Question about small caps on preamp tube pins

[loudthud]

One interesting thing about the TW type amps is that looong instrument cables are suggested as a way to tame the overly glassy top end at lower distortion settings. Glen Kuykendall (if you know who he is) uses fourty feet of standard instrument cable in front of his Express. I didn't see the sense in using extra cable as a needed compliment to the amp. So my little 4.7pf cap solves for that too.

I have one of Glen's CDs. Bought it from him on ebay. Just listened to it three times. Once on laptop speakers, once on headphones and the last time on a ghetto blaster. It gets better every time. Gotta put my real system back together. Glen's a frequent poster on Amp Garage.

[Chuck H]

Yes. I've read many of his posts. Even saved some to my "Express" file. He's a good player but it's also obvious that the amp sounds killer. Glen was practically pals with Ken. He's a bit of a gear tweak so he's always serviced his own amp with Ken's intruction. He built a couple of clones to take on the road so he could leave his 30k amp at home. I'm sure it helps that he already knows some service rituals and has an actual example to work from, but his clones sound better than average. He's not the most tech saavy but if your going to build this type of amp I would definitely read his posts over at AG.

I don't have a good "console speaker" system right now either. I have an inexpensive 5+1 type thing for my cheap entertainment system. It sounds alright. But nothing like my old Marantz and a pair of big Pioneer colsole speakers.

[Steve Conner]

you ever see the schematic for wreck steve?

it's a wonder they sound the same between power ons.

Yes, I've seen a couple of the Trainwreck Express schematics. I could build them and make them run stably and consistently. But from what you guys are saying, that would make it sound wrong!

I think to do the concept justice, I'd have to look at any real Wreck gut shots I could find, experiment with the layout a lot and find where the feedback was mostly happening, then fit a tiny variable capacitor with a knob marked "Tune for maximum mojo". That was what Philbrick did.

I wonder if Chuck's edgy chip pick attacks were bursts of oscillation?

[soundguruman]

"Glen kuykendall (if you know who he is)"
Ask Glen: "Who built the Purple Amp?" Where did you get your first Gibson SG, Glen? (What's the recipe for Dragon Soup, Glen?)
That's funny, did not know he was still around. Ask him to PM me. soundguruman@yahoo.com

[Chuck H]

[soundguruman]

Thought you knew Glen kuykendall.
Would like to say hi to him, that's all.

[Steve Conner]

Soundguruman, are you an authorized Trainwreck service centre? That would explain a lot of things!

[soundguruman]

No, there is no train wreck here. They want me to be the Fender Service Center, but so far I have avoided it.
You see the closest service place is like 150 miles away.
you see when you are a service center, you wind up doing a lot of stuff for free / not much money, and I enjoy the freedom....
But I have been (years ago) the service center for Fender, Marshall, Crate, Ampeg, PV, etc...etc...etc...
But anyhow I have not talked to Glen in about 30 years, and I thought it would be cool to tell him hi. He would be surprised to hear from me...know where he is?

But what does Trainwreck have to do with it?

[soundguruman]

Thanks for helping me find him. He says he has 2 Operadio amps out in the shed, and he wants to chrome one of them. Right on Glen, keep grinding.

[Chuck H]

Yep. He's definitely a gear head. His old stage rig was a vintage Marshall (set with the old right hand roll across all the knobs), a cord and his tre bitchin' LP. But you asked:

But what does Trainwreck have to do with it?

His main amp has been a TW Express for a long time now. His CD (Can You Hear it Ring) is sort an homage to the TW amp. The guitar tracks are all done with his Express, a cord and a guitar. No effects. He's a very talented player. Especially WRT using his guitars volume control for effect. This makes him a strong proponent for the TW amps since recorded examples are hard to positively ID. So his CD, while worth getting for the material, is also a great intro to just what a TW amp sounds like. I wouldn't think it would be so hard as it is to find iconic recordings of a TW since they sell for about 30K. But there you go. That's what TrainWreck has to do with it.

I really don't see how I eluded to knowing Glen personally.?. But I'm glad you got in touch.

[jamesmafyew]

Connecting a capacitor between grid and plate increases the miller capacitance of the tube, and depending on the tube type the value of this capacitor will do different things. Basically the effective capacitance of the capacitor is multiplied by the gain of the stage (plus the stages initial miller capacitance). From here, you can view the miller capacitance as essentially a capacitor from grid to ground, and it forms a RC filter with the grid stopper.

Sorry to resurrect an old thread---

I'd be interested to see some math on how to calculate the -3dB point of the plate-to-grid cap. If I understand what you're saying, the value of the cap is multiplied by the gain of the stage, then you add that to the Miller capacitance of the tube, then you plug the resulting sum -- as the shunt capacitance -- into the F3 equation for a plain old RC filter with the grid stopper.

Is the slope just a normal 6dB/octave here? I guess I'm confused because intuitively I would think that the negative feedback arrangement from plate to grid would affect the attenuation level (and therefore the slope of the filter) more than the cutoff frequency. But then I guess the whole thing isn't that different from a NFB/presence circuit in some ways.

The reason I'm interested is that I have a build much like Chuck H's, where the input stage is very close to the input jack, and I just used the grid stopper resistor leads to connect the jack to the grid of V1. And like Chuck's build, it oscillates with all the controls maxed (but ONLY with all controls maxed, and a cable plugged in connected to my signal generator or a guitar with the volume down), at about exactly 40kHz. It's not an issue in practice because I never play the amp that way, but I'd rather it not oscillate at all. I was hoping to calculate the exact value of Ca-g1 needed to kill the 40k oscillation without changing the tone of the amp -- which I love.

[Chuck H]

By my experience, sans any math support, the correct value to squelch a 40kHz oscillation would be very very small. Since you need to open up the amp and heat an iron anyway, you could just buy a few very small caps and experiment. I do think knowing the correct formula is a good and fun exercise and worth knowing. But don't let that stop you from fixing your amp now. My cap, at 4.7pf, does have an audible affect. But just barely. I like the effect. Which is why I stuck with that value. Seems to take off the glassy shard tone and still keep the more useful harmonics. This as it applies to the typical 12ax7 triode gain stage. If you absolutely don't want to hear the cap and just squelch the oscillation you might just get caps ranging from .5pf to 2.7pf and use the smallest one that stops the oscillation. If this is a "tubes hanging" amp I would get 1kV caps with a respectible temp rating to avoid any thermal derating or value differences. If it's a "tubes sticking up" amp any 500V cap should do.

[ivey]

Ringing, high freq. oscillation, high freq, and low freq. wave shaping etc., all are used to correct the freq looping in tubes. When the player plays, the vibs from the music effects the electronics, causing movement, hence oscillation. That is why the small and medium cap seizes are there.

30 to 40 years ago, we used "spark gap" capacitors, that really did a good job. You may be able to come across some in the very older TV sets. I have not seen them since the 1990"s.

I one time I had a whole jar load of used ones. But I screwed up and loaned it a "friend". End of story

[jamesmafyew]

Since you need to open up the amp and heat an iron anyway, you could just buy a few very small caps and experiment. I do think knowing the correct formula is a good and fun exercise and worth knowing. But don't let that stop you from fixing your amp now.

Yeah, of course I should do that. I guess my main issue is that I don't actually trust my ears... too much time spent building hi-fi components for tweako dweebs. I'd almost rather look at a graph. I think I could put a cap in that spot and convince myself it compromised the tone even if it actually didn't. I'm the sort of person who wants to sweep the thing into a spectrum analyzer and KNOW where the -3dB point is, and plot the phase response of my amps, blah blah blah. It's awful.

[Steve Conner]

Well then, yes, the rolloff is calculated just the way you said. And it is 6dB per octave.

I had a build that oscillated in a similar way, except worse because I didn't use a shorting jack for the input, so it would take off when the guitar lead was unplugged. To shut it up I used 47pF from grid to cathode of V1, which is equivalent to about 1pF from grid to anode. I found the value by experiment: I had a couple of 47pF caps handy, so I tried one and it worked.