EL 6420

Burns, Selmer, WEM, and more

Re: EL 6420

Postby Merl1n on Mon Dec 14, 2015 7:24 am

Hi

It haven't checked the schematics so thoroughly about rectifier delay. What i know is that there is manual power switch with two positions, first for tube filaments and second for power stage high voltage. Caps I have tested with isolation resistance tester. It shows if capacitors have internal short grounds and it is also good for old capacitors as is supplies higher dc measuring voltage which in turn gently re-charges old capacitors instead of high current surge when firing up an old amp after years of sleep. Old caps seemed to be in working order though will probably be changed in future. Amp sounds great with guitar being played through it.

-Hente
Merl1n
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Re: EL 6420

Postby Racing on Mon Dec 14, 2015 8:55 pm

@ Merlin.
With amps of this age you might as well plan to replace the electrolytes of the amp right off the bat. Working or no.
Point being that so much can be had from mere modern performance and in turn size.... No point in beating a dead horse if practical use is the intended goal.

Type of e-lyte to replace with i guess to a large degree is a matter of personal opinion and taste. Suffice it to say that as you clean the innards of the chassis out from parts "not needed" these days (incl that 16mF cap) there will be ample room to basically go for a swim in there.

As for older style e-caps Trobbins is on the money. Some of them are truly enviromentally hazardous and should be handled accordingly. This holds true for a variety of old electronic components.

When replacing some of the smoothing caps be aware of limitations set forward by for instance tube rectifier type. Maximum values can be had via datasheets found online.
Further,as i point out in previous text,it might be an idea to use tube rectifiers that are not of mercury type if possible. Mercury isn´t exactly to be fooled around with and if the amp in case will see regular use and/or will be touted around...it´s a good idea to replace.
Racing
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Re: EL 6420

Postby Racing on Sat Dec 19, 2015 12:12 pm

Right.
So i tipped this one up on the bench. Basically cleaned house,completely.

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At first i started by replacing the HT fuseholder and checking the mains one...but TBH i think i´ll rewire those to be panel mount ones instead where we "capture" anything dangerous to be within the realms of the chassis.

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I started looking into the rectification of the unit,and it IS peculiar to say the least. Have wired the entire thing up again and...it is different. No argument.
Have checked the various voltages outbound and...

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It sure carries with it a crapload of ripple. No argument there. The entire thing is indeed setup as a voltage doubler of sorts. Just very peculiar to layout.

That said.
There´s a number of high power resistors that the amp depends on,to the letter,to work.
I´m replacing them as best as can be and while at it using chassis mounted such in contrast to the stock wirewound ones. Point being that as i install them vs the chassis i get a downright fantastic heatsink seing the chassis is out of cast aluminium
Racing
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Re: EL 6420

Postby trobbins on Sat Dec 19, 2015 10:18 pm

The 'spring' has got me intrigued. It looks like it has an additional 'wire' in the middle. Is it a type of fuse?
trobbins
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Re: EL 6420

Postby Racing on Sun Dec 20, 2015 12:25 am

Image


Yeeeeeeeep.
Racing
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Re: EL 6420

Postby Racing on Sun Dec 20, 2015 1:59 pm

Image

Like many an other old toober this Philips sports a rather massive pulldown resistor too. Old components are normally of a larger size physically,and as my intention is to cramp anything "dangerous" together under the hood i took to replacing those various power resistors.

Idea here is simple really. The stocker is of 39k/25w and what we´ll see now is all in all 40k with resistors of 10k a pop and 25w. IOW lesser unit loading,and as the chassis in turn is out of cast aluminium i´ve installed them such that the chassis becomes a,hopefully,heatsink.
In final install they´ll see cooling paste too.

Have debated the tube rectifier setup with both myself and my mentor. It is a peculiar setup no doubt and indeed is a voltage doubler setup. What the power transformer hands us is in essence 300VAC (well..plus) outbound. A rather massive powertransformer in turn.

Seing that and seing how the mercure AZ-50 rectifiers has already been replaced by AX-50´s in this unit and in turn reading THEIR datasheet..i think i´ve arrived on a reasonable setup for first hit.
Racing
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Re: EL 6420

Postby Happy Face on Mon Dec 21, 2015 1:12 am

"Like many an other old toober this Philips sports a rather massive pulldown resistor too. Old components are normally of a larger size physically, and as my intention is to cramp anything "dangerous" together under the hood i took to replacing those various power resistors."

Nicely done.
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Re: EL 6420

Postby Racing on Mon Dec 21, 2015 7:40 pm

No.
No it does not sport such a relay.

Am fully aware of that them lytes are toxic waste.

Gear. High gear even,so...

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Basically i true to myself wiped the amp clean within. Screw it as not all that much was to be kept anyways. So. Wanted to get it up and kicking again so..
Yeah. Fresh fuseholder,the old one was basically shot. Keep that thought tho..

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Disregard the three wires to the left. They´re part of the primary. The rest of them though.. Yep. Thicker gauge wire means heaters,in this case 4VAC. The one to the left part of the HT windning. The gray and black tho..separate.
See. This amp is a half peculiar beast in that it uses a rather odd setup for a voltage doubler. That brings that there´s approx 300VAC or so at them joints there and that´s about it.
..and that´s not even half of it!

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Rectifier-S in this case is a pair of AZ-50´s. Stock it calls for a pair of AX-50´s but as these are..toxic as well (mercury) and they had already been replaced. Yeah well. For better for worse and from what it seems the ones in there are A ok as far as nick.

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Component size has changed a bit since 1952 and see this amp as several other highball ones of the time use a pulldown resistor for the halftap. Mainly to control cold voltage spikes to my reasoning. It calls for 39k in this case and now got replaced by 4*10k instead. Alumnium housed ones,and hence the cast aluminium chassis will work well as a heatsink while at it. Ample dimensions to say the least.

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Debated a bit back n forth as far as cap capacitances. Arrived on a solution where i installed caps both sides of that there tagboard....and fired the thing up.
It sure works. That much is for sure.

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The two resistors i didn´t replace was the two 120 Ohm ones running from the one rectifier vs ground. Works tho (they speced fresh) so..

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Seing that safety thinking has progressed a bit too since 1952 i opted to drill into the rear of the chassis for a whole series of panelmount fuseholders. Takes regular 5*20mm glass ones.

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The working end..

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´N yeah. Voltages are right up there. This came to bump a bit more as i got to redo the main filtering section. Powertransformer set to 245VAC and reading taken at wall was 224VAC.
In other words,expect that there to jump even more need be.

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Yes. Tube setup has been changed a bit. True. As much as i´ve fooled around with old rimlock tubes before..not this time out. Don´t get me wrong ´cause basically all tubes can be had to roar,rimlocks are however not what i have in mind this time out.
So.
One EF-86 entry channel (optional) and one 6SJ7 entry channel (optional)
The AZ flask for recifying bias voltage has been canned,and see...let´s dwell on that for a second. The amp uses a rather intrictate,for the time,setup to ALTER bias voltage with load. Thus there´s a small transformer between the powertubes and the chassis,and the variance in bias voltage is handled accordingly.
Nifty,and really nice thought out,solution...and the basic idea is to keep distortion to a miminum. Now..that becomes a sort of contradiction in words this time out so....the AZ flask has been replaced by a common silicon rectifier setup.

There´s YET another rectifier onboard though and it´s an EZ one. This isn´t used per normal thinking but more to to offer readable signal under varying conditions for the EM "magic eye" tube.
Hence,the EZ tube stays.

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So. I came to challenge myself as far as the main smoothing. Seing how the principal layout of the HT side is done i came to differ quite heavily in value for "upper" and "lower" capacitance.
Look the schem up gents and give it some thought :wink: This and the swap of a resistor made the B+ voltage touch down at 800VDC spot.

At the same time that rather crude but well working bias setup was put together. Hooked the amp up and...yep. Works as god intended.

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What?
Well gentlemen....the amp IS from 1952 and common sense tells that replacing tube sockets,as a whole,is a good idea. What i haven´t been able to dig up yet is a pair of sockets for the AZ-50 rectifiers tho. Coming up however..

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Yeah. I´m an idiot. The amp sports an EF-22. In all haste i took that for a commonplace octal socket. Not so...

Anyways. Both the entry smallsignal pentode sockets are rubbed suspended. Per usual...
Racing
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