The 6550 Experience

[daveweyer]

No you are not missing anything. I just didn't complete the subject matter sufficiently. Yes, Tazin was talking about grid#1 resistors, but the same technique can be applied to the screens, or the plates for that matter in amps with plate stopping resistors.
The point about "lumped constants" remains. And the idea of staggered resistors is a good one.
I should have been more specific.

[daveweyer]

As an example of Marshall using plate stopping resistors look at the early 200 watt models. They also used cathode resistors, every active element of the tube was isolated somewhat. Later they abandoned all that.

[Tazin]

I am also considering removing the V2 bypass cap...what say you, Tazin?

If you take out the 0.68uF bypass cap on V2a then you'll loose a little gain in the mid frequencies, but it will slightly thicken up the the tone which can be good when it comes to strats. Are you using a 0.0022uF (standard Super Lead spec) de-coupling cap for V1b (Bright channel)?

EDIT: I think the sound difference between running only two 1K5 swamp resistors on the outer two valve sockets compared to using four 5K6 swamp resistors (one per output valve socket) would be very minimal.

[Xplorer]

very interesting results thorleif, thanks for the clips you've posted.

some parts of it are quite bog but some others are something else to my ears, interesting too.

i see that with these wima caps you were close to a Dickinson and a few others isn't it ?

here is a temporarily layout for a west coast 45/100 . what do you think, compared to yours, based on superleads ?
in red, what's changed. and i'm joining the classic layout too, for comparison.


https://drive.google.com/folderview?id= ... sp=sharing

[Xplorer]

Dave,

about this Dynaco A431S http://triodeelectronics.com/a4dyoutr43oh.html

it says 120 watts, correct me if i'm wrong. and you said that the amp would deliver 180 watts after the mod so i'm a bit worried about what i'm missing there, electronicaly. is everything normal ?

[daveweyer]

Xplorer, the transformer is rated at full bandwidth for 60 watts, slightly narrower bandwidth at 120 watts.
You can get, if your PT regulation is good, up to 180 watts in full-on square wave clipping from the transformer.
Of course your output depends on the power supply and the plate sat ratings of the tubes you use. The plate sat rating tells you how much voltage will exist across the tube when its grid #1 value is zero and the tube is at full conduction.
This figure varies, but could get down to 40 volts on the best American tubes. So in other words, if you have a plate voltage of 560 volts at idle, and at full power it drops to 500 volts, then you can use 460 volts of the supply to generate your power--the tube will drop all of the voltage through the transformer but 40 volts. But many tubes in this world will not pull all but 40 volts, some will only pull to 80 volts, in which case you will never get that kind of power.
Your power depends on how much voltage the tubes will pull through the load. If you are working with a net available swing of 460 volts, double that for the plate to plate swing, square it, take the RMS value of that, then divide by the load, say 3600 ohms, and you get 70 watts RMS output sine wave power.
But if your PT had really good regulation, say 5%, and your B+ voltage dropped to 532 volts, you would suddenly have 90 watts output RMS square wave.
If your voltage could stay at 560 v with a load, you would get 104 watts output RMS sine wave. And, if you mismatch the impedance of the OT to get a 2200 ohm load, you would have 171 watts RMS sine wave output.
Further, square wave clipped output would give you another 30 to 50 watts of power, since a square wave has the most energy of any wave, now you are over 200 watts RMS square wave output.
As I have mentioned, the actual power out depends on the total available voltage in the supply, and the regulation and losses in all the components. Good tubes are vital.as well.

[Xplorer]

thanks !! so, full bandwidth at 60 watts, ... much narrower bandwith at 180 watts but it doesn't mean that it'll blow the transformer, right ?

i certainly have some big blanks in my electronic education, to fully understand what you said but with some time ...

if i don't mismatch the output transformer , say 16 ohms into 16 ohms cab , it won't reach 180 watts rms, right ? so ... the cab is safe then ?

on a 100 watts amp i often heard that there could be some pics of 160 - 200 watts sometimes ... and it means that you must beware and make sure your cab can handle about 200 watts for a 100 watts amp ( maybe i'm saying BS, i don't know )
so with a 180 watts amp well, it could get even higher i suppose ...

also, after the watts calculation formula that you explained, i don't know why i figured a measurment taken at the output of the amp, with whatever tool that i don't know ...
this measure must be done while the power of the amp meets a resistance/load/cab , right ?

sorry guys ... newbie here ...

i don't understand how a 2 / 8 / 16 ohms load becomes a 2200 or 3600 ohms ..

mixing up everything i guess. sorry ..

if you have a plate voltage of 560 volts at idle, and at full power it drops to 500 volts

at full power ... why does it drop ? i thought that the power transformer was delivering a constant 560 volts ..
does this mean behind it that a portion of the 560 volts is converted into an output power, according to the variations of the signal amplified ? am i a bit closer now ?

[shakti]

I am also considering removing the V2 bypass cap...what say you, Tazin?

If you take out the 0.68uF bypass cap on V2a then you'll loose a little gain in the mid frequencies, but it will slightly thicken up the the tone which can be good when it comes to strats. Are you using a 0.0022uF (standard Super Lead spec) de-coupling cap for V1b (Bright channel)?

EDIT: I think the sound difference between running only two 1K5 swamp resistors on the outer two valve sockets compared to using four 5K6 swamp resistors (one per output valve socket) would be very minimal.

Yes, I know what it does from a techincal point of view, but I was curious if you had any hints on what you've seen in the purported Hendrix amps? But of course, best is to try it both ways and see.

My amp has the stock 0.0022uF coupler on the first stage for bright channel, yes. I can try to parallell in a bigger cap and see.

As it is now, I think the 6550s are a step in the right direction, but we're still not there. For BOG tones, there is such a smooth transition into sustain and overdrive, yet it has amazing clarity and cuts through very well. With my setup, I get too much harshness still, and a kind of hard transition into overdrive. I need to experiment more with preamp tubes, maybe mess with V1b coupler and the bypass cap on V2a. With my latest tests, I seemed to prefer 8 ohm feedback though. But I need to actually cut back on the bass, it tends to clutter up things too much when I turn it above 2, so maybe I will go back to split cathode on V1.

Mind you, the fuzz I am using has incredible clean-up, but cleans up to that very sparkling clean that a good germanium fuzzface does, and it doesn't blend well with an already bright amp. The BOG fuzz, if it was a West Coast mod, probably had more of an even tone across the range off fuzz/sustain, at least if I interpret Dave correctly.

[bill bokey]

I made a few changes on my guitar : lowered the action and pickups and used a much thinner pick (Fender Medium), I also tried to play softly...
Then I tried with the OT matched to the cabs and I prefer it, there's a bit more volume volume but definitely more punch and clarity.

https://soundcloud.com/castle-made-of-s ... matched-ot


Next I think I'll try to increase the NFB, maybe 27k on 8 ohm ? Should tighten the bass and decrease gain.

[Tazin]

Yes, I know what it does from a techincal point of view, but I was curious if you had any hints on what you've seen in the purported Hendrix amps? But of course, best is to try it both ways and see.

My amp has the stock 0.0022uF coupler on the first stage for bright channel, yes. I can try to parallell in a bigger cap and see.

All I can tell you is that three out of four dentists recommend the 0.0022uF de-coupling cap for V1b.....

[bill bokey]

Following shakti's (very good) idea I had a go at daisy chaining :

https://soundcloud.com/castle-made-of-s ... y-chaining

(first part is with daisy chaining then back to normal for the second part)

The second amp is not switched on but I find the main amp cleaner and punchier that way. There's less preamp gain and therefore a tad more clarity.

[daveweyer]

Xplorer,
All these guitar amps use "capacitor input" rectifier circuits. The filter caps charge up to their peak value with no load, which is 1.414 times the AC delivered across the PT secondary. When current is drawn, the caps discharge and the voltage available drops; if enough current is drawn, the value approaches the AC value across the PT secondary coils.
If your transformer delivers 560 volts DC with no load, then the voltage under load can go down to 395 volts. It usually doesn't make it down that far because the tubes can't draw that much current.
So, for instance, your PT probably is a 400-0-400 AC model. 1.414 times 400 equals 565 volts, the peak value for a capacitor input rectifier system with a 400 volt secondary.

The output power is the voltage squared divided by the load resistance, the AC RMS voltage across the OT primary when the tubes are running at full current. Of course there are some losses in the transformer to figure into the equation, but that is the basic calculation.
So the important figure to know, is the available DC voltage while the circuit is loaded--that's the real plate voltage--which in your circuit is probably going to be around 500 volts, maybe a little lower depending on the regulation of your PT, possibly down to 450 volts. That's what you really have to work with to develop output power.
That's why I tried to pick power transformers for Jimi Hendrix' amps which had real good regulation. The stack of transformers at West Coast Organ and Amp were sorted that way. You could have a very "Hot" transformer, but its voltage would not hold up under full current, meaning that it was not a good candidate for a hot rod amp upgrade.
So I didn't care how big the transformer was, or how high it's unloaded output voltage was, just its regulation, because if its voltage also falls when you apply the load, then you have two sources limiting your output power, the DC drop from the discharging of the filter caps from their peak value, and the AC voltage drop from the limits of the power transformer.
If you are just going to play in your bedroom, then it probably doesn't matter, but if you need to have the speaker cabinets vibrate your guitar from 35' away to get harmonic sustain, then that regulation helps.

So I doubt you will have to worry about blowing your output transformer, although I should hasten to add that I have replaced plenty of them, usually from super high voltage spikes which would cause an arc to jump from one of the coils to the case. When an amp is in square wave clipping, the peak voltages in the OT can reach several thousand volts.

Put a signal in your amp, turn it up as loud as it will go; use a resistor for a load if you need to keep it quiet, then measure the B+ voltage. Don't try to measure the plate of the tube, just the first filter cap. What you have then will determine the actual output power available. Then, on the secondary of the OT, measure the AC voltage with a resistor as a load. Square that voltage and divide by the value of the resistor and you will have the ACTUAL power output of your amp, with all the losses built in.

[shakti]

Getting closer! Tell me about your guitar.

[shakti]

Yes, I know what it does from a techincal point of view, but I was curious if you had any hints on what you've seen in the purported Hendrix amps? But of course, best is to try it both ways and see.

My amp has the stock 0.0022uF coupler on the first stage for bright channel, yes. I can try to parallell in a bigger cap and see.

All I can tell you is that three out of four dentists recommend the 0.0022uF de-coupling cap for V1b.....

[shakti]

Dave,
this photo is of an amp reputed to have been used by Hendrix. Do you recognize any of the work here as something you would have done? The photo is quite blurry, but we can clearly see that the bias circuit has been messed with. It also has what looks like replaced output coupling caps, unknown value. The bypass cap on V2a is not original, but it's impossible to read the value. V1 has been rewired to shared cathode. There are other little parts that may have been replaced, but impossible to read the value (like plate resistors on V1). Lastly, the PT looks "worked on", but still looks like a Dagnall T2562 to me.

Oh, and the PI tail resistor has been changed as well......hmmmmm....

edit: actually, looking at it again I am not sure it has been changed. The solder dye on the junction with the 470R and 1Ms looks untouched, and Marshall occasionally used a carbon comp 10k, so it could be the stock resistor.

http://amparchives.com/album/Marshall/1 ... 9new7.html