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Radford bias: options for ultralinear output

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#81
billshurv said:
These are all valid options. front panel space is a bit limited, but should be able to put a couple of extra switches in.
Click to expand...

Couple?

Can't you get a Maplin rotary switch in with 3 poles 4way which also has a plastic shaft to isolate the knob?
Something like this and switch it all at the same time?

3 pole 4 Way Rotary Switch | Maplin

The contact rating is across the poles so it should be easy..

Works fine on my SE..silver contacts.

Why are you so worried about the curves? U/L if you are switching then will you run U/L? I tend to run my switchable amps most of the time in Triode. If you set the Bias for U/L at 40mA it should be OK in all other positions but you can check to see.
Set bias forget>> switch to suite as and when you like. (just don't switch when its on)<<You can fit a HT switch for quick change with a 0.01uF polypropylene across it but you have to watch the B+ doesn't rise (above working cap voltage)on the PSU caps with no load.
Don't forget the 1K on the screen grid if you are switching..

You would have to re-bias when you change the tubes to a different type.
Just passing through again..😀..

The 10/12 ohm cathode resistors can even be Maplin as well cement type sound quite good..(mills if you must)
You need the wattage in case of flash over and they take the shock out of the system on power up.
(You know the accidental off and On while warm situation)

Regards
M. Gregg
 
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#82
If your not a fan of the rotary sw you can use miniature toggles 2way.
There is little current and the switch operator is grounded via the mounting nut. The voltage rating is quite good.
Something like these work OK (look at the insulation resistance) The actual voltage across the poles is small.
You can get better voltage ratings in miniature toggles, however I used miniature toggles for Many years in an 8 X EL34 amp with no problems. You might find a better rated switch at Rapid Electronics etc.

Sub-Miniature Toggle Switch F On-Off-On DPDT | Maplin

Regards
M. Gregg
 
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#83
M Gregg said:
Couple?

Can't you get a Maplin rotary switch in with 3 poles 4way which also has a plastic shaft to isolate the knob?
Something like this and switch it all at the same time?

3 pole 4 Way Rotary Switch | Maplin

The contact rating is across the poles so it should be easy..

Works fine on my SE..silver contacts.
Click to expand...
Use them on my preamp. Not the nicest switches on the planet for feel tho. Could set fixed taps on the output and use the impedance selector switch of course.

Why are you so worried about the curves? U/L if you are switching then will you run U/L? I tend to run my switchable amps most of the time in Triode. If you set the Bias for U/L at 40mA it should be OK in all other positions but you can check to see.
Set bias forget switch to suite.

Regards
M. Gregg
Click to expand...

Just so I understand exactly how it was designed before I dick around with things. Bailey and Radford were clever, certainly more clever than I. As the amp is busted I can't measure it so need to reverse engineer. For example radford specify 50mA bias, which is high for a 5K plate to plate transformer according to the curves I have. most certainly a mistake in my maths but a backup set of curves would help prove that.

Just had a peek at the serial number D00006, so 1966 vintage. If the case weren't ratty sure some japanese collector would pay good money for a single digit serial number. Shame its not 8.
 
#84
😀..I was just passing through while making the tea!

Its nice to try different things...that's why Most of my amps are switch everything I get bored..then change the tubes which is great on a SE because you can just use a CCS (class A) not even any bias setting.
Plug and play tubes switch Triode/Pentode U/L ES..

But I still get bored..😀
So the driver has to be multi change as well..But I try to think of an excuse to pull some parts out and change them..
When all else fails its time to build something.

Regards
M. Gregg
 
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#87
billshurv said:
rechrome and new powder coat. other than the transformers everything else needs ripping out an replacing.
Click to expand...

Then would you have an amp you might want to use and keep?
However you would need to fault find and get it going even if only lashed together so you know the Output Tx's are OK.
Otherwise it could be an expensive white elephant!

NB.. we haven't seen any pictures of the sad forsaken animal..so where are they..😀

So its a lash up test and rebuild..then you have to ask do you want a retro amp? Or can you use the iron for a nice build.
Using tag board and hard wire.


https://www.google.co.uk/search?q=radford+tube+amp+modern+build&biw=1280&bih=585&source=lnms&tbm=isch&sa=X&ved=0CAYQ_AUoAWoVChMIw-6E4sLsxgIVSuwUCh1KvAAe#imgrc=zZf5WeUXa5-7RM%3A

Could it be a Phoenix?


Regards
M. Gregg
 
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#88
If I was going to rebuild a project,

I would probably do something like this..(thinking of the shape)
https://www.google.co.uk/search?q=radford+tube+amp+modern+build&biw=1280&bih=585&source=lnms&tbm=isch&sa=X&ved=0CAYQ_AUoAWoVChMIw-6E4sLsxgIVSuwUCh1KvAAe#tbm=isch&q=papworth+valve+amps&imgrc=Uf-DNcKUs-CivM%3A

Probably with mixed case Oak and brass..but each to his own..
Weld polish and then chrome..depends...😀 (Turn it around side to front)
Ditch the oribble handle protectors.

Regards
M. Gregg
 
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#90
billshurv said:
I'll take some pictures tonight. Shame I lost the preamp somewhere in a move as that had a nice retro box.
Click to expand...

Any good at cut and shut? 😀

I wouldn't think twice to fit Triode/ U/L (Electron stream)/ U/L switches and adjustable increased adjustable bias for tube rolling.
Can you move the transformers back and bring the tubes to one side then use it as the front?

Regards
M. Gregg
 
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#92
billshurv said:
cutting yes...
Click to expand...

Move the transformers back and bring the tubes to one side then use it as the front. (if there is enough room)..
Depends what you want to do..plan it using a full size piece of cardboard first.

However lash it up and make sure the transformers are ok..

Cut a rectangle out of the top and fit a new top plate..so you just have enough to support the new plate.
You could weld grind back and chrome..or fit a brass or copper top plate. or just use aluminium..etc
Then its up to you how you want to lay it out..at least then you wont have the PCB's to deal with..

Or restore the original..even then I would fit plates over the chassis and ditch the PCB's.
Chrome the chassis and powder coat the plates.

Its just a few Ideas..🙂

Regards
M. Gregg
 
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#93
I don't know how complex the PCB's are,

Can't you make hard wired PCBs?

I did it on my OTL.. like this..

Regards
M. Gregg
 

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#96
Couple of pics.

And a smoking gun. One of the ground studs came loose on the HT supply. Certainly consistent with the charred EL34s I ended up with. I guess a cack handed conversion to triode operation followed by shipping to USA and it dropped off.

Ah the joys of youth. To think the person who butchered that in 1988 is now an IEEE fellow 🙂.

There is actually more real estate there than i remembered.
 

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#97
billshurv said:
Couple of pics.

Ah the joys of youth. To think the person who butchered that in 1988 is now an IEEE fellow 🙂.

There is actually more real estate there than i remembered.
Click to expand...

Well the IEEE fellow would have to unbolt as much as possible so the chassis comes away leaving a pile of bits and top plates complete with circuit boards. Taking the pots out without disconnection.

Then a new powder coat and take it from there. 😀

No institute of electrical engineers would be able to just look at it.
OK here is your task for today strip this thing and get the chassis reconditioned.
Next while its away check everything on the circuit boards.

Clean the pots with servisol and replace all the caps. LMAO

Are you sure the output Tx's are any good?
Put it all back standard change the caps and get it working even use the old valves and see if you can get anything out of it.
Remove the mods..get a known situation ie all the transformers are working by putting all the circuit boards into normal standard situation.
Once its working you can choose to sell it or do it up.

The two PCB's look the same strip one out and replace the caps and put back standard. Then do the other side..
When it stopped working did both sides fail? or just one side.

You could make a really good job of the boards with a view of using them in the final build. Then if it turns out the transformers are shot at least it hasn't cost very much just time.

Regards
M. Gregg
 
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#98
Use a couple of cheap speakers to test it..think of it as an old machine it either works or it doesn't..

Its not to bad inside I have seen much worse..recon chassis and rebuilt boards new power leads and earthing should be good to go!
Assuming the initial test is OK. If the PVC has gone hard then you could rewire but I wouldn't entertain anything until its been running.

Regards
M. Gregg
 
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#99
Well I replaced one Tx in 1988 myself, so at least one of 2 is good. I haven't taken the board out to check, but knowing what triggered the failure I can trace back. I've moved 4 times so still not located all the tubes but the one pair of EL34 I have found look beyond economic repair which is consistent with a bias failure ( SY can be smug now).

400V and 48 year old PCBs is not a combination that fills me with Joy though. Whatever I do they will be replaced before it gets put back into service.

Just to be clear, we were sharing a room in Uni at the time. So he did the triode mods one weekend whilst I was away.
 
#100
billshurv said:
400V and 48 year old PCBs is not a combination that fills me with Joy though. Whatever I do they will be replaced before it gets put back into service.
Click to expand...

I have radios older than that and they work fine with original everything except new caps..so from the point of a test it seems the logical way to go.

Have any fuses blown?

It depends if the unit has sentimental value? If it doesn't then if the transformers are shot, I would probably just build a new amp.
Its easier..😀

Regards
M. Gregg
 
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