DIY Valve Guitar Amp Head

[DrNomis_44]

Update:

I was just doing some searching on google for how to make an 8 Ohm/100W Dummy Load using some 5W Ceramic resistors, and found this useful webpage, so I thought I would post a link to it here just in case other forum might be interested, anyway, here it is:

https://www.omnicalculator.com/physi...allel-resistor


As it turns out, by my calculations if I put a total of ten 82 Ohm/5W Ceramic Power Resistors in parallel with each other, that will give me an 8.2 Ohm/50W Dummy Load resistor, so, maybe if I double the value of all the resistors, and put twenty of them in parallel, that'll get me closer to what I want.

So, 2 X 82 gives us a total of 164.


Now, here's my thinking, if we put ten 164 Ohm/5W ceramic power resistors in parallel with each other, we effectively get a 16 Ohm/50W Dummy Load, and, if we put two 16Ohm/50W Dummy Loads in parallel with each other, we effectively get.......an 8 Ohm/100W Dummy Load.

The question is, are 164 Ohm/5W Ceramic Power Resistors available? most likely not, I'll consult my Jaycar Electronics Catalogue to find out.

Turns Jaycar don't stock a 164 Ohm/5W Ceramic Power Resistor, the closest next value up that they do stock is a 180 Ohm/5W Ceramic Power Resistor, Cat. No. RR3280.

So, how close can we get to 8 Ohms with a resistor value of 180? let's see.....

If we put ten 180 Ohm/5W resistors in parallel with each other we get an 18 Ohm/50W Dummy Load.

If we put two 18 Ohm/50 W Dummy Loads in Parallel we get.....a 9 Ohm/100W Dummy Load.

This is one Ohm larger than we need, but, we didn't take into account the resistor tolerance, or how much +/- of the marked value the resistor/s actually measured, and we also didn't take into account the very small resistance of the solder leads either.

[Trevor Davies]

What is more important - the 8 ohm or the 100 W?

If you have two 18 ohms/50 W in parallel with an 82 ohm/5 W you can get 8.1 ohm /105 W? Is that allowed?

[DrNomis_44]

What is more important - the 8 ohm or the 100 W?

If you have two 18 ohms/50 W in parallel with an 82 ohm/5 W you can get 8.1 ohm /105 W? Is that allowed?

Yep, that'll work because the power dissipation will be divided between all the resistors.


The 8 Ohms is important because that's the load the amp wants to see in order for the power amp circuitry to operate properly, as long as the power dissipation of the Dummy Load is more than what the amp will put-out there won't be any risk of burning out the load.

[ian_macpherson]

Or you could use 11 180 ohm resistors rather than 10, which would give yo 16.4 ohms, or 8.2 for 2 sets in parallel.

[DrNomis_44]

Or you could use 11 180 ohm resistors rather than 10, which would give yo 16.4 ohms, or 8.2 for 2 sets in parallel.

Yes, you're absolutely right, and that's something I didn't even consider doing, as long as the Dummy Load is close-enough to 8 Ohms it should work fine, so, if we put eleven 180 Ohm/5W Ceramic power resistors in parallel, that gives us a 16.4 Ohm 55W Dummy Load, and if we put two 16.4 Ohm 55W Dummy Loads in parallel, we get an 8.2 Ohm/110W Dummy load which is perfect for what I want to use it for, basically as a dummy load for when it comes to setting the bias of the two KT 77 power tubes prior to test-out to see if it all behaves correctly, without disturbing my neighbours, I'm sure they won't appreciate having to put-up with hearing a 1kHz Sine Wave at say 110dB.

So, it looks like I'm going to be ordering X22 180 Ohm/5W Ceramic Power Resistors online from Jaycar, or maybe Mouser Electronics soon.


Maybe I should start a new thread in the Pitbull Guitars Amplifier section for building Dummy Loads for guitar amps? I'm sure lots of forum members would find it a handy device to make.

[DrNomis_44]

Update:

Just started work on making all the tagboards for mounting the components this afternoon.

[DrNomis_44]

Update:


Popped-by my local Jaycar Electronics store to buy most of the resistors I will need to build all the circuitry in my diy scratch-build Sloclone 50 amp, there are some resistor values I needed which Jaycar did not have in stock, so I will be ordering them, and all the caps and etc online from Evatco, and another online Electronics Components supplier, it's going to be a real fun ride this year, hopefully my build will sound similar to this Soldano SLO 100 amp:

https://www.youtube.com/watch?v=hhgV...oodRareGuitars

[DrNomis_44]

Update:

I stripped the Sloclone 50 chassis down so it was just a bare chassis and spray-painted the front and rear panels with some matt-white spraypaint I bought from my local Bunnings store, then I left it to dry overnight, today I went through the process of re-labeling all the controls and etc on the front/rear panels with some white paper dymo labelmaker tape, once I was satisfied that everything was good, I gave the front/rear panels two coats of satin clear-coat, also bought from Bunnings, seems to look pretty good so far, I'll see if I can post some pics later on for you to check out.....stay tuned.....

Here we go, this is what the front and rear panels look like now after I spraypainted them matt white and did all the re-labelling:

The Front:



And the Back:

[DrNomis_44]

Update:


The progress on my diy scratch-build Soldano SLO-50 valve guitar amplifier seems to be going really well, today I managed to get all the resistors soldered onto the two main Tagboards containing the majority of the preamp and power amp circuitry, all I need to do now is order some parts online such as the new HV Electrolytic caps, all the new Tone and coupling caps, Optocouplers, and Cliff Jacks, I'm also going to order a new 3-way Impedance switch while I'm at it, and anything else I need, but for now, here's a pic of the two Tagboards with the resistors all soldered in place, there are wire links on the underside linking resistors where necessary:

[DrNomis_44]

Thanks Doc... the circuit is very useful as the circuits I have are difficult to read hand drawn versions and don't include the wiring in the actual foot switch.

Initially I was only interested in how the switching was done so I could incorporate something similar in my AC18W build to swap from a 12AX7 input tube to the EF86 tube, but over the course of the afternoon I've been increasingly tempted to gut my 2204 build and step it up to a build not unlike yours as a 50W SLO-100 type build or more probably a 50W hot rod. As my 2204 build originally in a former life had 6L6's and the output transformer is impedance matched for 6L6's I'll probably pull the EL34's it's currently fitted with mostly as due to Z issues there have been moments where my 2204 is not happy with the load it get presented with. I have a Marshall with EL34's so some new tones will become available if I do make the swap in this build from EL34's to 6L6's...

Keeping the T/P switch is a good idea, If I go ahead then I'll probably keep the PPIMV control that is already in my 2204 build.

I've definitely decided to keep the T/P switch but I relabelled it so that in T or Triode it is labelled 25W, and in P or Pentode mode it is labelled 50W, I also re-labelled the Depth control to Resonance, I've also decided to switch the power tubes to a matched pair of JJ KT-77 tubes, from what I read on the Evatco website KT-77 tubes are a drop-in replacement for the EL-34 with a bit more power handling/headroom, and a tone that's a cross between an EL-34 and a 6L6, I'm also going to try a set of five JJ 12AX7/ECC83 twin triodes, I made a very simple mod to the tonestack where I can switch between a 33k slope resistor and a 47k slope resistor using a panel-mount DPDT toggle switch I labelled Mids, I used an app called Duncan Amps Tonestack Calculator (you might know about it) and I generated a frequency spectrum plot of the tonestack with the two different slope resistor values, from comparing the two spectrum plots, I noticed a shift in the midrange notch between the two resistor values, so I thought that would be an interesting feature to have.


I've figured-out how to wire the T/P switch so that it switches one end of the 1k screen-grid resistors from pin-3 of the power valve octal sockets for the Triode/25W setting, and the common screen supply voltage node on the power supply circuit for the Pentode/50W setting, the other end of the two 1k screen resistors connect to pin-4 of their respective power valve octal sockets.