Author Topic: Philips AG9015 - interesting output stage  (Read 301 times)

Offline El Sid

Philips AG9015 - interesting output stage
« on: February 18, 2021, 01:38:49 PM »
So I spotted this Philps AG9015 amp for sale in a radiogram: https://www.avforums.co.za/index.php/topic,8971.msg1044072.html#msg1044072




Took a bit of searching to find it - there are a few similar models, some of which are output transformerless. So I looked around more and saw they can have OPTs, but examining the schematic shows an interesting output stage topology. Discuss for 25 marks.



The OPTs are quite weird-looking things:



Full circuit:

« Last Edit: February 18, 2021, 01:41:21 PM by El Sid »

Offline handsome

Re: Philips AG9015 - interesting output stage
« Reply #1 on: February 18, 2021, 05:28:45 PM »
Its a very elaborate SRPP. Phillips built speakers and tubes for these amplifier designs originally to avoid using output transformers - speakers were high impedance (i think ~80R?) and the valves were low impedance. The OPTs look weird because they have the extra windings for screen voltages. Beautiful looking amplifier though - trust you snapped it up?

Offline Steerpike

Re: Philips AG9015 - interesting output stage
« Reply #2 on: February 18, 2021, 05:39:41 PM »
This is what Philips called their Series Balanced Output.
The AG9014, and the radiogram I have (model lost to memory) have no output transformers and require the 800R loudspeakers.
It has an astonishing power bandwidth for a 'home' hi fi of the 1950s of 7Hz to 50kHz. (THD 0.2%)
The AG9015 and 9018 had transformers so they could be used with 8R loudspeakers too, with lesser output power (12W).

The transformer offers the added advantage of screen grid modulation (a kind of "ultra linear" operation), which is impossible in the capacitor-only output stage.

The EL86 was created specifically for this topology - high anode current at moderate anode voltage, but particularly, the heater insulation is very good, since the "top" valve's cathode can rise to several hundred volts above the heater supply - which something like an EL84 will not survive. Sometimes an EL84 was used as the "bottom" valve.
You can get quite a shock from the speaker terminals (for safety they are somewhat like deeply recessed 3-pin XLR sockets)

Offline El Sid

Re: Philips AG9015 - interesting output stage
« Reply #3 on: February 18, 2021, 06:17:46 PM »
Its a very elaborate SRPP. Phillips built speakers and tubes for these amplifier designs originally to avoid using output transformers - speakers were high impedance (i think ~80R?) and the valves were low impedance. The OPTs look weird because they have the extra windings for screen voltages. Beautiful looking amplifier though - trust you snapped it up?

Thanks - good explanation. I didn't buy it because I really do have enough amps (can't believe I just said that!!) and also it's in Mbombela. I wonder if those are original Philips speakers too?




Offline El Sid

Re: Philips AG9015 - interesting output stage
« Reply #4 on: February 18, 2021, 06:20:45 PM »
This is what Philips called their Series Balanced Output.
The AG9014, and the radiogram I have (model lost to memory) have no output transformers and require the 800R loudspeakers.
It has an astonishing power bandwidth for a 'home' hi fi of the 1950s of 7Hz to 50kHz. (THD 0.2%)
The AG9015 and 9018 had transformers so they could be used with 8R loudspeakers too, with lesser output power (12W).

The transformer offers the added advantage of screen grid modulation (a kind of "ultra linear" operation), which is impossible in the capacitor-only output stage.

The EL86 was created specifically for this topology - high anode current at moderate anode voltage, but particularly, the heater insulation is very good, since the "top" valve's cathode can rise to several hundred volts above the heater supply - which something like an EL84 will not survive. Sometimes an EL84 was used as the "bottom" valve.
You can get quite a shock from the speaker terminals (for safety they are somewhat like deeply recessed 3-pin XLR sockets)

More great info! I have subsequently seen on the internet anything from high praise and love to absolute slagging off of these amps. How do they sound?

Offline Steerpike

Re: Philips AG9015 - interesting output stage
« Reply #5 on: February 18, 2021, 06:52:19 PM »
More great info! I have subsequently seen on the internet anything from high praise and love to absolute slagging off of these amps. How do they sound?

Since they ONLY work at their best with loudspeakers such as the AD9017AM, these amps are often/usually not heard for what they can do. The 9017AM doesn't have spectacular bass - even though the amp can deliver full power (20W) at subsonic frequencies. With vocals or chamber music, they are spectacular - makes one think there has been zero progress in amplifier technology in 70 years.

Offline Katji

Re: Philips AG9015 - interesting output stage
« Reply #6 on: February 18, 2021, 08:03:22 PM »
hmmm, that progress is sort of contextual.

Those might actually be the original speakers - I didn't think so when I first saw the pic, but now I see there is no usual grille or speaker cloth. That would make it very unusual, separate speakers. I was already thinking I'd never - afaik - seen a radiogram with a built-in cassette deck. Not so sure though, I vaguely remember one that had a cassette deck that was sort of tilted up.

These things really do need to be preserved.  .....The amps, I mean. The turntables I couldn't care less.

Offline Steerpike

Re: Philips AG9015 - interesting output stage
« Reply #7 on: February 18, 2021, 08:56:05 PM »
I don't think this thing as shown is how it came from the factory - it's had bits removed and added.
That cabinet would have had a multi-band radio/amp combination. (similar to https://i.imgur.com/MQf8GyI.jpeg)
That's not a Philips cassette deck, and cassette had not been invented during the time the AG9014 (or the AG1015 record player) was in production.
The amplifier is a step up in quality, and a retro fit I suspect. Those styles of cabinets always had a compartment in which to store your records - seems to be where they put the amplifier.
« Last Edit: February 18, 2021, 08:58:24 PM by Steerpike »

Offline Katji

Re: Philips AG9015 - interesting output stage
« Reply #8 on: February 18, 2021, 09:09:13 PM »
ok, that explains/clarifies.  I hadn't quite realised it was not the usual radio+amp, with a glass face...the photos of it separate were puzzling.

Offline Zilch

Re: Philips AG9015 - interesting output stage
« Reply #9 on: February 24, 2021, 10:21:05 PM »
I don't think this thing as shown is how it came from the factory - it's had bits removed and added.
That cabinet would have had a multi-band radio/amp combination. (similar to https://i.imgur.com/MQf8GyI.jpeg)

@Steerpike thanks for linking to my Philips radio :thumbs:
If you don't understand the difference between "their", " there", and "they're", then your an idiot.