There's two mods there, I'm not sure which one you're asking about... So...
One thing I did to it last night was changed the tone pot to that concentric one, and pulled the tone knob and the voltage divider and other resistors which fix the response of the first tone stack out, so I could adjust the treble and bass like, uh, this, which is awfully close to the special 6 minus the vib.
http://www.ampwares.com/schematics/c...ibro_aa764.pdf
That mod is pretty badass, the amp is much friendlier now.
The next one I mentioned is the way VHT did the "boost"- it lifts the gnd on the first tone stack way up, basically taking it out of the circuit. This is cool but kinda brutal, a much sweeter way to reduce the loss of the tone stack involves placing a cap across the top of the stack, like a 680 or 750pf. You can see that here:
http://tubefreak.com/mk3-1.gif
one cap, one switch and one say 10m resistor to keep it from popping.
Next will be NFB-
Some improvements i made, mostly to get rid of the remaining hum (in pink :Self between B1 & B2, R5 from 10K becomes a 33K etc)
I tested EL84 with the VHT adapter (and the "6P3S/6L6GT" bias resistor) and it's cool, hot distortion and gain, but i can see G2 glow in the tube, will have to drop the HT a little (50 volts) to use them on a daily base.
Tested 6P1P too, with another adapter (Ebay) and it's a cool tube too -will need a lower voltage too-
Added a 50 volts drop zener with switch and led, to get a more vintage feeling, and to be able to use EL84 and 6P1P without frying them.
Back of the amp, with the self and the Vintage switch to the right.
Vintage mod's Led
Last edited by kleuck; 05-23-2011 at 06:47 AM.
Last edited by kleuck; 05-29-2011 at 09:13 PM.
why did you cut the second cathode bypass cap and increase the first one?
I would think that would make the amp overly bass heavy.
It's better to fully bypass the first triode (only) for a better 50 hz rejection, and i cut the second one to limit the lows,
Grid cap of the power tube is small too, same goal.
It's almost perfect with my open cab with a Emi Ramrod, but you can tune the circuit to your needs, i will perhaps limit a little more, but not on the first triode, only with the second and power tube grid cap.
You can too with the power tube's cathode cap, but i prefer to have a big cap here, to have a rock-solid bias, and it's necessary as i can use different tubes and different cathode resistors.
EDIT : i actually replaced the second triode cap with a 1µf one, tighter lows, and the tonestack (R9 >> 68K ) : sligthly tighter lows in Hi mode, and still enough in clean mode.
Last edited by kleuck; 05-30-2011 at 08:55 PM.
La nouvelle self :
et les nouveaux chiffres.
Last edited by kleuck; 05-31-2011 at 05:46 PM.
Mistakes in my voltages/power chart, here is the good one :
The two leds :
Some tubes i use (6V6G ; E84L ; 6P3S ; 6P1P ; ECC83 smoothplate Siemens or TFK) :
BTW, i compared one more time 6P3S and 6L6WXT this morning, and the WXT doesn't sound as powerful, rich or musical as the 6P3S, so with my bias, i think that modern 5881 or 6L6GC are not a good choice, choosing a resistor for a 6L6GC would lead to a huge idle current, not sure the PT can stand it ; perhaps the best solution would be to use 5881 and bias for 23 watts.
I tried a 6P3S-E too a few minutes ago, and i sounds slightly more powerful than the 6P3S, more modern too (less musical compression) probably the best choice for max headroom and power, would need a slightly different bias to be at it's best i think (did not measure)
Oh! , the might of the Placebo Effect !
Did you measure the actual RMS power increase?
Just wondering.
I'm not equipped for that, and actually not really interested, but why do you want to see a placebo effect ? I made repeated tests, i takes me a couple seconds to swap tubes, the amp is not powerful enough to make me deaf, so, i'm sure of what i say.
You can just give it a try you know ? Just a couple resistors and switches -and tubes of course- are needed.
Even different 6V6 do not provide the same power -by ear- i love the old RCA in metal case, but they do not sound as powerful as Mazda or Brimar 6V6 for example.
BTW, i was probably wrong, my anode current for 6P3S is already 67 mA, a true 6L6GC would need about 80 to 85, i think the PT cand stand this extra juice.
Tried an old 6L6GC 'Dario Miniwatt) today, sounds as powerful as the 6P3S-E, muuuuuuch better sound.
I read a similar test on another forum.Originally Posted by kleuck
Another Tube Favored, was the Tung-Sol 5881 New issue.
Terry
Keep Rockin!
Terry
Interesting, with the stock bias or with same kind of bias and impedance trick as i do ?
Tried a 6K6GT (RCA) today, with the amp on the "vintage" position (lower B+, around 300-310 volts) but also on on the regular position (not for too long, no redplating, but i will have to measure voltages an currents to see if at least the tube operates more or less in a safe zone), these tubes are nice, sligthly less power than the 6V6, a little "pentode fizziness" added
you could try to mod the tone stack to a more "Ultra" topology and maybe add a few "depth" caps
https://docs.google.com/open?id=0Bx4...ViYjdkMzYwMzBl
^^big .png file
^^preview
Why ?
A question for Klueck,
First...thanks for sharring all of your notes on these mods....I've learned quite a bit from your experience...I'm fairly new to tube amp modding so I've been trying to understand all that you have suggested...
Can you explain the reason for referencing the Heater to the Cathode....I noticed that early on you did not reference the two...simply adjusted the Cathode resistance to achive the correct bias...but later you decided to refrerence the two...From the tube datasheets the spec indicates a maximum +/- 200V...which we will never see...what other reason for referencing heater to cathode ?/
thanks,
Ray
Just an old trick to reduce hum.
Cathodes pick hum from the heaters when their voltage is higher than the voltage of the heaters, which is actually 0 (average, as it's AC)
Connecting the virtual ground of the heaters wiring to a higher DC voltage (instead o ground) reduces the hum, you can do that with a divider from the high voltage line (would be more efficient, as you can achieve a floating 40 volts or so) but it's so easy with a cathode-biased amp to simply use the power tube cathode voltage
Merlin has a complete page here : The Valve Wizard
Did not do this at first, cause i was finding it a little tricky -first time i've done so much mods in a tube amp, did not want to fry it or myself etc.
Last edited by kleuck; 01-09-2012 at 11:10 PM.
Bias was a little cold to my taste (especially when using a 6L6) -not center-biased- so i modified the cathode resistors. 6V6 are now between 12 and 13 watts and 6P3S between 20 and 21 watts. I would prefer 14 and 22 so will try to lower the resistors a little more.
Just curious: what does "center biased" mean in this case?
Thanks.
The OT is 9K a so center bias ("max power") with the anode voltage i measured (342 volts if i remember well) would be around 14 watts static for a 6V6, for a 4K5 OT and with the 6P3S, it would be 22 watts (310 volts on the anode) so i try to nail that, not really for the tiny difference in power (not sure to hear one) but to try to have a better sound when using 6L6 or 6P3S-E, which doesn't sound really good with the cathode resistor meant for the 6P3S.
I try to find the best accomodation between the two bias resistors, for all my tubes (EL84 with the socket are runnnig pretty cold too, 9 watts)
Did not try yet with, but i found the 6V6 and 6P3S to sound better ("rounder") and slightly more powerful, but it can be a placebo effect, i did not have time to really play, just a few minutes to be sure nothing was burning or redplating
Last edited by kleuck; 01-23-2012 at 09:47 AM.
Tested the 6P3S-E and a vintage 6L6 this morning, indeed the 6P3S-E sounds way better now.
http://youtu.be/DBodXr4docc
My friend Marc is playing an alder Telecaster, with Tonerider pickups (alnico III) and a custom electronic : 1M pots and 5 nf tone cap.
The VHT is (obviously) modded, bias is a little higher than on my last schematic (430 ohms for the 6V6 and 235 for the 6L6-like) R5 is lowered to 22K.
At the beginning of the video, the SP6 is fitted with a 6P3S-E, at the end (Hendrix) it's a 6P3S (slightly less powerful, but much more musical imo).
Preamp tube is a Telefunken smooth plate.
Cab is fitted with an Eminence Ramrod (awesome speaker) which is a 10".
Video recorded with a Sony HxV7 (or H7Xv ?) well, a good APN, on YT you can hear in HD, it's really better, but 8-10 watts in a small room was too much, the sound is squashy.
Last edited by kleuck; 01-27-2012 at 08:06 AM.
Yikes that tone in first part hurts my ears....
Liked it very much.
Don't know what the difference with a stock VHT is, though.
The speaker is killer, but don´t 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.
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
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.
Last edited by kleuck; 01-28-2012 at 08:37 PM.
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.
VHT SP6 Ultra "Tweed mods"
Uploaded with ImageShack.us
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
Keep Rockin!
Terry
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.
Last edited by kleuck; 04-28-2012 at 10:48 PM.
The SP6 Ultra "5F2A modded"
Félicitations, kleuck. Très bon son!
Thanks !
I would love to have advices from people who owns a 5F1 or 5F2 (i do not)
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