Philips output triode HELP please

[trabant]

I have a 1930s Philips and it's been robbed of its AC044 in the past. I need to replace this with another valve. I have a couple of new AC/HL kicking about I could do with someone giving me an answer as to how to alter the circuit please.

[snowman_al]

Unfortunately the AC/HL is not an output triode, just an amplifier triode. (For stages before the output valve.)

There have been a few suggestions recently about 'replacements' for the AC044 / PX4 etc. (They attract good money which is most likely why it is missing...) Did anyone have success with them?
Alan

[ekjdm14]

I'm one of the others in need of an AC044 equivalent, although other projects have taken precedent for now so I've not looked much further into the options suggested nor can I even remember what the suggestions were without referring back to my Bush SAC35 thread. (I do recall one suggestion being to build a solid-state substitute, although would very much like to stay with a 4 volt valve with around 1A filament draw if possible).

[trabant]

Ok let's go with plan B fitting an E443h pentode instead

[PJL]

Might be a good idea to merge this with your previous thread
https://www.vintage-radio.net/forum/...d.php?t=156789.

The AC044 handles 250V @ 50mA with a filament of 4V 1A but directly heated pentodes with this low a filament power have lower maximum anode current, for example the PM24M is 250V @ 30mA but the E443 and even the PT4 are similar. In these, some power is dissipated by the screen grid when wired as a triode but I don't think that would account for all the difference.

The Philips service sheet says the AC044 is 250V @ 48mA but more alarmingly the Trader says 280V @ 53mA.

You can fit a E443/PM24M and wire as a triode then adjust R7 (this sets the bias) to reduce the combined anode/screen current to 30mA. The 575A uses a magnetic speaker so we don't have a field coil that would cause a significant HT rise but you may still need to put a series resistor between the rectifier and reservoir capacitor to keep the anode voltage down to 250V.

I assume you have an E443H? The datasheet is here https://www.radiomuseum.org/tubes/tube_e443h.html BUT the anode voltage is 250-15V in that circuit so you might want to start with 470 ohms for the cathode bias (R7).

[Radio Wrangler]

If you can sneak in an added heater transformer, then you get a huge choice of pentode or beam tet bottles. You can triode strap them and get an anode characteristic close to the robbed rhino horn.

David

[Ed_Dinning]

Hi, note based on David's comment ; you can use a low voltage auto transformer which will be physically quite small to fit under the chassis is you wish to preserve original above chassis looks

Ed

[Argus25]

If I was confronted with this, I would use the highly capable, economical and easy to get RCA 6N7. Strapping the two triodes together, then step up the 4V to 6.3 using a small autotransformer. Very easy to design & wind on a 1cm(squared) cross section laminated iron core.

[Hartley118]

The 6N7 is the forgotten medium power output triode. It's a double triode designed by RCA primarily for a zero bias class B output stage, specified for up to 10W audio output. That 'class B' intention has fortunately protected it from audiophool price escalation.

However... connect the two halves in parallel (cathodes are already common) and bias it into class A with a few volts grid bias and it becomes a decently linear triode output valve which will take upto 300V and dissipate up to 10W on the anode.

I've successfully used a 6N7 as the output valve in my modified R1155. I wanted a triode for my application mainly because, unlike a pentode or tetrode it copes with accidental disconnection of the output transformer - there's no screen grid to suffer excess dissipation.

The 6N7 is certainly worth a try if you want retain the triode output feature of your radio. I gather that the 6A6 is an equivalent on a UX base instead of octal.

Martin

[Radio Wrangler]

Shhhh, Martin, quietly... we don't want the wrong people to hear.

To throw them off the trail, we ought to mention that the 6N7 suffers from terrible granularity, and almost complete lack of air and its bass is dreadfully slow. AND it will pollute all their cables, requiring them all to be replaced/upgraded.

David

[Hartley118]

It’s OK. Don’t panic David. I see that audio forums already deal with those 6N7 issues. Its Class B genes mean it’ll never make it as a high flyer.

Martin

[PJL]

The audio folks are correct in this case as the 6N7 will be in grid current just to get the two sections to reach 30mA quiescent let alone the 48mA of the AC044. Good for Class B Push Pull but useless for a single-ended setup.

The step up transformer would also be a problem as, even if the transformer was 100% efficient, the 1A @ 4V corresponds to to 630mA @6.3V. If you went down this road you would be far better off using an EL84 that is capable of handling the plate dissipation and there would be no grid current.

Stick with the E443H if you already have one.

[Hartley118]

Say no more on the 6N7......Not a word to the triode aficionados.....Remember that in single-ended configuration, most of that distortion will be lovely bright even harmonics......

It's been a while since I looked seriously at the 6N7, but I've just refreshed my memory from its somewhat unusual data sheet. With zero bias, the pair of triodes aren't really in class B at all because they're drawing 35mA quiescent current. That's why some 3-4V negative bias turns it into a half decent single-ended class A output stage. No need to run into grid current.

I guess that in its originally conceived zero bias push-pull application, it would more properly be classed as AB2 rather than B because

1) It has a substantial standing quiescent current
2) It can actually be pushed into some 20mA grid current if it's required to achieve its maximum 70mA anode current.

A further paralleled 6N7 is suggested by RCA as a driver. Its original application back in 1935 was intended for a compact portable PA amplifier.

To begin to converge back on topic, I can confirm that the 6N7 makes a remarkably cheerful sound in my R1155.

Martin

[Argus25]

One other reason I mentioned the 6N7 is that it was the output valve I used in the very first stereo class A audio amplifier I constructed when I was about 15 or 16 years old. It was a wonderful performer and the type number got etched into my brain. Yet I cannot now recall the driver valves I used. I had two small speaker cabinets made of 3/4" chip board, painted purple, with 4 inch Philips drivers with big ceramic magnets and a terrible turntable . I wore out my Deep Purple LP, but I loved the sound of this thing. I had no idea about distortion figures, or even knew what output power it had, I didn't have a scope or a dummy load then either. But I have very good memories of the 6N7. Later I found them being used in the deflection and oscillator stages of American Pre-WW2 televisions.