Regentone radio identification please

[Gabe001]

I was wondering if I could get some help with this radio, which came to me indirectly; rescued from a skip.

I was planning to try to tackle it.

Can I get some help identifying the model? I thought it was an A353 but the latter has 2 speakers. The markings on the chassis say A358, but according to radio museum the valve lineup doesn't tally. This doesn't exclude that they could have been replaced at some point. Has anyone got any ideas? I'd just like to make sure I'm working to the correct trader sheet.

Thanks in advance
Gabriel

[crackle]

There is no image for the 358 in RM. So what valves are in your radio then?

Mike

[PaulR]

I have an A353 which has only ever had one speaker so maybe some examples didn't have the second one for some reason. It is a great set once restored.

[Gabe001]

Mike: 5z4g, 6v6g, 6k8g that I can identify

[Gabe001]

Originally Posted by PaulR

I have an A353 which has only ever had one speaker so maybe some examples didn't have the second one for some reason. It is a great set once restored.

Thanks paul

[McMurdo]

Funnily enough my 1951 Regentone U25 had 2 speaker holes and mounting points in the baffle when I got it (all original, and from original owner) but only one centrally mounted 3R speaker. When I overhauled it as a lad I only had 8R speakers to hand and the original was open circuit so I fitted 2 speakers to the baffle in parallel to maintain impedance.
The trader sheet showed 2 speakers.

Wonder if it was just a Regentone thing! (my diagram for the 353 shows a pair of speakers as well!).
Reminds me of Pye's trick of making cheaper versions for wholesale and better ones for main dealers. A practice that I'm reliably informed goes on to this day.

[Gabe001]

Originally Posted by Gabe001

Mike: 5z4g, 6v6g, 6k8g that I can identify

This is the RM lineup:
ECH35 EF39 EBC33 EL33 AZ31

[crackle]

It looks like you are going to have to examine the chassis carefully to see if alterations have been made.
Mike

[cathoderay57]

You will quite likely find inside the screened cans a 6K7G (IF amplifier) and a 6Q7G (detector/AF preamp). These will all work OK even if the intended line-up was ECH35 (freq changer), EF39 (IF amp) and EBC33 (det/AF preamp). The one to watch out for is the rectifier - if the set was designed for use (as per early line-up in Trader Sheet #997) with a directly heated AZ31 rectifier you might find the HT is somewhat higher than expected with a 5Z4G indirectly heated rectifier fitted in lieu. The reason being the 5Z4G usually seems to have higher emission but conversely a slower warm up time than the AZ31. VERY CAREFULLY check the ac voltage across the rectifier heater pins, first removing the rectifier before checking the voltage. MAKE SURE that you get the heater pins right - pins 2 and 8 viewed from underneath counting clockwise from the spigot http://www.r-type.org/static/baseio.htm. (the other pin connections could have 600v+ between them) If the heater voltage is closer to 4v ac as opposed to 5v then the intended rectifier should be an AZ31 and a 5Z4G, requiring 5v heater, would be under-run. Not a disaster, but less than ideal. Cheers, Jerry

[Gabe001]

Originally Posted by crackle

It looks like you are going to have to examine the chassis carefully to see if alterations have been made.
Mike

Not just rewiring I think. The original az31 in the a358 has a 4v 1.1a filament. The 5z4g is apparently used in the model released the following year, the a478 which requires 5v 2a. (Just saw the comment above!)

This is going to be a tough one.

Gabriel

[Gabe001]

Originally Posted by cathoderay57

You will quite likely find inside the screened cans a 6K7G (IF amplifier) and a 6Q7G (detector/AF preamp). These will all work OK even if the intended line-up was ECH35 (freq changer), EF39 (IF amp) and EBC33 (det/AF preamp). The one to watch out for is the rectifier - if the set was designed for use (as per early line-up in Trader Sheet #997) with a directly heated AZ31 rectifier you might find the HT is somewhat higher than expected with a 5Z4G indirectly heated rectifier fitted in lieu. The reason being the 5Z4G usually seems to have higher emission but conversely a slower warm up time than the AZ31. VERY CAREFULLY check the ac voltage across the rectifier heater pins, first removing the rectifier before checking the voltage. MAKE SURE that you get the heater pins right - pins 2 and 8 viewed from underneath counting clockwise from the spigot http://www.r-type.org/static/baseio.htm. (the other pin connections could have 600v+ between them) If the heater voltage is closer to 4v ac as opposed to 5v then the intended rectifier should be an AZ31 and a 5Z4G, requiring 5v heater, would be under-run. Not a disaster, but less than ideal. Cheers, Jerry

Thank you very much, this is really useful. Also noted that the 5zvg draws much more current. Can I ask about the 6v6 replacing the el33? Pinout looks the same but the valve performance seems substantially different

[cathoderay57]

Hi Gabe, the 6V6G and EL33 will work as plug and play replacements - the base connections are identical (pin 8 on the 6V6G beam tetrode being cathode whereas on the EL33 pentode it is cathode/grid 3). However, if you intend to continue using the 6V6 it would be wise to replace the 150R cathode resistor with a 330R or 390R rated at 2W so as to get the biasing right. The cost of an EL33 is getting substantial now and fitting one instead of the 6V6 is unlikely to make any audible difference. Do check the 6V6 cathode resistor - you might find it is already the higher value. Some manufacturers did produce the "same" model with different valve line-ups (the Barker 88 being one example). Cheers, Jerry

[Gabe001]

Thanks Jerry I think I have enough information to get this done. Will post updates as I go along.

[Gabe001]

Hello again, I've taken this radio apart and have started having a look at the chassis and working on the case. The service sheet for the a358 is a bit lacking in that there's only a circuit diagram and a bit more info on alignment. So I'm slightly outside my comfort zone, hence the questions. The T1 and T2 resistance reading look reasonable so far judging by the A353 service sheet

I'm posting some picture of the underside of the chassis

There is an electrolytic capacitor (I cannot see the value without removing it) that seems to be a later addition. It's odd in that the positive end appears to be grounded to chassis and the negative end ( yellow wire) going to pin 6 of v4. Allegedly this pin is unused in a 6v6g, but I've never seen an electrolytic capacitor connected the "wrong way round" before. Am I missing something?

The circuit diagram Indicates 4 electrolytics in total. 2 are in the can, one referred to above, can anyone spot the last one?

The 150 ohm resistor Jerry asked me to check - I can't find it. Pin 8 where it's supposed to attach to goes to ground directly, and it's not under the waxy capacitor either.

Thanks in advance for your help.

Gabriel

[jjl]

This set may use resistors in the negative line of the HT supply in order to develop a negative bias supply for the output valve and possibly earlier stages. This would explain the output valve cathode being connected to chassis and the positive lead of the electrolytic you mentioned being connected to chassis.

This arrangement was common in '20s and '30s sets that used directly heated output valves such as the PX4 and some sets that have an energised 'speaker field. It was less commonly used post war.

It may be that the set wasn't originally designed this way and that this arrangement is a modification made by a repairer.

John

[cathoderay57]

It's hard to tell, but the components look original, or else replacements/modifications were done a long time ago. I'm inclined to the former. If you take a look at Trader Sheet #997 for the Regentone A353 it probably reflects similarities to the circuit you have got. https://www.service-data.com/search....&search=Search. In particular, you can see that, as described by JJL, V4 cathode is connected to chassis, and C25, a 25uF (probably 12-25v) electrolytic is shown with positive to chassis and negative to the mains transformer HT winding centre tap. C25 is almost certainly the yellow waxed electrolytic in your images hiding below what looks like the audio output transformer. The thick black cotton covered wire from V4 pin 6 (being used just as a solder tag) is probably the wire to the mains transformer HT centre tap. Not sure if your radio has an HT choke (L16 in Trader #997) or whether you have an energised speaker in which case L16 will in effect be the speaker field winding. V4 grid 1 resistor (R21 in Trader #997) returns to the mains transformer HT centre tap and therefore R18 + R19 act as cathode resistors for V4 providing the necessary negative bias at V4 grid 1. Sorry this has got into the weeds but the good news is because the components look to be original, IMHO you should leave the resistors and valves as-is (except for checking the rectifier heater voltage), test or replace the waxies and the electrolytics and away you go. I'm not sure about the fourth electrolytic - in Trader #997 there is traditional cathode bias on V3 with a decoupling electrolytic but I could not see one in your images. The easiest way to help further would be to trace out the circuit you have actually got using the closest match printed one as a starting point. Cheers, Jerry

[Gabe001]

Thanks John. I've just looked at the trader sheet for the 353 as opposed to the 358 and it seems it was modified to a 353 spec, which employs a capacitor in this way. Makes a bit more sense now. If I have time tomorrow I'll replace the grid coupling capacitor and try a power up, initially without the rectifier as advised above. I'm not very hopeful the smoothing electrolytics will reform, but if they do, bonus. Meanwhile the case is almost ready after a sand down of top and one of the side panels, stain and varnish.
Gabriel

[Gabe001]

Thanks Jerry. Your reply was visible to me after I wrote my last comment, but we're thinking along the same lines. I managed to find a trader sheet for an early a353 which used a 6v6g and there are lots of similarities. The black cotton wore is the transformer centre tap.
Gabriel

[PJL]

As far as I can make out, the 353 and 358 use the same chassis and the only difference is the twin speaker in the 353. Marketing are simple folks and decided that the twin speaker version should have 2 bars and the single only one. The layout of your set does not correspond to trader 997 (353) or trader 888 (358) which are basically the same. It sounds like your circuit is probably identical and frankly, the trader layout looks like it places wax capacitors next to power resistors. There is also a comment that component values differed due to shortages which does not bode well for consistency.

So I would be inclined to think yours is original and undisturbed as the parts look right for the period and I would guess it would be later than the trader models as the part dimensions are drawn smaller.

[Gabe001]

Jerry, the AC voltage on the rectifier pins is 4v, so I need to look for an az31 valve. I'll ask around in the appropriate section.

Can I ask about the value of the resistor in the picture attached. I'm not getting a reading on the multimeter, but my very amateurish interpretation of colours (blue, gray, green) adds up to 6.8million ohms so maybe that's why I cannot get a reading.

Thanks again for your help. Generally I just get on with it, without asking so many questions, but usually I have a proper circuit diagram to refer to.

Gabriel