My head amp

Anything to do with the things that make the music we listen to.
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Paul Barker
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My head amp

Post by Paul Barker » Tue Mar 13, 2012 8:49 am

OK so it has been evolving. The kt88 output in pentode mode with one stage feedback (I can't try U/L don't have the right transformer) sounds about the same as kt88 triode mode without feedback. But has more power as expected and more low level detail across all the frequencies plus, punches out the music very fast. Attack good I think you would call that.

But there is sibilance and there is too much brightness tilt.

So I tried the 300b. Better middle tones but not significantly better enough, and not worth the loss of low level detail across the board compared to the pentode, (remember this amp doesn't need to go below 80hz) attack non existent next to the pentode. loudness not much less.

So then I tried the 801a. Much nicer, and a keeper, as good as the pentode at everything but with a better tone than the 300b. Obviously the flaw here is power output. Great for a practice amp. the pentode has enough power for a hotel lounge balad style gig or a small pub blues gig. Not enough for a prog rock gig. But I recon it would do stairway to heaven in a lot of rooms.

But the flaw in the design in terms of my hifi experiences is the sibilance and the peakyness. But these things possible accompany guitar amp designs.

My comments on output valves relate only with respect to this guitar amp circuit for the guitar. I have left my prejudices behind, it has been with the aim to improve the tone and retain the volume and the punch.

My next experiement which will take some time, will be to build the Rosenblit Glass Audio OTL once more. My memory of it in hifi terms was that it filled the room from ceiling to floor from wall to wall from corner to corner both ways with a huge wall of sound. It completely ibliterated what I was doing at the time with 300b SE, 813 SE 2a3 and 45. Left them sounding like little mice trying to occupy a small part of the room with sound.

SO I am thinking this attribute of filling a room with sound should bode well for a guitar amp.

I am hoping to recover some of the tone I am after by using my German Field coil speakers. If I make a 4 X 8" open baffle arrangement that looks like a standard guitar cabinet because it will be fully open backed, with grille cloth across the whole back. (The damping so provided may help things anyway).

They are 8 ohm each so with four in series I should get 40 watts or so.

Because the Rosenblit has global feedback I shall probably built a small lowish voltage valve preamp as a seperate plug in which will provide initially the overdrive sound, and later tremalo and reverb, I may incorporate tone controls.

We shall see how that works out.

Not sure where the sibilance is hiding at present but the guitar amp design breaks so many rules I am lost..

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Post by ed » Tue Mar 13, 2012 9:15 am

Paul

is this of any interest?


http://www.ampage.org/

it's a fertile field of info and feedback if guitar amps are your thing, the raison d'être is different.
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Post by Paul Barker » Tue Mar 13, 2012 9:28 am

Thank you Ed. Great place.

Look at the Champ, it just has a 16 microfarrad as a smoothing filter.Image

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Post by ed » Tue Mar 13, 2012 9:58 am

An historic observation...

The thing about prosumer kit, and I think the champ falls into the category because loads of youngsters own them on their journey to stardom.....

is that when you plug an affordable guitar into one of these amps, the first thing you hear is HUMMMMM. Every kid knows it. It's not the amp or it's design, it's the guitar-amp interface......it's wanting!.

If the amp hums because of it's design you're very unlikely to know, cos it's swamped by a load of other factors...leads, jacks, plugs, internal connectors...IME mostly leads....

old champ should consider himself lucky that he's got 16uf.

btw those fenders had loads of variations...there may be a later champ with 16000uf(joke)

in the 1960s I had a futurama which hummed into every amp I tried(including an ac30)..it took me ages to discover that the cost of the lead was proportional to the hum(inversely).......smoothing caps?, nah totally unimportant.
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Post by Paul Barker » Tue Mar 13, 2012 10:10 am

That is interesting.

I am aiming to use telecaster with single coil pickup. Presume the humbuckers of later eras don;t hum so much.

So Telecaster into champ; buy good leeds!

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Post by ed » Tue Mar 13, 2012 10:20 am

continuing from my joke above....

the 5E1 champ model had 450uf 10H 450uf

****edit : misread the circuit - it's 8uf ?H 8uf *****

there maybe more..I have loads of champ circuits, I will check..

do you want me to send you some?
Last edited by ed on Tue Mar 13, 2012 8:04 pm, edited 1 time in total.
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Post by Dave the bass » Tue Mar 13, 2012 10:39 am

Talking of 'pros' and Super Champs.... IIRC Alan Murphy of Go West/Kate Bush/Level 42/+ever other half reasonable 80's band used to use a super champ as his primary overdrive/distortion source.

He then fed the likkle Super champ into whatever big amp he was using on stage.

Read all that in a Guitarist mag years ago, pre-interwebs so it must be true then! :-)

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Post by Paul Barker » Tue Mar 13, 2012 1:31 pm

ed wrote:continuing from my joke above....

the 5E1 champ model had 450uf 10H 450uf

there maybe more..I have loads of champ circuits, I will check..

do you want me to send you some?
Yes it would be interesting, or you could put picture in photobucket and give me a link so I can print them off?

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Post by Paul Barker » Tue Mar 13, 2012 1:33 pm

Dave the bass wrote:Talking of 'pros' and Super Champs.... IIRC Alan Murphy of Go West/Kate Bush/Level 42/+ever other half reasonable 80's band used to use a super champ as his primary overdrive/distortion source.

He then fed the likkle Super champ into whatever big amp he was using on stage.

Read all that in a Guitarist mag years ago, pre-interwebs so it must be true then! :-)

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That particular version linked is elegant in it's simplicity.

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Post by ed » Tue Mar 13, 2012 1:46 pm

redundant post
Last edited by ed on Tue Mar 13, 2012 2:18 pm, edited 1 time in total.
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Post by Paul Barker » Tue Mar 13, 2012 2:04 pm

Thanks i have downloaded them.

The SE version with 6sj7 front end that uses degenarative bias across a 5 meg grid leak resistor looks interesting. It is also the first SE circuit in the series without output stage feedback.

they certainly have played around with it.

I bet that early one with 16 uF and later 8 uF pie filtering (undoubtedly paper in oil) would sound great!

I really should build it for fun.

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Post by Mike H » Tue Mar 13, 2012 6:32 pm

Edit ~ somehow I think they were more inclined to be electrolytic.

16 & 8 uF are archetypal values from long ago, 16uF was the biggest value available, that was it, must've been flippin' expensive too it would seem as you rarely saw them doubled up to make the value bigger.

Pretty likely it was a 16+8 in a metal can as well. Used to mess with some of these years ago but they'd all mostly expired from old age.
 
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Post by ed » Tue Mar 13, 2012 6:39 pm

Mike H wrote:Edit ~ somehow I think they were more inclined to be electrolytic.
were electrolytics invented when these amps were on the go?
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Post by Mike H » Tue Mar 13, 2012 6:43 pm

A year or so ago I was in correspondence with a musician chap who was really into Champs, he sent me this, a variation on the above, it's the Champ 5F1:

He liked the feedback bit so he could have it clean for blues type stuff (I think), with a switch for it I believe as well so could be used either with or without.
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Post by Mike H » Tue Mar 13, 2012 6:53 pm

ed wrote:
Mike H wrote:Edit ~ somehow I think they were more inclined to be electrolytic.
were electrolytics invented when these amps were on the go?
Well they were in an Ekco AC85 wireless I repaired about 11 years ago and that was made in 1934 :D

Some of the ones I played with came out of other wirelesses one of which was around circa 1940's -ish I would think. Or maybe slightly earlier. At least me dad got it 2nd hand after WW2 after he was demobbed and moved down here to Essex A. cos money was short in those days B. nobody was making any new ones yet :D

He also had a small plastic cased American one, adapted for the UK mains by a resistive mains lead (HT = that rectified, no transformer), could be wrong but problee what he was using while still in service. That had lytics in it as well.
 
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