Cathode Follower in Single Ended Amp?

[bw1985]

Hey all,

I was hoping for some advice on adding a cathode follower circuit to act as a faux phase inverter stage in a single ended amp.
I have a single ended project that I build years ago, but would like to try to convert it to a TW Express style preamp.

I've put together a schematic I cobbled together from existing circuits on the web, would my placement of the cathode follower work, or is there anything else I'm missing?



Thanks!

[pdf64]

The CF placement looks good but it needs a dc blocking cap after it to prevent putting >100Vdc on to the power tube g1.
The g1 would also need a grid leak resistor, eg 470k.
Making it a pot, to act as a gain control (aka master vol type 4), may turn out handy.

[stokes]

I did the same thing with a pair of 6L6WG's in single ended,parallel.It was a long time ago,but I think I used a .047 cap,I used a few and really forget where I settled.I def used a 220 grid leak.I have a couple of TW schems,dont recall which one I used.

[bw1985]

Thanks for your responses! I’ll make sure to include the grid leak resistor and coupling cap and update the schematic.

Also, would it be a good idea to connect the prescense pot (don’t think it’s in this schematic) with negative feedback to the cathode follower, or before?

Is there any reason a cathode follower doesn’t require a plate resistor? I just haven’t ever incorporated one of these in a design and still learning the theory.

[loudthud]

The "Cold Clipper" third stage will have a high plate Voltage. With a 250V B+, plate Voltage might be 220V. (there are two different Voltages indicated for B+2) The high plate Voltage makes direct coupling to a cathode follower impractical unless a higher B+ can be used for the follower. The grid to plate Voltage for a 12AX7 should be at least 100V.

The Cold Clipper doesn't output much on the positive side, but big negative swings on the bottom of it's output. IMHO you don't need a cathode follower.

[bw1985]

The "Cold Clipper" third stage will have a high plate Voltage. With a 250V B+, plate Voltage might be 220V. (there are two different Voltages indicated for B+2) The high plate Voltage makes direct coupling to a cathode follower impractical unless a higher B+ can be used for the follower. The grid to plate Voltage for a 12AX7 should be at least 100V.

The Cold Clipper doesn't output much on the positive side, but big negative swings on the bottom of it's output. IMHO you don't need a cathode follower.

I noticed some pretty high voltages on V2...I believe they were exceeding the 250v+, I'll need to double check my numbers again when I'm in the shop tomorrow. I believe I may have fried the preamp tubes because now the amp has barely any volume and sounds super fuzzy. First order of business will be to test the tubes.

However, the first time I fired the amp up...it sounded incredible! So, is there any way of using this circuit but modifying values to bring the voltage down?

[Enzo]

The cathode follower does not need a plate resistor because, well, it is a cathode follower.

250v is not going to hurt a 12AX7. The only potential trouble is that 100 volts on the cathode. Tubes have a maximum heater to cathode voltage.

[pdf64]

I noticed some pretty high voltages on V2...I believe they were exceeding the 250v+, I'll need to double check my numbers again when I'm in the shop tomorrow. I believe I may have fried the preamp tubes because now the amp has barely any volume and sounds super fuzzy. First order of business will be to test the tubes.

However, the first time I fired the amp up...it sounded incredible! So, is there any way of using this circuit but modifying values to bring the voltage down?

A Marshall Super Lead may put ~400V on its CF, with a consequent cathode voltage of ~200V, which can damage the tube as Enzo describes.
But 250V on its plate should be no problem.
Are you sure that the circuit isn't oscillating?

[Roberto]

It depends on the 12ax7 indeed.

Sovteks do not go over 100 Vhk, while EI, Tung-sol and many others are rated around 180 Vhk like old datasheets show.
Raising the heaters by 40-50V can't be bad for noise reasons too, so why not?

[bw1985]

It depends on the 12ax7 indeed.

Sovteks do not go over 100 Vhk, while EI, Tung-sol and many others are rated around 180 Vhk like old datasheets show.
Raising the heaters by 40-50V can't be bad for noise reasons too, so why not?

Well, I've checked the tubes and they are fine! I found one of the pins is acting funny and if I wiggle it around I get my signal back.

I'm noticing something very strange, seeing 40vdc on the heater windings and hear a weird buzzing in the output. I'm assuming this is related...

I've updated my schematic with the additional components.



Here is a list of my voltages:

V1
1 - 195v
2 - 0v
3 - 2.38v
4/5 - 6.6vac
6 - 171.6v
7 - 0v —-> 7vac??
8 - 1.65v
9 - 0v ——> 40vdc


V2
1 - 336v
2 - 0v
3 - 3.71v
4 & 5 - 6.51vac
6 - 377v
7 - 0v
8 - 4.7v
9 - 0v ——> 40vdc?

V3
1 - 0v
2 - 42vdc / 6.5ac
3 - 378v
4 - 394v
5 - 0v
6 - N/C
7 - 43vdc
8- 43vdc

[g1]

Your tube numbering on your schematic is funny and I don't think it matches your measurements.
I think the measurements you call v2 are for the 3rd and 4th triodes shown. One of them should have a higher voltage at the cathode where the 100k cathode resistor is.
Perhaps you have the cathode to heater breakdown that was mentioned in post #7 & 8. That may cause the buzz and extra DC on the heaters (beyond the voltage from point 'A')

[bw1985]

Your tube numbering on your schematic is funny and I don't think it matches your measurements.
I think the measurements you call v2 are for the 3rd and 4th triodes shown. One of them should have a higher voltage at the cathode where the 100k cathode resistor is.
Perhaps you have the cathode to heater breakdown that was mentioned in post #7 & 8. That may cause the buzz and extra DC on the heaters (beyond the voltage from point 'A')

Yes apologies, the pin outs don’t match the schematic. They are reversed for the 2nd tube.

So I measured the cold clipper and am seeing +300v across the grid and anode... so should I just chop the cathode follower out of the mix?

[pdf64]

The CF stage grid needs a dc reference.
It came from the preceding plate in the previous schematic version (direct coupled cathode follower, see valve wizard).

[Steve A.]

I've put together a schematic I cobbled together from existing circuits on the web, would my placement of the cathode follower work, or is there anything else I'm missing?

Boy, the original schematic you used of the preamp section sure looks familiar... Yikes, it was mine!

Back then I was using TurboCAD 10.5 with a custom library of electronic symbols I drew up myself. That was like 6 computers ago and I'm sure I backed up the library somewhere but I have no idea which hard drive I put it on...

One question for the electronic experts here... is there a reason that the "bottom" noninverting phase of a cathodyne PI should be used instead of the "top" phase which is inverting?

I put a cathodyne PI in my untested Trainwreck Express "PR" Version A5i drawing which was posted strictly for discussion as it compared the Trainwreck design to that of a Princeton Reverb with the reverb circuit bypassed with a jumper. It was inspired by the report that Ken Fischer's initial pre-Trainwreck prototype was a modded Princeton Reverb. Although the specific values differed as shown in red I thought that there were a lot of similarities in the overall architecture.

In any case since the PI in my drawing is from the tried and true Princeton Reverb design I would suggest that either the inverted or non-inverted phase be used for a SE output stage. Is there any reason to use one or the other? Of course the push-pull power amp section would have to be modified for a single output tube. In particular you would want to use a different cathode resistor. A Vox AC15 uses a 270R for two EL84's; without doing the math I'd try a 270R resistor for starters, but that is assuming that you bring down the B+ a bit. I was told that EL84s are "happiest" with a B+ around 320v although they can handle more than that. You can insert a string of zeners in the CT of your Hi voltage PT windings going to ground (or in the circuit to ground from a rectifier bridge) to bring the B+ down.

My Trainwreck Express "PR" Version A5i drawing...





Steve A.

P.S. I see that your schematic does not show a center tap for the heater windings. You can create a "virtual" center tap by wiring the two heater secondaries through a 100R resistor to ground... or better yet to point A in your drawing which will "float" the 6VAC at whatever DC voltage is present at the cathode of your EL84 and reduce some noise in the preamp tubes as mentioned by Roberto in Post #9. Not as effective as a DC heater voltage to the preamp tubes but so much easier to implement!

P.P.S. So what software program are you using to create and edit your drawings? They look pretty slick!

EDIT: I redid the schematic upload because it wasn't working right.

[bw1985]

Boy, the original schematic you used of the preamp section sure looks familiar... Yikes, it was mine!

Back then I was using TurboCAD 10.5 with a custom library of electronic symbols I drew up myself. That was like 6 computers ago and I'm sure I backed up the library somewhere but I have no idea which hard drive I put it on...

One question for the electronic experts here... is there a reason that the "bottom" noninverting phase of a cathodyne PI should be used instead of the "top" phase which is inverting?

I put a cathodyne PI in my untested Trainwreck Express "PR" Version A5i drawing which was posted strictly for discussion as it compared the Trainwreck design to that of a Princeton Reverb with the reverb circuit bypassed with a jumper. It was inspired by the report that Ken Fischer's initial pre-Trainwreck prototype was a modded Princeton Reverb. Although the specific values differed as shown in red I thought that there were a lot of similarities in the overall architecture.

In any case since the PI in my drawing is from the tried and true Princeton Reverb design I would suggest that either the inverted or non-inverted phase be used for a SE output stage. Is there any reason to use one or the other? Of course the push-pull power amp section would have to be modified for a single output tube. In particular you would want to use a different cathode resistor. A Vox AC15 uses a 270R for two EL84's; without doing the math I'd try a 270R resistor for starters, but that is assuming that you bring down the B+ a bit. I was told that EL84s are "happiest" with a B+ around 320v although they can handle more than that. You can insert a string of zeners in the CT of your Hi voltage PT windings going to ground (or in the circuit to ground from a rectifier bridge) to bring the B+ down.

My Trainwreck Express "PR" Version A5i drawing...




Steve A.

P.S. I see that your schematic does not show a center tap for the heater windings. You can create a "virtual" center tap by wiring the two heater secondaries through a 100R resistor to ground... or better yet to point A in your drawing which will "float" the 6VAC at whatever DC voltage is present at the cathode of your EL84 and reduce some noise in the preamp tubes as mentioned by Roberto in Post #9. Not as effective as a DC heater voltage to the preamp tubes but so much easier to implement!

P.P.S. So what software program are you using to create and edit your drawings? They look pretty slick!


.[ATTACH=CONFIG]46485[/ATTACH]

Hi Steve,

Thanks for replying, and sorry for adapting your schematic without credit, I actually found it through google image search! But it has been very helpful in working on this project so thanks for creating it. I'm honestly just hacking these schematics together in Paint for now. I'd like to redraw everything once I have it finalized.

Also, I realized that the 40Vdc that I'm seeing is actually the positive bias voltage coming in on Pin 8 which is also tied to pin 7. I'm still trying to understand the point of this, I'm still quite the amateur amp builder but slowly improving haha.

It's funny you mentioned the virtual center tap, I was thinking of adding this to the amp. I think it will fix the issue as the sound I'm getting sounds like a ground buzz (I haven't found any other grounding issues in the amp yet).

I also need to remove the coupling cap before the cathode follower to get that DC reference back.