Author Topic: Philips EL6411  (Read 9330 times)

Offline Mars

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Philips EL6411
« on: July 14, 2014, 11:30:13 AM »
I have this 1950's PA amplifier that have been waiting to be used for something. I've decided to change it to a stereo hifi amplifier to test my latest C-core output transformers. I took some photos before I stripped the amplifier.

I do not have photos of the amplifier outside. This I got from the 'Net




The inside is very beautiful:



Microphone inputs


Here is the bias supply


ECC40 gain stage


More of the same with the phase splitter in the foreground


The EF86's have been removed. A shot of the power supply arrangement:


Power amplifier coupling caps and grid leak arrangement


That's it folks!





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Offline valvesound

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Re: Philips EL6411
« Reply #1 on: July 14, 2014, 12:10:47 PM »
Hi Mars, man I love these rats nests.....what do you intend changing to make it 'hifi'?
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Offline Mars

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Re: Philips EL6411
« Reply #2 on: July 14, 2014, 12:19:33 PM »
I have actually already done the modifications, and the amplifier is playing music.

I removed the EF86's and the mic xlr inputs. The OPT was removed and two Mars 10k:8 ohm OPT's were fitted. I also removed the electrolyic caps and old rectifier bridges.

Kept the fixed bias setup and the two ECC40 tubes. Each tube was wired like the original phase splitter. Kept the 0.068 mustard coupling caps.

I am running the amplifier as a power amp driven by a CCDA 6SN7 preamp.
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Offline valvesound

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Re: Philips EL6411
« Reply #3 on: July 14, 2014, 12:35:38 PM »
and...are you happy with the result? I like the 6sn7 CCDA as a pre-amp!


On a different note, sorry its off topic....You mentioned your [size=0px]Mars 10k:8 ohm OPT.[/size]
[/size]
[/size][size=0px]Do you have something I could use i.s.o Edcor CXPP25-MS-8K?[/size]
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Offline Mars

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Re: Philips EL6411
« Reply #4 on: July 14, 2014, 01:18:50 PM »
I have a design that sounds excellent. I've used it here:


It is smaller than the EDCOR transformer. Do you need all the secondary taps, or is 8 ohm sufficient?
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Offline valvesound

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Re: Philips EL6411
« Reply #5 on: July 14, 2014, 01:29:52 PM »
I think a 4 and 8 ohm tap would be required to keep it flexible.
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Offline Qualityten

Re: Philips EL6411
« Reply #6 on: October 03, 2015, 07:01:24 PM »
Is there any chance that you have drawn a schematic of your design?  I also have one of these that I'd quite like to re-purpose.

Offline Mars

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Re: Philips EL6411
« Reply #7 on: October 06, 2015, 10:48:53 AM »
Hi

I basically used the published design. The two ECC40's became preamp/phase splitter for each channel. Here is a cropped schematic:

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Offline Steerpike

Re: Philips EL6411
« Reply #8 on: October 06, 2015, 05:43:24 PM »
What is the EL81 anode voltage ?

Offline Qualityten

Re: Philips EL6411
« Reply #9 on: October 07, 2015, 11:13:36 PM »
Thanks for your reply Mars. 

Steerpike, the Dutch service manual is available here: http://frank.pocnet.net/instruments/Philips/EL6411/EL6411.pdf

It gives the anode voltages for the EL81s as: 'nullast.' (=unloaded?) 297-315V and 'vollast' as 263-279V.  It says that the amp should be run within these limits.  I haven't had mine working for a few years, so can't do a live measurement right now, but last year I measured 290V on the anodes.
« Last Edit: October 07, 2015, 11:20:33 PM by Qualityten »

Offline charles

Re: Philips EL6411
« Reply #10 on: October 08, 2015, 07:48:29 AM »
According to Philips Electron Tube manual (1963):
El 81
Limiting values
Anode voltage in cut-off condition (Vao) = 550 V
Anode voltage (Va)                                 = 300 V

Offline Steerpike

Re: Philips EL6411
« Reply #11 on: October 08, 2015, 02:55:00 PM »
Sometimes Philips really pushed their valves to or over the limit! I can't recall which model, but one of their EL34 amps has 1000V anode supply.

Offline Mars

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Re: Philips EL6411
« Reply #12 on: October 08, 2015, 06:44:22 PM »
Yes, Philips knew how to get most out of a tube. The PA Steerpike is talking about is the EL6431







EL34 can actually handle 800V on the anodes:

EL34 Ratings (from http://tdsl.duncanamps.com/show.php?des=EL34)
Vh    Ih    VaMax    Vg2Max    VhkMax    PaMax    Pg2Max    IkMax
6.3    1.5    800                500      100       25         8      150

This amp runs at that voltage, and keeps running.... still running.

You'll probably need a quad of these tubes:

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Offline Qualityten

Re: Philips EL6411
« Reply #13 on: October 11, 2015, 12:20:40 AM »
Mars, thank you for your idea of making this old PA integrated amp into a usable stereo power amp.  I've dug out my old EL6411, disconnected the EF86s, removed the mic inputs, etc. to do the same.

Would you be willing to post a couple more pics of your new build, especially from underneath, to prevent me from making errors?  Those C-Core transformers of yours look beautiful.  Do they come in a range of Z ratings.  I was interested that the 10k ones you used work well, as I have read that EL81s prefer 6.6k.  I'm still learning so I might have that wrong.

p.s. I notice your fuse holder is also exposed--it looks as scary as mine!

Offline Mars

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Re: Philips EL6411
« Reply #14 on: October 12, 2015, 10:51:36 AM »
Qualityten, the c-core transformers come in any Z you ask for. I do custom manufacture.

I'll take some photos of the EL6411 and post it.
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