Help with EL504-based amplifier

[Socram8888]

Hello. This is my first tube-based amplifier I've ever built, and I'm experiencing a strange issue with it.

The amplifier works well when the EL504 is heating up, but it stops working when fully heated. It works also when cooling down.

I'm following an schematic made for the PL504 but modified (I've just added another transformer for the 6.3VAC the EL504 uses)

Here's the schematic:


The output of the rectified voltage measures about 31VDC (vs the original 27V the original amplifier used)

Thanks

[Kay Pirinha]

Hi,

three issues to consider:

1. 31 volts B+ supply ist way too less for your tube.
2. EL or PL504 don't work too well as a single-ended class a output device.
3. A mains transformer isn't a good output transformer.

Best regards!

[Socram8888]

I don't want (or need) an HiFi quality, I'm just interested in building a very simple amp with tubes, then try building some more complex

The schematic is from the famous electronics magazine Elector:
http://www.gtlab.net/gtlab2/filemana...ow_anode_1.pdf
http://www.gtlab.net/gtlab2/filemana...ow_anode_2.pdf

Also there are videos on YT with PL504 working at an even lower voltage, this one for example: Amplificator cu tubul PL504 la 27V - YouTube

Thanks for your answer

[Zibi]

No bias at all.
Biasing Power Tubes

[Socram8888]

I have no 1 Ohm resistors. The lowest resistor I have are 75 Ohm resistors. Can I use these to obtain for example 7.5Ohm, then bias with that value?

So the correct schematic is this one, right?

Which resistor adjusts the bias?

Sorry if these questions seems too basic or stupid, but it's my first project using tubes. Thanks for your patience

[quikie22]

put in a 30 ohm resistor at the cathode. that should work

[quikie22]

since u have 75 ohm resistors, put a pair in parallel at the cathode...

[Socram8888]

Thanks! That worked. I've put a 1kOhm resistor on the G1 and removed the capacitor. Now it sounds more clear than before.

A bit offtopic: would be possible to use this exact circuit with this other tube? http://tubedata.tubes.se/sheets/127/1/17DQ6B.pdf . They're both dirty cheap, but these use a more common octal socket

[SoNic_real_one]

The difference between "working" and "working well" is made by the right supply voltage.
Sure, it "works" with 30V but that is still in the unlinear region of the tube (knee), so you will have lots of distortions, especially at low level passages.
Phillips recomens supply voltages between 170-230V for 504 tubes.

The second tube is a horisontal deflection tube. Very high voltage, low current, works best in pulses... not an audio device.

[aardvarkash10]

actually, the (X)dq6b has been used successfully as an output tube, but only in PP circuits (or OTL). It seriously sucks as an SE tube...