Bogen K10 Amp

[Willabe]

Sorry I don't but you have 4.7K R there now.

Use a couple of gator test leads and pop in a 100K or 250K and see how that sounds. 

When your doing this disconnect 1 of the test leads and you'll hear what it sounds like with no NFB. Then you can compare by ear how much resistance you need to shut down the NFB. 

[Paul1453]

The more I play with this one the more I like it.

I think this one is a keeper!   

[DummyLoad]

The more I play with this one the more I like it.

I think this one is a keeper!   

it's a pure evil little thing... in a good way. those miniature 6V6 are sweet sounding.


--pete

[HotBluePlates]

Any idea what amount of resistance added there is effectively no NFB?

Rev 1.3's 25kΩ pot in series with a 2.7kΩ resistor is probably pretty close. But SW2 in that schematic is certainly no NFB.

If you just have to try, grab a 250k-500kΩ pot and see where you stop hearing a difference. It will be approximately where the series resistance/330Ω equals the gain of the phase inverter & output tubes. I'm gonna guess total series NFB resistance at ~185-190kΩ is where you'll stop hearing a change.

[Platefire]

This was my last NFB control I installed--a switchable pot. I think it's a good option to dial it in or cut it off completely. Platefire

[Paul1453]

I put that switch/pot combo on the front, in the hole left by the TSFH.

The original knob actually fit my switch/pot.  Looks good.   

[DummyLoad]

I put that switch/pot combo on the front, in the hole left by the TSFH.

The original knob actually fit my switch/pot.  Looks good.   

that's where i put mine.


--pete

[Ed_Chambley]

Just for fun once I put the NFB on a jack and with nothing plugged in there was no NFB.  I hooked up a Ernie Ball Volume pedal to it.  It was too noisy the way I did it, but it was way cool.  Push forward and it would get tight and clean which was great for Crunch using a overdrive.  Back it off and it would get greasy.

I used a JCM800 for the test.  Of course now it has a pot.

[Paul1453]

That is a unique and interesting idea, Ed!

My amp will give howling feedback, when you point the guitar at the speakers, with the NFB off.
I think it would be kind of cool to be able to control that feedback with a foot pedal.   

[DummyLoad]

That is a unique and interesting idea, Ed!

My amp will give howling feedback, when you point the guitar at the speakers, with the NFB off.
I think it would be kind of cool to be able to control that feedback with a foot pedal.   

use a switched jack and keep the pot. pot is disconnected when pedal is plugged in.

--pete

[BLUEGLOW]

I've been toting a K10 around with me since the late 60's(yes I can remember). It was part of my fathers shop that shut down in '67.
Then, just about five or so years ago, I started on my own K10 to guitar amp conversion, tinker-trip. Mostly for fun, but also for another place
to plug a guitar(doesn't everyone own at least 5 ?) into.

So I'm more than just tickled and amused to discover this Excellent Forum and the many others who have been down the exact same K10 path(s).
"The Tone Stack from Hell" what a perfect description! Took me forever to ring it out. FYI - I think Marshall had a guitar amp with a tone stack in the NFB.
 
Anyway - My first question.
The 6AU6(V1) on my amp is still in it's original configuration. An added 1/4 inch guitar input jack connects directly to the C1 input coupling cap.
What is the source of the negative 1.06 vdc on pin 1(G1) of the 6AU6 ?

Is this some kind of sharp cutoff pentode or mu amp thing ?

[Paul1453]

I'm pretty sure that -1V on Pin 1 comes from the 1M grid leak resistor.

It is pretty easy to make the changes to the V1.3 circuit.  Best choice, IMO.

Well worth the effort!

The switched NFB pot goes in the TSFH spot.

With those simple mods this little lunch box amp kicks some serious ass.   

The heavy metal players love it, and can't believe the sound that comes from this little amp.   

[jjasilli]

There are a number of ways to bias a tube, such as cathode bias or fixed bias.  But this is Grid Leak bias.  The input cap is necessary to the circuit.  For more info, see:  http://electriciantraining.tpub.com/14178/css/Grid-Leak-Biasing-45.htm  note that page navigation is at the top right of that web page.

[Paul1453]

There are a number of ways to bias a tube, such as cathode bias or fixed bias.  But this is Grid Leak bias.  The input cap is necessary to the circuit.  For more info, see:  http://electriciantraining.tpub.com/14178/css/Grid-Leak-Biasing-45.htm  note that page navigation is at the top right of that web page.

Yeah, I got it right.

Thanks for that confirmation jjasilli!   

I am studying some Dummyload, but the Radiotron 4th edition is like a very strong sleeping pill.   

I much prefer to try to pick up info by reading the distilled clear explanations posted here by HBP, PRR, jjasilli, yourself and others!   

[PRR]

> Radiotron 4th edition is like a very strong sleeping pill.

Really should be chewing on the 3rd.

But this may not be in 3rd.

http://www.ax84.com/rdh4.html
CHAPTE02.PDF Valve Characteristics
Fig 2.9 (page 7 of PDF, page 19 of book)

This will need some head-scratch. Curve D is the sum of all grid current effects. If there were _no_ grid resistor (infinity), the grid might tend to drift to point "X", which is slightly negative. With a "too large" grid resistor (such as 3Meg), it will be a little off of point X, but still negative.

In triodes, this grid-leak point will depend on plate voltage; in pentodes, on screen voltage. Large plate/screen resistor also tends to stablize the operating point (tube sucks hard, plate/screen falls, sucks less.... it finds a balance).

This grid-leak bias "tends" to be a high gain point. The uncertainty of bias (varying with tube chemistry and age) makes it a poor choice for a high-level stage (and ultimately, most stronger guitar pickups).

And yes the input cap is essential.

[BLUEGLOW]

Yes, a redesign or complete trashing of the TSFH network has tempted me before.
And that TSFH spot just begs for a re-purpose.

Not quite ready to stop tinkering yet.

Fyi - V1 on my amp is in the stock K10 config, with its cathode and G3 directly grounded, a 3.3M G1 grid leak resistor and 33nf input coupling cap.

I see jjasilli has just pointed me to a Grid-Leak-Biasing site. Yes I will go there. 


Even though my little K10 box is on the dark side, I can pull much out, just from the guitar levels.
Touch sensitive. And this is just first stage pre-amp distortion.
 

[Paul1453]

With the TSFH and it's NFB connection intact, I just couldn't seem to get much output out of the K10.

It seemed to be slightly less loud than a SE 6V6 Champ in comparison.    

I removed the TSFH, put a switch/pot on the NFB, and completed the other mods to get to v1.3

Then that little box really kicked some ass!   

Way, way, way louder than the Champ with nice harmonic feedback available by pointing the guitar at the speakers.