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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Douk EL34 Amp kit

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W

wdecho

Member
Joined 2014
Avo111 said:
Folks,

Well the kit turned up today having had a nice tour of the far East thanks to mis routing by Fed-Ex! Overall it seems OK, many of the comments I have seen about build quality seem fixed and it even comes with a well ventilated bottom plate. Here are some pictures of the components attached ( i hope:)

First question is the capacitors. The two yellow ones are the coupling between stages. They are labeled BENNIL will they be OK or what would you suggest I should get to replace these? The carbon resistors seem all to be brown where I am expecting the tolerance band. does this likely mean they are 20 % if so should I get tighter tolerance for any specific resistors in the schematic?

I am planning to build the one I first posted however I want to add additional line inputs so I was proposing just to add a rotary switch and some more RCA connectors.

i am also wondering about adding an alternate value resistor in the feedback loop just to see the audible difference. Any suggestions what I should have as alternate for the 2K in the schematic?

Thanks AVO 111
Click to expand...

It will not hurt to build with the parts you have to start where you can compare the sound later if you replace some components. The ones I would replace first would be the coupling capacitors with some orange drops or comparable cap. Of late I have been using some of the Russian PIO that are selling on Ebay. Measure everything before soldering is good advice and just build. I look forward to hearing what you think when finished.
 
A

Avo111

Member
Joined 2015
Ok it's built! I followed the original schematic powered it up and no smoke came out! It works sort of. There is a massive difference between channel outputs and swapping the input valves moves the problem so I guess I have a dud valve. Just to confuse me there are two resistors and a capacitor left over that do not appear on the schematic! Doug agreed to send a replacement tube and said the odd parts were " spares" now looking for alternate input valves and probably a different pair of EL34s to,see how the sound differs. There is a small amount of mains hum but not sure where this is coming from could it be associated with the dodgy valve?

Thanks
AVO 111


Caber :)
 
A

Avo111

Member
Joined 2015
Now it is broken:( as above it was working and seemed to have a bad valve so I got a pair of Russian 6h9c that are supposed to be equivalent to the 6n9p that came with the amp however I put them in and now the amp is completely dead with these or the original valves. Any idea what might have gone wrong and where I should start looking for failures?

Thanks AVO 111
 
C

celestar32

Member
Joined 2013
Avo111 said:
Now it is broken:( as above it was working and seemed to have a bad valve so I got a pair of Russian 6h9c that are supposed to be equivalent to the 6n9p that came with the amp however I put them in and now the amp is completely dead with these or the original valves. Any idea what might have gone wrong and where I should start looking for failures?

Thanks AVO 111
Click to expand...
If I were you I should start testing it in various stages from power supply output to the power tubes. First should check power supply is normal at required working voltages usually no load is between 380- 480 at no load depending your main transformer output voltage. Secondly, start put in the driver tube and measure the required working voltage. Lastly, the power tubes as usual check the cathode current and cathode voltage and plate voltage. I think is much easier to troubleshoot in various stages.
 
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H

Hyperman75

Member
Joined 2014
Hi

I would like to make 2 points :

First: there are EL34 pentodes, europeen original design with acces to supressor grid g3 on pin 1 and there are EL34 beam tetrodes designed in the USA and called 6CA7, with no acces to supressor grids that don't exist (beam plates). They sound completely different.

Second : in the original schematics, the pentode is used in pentode mode in a Single-Ended power stage - this is the worst solution for hifi ends because mainly odd harmonics.

So, IMO, changing the OPT for a better one is not the only improvement to do but you also have to change the pentode mode to triode mode ( screen grid connected to plate ). Of course, you will have a noticeable lost of maximum available power.

Regards

Jacques
 
M Gregg

M Gregg

Disabled Account
Joined 2010
Are the heaters on and working?

I haven't checked but its OK saying some valves are a direct substitute but the heater current can be a lot higher on some tubes.
And it is possible to over load the heater winding on the power Tx. (Tx= transformer)

If the heaters are working.

Check every component that it is in the correct place and the right way around. Pay particular attention to electrolytic's in the PSU.

NB BE VERY CAREFUL IF THE TUBES HAVEN'T CONDUCTED AND THERE IS NO DISCHARGE FOR THE PSU IT WILL STILL BE UP AND LIVE AFTER POWER OFF!
ALWAYS TEST WITH A METER BEFORE YOU TOUCH ANYTHING!

Regards
M. Gregg
 
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M Gregg

M Gregg

Disabled Account
Joined 2010
Post the circuit you have built.

So we know exactly what you have.

You are aware of the stored charge after power off? Ie its still up at HT after power off..especially if the tubes don't conduct!

Always teat before you touch anything<<<or it will catch you out!

Regards
M. Gregg
 
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M Gregg

M Gregg

Disabled Account
Joined 2010
What voltage do you have across the two PSU electrolytics.

DC volts from one side to the other<<its up at B+ warning!

The diagram says 350V.

NB you have got them the right way round and the 47uF after the rectifier NOT the 330uF after the rectifier?

Regards
M. Gregg
 
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