• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

OTL amp with EL504 for 800 Ohm speakers

Status
Not open for further replies.
Jump to Latest
3

32860

Disabled Account
Joined 2009
#1
My nickname being EL504, i found it appropriate to show a picture and the schematic of an otl amplifier i have built some years ago. It is meant for 800 Ohm loudspeakers.

Philips in the Netherlands used 800 Ohm speakers in radio's, tv's and HiFi equipment in the 50/60's. By using tubes with low Ri in combination with the 800 Ohm speaker impedance, they could skip the OPT.

The EL504 is a power beam tetrode for horizontal deflection in tv's.

Klaar+4.JPG


Schema.png
 
#5
Philips made (or branded) them in the 1950's and 60's to match OTL amps they developed in a number of products. They ranged in impedance from mid-600 ohms to around 1.2kohms, 800 ohm being the more common.

The windings are VERY fragile and along with the natural decay of speaker cones and other components, ensured not a lot survive.
 
#7
Thanks for the feedback.

I could (should?) try SRPP but for a period i was simply very into building OTL amps like this because i like the way they sound very much. Because of the way i built this serie of amps, it is very easy to change them into SRPP though so i will try that out in the coming time.

It is not very hard to get your hands on 800 Ohm speakers in the Netherlands (i did buy some the ones i own trough: https://www.marktplaats.nl/z/audio-tv-en-foto/luidsprekers.html?categoryId=38 and then search for Philips.

The only real woofer for 800 Ohm is the Philips AD5201A "Bombardon" wich is made of styrofoam and has a special kind of support of the front side of the conus. See: OTL800: De Bombardon.
The Philips 9710A (no whizzer cone) and 9710AM (with wizzer cone) are 800 Ohm broadband speakers with a good reputation. I have some myself and like them a lot but they do not give very much low bass.

In general, the Philips codes for 800 Ohm speakers have an "A" after the the number. Information about the speakers of Philips can be found here: John's Radio Web then click on "luidsprekers" on the left.

I personaly have no bad experiences with windings of 800 Ohm speakers going bad. I do know however that the three screws in the magnet of the AD5201A woofer are prone to break in time and by doing so, the windings will also break. For that reason i supported the magnets of my two AD5210A's in the enclosures i have built for them.

My style of building started with breadboarding but got a bit out of control. I don't have the tools to make nice looking metal chassis. Since i don't have children or pets, and because at line level shielding is not necessary (the amps have no hum or hiss at all) this style of building is ok for me.
 
#8
The attached table may be useful to source a vintage 800 ohm Philips loudspeaker. Philips Hi-Fi drivers seems to have been popular at some point in the '60, but only in Netherlands, I believe. It is quite difficult to source them online at reasonable prices on non-dutch sites (as many other Philips items), and I never saw them on any thrift store in my country. The main source for the rest of us are old tube radios, mainly from the Jupiter and Reverbeo range. They are unfortunately very expensive compared with other radios of the same era. When I see a cheap ebay listing for this type of radio, the 800 ohm speakers have always been lifted. Not surprising, since I believe that the current market price for the AD9710AM is around 300/400 euro for a pair. I have a pair of smaller AD3800AM on one of my radios and they sound nice, but there are other cheaper and more conventional speakers that sounds better (for me).
This EL504 OTL amplifier is a peculiar design anyway. The PC86 input tube with both filament and B+ independent power supplies is a unconventional choice.
 

Attachments

  • philips_loudspeakers_1966.jpg
    philips_loudspeakers_1966.jpg
    300.1 KB · Views: 715
#9
This brings back memories of my own 800 ohms speaker amplifiers I build in, indeed, the 60-ies. I opted for a cyclotron, first with 807's, later with EL36's.

The nice thing with this is that there is just a center-tapped choke across the outputs, in parallel with the speaker. The grounded center-tap of the choke provides ground reference.

Jan
 
#15
A link to the book "Hi-Fi Amplifier Circuits" (Philips, 1965). See pages 85 - 93 for a description of an otl amp for 800 Ohm speakers with ECC83 + 2 x EL86 with both positive and (very much) negative feedback and an special way to feed the screens of the EL86's (with a double choke). On pages 16 - 20 and 24 - 28 some theory on otl amps:
http://www.introni.it/pdf/Rodenhuis%20-%20HiFi%20Circuits.pdf

Philips used this end stage in a series of Hi-Fi amplifiers (AG9013, AG9014, AG9015, AG9018). I have built two of them for bi-amping with an active crossover, see:
OTL800: OTL Versterkers met zeer veel tegenkoppeling
 
#16
..........from Klein/Zaalberg ; Precision Electronics :

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


and here are my NOS AD1250/M800 :

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


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


I also have a "Bombardon" but none of this projects get ready 🙁 .
Kind regards , Alexander .
 
Status
Not open for further replies.
Top Bottom