PMsix61 Limiter DIY

[alexc]

My first attempt at measuring the output impedance of the control amp with the tertiary winding connected as described above.

- open-circuited the secondary (disconnected 4K7 load resistor and the parallel bridge network)
- applied 1KHz sine to control amp input measured the O/C voltage across the control amp output secondary of around 16Vpp
- connected a 2K5 3W pot with left and centre terminals strapped across the secondary
- adjusted pot to 3dB down on 16Vpp which is 11.3Vpp
- measured pot resistance

I see 195 ohms as the pot setting.

Dave, could you correct me if it's not the right procedure 

Thanks

 

[DaveP]

Alex,

I thought it was 6dB down, I.E, half.

You can also use this formula:-

Zout = Rtest (Vout - VRtest)/VRtest

So leaving the stock 3.9k in place, you simply note the change in voltage when adding another test resistor (say 4.7k) in parallel.
Then put it in the formula

For example:-

Say Vout is 16V, when you put 4.7k in parallel it goes to 15.65.

16-15.65=0.35

0.35/15.65 =0.0223642

0.0223642 x 4700=105 ohms Zout

best
Dave

 

[alexc]

Thanks for that - tomorrow I'll be looking more at the nfb, output Z and the effect of connecting the bridge.

So far, so good.

Cheers

 

[alexc]

I tried a few different signal levels and measured the Zout  - looks like it's around 250ohms approx.

That seems reasonably low so I'll leave the nfb network as is for now.

Bridge seems to be working, but my max CV seems to be too low at some -40V or so.
It was a lot higher but that was when there was no nfb, so I guess I probably have too much nfb going.

Moving along to the signal amp, with no CV applied, some early indications are

- cathode balance is working nicely (no tube matching done at all)

  ie. basic cathode dc voltages can be trimmed to equal quite accurately

- I need to figure the right value of each phase cathode common resistor to balance pot

  ie. my balance pot is 300R not 100R as in the original
      I figured I need then total each phase cathode resistance to ground of 730R at mid-adjust so 580R resistors required
 
  Using 470R is giving me way too high a cathode voltage. So I am working on that.

- the PSU regulated HV is holding up fine when supplying max 'idle' quiescent current at the GR tubes of 100mA total.
  Regulators aren't getting too hot when cathodes set for 50mA per phase
  Rails are dropping around 3V or so at full rated current which is pretty good

  ie.  PSU traffo can do 200mA total which is 100mA for side chain HV and 100mA for the GR, 50mA for each phase


- Indications are the 'idle' signal amp gain is pretty big. At 28mA per phase (1/2 my intended idle current) :
   
      2Vpp input to signal amp with no atten at the t-pad is giving me 44Vpp output, x22 gain (27dB)
      distortion on the signal amp is quite low even at reasonably high output levels
      symmetry on the output is quite good so far

- mains harmonic noise components are a little high right now at around -80dBu for 50,100 Hz with no grounding optimisations or 'praying to the stars' as yet.

- mains harmonics noise components are reasonably independent of  signal amplitude and seem quite stable  ie. not 'twitchy'
  So those are good signs.

Anyway - still issues to resolve before applying CV to signal amp :

- find the right amount of nfb in control amp and iron out some possible instability
- figure out the right signal amp common cathode resistors to balance pot so as to achieve the desired idle cathode bias

That's enough for today. Have a bunch of things to think about.

 

[DaveP]

Sounds good Alec,

If your Zout is 250 ohms then your attack time will 2.5 times the Fairchild's (100 ohms).

On the signal amp, it's the currents that need to balance not the voltages or you will have dc in the output TX.

The signal amp tubes are pre-biased to -6.2V from a separate -ve supply to get them onto the "straight" section of the gm curve.

I think the CV should be able to go down to at least -70V

best
Dave

 

[alexc]

Thanks Dave  - it is moving along but there are a number things not working right.

That cathode voltage I mentioned is actually voltage(s) across the common-cathodes-to-balance-pot resistance of 470R.
So that gives me my cathodes-current-per-phase and they match pretty good at this one GR tubes gain setting.
It's a start

I'm using the 670 style of biasing - I have a variable -ve reference locked with the 'analog clamp' feeding the cathodes-balance-pot (other side grounded) as well as a seperate variable -ve reference at the common of the bridge which sets the minimum CV to the signal amp transformer CT

I should complete my first pass thru the whole thing tomorrow then start a more detailed pass to correct my issues.
I'm getting a better feel for the detail of how it's meant to work so with some luck I can get there soon.

I'll be having a break for a few days after tomorrow, which is probably a good idea!

 

[alexc]

Little bit more progress but not much - added some balancing resistors across the secondary of the signal amp transformer with the CV to the junction of the two resistors.

That certainly tamed the large cathode voltage I had previously - the grids were more or less floating before.

I also connected up the CV but disabled the input to the control amp, so I only get a CV which is a dc voltage comprising the output of the bridge/TC network with just the -V ref applied.  I started with -6.5V or so as the bridge ref voltage.

I have a 10K secondary so I started with a pair 4K7. I did a quick check of the signal amp output and noted the amplitude.
I then tried some 150K resistors and checked the signal amp output amplitude again.

Although the GR tubes current didn't change much, the output amplitude was much lower with the 4K7 pair as compared with the 150K pair. It didn't appear to be an issue of loading the primary of the signal amp traffo but instead an issue of the voltages around the balancing pot.

Not yet sure of that, so add that to my list of things to understand

I'm just having a look at some other designs by way of comparison - the original 660 has 100K but the secondary impedance of it's traffo is a lot higher than 10K which I have.

Apart from that, I satisfied myself the CV is working to reduce the gain as expected when the control amp has signal.

The interaction between CV-no-control-signal, cathode balance pot reference voltage and cathode currents is fairly interesting, so I'm playing there for a while to understand that some more before settling on my best guess values.

The 'analag clamp' is doing what is expected - locking the voltage applied to the balancing pot independantly of cathode currents. It's not moving at all. I had it sinking the 100mA max idle case with no troubles. I have a heatsink that can handle 5W or or so dissipation. It gets a little warm but not hot.

Now, to start again at the top!

 

[alexc]

Looking again at the control amp BH7 feedback, I measured again the Zout at the output of '70V-common' taps of the Hammond
(reasonably close to a 600R winding - edit: more like 800ohms. The voltage step down is around 620/200 and primary is 8K) terminated in 3K9 1W (stock for a 670)  and the bridge disconnected

Using a 1% resistor substitution box and with the best resolution I can on my CRO, I now get 125R which is closer to the orig. Method is to add a parallel resistance such that the output signal voltage drops 6dB.

That is with the stock 2K7 series resistors in the feedback connections. I also get 2.33V at the BH7 cathodes, which is a bit high compared to the 2.6V of the original.

Tried increasing the 2K7 to 6K7 -> increased the Zout to  around 250R and decreased the bias to 2.63V
So I'll stay with the original 2K7 series resistors.

Doing some more checks with the bridge.

 

[alexc]

Have been spending more time looking at the control amp and my problems therein ..

I have checked eveything over and over - pretty sure I have everything wired up the way I intend.

My major malfunctions are :

- I can only get some -38V dc max CV
- the range of CV achievable is very narrow  ie. around  -15V to -38V dc or so

My goal is :

'smooth' variable CV from some -3V to -70V dc or so

Some observations and issues I have noted :

1. the gain staging of the control amp is very sensitive.

- the 6V6 finals have a bias of around -14V which would indicate a max input of around 26Vpp or so

- the BH7 driver has a gain of around x5 and a bias of around 2.4V dc with feedback applied to the cathodes
  That would imply a max input signal of around 4.5Vpp or so for a max output of 19Vpp or so
 
  I have indeed seen that.

  Any higher input causes clipping on the feedback signal which in turn makes the BH7 output 'weird'

- the AX7 pre-driver has a gain varying from some x9 down to x4 or so depending on amplitude of input and bias

So for a 1Vpp typical input to AX7 (which is pretty small), I can easily end up with 30Vpp or so at the grid of the 6V6
which clips the 6V6 output, clips the control amp transformer feedback to BH7 and the whole thing gets completely out of hand. 

2. the input to the control amp needs to be small when the 'dc bias is trimmed to 0V)

- as I said, even with feedback applied, the control amp is pretty sensitive

- the input to the control amp is :

    signal amp -> resistive pad -> control amp transformer primary -> step up at secondary -> potentiometer -> grids

A typical output level from the signal amp, for me, is around 8Vpp or so
Not crazy over the top loud, not pissy either

The attenuator on primary is something like say -6dB
The 600:10K stepup is some +12dB
The variable potentiometer is 0 -> -24dB.

That blows the hell out of the control amp such that is is basically 'off' or 'clipped to the max' and not much in between.

I find I need to feed a very small input of some 2Vpp or less to get any thing approaching what I expect.
And then, I see the CV vary only really from -20 to -38V dc max

Again - this is when operating with 0V dc bias applied  ie. in class A push pull, not class B which is the typical operational case

--------------

Easy error points I'm sure I've got right are :

- phase of feedback to BH7 (reversing causes insane positive feedback driven gain)
- basic biasing of each of the 3 stages
- stepped threshold potentiometer is as expected with 100K max resistance, linear 0-24dB is approx 1dB steps
- wiring of control amp primary, secondary and tertiary
- feedback and output impedance of control amp with no bridge at 125 ohms

--------------

So I am looking further at :

- the terminating resistor at the control amp output transformer secondary
- the bridge circuit
- the CV to signal amp input transformer network
- feedback factor from tertiary winding to BH7
- local feedback at the 6V6 plate to grid network

as well as further characterising the control amp in general

The good news is things are broadly doing what I expect - PSU rails are good, amps are 'amping', CV is 'GRing' and so on.

But I'm sure something pretty major is not right in the control amp.
Anyway, I like a challenge

Cheers

 

[DaveP]

Alex,
Have you got the timing components attached to the bridge yet?  Maybe they should be attached to soak up some power, 2uF is a pretty big load. ???
best
DaveP

 

[alexc]

Yes, I do have the timing components attached to the bridge.

I did some comparisons with the timing components attached and not attached - they did do the expected 'smoothing' of the voltage waveform but did not change the max amplitude of the 'unsmoothed' voltage waveform.

But I'm not convinced I'm getting the right rectification, so I'm going to replace the bridge with some fresh diodes.

 

[alexc]

Success!  8) 8) 8)  Bad diode(s) meant only 1/2 wave rectifcaze. Hence only 38V dc.

Replaced and bingo - perfect -87Vdc CV rock solid.

Now I have to find some better diodes. I have 1N4148 which are a little lightweight. The original uses 1N538 according to my schematic, which are much higher voltage ratings -  so need to find a good replacement that can handle up to the 200Vpp which the control amp transformer can output.

Anyway - now to decide on the appropriate sensitivity of the sidechain. For now I have like Dave, a pad at the control amp input transformer primary of 300+1K3+300.  That gives good control, along with my 24dB threshold pot for input signals of around -24dBu or so applied.

I will need to up that pad I think - should become clearer now as I play with the signal amp and set the dc bias to a more typical operational value.

OK - so now I have a working control amp with a heavy, heavy punch!

---------------

The time constants are working well, as I can confirm 'visually'. The faster release is indeed faster, and the slower surely slower. Same for the attack. I have 2 seperate switched networks in parallel for release and attack. I will tune those later.
I can clearly see the CV can very quickly go to the full 85Vpp or so with my current settings.

It is nicely controllable with the Threshold rotary switched attenuator to a minimum value which depends on the net amplitude of the control amp.

-----

Now examining the the signal amp / CV operations. So far all I can achieve is signal amp shut off  

I have a tiny applied signal of 350mV pp and with the 'Level' t-attenuator at max signal I get some -23Vdc CV.
As I reduce the signal into the signal amp, my CV does go to my -ve bridge reference, which is as expected.
The 'dc bias' setting does indeed set the threshold of control amp 'amping'; as it in increased, only the peaks of the control amp signal are amplified.  All good.

Now a question of balancing the CV range with the desired signal amp range and dc bias range.

Once that is in the ballpark, I will spend some time optimising the ground scheme to get noise floor as good as I can.
There is no scope for transformer moving or rotating in this build - it will all come down to the placements I have and what I can achieve with wiring etc.

 

[alexc]

Something BAD I can't explain - I have a +1.5V to 3.5V dc offset identical on each phase of the signal amp output transformer secondary with respect to ground. It is independant of signal amplitude and is same with no signal.

WTF? Never seen that before and I am mystified.

I have been running the signal amp loaded with 600R 5W resistor. It happens open circuit as well but worse +3.5Vdc on each.  On power up, it increases at about the same rate as HV rail. On power down, decreases similar rate to HV.

I have some others for my other vari mu build, so I'll have to replace it and check.

-> OK I checked with another 8K:600 edcor - same thing. WTF?  Am I missing something?

The secondary is completely isolated from any other circuitry - open circuit or terminated in 600R. with signal or no signal.
I get each phase with identical dc voltage of up to 3.5V.
Checked with 2 different meters and the cro, all with respect to ground.

Can't be any kind of inductive coupling - no signal still shows issue. (possibly could be a 50Hz supply noise thing - but I didn't see any indication of noise signal on the CRO - just a solid dc charge).

So it's a dc thing - either resistive, meaning some kind of insulation breakdown, which is unlikely (tends to be an all-or-nothing affair and I also get it in 2 identical units) - or a capacitive thing.

If it's capacitive between the secondary to ground (as indicated by the presence of a charging type voltage wrt ground on powerup and a decaying type on powerdown) then grounding the traffo chassis may help.

It doesn't have a dedicated core/frame ground connection, so I'll try cobbling one up with the frame bolts and chassis connection.

I also have a Sowter output traffo which is reasonably beefy - I'll toss that in and check if that is similar. I believe it has proper traffo chassis ground connection.

I need to figure it out because I'm not keen on dc-to-ground either at the control amp input or my Motu converter inputs 

If anyone could enlighten me I would be grateful.

As usual, 2 steps forward, one step back!

 

[alexc]

On the issue of the signal amp output transformer secondary floating with a dc offset, grounding the traffo frame did nothing.

As might be expected, when connecting the winding to a ground referenced circuit, like the inputs CRO or my Motu, it shunts the offset to ground, so it disappears. I'm assuming the Motu has cap-coupled input, so doesn't seem to be any problem there.

So probably not an issue to have a floating voltage when measured wrt ground. Anyway, I'm going to keep an eye out in the future when I do other similar applications.

For now, I move on

 

[alexc]

So today, I am focussing on setting up the operating points of the signal amp.

To facilitate this, I'm driving the control amp input from a signal generator instead of the signal amp output.
I'm driving the signal amp with a different signal generator.

That way I can set the CV applied to the signal amp independently of the the signal amp.
Will facilitate characterising the signal amp as well as the control amp

-----

Just playing about with the signal amp where there is no control amp signal.

Trying to achieve an idle current of around 12.5mA for each triode-connected pentode 6K4P for a total of 50mA per phase and 100mA all up at +135V regulated. That's the max current my PSU can safely supply I figure (not yet fully confirmed!)

It should happen at grid-cath difference voltage of -3V approx according to my datasheets.

So I have several variables to set the idle - similar to orig 670 :

1. CV (no signal) reference                          ie. the bridge                -ve reference voltage, trimmable 0V to -17V
2. cathodes balance pot reference voltage    ie. the cathode-bal pot  -ve reference voltage, trimmable 0V to -15V

Those two trimmable reference voltages acting thru the dual cathode-bal pot + feeding cathode resistor set the overall idle current.

Phew - quite a mind bender when you sit down and try to follow it.

-------

Right now, the upshot is I need to vary my feeding cathode resistor from the current value of 470R. The range of control from the trimmers is too narrow to get me to the desired idle. I noted previously that no resistance caused my idle current to go up a lot, so I'll try decreasing first.

The original 670's 6386 tubes idle at 24mA per phase, +240V B+ and some -19V grid-cath difference.
Also, my balance pot is 350ohms not 100ohms (what I had on hand).

So I get :

470R -> 13.8mA
235R -> 24.4mA
120R -> 31.3mA
47R  -> 41.4mA
24R  -> 44.3mA

And I'll stop there - things are warming up on the HV reg heatsinks and the cathode clamp heatsink.
Just warm, nothing alarming.  Thats around 90mA of HV for the signal amp.

I did a check with an ammeter on the +132V HV rail feeding the signal amp : 91.1mA so all as expected.

I also notice that the grid signal waveform is starting to flat top on the positive going peaks.  Clipping input by grid current probably.

Now the timmers do affect the current but there's a lot to it than that, so I'm going to give it all some thought before I go too far further.

Just out of interest, the signal amp gain structure is something like this (CV is at -Vref  -3.0V, cathode-bal ref -10.0V) :

1.58Vpp input to XLR (phase to ground) -> 1.58Vpp at sig amp traffo primary  (phase to ground, Level t-pad at min atten)
8.9Vpp at sig amp sec (phase to ground, x5.6 step up traffo is 500:15K)

Gain with various loads :

open circuit : 67.1Vpp or 17.5dB
10K            : 67.1Vpp or 17.5dB
1K              : 51.1Vpp or 15.1dB
600R          : 43.9Vpp or 13.9dB

The cathode is  at  -0.015V wrt ground, the grid at -3V wrt ground and the cathode-grid difference is -2.985V (measured).
So that matches pretty well the long-since-contributed 6K4P curves triode connected which say 11mA at 130V for -3V bias.

So these are some first numbers for the signal amp. Seems a little low in the gain dept so far!

Just out of interest, it looks like around -45V of CV pretty much shuts the signal amp off.
I ended up using 56K+56K resistors across the signal amp secondary with CV feed to the center of that resistive network

Now to stop and think about it all for a bit. 

 

[alexc]

So tomorrow, I'll be doing some more tweaks to the signal amp.

Based on these initial observations , looks like signal amp gain will be a bit on an issue.

I'm going try to maximize signal amp current to the limit of what I have available.
ie.  bias the side chain to use around 85mA so I have a max 115mA for the signal amp at idle.

To 'squeeze' the most GR capability out of the signal amp, I'll bias the idle to 14mA each.
Not so much to burn up when not compessing, but as high as I can for as much 'starting gain' as I can get.

I am hoping to get something like 20dB of idle gain into 10K (my intended load).
I like to use up to 20dB of gain for instruments (as as EFFECT alright already!) and still have reasonable output. 

Failing that, I may downshift the 6V6s to 6K6s and 'gain' some 10mA there.

If I were doing it again, I'd find a PSU with more regulated HV current - like 150mA and run these GR tubes harder.

Of course, a signal amp output transformer more 'designed' for the application would help enormously too.

And doubly of course, this all pre-supposes that I can achieve a noise floor such that 20dB of gain leaves me still with an acceptable noise level.

That would be my PMsix661 Mk II    If I had the where-with-all, I'd buy emrr's super A-26 for it's input.  
And a Sowter for the output.

But alas, I am but a mere mortal and I'm already drowning in a sea of prototypes.

Maybe I *should* try paper designing more! 

Nah. Where's the fun in that ? Only serves to illustrate what you *should* do if you had more cash.

The real challenge is to make what you can afford do as close to what you *think* you want : ;D

 

[DaveP]

Hi Alex,
I like how you've gone about the problem solving...nice work.
If you PM me your email address I will send you the manual for setting up the 660.  I can't post cos its an 8.5Meg pdf.
best
DaveP

 

[alexc]

Thanks again Dave   

Before I go any further, I need to work on the noise floor.
No point in having more gain if I'm getting a lot of hum with what I have now.

And I am. 

So - now to prod, poke, jiggle and reseat. Looking for source of the noise and focussing on the grounding.

---

First things I can see is there is a little hash on the ground, some 1.4mVpp of 50Hz and 100Hz mixed and that corresponds to around -64dBu right there. That stays pretty much same at the various local ground points.

Next I look at the ripple on the heaters : the regulated heater  (2.4A of 6.3Vdc) is not so good with 219mV of 50/100Hz  and the unregulated (1.8A at 6.3Vdc) is better at 100mV. So I need to improve the unregulated at least.

Regulated has 20KuF before the pcb, 10KuF + 0.22R before the regulator, 10KuF after the regulator.
Perhaps I can rejig this - I need to do some PSU sims and review some other results to check.

Unregulated has 10KuF + 1.5R + 10KuF and is about what I would expect.

HV ripple at the +132Vdc is quite low at 3mVpp or so.

The CV (no signal) has about 4.5mVpp of ripple

The grids have some 3.5mVpp again a mix of 50/100 Hz. That is off an input transformer with x5.5 step up.
The signal amp alone is doing around x5.

The plates have a lot  63.6mV. Now that is more than 3.3x what I would expect based on the grid noise and gain.

The XLRs have around 19mVpp each phase wrt ground, which is the x3.3 step down of the output transformer.
Across the phase is much higher (CRO has one side grounded) at 36mVpp or so.

Normally I have a noise floor of around -83dBu loopback with my Motu, calibrated to absolute voltage.
With the 14dB or so of gain into 600R, I would expect that to rise to maybe 73dBu or so.

I'm seeing a signal amp noise floor of around -41dB or so, that's thats a mighty wallop of noise!

Either I have a major ground loop or it's heater ripple. Or both

-----

 

[alexc]

I rearranged the regulated heater circuit with the 20KuF on the output side rather than input side.
That dropped the ripple to around 100mA - the same as the unregulated. Didn't help much, tho' 

Also noted that applying strong CV to the signal amp, which shuts down the gain, doesn't change the noise much.
Same with briefly shorting the grid to ground.

Tried some temporary steel shielding between psu traffo and signal amp - no difference there.

So it's not anything inherent in the signal amp operation or even induced psu noise [WRONG!]

I also did some rough freq response testing just to make sure I don't have a generally rising hump at the low end due to bad terminations and what not - nope. Low freq response with signal dips a little at the low end as I would expect.
----

A merry chase indeed - successively disconnecting all the psu sections and I stil have some 19mVpp across my output
Even when the unit is disconnected from power! Enough to show a noise floor of -49dBu.

It was the soldering iron nearby

OK - repeating the exercise with no nearby devices powered up.

With all secondaries of the psu disconnected, I'm still seeing -60dBu across the output XLR. That's a worry!
The output traffo is 8K:600 connected to balanced Motu, probably 10K or so input impedance.

Looks like some pretty significant coupling of psu traffo into the primary of the output transformer then to the XLR output.
I can see the voltage at the plates even though all the internal circuits are powered down.
Some 0.414Vrms or 1.242Vpp of hash just by powering up with disconnected psu secondaries.

So I've taken my other 8K:600 and connected secondaries to my Motu in the same way and doing some checks of induced noise nearby to my rack unit. This is a completely discombobulated traffo just hooked up to Motu and moving around near the rack unit with the primary of it's PSU powered up.

Same thing exactly.The high-ish primary impedance coil is picking it all up. Short the primary  together and it disappears. Loading primary with anything more than 1K or so and the pickup goes very high. Even more than 50 cm away. Changing the orientation doesn't achieve much at all..

Did the same check on the Sowter 8650e output from the 175b build - it's much higher primary impedance.
With open circuit primary, and there is only small pickup at at 25cm, maybe 2dB or so.

At same 50cm distance as the Edcor, < 1dB pickup and mostly at the fundamental, not all the harmonics as the Edcor does which seems to be  - more like 18dB pickup SPL all the way to 2KHz or so.

Maybe I'm missing something here - I have used these reasonably big PSU traffos before with no big probs.
My big edcor tube amp pp output traffos don't seem to give me  hum problems .

But this is my first of these 8K:600 and so far I am wondering where to go from here.
I have models in 2 different frame types, so it's not just a one-off thing.

Perhaps these 8K:600 are fine when connected to a 2K primary source and loaded down to 600R on the secondary.
But in my application, I figure the source Z is much higher and the intended load is also much higher.

All that in addition to the inexplicable (to me) but utterly reliable floating dc I am seeing on the secondary makes me think - "boy, you f**kd up". Why do I keep hearing 'you gets what you pays for' ringing in my head ?


Cripes! I need to lie down!

 

[alexc]

After a brief rest I really do have the feeling that this is not going to end well. 

I have a lot invested here; giving up is not an option

So it's back on the hunt for a suitable output traffo, I guess. I'm not at all sure a high ratio primary would work here (still quite ignorant really), so the Sowter 660 type may not be a good choice.

And I don't really know how much primary dc it can take, although I think it would take a fair bit, with a bunch of 6386s feeding it.

Usually something that can handle 40-50mA dc (balanced hopefully!) per side is used with a  6V6 pp or higher. Like rated 10W to 20W. Beefy enough to take some 10% unbalanced dc that likely will happen.

The one I'd like to try is of course the UTC LS series. But $$$$$, even when compared with $owter
These guys are unusual in that they are plate-voice coil (meaning beefy) but also with the line 600R tap. Not easy to find.

Funnily enough, the only other cheap beefy one that comes to mind with a line tap is the Hammond again!
5K primary is too low for comfort, though. Although loaded up high it might work.
Almost worth trying, as I could pretty easily divert the control amp output traffo for a look-see exercise on the signal amp.

At least I can use this one to learn more about the conflicting impedance match issues at the min-GR (max current, min plate resistance) and max-GR (min current, max plate resistance) conditions.  I still don't fully grasp that part of things. But I am getting there.

Apart from the extra cash to finish this one (more things to sell, I think!) I'm wondering if my dual rca type builds, which also have this traffo, will work. I have three of these traffos and thats around $200 so far. To replace with good stuff is going cost! I think the single Sowter cost me around that.

I guess I'll find out soon enough.

Tommorow, I'll sub the output traffo with a couple of others I have lying about to check for induced psu noise as well as to get an idea of what  different ratio traffos do in this application.