Model T feedback factor

[jnick1947]

The schematic for the original version power amp has a 22K feedback resistor with a 10K shunt, in parallel with the 25K presence control. The 2nd generation is built the same way. Since this is a non-inverting amp, that would give it a gain of a little over 3... something generally not a good idea with tube power amps! Since the feedback is taken off of the 16 Ohm tap, that means that ~40/3 volts, in, are needed to get 100W out!
Any comments

[loudthud]

I tried to add a resonance control to a Model T. It didn't work very well. The ultra-linear output stage just doesn't have very much gain and neither does the long tailed phase inverter.

[jnick1947]

Hi, the PI in this is just like a Fender... or almost anything else, and would therefore have the same gain. It must supply a swing of 110 volts maximum (twice the bias voltage) to the output tubes - about the same as with say 6L6s running at ~500 volts. If one shorts out the feedback shunt (10K resistor), then the amp will be running open loop and one can note the increase in signal level to figure out the feedback factor, and the open loop gain. However, the signal input to the PI is an extremely high impedance because it is "bootstrapped." One cannot simply hang a meter probe - or scope probe - on the input without seriously loading down the signal. For instance, putting the probe of a Fluke 45 meter on the output of the MV caused the amplifier output voltage to drop to 1/2 of it's level!

[rick.heil]

On a related note, I'm taking care of my pal Schroedinger's kitty while he's away...

[JoeArthur]

On a related note, I'm taking care of my pal Schroedinger's kitty while he's away...

Oh please... please... please - let me open the box!!! 

I've stayed away from this - apparently some tube tone guru tech or ex-spurt somewhere, sometime has decided that the power amp must have some certain closed loop gain to meet his obviously strict requirements. And I'm not quite clear if they really think it should be some sort of tube based op-amp or not.

When any real post says something like "tone = <some obscure calc goes here>" it might get interesting... otherwise it remains in the ever present twilight zone of "personal preference".

I've never played a model T thinking the gain of the power amp was lacking - and I don't know anyone that has.

Maybe it's simply a worship offering to the previously mentioned un-named but implied tube tone guru.

Don't know, don't care... so far.

[loudthud]

A wise old instructor once told me, "Paper does not refuse ink." He was referring to the little blue numbers (indicating voltage) that appear on Tektronix schematics. I don't know about you guys, but the 6550's I've been getting lately would probably red plate in a model T at -55V bias. I think they want something around -70V.

Just for grins, search for the schematic of a Traynor YBA-200. Four 6550's driven by a 12AU7 LTP.

[jnick1947]

Hi again, I have done some playing around and exploring with the beast. First of all, the meter is the culprit in loading down the input... a 10X probe on the scope works fine, just remember to calibrate the scope channels. Yes, as the first responder noted, the power amp does have a low gain - this despite the 12AX7! The O.L. gain 1s 16-17; closed loop 7-8. The PI typically provides all of the gain, as the gain from the power tubes is lost by the OPT ratio. So, what's going on? Try this on for size: the feedback node is looking at the 10K shunt... and the parallel cathodes through the two biasing resistors. Figure about 600 ohms for each of the cathode impedances, and it works out. Now those resistor values in your BF Fender don't seem so strange.

Another thing I noted: when one cranks the amp, there is little if any blocking distortion visible on the scope. The thing puts out a good 100 watts @ clipping: therefore, the 150W rms on the gen. 1 schematics is rather over optimistic for this.

Tubes. I ordered the quad set of J&J KT-88s from CE distributing and asked for the highest current ones they had. What they sent me were marked 25/3.0. These dial in at about 56ma with -55 volts of bias.
The guy @ Eurotubes - who lives up in Sunn country and collects the amps - says that this is normal - at least with the J&Js he sells.

The amp - with Joe's mods, and the totally rewired "rat's nest" is almost dead quiet. With the MV all the way up and the volumes down, I had to stick my ear up to the EV Force 12"s to tell that it was on. The response with the tone controls at 1/2 has two peaks - one at about 50Hz and the other at 1.3kHz. There is a slight "scoop" in the mid range determined by the 'mid' circuit's slope resistor at either 250 or 400Hz (I installed a normal sized 1M log push-pull pot - allegedly used in Marshalls and available from CE Dist./Antique Electronic Supply, and others - for the bass pot to switch this for bass/guitar). BTW, the two (1.5K/3K) positions on the mid switch may not be all that usefull with the modded ckt. as the amp is rolls off around beyond the 1.3kHz peak and is down by 1/2 at 2.5kHz.
John   

[jnick1947]

Hi again,
controls:
both volumes at 0
bass (now an "audio" pot), mid , treble, presence at mid (5-6)
MV at 10.
mid switch at 750~
I have not touched the "hum" control, yet.
The main filters are now 100uF/350v F&Ts stacked, with the rectifiers connected to the first section, followed by the standby switch, the chokes, then the second filter. BTW, all of the 600v caps that I have seen are simply two lower voltage units stacked in the cardboard tube, without the benefit of voltage sharing resistors. The filament wires are now twisted and routed separately. The filament wires to one of the output tubes were reverse polarity to the other three.
I did not bother firing it up before rebuilding it, so have no idea about the original noise level.
re: output tubes.
I will try and remember to ask the people at CE Distributing what their range is... and everyone seems to have a different grading system, that they use. The popular "off the shelf" computerized testers only go up to 400 volts, and some - like EH run a much lower screen voltage when they test. So, it is a good idea to just ask if they are at all familiar with your amp when you order. Generally, power tubes will have at least a 2 to 1 current spread.
John

[jnick1947]

I talked to the folks at CE Distributing (Antique Electronic Supply), www.tubesandmore.com.
They were kind enough to walk out in the warehouse to see what they had. KT-88s are running 19-39ma.
Next month... who knows. They do not keep any statistical records on tube lots.
As I said, the 25ma set that I have pulls about 55ma @ -55v, so one would want these, or a bit less.

On a related subject: so many tube gurus... so little time.
Sometimes we make the mistake of trying to stuff our own thoughts into someone else's head. I believe the correct term is transference! When I first eyeballed the model T feedback arrangement and made the quick - and erronious - calculation that it only had a gain of three, I was basically concerned with the obvious problem of shoving enough clean signal through the tone stack and MV to drive the output section.
And... since you asked, I would have to go with Vernon Cooper, or to be less esoteric and more contemporary, Menno van der Veen.