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6c33 - ready, SET, GO

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Q

quikie22

Member
Joined 2009
rvrazvan said:
well. i don't like push pull designs. and otl means push-pull in 99% of the cases.
I would like a SE otl :D.
otl3_-1dbr.bmp

Check the third harmonic....
I would rather have 10% distortions with 9% of it beeing second harmonics :D.
Click to expand...

have tried both.. My experience thus far...

P-P gets me edgy

S.E. puts me to sleep :)
 
Shoog

Shoog

Member
Joined 2002
Alastair E said:
All I have to say, is Dont knock a well designed OTL till you Try it, and really Listen to it for a while!

Ive done Circlotron and Inv. Futterman, both have their merits and drawbacks, but without a doubt, to my ear they'll knock spots off Any S.E. amp, running the same tubes as O/P!

Hans,--'TubeTVR' has Huge experience and knows what he's talking about, he's been making OTL amps for years....(Interesting the comments on the distortion graphs BTW Hans...!)

SE is all very well, but you Still at the mercy of that huge chunk of Iron and copper between the Tube and the Speaker....

--As the old saying goes, 'The best cap is No cap'--I believe the same holds true for Transformers too...

Gave up on SE after I saw its limitations--Unless you have a Rothschilds' Bank-Balance to buy a 'perfect' OPT, made with well exotic materials, costing an average man a years salary!
--But, Thats just My opinion, and I'm a nobody....
Click to expand...
But what you get is protection for your very expensive speakers.

It doesn't have to cost the earth to get performance out to 60khz (adequate) and down to 10hz with an output transformer. No need to run more triodes than you actually need either - just to get the output impedance down to a respectable level. Also a well designed PP OT will allow feedback free operation, or just a tad of local feedback.
Have you actually tried all of the available OT options - not all are made equal. C-core and toroidal will outperform an EI by a long mile.

There are many advantages to going Push Pull with output transformers, and only a few to go the extra mile of an OTL.

Shoog
 
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rvrazvan

rvrazvan

Member
Joined 2010
SE are not accurate but i like the soundstage, the feel of the music. And right now i'm hooked up on instumental music, classical music, and some jazz.
SE are horrible when confrunted with pop or other modern music. The bass just does not have enough damping. I don't know how OTL's sound...but the few PP i have heard till now did not impressed me in any way. I like the ideea of no OPT..but i hate push pull :D.
Anyway ... SE, PP, OTL PP....they all have flaws. So the best thing you cand to is to combine them all :)).
PP otl for bass, SE for mids and tops. The only problem with this....100kg for two monobloks ?

And now back to 6c33..cuz this is what i'm building right now...

Any opinions about using a pentode as a driver for the output stage? would that make the sound worse or better compared to a SRPP driver stage?

Electrolythics in the power supply or motor run caps?
 
Shoog

Shoog

Member
Joined 2002
One last point about PP (before we leave it alone). Most PP you will have heard are built on the principles of Mullard or Williamsons. Because of this they are in no way comparable to the design principles applied to your typical SE amplifier.Build your PP amp to the same principles as an SE and it will walk all over the SE, stamp in its face, spit on it and then chuck it in the rubbish pile. That means Class A and No loop feedback. Find me to many PP amps which fit that spec and we can begin to discuss whether they sound as good as SE.
I run a 6080 PP amp and it will suck up any style of music without flinching - including techno dance.

Rant over:D

On the subject of power supply caps. When drawing so much current from your power supply its going to be quite difficult to throw enough motor run caps at smoothing (especially in SE), so reserve your Motor runs for last position duty - where you will hear them.
As to pentodes. Probably a good idea if they are high transconductance examples. The grids of the 6C33 will probably be quite leaky and the pentode will resist letting this effect its output behaviour. On top of that, it allows you to think about a bit of Schade feedback to improve the damping factor. I would be thinking of something like an EL86 as a driver - plenty of grunt to cope with any grid behaviour.

Shoog
 
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Shoog

Shoog

Member
Joined 2002
I don't think the EF86's will have anything like enough grunt. Look at the EL86.

Ultimately that may not be the best choice - but something like a 6AU6 or anything capable of at least 10-20mA of plate current should be considered.

Here's a good place to start searching;

http://www.pmillett.com/pentodes.htm

Heres what he says about one of the prime candidates;

The main peculiarity of the amp circuit, I guess, is the single-tube pentode driver. I experimented quite a bit with the 12HG7 and 12GN7A tubes, both of which are high Gm pentodes designed as video output amplifiers for color TV's. They are amazing devices, capable of providing very low distortion, high gain, and low output impedance all at the same time... much better than you can get with any single triode I've seen.

Here is some performance measurements I took of the driver stage, by itself:

Conditions:

B+ = 400V

Vg2 = 150V (regulated)

Plate load = 4000 ohms

Cathode resistor = 50 ohms, unbypassed

The driver has a very nice harmonic profile, with the 3rd harmonic always below the 2nd. At lower levels, the 2nd is about the only harmonic that's measurable. For some reason pentodes seem to have the reputation for creating more high-order harmonics, but it's simply not true in a single-ended design. This driver tests, and sounds, better than any of the possible two-triode drivers I tried, like 6SN7 direct-coupled, or 6SL7 SRPP. It also does much better than a 6C45pi-E.
Click to expand...

And look at the beast its driving;

http://www.pmillett.com/813_se_triode_amps.htm

Cheers

Shoog
 
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S

Stuben

Member
Joined 2005
6c33 filament current & plate R

Wow these puppies can pull some current. I'd look at the 12.6 v serial connection for 3.3 A...else you're looking at 6.6 A....;>{

Because of the way low Plate resistance, a Parafeed configuration might be an economical solution considering you can probably load the plates with 10 or 15 H and you don't have to worry about OPTs gapped for that mambo current...Your cash can go in to a good parafeed OPT.

Anyone tried this?

Stuben:)
 
Globulator

Globulator

Member
Joined 2004
cutestudio.net
rvrazvan said:
SE are not accurate but i like the soundstage, the feel of the music. And right now i'm hooked up on instumental music, classical music, and some jazz.
SE are horrible when confronted with pop or other modern music. The bass just does not have enough damping.
Click to expand...

My SE sounds just fine with AC/DC, Rolling Stones, Dire Straits, Pink Floyd, Black Eyed Peas, Megadeth etc!! It's also silky smooth and subtle with Katie Melua, Sheryl Crow, Norah Jones and classical pieces.

What I did to make it this way was to use an SRPP driver into a GU50 pentode output, and then wrap masses of feedback from the GU50 anode back to the SRPP cathode resistor. If you don't have enough the bass is a bit floppy, so you need loads ;). This presents a very low impedance drive to the OPT, without the traditional stability issues of GNFB. There is no global feedback.

This idea was based on a thread here by Kuei Yang Wang (Thorsten Loesch) about partial feedback and works a treat :)
 
rvrazvan

rvrazvan

Member
Joined 2010
Globulator said:
My SE sounds just fine with AC/DC, Rolling Stones, Dire Straits, Pink Floyd, Black Eyed Peas, Megadeth etc!! It's also silky smooth and subtle with Katie Melua, Sheryl Crow, Norah Jones and classical pieces.

What I did to make it this way was to use an SRPP driver into a GU50 pentode output, and then wrap masses of feedback from the GU50 anode back to the SRPP cathode resistor. If you don't have enough the bass is a bit floppy, so you need loads ;). This presents a very low impedance drive to the OPT, without the traditional stability issues of GNFB. There is no global feedback.

This idea was based on a thread here by Kuei Yang Wang (Thorsten Loesch) about partial feedback and works a treat :)
Click to expand...
SCHEMATIC please :D
 
Globulator

Globulator

Member
Joined 2004
cutestudio.net
Jaap said:
What OPT did you use ?
ciao
Jaap
Click to expand...

Just the ones that came with the Sweet Peach FU50 amp - for this is a re-working of that:

Sweet Peach-FU50 Tube Amplifier [Sweet Peach-FU50] - $310.00 : DAC,Audio Amplifier,Hifi Amplifier,Tube Amplifier,Hlly Audio, Hlly Electronics

It starts off life with a mundane 6N3/6N1 front end driving a triode strapped FU50, so I put a new top plate on for the VR tubes and others, and built it point to point to my new design.

I did try a few different topologies for driving the GU50, but the only one that really lit it up was the SRPP, and the nicest/liveliest sound was from the 6N2p. What I wanted from the driver was a big voltage swing (The pentode bias is about -70V) and the ability to drive. Then I wanted a large gain of 6N2p SRPP + Pentode GU50, so I could wrap a very tight feedback on it.

A possible mod is to load the SRPP stage to it's ideal impedance - to reduce distortion, but it's good enough as it is for me, I haven't felt the need to mod it further. You can do this because it is driving a pentode, not a triode so there should not be too much load there to drive bar the capacitance.. which is sort of what SRPP was designed to drive IIRC.

This leaves the input tube wich must be protected against PSRipple and has no feedback, so it's very 'rollable' and tweakable. I ended up on an ECC88.

BTW the top SRPP triodes are in the same envelope off a separate 6V transformer - a small potted one I picked up for about £10, so cathode/heater voltages are safe even when the SRPP is pumping a big signal.
 
Globulator

Globulator

Member
Joined 2004
cutestudio.net
rvrazvan said:
I now know what the next project will be :).
Can you take a look at my schematic: http://audioproject.net/pdf/15wse.pdf.
Can i use the same principle to bring down the damping factor?
Click to expand...

I reckon so, but it would be quite a different schematic!
Which of the schematics in your link is the favourite?

TBH I'm better with concepts than implementations so I'm sure there is room for improvement in my schematic. Eliminating the coupling capacitors by using different/multiple power rails would be one idea. A shunt regulated supply another.. I'm listening however to 'Lilac Wine' sung by Katie Melua and the dynamics and fluid sound is very fine!

My philosophy would be to up the overall gain, disconnect the global feedback and stuff it into the driver tube via a big high voltage (poly/elec-poly bypassed) capacitor - but you knew that already ;). Schade feedback works in the same way of course - giving a nice low impedance drive to the OPT. Schade with an SRPP driver would be interesting - you'd need a resistor network to load the SRPP and decouple/convert the output to the next stage so the schade FB could actually work and not fight the low impedance drive.

My view (after my experimentation) is that Thorsten was right: The OPT should not be in the feedback loop, and that lowering the driving impedance is the best/better way to get the most out of a given OPT. In my case it gives an apparently cheap and feeble SE the guts of quite a pokey beast!! Of course it's helped by using the superb GU50, a tube of remarkable quality.

In my experience you can play about quite a bit with tubes, operating points, load lines etc and they all behave well. The only issue I ever had (not an issue for your tubes I think) was with VR tubes oscillating, and I could only escape that by buffering it with an IRF730 MOSFET. It was worth it to run the GU50 in pure pentode mode though :)
 
rvrazvan

rvrazvan

Member
Joined 2010
Your schematic has been cathegorised as "SMART" by the man who build my OPT's. And he knows a few things :p.
Anyway...i'm surprised by your addiction to russian stuff. Not so common in the west :p.

As for my schematic.. right now i'm learning about interstage transformers. So i might end up with a SRPP driver stage coupled via an interstage transformer. Fixed bias for the power tube.

Bleah. S**** to be a newb. You just don't know which way to go.
 
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