Knight reverb unit KN-701

[punkykatt]

Hey Guys,  I was cleaning up the shop the other day uncovered a Knight stand alone reverb unit  that one of my clients gave me a while back. He said someone gave it to him years ago and didnt know if it worked or not?  By the A&B  RCA input and output jacks Im thinking it was used with hi fi stereo systems. Has anyone here on the forum  ever used, worked on, or modded one of these for guitar?  It came with a Gibbs 4AB_ _ _ _ tank.  I drew a quick schematic to get an idea whats going on in the circuit. My eyes are still tired LOL.  Any input is much appreciated.  Punky

[punkykatt]

More pics

[sluckey]

I drew a quick schematic

Can you post that schematic?

[punkykatt]

Click on the paper clip just above the first picture in the first post.

[sluckey]

Ah ha! Totally missed that.

[punkykatt]

I cleaned all the jacks, tube socket pins and reverb pot.  I fired it up using the light bulb limiter with out the tubes in.  No shorts. Installed the rectifier tube and did some voltage checks for high voltage  in places  there should`nt  be high voltage. Ok. Installed the rest of the tubes one at a time, no shorts. Connected the tank, and using 1/4" to RCA adapters connected the guitar and a guitar amp to the unit.  I get guitar signal through both A and B channels once the tubes in the unit warm up but no reverb on the guitar signal. If i tap on the tank I can hear the reverb and control it with the 1M pot.  Voltages on all the tubes look to be normal except for the 6FQ7.  Pin 1 (plate) is 70vdc pin 3 (cathode) is 2.83vdc, pin 6(plate) is 3.2vdc and pin 8(cathode) is 11mV.  On the schematic, what is the purpose of the .003 cap from node A to pin 6 of the 6FQ7? I tried known good tube also.No change.  Thanks in advance .  Punky

[eleventeen]

You ain't gonna get a tube to work with 3 volts on the plate, that's for sure. Are you getting good volts to (and through) the rev driver tranny? 


The .003 is probably to squash oscillation.


70 volts seems kind of low for the 6CG7/6FQ7 but with 2.8 volts on one cathode (pin 3) clearly it is conducting.

Still, we are used to big-time B+ showing up on the plate(s) of the 12AT7 reverb driver in a Fender amp...2nd highest B+ in the whole amp. 


My suspicion is that you aren't achieving good drive to the tank. Possibly the transformer, maybe that .003 is NG and shorting out the tranny. You could snip one end of that cap for a temporary test to see if the tranny works.

[punkykatt]

 Thanks for the reply.   Tomorrow I`m going to check the reverb tranny primaries. They might be open. i should be getting 161vdc on pin 6 of the 6FQ7.  I bet that .003 cap from node A is leaking the 3.2vdcv I`m reading?

[HotBluePlates]

... 6FQ7.  Pin 1 (plate) is 70vdc pin 3 (cathode) is 2.83vdc ...

Do you get similar voltages on pin 6 & pin 8 of V2?

You didn't give voltages for the power supply, so there's no context for plate and cathode voltages of the tubes. But V2 pin 6/8 has power supply node and plate/cathode resistors and could be a guide (in spite of being a 12AX7 instead of a 6CG7).

... Possibly the transformer, maybe that .003 is NG and shorting out the tranny. ...

I think the reverb transformer primary is open (or near-open).

... pin 6(plate) is 3.2vdc and pin 8(cathode) is 11mV. ...

The plate is at 3.2vdc; if the tube's plate load is shorted, you would expect the full supply voltage at the plate. But this supply node can't be at 3.2vdc, because you're saying other tubes have normal voltages which includes V2 (fed by the same supply node as the 6CG7). So we can conclude the plate load (the reverb transformer & parallel cap) aren't shorted.

When there is plate voltage but it is very low, you would expect that the tube is drawing very heavy current causing a very large voltage drop across the plate load. But the plate load is a coil of wire, and might only be 100-200Ω (maybe less, maybe a bit more). If the supply voltage is 150-200v, that would mean 147v or more is dropped across the reverb transformer primary which implies 147v/200Ω = 735mA or more. The reverb transformer primary would have burned open by the time that much current flowed.

If the plate voltage was dragged low by excessive current as described above, the cathode resistor should have a large voltage across it because of the large current flowing through it (Ohm's Law). The cathode resistor is 680Ω, so if 735mA was flowing through it as calculated above there would be 680Ω * 0.735A = 499.8v across it. Again, this is impossible due to the assumed supply voltage; it also implies 499.8v * 0.735v = 367w+ dissipated by the cathode resistor. It would have long since burned open.

If the reverb transformer primary were completely open, there would be 0v on the plate and cathode (infinity ohms gives infinity voltage drop, 0v left on the other side connected to the plate). It's possible that 11mV measured with some meters is just error and really represents 0v, but the 3.2vdc on the plate is more than just error.

If the 11mV is an accurate reading, then there is 11mV/680Ω = 0.016mA passing through the cathode resistor and also through the "plate load". You didn't give supply voltages, but if this node is indeed 150vdc that leaves ~147v across the reverb transformer primary while flowing 0.016mA so its resistance is 147v/0.000016A = ~9MΩ. Close enough to open to not work but not infinity so some voltage leaks through.

Turn the reverb unit off, make sure filter caps are drained and measure the resistance of the primary. I can't imagine a good one being over 1kΩ, maybe much less. It will be obvious if it's closer to 1kΩ or 9MΩ. If the resistance seems good & low, the remaining possibility is a partial short-circuit from the plate-end on the reverb transformer to ground external to the tube socket and the transformer. The other failure that could fall in this category is a short from the primary to secondary, which is grounded.

... what is the purpose of the .003 cap from node A to pin 6 of the 6FQ7? ...

It reduces high frequency response. Maybe to tame a tendency to oscillate, maybe to shape the tone of the reverb signal.

[punkykatt]

HBP, thanks for that informative reply.  I will do some more testing and get back with more needed  voltages and my findings.

[kagliostro]

Hi Punkykatt

in order to try to understand how this thing works I tried to redraw your schematic as to have it more readable

(also drawing something to me is easier to understand it)

Can you revise my drawing with yours ?

There is an odd thing that I'm not able to understand at all, the 0.003 cap connected to the plate of V3a and to the A node of the PS

Ciao

Franco

[sluckey]

There is an odd thing that I'm not able to understand at all, the 0.003 cap connected to the plate of V3a and to the A node of the PS

It reduces high frequency response. Maybe to tame a tendency to oscillate, maybe to shape the tone of the reverb signal.

Franco, node A is AC ground via the filter caps, right? The .003 does the same thing it would do if connected to ground instead of node A.

[kagliostro]

Ciao Steve

Ti ringrazio molto, adesso ho capito

When an AC signal is shunted to ground via the PS capacitors I'm always in trouble, now I've understand

Thanks again

Franco

[punkykatt]

Kagliostro,  Thank you for that beautiful schematic.  It matches exactly with what I have drawn, and is so much easier to read.  I am surprised Knight built this unit with no ac line fuse. Perhaps it was to intended to be plugged into a piece of equipment with a fused receptacle???? 

My testing today revealed an open primary winding in the reverb transformer.  I clipped in another transformer and I now get reverb on both A and B channels. The transformer thats in there is smaller in size compared to the Fender reverb transformer and has no end bells. There is no way a Fender transformer will fit. I need to start hunting for a smaller transformer that can power a 4AB_ _ _  tank.  Can not be bigger than:    height 1 1/4" depth 1 1/4", and hole space  1 3/4".

Thanks again  Guys for all the replies.  Punky

[eleventeen]

Isn't the req'ment for such a tranny just a cruddy little speaker transformer?

Kagliostro, that .003 is simply connected right across the rev transformer primary; think of it like that. Swamps oscillation, would be my bet, perhaps a tone shaper.

[kagliostro]

Ciao Punkykatt

So the schematic is as you have draw, thanks

here a version with the 0.003 cap draw in a bit different way, same function, may be more intuitive (as Eleventeen say)

if you want to add something or change something on the schematic, just say it

---

May be that unit was part of a bigger apparatus, as you say, so the absence of an AC fuse may be justified

---

It is strange that it has a pair of inputs, a pair of outputs, but only a reverb tank and reverb driver

 

Franco

[sluckey]

It is strange that it has a pair of inputs, a pair of outputs, but only a reverb tank and reverb driver

It's an add-on feature for a stereo unit. That's why the two channels. As a cost saving feature, the two channels were mixed together for a mono signal to send through a single reverb circuit. Then the mono reverb signal was mixed back with the two stereo channels so the reverb would come from both channels.

[kagliostro]

Ah, OK, I understand, it was a bit odd to me because I remember the schematic of a stereo unit that had two reverb unit inside (but I'm not able to remember the name)

Thanks Steve

Franco

[punkykatt]

Found a tiny whiny Hammond 125A output transformer I had in my stash from an old Fire Fly amp build I built  back when. 3w with adjustable primaries 1.2k to 27k and secondaries 1.5 ohms to 15 ohms.  It will be cool to experiment with clip leads to hear how the different Z`s change the reverb tone.  Punky