Hi,
I'm working on an amp with CV1135 input tubes (~2C22 or DET20, or just 6J5 with top caps for plate and grid).
The grid top connection bothers me a bit since I want to install 2k2 carbon comp grid stoppers but I can't think of a way to do it properly without ruining the aestetics.
According to common knowledge, grid (and gate) stoppers should be installed as close as possible to the grid pin on the tube socket.
Would it be terribly wrong to install the stopper resistor inside the chassis and have a 3-4" long wire between it and the grid connector?
CV1135 is not a high gm tube, 3mA/V or so, so my gut feeling tells me that I might get away with it.
If it matters, the tubes will run at about 250V/9mA with LL1660S ITs as plate loads.
/Daniel
I'm working on an amp with CV1135 input tubes (~2C22 or DET20, or just 6J5 with top caps for plate and grid).
The grid top connection bothers me a bit since I want to install 2k2 carbon comp grid stoppers but I can't think of a way to do it properly without ruining the aestetics.
According to common knowledge, grid (and gate) stoppers should be installed as close as possible to the grid pin on the tube socket.
Would it be terribly wrong to install the stopper resistor inside the chassis and have a 3-4" long wire between it and the grid connector?
CV1135 is not a high gm tube, 3mA/V or so, so my gut feeling tells me that I might get away with it.
If it matters, the tubes will run at about 250V/9mA with LL1660S ITs as plate loads.
/Daniel
Put the resistor and connecting wire in heat-shrink tubing running from the cap into the chassis.
I'm very fond of the CV1135 - I thought it the best of the 6J5/6SN7 types.
I always put the grid stopper in the top cap. Soldered one side inside the top cap, made a hook in the other end and soldered the grid wire to the hook. The hook made it more solid long term. You could cover what shows of the resistor with heat shrink. I didn't bother. I think I used a 1K Holco or Dale metal film resistor. Worked fine.
If you or anyone is interested I have a few of these I could sell on.
I always put the grid stopper in the top cap. Soldered one side inside the top cap, made a hook in the other end and soldered the grid wire to the hook. The hook made it more solid long term. You could cover what shows of the resistor with heat shrink. I didn't bother. I think I used a 1K Holco or Dale metal film resistor. Worked fine.
If you or anyone is interested I have a few of these I could sell on.
Nice valve at 150MHZ, 5Watts PEP, not sure it will work well at less than 30MHZ though.
I would place a 2k2 MF 1/8W inside the cap cover with just the screened feed wire coming out of it. The resistor will not be on show in that case but not sure if you really need one.
I use a lot of 807s in my designs and the top caps I use are Bakalite with plenty of room inside. There is no need for a larger Wattage resistor as there is practically no current flowing.
I would place a 2k2 MF 1/8W inside the cap cover with just the screened feed wire coming out of it. The resistor will not be on show in that case but not sure if you really need one.
I use a lot of 807s in my designs and the top caps I use are Bakalite with plenty of room inside. There is no need for a larger Wattage resistor as there is practically no current flowing.
Thanks for your replies, gentlemen! Hiding the resistors inside, or close to, the top cap connectors is plan A, I just got the idea that I possibly could get away with moving it down into the chassis in this particular application. Due to the low gm/gain I could probably get away without any stopper resistor at all but I wouldn't be comfortable with that.
I have a few old bare metal connectors somewhere, perhaps I can practice a bit of "reversed psychology" and build an intentionally Frankenstein-esque grid connection since there is no high voltage present at the grid.
I have a few old bare metal connectors somewhere, perhaps I can practice a bit of "reversed psychology" and build an intentionally Frankenstein-esque grid connection since there is no high voltage present at the grid.
Heat shrink tubing lacks sufficient voltage isolation. HV tubing is required for safe operation.
Raychem MWTM is specified for low-voltage connection, shrinks to 1 mm wall thickness or more. Dielectric strength of 10KV/mm seems adequate to me, but there is a thick-wall version too.
Heat shrink should be fine for a grid, unless the cathode is floating at a few hundred volts. I presume the plate connection gets its own wire with adequate insulation.
benb is right, the grid will be at ground level plus minus a few volts of input signal. The plate connection is a whole different matter but since I probably don't need a plate stopper it shouldn't be a problem.
Thanks for the replies, I'm laying out the input stages now and just needed to be talked out of that bad idea.
Thanks for the replies, I'm laying out the input stages now and just needed to be talked out of that bad idea.
It depends more on the overall circuit / amplifier. In a conventional design that isn't super-high open loop gain, I would not expect any problem with putting the grid stopper inside the chassis with a few inches of wire to the grid.
If the grid wiring is kept short, and arranged so that it does not couple to the anode or anode-wiring, or screened, if your circuit can stand the capacitance;
AND the anode wiring is short and straight;
AND the IT anode supply voltage is decoupled using a standard electrolytic (Panasonic M, Nichicon VR - not a low-impedance type)
- then at 3mA/V you should not need a grid stopper at all.
AND the anode wiring is short and straight;
AND the IT anode supply voltage is decoupled using a standard electrolytic (Panasonic M, Nichicon VR - not a low-impedance type)
- then at 3mA/V you should not need a grid stopper at all.
Left-field esoteric connection would be teflon coated/insulated nichrome wire - similar concept to auto ignition leads, or scope probe innards 😎
I did this on my 2C22s. Slid a covering of white fiberglass insulation and a bit of heat shrink to hold it in place. The whole resistor and connections are fully inside the ceramic cap. Seems fine.
Since the tube is low gm type, I would investigate if any grid stopper resistor is needed at all.
As the CV1135 is the input tube, it would feel safer to have something between the grid and the unspecified outside world, so to speak.
I soldered the input stages last night (will post pics soon) but didn't put any stoppers inside the chassis. It won't be any problem to add stoppers close to the grid connectors, I just thought it would look better with a plain wire.
While we're at the subject of grid stoppers: The output tubes (6S4S / 6B4G) will be driven by direct coupled Mosfet followers to allow A2 operation. Any stopper resistors larger than a few tens of ohms will interfere with the grid current and cause distortion. I've read that the ham radio guys use carbon comp resistors in parallel with small air core inductors (~20 turns of magnet wire around the resistor) as stoppers, is there anyone who can elaborate the science behind this?
I soldered the input stages last night (will post pics soon) but didn't put any stoppers inside the chassis. It won't be any problem to add stoppers close to the grid connectors, I just thought it would look better with a plain wire.
While we're at the subject of grid stoppers: The output tubes (6S4S / 6B4G) will be driven by direct coupled Mosfet followers to allow A2 operation. Any stopper resistors larger than a few tens of ohms will interfere with the grid current and cause distortion. I've read that the ham radio guys use carbon comp resistors in parallel with small air core inductors (~20 turns of magnet wire around the resistor) as stoppers, is there anyone who can elaborate the science behind this?
Ah, plate stoppers.
I vaguely remember using such resistor/coil combos at the grids in my 807 A2 mono blocks when I thought I had an oscillation problem (turned out it was the cathode chokes in the CF driver stage that couldn't handle large signals at higher frequencies). Can't remember if I removed the stoppers later or if they're still in there.
I vaguely remember using such resistor/coil combos at the grids in my 807 A2 mono blocks when I thought I had an oscillation problem (turned out it was the cathode chokes in the CF driver stage that couldn't handle large signals at higher frequencies). Can't remember if I removed the stoppers later or if they're still in there.
