Re: assessing resistance on an amp PHOTOS ADDED

surfguy13

Hi Guys

Just bought a Linear L45A amp on Ebay, bit of a whim really (I blame ESChap....damn you Gordon!!) but very sweet sounding little amps.  Just 4.5 watts and very basic indeed.....one vol/treble/bass and that's it.  Single 1/4" input.

             
http://img802.imageshack.us/img802/1465/nnlv.jpg

http://img826.imageshack.us/img826/4259/0cia.jpg

http://img407.imageshack.us/img407/3003/cz07.jpg

http://img109.imageshack.us/img109/7241/7mcx.jpg

Anyway, the guy I bought the amp from isn't sure if the output is 4 or 8 ohms.  I've had a look around and can find very little info.  The only thing I could find is a couple of schematics from John Chambers website....one is the straight circuit diagram and the other modded.  I really don't know what I'm looking at with circuit diagrams but it seems to me that the output appears to be 3 ohms which I assume would be classed as a 4 ohm output?

Wonder if someone could have a quick look at the schematics and confirm what the output is? 

http://www.chambonino.com/carlsbro/linear-l45a..html

Many thanks!

Guy

Misterg

Yep, if it's built to the schematic, it looks like 3 ohm to me - I'm sure that 4 ohm would be OK.

If you can see any markings on the output transformer (the one sitting on top of the chassis), they might be useful to confirm this. I highly doubt that you'd do any damage in any case.

(The modified version has different tone controls and no negative feedback.)

surfguy13

Brilliant, thanks Misterg.  I think a nice 4 ohm Weber Signature would be perfect in that case!!


Misterg said:

Yep, if it's built to the schematic, it looks like 3 ohm to me - I'm sure that 4 ohm would be OK.

If you can see any markings on the output transformer (the one sitting on top of the chassis), they might be useful to confirm this. I highly doubt that you'd do any damage in any case.

(The modified version has different tone controls and no negative feedback.)

ICBM

You could calculate it from the turns ratio of the OT if you can find the spec or measure it, but the best way to be absolutely sure is to measure the power output at the point of clipping (using a scope) at several load impedances, and the one which gives the highest is the correct match.

But since it hardly matters - even a 2:1 mismatch usually only reduces the power by about 20-25% usually - I would just assume it's 3-ohm, which is more or less the same thing as 4 anyway. It's all a bit of an inexact science really.

I also doubt the amp will be anything like 4.5W real output (ie clean) if the EL84 plate voltage is as low as 167V. (Or possibly even at the modified 220V.)

surfguy13

ICBM said:

You could calculate it from the turns ratio of the OT if you can find the spec or measure it, but the best way to be absolutely sure is to measure the power output at the point of clipping (using a scope) at several load impedances, and the one which gives the highest is the correct match.

But since it hardly matters - even a 2:1 mismatch usually only reduces the power by about 20-25% usually - I would just assume it's 3-ohm, which is more or less the same thing as 4 anyway. It's all a bit of an inexact science really.

I also doubt the amp will be anything like 4.5W real output (ie clean) if the EL84 plate voltage is as low as 167V. (Or possibly even at the modified 220V.)

Sadly don't have access to anything like a scope and if I did I wouldn't have a clue how to use it.  However, it's good to know that using a 4 ohm speaker will work OK without damage to anything.

Interesting that you say the output won't be anything like 4.5 W, I assume it will be lower?  I had assumed it would be a very low power amplifier, that was one of the reasons I bought it (possibly better as a hi-fi amp ?), so that's fine.  I have an old tweed princeton which is 4.5 W output and that's fairly loud for 4.5 W but once the volume gets to, say, 2 o'clock it just gets dirtier rather than louder.  However, it uses a 6V6 output valve which probably makes a difference? That's also a 4 ohm output coincidentally.

ICBM

The 6V6 is roughly similar in spec to the EL84, but the Princeton will have much higher voltages, typically around 300V. Leo Fender believed in running things hot to get the most power from the least expensive components he could. (Often above the valve ratings in the bigger amps.)

I have an old Centurion amp which is quite similar circuit-wise to that Linear, and even with somewhat higher voltages in the 250V range it was well under 4W clean. I raised it to around 275V and got about 4.5W, but it's very "about" since the definition of the point of clipping is open to interpretation! It's noticeably distorted - even though not actually clipped - a lot lower than that.

ESchap

Hi Guy, glad to see you delving into more dusty stuff from the 50's.    As MisterG confirms the schematic is definitely indicating a 4ohm output.   Those small single ended output transformers tend to be 5K ohm primary with a single 4 ohm output.  They were used in large numbers for single ended mono amps in radios, record players etc. 

The unmodified circuit looks as if it will be quite tame, been designed to keep distortion down.  The modified circuit is quite the opposite! 

If you've got a 4 ohm and an 8 ohm speaker available I'd try both and see which you prefer.  Running into an 8 ohm load, with the EL84 running so "cool" on its anode, the higher reflected primary impedance on the O/T shouldn't do the valve any harm, even long term.  The 8 ohm should be slightly louder, though you may not be able to detect it.  Whichever ohmage speaker you plump for, if volume is an issue, the best thing would be to get the most efficient speaker you can.

 

 

surfguy13

ESchap said:

Hi Guy, glad to see you delving into more dusty stuff from the 50's.    As MisterG confirms the schematic is definitely indicating a 4ohm output.   Those small single ended output transformers tend to be 5K ohm primary with a single 4 ohm output.  They were used in large numbers for single ended mono amps in radios, record players etc. 

The unmodified circuit looks as if it will be quite tame, been designed to keep distortion down.  The modified circuit is quite the opposite! 

If you've got a 4 ohm and an 8 ohm speaker available I'd try both and see which you prefer.  Running into an 8 ohm load, with the EL84 running so "cool" on its anode, the higher reflected primary impedance on the O/T shouldn't do the valve any harm, even long term.  The 8 ohm should be slightly louder, though you may not be able to detect it.  Whichever ohmage speaker you plump for, if volume is an issue, the best thing would be to get the most efficient speaker you can.

 

 

Hi Gordon

Many thanks for that and yes, it seemed like a nice little inexpensive amp and it may or may not work for guitar.  The guy I bought it from was using it for guitar and said it sounded amazing but I'll wait and see.

GREAT to know that I could potentially run it into 8 ohms without any major damage, that's great.  I have 3 2x12 cabs that are currently wired for 8 ohms so I'll mess about. Unfortunately I don't have any 4 ohm cabs but do have the 4 ohm Weber in the princeton and as the speaker is connected to the chassis via a phono I can just make up another lead and try the Linear through that.  I can then compare the 4 and 8 ohm cabs.

You mentioned you had gone down the valve route with hi-fi Gordon, if this doesn't work out well for guitar could I use it in conjunction with a hi-fi?

ThePrettyDamned

Totally unrelated, but I love threads like this, even though I have no idea what is going on.

As you were.

surfguy13

ThePrettyDamned said:

Totally unrelated, but I love threads like this, even though I have no idea what is going on.

As you were.

Yup, know the feeling!! 

Bogwhoppit

Reading this thread sent me down memory lane.

In the mid 70s there was an electronic surplus shop in my area that sold Northcourt Cage amps (the Thirty and Fifty) . I believe the Linear and Northcourt brands were a rebadged generic amp made by some company in Essex back in the 60s. To us impoverished kids  [back then], these were pocket money amps (the Northcourt Thirty was £11.50) compared to a Marshall 100W MV head which was around £200.

Those amps sound amazing,

 

Oh how times change !

surfguy13

Bogwhoppit said:

Reading this thread sent me down memory lane.

In the mid 70s there was an electronic surplus shop in my area that sold Northcourt Cage amps (the Thirty and Fifty) . I believe the Linear and Northcourt brands were a rebadged generic amp made by some company in Essex back in the 60s. To us impoverished kids  [back then], these were pocket money amps (the Northcourt Thirty was £11.50) compared to a Marshall 100W MV head which was around £200.

Those amps sound amazing,

 

Oh how times change !

Yes, me too!  I remember the old Northcourt and Linear amps.....they were excellent little amps, in some cases not so little !, and as you so rightly say an alternative to mainstream brands like Marshall which were impossible to afford.

I was a struggling guitarist in the early 70s and as I've tried to explain to so many people, there wasn't such a thing as vintage amps or speakers or guitars back then....they were either 'just' affordable or impossibly expensive.  Nobody I knew had new gear.  It was bits and pieces put together that probably sounded terrible but it didn't matter.....you were playing!!    Those were the days........................

Bogwhoppit

surfguy13 said:

Bogwhoppit said:

Reading this thread sent me down memory lane.

In the mid 70s there was an electronic surplus shop in my area that sold Northcourt Cage amps (the Thirty and Fifty) . I believe the Linear and Northcourt brands were a rebadged generic amp made by some company in Essex back in the 60s. To us impoverished kids  [back then], these were pocket money amps (the Northcourt Thirty was £11.50) compared to a Marshall 100W MV head which was around £200.

Those amps sound amazing,

 

Oh how times change !

Yes, me too!  I remember the old Northcourt and Linear amps.....they were excellent little amps, in some cases not so little !, and as you so rightly say an alternative to mainstream brands like Marshall which were impossible to afford.

I was a struggling guitarist in the early 70s and as I've tried to explain to so many people, there wasn't such a thing as vintage amps or speakers or guitars back then....they were either 'just' affordable or impossibly expensive.  Nobody I knew had new gear.  It was bits and pieces put together that probably sounded terrible but it didn't matter.....you were playing!!    Those were the days........................

Indeed, my first amp was my mothers old valve gramophone which she had just replaced with a 'music centre'.   I ripped the chassis out of the gramophone ( I think it was a Decca ), bought a cheap germanium transistor pre-amp that I stuck in the 'arm pickup socket' with matchsticks and spent two hours every night after school learning tracks from The Song Remains the Same.

Bogwhoppit

It appears they were made in Leeds.

 

http://www.chambonino.com/work/linear/info1.html

surfguy13

Added some photos in my initial post back up at the top and here's some links to a load of more detailed photos:

http://imageshack.us/g/1/10383589/

The amp is really quite remarkable....as ICBM and ESChap said, it's not loud but it's sweet as you like.  Turned up full the drive/crunch, whatever you want to call it, is just wonderful through a 2x12 at 8 ohms.  With a treble boost in front it really makes it a much more viable option with bags more volume but without compromising the original tone.

The tone controls are pretty amazing too.....they really do have a good sweep but not radical so you can get the tone 'just right'.  I often find that EQ is either interactive which doesn't suit me or has far too much sweep. 

I know ICBM asked if there were any markings on the transformer but sadly not.  The wiring is pretty much perfect and done to a high standard.  The chassis looks almost new and the pots are original but feel like they're brand new so I suspect this amp hasn't been used much in its lifetime.

The solid mahogany cab is a nice touch!!    To give an idea of scale........this on top of a 100 watt head:

http://img843.imageshack.us/img843/5913/iwvj.jpg

Misterg

That looks really cute!

The tone control network (James / Baxandall ??) is different to the "normal" arrangement used by Fender, Marshall, etc. so interesting to hear your comments on it.

surfguy13

Misterg said:

That looks really cute!

The tone control network (James / Baxandall ??) is different to the "normal" arrangement used by Fender, Marshall, etc. so interesting to hear your comments on it.

Yeah, that's it in a nutshell....really cute! 

It's rare that you grab a tone control and it does what a tone control should do......give you a nice graduated and unsaturated response.  Comparing it to a Marshall......well, I don't think you can compare it they are so different.  The tone on this little fella really does graduate so perfectly and the bass and treble work really well together. 

To be honest the volume control is great too.....although the range isn't great in terms of volume available it really does lay it down so smoothly.  I guess this is a characteristic of a low volume amp.......the difference between zero and full power is relatively small.

Sparks

I just picked up one of these and am hoping to get an amp tech to service it and pop a 1/4" socket in place of the phono input for me. Interesting/reassuring to see it has been done before.