer who is curious about me building him amp similar to this one:dano_commando.pdf (el34world.com) It's pretty straight forward except for the biasing scheme for the power tubes which to me does not look like a great idea.  The cathode of the power tubes runs to the heaters of the 12AX7 which are wired in series to the ground along with a 10uf cap; a odd ball form of cathode bias.

I could be wrong but it seems to me if the you ever had a 12AX7 heater go bad you would have real problem and not develop voltage on the cathode and the power tubes could go in runaway.   I'm not sure if this type of bias system has any tonal properties.  Does anyone know?  Would I be better off making the amp a more standard form of cathode bias or fixed bias?  

Thanks for the input!

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  FX Loop Bypass - Signal Loss
Posted by: NGW - 04-25-2021, 04:56 AM - Forum: Preamp Mods & Design - Replies (2)

Hi Fellow TUT Followers,

Attached is a schematic showing the FX Loop I used in an amp that I built for a local pro. (The preamp shown is not in his amp, which has switchable Clean and Drive modes.)
He wanted a serial loop, for a Delay pedal, followed by a Reverb pedal (fitted inside the cab), instead of built in reverb and a panel knob to control the reverb level.
I achieved this with the attached circuit.

He recently asked if I could make a foot switchable bypass for the serial loop so he could mount his Delay pedal in the cab also and then just use the amp foot switch box, instead of adding the delay pedal out front. I was able to do this by repurposing one foot switch's function that he didn't use and changing that relay's wiring, as shown and adding a panel switch, as well.
Initially I had it wired so that the loop return was switched in and out, but that meant I lost the Delay Tails.
I rewired it to switch the send in and out, which retained the tails, but the Delay pedal output Z is loading down the bypass signal and there is a volume drop; it works great with no pedal, which is not the object of the exercise.

I would like to find a way to have the Tails and no signal loss in bypass, but I am struggling to find a solution.
If any of you can suggest a suitable way to fix this, I will be very grateful. Otherwise I will change back to loop return switching and he will have no Delay Tails when bypassing the loop.

Cheers, Noel


.pdf   40C110 - FX Loop Bypass. 2302021.pdf (Size: 709.88 KB / Downloads: 5)

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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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