An Admission - I know nothing about valves

ecc83

monquixote said:

It has a built in attenuator so perhaps it's that.

Hmmm? Possibly but that gung ho reviewers refers to "power soaks" which I would take to be big ass resistors.

Dave.

ICBM

monquixote said:

No it doesn't have an impedance selector. 

The speaker out says 16 Ohms 25w 8 Ohms 40w.

How odd! In that case it is a 40W amp, not 25, and must be Class AB. It's presumably matched accurately to 8 ohms or even lower, which would explain the low power at 16.

monquixote said:

That's a gut shot, not sure if it tells you anything

Does look a fairly simple and well made amp as the marketing blurb suggested.

Looks like cathode bias. I would guess the pairs of caps and large white resistors (one of which appears to be detached from the board…) at the left-hand end are the cathode networks.

monquixote said:

The Puretone is (supposedly) class A and EL34 and yet it sounds pretty much like a Fender amp when it's clean which I put down to the tone stack more than anything because if you bypass it, it really doesn't anymore.

Exactly!

ecc83 said:

I am also intrigued by the two small PCBs either end of the chassis? They each have a ferrite inductor/traff on them. Anyone shed any light?

Since they appear to be identical, I'll make the bold guess that they may be some sort of voltage control units, one for each power valve. (Since the cathode bias appears to be separate as well.) That would correspond to the claim of a "built-in attenuator" - which it certainly doesn't have in the conventional sense. But that is just a guess.

ecc83 said:

H&Ks blurb does state that the amplifier is "Pure Class A". Make of that what you will!

I am always highly suspicious of the word "pure" in any description, as you know . As well as "Class A".

monquixote

It's not my actual amp in the picture so I won't worry about the broken bit!

It has an 80 and 50 percent power mode which I assumed was a load of resistors but could be wrong.

ICBM

monquixote said:

It's not my actual amp in the picture so I won't worry about the broken bit!

It has an 80 and 50 percent power mode which I assumed was a load of resistors but could be wrong.

I assumed it wasn't your amp .

The only large resistors I can see are the two which I think are the cathode resistors, and four which are grouped in two pairs at the upper right and appear to be part of the power supply. What is the third switch? If that's the power setting switch, those resistors could be involved since there appears to be a green wire going from the junction of them to the switch.

The two small PCBs Dave spotted look very roughly like switch-mode power supplies, which is why I'm guessing they're voltage controllers of some sort. I can't think of any other reason you'd need anything like that inside a valve amp, and it could fit if there is some sort of power control. Interesting though...

monquixote

The top panel looks like this

http://www.audiofile.co.uk/images//Puretone_Combo_Panel-1024x369.jpg

The switch called "Output" is a three position which the manual says is 100% 80% and 50%. My assumption was it's a soak, but I don't know for sure. 

This is a pic of the other side if that helps:

http://carvinbbs.com/userpix/7879_6159_1.jpg

ICBM

Looking at the inside pic, it could be as simple as the output switch bypassing either of those pairs of resistors, which would allow power control if they change (eg) the screen voltage for the power valves. There are definitely no resistors on the other side either, just the standard power and output transformers and choke.

Which takes us back to square one regarding the extra PCBs! Unless they're supplies for the pretty blue lights .

monquixote

Oh I suppose it could be that. Like most h&k amps it lights up blue.

ThePrettyDamned

monquixote;323306" said:

Oh I suppose it could be that. Like most h&k amps it lights up blue.

Possibly the single coolest thing an amp can do.

Killswitch engage use Laney ironheart heads, but they had blue leds custom installed because it looks way better than red!

Interesting thread btw. I don't get how they can advertise as pure class a if it isn't?

ICBM

ThePrettyDamned said:

I don't get how they can advertise as pure class a if it isn't?

Because advertising standards legislation does not seem to apply to this industry on that sort of technical matter.

Or the hi-fi industry.

It would probably need someone with a lot of money to make a complaint and take them to court.

ecc83

ICBM said:

ThePrettyDamned said:

I don't get how they can advertise as pure class a if it isn't?

Because advertising standards legislation does not seem to apply to this industry on that sort of technical matter.

Or the hi-fi industry.

It would probably need someone with a lot of money to make a complaint and take them to court.

Too true! If we could not "get 'em" during the 70' and 80s with their "Music Power" "Peak Power", "Total Peak Music" power that inflated a 6W per channel car radio amp to 100watts, the relatively mild porky inherent in "Class A" guitar amp is never going anywhere!

In any case, nothing is class A if you drive it hard enough.

I have however resigned myself to the mathematical absurdity that is "rms" watts!

Dave.

Dave_Mc

ICBM said:

I am always highly suspicious of the word "pure" in any description, as you know . As well as "Class A".

LOL

ICBM

ecc83 said:

In any case, nothing is class A if you drive it hard enough.

It's particularly ridiculous for guitar amps, when Class A *by definition* only applies to clean sounds. Marketing BS like "Class A distortion" is an oxymoron before you even start.

It's even questionable whether a lot of single-ended guitar amps are truly Class A because they're biased so far away from the correct operating point for a symmetrical waveform.

ecc83 said:

I have however resigned myself to the mathematical absurdity that is "rms" watts!

Same here. And to a few other technical mis-descriptions...

57Deluxe

Valves tell you when they aren't fully fit... Sometimes is just a question of popping them out and reseating them, or applying a small amount of contact cleaner on the seats and /or the jack sockets or even swapping their positions accordingly.

Unless you on an intensive gigging regime, casual home use can mean years between attention... or two weeks!

jpfamps

As ICBM says the difference in power outout I'm fairly certain will be due to the fact the output transformer loading is optimized for an 8 ohm load, so you are simply mismatching the loading at 16 ohms, and hence getting less power.

re the small PCBs, they are for powering the front panel LEDs.