Setting Up High/Low Inputs On 6SC7s (5C8 Clone)

[rp]

I want to make the inputs more useful in a 5C8, so far I've left Channel 1 as stock w/ grid leak and set up Channel 2 as cathode bias with a 2700R / 250mf cap. I'd like to have a High /Low set up on V1 (the grid-leak side) and on (V2 the cathode bias side). I can't just copy the typical fender 68K/68-1M switched jack voltage divider since w/ the 6SC7 each jack of each channel goes to its own grid, but I want to end up with the same effect of a 6db drop on one of the jacks on each side. I think I got how the 68/68-1M switched jacks work: High sees a 1M load with a 34K stopper and Low sees a 136K load and no stopper(?) so I think I know how to implement this on V2, the cathode biased 6SC7. Not sure how to approach the grid-leak side, and very much not sure if I should add a 1M load resistor here and/or stoppers before the input cap. Built one amp w/o stoppers in my life and it immediately picked up German U-Boats, a 33K ended that.

I realize what I want to do should be even easier than the switched inputs, just a matter of adding a stopper and a load resistor on one grid on each channel, but as always inputs make my head hurt.

I've added Fender inputs images to save peoples' time. I'd draw up what I think I ought to do but presently no scanner and w/ schematics I'm still a paper and pencil guy.

MM, I think this on might be all yours.

[jon]

Having built both the 5C8 and the 5D8 as well as many normal fender/marshall input (5F6A) style inputs I always liked the second side of the 5D8. This is an unusual set up as it has a feedback loop there. this input take pedals really well, and has a very smooth feel to it.

Just my $0.02

[rp]

Having built both the 5C8 and the 5D8 as well as many normal fender/marshall input (5F6A) style inputs I always liked the second side of the 5D8. This is an unusual set up as it has a feedback loop there. this input take pedals really well, and has a very smooth feel to it. Just my $0.02

Good bright and early morning! How are the two sides of the 5D8 different, or did you mod the second channel? The only difference the schematic shows is a 100pf bright cap on one channel. Both channels then feed a cathode follower 2nd stage.

[jon]

I was speaking of the input jacks.

[martin manning]

Hi rp, for the cathode biased side it's as you suggest: high side is 1M to ground and 33k (standard value) in series to the grid, low side is 68k in series from the jack to the grid with another 68k to ground from the grid-end of the first 68k.

A -6dB input on the grid leak biased side is a bit of a problem because you can't put a 68k-68k resistive divider next to the 5M grid leak (it would then be 5M//136k). If you put the divider at the jack and isolate the 5M from it with a cap it will work, but then you will load the guitar with 136k instead of 5M. You could try a 3M3-3M3 divider followed by a series cap and then a 5M to ground for the grid leak. That would look like 5M3 to the guitar. I've never seen anything like that before, though.

[Cliff Schecht]

Go find my 5C8 thread, I did almost exactly the same thing when I built mine (minus the padded inputs, didn't need 'em). I made one side stock grid-leak biased and the other side as cathode bias with (IIRC) 1.5k//22uF. The grid leak side is nasty and distorted (in a good way), great for slide or old Zeppelin tunes, and the cathode biased side is clean and clear with only a hint of breakup when cranked.

As I mentioned in my "getting back to my roots" thread, this is the amp I've been using to jam with at my drummers house. No pedals needed, it gets up on it with that grid leak channel (cleans up nicely with the guitars volume knob) and for cleaner stuff I just switch to the cathode biased input. It's a fun little amp!

[rp]

low side is 68k in series from the jack to the grid with another 68k to ground from the grid-end of the first 68k.

Martin, Cliff's success has me thinking I'll leave off the 68K stoppers and go naked as in the original. So, I would use a 150K load only on the Low jack, and a 1M load only on the High jack? Seems easy enough or does that not get me the 6db drop anymore? BTW I've decide to forgo the power string upgrade. IME getting the voltages right has proven to be at the top of the list for a good clone.

Cliff, your old thread makes me optimistic, and anxious for my euro-parts order to arrive. So, you didn't even set up the cathode biased side with a 1M load resistor like the 5D8, or 2M like the 5B6? Nada? Plus, with no stopper you never pick up radio anywhere? This is good to hear. Guess I should build it straight to get the feel of a real 5C8 first. Though I really want to do H/L inputs, seems silly to offer all identical inputs in 2012. Musicians always ask what the inputs do - can't just say, "Nothin'!'" Of course I could give them the 1955 sales pitch, "You can plug a whole dang band - bass, guitar, mic, and accordion into it, what more do you want!"

Also, noted you chose 1500R for the cathode resistor on the 6SC7? I had a post about this and Martin's opinion was 2500-2700 as was Fender's looking at the 5B6. 1.5k seems low by about half, like using 820 on a BF amp instead of 1.5k to goose it up. Does yours sound extra-charged on that channel? Did you check the bias, did the 1.5k give you the optimal -2V? I'm actually trying for a hot channel 2.

I know, seems like silly questions as I can go back in and play around all I want but it's developing as an awfully pretty PTP build and I'd like to avoid going back into this one and chewing it up too much.

[rp]

Martin, what about this, it's the 5C1 inputs? Doesn't this affect the 5 Meg and screw up the bias?

[martin manning]

low side is 68k in series from the jack to the grid with another 68k to ground from the grid-end of the first 68k.

Martin, Cliff's success has me thinking I'll leave off the 68K stoppers and go naked as in the original. So, I would use a 150K load only on the Low jack, and a 1M load only on the High jack? Seems easy enough or does that not get me the 6db drop anymore? BTW I've decide to forgo the power string upgrade. IME getting the voltages right has proven to be at the top of the list for a good clone.

To get the -6dB on the low jack you need a divider so you need to have a 68K from the tip to the grid and another from the grid to ground like this. If you find out that you need grid stoppers on any of the inputs you can put them on the end of the lead going to the sockets (where they should be).

[martin manning]

Martin, what about this, it's the 5C1 inputs? Doesn't this affect the 5 Meg and screw up the bias?

No the cap isolates the DC bias on the grid leak from the guitar. The AC load on the guitar using the high input is 5.075M, so that works! Note that the low input loads the guitar with 150k, though. Above I was trying for a -6dB input with a 5M input impedance.

[Cliff Schecht]

TBH I don't remember if I used grid stoppers. Might have to open up the amp.. Did that thread of mine have pics of the guts? If I didn't have the resistor in there then it's not in there now (if you are looking at the rebuilt version I did with a turret board instead of my first point-to-point mess).

I went with 4.7M (essentially 5M) on the grid leaked input and 1M on the normal input. Also I don't ever use 6SC7's, I have very few whereas I have two boxes full of 6SL7/6SN7 old manufacture that I've picked up for little to nothing. IMO the 6SL7 tube is choice for this amp and super cheap in a lot of places (although that's changing!). Save the 6SC7's for the amps that already need them.

Also I found that lead dress is critical in this amp design (and any with non-humbucking heaters). I'd say study my build closely, as I mentioned in that thread I managed to get the noise floor lower in my 5C3 rebuild than I did when I went with all DC heaters in my crap point-to-point attempt.

[rp]

Ok, one more quickie, which way to orient the foil on the grid leak caps - to guitar or grid?

Hey, Cliff, mine is PTP but very tight and clean, as for the heaters they sure look neat and clean but I've got my fingers crossed tight - makes it hard to solder should I offer a preview or do an Grande Unveiling? Would sure suck to unveil a humming amp

[martin manning]

Ok, one more quickie, which way to orient the foil on the grid leak caps - to guitar or grid?

Guitar. The impedance to ground is much less.

[rp]

Ok, one more quickie, which way to orient the foil on the grid leak caps - to guitar or grid?

Guitar. The impedance to ground is much less.

Top of the morning to you too Martin. Thanks.

[martin manning]

Hi rp, looking forward to seeing this p-t-p creation of yours!