Cornford RK100 Richie Kotzen

sinner · Post by **sinner** » 11 May 2013, 11:24

Any Cornford RK100 owners on board?

I just need to confirm few things...

sinner · Post by **sinner** » 11 May 2013, 19:56

Alright...

I've posted this in SLOclone forums, so I can do it here as well - FSB is my home and platform Im learning from since 2008 after all

Here's schematic I drawn from detailed picture found @ SLOclone

I'm not sure how close I am, as till now I was interested in simple constructions mainly (old Marshall's, Hiwatt's, Fender's), and quite frankly - I am dump ass newbee when it comes to tubes...

I'm very confused with that jfet part of the preamp, and totally unsure about that part

Have fun

Groovenut · Post by **Groovenut** » 12 May 2013, 00:43

Hey Sinner,

If I had to guess at the JFET, it's for high gain/low gain switching. The ? is probably attached to 15-24 VDC or so somewhere. When the switch is grounded it pulls the gate low and the drain high. I would assume there is an LED in the footswitch pedal that is turned on through R20 when grounded. This would be high gain mode. When the switch is not grounded the gate is pulled high via the R19 pulling the drain low and putting R33 in parallel with the gain pot. C14 forms an RC filter with R19 that limits the speed the JFET can switch at, reducing popping when the switch activates.

There is a tone of gain and compression available in that preamp circuit. The dual bass pot is an interesting idea. Also the fixed resonance circuit, very cool.

sinner · Post by **sinner** » 12 May 2013, 15:03

Groovenut wrote:Hey Sinner,

If I had to guess at the JFET, it's for high gain/low gain switching. The ? is probably attached to 15-24 VDC or so somewhere. When the switch is grounded it pulls the gate low and the drain high. I would assume there is an LED in the footswitch pedal that is turned on through R20 when grounded. This would be high gain mode. When the switch is not grounded the gate is pulled high via the R19 pulling the drain low and putting R33 in parallel with the gain pot. C14 forms an RC filter with R19 that limits the speed the JFET can switch at, reducing popping when the switch activates.

This is what I was thinking, and it was mentioned this at SLOclone, but still I know no theory of jfet switching - thanks for explanation

That jfet part of this amp can't do anything else really, the rest of the first tagboard is just a tube gain stages, nothing groundbreaking, and the 'footswich' jack needs to be connected someware

There is a tone of gain and compression available in that preamp circuit. The dual bass pot is an interesting idea. Also the fixed resonance circuit, very cool.

The dual bass and master was that push for me to move my ass and try to trace it. I was expecting some kind of PPIMV by Ken Fisher really for Master pot, and was surprised is not.

For bass control some kind of double function - like typical bass + something like cathode bypass shift, or PI coupling caps mix... What's the difference between Cornford tonestack topology and typical Fender/Marshall tonestack?

sinner · Post by **sinner** » 12 May 2013, 15:07

There is one more thing on the layout that I cant understand.

It's two diodes and one large 2200uF low voltage (I think, it's quite small in diameter). Cant crank where that should be connected, and what function does it serve... I'll try to upload the layout later today..

Groovenut · Post by **Groovenut** » 12 May 2013, 17:19

sinner wrote:There is one more thing on the layout that I cant understand.

It's two diodes and one large 2200uF low voltage (I think, it's quite small in diameter). Cant crank where that should be connected, and what function does it serve... I'll try to upload the layout later today..

Just guessing again, but it's either for DC heaters or generating the DC voltage for the switching.

Edit: looking at it again I would say it's generating the switching voltage. It looks like its using AC for the heaters. The two diodes look to form a voltage doubler.

ppluis0 · Post by **ppluis0** » 12 May 2013, 17:20

sinner wrote:What's the difference between Cornford tonestack topology and typical Fender/Marshall tonestack?

Hi Max,

As I can see, the dual pot to adjust the bass freq is acting merely as a single one, so the tonestack does not differ from a traditional Fender amps.

Also this amp uses dual pots to adjust the preamp gain, one section in each triode stage, but again seems to be employed as a single potentiometer in the master volume control. I think that is a mere reduction in inventory at the moment to buy parts to build this units and single potentiometers with a total resistance equal to the dual units wired in series, must work well.

sinner wrote:...two diodes and one large 2200uF low voltage (I think, it's quite small in diameter). Cant crank where that should be connected, and what function does it serve...

Perhaps the heaters of the preamp tubes are supplied by DC to avoid hum...

Cheers,
Jose

Groovenut · Post by **Groovenut** » 12 May 2013, 17:27

sinner wrote:
Groovenut wrote:Hey Sinner,

If I had to guess at the JFET, it's for high gain/low gain switching. The ? is probably attached to 15-24 VDC or so somewhere. When the switch is grounded it pulls the gate low and the drain high. I would assume there is an LED in the footswitch pedal that is turned on through R20 when grounded. This would be high gain mode. When the switch is not grounded the gate is pulled high via the R19 pulling the drain low and putting R33 in parallel with the gain pot. C14 forms an RC filter with R19 that limits the speed the JFET can switch at, reducing popping when the switch activates.
This is what I was thinking, and it was mentioned this at SLOclone, but still I know no theory of jfet switching - thanks for explanation That jfet part of this amp can't do anything else really, the rest of the first tagboard is just a tube gain stages, nothing groundbreaking, and the 'footswich' jack needs to be connected someware
There is a tone of gain and compression available in that preamp circuit. The dual bass pot is an interesting idea. Also the fixed resonance circuit, very cool.
The dual bass and master was that push for me to move my ass and try to trace it. I was expecting some kind of PPIMV by Ken Fisher really for Master pot, and was surprised is not.

For bass control some kind of double function - like typical bass + something like cathode bypass shift, or PI coupling caps mix... What's the difference between Cornford tonestack topology and typical Fender/Marshall tonestack?

Personally , I think the dual pots on the bass control and master volume are serving to extend the taper of the control in a sense as far as how the user will hear it (obviously not how its actually working). I believe this gives the impression of having a smoother control throughout the range of the pot. Pretty brilliant if you ask me. The dual Master volume also serves to change the HPF knee as the control goes from min to max. I think this may be an attempt to offset the issue of tone thinning at lower master volume levels.

The dual gains in one pot I have seen before on almost all of the AX84 amps going back quite a ways. This serves to have a larger control over the gain of the amp. You get less gain at lower settings and more at higher settings than you would with a single control and a fixed divider on the later tube stage.

sinner · Post by **sinner** » 12 May 2013, 20:02

Thanks for explanation guys

This reminds me how large learning field I have to discover

I've attached uncompleted layout for this amp. I did it to make my trace easier, and if I only ever decide to build it, I'll use my own layout - so this one will stay unfinished. In fact I had it more developed, but apparently I had some problems with save function last time I was working on it. Anyway, PA and transformers are undone.

On the second board where PI and bias circuity is you see that big cap and two diodes. These has no connection to anything (except ground) and I never seen anything like this in an guitar amp - well no wonder, I am noob

Groovenut · Post by **Groovenut** » 12 May 2013, 21:49

sinner wrote:On the second board where PI and bias circuity is you see that big cap and two diodes. These has no connection to anything (except ground) and I never seen anything like this in an guitar amp - well no wonder, I am noob

I believe that is the DC supply for the switching fet. I cannot see a connection in the photo at either the 2k7/1M junction or at the diodes/cap junction, but there is a suspicious peach colored wired bound to the orange wire running from under the bias board to under the board with the fet on it. Just my guess. There are two white wires that run from the PT to each of the diodes by the 2200uF cap. I believe this is a 15 VAC tap. When rectified and smoothed by the 2200uF cap with the miniscule current draw of the fet and LED, it would yield ~20VDC. This should be plenty to keep the fet from signal limiting when it's disengaged. Again just my guess but it sounds within reason of the design.

GJonesy · Post by **GJonesy** » 19 May 2013, 09:50

Hi sinner,
Groove nut is spot on with his comments. I've quickly replied to you on sloclone but your getting more from the guys here at the moment. Let me know I you need any info, I don't have the amp but I can get access to it (check those other schematics I mentioned, mk, laney)
Good luck G

sinner · Post by **sinner** » 21 May 2013, 12:57

GJonesy wrote:Hi sinner,
Groove nut is spot on with his comments. I've quickly replied to you on sloclone but your getting more from the guys here at the moment. Let me know I you need any info, I don't have the amp but I can get access to it (check those other schematics I mentioned, mk, laney)
Good luck G

Thanks to all of you guys

and... I just noticed that this is you who did that RK100 picts - thanks a lot mate, without you I would be noware...

Meanwhile I've checked MK50H schemo's (two versions) and yeah, it's a part of switching supply. The schematics on SLO show it differently wired (different transistor pinout than mine). I'm quite sure of connection scheme I did, your detailed pictures shows it clearly, and I draw it with jfet datasheet on hand.

If it's possible to have more picts of this part, as well as footswitch internal shoots I would be grateful. Also, I'm not sure, but I think I see miniature toggle switch near to the cab jacks... Can you do the picts of it as well?

Also, I'm not sure of PS, picts and values would help

Thanks again guys

EDIT: Do you think you could help me to draw grounding scheme of RK100? As I said, I don't think I will fallow Cornford layout, as I'm able to easily draw my own, but I'm just curious how he does it

lead2203 · Post by **lead2203** » 21 May 2013, 20:39

sinner wrote:
GJonesy wrote:Hi sinner,
Groove nut is spot on with his comments. I've quickly replied to you on sloclone but your getting more from the guys here at the moment. Let me know I you need any info, I don't have the amp but I can get access to it (check those other schematics I mentioned, mk, laney)
Good luck G

Thanks to all of you guys and... I just noticed that this is you who did that RK100 picts - thanks a lot mate, without you I would be noware...

Meanwhile I've checked MK50H schemo's (two versions) and yeah, it's a part of switching supply. The schematics on SLO show it differently wired (different transistor pinout than mine). I'm quite sure of connection scheme I did, your detailed pictures shows it clearly, and I draw it with jfet datasheet on hand.

If it's possible to have more picts of this part, as well as footswitch internal shoots I would be grateful. Also, I'm not sure, but I think I see miniature toggle switch near to the cab jacks... Can you do the picts of it as well?

Also, I'm not sure of PS, picts and values would help

Thanks again guys

EDIT: Do you think you could help me to draw grounding scheme of RK100? As I said, I don't think I will fallow Cornford layout, as I'm able to easily draw my own, but I'm just curious how he does it

Just to give you something that is a similar idea in a pedal for the FET switching

sinner · Post by **sinner** » 21 May 2013, 21:04

...so pinout-wise i got it right

GJonesy · Post by **GJonesy** » 22 May 2013, 07:41

sinner wrote:
GJonesy wrote:Hi sinner,
Groove nut is spot on with his comments. I've quickly replied to you on sloclone but your getting more from the guys here at the moment. Let me know I you need any info, I don't have the amp but I can get access to it (check those other schematics I mentioned, mk, laney)
Good luck G

Thanks to all of you guys and... I just noticed that this is you who did that RK100 picts - thanks a lot mate, without you I would be noware...

Meanwhile I've checked MK50H schemo's (two versions) and yeah, it's a part of switching supply. The schematics on SLO show it differently wired (different transistor pinout than mine). I'm quite sure of connection scheme I did, your detailed pictures shows it clearly, and I draw it with jfet datasheet on hand.

If it's possible to have more picts of this part, as well as footswitch internal shoots I would be grateful. Also, I'm not sure, but I think I see miniature toggle switch near to the cab jacks... Can you do the picts of it as well?

Also, I'm not sure of PS, picts and values would help

Thanks again guys

EDIT: Do you think you could help me to draw grounding scheme of RK100? As I said, I don't think I will fallow Cornford layout, as I'm able to easily draw my own, but I'm just curious how he does it

Hi Sinner
heres a pic of the foot switch
https://dl.dropboxusercontent.com/u/121 ... .37.34.jpg

I dont have the amp with me, it belongs to a friend. he's gigging it a lot so I'll have a look next time I get it.
the mini switch is for EL34 to 6L6, I'll have a look through my notes. As for the grounding scheme stick to the tried and tested methods and you should be fine.

G

67Mopar · Post by **67Mopar** » 01 Jun 2013, 12:59

This is my first posting, so hello!

This is my SE built Cornford RK100 that I decided to rebuild. These are also posted on the Cornford forum.

For those who need the pot values:
Preamp: 500K Tandem. This dials-in the gain stage vs. doing it via a foot-switch. The Motorola JFET attenuates the signal.
Bass: 1M Audio
Midrange: 25K Linear
Treble: 250K Linear
Presence: 25K Linear
Master: 1M Audio

Here you can see the boards as they were stock. Burnt insulation, bent leads, incorrectly sized slope and PI cathode resistor(s), and flux galore!

Here it is with Dale RN65D resistors, Philips MKT368 and WIMA MPK-4 polypro coupling capacitors. Two of the CTS pots are so loose that they will turn with vibration. I have never experienced this with CTS pots before. I have ordered a full set of PEC pots to replace the stock pots. I'm going to also get rid of the soldered buss-bar on the back of the pots... The new ground buss will be fitted at the terminals, and Kynar sleeved signal buss where applicable. I'm waiting on Dale CMF60 resistors to complete the refit, but she's looking pretty darn good so far... The tone is CRUSHING!

The Nichicon capacitor that is strapped to the side of the board will be properly secured to a tag-strip.

I replaced the RUBY brand filter caps with ARS brand filter caps. Notice how SE used a .5W Xicon metal-film as a power resistor... That resistor is supposed to be a 10k/3W metal-oxide resistor.

Here are the latest pics... I've replaced the generic Jameco filtering caps with Nichicon brand and the 10K/3W resistors with KOA Speer resistors. I'm still waiting on the PEC potentiometers.

sinner · Post by **sinner** » 01 Jun 2013, 15:54

First of thank you for accepting my invitation, and big thanks for pot values

Secondly nice job, for that money Cornford should do it that way, instead of using crappy of the shelf parts all the way up.

Could you also give us some input about transformers spec? Any part numbers, voltages perhaps?

Ones again, thanks a lot mate, I hope you stay with us for longer

67Mopar · Post by **67Mopar** » 01 Jun 2013, 20:02

sinner wrote:First of thank you for accepting my invitation, and big thanks for pot values

Secondly nice job, for that money Cornford should do it that way, instead of using crappy of the shelf parts all the way up.

Could you also give us some input about transformers spec? Any part numbers, voltages perhaps?

Ones again, thanks a lot mate, I hope you stay with us for longer

Thank you! I don't have any info on the transformers - yet. I put the capacitor values back to stock today... Sounded great with the .022uf caps, but I prefer this amp with the stock values. I still need to replace the pots, but other than that it's complete. Yes, I agree on the quality of parts used in these builds...

sinner · Post by **sinner** » 01 Jun 2013, 20:28

67Mopar wrote:I agree on the quality of parts used in these builds...

I was even more amazed then I saw the posts all around the guitar related forums about amount of issues with Cornford amps - buzz and hum problems, relay faults (mk50II specially), transformer break downs (specially in Hurricane series)

Cornford forum was partially helpful with transformer info, but I'm not sure how does it apply to RK100

Transformers for Cornford were custom wound by Transformer Equipment Ltd in Kent

http://www.transformer-equipment.co.uk/

My 2001 metal grill Hurricane had a faulty EL84 which caused a short in the mains transformer. I was very fortunate in finding an excellent tech who is a guitar enthusiast and has confirmed that the transformer was manufactured by Transformer Equipment Ltd. They are making a new one for the Hurricane at a cost of L58.08 (including VAT, Carriage etc) which I think is fairly reasonable. Most other components are fairly easy to source so being able to replace the custom transformers has made me feel a lot easier about my Hurricane and Hellcat.

I have a favor - could you look on my schematic posted earlier and spot possible errors? I'm not sure about the resistor value from input to V1b grind as I cant see the value, as well as I'm not sure the PS. Other places wouldn't hurt to be verified by another pair of eyes too, specially the eyes in possession of THE amp.

Also - preamp pot is 500k AUDIO taper, right?

Thanks again, all you guys are great!

67Mopar · Post by **67Mopar** » 01 Jun 2013, 22:47

sinner wrote:
67Mopar wrote:I agree on the quality of parts used in these builds...
I was even more amazed then I saw the posts all around the guitar related forums about amount of issues with Cornford amps - buzz and hum problems, relay faults (mk50II specially), transformer break downs (specially in Hurricane series)

Cornford forum was partially helpful with transformer info, but I'm not sure how does it apply to RK100

Transformers for Cornford were custom wound by Transformer Equipment Ltd in Kent

http://www.transformer-equipment.co.uk/

My 2001 metal grill Hurricane had a faulty EL84 which caused a short in the mains transformer. I was very fortunate in finding an excellent tech who is a guitar enthusiast and has confirmed that the transformer was manufactured by Transformer Equipment Ltd. They are making a new one for the Hurricane at a cost of L58.08 (including VAT, Carriage etc) which I think is fairly reasonable. Most other components are fairly easy to source so being able to replace the custom transformers has made me feel a lot easier about my Hurricane and Hellcat.
I have a favor - could you look on my schematic posted earlier and spot possible errors? I'm not sure about the resistor value from input to V1b grind as I cant see the value, as well as I'm not sure the PS. Other places wouldn't hurt to be verified by another pair of eyes too, specially the eyes in possession of THE amp.

Also - preamp pot is 500k AUDIO taper, right?

Thanks again, all you guys are great!

Grid leak R is 1M. Input grid R is 68k. Cathode 1k5 w/2.2uf bypass / Plate 220k. Yes, the preamp is 500k tandem. The preamp is similar to the Soldano SL60 preamp in terms of values.

Speaking of transformers, should each tap of the OT reference to ground when buzz tested on the impedance switch? Which wires are which on a brit OT? I don't know which is 4/8/16 Ohm tap.