Do "cold cathode" stages maintain their tone when bypassed?

[NorCalTuna]

I'm building an amp, which is how many threads start here, with a moderately cold cathode stage-10k. I'm thinking of adding a parallel cap with a var. Resistor to make that stage have a variable gain boost, like a .68u and 100k pot. I'm also thinking of chilling it down more, like 25-39k. I notice both the jcm800 and the 5150 have unbypassed frosty grounds.

My questions are, do cold stages lose their signature tone when bypassed? And, what effect could the circuit being there when the var. is at max resistance, should it be isolated with a vactrol or relay?

Thanks forum!

[d95err]

I think if the pot is about 10 times bigger than the cathode resistor, the cap should be pretty much out of the circuit. So 100k pot and 10k cathode resistor should work fine.

The pot needs to be log or reverse log (depending on which direction of turning the pot takes the cap into the circuit) so that the main part of the pot range is when the resistance is very small. A linear pot will work pretty much like an on-off switch for the cap.

[Steve Conner]

A linear pot will work pretty much like an on-off switch for the cap.

That "on-off-switchness" is only made worse by making the pot 10 times bigger than the fixed resistor. I'd make it twice as big, at most, and just accept whatever tonal change that gives. It shouldn't be drastic, and it might even be an improvement!

I once did something like this. I couldn't find a reverse log pot in the value I wanted, and every pot I tried would crackle violently in that circuit, even with a DC blocking cap. So I ended up using a rotary switch with a string of resistors that gave an approximation to a reverse log taper.

[cbarrow7625]

I have bypassed 10k stages in a number of my amps. The same charachter is there, more or less, with higher gain. If you do it without other circuit changes you may run the risk of too much gain with the resulting squealing and fartiness issues you might expect.

You can partially bypass the 39k resistor in a 4 stage amp. I would not recommended a full bypass. The problems listed above will just be exacerbated. If it is a 3 stage amp, anything larger than 10k seems to reduce the gain too much to be terribly useful.

[defaced]

The VHT Deliverance I'm building is partially bypassed. 325 B+, 100k anode resistor, 22n coupling cap, cathode - 2k2||1u - 6k8 - ground.

[dai h.]

I once did something like this. I couldn't find a reverse log pot in the value I wanted, and every pot I tried would crackle violently in that circuit, even with a DC blocking cap.

does it matter if the pot is one of those "RV4" sort of style (such as older Dunlop Hot Potz, Clarostat, Allen-Bradley sealed which all(?) seem to have a big piece of copper for the wiper with a small rectangular block of black (carbon?)) pots? I tried a reverse audio 50k in series with the 470 ohm in the phase inv. in a Marshall with that style of pot (to try out a PI gain reduction control) and as far as I can remember I don't recall any major crackling. Also one of those types *seemed* to be comparatively quieter also for the 5k presence pot. Just wondering if there would be some technical reason such as more (as well as more constant contact) between the wiper and track compared to other style pots (say, those that have more of a "blade" shape wiper"). (Crackles make me think that discontinuities are happening such as pops from say switching caps.)

[Chuck H]

To address the original Q...

I think the character would be the same, but with more gain. And if you use a bypass cap that shelves at an audible frequency there would be that also added to the post stage EQ.

I have never done tests to verify my belief but the "cold clipping" or "cold cathode" stage does what it does (clip predominantly in the tubes cuttoff region) because of bias. A bypass cap won't change the bias so that characteristic should remain the same.

Chuck

[Wilder Amplification]

I have never done tests to verify my belief but the "cold clipping" or "cold cathode" stage does what it does (clip predominantly in the tubes cuttoff region) because of bias.
Chuck

That is correct. It can draw more current on the input signals positive swing, but since zero signal current is low, it will go into "cutoff" sooner.

[NorCalTuna]

Well, the circuit went onto the board tonight. I kept the 10k cathode resistor and then bypassed it with a .68uf on an 25k pot, which i'll wire as a variable resistor. The amp should (my old boss when I worked on race cars would say "well, you just f'ed us" because, as carroll smith said, murphy lives at the racetrack and he was very superstitious!) be up and running sometime this weekend so I'll let you know how the "ANGRY" knob works out on this lightly modded JCM800 clone. If it's nasty, it'll get rewired into a "FROST" knob to jack the 10K up to 35K, I suspect. It'd be cool to see how that would work out anyway, dammit... I'm sure somebody's been there...

Not to hijack my own thread, but instead of a traditional presence knob, I wanted to try the pres and res circuit from the 5150. It was my favorite feature of that amp, HUGE, tight bass! I hope it transfers well, as that'll be on there as well-

thanks guys!

[martin manning]

I'm thinking that when the Rk is un-bypassed, the resulting NFB limits the excursion of grid voltage WRT cathode and keeps the stage from distorting too much in spite of the cold bias. Bypassing Rk will increase gain and distortion, and if the Ck is not large enough to bypass all audio frequencies, the extra gain and dirt will be on the high-end. If you make the Ck 5uF that will pretty much cover the audio range, but you could try 10uF too. Of course the cap should be connected to the cathode, and the pot to ground so that there is no DC on it.

MPM

[xs1b]

I know what you mean but I think we need a new name because cold cathode already means something very different. Cold cathode - Wikipedia, the free encyclopedia

[Steve Conner]

They're called "cold biased" stages, the original poster got the name wrong.

[Wilder Amplification]

and if the Ck is not large enough to bypass all audio frequencies,

Since when are ALL frequencies within the audio pass band usable to a guitar???

This is EXACTLY why DSL/TSL amps fart out on the lows. The very first gain stage uses a 1k8 cathode resistor bypassed with a 4u7F bypass cap, which starts to gain boost around 18Hz! WAY more gain than you want to be adding that low in the pass band that early in the gain chain.

Bypassing for a higher frequency than the low E string keeps low freqs in check early on, which keeps things tight and defined. Add the lows in AFTER the overdrive circuitry, not right at the beginning. Adding it at the beginning is what causes farty lows.

With a 10K, if you're going to bypass I would recommend no higher than a 220nF (0.2uF). Start there and go lower, and let your ears decide.

[Gaz]

Since when are ALL frequencies within the audio pass band usable to a guitar???

This is EXACTLY why DSL/TSL amps fart out on the lows. The very first gain stage uses a 1k8 cathode resistor bypassed with a 4u7F bypass cap, which starts to gain boost around 18Hz! WAY more gain than you want to be adding that low in the pass band that early in the gain chain.

Bypassing for a higher frequency than the low E string keeps low freqs in check early on, which keeps things tight and defined. Add the lows in AFTER the overdrive circuitry, not right at the beginning. Adding it at the beginning is what causes farty lows.

With a 10K, if you're going to bypass I would recommend no higher than a 220nF (0.2uF). Start there and go lower, and let your ears decide.

Martin's post seemed concerned with loss of NFB when bypassing the stage. Most folks do not like the sound of bypassing a cold biased stage, and describe it as 'ragged' or the like. This is quite a reasonable description considering the loss of NFB.

You are making gross generalizations about low frequency roll off when bypassing - there are plenty of amps that fully bypass the first stage which have 'tight' and 'defined' bass.

Personally, I would leave the cold-biased stage alone, and instead use the variable bypass on a more center-biased stage that would typically be bypassed anyway

Not to hijack my own thread, but instead of a traditional presence knob, I wanted to try the pres and res circuit from the 5150. It was my favorite feature of that amp, HUGE, tight bass! I hope it transfers well, as that'll be on there as well-

I like this arrangement very much, and it can also be found in the Soldano SLO. You can vary the values of the pot/cap combo of the resonance control to vary amount and frequency cut off. If you find the .0047uf/1MA combo too bassy or muddy, try .0022/250kA, or maybe just one of those at a time. It's funny that you describe the bass as 'tight', when turning up the resonance actually decreases global NFB in the low/low-mid frequencies, which is really more of a 'loosening' of the bass. It's funny though, there are a lot of folks who perceive it as tighter. Awesome!