Monitor Amps - Class A Monster Madness ?

[alexc]

The fender super 4x10 style I did ages ago -  I use it for bass mostly,  with a delay pedal into the vibroverb channel

It's there in one of the pics  (left side) - its has a 'direct out' which I feed to a balancing iso-box and then to the M-Audio fw1814 front panel bal input

Super quiet, fully out there clean and vibroverb mix to daw  with 'live' monitoring!

 

[alexc]

I still think Class A sounds best    for the detailed work.

My particular liking is parallel SE amps ... giving like 10 thru 15 and on to  22Wrms,  depending on ones's speakers.

I personally like combining the tubey amps with the clean ss amps ..  as I go.  Together, in a decent sized space  ,, can get really intimate sounding.

If you have a climate that leans to the colder ... all the better !

 

[alexc]

My next build in the field of ' tube stereo power amp' is going to some SE compactron  cathode follower OTL  *with* output transformer!  type of  biz   

I'm thinking a low-ish  Z output traffo (600:8 or so), with some beefy compactron current driving it  ... cathode follower style ....  and  of course the requisite high swing, low thd driving stage.

Or I could go  with lotsa parallel smaller noval types ..    Either way, a CF type SE build is next for me!

A seperate psu box with a cable and I'd like to try an all tube regulated circuit this time ..

Mostly Edcor transformers .. again    It's just that they make exactly what I want that makes it easy  :
..

Of course, one needs to keep a speaker set in mind when doing amp scenarios ....  ideally you add another pair of speakers when you do another ..  *yet* another amp .... to keep it real  8)

Happily, I do have a speaker build in the works  ....  looking over passive crossovers and such - I have the drivers  ... 12" alnico paper cones, to be paired with some 6" fostex full-range paper coners

 

[alexc]

A toast to both ss and tube Class A amps, both pre *and* power!

Along with the JLH ss amps I have in the works, my desire is to do a last  snazzy  'se tube'  amp, to replace my old-time  daily driver  6L6 spudster
....

Decisions, to make, I must    ...  a 'super dooper' SE or  something different  ..  a 'cathode-follower'  type, or  even a 'more expoorimental' off the beaten track type.

Any which way I'm thinking some super nice finals  ..

....

Been  doing some 'book learning' in the field of 'sweep tubes'  ...  which once were 'cheep' but now not so much. 

...a couple new 'modes' of driving  big finals  ....  twixt I tweren't  aware of betwain  ... the 'enhanced triode'  mode and variants therof ...  'screen drive' and  'screen+grid' drive  . sometime called  'crazy drive' and of course .. 'power drive' from the well known 'tubelab' .. tube .. lab.

Here in Oz, the local fav tube guitar amp was the 'goldentone'  pp 6dq6 combos..  kind of the poor mans fender lineup . Nice tube, and a candidate for some 'tv tube' fun.

...

If I have my way ... and can raise the dinares,  ... I think I'll do something with the  Svetlana SV range of tubes - prolly the sv572/10.  The modestly crazy scale of such a build is quite apealing

I mean .. where do you go next, after the el86, 6v6, el84, 7815, el34, 6l6 and kt88  classics  ? 

Either sweep tubes or the dht triodes  8)  Something a little different :

The goal ... 10Wrms at low thd ... simple like  ... 1% or 2% mostly h2 ... praps  some local nfb and hopefully ..  a good matching speaker,  nice  'room'    ...

with some time to enjoy

 

[Andy Peters]

The goal ... 10Wrms at low thd ... simple like  ... 1% or 2% mostly h2 ...

You consider 2% THD low?

 

[alexc]

2% (mostly h2) at power  is low for a single ended traffo coupled tube power amp with no negative feedback !

Applying global nfb will bring that right down of course, as will local negative feedback at the finals (plate-to-grid).

But for zero nfb, 4% at power is my 'max output' test. 2% is my goal at reasonable power.

And that will require some serious plate! Something like 80 to 100 W dissipated for 10Wrms of < 2% thd

For example, for a zero nfb SE single 6L6 ultra linear, I get 4% thd+n at around 4.5Wrms into 8ohms (off of 17W of dissipation at the plate).  Thats around 19% efficient.

The super cheap and small-ish sino output traffos  are surprisingly adequate - flat response all the way up and down, but the psu craps out pretty early. They are not too efficient tho', and thd is a bit high too!

The psu traffo sags pretty good beyond 50mA each final. I sub'd a 'diode rectifier' for the tube rectifier to gain some more volts at the plates - added 0.5Wrms

The 6L6s I have in the stereo SE amp are a little tired too ... I think some new tung-sol 7581a would give a bit more power and/or headroom  for similar bias conditions. For a reasonable price of 60usd a pair.

 

[alexc]

Here's what happens with SET amps when things get way out of control!

... from the Electra-Print shop on ebay 

 

[alexc]

On the subject of *zero nfb*  Tube power stages, of the *Single Ended* variety .,... 10Wrms into 8ohms  .. at low thd ...

And apart from the time tested  method of getting lowER  thd that is    ..  turning  it down  ...  and/or  .. good quality parts/design  ..

It seems  among possible  ways of reducing thd are :

- solid state devices mixed in with tubes

or
- cap coupled parafeed traffo  .. either plate choke loaded or cathode follower

or
- screen driven 'extended triode' mode sweep tube finals

as well as
- cathode follower finals with se traffo

[not considering output-traffo-less OTL types]

Beyond those, it seems then, local nfb and / or global feedback is the right path to low THD  SE.

And of course, Intermodulation Distortion IMD issues would be next on the list of things to look at 

Then there the question of a bunch of parallel'd tubes or single tubes to consider ... given that se tube power stages are 20% efficient more or less!

Such is my thinking for this amp  8)  and price . of course.

 

[alexc]

I think I'll get one of Tubelabs SE pcbs to do some further testing    ..

should give me a better idea of how-low-can-you-go wrt to an inline    SE+sand  stage

I can test it with my current do, then make some decisions on the tubes'n'traffos going forward.

...

Right now, I'd spend on a pair of sv572-10 and se 3K/200mA ElectraPrint  traffos  ....

Time to sell some bits'n'pieces 

 

[alexc]

Another few weeks (months) of daily usage on this 'lounge rig'  .. 

and I'm happy to say that the 8" monitor refurb of some sad 90s jbls has exceeded my expectations and is well on their way to .. best little hifi rescue ever 

- the pair of new peerless drivers 8" and some cheapish dome tweeters  (details are earlier in thread I think)

WOW! The first time I've heard speakers 'break in' from 'not sure' to 'way pleasing' over 3 or 4 months with a solid bunch of hours in most day

It's  like the bass has freeed right up, and the mids are now the focus. And they are really snappy, paper cone intimate ..  kind of way. Like a brisk snow capped but sunny morning !

The tweets I pad'd down significantly,  and they are much better blended in now 

I have them on a 90s Yamaha A-500 hifi with an MAudio fw box playing Cubase pc 

 

[alexc]

Then, I blend in some even more 'lightweight' speakers .. 

- some Realistic Nova 8s - light weight, timber, lattice fronts with 8"+tweet

Funnily enough, I loaded up the jbl woofs from the above mentioned refurn into these and they *just* took off some .

SO once again, the name of the game in awesome audio componentry is to 'do your best' on each build, but *then* find the combination that makes it shine bright.

In this case it's all quadraphonic low watts intimate paper with g-pultec boosted tube bottom end

 

[alexc]

Nothing much new to report, 

but an upgrade of my  ' single ended spud' stereo amp (6sn7+6l6)  finals' with some new tung-sol 7581a

Damn fine! They *sound* like lower distortion to me, than the previous 6L6s  (some early 70s RCA clear tops)

Matching across L-R of the new 7581a tubes, with same fixed bias pot setting,  was within 2mA at the cathodes  .. pretty good match!

I won't be 'measuring' proper, until spring when my bench thaws out, but I would say the new t.s.  7581a finals would do 3% thd (1KHz sin) 5.0Wrms @ 8ohm load  ...  up 0.5W from my current 6L6 ..  in these very spartan , zero feedback spudsters 

The cheap sino se traffos (70ma rated) work quite well 20-20Khz wise, but are fairly inefficient, with some decent losses beyond expected. But running conservatively they do OK. I measured them on the 'way up' (current wise) and they get very ordinary, very quickly.

[ 7581a plate +390V, screen UL +387V and 53ma at the cathode, fixed bias,  with a plate dissipation of around 21W (from 35Wmax rated)]
[ 8" paper cone, timber ported cab with silk tweets, 30Wrms rated]

Of course, running a higher voltage, up to +465V or so would clean things up 'a little'. 

The new tung-sol 7581a have a nice modulation of the 'blue glow' with the music  ...  damn fine, next to the purple lava lamp  8)

These modern ts 7581a are my 'current' favs'

....

With decently matched speakers, 5Wrms can be quite 'big' With some 'gyraf tube eq' on the way in, it really moves me.

One day, the ' se 6L6 spudster' amp will be replaced with an  'pse 300b  interstage Sowter' build    to gets me 10Wrms  a  channel at <3% thd with low-ish overall nfb.

I'm thinking to do a compare of an all tube path against a tube+solid state 

....

I'm just working on the speaker boxes just now ...  some 12" cast frame paper coned 70's pioneer alnico woofs (60Wrms)  +  some 6" fostex full range drivers in large timber,  ported cabinets.  And old stock sansui timber lattice fronts 

..

For crossovers, I'm starting with a low cost automobile 'multi-channel active crossover'  box    and a bunch of ss amps.

The 'listening' should guide me as to the 'freqs and slopes' enough to try my hand at passive crossover networks from scratch.

In the meantime, I have to sell some surplus parts to raise the cash for such   

 

[alexc]

Just another word on my lounge room diy speaker 'refurbs'  ...

These Peerless Tymphany SLS series 8" drivers are just phenomenal ...  never mind that they are 'affordable'.

They are a very nice contruction, sort of 'sculpted stamped' frame with some nicely engineered parts, strong magnetics and a paper cone with heavy duty surrounds and vented spiders  .. rated 80Wrms I believe.

These drivers I put in some old jbl boxes : 34cm x 27cm x 55cm (outside dimensions)

The tweets are quite cheep vifa units.

The JBL crossovers were really good however  ... with proper big, iron cored inductors. I re-capped with decent polyprop caps and padded the tweet by around -4dB.

I also stuffed alot of good quality damping material in there - they had a tiny amount of thin and crappy stuff before.

I glued a really  dense matting on the inner surfaces (about 1.5" heavy, wadded cotton) and lightly packed the cavities with 3" thick, light-weight acrylic fibre fill.

That made a huge difference 

...

The bass is now so totally awesome - kicks sound ultra realistic, like you can really hear the beater attack as well as a satisfying thud  ...  and the bass guitar ..  is all there too .. for once. The Hofner is sublime, and the J-bass with a bit of overdrive is to cry for - solid and scary, like it should be

Completely controlled bottom end, but really strong at the same time.

My guitars just pound thru these .. I *am* running my own fender pre+la2a box ... which is the best thing imaginable .....

Playing along with the stereo track, my Guild jazz box just sounds like a million dollars   the Strat is truly hifi, and the SG with some fuzz is pure 70s heaven 

I would really like to try these SLS drivers in a tube guitar cabinet too ..  no crossovers 

..

So, I'm running these SLS refurbs  off my sum box+finalizer (minimal processing, mostly gating) ... and a solid state amp doing about 10Wrms I would say, on average ... with a lot of dynamic headroom  (70Wrms max power at the amp, turned down at around 10 o'clock)

... not crazy loud, but certainly enough to rock my room  ..  which is around  8m x 4m x 3m

I have about 200hrs on these Peerless Tymphany SLS drivers and WOW they are amazing! Best 200aud bucks I ever spent 

They did start out pretty lame but opened up at 70hrs or so and are still getting better. I'm running mostly flat EQ now ..  whereas I used to run a heap of bass boost and treble cut before the refurb.

During my winter, which is just now ending, they are going all day and night ..  along with the wood burner 

I really recommend doing a refurb  like this for yourself - the old 80s JBL boxes sucked totally before.

...

SO - try these Tymphany SLS drivers out .. the bass is so tight and impact-ful 

I'm going to try some 12" SLS units next  in my Sansui SP2500 speakers  ...  they are beautiful cabs with super awesome crossovers  and quite good at the mids and highs ..  they have  interesting horn flares ...  but the woofs are a bit tame  ....  'japanese' tame    SOunds great with The Carpenters tho'   

ahh, Karen  .. sigh ..

I think  they will become raging monsters of rock  with some Peerless Tymphany SLS 12" drivers 

(around 200usd a pair for the larger model SLS drivers  )

ps - a little white wine makes the whole thing worth recording ;D

Download some classics from YouTube and thrill all over again to the sublime sounds of History

Like some Vince Guaraldi doing 'Peanuts'  stuff  ..  best .. piano .. ever  !

 

[alexc]

The twin gods of audio and electronics are whipping up some headwinds in my direction ..  again ..

Some good deals available on some big ass direct heated triodes ..  and some nice lay-down psu traffos  ..

Looks like I'm going to fold my common sense away  ..  and go for it ..  a pair of no compromise mono bloc builds   No Edcor or Hammond iron this time! (as much as I love them both)

with a couple years  hence  ETA  ...

...

I've tried to resist, finding *very real* pleasure in my cheap sino SE TungSol Russian 7581a zero nfb spudster stereo amp  8)  ....  it's perfectly adequate for my needs  ..  4.5Wrms at 3.5% thd (1KHz)

BUT as they say, resistance is futile ...  the lure of znfb SET 10Wrms at 1% thd is just too much 

I blame the good folks of Sowter, Svetlana and VTV 

...

I'm looking at monoblocs with  600Vdc B+, and on each chassis, two large psu chokes, one driver choke, a big rectifier tube,  a 125W carbon plate DHT + Sowter SE output transformer AND some low uF oiler caps.

At the input, a Sowter debalancing traffo, and for driver duties,  a 12sx7 dual triode.

...

Completely insane!  But truly, one only lives once, and my ears only have a few good years left

Target price is around 2.5Kusd, THEN there is the small matter of 'to interstage' or not.  That can add another 0.5Kusd to the tag, easy!  :-X

So, a nice round figure of 3Kusd for the pair. 

Getting some nice prebuilt Japan Tangos 'downunder' would run to twice that, what with shipping and taxes and all. And they still wouldn't have the same impact, I'll wager.

Such are the passions of an old ee and guitarist like me.

I simply *must* experience some zero bias DHT Sowters before I fold my cards for good 

...

Looks like I'll begin the spend tomorrow ...  and the selling of stuff the day after  : and for a  couple more  years hence.

 

[alexc]

I've  moderated my plate dissipation spec somewhat down to 65W  - some svetlana sv811-10 are on the way

I'm not game to go beyond B+ 600V      I'll be using familiar motor run caps of 450Vac rating, which should be good for 630Vdc - my expected unloaded B+.

The extra plate of the sv572-10  is mostly applicable at higher B+, so not really required for a chicken-little builder like me 

Prolly a C-L-C-L-C-L supply filter like  10uF -> 200mA 10H -> 40uF -> 200mA 10H -> 50mA 20H -> 40uF  providing B+ 600V  with some resistive drops for B1+ 430V for the 6SN7 and B2+ 300V or so for the 12SX7

....

Now for the long slog to save for some Sowter SE traffos  ...  5Kohms and 140mA max will work well.

At near-zero bias, I'll be doing some 85mA quiesscent current at +600V HV and expecting something like 7Wrms into 8ohms at 1% thd (1KHz), going up to maybe 15Wrms at 3% (no global nfb)

Not quite my goal of 10W at 1% but that would likely require higher B+ ... like 800V.  I'm not so great a craftsman that I want to risk those voltages!

I'm still mulling over the driver stage and coupling to the finals, but I think I'm going to go all the way with some interstage Sowters   

I'm thinking  a 12SX7 parallel'd for low thd voltage gain and a 6SN7 as a cathode follower driving a step down. Possibly the first half of the 6SN7 for additional gain with some local ndb (plate follower) if required.

The finals will need something like 120Vpp of low Z drive. The stepdown interstage is a good way to get the driving Z down low whilst still providing an easy way to provide zero-bias.

I think    ;D

I'll start with a coupling cap and measure before deciding on the interstage.

Looking foward to quantifying the performance envelope  with the various options of cap-coupled, traffo coupled driver and/or conventional fixed bias/zero bias/cathode bias of the SV811-10 

...

For PSU traffos and filter chokers, I'm going with Raphaelite laydown and potted models. The PSU traffo has a bunch of primary and secondary taps to help me find my optimal voltages, including a bias tap which I can use in 'prototyping' the bias to the DHT final.

..

There's some nice USA chassis on ebay I like ...  beautiful folded thick alu with timber sides  for a decent price with international shipping 

...

Still looking at 3Kusd for the monobloc pair all up with all the options    About 4K3aud after sales tax!

That would represent my single largest investment in audio madness to date ..  I started in about 1977 

 

[alexc]

And of course, props to Eric Barbour for his Svetlana SV811-10 design ..      and of course  ..  VTV    ...  best  .. mag ..  ever 

Right up there with the greats of USA and Russia  audio