6l6 in VHT Special 6

[J M Fahey]

Liked it very much.
Don't know what the difference with a stock VHT is, though.
The speaker is killer, but donīt know why they paint it red and enroll it in the British army, sounds very American to me.
Like a 10" Black Powder in fact.
*Very* bright and punchy, not British at all.

[kleuck]

Well, the axe is bright, the player (Marc) likes it bright, and the amp is too (at least now)
I think i like the in-between, and always end with VoxEnder amp
Actually, the 6P3S is the best 6L6-like i have used in the amp, almost a big 6V6, the 6P3S-E is harsher, less compression etc.
Do not have a lot of vids of the stock SP6, but here's one with the Ramrod, and only good tubes (Mazda preamp and power) begins with the amp only and a booster of my own in the second part : Ballast Trouble Booster Germanium revisited quick and dirty vid - YouTube

[kleuck]

And indeed, the Ramrod is a Killer speaker, punchy, touch sensitive, works well at very low levels, high efficiency and cost-effective : i love it.
It exhibits a juicy midrange, i think that is why it's part of the Brit Speakers, very different from the speakers from the Legend or Patriot line i have heard or owned.
My friend Marc uses a Emi Wizard in his BJ, and the amp stills sounds fenderish, despite the EL84 and the "Brit" speaker.

[kleuck]

Forgot : my son's combo was prone to fizziness.
My last update :



C8 & C13 are now NP (supposed to sound better than polarized caps, will tell in a few days) and added a 430 pf cap across R13 to soften a little the highs.

[kleuck]

VHT SP6 Ultra "Tweed mods"



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[big_teee]

VHT SP6 Ultra "Tweed mods"



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Great Schematic!
Could You, and would you mind giving a list of what the Mods on the Schematic address?
List the Advantages, or ++s.
Thanks In advance.
T

[kleuck]

Well the owner wanted a "Tweedesque" sound, so i tried to mod it in a 5F2A way :
I got rid of the fixed tonestack
Put better, and supposedly more "correct" caps (metalized polyester, and a PIO, just because i love the sound in the Tweed Tonestack) and that alone is a great (and cheap) improvement.
Most important thing was to tame the power supply : the SP6 does not use a Tube rectifier like a 5F2A for example, caps are of lower capacitance, and the first resistor before the stopper is way higher.
So i put two 100 ohms resistor before the diode rectifier to more or less simulate a tube rectifier sag (even though sag is not obvious with a SE amp) a higher R48, lower caps (F+T)
The result i a way smoother, musical amp, not "in your face", with compressed and somewhat lousy lows (like a tweed)
Next, i tuned the bandpass (// cathode and anode caps) and put a light NFB, tuned it too (inspired by the AA764, not the 5F2A to save the gain and crunch)

I modified the Ultra triode too, the owner did not ask for it, but more gain, less lows gives a more marshallesque sound, so the amp is a little more versatile (cheap mods too)

The original schemo is in the SP6 Ultra manual

Please excuse me if i'm not precise enough, i'm french, not an easy task to explain these tricks in a foreign language.

BTW, experimenting with the Ultra lent me to modify one more time my own SP6 for more lows.

[kleuck]

The SP6 Ultra "5F2A modded"

[J M Fahey]

Félicitations, kleuck. Trčs bon son!

[kleuck]

Thanks !
I would love to have advices from people who owns a 5F1 or 5F2 (i do not)

[kleuck]

Last updates of my own SP6.
By comparison with the modded Ultra, if found it lacking lows after all (slighly) too gainy and "in your face".
So, i tuned the lows, added some screen compression (1K5 resistor between B+1 and B+2) and put lower plate resistors on the triodes.
I drew the Zobel in the amp, but i actually built it into the cab.
Last mods are red.

BTW i took new measurements with a 6P3S (same as 6L6GT) and calculated the power at 6,25 watts with this tube (did not yet with a 6V6).

[kleuck]

[kleuck]

BTW, i tried as i said to estimate the output power of the amp (graphical method) and found 6,25 watts with the 6P3S, and now 4,7 watts with a 6V6.
Only an estimation, as the curves are for a screen voltage of 250 volts, where we actually have here 340 / 320 volts.

[J M Fahey]

You know you *can* measure real RMS power out at the onset of clipping
Just need a test tone (1 KHz MP3), a load resistor and a software "PC scope".
You must attenuate speaker signal 100X (easy: 100K/1K resistors) before feeding it into PC Soundcard Line In.
The scope is not for measuring, just to see waveforms and the onset of clipping; then you measure Audio voltage with your multimeter.
Sure beats "estimation" by a long way
Download this *killer* PC scope:
PC Based Osciloscope : Winscope 2.51 Ŧ Invobot : Artificially intelligent
It saved my bacon more than once, specially when away from home/shop.
Can be installed in 5 minutes; the crocodiles to (2 resistor) attenuator to 3.5mm plug cable another 5 minutes.

Just sayin'

[kleuck]

I know, and i have a scope under Linux, but anyway, a true rms DMM is mandatory i suppose ? Mine is not.
A friend (an amp tech) measured the stock SP6 at 4 watts.