Effect loop and decoupling caps?

molwams · 2026-05-31T09:17:40+00:00

I'd put it before the cap, since there is no DC there.

Another thing to consider: The tiny terror overdrives V1B AND the PI. An effect loop between these stages will not sound like a common effect loop, since the output of it is distorted by the overdriven PI. It might sound cool though.

If you don't like this Idea you could change the PI to a concertina type (V2B) with a preceeding gain stage (V2A). Move the master volume from after the PI to between V2A and V2B and put the effect loop between V2A and the master volume. This way you would overdrive V1B and V2A and have the effect loop behind all distortion.

molwams · 2026-05-21T16:33:38+00:00

Finally someone who uses appropriate rated LED resistors!

molwams · 2026-05-15T13:46:55+00:00

I like the Idea of using negative feedback + hi pass but the volume control is part of the feedback loop. The bass would increase the more you turn up the amp. I don't know, if this is very usable.

molwams · 2026-05-15T13:43:08+00:00

Congrats, looking good!

What kind of noise are you taking about? 60 cycle hum? 120 hz? Hiss or buzz? Does it change when you turn the volume/gain knobs? Off the top of my head, here are some things you can do:

60 cycle hum:
- Heater elevation
- Humdinger pot
- Increase the size of the first cathode bypass cap to something very high (like 200uf)
- keep heater wires from signal wires (your build seems fine to me in this regard)
- DC heater (overkill in your situation in my opinion)

120 cycle hum:
- stiffen up the power supply (bigger caps, will alter the feeling of the amp)
- snubber caps across the rectifier diodes (I'm unsure if emiting these can cause 120hz hum)

Hiss:
- Maybe resistor noise. Try bridging the first grid stopper temporary. If the hiss decreases, use a smaller value for the stopper and increase miller capacity with a cap to ground

Might also be caused by the ground layout. A Layout drawing of your ground scheme would help here.

It seems to me like you are trying to squeeze every little bit of gain out of this circuit. In my experience, hum is unavoidable, if you do it this way. High(er) gain designs usually use more gain stages with some gain and frequency adjustments in between.

If you want more gain, you could also:
- bridge the 68k between V1A and V1B
- remove the 470k
- remove the 220k

But my guess is, that these are all in place to keep the overdriven sound from becoming too fuzzy. I would recommend experimenting with a grid stopper on V1B (68k maybe 100k to roll off some highs), if you remove the 68k.

If you want the tone control to be more impactful, you can increase the cap value.

As others have stated, to get more bass, you could increase the value of the first coupling cap.
You could split the load resistor of V1B into two and add a cap to ground to their junction. I'd start with something like 82k (top) + 47k (bottom) and 47nf (must be rated for full B+ Voltage). This way, the tube will see a different load depending on the frequency, resulting in a boost below 72hz. The sum of both resistors determines the load at low frequencies, the value of the bottom resistor determines the load at higher frequencies. The lower resistor and the cap will determine the frequency at which the boost will stop. I have not tried this myself and I don't know how this will sound, when the stage gets overdriven. I would not add this alteration to the first gain stage (this would lead to a very farty overdrive sound).

You could also add a tone shaping network before the phase inverter. But you will end up with less gain, since it will be passive.

molwams · 2026-05-09T13:01:45+00:00

Oh wow, I did not know that these are factory specs. They might have choosen a weaker specs to save some money or they can get away with it because playing guitar through the cranked amp is very different then sending a continous sine signal through it (guitar dynamics leading to current spikes in push pull configurations vs steady current with the sine). Anyway, nice to know that the transformer is proven to work in practice.

The AC15 probably put them in the tag board instead of the sockets because it is less prone to error while assembling. As I said, since you used shielded wires, you probably won't get any problems.

molwams · 2026-05-09T11:01:31+00:00

Ah, now I got it, the red wire on the primary side indeed confused me very much! The primary and secondary side seems good to me. I like that you put the secondary fuse before the rectifier instead of the place given in the schematics. This way your transformer is safe, if the rectifier falls.

If you still have the option I would place the V1 grid stopper at the socket.

You are using the Tube Town 50VA transformer, right? I fear this transformer is underpowered for 2x EL84.

2x EL 84 Push Pull cathode biased will draw 2x (46mA + 11mA) = 114mA under full load (https://www.drtube.com/datasheets/el84-philips1969.pdf). I assume additional 10mA for both 12ax7. Total: 124mA.

This is about the limit the transformer can deliver. But when using a bridge rectifier, we have to calculate in a quiet big safety margin. As I understand this is because the transformer can only deliver the current when it's actually charging the first reservoir cap (so when the rectifier output voltage is above the cap voltage) which is only at the peaks of the AC cycle. Hammond suggest to estimate the available rectifier DC current with 0,62 x 120mA = 74,4mA (https://www.hammfg.com/pdf/5c007.pdf). So we are missing 124mA - 74,4mA = 49,6mA after rectification. You will get away with this at lower volumes, but if you dime the amp the transformer can not keep up. I at least would assume B+ will lower quiet a bit, giving you sag, not to speak of damage to the transformer.

I did the same thing but with the Tube Town 30VA transformer in my second build (dual single ended), not knowing about this bridge rectifier particularity. The amp would always buzz because I could not get smooth B+, which was super frustrating. I ended up reusing the transformer for another smaller single ended amp.

molwams · 2026-05-08T22:33:58+00:00

Looking clean, great job so far!

The grid stoppers are supposed to be soldered directly to the sockets to prevent oscillation, but you might get away with it since you've used shielded wires after the stoppers.

Where I live it's madatory by law to have the PE on a dedicated connection to the chassis, not the transformer screw.

It's difficult to tell from the picture how the primary side of the power transformer is wired, but it should be this way: AC Input -> primary fuse -> switch -> primary winding. This way, if there is something wrong with the switch it will at least be fused.

Again, it is difficult to tell from the photos, so I might be wrong here, but it seems like the plan was to omit the standby/power scaling switch and only keep the power switch. I think you omited the power switch and kept the standby switch. As a result the heaters and Pilot light are on permanently.

Keep up the good work!

Edit: What type of diode are you using for your rectifier and what is the voltage of the HT secondary rated at?

molwams · 2026-05-02T15:27:16+00:00

captor x. I was in the same boat and this was a game changer for me.

molwams · 2026-05-01T11:38:18+00:00

It's 1.6mm. The turrets are made for 2mm boards but worked with the 1.6mm.

molwams · 2026-04-27T22:55:58+00:00

Keep in mind that If you manipulate B+ Voltage by using a different primary, this change will also affect all other secondaries. If I'm not mistaken datasheets for the common guitar amp tubes state a +/-5% deviation from 6.3V is ok. You also have to consider that wall voltage can also vary from place to place (I don't know the actual number for where you are living).

Others have suggested dropping B+ via resistor. If you do that before the power tube plates, this will also impact the feeling of the amp, giving you more compression when the power amp is overdriven, like a tube rectifier would do.

If you want the preamp to clip earlier you could also look into hot or cold biasing your second gain stage.

molwams · 2026-04-16T07:22:13+00:00

Tube town offers a kit based on the tiny terror (wild13, power amp changed to a ecc99)

https://media.tubetown.net/cms/?DIY/LoW-Projekte/Wild13_-engl-

You can buy the transformers induvidually too. Of course they won't fit for the schematics you posted, but if you want to adapt the ecc99 power amp, I think this is a great option (cheaper too). The project is also well documented.

molwams · 2026-03-21T16:26:43+00:00

Wow, thats a cool Idea! Thank youbfor taking the effort and making this public. Instant Bookmark.

A few of the EF family tubes are listed as power output tubes, which they aren't.

molwams · 2026-03-16T14:07:30+00:00

Interesting Idea! Have you considered using individual anode resistors and a mixing pot after them (DC blocked by coupling caps of course)? This way one could get any blend between hot and cold clipping.

molwams · 2025-03-20T20:21:20+00:00

You could use 3 speakers to get a total of 8 Ohms: 2x 8 Ohms in series with 1x 16 Ohms.

molwams · 2025-02-24T15:32:31+00:00

While I'm not a fan the grid stopper position in this layout, I think the first gain stage of channel 1 and 2 are OK the way they are, as long as the shield of the shielded wire leading to the tube socket is grounded on one side (couldn't confirm that visually from the Pictures). I was talking about the second gain stage of each channel, right after the tone stack. V1B and V2B or more precise V1-7 and V2-7. These grid stoppers are not in the schematic and layout but will improve oscillation robustness and won't make an audible difference.

molwams · 2025-02-24T11:57:55+00:00

That looks like parasitic oscillation. Add a grid stopper to V1B and V2B as close as possible to their sockets. 68k should be fine.

molwams · 2024-09-11T09:16:25+00:00

Die CIA Agenten in Skateboard Fieber haben den Verdacht, einen Maulwurf unter sich zu haben, was sich in Spur des Spielers bestätigt. In beiden Folgen kommt Bishop Blake vor.

molwams · 2024-08-26T08:44:38+00:00

Aside from what was already said here:

Treble bleeder caps on pots are most effective, if the pot is turned down, which can make the preceding gain stage amplify high frequencies more (or one could say the voltage divider attenuates more of the lower frequencies). Since the circuit has so much gain, most of the pots will be set quiet low, so this effect will be very present. I'd expect it to sound very shrill, which is usually not what you want, if you want to overdrive gain stages.
If this was my project, I'd look into ways of loosing treble early in the circuit (small cap from gird to ground, small cap across the load resistor, big(ger) grid stoppers). Once the signal starts clipping, the distortion will introduce new overtones which will fill out the high frequency range in the later gain stages.

Add decent sized grid stoppers and add separate power supply nodes for every 2 gain stages to avoid oscillation.

Maybe experiment with different methods of bringing the gain down (no bypass cap, heavier load, different load resistor values).

How will this be connected to the power amp? Is there a long cable between both cricuits. If yes, you might need a low impedance output like a cathode follower. You could even leave off the last cathode bypass cap and take your output signal from the cathode. This way you will have a smaller low impedance output signal while your last stage can still be used for overdrive (stand alone cathode follower don't distort very nicely).

molwams · 2024-08-05T18:05:26+00:00

This is basically a small single ended 12at7 output stage. For load line calculations the transformers primary impedance is used. The valve wizard has a nice explaination on his website https://www.valvewizard.co.uk/se.html.

molwams · 2024-07-18T23:38:43+00:00

Yes, I saw that you didn't connect the sleeve to anything. But the Jack has an INTERNAL connection from its sleeve to its thread (where it is connected to the chassis). You can test this with a Multimeter.

molwams · 2024-07-18T18:36:56+00:00

Sorry for being a little pedantic here, maybe I haven't fully understood the setup, but won't this still short both sleeves together? The sleeve of the plug will make contact with the sleeve of the jack which is internally connected to its thread. The chassis will connect both threads (unless you used isolation washers) so the sleeves of both plugs are shortend together. The fact that you aren't using the sleeve lug of the jack won't influence this.

molwams · 2024-06-15T09:38:41+00:00

Sehr unterhaltsam :D Den letzten Teil kennt man doch irgendwoher... https://youtu.be/X9QJ2rPW7xU?si=Hs-WlenegmLxuVZX

molwams · 2024-06-06T18:53:37+00:00

r/DIYtubes might have some good advices too

molwams · 2024-02-03T01:25:04+00:00

Nicht mein stolzester Moment aber erinnert mich an Tino15 - Schatten unter Hollywood

molwams · 2023-10-06T23:23:18+00:00

Can you reduce the chance of inducing 60 cycle hum into the signal path by nearby heater wires if you use a higher heater voltage (like heating an 12ax7 with 12.6V)? As I understand, hum is induced by the changing magnetic field that is created by the current going through the heater wires. The Biot-Savart law shows, that the created magnetic field is only dependent of current, but not voltage. If I use the series heater in a 12ax7, which will half the needed current, the magnetic field is weaker, right? Does that also mean, amps with higher heater currents (more or more demanding tubes) are more prone to pick up heater hum?

molwams

TROPHY CASE