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    Building, troubleshooting and testing of these amplifiers should only be
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Help with EL504-based amplifier

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Socram8888

Socram8888

Member
Joined 2008
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:
An externally hosted image should be here but it was not working when we last tested it.


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

Thanks
 
Socram8888

Socram8888

Member
Joined 2008
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/filemanager/userpix/igor/low_anode_1.pdf
http://www.gtlab.net/gtlab2/filemanager/userpix/igor/low_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
 
Socram8888

Socram8888

Member
Joined 2008
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?
An externally hosted image should be here but it was not working when we last tested it.

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
 
S

SoNic_real_one

Member
Joined 2008
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.
 
Last edited:
Socram8888

Socram8888

Member
Joined 2008
SoNic_real_one said:
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.
Click to expand...
AFAIK, both the xL504 and xDQ6B are horizontal deflection tubes :/

Also, I'm using ~30V because it's my first tube project, so I don't want to play with high voltages yet.
aardvarkash10 said:
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...
Click to expand...
I can get these locally for less than 2€, so the amount of tubes is not really a problem. I'm looking here using the search for circuits using the 17DQ6B or 6DQ6B, but I haven't found any. Do you have a link?

Thanks to everybody for your help :)
 
Socram8888

Socram8888

Member
Joined 2008
Actually I have a 6AR11 double pentode compactron.

This is the full list of tubes I have:
- 2 EL95 small audio pentode
- 2 6AS6 RF pentode
- 3 EL504 horizontal deflection pentode
- 1 17DQ6B horizontal deflection pentode
- 2 ECC89 VHF double triode
- 2 6N1P double triode
- 1 UCH84 RF stage triode & pentode
- 1 UBC81 RF diode triode
- 2 Z806W neon discharge tube
- 4 0RP30 selenium light sensor
And the said 6AR11

If there is anything interesting let me know. There is a local store who sells old NOS tubes (but they don't have any classic audiotube, EL84, EL34, KT88, ...
 
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Kay Pirinha

Kay Pirinha

Member
Joined 2011
UCH84? Never thought that ever has been there any! It's - presumed- counterpart ECH84 once has been derived from the elder ECH81 especially for TV services (line sweep oscillator, sync separation), not for radio sets, and never has been there any TV set with a 100 mA series heater chain. Also can't find any datasheet of this tube...

Maybe you might have an UCH81?

Best regards!
 
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aardvarkash10

aardvarkash10

Member
Joined 2007
30V B+ is going to be VERY disappointing, - to all intents and purposes impossible and not a learning step at all (unless you count learning that 30V is not enough).

At 150V, you start getting into a usable operating point. It will certainly do more than tickle if you accidentally touch it, but is not likely to be deadly unless you are already unwell and you take extreme risks eg using a line supply
 
Socram8888

Socram8888

Member
Joined 2008
aardvarkash10 said:
30V B+ is going to be VERY disappointing, - to all intents and purposes impossible and not a learning step at all (unless you count learning that 30V is not enough).

At 150V, you start getting into a usable operating point. It will certainly do more than tickle if you accidentally touch it, but is not likely to be deadly unless you are already unwell and you take extreme risks eg using a line supply
Click to expand...
I have a 230V to 125V transformer. I only have to replace the input transformer, or I have to change something more?
 
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