Bias-Vary Trem with Level Shift CF

[tubeswell]

This amp (I built a little while back) is going through trem tubes at a fantastic rate of knots. At present the CF is direct coupled to the LFO stage and the CF cathode is about idling at about 220VDC (with an HT of 426V and a cathode resistor of 470k), and the heaters are elevated to 43VDC. Rather than elevating the heaters some more and possibly risking endangering other tubes, I thought about implementing a level-shift in the CF part of the LFO circuit to get the cathode voltage down to a 'safe' level. I have no idea how much you can level-shift a DC coupled LFO and CF and still have it working reasonably well, but I assume that as long as one engineers it to keep the CF voltage swing potentially bigger than what you need to affect the output tube grid bias voltage, it will work. Is that right?

Here is my first attempt at a schematic. I have not given much thought to impedance bridging in the level shift. What are people's opinions in this regard? (I suppose bootstrapping the CF stage's grid would increase the input impedance there, so that is always another resort)

The components in particular I am thinking of changing/introducing are shaded in green and I have put questions marks after the component values and question marks after the prospective voltages.

1) I haven't done a proper analysis of the cathode resistor value required to get approx 85VDC. But my reckoning is that with the LFO plate at 213VDC, I need to knock the grid of the CF stage down to about 77VDC - which is 36.5% of 213 - and estimating that the cathode would need to be biased at about 8V above that, I reckon I'd need to aim for 85V on the cathode, but I didn't yet go to the step of calculating the cathode resistor needed to get that with an HT/CF plate of 426VDC, (Edit - I think that 100k - which I showed on the schematic - is definitely wrong; methinks now that it should be more like 180k to get 85V at the CF cathode).

By my reckoning with the existing DC coupled pair (using Merlin's cook book as a guide):

Quiescent plate current of LFO stage = (426-213)/470000 = .00045A (.45mA)

Current through LFO Rk = 2.03/4700 = .00043A (.43mA)

.02mA of current is 'missing', and the bias on CF stage is:

220-213 = -7V,

so the 'missing' .02mA of current (big deal) is being added to the current flowing through the (existing) 470k Rk of the CF.

No doubt this changes when you put a level shift voltage divider in between the two stages.

2) Also I'm not sure about the build-out resistor - so I tentatively put that at 100k.

3) The impedance bridging is probably inadequate given there is 470k plate resistor on the LFO stage, and the ra there is about 62k(? - well I'm not sure about that ra value - since the stage is biased a little on the cool side and there is a 470k load line). I guess I could use something like 4M7 and 2M7 in the interstage voltage divider to get the level shift mentioned in 1) above (i.e.; if the impedance bridging with 470k/270k divider isn't enough)? (Or bootstrap the CF stage's grid to get a higher input impedance there?) Please correct me if I'm wrong.

4) haven't calculated the proper freq for the bypass cap in the voltage divider yet - .1uF is a stab in the dark. Any opinions welcomed.

Cheers

[txstrat]

I'm not an electronics engineer and I usually copy already working circuits and put them in my amps. I sometimes tweak, but most of the time it's not necessary (a resistor or cap here and there to change the tone to some degree).
If you look at the 6G11 schem you'll find the trem to be almost identical with yours. And the cathode voltage is at 200V as well. I don't assume the voltage would be unsave, cause these amps run fine since the early 60s.
BTW: I find the 6G11 trem way better than e.g. the 6G3 trem (with only a half tube). Lusher and smoother.

[tubeswell]

I've been through 3 trem tubes in the last 3 months. Each time the symptoms are the same, it starts with a popping noise when I switch the trem (footswitch) off, and then eventually (quickly) builds up to a staticky thump that's in time with the speed control. And it is cured when I pop a new trem tube in, which lasts for okay about 20 hours, and then the same thing happens over again.

I had a word with Simcha Delft (who lives locally here in Wellington), who recommended I try the level shift. But its a matter of trial and error a bit. I have some fundamental appreciation of concepts and Merlin's book is helpful.

I do wonder about how a level shift will affect the characteristic bias balancing act that otherwise takes place in a gain stage-CF DC-coupled pair. The introduction of a voltage divider in between the two stages has to have an effect on how much grid current will be 'stolen' from the first stage's plate resistor by the CF stage's cathode resistor (which takes place in a typical DC coupled pair), and it will no doubt affect the CF bias.

I was thinking another option might be to take the HT for the whole LFO/CF circuit from a separate 10k supply resistor (that would be in parallel with the 10k supply resistor after the screen node) and use another 20uF to decouple that. That would at least lower the HT to around 360 for that circuit. Anything has got to be better than continually forking out for trem tubes

[txstrat]

What's the actual value of the cathode resistor in your CF stage? In your schem I see 100k with ?? marks.
In the 6G11 it's 470K. That's a significant difference which lowers the 6G11's gain at that stage. Also the grid leak resistor. 270K compared to 470K in the 6G11, although 470K should give the grid more drive. The slope resistor to the intensity pot (100K compared to 220K) shouldnt affect the previous stage.
Just my 2C

[tubeswell]

What's the actual value of the cathode resistor in your CF stage? In your schem I see 100k with ?? marks.
In the 6G11 it's 470K. That's a significant difference which lowers the 6G11's gain at that stage. Also the grid leak resistor. 270K compared to 470K in the 6G11, although 470K should give the grid more drive. The slope resistor to the intensity pot (100K compared to 220K) shouldnt affect the previous stage.
Just my 2C

Yeah there is 470k there at the moment - in fact it is set up exactly like a 6G9B trem circuit. The ?? marks are my guesses as to what I would do to implement a level shift for the CF. (Except that now I think that the cathode resistor there needs to be more like 180k to get a quiescent bias voltage of around -6, -7 v. with the grid voltage at 77V)

The slope/build-out resistor is currently 220k, and I'm guessing it may need to go down to 100k with the level shift implemented (because I'm anticipating there will be less signal swing)

The grid leak/lower part of the voltage divider in the level shift in the schematic is 270k, because that works with the 470k upper resistor to give the CF grid about 36% of the voltage from what is at the LFO plate. (I think this gives a total of 740k - in parallel with the 470k plate resistor for the LFO stage - as far as AC is concerned. Somebody please correct me if I'm wrong - because I intuitively suspect the bypass cap in parallel with the 470k would alter this to some extent).

If this 270k needs to be something like 470k, then I would need to make the upper resistor 820k to get the same voltage division (which I think would then make 1.3M in parallel with the 470k LFO plate resistor - as far as AC is concerned - when I go to determine the output impedance of the LFO stage. Again somebody please correct me if I'm wrong).

I was wondering what the whole effect would be on the characteristic form of the LFO wave compared to what you would get out of a 'normal' DC coupled pair. With the latter, you would get a more asymmetric wave - because the CF Rk steals current from the LFO plate one one side of the signal swing.

However with this level-shift idea, it is introducing another route for the current to take to get to the LFO plate. I guess that if the total resistance of the level-shift voltage divider is increased from 740k to 1290k, that would slightly affect/reduce(?) the overall output impedance of the LFO stage.

I am curious to get some comment/feedback what others think.

[tubeswell]

Ok since there has been no further comment for a few days, I was also wondering about the effect of using AC-coupled CF stages after the LFO.

As you can see there are a couple of options - fixed bias vs cathode biased. I have guestimated the resistor values needed to get a moderate cathode voltage from an HT of 426V.

I have never tried this before - Can anyone please comment on the the effect on the trem wave-form from using AC coupled CF (as opposed to DC coupled)?

Cheers

[tubeswell]

Bump

[tubeswell]

Well I ended-up doing an AC coupled CF stage FWIW. (See schematic)

Curiously the idle voltage with the 180k rk on the CF stage was something like 230V, and the trem sounded mighty fine like that (with a 100k 'slope' resistor).

However that wasn't achieving my objective (of lowering the h-k voltage). So I ended up changing the rk to 100k and that brought the CT voltage down to 172. With the heaters at 40V, this makes the h-k voltage about 130, so we'll see how that goes. The trem is obviously not as intense this way, but maybe the 'slope' resistor can go down quite a bit further yet (47k at the mo').

(Incidentally at Merlin's suggestion, I tried adding a 1N4007 from grid to cathode before I changed the CF to an AC coupled version, and the trem didn't work. Don't know what I did wrong. Merlin said to install it without any resistor in series, so that's what I did. Never mind undone that mod now)

[tubeswell]

I changed that CF 'slope' resistor to 26k and the trem is really intense again. I'm going to monitor the performance of this AC coupled CF in the trem circuit for a while. Hopefully I have overcome the problem of short-lived trem tubes (fingers crossed). I better stick a new recording up

[Chuck H]

I'm not experienced with trem circuits but I can say that there have been many problems with currently MFG'd 12ax7's and their low tolerance for cathode abuse. There are many vintage amps that can't use some currently MFG'd 12ax7's in certain stages for this reason. I don't know many specifics. I only know I've seen enough mention of the phenomenon to make it worth considering. Perhaps find out if the tubes you've been using may not be up to the task and, if not, find some that are.

Chuck

[tubeswell]

I agree Chuck. However I think (touch wood) that I may have a bit of a work-around with the AC-coupled cathode-biased CF running a lower cathode voltage. With the slope resistor at 26k it is sounding pretty intense/tremtastic. I just want to monitor it for a while and see how it stacks up.

As I said earlier, the heaters are elevated at about 40V and the CF cathode is now 172 at idle, so that's a net difference of 132, which theoretically brings it under most manufacturers 12AX7 specs (of 180 hk voltage).

I have taken a couple of weeks off work (summer vacation over here), and I have been busy tinkering with this the last couple of days (as well as building a BFPR clone - but that's another story), and tomorrow I thought I'd mic it up and burn some .mp3s.

Cheers