Cust showed up carrying two old tube radios. One of them a more contemporary Luxor and then this Philips.
We agreed on me building him a cool little amp out of the Luxor for him and...i could do whatever i wanted with this old worn into the ground Philips.
So. I started to check it out. At first i had guessed it was from somewhere -45 to -50. Ie;built just after the war but...was i ever dead wrong.
That´s what it SHOULD look like. This Philips model 521-A.
As it turns out,as i use the internet to background check the piece,this old radio was built down in Holland back in....1934.
Goes to show ya! But? Can something electronic be trusted in the 21st century? I mean,really?
Yeah well,it can. If it holds an AC powertransformer it can as that brings that we have no galvanic contact with the mains net and see...this old radio does! It´s even multitap. Ie;has a voltage selector.
Tubes in there,as i start to check them too,turns out to be presented to the public in an era where much tube development happend/was going on.
(Pic stolen off of the net)
That´s what it looks like within its cab.
Now.
"if it holds an AC powertransformer"! Huh? Yeah well...at the time there were no downright standard as far as wall voltage in Europe. Running both ways...AC and DC (sound familiar ?). Here in Sweden for instance the town of Lund was the last of them to convert to at the time Swedish standard of 220VAC. They used 100VDC at the wall as late as 1952,and in essence that´s how it looked across Europe.
To cater to all them various wall voltages the norm at the time were radios and what not that were "all current devices". Ie; they were in short to run whatever was available at the wall and as this from time to time didn´t conform to anything really...and as DC current can´t be transformed...these units often lack a powertransformer,thereby making them downright lethal to work on as their chassis often is kept at potential. Ie;there´s for instance 220V at the chassis.
Hence why old radios and what not often have cabs out of wood or backelite and knobs the same. Reason is simple...as insulation.
This 521-A DOES sport a powertransformer however and that brings that we can at least set chassis to be zero. Ie;become safe.
In fact that was how the appratus was setup from the get go so...all good IOW.
That´s one OLD "electrolytic" cap (often these were oil filled ones) and one carbon resistor of age!
To be honest you don´t even consider trusting anything capacitor of this age. It is simply retirement time,for real.
Granted,owning ESR and LCR meters let me check the actual caps,and sure enough..dead as doornails. The lot of them.
So. Replaced all caps...and sans the 506 rectifier tube installed i flicked the switch. Yep. Life. I saw like 260VAC vs ground for each HT tap and in turn a few decimals to high for the 4 volt heaters.
Yes. 4 volts. These units were made in a time where 4 volt heaters were the norm.
In the 506 rectifier went and....sans ANY and all load...55VDC. Right. Something was amiss here. Quite evidently.
That made me look for a replacement and checking the various European e-bays i soon enough realized that a 506 rectifier isn´t exactly expensive but...it couldn´t be picked up locally-to me.
Now. I happen to have half a crapload of old AZ-1 rectifiers at hand. These take a different socket so out one of these old socket came,and apart to be cleaned. That done i jumpered that thing in there and...sure enough.
Approx 270VDC under load! Time to bring out the champagne!
Now. An AZ-1 is a SOMEWHAT later tube and hence isn´t really period correct but on the other hand...AZ-1 is a FEW years more "modern" vs the 506 so..to hell with it. Radio was that run into the ground anyways so any kind of restoration was out no matter.
In short? Out the rotary file came...
So. What could be said thus far? Well. There was an ACH-1 in there,which isn´t really usable to us,and that worked. There was a pair of AF-2 pentodes...and they both worked. In turn an AB-1...which again is of no use to us and then in turn a RES-964 powertube and that shot 506 rectifier.
The RES-964 then? Well. An RES-964 is in essence the same tube as AL-1,just carries a different socket,and see while the RES might not be held to the skies the AL-1 is. Why?
It is the grandfather of them all. The one that led to what we today call EL-34 for instance,and then onwards. A for 4 volt heaters and L for power pentode. No1 in turn..yeah well.
it was superseeded soon enough (less then a year) by AL-2..which turned into AL-3..and in turn AL-4,and by that time we´re into "almost" modern power pentodes. As stated above LOTS happend in those years in the -30´s as far as development of power pentodes.
The RES-964 is specified to 9 watts total anode loss but see...efficiency isn´t exactly top notch so..them 9 watts make for a mere 3+ watts total in output power. Cool enough.
Well. I fired the thing up all tubes in place and..first AF-2 in the signal chain was dead. Traced that to a shot cathode resistor so replaced that. There was a reason for the shot cathode resistor tho so..saw a mere 10 volts at the plate of it. The AF-2 has its plate entrance at the top of the tube in the form of a nipple...hence there´s wiring heading for that and this wiring had deteriorated within and was thus shorting against its shield. Replaced that and...now that AF was up and running.
That done the "amp" came to life. Didn´t sound of crap but..sound was at least there. Checked the other tubes and in turn signal amplitudes and realized much work was still ahead.
Something was still amiss with BOTH of them AF-2´s tho and i kind of took that with ease as i had realized that it wasn´t the actual tubes that were at fault at least.
Now,PLEASE be adviced that this there is for benchwork ONLY. Ie;i don´t really care how it looks as what´s up is to make the SOB come to life and..all of it will be pulled apart shortly anyways. IOW as long as I know that it´s safe and know where NOT to put my mittens it´s all good.
What was "radio" of it was in short cut out and tossed in the bin. Piece by piece.
Well. At that point i took to cleaning the thing out. Rubbing alcohol,windex and a stiff brush coupled with water and large doses of compressed air.
..and.. then i hooked the stock speaker up to check if it was alive and lo n behold...it was! Damn!!
Time to set this thing right. That "e-lyte bank" was put together. An AZ-1 will take a capacitance of 60µF for first hit..so i dreamed that there up to cater to the origins of the thing. Sure..guess i could have made it more "pro" looking but..that there will suffice and then some.
All of the e-caps are held together by glue in turn and..what´s more apart from the ty-wrap the whole shabang is glued to the amp chassis too.
Most of that yellow wiring in the pic...be gone.
Old techniques were used to splice the caps together where so was called for. In turn that black resistor there is the only one i kept out of the stocker and it is the cathode resistor for that RES-964 powertube.
Now..that takes a bit of explaining too. RES-964 as the AL-1 is DIRECTLY heated meaning that the cathode hits ground via the heater. To make that happen we use the centertap of the heaters as our hookup point for the cathode resistor.
Different to say the least huh? That ALSO brings that a certain degree of hum will be present no matter... In turn the amp being single ended,as in contrast to push/pull,no phase cancellation will be present and thus..it WILL hum. To a degree.
As i checked the various datasheets out for the tubes i took to heart that rail voltages could be reset a bit seing the different goal of the build vs a radio.
Doing this i also opted to reroute the various grounds of the amp and by that taking Mr Kirchoffs laws into account. For the better. Have in mind when this piece was manufactured..please.
All components catering to the AF-2 small signal pentodes were replaced. A common ground point was selected and..yeah well. Net result is what you see right there.
Most of that yellow wiring there has been replaced and in turn a few fuses were added. Stock the amp lacked any and all fuses,which is more the norm then anything seing its age.
Indeed! Up and running! Still need to reevaluate the resistor values for the AF-2 tubes screens but...up and running at least. I opted to run 220k plate resistors and in turn 1M screen dittos and the latter value is simply to high and it needs to come down. Approx 130VDC for the plates and in turn a mere 35 volts for the screens tells the story if you ask me. One thing at a time tho and..it sure as hell works as is!
..and lemme tell ya. This old piece has a fair amount of rocknroll inherent. No doubt.
No matter how we regard this that there thing is 82 years old. EIGHTY TWO!!
As is there´s one pot in there and that is a 500k volume one. Be that as it may i found the amp to be well enough shrill to the tone so i opted to install a small ceramic cap of 1,5nF across the output transformer,and that sure enough did the trick. On the hefty side even so i´m going to try a 1nF or 470pF one too...
https://www.youtube.com/watch?v=ch4N4YQewkw
That there is what it sounds of at wide open throttle ATM. Engl eat your heart out!!!
First up through the stock speaker and then in turn through the 212 box i keep under the bench for...bench work that contains a pair of the most ill fabled Celestions there is. The G12-50.
No matter. That there is basically an EIGHTY TWO year old amp roaring its heart out.
There´s times when owning a lathe comes in handy. I had picked that old stock volume pot apart and..well,after 82yrs i´d say it´s retirement time no matter. The norm these days though is that pots have WAY shorter shafts then back in the day so..opted to turn these "adapters" out of 7075T651 aerospace aluminium.
Two?
Well,yeah. The one dial is volume right...the other dial is to dial in radiostations,and we´ve got no need for that so i opted to replace that with a regular pot too. One that was set up for a simple Garnet tonecontrol.
Did i mention that it was worn into the ground? Did i?
Gonna turn a pair of "bushing" inserts out of plastic for them shafts i turned.
Took the noodling around with it a bit when volume all of a sudden took a dive. As it turned out the second AF-2 all of a sudden had a mere 10 volts at its plate. That tho..was just due to a bad solderjoint. Easy fix IOW.
Then i hooked an MXR OD up infront of the thing and...DAAAAAAAAMN!!!
Volume was back higher then ever and..what came out the speaker end was.. This thing was all of a sudden in heavy metal territory!
That concludes the first part of the story...we´ll see where this ends up. No matter...it isn´t just a conversation piece anymore,it is a usable guitar amp.
Cheers!
