REDDI DI

[abbey road d enfer]

Okay thanks for your answers!

I mean the Alpha 1k audio taper in the schematic.

OK, I didn't see it, it's a matter of the forest hiding the tree.

I used a rev log potentiometer for it -- would linear potentiometer be better?

Revlog at 10% rotation is only about 0.5dB down. Use a log one.

Oh, and I use a UA Arrow interface in the mic input, but I almost never turn up the gain

I couldn't find any specs regarding input sensitivity/impedance for this Arrow interface, but I would bet anyway your isue is the wrong potentiometer taper.

 

[ruffrecords]

Not wanting to cause more confusion but in this application the pot is not wired as a potentiometer.

Cheers

Ian

 

[rock soderstrom]

Not wanting to cause more confusion but in this application the pot is not wired as a potentiometer.

Yep, it think its called rheostat.

"The difference between a rheostat and a potentiometer is that the potentiometer uses three connections and forms a voltage divider, whereas the rheostat only uses two connections: One end connection and the wiper contact. It is therefore an adjustable resistor whose current flow depends on the position of the wiper contact."

 

[buchholzstudio]

Yep, it think its called rheostat.

"The difference between a rheostat and a potentiometer is that the potentiometer uses three connections and forms a voltage divider, whereas the rheostat only uses two connections: One end connection and the wiper contact. It is therefore an adjustable resistor whose current flow depends on the position of the wiper contact."

Not wanting to cause more confusion but in this application the pot is not wired as a potentiometer.

Cheers

Ian

I am grateful for any help!
I have never heard of rheostat in my life. Can anyone tell me exactly which component is best for this application?

 

[merlin]

OK Grandad, we've all seen your micro-micro-farad condensers. Nobody says rheostat anymore. It's just a variable resistor.

 

[buchholzstudio]

I have now grounded the cabinet and now the hum is really minimized
I have an A/B comparison here in my dropbox
DIY DI Box
https://www.dropbox.com/scl/fi/r4uk...K_DI.mp3?rlkey=022j3l3x4fe3rw665ikmc92l2&dl=0
Millenium DI Box (~15€)
https://www.dropbox.com/scl/fi/2dkx...m_DI.mp3?rlkey=08g8c32v5c3yn37q27wnk4efn&dl=0

 

[abbey road d enfer]

I am grateful for any help!
I have never heard of rheostat in my life. Can anyone tell me exactly which component is best for this application?

Today, a rheostat is more a concept than an actual part.
Etymologically, it's a "flux regulator". In jurassic times, rheostats were made as wirewound resistors with a wiper.
Originally they were used only to control current, so only two terminals were accessible, but when the notion of voltage divider and its extension, the potentiometer became desirable, the three terminals were accessible.
You make a rheostat by using only two terminals of a potentiometer. One has to be the wiper, the other either one of the others, typically the CCW one, but not always.

 

[rock soderstrom]

OK Grandad, we've all seen your micro-micro-farad condensers. Nobody says rheostat anymore. It's just a variable resistor.

We really learnt this in basic training, which must have been a while ago. Time goes by, son.

 

[Wilhelm]

After a long forced break, I was finally able to continue! I have set up everything and have now been able to test it.
Basically it works, but I have a few questions:

1. if I turn the regulator up 10%, it is actually already too much - what could be the reason for this, or is it the same for you?
I noticed that I have 200 VDC with the B+ (instead of 170 VDC as in the circuit diagram) ... can this lead to problems?

Could the output-transformer perhaps have become connected in reverse by accident ? If that's not cause, i would suggest reducing the gain in the circuit by removing the cathode electrolytic capacitor (470 yf) - and increasing the cathode-resistor value to 820 Ohm - or even better : some 470 ohm for each of the cathodes of each triode-half. If you then still have too much output, you can reduce the gain further by adding some 2k2 to 8k2 ohm in series with the cathode resistors. For this to work though, you will have to "lift" the input by dc-insulating the input, using fx. a 100 nf/200v capacitor in series with the input-lead AND disconnect the 470 Kohm grid-bias resistor from ground - and then reconnect that end of the grid-bias resistor to the point between the extra gain-reducing resistor and the new 820 Ohm resistor - (otherwise, the biasing will be getting absurdly altered.) Perhaps you will even want to install a variable resistor for this gain-adjustment, instead of a fixed resistor

2. there is also a very loud humming noise, which decreases when I touch the housing while playing the instrument - so I still have to ground the housing, right?

Why did you place the Tube right next to the power-transformer ? Could you perhaps rearrange the layout in your very nice box, so all signal-carrying wiring ( which must be shielded) and active circuitry ( the tube) is as far away from the where you have 50-60 Hz high-voltage happening ? Often it helps a lot !

3. i have noticed that the tube only lights up very weakly... i know it differently from my amps. The tube is very old (probably built at the end of the 80s...) is that normal?

Best regards

Check the voltage at the filaments - +- 10 % is normally acceptable. Are you sure there's enough voltage - for the series regulator to work ? Please report back - if none of this helps. Best regards

 

[mrclunk]

How much gain does the circuit have? I'm sure mine had about 20dB at one point but now i'm only getting +12dB...

 

[abbey road d enfer]

How much gain does the circuit have? I'm sure mine had about 20dB at one point but now i'm only getting +12dB...

Typical gain is supposed to be about 16dB with a 1k load.

 

[CJ]

that pot will drag more AC current through the transformer primary which has a dcr of about 800 ohms,

and the secondary will eat some too but the sec dcr is much lower at least on the ones i wind as it has three secondaries in parallel, like the Telefunken stuff which is where i stole, i mean borrowed the idea.

so if you load the secondary with that pot you will get a voltage reduction via the I x R effect,
and some tone shaping.