Tiny Terror Clone with TB (James Baxandall)

[St4rP0wd3r]

Update:

I finally finished my project, after buiding it and testing many changes/alternatives to make it sound right, given that I used my own tentative selection of PT and OT.

The James Baxandall works like a charm and having it switchable was definitely right as it drops volume a lot! Anyways, there is still plenty of power and gain there and the amp gets more on the range of a vintage JTM 45.

Talking about Marshalls, the amp initially was already sounding amazing, but it was too trebly like a modern Marshall, instead of having thet mid-roar like the oranges. To achieve it I finally settled on changing the drop resistors values to have an overall acceptable voltage on the power tubes grid, which now are about 312V, and keep a better voltage on V1 and V2 plates, 188V and 224V respectively.

I also set the bias slightly colder to achieve some sag and bassier tone. The widely available schematics on the web indicated a 120R cathode resistor, but it from the beginning set my bias over the tuve max! I tries 150R, which was good but Marshally, and ended settling on a 180R resistor with a healthy colder bias.

As for the transformers I'm using Tube-town 50VA for the power, and theis 18W Marshall model as the OT.

I'm pretty happy with the results and will leave some pictures and layout here for your review. Ah, the V1A cathode bypass switchable alternative I used is a waste of time as it is quite unnoticeable. I'd stick with the original 22uF and 1,5K combination only.

Now I just have to redo some of my soldering works as they ended up lame with the many changes I had to make for troubleshooting and tone search!

Also I'm capturing all my handmade amps with Tonex, so let me know if you are interested in hearing them!

Cheers guys!



"Hi Guys, I just buit my first amp, a RobRob Deluxe. I decided to go full wild and selected and ordered all the parts by myself which I bought mostly on Tube Town. I was able to troubleshoot it quite easily when my 5Y3 malfunctioned and added a solid state option switch as a new rectifier valve would take some time to be delivered.

I've already ordered the parts for my second build, which will be the conversion of a Orange Crush 12 combo into a Tiny Terror Head (+ will keep the 6 inch speake because it is there).

I've found all the schematics for the tiny terror, but I've been thinking about including a few tweeks and would like to know your thoughts:

1- Most of the character of the TT comes from the coarse high gain from V1A and V1B, combined with the simple tone adjustment. I'd like to give it more flexibility of a TMB adjustment, without loosing too much gain. I've been researching and thought about using the Dumblelike solution of Mat's Mini HRM amp, on which the TMB enters before anything on V1A anode, and the original tone knob will be kept as sort of a cut knob. I plan to make the tone stack switchable so I can always turn back to the original circuit.

2- The secont mod is easier, and will be to add a switchable option on V1A cathode bypass, with a lower 4.7uF cap with 2.2K resistor, so I can achieve a more trebly-mid tone on the amp. The original 22uF and 1.5K resistor will be kept as swichable of course.

Let me know tyour thoughts, mainly about the Tone Stack and the complications you think it may generate in my build.

here is my layout in case anyone can help with any corrections or improvements:

Cheers."

[pineapple]

Nice job on the Layout. Did this work out well? I have an OTT build that I may get the TMB treatment.

[cdemike]

Cool design, and congrats on the first build! Before getting into the layout, I wanted to raise a safety concern with the SPST power switch setup; many buildings are wired incorrectly, reversing "hot" and "neutral," so that it'd be possible with this switching arrangement to have an always-on hot lead. Using a DPST power switch that disconnects both the neutral and hot mains connections avoids this problem. I have more suggestions regarding the layout below, but another safety concern I have has to do with the B+ present on un-insulated resistor and capacitor leads (as is the case at the junction from the first gain stage's load resistor and the coupling capacitors, i.e., at the TMB end coming out of the tone stack switch). In addition to having lethal voltages on uninsulated flying leads, those leads are very close to the only part of the amp that users consistently touch and manipulate. In other words, very high voltages are on uninsulated leads in a part of the amp most subject to having loose fasteners. Additionally almost no switches on the market are designed to stand up to high DC voltages, so that switch is likely going to fail prematurely due to arcing across its terminals. If you use a DPDT switch, you can use a series coupling capacitor after the first on one pole and then send the signal to either tone stack using the second pole. That'd involve minimal switching noise and would mean all controls are at ground potential, eliminating the safety hazard and eliminating a failure point for the switch itself.

I did want to warn that mod #2 described in the original post will introduce very noisy switching since the operating point of V1 is changing with the different cathode resistors. You can achieve the same frequency response range without the noise by making only the cathode bypass cap on the first gain stage switchable, i.e., leaving the cathode resistor alone and picking either 2k2 or 1k5. 1k5 will offer more headroom. However, with that load 68k load resistor you'd have to hit the front of the amp pretty hard so I'd personally go with the 2k2 with a switchable 22uF bypass cap in parallel with the 4.7uF cap on the cathode bypass resistor.

As far as layout goes, here's my input:
The grounding scheme for the power supply will likely be very noisy. I'd strongly recommend reading this article: https://valvewizard.co.uk/Grounding.pdf
In short, you have the filtering for the first gain stage connected directly to the ground reference for the bridge rectifier, which is more than likely the noisiest ground reference in the amp. Most "best practice" grounding schemes I've seen connect that ground (or the power transformer center tap, if applicable) to the negative terminal of the reservoir capacitor. The fix that involves the least changes to your layout is to connect the rectifier ground to the reservoir ground and from there run a lead to the ground tab. You'd need to disconnect that filter node's ground to the screens' filter node ground, but you can leave the downstream nodes' grounds connected as-is beyond that. You'd then want to ground the string of filter caps' grounds to the ground tab. The grounding setup on the signal board similarly should be rearranged such that the grounding tab is connected closest to the grounding points for the V1 grid and cathode resistors. Relatedly, there is no actual connection to ground for any of the grounds on the signal board (only the power board has a connection to a grounding tab). I would also suggest not using a ground bus as is shown; I know Marshalls have used grounding busses on the backs of pots, but it's really bad practice and while they might get away with that scheme with an acceptable noise floor to some, it won't be clear if that'll work for your amp until it gets built, so I'd save myself the trouble and just use discrete wires running to ground from the controls.

I don't think you need both the grid leak resistors if you use a PPIMV. Sometimes it's a good idea to include a resistor between the PPIMV's wiper and ground in case the pot fails so that you have a redundant, stable ground reference that keeps the output tubes biased. Since it's cathode biased you just put a safety resistor with a value of your choice referencing the wiper to ground as long as the total resistance from G1 to ground is less than 1M per the EL84 datasheet.

[St4rP0wd3r]

Thank you very much for the time you took yo look into it, it was very helpful! I see the B+ concern you pointed out and will fix it right away. I totally see that I should have the TMB after a coupling capacitor and will amend the design.

I will also take into consideration all other suggestions as to add security and avoid noise!

Will update you guys once the build is completed! Cheers!

[St4rP0wd3r]

Nice job on the Layout. Did this work out well? I have an OTT build that I may get the TMB treatment.

Hey pineapple. See my update! It turned out very good!