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EL84 Amp - Baby Huey
EL84 Amp - Baby Huey
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Old 18th January 2018, 09:06 AM   #1881
bandol83 is offline bandol83  France
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Join Date: Nov 2004
Location: near Bandol, French Riviera
Hi all,

I had a request from meanman1964 on a private message :

Hi Marc ,
Looking forward of building this amp .Ordered the parts at Mouser and Banzai music .
Gonna use my EL 34 Golden Dragons for this amp .
The only sad thing for me regarding your board is that I can't use a separate HV-PSU .
Are you planning a new layout that's having this possibility ?

I will answer here since as I have already said: "Please put the technical questions on this forum because it can help every body"

There is absolutely no problem to use a high voltage DC power supply with this PCB, the input bridge will be redundant, you will only loose 2 x o.7 V through the two rectifiers ! This is the safe solution

If you want to avoid this, you can replace D1 and D3 by a wire and DO NOT CONNECT D2 and D4, you will have the + HV on the right input os the connector (indicated AC) and the -HV on the left side (near R27). This is not a safe solution because if you make a wrong connection, you will destroy the amplifier

You can see the PCB layout in the attachement.

Attached Files
File Type: pdf PCB EL34 rev2 14 jul 2017.pdf (160.3 KB, 46 views)
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Old 18th January 2018, 09:57 AM   #1882
meanman1964 is offline meanman1964  Belgium
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Thank you Marc ,
When I leave the input bridge in place I've to connect the negative lead(seperate HV supply) to the side near R27 ?
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Old 18th January 2018, 01:14 PM   #1883
bandol83 is offline bandol83  France
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Join Date: Nov 2004
Location: near Bandol, French Riviera

I have a new request on PM Please use this forum for technical question ! PM are only for confidential private message, I will not respond to technical questions sent by PM anymore and my box is nearly full...

Question from potepuh:


I am about to built a baby hue with the 6550 or 6V6 will this transformer be apropriate? FS: Pair of Hammond 1628SEA HIFI Single-Ended Output Transformers In EU. or I should by the toroidy?

Thanks! Sebastjan"

No this transformer is for single ended amplifier as indicated and it is not designed for Push-pull output You need the recommended Toroidy or if you prefer Hammond : the 1650N.

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Old 18th January 2018, 03:08 PM   #1884
Elerion is offline Elerion  Spain
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Join Date: Sep 2017
Nice design!!
There's something I don't get from the wiki.

"The outputs must be high gm devices which will tolerate largish Rg1 values. That limits us to Cathode biased EL84 or if using fixed bias".

High gm means high maximum grid resistance?

Why only EL84?
EL34 has same or slightly higher transconductance, and can use a 700 kohm grid resistance.
Even KT88 can use 470 kohm, cathode bias (and its gm is similar to that of the EL34)
Just 6L6 and 6V6 are not suitable, according to that requirement.
Even power triodes like 2a3 and 300b can use 470k grid resistance (even though their gm are less than half of that EL84).
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Old Yesterday, 08:42 AM   #1885
bandol83 is offline bandol83  France
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Join Date: Nov 2004
Location: near Bandol, French Riviera
It is not only EL84

The thread started with EL84 because the original design was based on an ECL86 which was unfortunately no more produced and therefor Ian replaced the two difficult to find ECL86 by an ECC83 and two EL84...

I have designed 3 different PCB for the Baby Huey :

1) A simple version with two cathode biased PCL86 (equivalent to the ECL86 with a different heater voltage and easier to find) see photo...
2) The official EL84 third version with MOSFET source follower and fixed bias...
3) The octal output tube ultimate version with same MOSFET driver and fixed bias for EL34 or equivalent tubes !

There were many different version with the EL34 or 6CA7, the KT88, and even the 6V6 which work very well (see photo) and more than 200 PCB were produced with octal output sockets

Attached Images
File Type: jpg Ampli 6V6.JPG (413.5 KB, 73 views)
File Type: jpg PCL86 avec transfo Hammond 1603 10 kohms.JPG (414.2 KB, 75 views)
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