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Old November 26th, 2011, 10:12 PM   #1 (permalink)
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cathodyne vs. long tail PI in a 5E7 Bandmaster

I have a problem with my 5E7 when I gig with it. I love its clean sound which I use 95% of the time. But with heavy overdrive the sound thins out, especially with single note playing. I get a raspy low-fi sound that lacks body. Could it be that the cathodyne PI is giving me this type of distortion? I did do the Paul C mod (gave only little improvement) and it is not the speakers (which are 2 NOS C10N's and a G10 Gold that should be able to take the power). Earlier I modified the tone stack which resulted in a higher gain among others (the raspy distortion remained the same though). I could probably also convert the original PI and the preceeding gain stage to a Long Tail PI a la 5F6a and still have more than enough gain. Would this also give me a more solid overdrive tone? I like the amp's size and power rating so I don't really want to switch to a 5F6a. Did anybody try this (the classic narrow panel tweed circuit but with a long tail PI)?

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Old November 26th, 2011, 11:37 PM   #2 (permalink)
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Before you convert the entire PI, try a stopper resistor on the cathodyne grid, 10k-100k. Something like 33-47k should be tonally transparent.
The cathodyne gets a bad rap because too much grid current causes blocking distortion which sounds crappy. It sounds like you have too much current to the cathodyne grid.
Also try raising the power tube grid stoppers; if you double them you won't notice a tonal difference but should more effectively limit blocking current to the power tubes.
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Old November 27th, 2011, 02:56 AM   #3 (permalink)
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Fire, how much grid current do you think the 12AX7 can pull? And do you really think the cathodyne is being driven into class AB2 or B? Also, since the cathodyne is being driven by a cap coupled 12AX7 section, where is all that grid current going to come from?

Sounds like the OP may indeed be getting blocking distortion, something that happens with Fenders, but I'd suspect it may be in one of the outputs.
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Old November 27th, 2011, 04:39 AM   #4 (permalink)
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@BiggerJohn, I would be interested in your take on this piece:

http://www.freewebs.co.uk/valvewizard/cathodyne.html

which is sub-titled

Quote:
Always use big grid stoppers on cathodyne phase inverters!
I am unable to decipher much of the technical talk (having no electronics training or education). Do you think he is making a valid case for use of a grid-stopper?

Last edited by jhundt; November 27th, 2011 at 09:18 AM.
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Old November 27th, 2011, 10:18 AM   #5 (permalink)
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@BiggerJohn, I think you are not considering the amount of local NFB a cathodyne has. As you drive it out of linearity, NFB collapses and you get doubling on the anode output.
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Old November 27th, 2011, 12:55 PM   #6 (permalink)
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I ended up doing a mod called the 'PaulC' mod to my 5F4 - which is awful similar to your amp... changed the bias on the PI tube, and changed the overdrive behavior for the better.

Minimal (but noticeable to my ears) effect on the clean tone - and much better overdrive, was totally worth it to me. Only involves changing/moving a few parts.

Anyway, I'd suggest looking that up before totally redesigning the PI.
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Old November 27th, 2011, 01:31 PM   #7 (permalink)
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Thanks all for your replies! I will start by giving a 47k grid stopper a chance and maybe the 1k5's on the power tubes as well. Thanks for that advice. Somebody else already suggested the PaulC mod some time ago and after I did that the sound did get better, but only with a slight to modest degree of overdrive. Maybe I am just pushing the amp too hard but I do not find it particularly loud at max.. I will report back how the grid stopper worked out.
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Old November 27th, 2011, 01:51 PM   #8 (permalink)
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Please do. I was paraphrasing the Blencowe (valvewizard) take on cathodynes and not speaking from experience. I had no educated answer for Biggerjohn. So I'm real interested to see if a grid stopper will help, and where.
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Old November 27th, 2011, 02:23 PM   #9 (permalink)
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yes, please do let us know how it works. I have been wondering about this for a while myself.
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Old November 28th, 2011, 01:39 AM   #10 (permalink)
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Quote:
Originally Posted by jhundt View Post
@BiggerJohn, I would be interested in your take on this piece:

http://www.freewebs.co.uk/valvewizard/cathodyne.html

which is sub-titled



I am unable to decipher much of the technical talk (having no electronics training or education). Do you think he is making a valid case for use of a grid-stopper?
Thank you for the link, I found it to be a very good read. Excellent information there, +1.

I have actually seen (when working on my own Princeton Reverb) the so-called "nipple distortion" happening. You will see this type of phenomena often when instrumenting amplifiers which have capacitor coupled stages and you drive them hard, specifically the clamping effect. For example, as you drive the output tube grid slightly positive with respect to the cathode, it starts drawing grid current, which is a no-no in a capacitor coupled amplifier, so the grid to cathode starts acting kind of like a diode and clamps the grid voltage from going further positive. The then clamps the voltage on whatever is driving the grid (as showin in the article) and building more charge on the coupling capacitors, which then leads to the nasty "blatty" sounding blocking distortion.

However, following that the author goes on to tell what happens when the cathodyne is driven "really hard" I'm sure his results as shown are accurate, but i have never seen them in a Fender, and I think the reason is because the preceeding 12AX7 stage really can't drive the cathodyne "that" hard. Again, you are cap coupling into the cathodyne, from a high impedance 12AX7 plate, so it's ability to drive hard is quite limited. Rather, it to will clamp! I believe to achieve what the author shows with the frequency doubling, you'd have to drive the cathodyne with a low impedance source which is really capable of driving the beejezus out of the thing. Regardless, great information as to what could happen in the extreme or limiting case.

The author seems to draw conclusion that stopper resistors are the cure. In the example of the cathodyne itself, with hit's real high input impedance and low miller effect capacitance, that may indeed be a good idea and I might try a mod on the Princeton now. However, on output tubes you have to be real careful with stoppers, as if you go too big, you will start rolling off the high end because of miller capacitance, and then power tubes also have max ratings for grid impedance to ground, which if you exceed, the bias may get screwey.

Again, great read. Did I answer your question or just cause more confusion and mess?
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Old November 28th, 2011, 01:42 AM   #11 (permalink)
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One more point.

Oscilloscopes are marvelous things. I don't see how anyone can seriously work on amplifiers without one. Ditto for a signal generator.

No offense intended to anyone, but anyone trying to work on an amp with only a DVM and a guitar for a signal source, is not serious.
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Old November 28th, 2011, 02:54 AM   #12 (permalink)
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thanks for the response John. The tech aspect is still a bit blurry for me, but I am satisfied to know that you have read it and confirmed what he said, to a great degree. I don't always trust everything that pops up on the internet, and it's good to get a second opinion from a knowledgeable source.

Anyway it seems like an easy enough thing to try out - even without an o-scope! Good advice about the tools, too. Maybe I'll have to get serious about a scope. Years ago I was given a few old second-hand scopes but I could never get them to work properly, and I was spending more time working on test equipment than I was playing my guitar.
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Old November 28th, 2011, 02:09 PM   #13 (permalink)
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I spent some time reading other entries on that site. FINALLY someone does it right! No sales hyperbole or BS or other nonsense. Good technical meat with examples. No outlandish modification recommendations. No high gain freak crud. That whole site gets a big +1 from me. Let that site be Reference 101 for us. Whoever put that site up I'll buy you a beer. Good job.
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Old November 28th, 2011, 07:56 PM   #14 (permalink)
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Quote:
Originally Posted by BiggerJohn View Post

... However, on output tubes you have to be real careful with stoppers, as if you go too big, you will start rolling off the high end because of miller capacitance, and then power tubes also have max ratings for grid impedance to ground, which if you exceed, the bias may get screwey.

Again, great read. Did I answer your question or just cause more confusion and mess?
If the output tubes are pentodes, then there is almost no Miller effect because the screen grid "screens" the anode from the control grid. That was part of the design brief when tetrodes were developed. Using a 6V6 as an example, With a grid to plate capacitance of 0.7pf and a voltage gain of about 3 gives up a miller capacitance of 2.1pf so the grid resistor has to be huge before you get roll off down to 10khz, like over 100M

I just hunted down Preisman work on the cathodyne inverter. If the loads are balanced, a cathodyne is perhaps the most accurate inverter available. Signal level and Zout are very close to perfectly matched if the loads on each output are equal. If the loads become unbalanced (like a power tube driven into grid conduction, which happens below G1=0v) its balance goes all screwy and makes bad sounds. This alone is a good reason to use grid stoppers on the power tubes in the range of 10k.

As for the Valve Wizard site, Merlin Blencowe is perhaps the preeminent living valve authority. I am sure almost every thing he writes is from his own investigation, and not just repeating "common knowledge". I was asked to proof read some of his power supply book, and like the preamp book it is full of enlightening and accurate information that I have not seen in print anywhere else.
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Old August 7th, 2012, 04:41 PM   #15 (permalink)
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It has been a while but I promised to report back my findings regarding the cathodyne phase inverter in my tweed Super. I was unhappy with the overdrive sound which I found raspy and thin. I already read about similar complaints with the 5E7 and the BFPR and someone here redirected me to the Valve Wizzard article that is linked above. I tried a 500k pot in the grid stopper position and found that with around 100k the raspy sound indeed completely went away. It worked quite well with the sound changing to a 'regular' overdrive sound. I did not notice any appreciable difference in gain with a 100k grid stopper compared to the stock circuit. I liked it much better than the Paul C mod (that only gave a very modest improvement). The problem is completely cured with the grid stopper resistor.

But I still found that the circuit distorted too early and I was convinced that it was the cathodyne PI. So I took the plunge and replaced the complete PI circuit from cathodyne to Long Tail. This picture shows the result.



I had to add a extra circuit board on top of the existing one because there was not enough room on the 5E7. I used now obsolete eyelets in the old board to solder in little bolts that support and fix the new board. The extra board has only the extra parts, I used as many existing parts as I could. I think the sound has improved again. I have much more headroom and there is a lovely transition into crunch when I turn up the volume, without any nastyness. I feel that the clean sound improved as wel. Much more 'spatial' for lack of better words.

The whole reason why I switched from a 5f6a to a 5e7 was that the 5f6a became too loud on its sweet spot for the gigs I do. The 5f6a sound is the love of my life though. The 5e7 with its slightly lower B+, smaller OPT, less filter capacity and one less speaker was meant to function as a 'mini-bassman' but that did not work out that way. In hinsight due to the PI circuit. I already modified the tone stack to a 5f6 characteristic (with the mid scoop), increased the preamp voltages and changed the presence circuit a la 5f6. I also yanked out one of the channels to make room for the extra controls. The preamp is basically unchanged (and is not updated to 5f6 specs) and I noticed that for me it is not really critical for the sound. The preamp voltages and the PI circuit were. I am happy now, it is for me what it is supposed to be: a mini 5f6a Bassman
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