If my memory serves me well,,,Then with a HT voltage of only 12 it is unlikely that biasing is needed at all.
And also not much chance of blowing anything up by mismatching transformer outputs with only a 12 volts HT. [but don't do it with normal tube outputs]

Very unlikely you wil get a clean sound [but Hey,, I could be wrong ]
As these valves are no longer working in there linier region and hence all the standard values will be very different. [ heck 1 meg or 10 meg  grid R suck it and see]
The transformer will never be able to saturate [if your looking for that overdriven OTr fat sound].
IMHO I think there are far better thing to do with valves.
There was a circuit by Anderton "Stack in a box" [I think] and that ran 40 volts HT with no OTr,,, again not really worth it.
The "Mesa Vtwin pedal" would give far better results than most of these things.
but yes it uses normal HT 200 VDC.
Cheers,, Phil.

I think I follow you,
Ok,, if you have AC secondary And then re fit Caps delivering DC rails ,,,
Now Bang when you reconnect the power amp.

Sorry sounds like it's terminal.

You could insert a pair of say 33ohm 10/ or 20 watt Wire wound power resistors before the power amp rails.
This will limit the current and may give you some time to check out what part of the poweramp circuit is overheating,, there is usually something getting very hot.
Check TR3 and TR6 these are the predrive and usually work harder than the output devices.
If it stays working long enough then see how much DC is on the speaker terminal.
it should normally only be within a few mV like 30 /100mV if it is many times higher ,,then it will just go bang again real soon.

Sadly SState power Amps like this are hard to fix at home,, you could replace ALL active devices but these things often have design flaws and the problem may well resurface again.
If you can source a poweramp module designed to run on the 50/50 rails then that is another option.  look at ESP site ,, a Mosfet power module ultra simple and probably a lot better than yours. http://sound.westhost.com/project101.htm

I have often found design flaws in some big brand name gear,,, yeah should not happen but sadly they don't care.
Sorry I can't be of more help.
Phil,,, Nambour Australia.

Ok,
   If you simulate a triode Push pull output stage [firefly] you will notice the waveform becomes rounded at full power but NEVER Flat topped as It does not possess the grut needed to do so.

Pentode PPull is a very different animal,,
it just square waves with very slight rounded corners.
If you have access to one of those fancy tube amps that has
a triode/pentode switch on the output tubes then
see which one sounds full to you? Yes it works but is that what you want?

One reason the pentode was perfected was to get around this very problem,, so to me it's kinda going backwards to use triodes for a power stage.
Oh I admit I wanted that firefly amp to work but I'm glad I took the time to read stuff first.

TC [transconductance] for EL84 is about 11,200
now try and get that from a small signal triode.

I did listen to your little clip so if your content with what your getting
then disregard my ramble.
Also if your happy with Attenuators alone then that's ok too but
a whole new world awaits those how want to go the extra mile.

If you wish to hear this Reamp concept taken to the extreme then
go have a listen to the Guytron GT100 Amp.
It's a pair of EL84's pp driving a resistive load then Reamped via
a EL34 quad output stage, effectivly giving you 2 tone stacks and jezz
if that one does not prick your interest then nothing will.

Your last sentence tells me you may have missed something in the link. "The whole point is about EQ",, In fact EQ tweaks are sitting at every stage in those old tube circuits.
If you're content with the mass thing fine but it's a whole lot simpler my way. You obviously wish to process with DSP anyway so the mass is hardly worth the effort.
Just my two cents,,

Cheers Phil.

Quote from: armstrom on January 15, 2009, 02:04:50 PM
I just finished the first stage of my hybrid combo amp. My plan was to build a complete low-wattage tube amp, run it through a dummy load (probably a weber MASS speaker motor) and tap the speaker level to drive a passive tone stack followed by a DSP effects processor and finally into my LM3886 power amp. The combo will likely use a single 12" Lil'Texas neo speaker to keep the overall weight down.

I have completed the first phase of my master plan The tube amp is done and tested. It's a firefly from www.ax84.com
If anyone is curious I have a 2mb sound clip of my friend noodling around on the amp. Keep in mind that the amp was only driving a 5" Jensen MOD speaker so the bottom end is kinda muddy. Strange, who knew a 5" cone didn't do so well in the low range  . The guitar is a Gibson ES 335 with stock humbuckers.

http://www.mr2-power.com/firefly%20test%201.mp3

-Matt

Good on ya for having a go at it.
As I may have already perfected what you are trying to achieve I figure I may be of some help.

The problem with the fire fly is the triodes,, sadly they just don't have what it takes.
Look up the transconductance of some Pentodes for a start then compare it to the triodes you are using.
I use Pentodes [6GW8], a small 10 watt Push Pull into a resistive load then tap off to a line level.

I found this to be a very good read if you are trying to get your head around what is often made unnessicerily complex. http://www.amptone.com/truesecretofamptone.htm
Hey look no maths  :tu:

My setup is just expanding on this basic idea.
my input EQ module is TRUE passive input  [HiWatt style] > tubeamp > ReAmp box [or power soak] > 5band graphicEQ +-6Db > 120watt SState poweramp.
Graphic is 100HZ > 330HZ > 1kHZ > 3k3HZ > 10kHZ.

Assuming an 8 ohm output from say a pair of EL84's in PP,,The big soak resistor can be made from three paralelled 33R 10 Watt Wire wound resistors giving about 11 ohms. Exact ohms is not super critical a little more than 8R is likely to be better but don't go crazy.
If you want to spend the time with inductors, Caps or silent speakers at this stage feel free
but there are easier ways to get around the loss problem via EQing. [Read the above Link]

Here is a mud map of the basic setup I use.
Cheers, Phil.

Quote from: Boprikov on January 15, 2009, 05:32:52 AM
http://www.vintagehofner.co.uk/selmer/schematic/ssmerc.html

This is an interesting vintage design. If i understand correctly, the primary of the output transformer must stand 400 mA. I have no idea what the impedance ratio should be. Is it possible to somehow estimate it? And what should the power supply voltage be? I think 10-20 Volts is not enough as efficiency is poor in class A.

Sorry my english... but i´m quite interested about what type of output transformer was used at those SS Selmers.

Hi Boprikov,
I searched years back trying to find such a circuit and you found one,, Good find.
Here is one I built some years back similar in concept to Selmer Mercury.
I never went on to complete it because of the bass issue but insanely loud even though it was driving a small 100mm speaker.
I have no idea where the OT came from, just found it in some trash.
The output Tr has to be a fairly high voltage as the collector voltage can get quite high when using transformer coupling.
Q3 only needs a clip-on heat sink as it only runs lukewarm.

Jfetter, I do wonder about Exotic transistors and all that heat ,, my thoughts are go with transfomer coupling as it puts a lot less heat into the Amp,,heck line transformers are easy found.  Teemu is on the mark there,,
just my take on it.
Phil,  Nambour Australia

Hi glugster,
            Oh I see your not gonna give up but believe me you will eventually 
This is a bit like wanting a tow bar fitted on an F1 car so you can tow a boat.

The signal from the Rev Drive is designed to drive a transducer [not line level]
which is a coil of wire wrapped around a small chunk of steel.
Via magnetic transfer it then moves [vibrates] a suspended magnet connected to a long spring.
The principal is sorta reversed at the other end where the slightly delayed signal is picked up
and mixed into the original [Dry] signal.

The output at the Rev Driver socket is pumping maybe 5watts needed to drive the rev tank
and will kill most line level inputs connected directly without any serious
and probably expensive modification.

There's also a pile of complex impedence /inductance issues to deal
with both drive side and return side.
Obviously you don't want the reverb so just take it to a teck and rewire to a dedicated Effects loop, there should be enough space once you remove the reverb stuff.

There are already enough things that can go wrong just building a propper dedicated Effects loop,  Now add all these extra complexities,, and the odds of it working well are 20times worse.
This will likely end in tears for you and quite frankly,, IMHO
It's a darn easy way to stuff the resale value of a very good tube amp.

Hope you understand I'll have to go find a whole pile of complex equations for you and Having messed around with 100's if not more ways to wire things up inside and out side of Amplifiers I have learnt enough to know when Not to try certain things,, this is one of em.

Be warned Effx loops can easy bugga the whole magic of tube amps like yours.
They where never really designed for the modern efx junkies,, though some will agree.
If you want the bells and wistles  stuff buy an amp that has all that pleases you.

Cheers Phil.

Short answer is NO don't even think about it.
Long answer is yes but it's going to cost you so much as not to be worthwhile.

On the subject of Efx loops I never found a use for them anyway,
maybe half useful on SState Amps but kinda stuffs the whole magic of tube Amps.
Cheers, Phil Nambour Australia.

Hi JV,
I just simulated some of the Bandits circuit for a friend and the results where as sad as the tone/sound they produce.
You only get about 6/10 Db of boost/cut anywhere in the preamp circuit.  hum?  No wonder my friend is not happy with his Bandit.
There are obviously many models of this rig but I get the impression they are all much the same.
You need at least 20 Db of cut at around 300/500Hz to get a worthwhile tone that resembles a guitar tone.

I built a very simple "Stand Alone Tone Stack" which improves the Amp dramatically, if your into building your own gear I can post it up here. [battery powered if you so wish]

I tend to try and work  "Outside" of Amp circuits as much as possible as modding some of these circuits can end you up in more trouble than the Amp is worth.

Cheers Phil.

Hi again Teemu,
                       Have read the above links, some of which is a bit over my head but yes I do understand the basic principal.
                       
OK here is what I think keeps getting missed with these things.
Most if not all of these Compressor/Limiter circuits work on the basis of turning down the gain at the input "After" the signal has already passed through the Amplifier circuit.

To my mind this is very different to how a power tube stage works when it is at or near full power. "That is my main point."

So far for me at least It has proven to be rather futile trying to get SState power stages to compress in the same way that tube power stages do.
With regard to the "Clean sounds",
The only basic diff between SState powerstage and Tube powerstage  is some bandwidth limiting/colouring [partly valves and AC coupling, partly transformer] 
BUT as they get towards full power the Tube output section develops the magic mojo.
I think it is that part that I'm wishing to recreate/emulate.
[As I'm sure you know, SState power stages just keep on going up untill they run into the rail voltage, sonically not much changes.]

I'm the first to admit that with some modern music I don't know if you even need power tube compression effect as there is so much preamp crunch/fuzz or just plain noise that I doubt if one could hear the difference between tube or SS.
Quite frankly a lot of modern tube amps ALSO have so much preamp gain stages that it becomes almost impossible to attain that magic sweet point of the output tubes compressing and it's just fuzz to my ears.

Vox Valtronic sounded like a good idea until I realised that the gain curve of a triode is vastly different than a pentode. I think that one will ba a passing fad,, but then some rave about it.
Anyway that's my 2 cents worth.   Cheers, Phil Nambour Australia.

Quote from: Fretts on December 22, 2008, 08:18:25 PM
there is feedback off the speaker that is used to modify the impedance/damping of the power amp.  Personally, I can hear it, so I want to get that same effect with less earsplitting volume.

I lack basic familiarity with SS power amp topologies.  This unit uses a pair of 2n6254 power Q's, cascaded apparently, per rail.  It has an elaborate biasing scheme I don't understand very well, but it does not look like there is any quick n dirty way to operate the existing amp to produce less power.  Technically, is the only real way to reduce the rail voltage?  Can I artificially limit the available current by modding the power supply and introducing intentional current limiting there?

Hi Fretts,
              If I follow you correctly, then That Feedback Resistor is likely a very small value [ie less than 1 ohm] Large Wattage wire wound.  You can "Increase" the value of that R and it will turn down the Wattage somewhat.
If it is say .1R, or .22R go up to One ohm and try it. [Make sure you understand the circuit first!!]
The idea is called a "Defined impeadence" setup ,,trying to emulate a non zero imp to sound more like valves,, Yeah works kind of OK but IMO it does not do that much.
Depending on the circuit and more Ïmportantly the speaker used,, turning down the voltage rails won't work well as some poweramp circuits are designed to work at specified voltage. you could stuff up the Amp.

Note also for "daffe"
I personally would take a long look at speakers as use of Big name Drivers with SState Amps is kinda Daffed [no pun intended] if you really understand whats goin on. They are mostly designed for tube Amps remember and I'm sorry but SState Amps are vastly different in the way they work with speakers.
Tube Amps of old had limited bandwidth especially under full output and hence NEED the extra peak at say 2/3khZ but with SState it's a whole different story. 90% of SState Power sections have a dead flat freq response at any volume level,  so you really need to be cutting not boosting those freq otherwise they get painfully harsh at high SPL.
I just fixed the harshness of a Tradsman 60 [tech21] by changing the speaker from the over sensitive "Special design wank" to a much lower SPL driver. [note also lots cheaper]
Now the Amps tone makes sense, It was stupidly harsh and the guy wanted to get rid of it but I think I may have changed his mind
Cheers,  Phil Nambour Australia.

Quote from: gbono on December 02, 2008, 03:09:44 PM


Are these good units? Is there a way to bench test these tanks with an signal generator/scope?

THX

That won't tell much what they will sound like tone wise,, if that is what you wish to know?  My option is to build a driver and pu and play with the 3 tanks to see which one works for you.

Likely the tanks will all sound ok,,, be aware it's more to do with the way they are driven that makes them sound good or bad not the fault of the tank at all.

Check the schemo I just posted if your looking for clues on how to go about it.
under "Mosfet Spring Reverb Driver by josh 777.

Have fun with it. Phil, Nambour Australia.

Thankyou Teemu,
                           Alas way past my bed time now so I'll get back when I've done some reading.

I'm interested in the "How" but also looking for comments regarding the colouration [for want of a better word]
Some of these comp circuits seem to come out rather bland and I noticed you mentioned this tonal effect in your book while discussing these things.
catch up soon, Phil.

FWIW,
          I wasted many years [10 in fact] trying to build a decent Rev unit so here is my 2 cents worth.
Build a stand alone unit and be done with it.
Forget current drive you need voltage drive.
[Yes I know all the experts tell us that current drive is best but having built most of the circuits out there,, well they must have missed something.]
Don't  bother with Accutronics circuits [utterly useless]

I doubt that a big drive will kill the transducer unless you pass full bandwidth signal through it.

Rod Elliot [ESP] helped me out a great deal in the end trying to perfect my circuit so search his pages for the reverb drive ideas.

The problem with the booster circuits for opamps is that they need to run off a much higher voltage not the same 15/15 supply.
Rod E will agree with that if you care to ask him.
You really do need a big swing to get it to work as it should.

Anyway to save my babbling on about it,,
here is my "MaxiVerb Stand alone Reverb unit".

No doubt the experts will see flaws in this but still as is this is a killer tank slapping Reverb that will put a lot of other more expensive gear to shame.   [very little noise /hum].
Have fun with it,,, Phil, Nambour Aust.

Hi,
I'm new here but can I ask what U6 and associated components wrapped around the bases of the diff input do?  Yep I see it's a compressor but is U6 a Transconductance unit or similar opamp?

In my limited experience a lot of the comp stuff tends to lack something.
cheers, Phil. [Nambour Australia]

Quote from: Jack1962 on December 02, 2008, 02:56:13 AM
lmao-check out a Peavey Musician 400 Series B , it comes reall close to tube sound(real close).

                                   Rock On

Ok,, is this similar or same as to the Nashville 400 amp?
I noticed the schematic of that under "Steel amps post".

That Nash 400 is not that much different than the Bandit models,,,
[a higher wattage of course.]
At a glance nothing stands out to me but I may have missed something as I'm not the expert that others are.

A Schematic would be good though?

If it's anything like the bandits with all that "Defined output impeadence" stuff then they where nothing special? That "Saturation" thing was a bit of a joke.

I have a friend who has two of those 112 Bandits and he would gladly swap his two bandits for my ReAmp setup anyday 

Cheers, Phil Nambour Aust.