Cathode follower resistor value in Rocket

[Cliff Schecht]

What value do you guys typically use in your Rockets on the cathode follower? I've been using 68k but I haven't seen a lot of discussion on many of the unknown/variable Rocket values.

[Cliff Schecht]

Also for the first gain stage I have always used 1.5k without thinking about it much. Is this what others typically use here or do they go bigger? I'm thinking about upping this value to ~2.7k to try to alleviate some of the graininess in my Rocket builds. I think the 1.5k/220k combination is causing my input to clip even when the amp is set "clean".

[rooster]

I run the stock values, so 1.5K/220K, and then the 56K cathode follower resistor. 56K has less gain that 68K and the maximum 'gain' is 100K. Going to 2.7K will reduce headroom, 1K will increase it, but this is not distorting at 1.5K. Are you using a cap on V1a? If you are, try adding the switch between 'no cap' and 'add cap' like some here do. This is an area where too much gain can come into play.

What I might suggest for a less 'gritty sound' is to unbalance your PI. You can do this with a PI tube that is measured out of spec, one triode vs. the other, something like 85/110. Or you can run two matched pairs of EL84s, one about 5 ma out vs. the other. Yeah, I know it seems wrong but remember that - in the (Beatle) day - nobody was buying or selling matched quads of anything. Oh, and don't forget, the song that is known for AC30TB tone is 'Taxman'. You may enjoy it but it is quite grainy on a good stereo. And this was put down on tape and some pretty fabulous mics and mic pres, don't forget!

[Cliff Schecht]

Thanks! Yeah I scoped up the input stage and saw that it wasn't distorting too badly, but I could lightly clip it with my humbucker (SD Hot Rails have a lot of output). I did change the 1.5k to 2.7k but I'll probably swap it back and put the 22uF on a switch like you said. Again I think part of my problem is using vintage-type maple neck Tele's which are seriously bright guitars already, especially on the B-string. I'm going to swap to the 56k cathode resistor and see if this helps.

I thought the 100k/100k PI plate combo already unbalanced the power stage outputs.. I do have two different pairs of 6AQ5's in the amp I'm playing with (as well as unmatched pairs of EL84's in my actual Rocket build) which I do prefer over a matched quartet.

Edit: FWIW I just swapped out the 68k cathode resistor for the 56k and put back in the 1.5k//22uF resistor and cap combo and the amp did sound better, but there is still a harshness on that damned open B-string. It's frustrating because the amp sounds so damn good as is too, minus this one little quip. I'm wondering if other peoples Rocket's exhibit this same problem with maple-neck Tele's, both of mine do it and they sound like Rocket's otherwise!

[rooster]

Well, no B string issues to speak of from the amp, two bright Teles here, too. So look to your guitar saddles or something guitar related I think.

You could have some physical vibration thing going on I suppose, in a tube or chassis, etc, but I still think it way more likely to find it in the guitar. BTW, I run the Callaham brass compensated saddles for the D/G and the B/E, and then on the low E/A I use one of those aluminum compensated Glendale saddles. Man, easily suggest this set up for Tele tone if you are looking for some options.

[M Fowler]

In some I run 1k5/120k and 56k cathode follower gives more head room.

Mark

[keewee]

I had a harshnes on my rocket using the silver mica caps 50, 100, 500pf. I heard that using ceramic and polystyrene are the way to go. I replaced them with old school ceramics, a big help and I've been trying to locate some polystyrene to give them a try FWIW.

[M Fowler]

The following places have polystrene caps

www.turretboards.com

http://www.justradios.com/polystyrene.html

http://www.triodeelectronics.com/polcap.html

http://www.westfloridacomponents.com/Po ... itors.html

[Ian444]

I don't understand how changing the cathode resistor on a cathode follower can change the gain, which has to be less than one because its a cathode follower. I must be missing something...

Edit: thought about it a bit longer. With 100k cathode resistor less distortion, because there is less grid current pulled. Merlin explains this well. OK, I get it now. Sorry for interruption.

[joCCe]

Holy Resurrection Batman!!

Regarding the 100K v.s 56K cathode follower to ground. A 56K would yield more gain and compression compared to a 100K, or is it the other way around? I'm pretty sure a 56K gives more gain than a 100K, but I'll drop the question, because some threads states the opposite.

Thanks
Joakim

[fuzzheimer]

Holy Resurrection Batman!!

Regarding the 100K v.s 56K cathode follower to ground. A 56K would yield more gain and compression compared to a 100K, or is it the other way around? I'm pretty sure a 56K gives more gain than a 100K, but I'll drop the question, because some threads states the opposite.

Thanks
Joakim

That would be correct for an inverting amp gain stage, such as V1. But for a cathode follower the greater the cathode resistor value equals less gain loss (up to a point).

[surfsup]

Going to 2.7K will reduce headroom, 1K will increase it, but this is not distorting at 1.5K.

Rooster, looking at the load line, the 2.7k is biasing the stage at -1.5V and the 1.5k is biasing at -1V.

So the 2k7 would be most headroom since it is about 3.5V total grid swing. So why do you say the 1k5 will increase headroom? Is it because as more current is pulled through the resistor, the "headroom" increases due to more voltage dropped through the bias resistor?

[roberto]

It depends on the preivious' GS, but generally speaking a 56k ( vs 100k) on CF has more compression and adds more low order hamronics than a 100k. Works good (warm and full of nice mids) if you want to dampen some high order harsh harmonics, and you don't overdrive it too much.

About first gain stage, take care that if you partially bypass (let's say, Ck less than 1µF) a stage, an higher Rk means a brighter stage (and less lows means less fizz, usually), and a lower Rp means a brighter stage too.

At higher Rp you obtain less gain difference between unbypassed and bypassed frequencies, and the same it you lower Rk.

[joCCe]

Thanks guys!

I'll drop the 100K to a 56K and have a listen. The 100K was a mistake that I just discovered, so a 56K for less gain sounds promising.

Thanks
Joakim

[surfsup]

Going to 2.7K will reduce headroom, 1K will increase it, but this is not distorting at 1.5K.

Rooster, looking at the load line, the 2.7k is biasing the stage at -1.5V and the 1.5k is biasing at -1V.

So the 2k7 would be most headroom since it is about 3.5V total grid swing. So why do you say the 1k5 will increase headroom? Is it because as more current is pulled through the resistor, the "headroom" increases due to more voltage dropped through the bias resistor?

I scoped it. The ~1k5 (2k7 parallel w 4k7) waveform was rounder at the lower peak than the 2k7 and with less gain (63v pp vs 59.2v pp).

I couldn't get more signal into the stage, unfortunately. My wav file was at 0db, any more and the input signal would clip. Anyway...