Bob, I bet this amp doesn't clip either

[Matt Wiebe]

Bob,
Big is always better.
Matt

http://www.tubelab.com/833SE.htm

[rj guitars]

Did I read that correctly... 220 Watts RMS from a single ended amp?

[jsn]

Bill and Ted voice: "Gnarly!"

jsn

[Matt Wiebe]

Yea you read that correctly. The 833A is a radio broadcasting triode, too bad radio has gone digital, I miss the well engineered analog days.

I didn't mean to put you off experimenting with cathode drive. Just that the cleanness of the amp may be more to do with its "excess" power then its drive topology. Cathode drive does sound different. Another big amp using it is the Altec 1570B, http://www.freeinfosociety.com/electronics/schemview.php?id=1310

I got some of the little Russian 6P1P pentode power tubes but I haven't had any time to play with them.

Matt

[rj guitars]

I have redrawn a couple schematics and would enjoy your peer review of them... thanks

Yesterday I ordered some tubes - all Russian:
  63PS - 5881ish
  6n2P-ev - 12AX7ish
  6P6S - 6V6ish

If these sound good then make the possibility of providing amps and kits with good tubes a noption...

thanks again for all your help.

rjh

[Matt Wiebe]

Bob,

I'm not sure I will be any help as I don't really know guitar amps. But if you give me which tubes you will be using I can maybe of some use. Some of your grid leaks look too big but that could be due to your tube choice.

I hope you got the coin base 6P3S-Es, as they sound much better then the traditional octal base. While the 6N2P has similar mu to 12AX7 they are only 6.3V heaters and have a much lower plate resistance which is good. A stereo design optimized for the 12AX7 will not sound good with a tube substitution. I think I have some 6P6S that I haven't played with yet.

Matt

<SSS Power Amp Options 1 and 2 - Oct 18, 2009.jpg>

[rj guitars]

Thanks Matt,

Here are the tubes I am bought. Tell me what mistakes i made and what do do different next time...

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=260492161276
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250515393384
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=260482974368

Can you tell me more about grid leak resistors? I assume this is the resistor tied directly to the grid on each tube, What size should those be?

The 5881's, 6v6's, and the 12AX7's are all for use in the four output tube Trainwreck Express amps and also for similar amps in a two tube output scenario. I run a little bit of a reduced B+ on these amps.

I also want to make a champ amp using these tubes... still looking for that ultra low cost good quality amp.

Finally, if these work out they will also be useful for that other four output tube behemoth (the SSS) that I've been working on with the Fender Twin output section, although I am a little worried that the B+ might be too much for these.

Currently I m in Malaysia and hoping to get over to a local amp factory this afternoon.

thanks for your help!

rjh

[Matt Wiebe]

Bob,

Nice part of the world, enjoy your stay. Last time I was there I was amazed by the suburbs getting built up just north of the boarder with Singapore. Beautiful little houses and at very reasonable prices. Pretty easy to see where Americans spending money is going.

My only comment on the tubes you bought is to buy a few of the coin base 6P3S tubes for comparison,

http://cgi.ebay.com/6P3S-E-5881-6L6GC-tubes-NOS-NIB-Lot-of-4_W0QQitemZ290334568810QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item439949f16a

Max grid leak is usually specified on some tube data sheets. A lot depends on whether the grid needs a little current, how closely it needs to be ground reverenced etc. But the size of the resistor also sets the size of the coupling caps. This is where guitar builders may do something different then stereo guys. You want to minimize caps in a design to keep phase shifts in check for stereo. Part of the reason some prefer very simple topologies and direct connection. 

A further problem with stereo amps is too many caps in the signal path can lead to motor-boating or screw negative feedback.

Anyway on tube compliment are you using 6N2P as V1, V2 and V3? Then 6P3S finals?

Matt

<Rail Runner XP-80 Schematic Rev 1.jpg>

[rj guitars]

Matt,

Thank you for the note. I went ahead and ordered a quad of those other tubes. I am now building a few amps with quad outputs and I seldom find that I have four of the same of anything.

I'll still need a little help in grasping things like grid leak and such, but finally I am starting to make sense of some of the overall pieces.

I have a video to ask you guy a to watch. The guru of Trainwreck amps gives the statement that  the HiFi guys use solid copper wire in the preamp to avoid phase shifts... or something like that. Can you comment on how much sense this makes.... sorry, it's a kinda boring video.

http://www.youtube.com/watch?v=Dnbe3A3pC_s

Malaysia is great.... Rolex watch anybody?

thanks,

rjh

[Matt Wiebe]

Bob,

For some reason I couldn't get the youtube to play. I do use solid magnet wire as hookup wire for no better reason then it's cheap, easy to bend to look nice and it sounds good. An EE could maybe explain why solid wire doesn't shift phase, or say that's absolute nonsense, but I'm not an EE. 

Capacitors shift phase 90 degrees. Capacitors block DC but pass AC. Look at a sine wave, at the top and bottom of the crest the wave form is closest to "DC" as the slope is "flat". In the middle of the trough the slope is at its greatest. In my simplified model the cap will be passing its most current and voltage when the supply wave's slope is the greatest, the most change in voltage over time, or most AC. So in the middle of the trough the passed sine wave will be peaking if the slope is positive or at its minimum is the slope is negative, so 90 degrees off. It's no big deal since the AC passes through the cap just fine. The problem is when you have lots of coupling caps working on the signal at once you can get some weird phase shifts happening. 

From an audio view I hope you are well decoupling your 6 B+ taps, this could get nasty fast without isolating these power stages. I'm not sure why you are using 1 Meg grid leaks on V1. As potential cost savings you could up your grid leak on V1 to 5Megs or so for contact bias and save the cost of the cathode resistor and bypass cap. You may want a grid leak on Vb since as its drawn if the pot wiper lifts there will be no grid bias. The size of your bypass caps on V1 make no sense to me, C1 22uf  C7 .1uF, I guess its a guitar thing. I don't understand your phase splitter either, what's VR5 doing? I see it's inside the feedback loop but there is so much going on I cannot follow it at all. It looks like only the bottom grid is getting feedback. The V3 cathodes are 10,470 over ground, are you sure you will have any current flowing through there? Why the Zener stack for the plates? I would think the screens would benefit more from voltage regulation. And why place the zeners between the output trans and the plate, that puts all the zener noise right on the plate. If you place them upstream of the output transformer at least the inductance of the transformer will weed some of the noise out. You regulation would be the same. But to be honest I just don't understand these amps at all.

To save money on parts you may try using an LED for cathode load if your design calls for 1.5V Gbias. This is much cheaper then a resistor and a bypass cap. Contact bias on the first stage will also save you these parts. And this would be easier on the kit builder.

I brought my wife a Rolex from Thailand. After 8 years it broke. She's kind of curious why I always forget to take it in to get fixed.

Matt 

[rj guitars]

Matt,

Thanks for the good notes... I still have a lot to learn. I guess all guitar amps are weird to somebody who really knows HiFi electronics stuff... it's always challenging to try to sort out the real from the imaginary once somebody has been declared a legend. Ken Fischer from Trainwreck amps apparently made some incredible sounding stuff, but if you ever get that video to play, it sounds like he's got some different ideas going on...

http://www.youtube.com/watch?v=Dnbe3A3pC_s

Anyway thanks, I've got to put some of this to the acid test but that would of course require building the amp and then trying some of the ideas you've mentioned. Hoping to get more than one amp done this winter including the sub-mini that you inspired a few years back.

rjh

[rjgu...@gmail.com]

Hey guys,

I got a few messages again defining the ultimate clean power amp for the
guitar/Bass and it seems this Ampeg SVT is what they say is the real deal in
a Dumble SSS amp. THis is what I am trying to make a copy of.

I made up my own schematic based off of those that I had sent you earlier. I
don't really understand this -150V bias, does it make sense to you?

BTW - I received most of my russian tubes and will use them in some of my
upcoming builds. I have tried one iteration of the 6V6 tube (6N6C) and it
sounds very similar to a 6V6 to me. I have 8 of them if somebody wants a
couple to experiment with.

Thanks,

rjh

[Matt Wiebe]

Bob,
I don't understand this amp, so I cannot say much but point out a few
things. You are going to have your hands full if you implement this
topology with different tubes.
Looking at half the schematic it's an A2 design with the cathode of
V2 directly connected to the power tubes grid. The only return to
ground for this driver tube is through the grid of the power tubes
which will be conducting current all the time, as well as through the
bias supply. So it's a long way from a class A push-pull design.
The bias of the power tubes is only partly set by the -150V bias
voltage. The -150V goes through a voltage divider of two 47K
resistors. And some of the current flowing through V2 will bring this
voltage more positive. I couldn't even hazard a guess what the grid
bias of the power tubes is at idle.
As I mentioned before the cleanness of this amp is probably more to
do with its two pair of 6550 outputs then the robust driver.
Nevertheless having so much juice on the power grids means it's going
to sound much different then a class A amp at clipping. I've totally
forgotten what this drive topology, cathode to grid, is called. But if
you know you could probably google it and look at a few more designs
that would help you get a better fell for what is going on.
Matt


On Nov 2, 2009, at 1:33 PM, rjgu...@gmail.com wrote:

> Ampeg SVT

[John Galbraith]

On Monday 02 November 2009 1:33:36 pm rjgu...@gmail.com wrote:
> I got a few messages again defining the ultimate clean power amp for the
> guitar/Bass and it seems this Ampeg SVT is what they say is the real deal
> in a Dumble SSS amp. THis is what I am trying to make a copy of.
>
> I made up my own schematic based off of those that I had sent you earlier.
> I don't really understand this -150V bias, does it make sense to you?

I don't claim to understand this topology, but I have a lot of experience with
this amp. The amp is totally amazing sounding for bass guitar. It's rated at
something like 300W, and that's how we record it. It is so loud that it
shakes the control room window and overwhelmes the monitor mix. We record it
clean (probably still with loads of THD) and also overdriven. It is totally
punchy, and the low end is to die for. We agonize over mic placement, and
also the gain knob setting becaues it's pretty tweaky, but otherwise record it
straight. We don't mix any direct signal from the bass into the console (I
hate that sound); it's just this amp. Amazing.

The thing is the heaviest amp I have ever lifted. And that's just the head;
the cabinet is massive and heavy, too. This amp is the reason that bass
players went to solid state - to save their backs. Looking at the packaging
of the thing, I would guess that the guts are *extremely* complicated. You
haven't marked the B+ voltage, but with those 6550's I'm guessing it is quite
high; 500V or more? It's probably pretty dangerous to work on, but I know you
are looking to downsize the power tubes and that should help the safety factor
a lot. Aren't 6L6's kind of like a 6550?

I don't understand the bias scheme either yet. However, I'm not sure the
figure is telling the whole story. Is that -150V B- supply a high output
impedance or low output impedance? If it's low impedance, then the V2 cathode
follower is just uses the -150V supply to sink it's current, and in which case
I am mystified by R15, the pot, and R16. If it is high impedance, then it
appears that this bias voltage will vary quite a bit with the input signal,
which is equally mystifying because I've never seen a fixed bias topology that
is not, well, fixed at some DC value.

BTW, Aspen Pitman in his book lists this amp as one of the ten most coolest
amps of all time, and I agree. Except during load out and load in.

Also, I might respectfully disagree that you want a bass amp to be "clean".
While most music does not call for excessive overdrive in the bass, I am
assuming that the reason this amp sounds so bitchin is due to it's distortion
spectrum. It can easily be dialed in to not sound "distorted", but I'm sure a
quantitative measurement with a spectrum analyzer would tell a different
story. A lot of modern music comes with very clean bass tone, probably from
solid state amps (or console direct, for the worst offenders), and in my
opinion it sounds very sterile and boring. I would not personally record
music of any genre with a bass amp that sounds *too* clean.

Another choice, which would be a much simpler project, is the lowly Fender
Bassman. That's a sweet amp too, and smaller in every respect. The copy that
Jason had a couple years ago was particularly great sounding. I actually have
all the parts for one already, but have not yet gotten to the project. I
would be frightened, myself, to copy the SVT homebrew. The bassman, on the
other hand, could easily be somebody's first homebrew project.

John

[robert taylor]

Didn't those Ampeg SVT monsters use six 6550s?
And yeah, lotsa feedback loops, not that clean... 
Do you have the PSU schematic? 
I'd kind of like to see it; I'm thinking of building a parallel PP 6550 amp,
but a real clean one, for hi-fi, just for a sub, up to ~70Hz. 
 
r  

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