Help needed from someone who understands tube amplifiers / tube heaters

[industrialarts]

Also, if someone could explain - why are the two triodes run in parallel? Impedance matching? And, could they not use the same cathode resistor and bypass cap?

 

[beatnik]

True, in fact the schematic version I have has a common cathode resistor and capacitor. Makes sense if the cathodes are in parallel anyway

 

[TDM]

Was also going to ask what the concept is here? Why the quasi parallel triode? how much gain are you going for before the transformer and what type of "sound" are you after? A basic clean instrument DI? I've used prenty of real paralell triodes in amps, whats the advantage to "quasi" parallel like this? I'd use one gain stage then a cathode follower. Then a transformer can be used, or not used, for "flavor" choices.

 

[MatthewMachinist]

I am not planning on selling these, but I would like to make them available via osh park.

Attached is the updated pcb

Changes made include
- Moving the capacitors 12mm from the tube, this is the same distance as the Reddi DI unit.
- Terminals 4 and 5 should are connected by separate wires to the heater power supply
- Mounting holes insulated from the audio ground

Please let me know if there is anything else that needs changing.

 

[tony hunt]

The text for the tube pin-out still says B9A.
The correct pin-out designation for your chosen 6,3V heater +/- on pins 4 and 5 (with 9 as an optional screen) is called 9AJ.

 

[MatthewMachinist]

Are you sure it's a 9AJ

This site says B9A 6N1P @ The Valve Museum

The tube fits a B9A socket... are they interchangeable.

 

[Gus]

I just seached reddi DI schematic

What is the point of a DI using so much power?

The PS in the schematic in this thread needs help.

Whats the point of making a PCB design available that is an easy point to point or turret or terminal strip build?

Why the ground poor? is it to add stray capacitance or possible places for voltage breakthrough from adjacent conductors?

 

[abbey road d enfer]

Also, if someone could explain - why are the two triodes run in parallel? Impedance matching?

Sort of. It reduces the impedance seen by the primary, which is always good for the xfmr response. In addition it results in a small noise benefit of about 3dB.
Considering the application, I don't think noise is a real concern.

And, could they not use the same cathode resistor and bypass cap?

There's a (very) minor benefit in making the circuit less dependant on matching of both halves of the tube.

 

[ccaudle]

Sort of. It reduces the impedance seen by the primary, which is always good for the xfmr response. In addition it results in a small noise benefit of about 3dB.
Considering the application, I don't think noise is a real concern.

There's a (very) minor benefit in making the circuit less dependant on matching of both halves of the tube.

Is it usual for a tube buffer circuit to have such high output impedance? The note on the schematic says 130 Ohms transformer DC resistance, plus 600 Ohms discrete resistors, plus the reflected output impedance of the tubes. Seems very high for a balanced line driver, but I am used to dealing with solid state gear, so I don't know if that is normal for tube outputs or not.

 

[abbey road d enfer]

Is it usual for a tube buffer circuit to have such high output impedance?

Not really "usual" but usable in the context of bridging connections, where line inputs typically exceed 10k. Even if connected to a mic input (worst case 1500 ohms) it is adequate.

 

[moamps]

Is it usual for a tube buffer circuit to have such high output impedance?

Strictly speaking, it is not a tube buffer, because the electronic tube works in a mode where it amplifies the voltage. When you play with electronic tubes, you are always actually dealing with relatively high impedances, so electronic tubes need high power supply voltage to do something useful.

 

[moamps]

I just seached reddi DI schematic
What is the point of a DI using so much power?
The PS in the schematic in this thread needs help.

REDDI is a successful di box project (primarily) intended for bass guitar.
https://www.adesignsaudio.com/reddi-all-tube-direct-box
This means that the low frequency response is very important and that is why it is necessary to have a lot of current available and a large output transformer and consequently a heavy power supply. In the original version, the heater reg has a large heatsink.

 

[tony hunt]

Are you sure it's a 9AJ

This site says B9A 6N1P @ The Valve Museum

The tube fits a B9A socket... are they interchangeable.

9AJ is the correct pin-out for your chosen tube.
B9A is generically OK for a 9-pin noval base socket designation.
Perhaps you can add the 9AJ text to the underside.
Your decision.

 

[CJ]

It is a nice project, simple for a beginner, the power supply will benefit from a PC board, the actual triode circuit is so simple that there is really no need for a board, but since the pwr supply is going to have a board, why not add the few parts for the tube?

This circuit is being used in fair amount of studios and by a fair amount of well known musicians. Reviews have been very
. very positive.
Listen to the bass on recent Chris Stapleton songs for s sample.

 

[trobbins]

I would have though it wise to ask for all the information on the power system first - I can't identify the pcb for the power supply circuitry in this thread, which I would have thought needed initial review as well, especially as the topic of mixed grounds has been raised and the original schematic does not adequately show how the grounding is done in any detail.