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Power Resistors in kits

Posted by: Strelok - Yesterday, 04:53 AM - Forum: Preamp Mods & Design- Replies (2)

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

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!

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
Buju banton boom bye bye mp3. 40C110 - FX Loop Bypass. 2302021.pdf (Size: 709.88 KB / Downloads: 5)

Not So Subtle Sag

Posted by: Strelok - 04-15-2021, 08:24 AM - Forum: Preamp Mods & Design- Replies (2)

Hi all you FireBottle lovers!
FireBottle.. who came up with that name?
A long time ago I built a Sustainor Preamp.
Like I said so many times: this preamp rocks!
I tried new tubes in it, and they were doing a 'new tube' thing.
They gave a sag that was *not* subtle!
The harder I hit the strings, the slower the sound came up.
It was fantastic!
After a while the effect was gone though.
As far as I have seen, the SUS kit works time based, there are a few resistors that set the time.
Can it do this kind of sag?
I have an EHX 'Attack Decay', that one works by peaks.
When it detects a peak the preset attack and decay times are applied, so that is only based on preset times.
Fun Fact:
This thing can do something I have not seen anywhere else.
If you apply 'poly mode' each note that you play gets its own attack and decay, even if you play multiple notes and let them ring!
I have no idea how they did this.
You would have to sample each note seperately as they are played and then play back a sample with the correct envelope.
But how can you extract the different notes?
Nice riddle to think about, but no solution available!
Strelok

Using chassis-mounted sockets with the kits

Posted by: K O'Connor - 04-12-2021, 04:25 PM - Forum: London Power Kits & Mods- No Replies

Hi Guys
Hobbyists often ask if they can use a chassis-mounted tube socket with the kits, to which the answer is 'yes', but with caveats.
The PCB layout incorporates the tube sockets and this controls the physical form of the connections to the sockets. All the parasitic capacitances and inductances are then controlled and stability is assured for the assembly. There is a further benefit that unit-to-unit consistency is assured even though most hobbyists will only build one of a thing.
If you really must use chassis-mounted sockets but want to use London Power PCB project as the basis, then the grid-stop resistors should be mounted right at the socket pins and a wire link installed in those positions on the PCB. The wiring to each triode or pentode should just be #22 stranded wire, properly twisted. For example, each triode in a dual-triode needs three wires. You can solder the wires to the PCB end, say, then measure out more wire than the distance to the socket side to be wired; then cut the wires; then twist them starting at the anchored end and allow ALL 3 free ends to rotate as you twist; only then do you begin terminating at the socket. Of course, you can reverse this order and begin at the socket remembering to install the grid-stop between the grid wire and the socket pin. Each triode then has its twisted-triple.
Heater wires should be twisted unto themselves. London Power PCBs that carry dual-triodes accommodate both 6V and 12V wiring. 12V wiring is preferred for 12V-capable tubes, as TUT4 explains, and in that case you do not have to bring out the pin-9 wire. The PCB keeps the heater wiring away from the triodes but now you are changing the effective layout and need to do so with care.
DO NOT use multi-conductor shielded cable to the tube socket. The cable capacitance will kill the frequency response and you'll be replacing it with twisted hook-up wire, so save a step
The same guidance applies to octal tubes, pentodes and tetrodes. With the latter, especially with transformer connections, the transformer connections can still go to the PCB unless it is neater to have them go directly to the t

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