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snaarman
13th September 2018, 10:34 AM
So this is what I have been busy with this summer:

http://e-group.uk.net/gallery/data/614/Fender_benders.jpg (http://e-group.uk.net/gallery/showphoto.php/photo/100238)

That's a hand built Strat that I customised with a pickup swap and clever wiring.

Then there is a little 2 valve Fender Champ 600 that I rehoused in a larger Fender box with a radical pre-amp redesign. So its the Fender amp that Fender never made.

Finally in front is my delinquent 4 valve Fender Pro Junior that I have spoken to harshly. I think it is starting to behave properly now.

In each case I have been chasing that perfect tone rather than "how loud can I make it". I actually reduced the output power on the large amp to stop if burning up valves. Obviously they got re-covered in tweed, which makes them sound better immediately.

These are return to basics amps. They came with just one or two knobs. No fancy tricks. Maybe after years of designing microelectronics I am reverting to childhood?

Pete

Zuiko
13th September 2018, 02:59 PM
I know it's only rock n' roll but I like it. *chr

OM USer
13th September 2018, 03:20 PM
A labour of love.

snaarman
13th September 2018, 03:52 PM
I know it's only rock n' roll but I like it. *chr

Yes I do...

Dewi9
13th September 2018, 11:02 PM
Valves, eh. That takes me back. What has it got for the output - a pair of 6L6's in push-pull ?


David

snaarman
14th September 2018, 03:59 AM
Valves, eh. That takes me back. What has it got for the output - a pair of 6L6's in push-pull ?


David

Ah a fellow valve fettler... Well the little one is class A single ended with a 6V6 - that amp his just two valves. Lovely...

The big one is push pull EL84 which I re-biassed to class A. That amp has four valves and therefore counts as complicated :)

My next project with three valves will be single ended with a pair of 6V6s in parallel. No-one seems to do that so I thought I would design one..

Harold Gough
16th September 2018, 01:09 PM
That's a hand built Strat that I customised with a pickup swap and clever wiring.

It seems to have had an organ transplant from a telecaster.

Harold

snaarman
16th September 2018, 02:00 PM
It seems to have had an organ transplant from a telecaster.

Harold

Absolutely. It has Tele metalwork but Strat like pickup arrangement

Naughty Nigel
16th September 2018, 03:38 PM
Ah a fellow valve fettler...

Another one here. *yes


http://www.e-group.uk.net/gallery/data/500/thumbs/Quad2.jpg (http://www.e-group.uk.net/gallery/showphoto.php?photo=12222)

They look wonderful Pete. You cannot beat valves, just as you cannot beat a good film camera! I love tinkering with analogue circuits whether thermionic valve or solid state, but digital just leaves me cold.

I am interested in you comments about redesigning the circuits of your various amplifiers. Few people realise that it is comparatively simple to modify valve circuits from one purpose to another. The only real obstacle is the amount of heat produced.

Jim Ford
16th September 2018, 03:49 PM
I thought the 'received wisdom' was that as loudspeakers are low impedance current driven, then their best driven from a low impedance current source - transistors. Any introduction of a matching transformer can only introduce distortion. Still, whose going to notice distortion with popular music!

Is everyone going back to the 1947 Williamson amp, then?

Jim

Jim Ford
16th September 2018, 03:59 PM
They look wonderful Pete. You cannot beat valves, just as you cannot beat a good film camera!

Yeah - and 'San Izal' toilet paper, Morris 10s, Brylcreem, use once carbon steel razor blades and Styptic alum sticks!

I expect you wear a wing collar, Nigel!

Jim

Naughty Nigel
16th September 2018, 04:01 PM
I thought the 'received wisdom' was that as loudspeakers are low impedance current driven, then their best driven from a low impedance current source - transistors.

You might think so Jim, but a good output transformer introduces little or no distortion, and seems to remove 'digital artefacts' from digital sound sources such as CD's.

The majority of the (very little) distortion produced by valve circuits is third order harmonic distortion, which actually enhances many genres of music.

Valve amplifiers also have comparatively few stages compared with solid state designs, (usually no more than three or four from pickup to output) which also helps to reduce distortion. Some solid state designs using IC's seem to have dozens of stages!

Jim Ford
16th September 2018, 04:06 PM
I wish someone would say something positive about digital - it would give me the excuse to say "I 1100100% agree!"

;)

Jim

Naughty Nigel
16th September 2018, 04:12 PM
Yeah - and 'San Izal' toilet paper...

Jim

Oh yes, we had that at school until it was replaced after pupil protests! :D

I think it was called Izal Medicated. There was another called Bronco which was just as uncomfortable to use. :eek:

Jim Ford
16th September 2018, 09:32 PM
Oh yes, we had that at school until it was replaced after pupil protests! :D

I think it was called Izal Medicated. There was another called Bronco which was just as uncomfortable to use. :eek:

We had to use both sides at the school I went to! When the supply had run out we used old cement bags.

;)

Jim

snaarman
16th September 2018, 09:32 PM
So, here is the current part finished build then, for the anode bashers out there

http://e-group.uk.net/gallery/data/614/IMG_20180913_081816_443.jpg (http://e-group.uk.net/gallery/showphoto.php/photo/100281)

This is a Snaarman modified 5F2 amp design. That is to say a 1950s Fender Princeton amp. As you see, just Volume and Tone pots* There's an old style turret board under there. I have modified the output to run a pair of 6V6 as class A parallel - which makes it one unusual beast.

And what is all this talk about reduced distortion? I'm hoping for at least 5% THD at a couple of watts. I don't want to have to thrash this in order to get it to growl. (Hifi addicts turn away now) That blue push button actually adds even more distortion for those difficult moments...

Pete

*These actually go to 12

Jim Ford
16th September 2018, 09:35 PM
What! Old grub screw control knobs? You haven't even cut down the spindles so the pot. nuts are covered. What about some nice collet knobs to finish it off.

Jim

snaarman
16th September 2018, 09:37 PM
Another one here. *yes


http://www.e-group.uk.net/gallery/data/500/thumbs/Quad2.jpg (http://www.e-group.uk.net/gallery/showphoto.php?photo=12222)


Lovely looking Quad power amp there, old man..

Naughty Nigel
16th September 2018, 09:51 PM
Lovely looking Quad power amp there, old man..

That Quad II is actually older than me, built in 1953! My other Quad II was built in 1957, but is otherwise identical.

To the best of my knowledge they are still on their third set of KT66 output valves. I changed the rectifiers to GZ34's (from GZ32's) early on because of their better performance and longer life. They are still in there and working well!

Stereo wasn't really thought of in those days so it was quite normal to buy single monoblock amplifiers.

Naughty Nigel
16th September 2018, 10:20 PM
Actually the history of my Quad II's is quite telling.

The first was bought by my father from Shorts of London in 1953, but by about 1960 my mother thought it was 'too old fashioned' so they bought a new Kolster Brands sterogram; mainly I think because it looked like 'a nice piece of furniture'. :rolleyes:

The Quad II and its preamp was therefore stored in a box in the attic until I 'found' and started using it around 1970. I managed to find a second Quad II in Exchange & Mart for 21 from a seller who lived just a short walk from where we lived. I therefore had a perfect stereo pair. :)

A few years later, around about 1976, I struck gold at work one day when the Redifusion man came in to service the works PA system, which contained no less than 24 GEC KT66's! These were changed every year and the old valves simply thrown in to the bin. They were run 24/7 in class B, so were barely run in. I have used eight of them and gave some to a friend but hopefully have enough remaining to see me out! *yes

Edit: I should add that the sterogram was consigned to landfill long ago, which is a pity as it might have been worth something now.

snaarman
17th September 2018, 06:06 AM
What! Old grub screw control knobs? You haven't even cut down the spindles so the pot. nuts are covered. What about some nice collet knobs to finish it off.

Jim

Ah yes, I should explain. Old style "chicken head" knobs are the thing on guitar amps. You can see the settings unambiguously from across the stage on a front facing amp head. And on a small amp on the floor leaning back you can't see the knobs at all but you can feel the position of a chicken head easily.

Those pots are a temporary fit until I buy a pair of expensive CTS pull pots. The pull switch activates a couple of futher tone options. In that case you set the pot height low enough to hide the nut but high enough to finger pull.

The rules are different when it comes to guitar amps :)

snaarman
17th September 2018, 06:10 AM
That Quad II is actually older than me, built in 1953!.

Ah but not quite as old as me then. I had a look at the Quad II circuit on line and it is a thing of sublime beauty. They were and still are the bees knees. And they make a 21st century version of that mono block. If you have enough money...

PS. Did you know the KT part of KT66 stands for kinkless tetrode?

Naughty Nigel
17th September 2018, 09:06 AM
Ah but not quite as old as me then. I had a look at the Quad II circuit on line and it is a thing of sublime beauty. They were and still are the bees knees. And they make a 21st century version of that mono block. If you have enough money...

PS. Did you know the KT part of KT66 stands for kinkless tetrode?


I didn't. I know the KT66's are beam tetrodes, which provide greater linearity than pentodes; but I don't think I have never heard the 'kinkless' term before (which I presume refers to their linearity)?

The Peter Walker power amplifier designs are a masterpiece in elegance, and are exceptionally stable even when driving 'difficult' loads such as electrostatic loudspeakers, which were another Peter Walker speciality. If you look at the circuit you will see that it is totally integrated, rather than having 'sections' bolted together with perhaps a nominal negative feedback loop from the loudspeaker output.

What few people know is that the Quad II amplifier design allowed for quite wide tolerances in valve performance as valves were still in short supply following WWII.

snaarman
17th September 2018, 09:13 AM
There's quite a story behind Pentodes, Tetrodes and Beam Tetrodes - mainly to do with patents and valve performance.

In order to get pentode performance without paying Philips (?) a licence fee other valve manufacturers looked for a new topology that would give similar power. Basic tetrodes have an embarrassing transfer kink which renders them unstable in some circumstances. But adding beam forming plates to make a beam tetrode creates a monster like the KT66, KT88 and the humble 6V6. These make as much power as a conventional EL84.

In a previous life I borrowed the Quad 303 current dumpling method* to make a hybrid IC/Transistor 200W bass amp. It worked rather well.


Pete

*Amusing engineer joke there

Naughty Nigel
17th September 2018, 01:22 PM
In a previous life I borrowed the Quad 303 current dumpling method* to make a hybrid IC/Transistor 200W bass amp. It worked rather well.


Pete

*Amusing engineer joke there

Interestingly, the late Peter Walker actually encouraged amateurs to copy his designs, and was happy to give advice when things went wrong. He must have been quite a character.

He was also keen to dispel myths about expensive loudspeaker cables, and on one occasion used orange Black and Decker at an audio exhibition because that was what happened to be available.