I have nothing of value to add here yet, but I've been wanting to build a Sta-Levelish for awhile and love that this uses such a basic (and cheap) all-octal tube compliment. I'll be tinkering with this circuit when I get some free time (probably sans the M/S functions). But please keep us posted about your progress! I'm recentish to the forum and gather that Ian's PS PCB's are long sold and not in the group of files he generously open sourced. Does anyone know if those are available anywhere or if there's a similar solution floating around? They would be handy for a couple tube projects I'm sketching out. Thanks!
PureAnalog M/SLevel Vari Mu Compressor (manual + schematic)
- Thread starter zzzzz
- Start date
Help Support GroupDIY Audio Forum:
scott2000
Well-known member
Maybe check out ghr.fi
rock soderstrom
Tour de France
Thanks!
chrispbass
Well-known member
Ok, so quite taken with the idea of this one (thank you @zzzzz) and gathered some parts over the last few months. Decided to have a couple of hours today to start roughing it out on ones chopping board.
I’ve had 2 nice Earnest Turner 1mA meters in the parts drawer for eons…and they need a home.
Plan is to make a stereo pair. I’m going to adapt a PSU pcb I’d designed for another project, run DC heaters and generally/hopefully upgrade the psu.
Regarding Tx’s, I’m thinking of using a UTM on the output 15K:600
https://utmindustry.com/utm3580/
And try an oep A262a3e?? Or 1:4 on the input.
I need a 15k:600 for another project and have Oep’s and 1:4 in the parts drawer so not too much of a gamble in that respect.
Thoughts?
I’ve had 2 nice Earnest Turner 1mA meters in the parts drawer for eons…and they need a home.
Plan is to make a stereo pair. I’m going to adapt a PSU pcb I’d designed for another project, run DC heaters and generally/hopefully upgrade the psu.
Regarding Tx’s, I’m thinking of using a UTM on the output 15K:600
https://utmindustry.com/utm3580/
And try an oep A262a3e?? Or 1:4 on the input.
I need a 15k:600 for another project and have Oep’s and 1:4 in the parts drawer so not too much of a gamble in that respect.
Thoughts?
Mine has OEP’s on the input (I think) in the shielding cans they sell for them. The builder went to great trouble to obscure them. They sound good.
It’s really a great sounding unit as is. I use it nearly every day (almost always in mid side), to the point where I can’t find any downtime to rip it apart and rebuild the PSU lol…
chrispbass
Well-known member
I’m building without the MS matrix. Given the range of Oep’s, I think the a3e is the most likely candidate…But I could be wrong. DC resistance of primary and secondary would confirm if you have a mo.
Also ordered some of your “api” make up gain boards Z…thank you again
chrispbass
Well-known member
I wouldn’t go that farso I'm thrilled that sometime else smarter than me is taking it on first.
, but I’ll do my best! Will be interesting for sure.As you can see, it’s just being ‘bread boarded’ for now, before committing to a case etc. I’ve got some tag board/strip coming this week so I can get on wiring up the main circuit PTP, and that’ll be a a new experience for me.
Next manoeuvre is to fashion a bracket for the valve bases from some aluminium angle in readiness for the PTP work.
chrispbass
Well-known member
Made some progress today. Fortunately I have a fully equipped and heated metal fab shop Luckily I beat the rain!
And decided to go for a full “back panel” along with XLR’s and IEC.
And wired up the mains and heaters in readiness.
Luckily I beat the rain!And decided to go for a full “back panel” along with XLR’s and IEC.
And wired up the mains and heaters in readiness.
chrispbass
Well-known member
90 mile an hour winds here in the UK today, and as one is feeling a bit snotty and grotty, best isolate in the lab dosed up with a glass of Chardonnay!
Tag boards arrived earlier so moving forwards. Each stage built on a separate board to make it modular in a way to hopefully make testing and learning easier.
You are live in the lab with Chris P Bass!
Tag boards arrived earlier so moving forwards. Each stage built on a separate board to make it modular in a way to hopefully make testing and learning easier.
You are live in the lab with Chris P Bass!
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chrispbass
Well-known member
Had a little play with oep I/P tx.
If we are indeed dealing with OEP A262 series, then the A3E does indeed seem the most likely candidate, so I’ve put him in his little house (good job in this weather).
If we are indeed dealing with OEP A262 series, then the A3E does indeed seem the most likely candidate, so I’ve put him in his little house (good job in this weather).
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I'm trying to understand the necessary specs for the output transformer. Would the Sowter 9530 be a suitable choice?
chrispbass
Well-known member
As far as I’m aware (where I’m at), a typical O/P tx for this type of application would be a 5:1, or 4:1 un-gapped without a feedback winding. Z mentioned that his looked like an Edcor. WSM/XSM probably?!
The UTM I linked to earlier, should be here by the end of the week, so will post when I’ve done some experimenting.
Edit: looks like it prob would be ok.
Do you already have one?
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chrispbass
Well-known member
And a small update for anyone interested.
Current state of play…
Current state of play…
Edcor would certainly be less expensive, as would the UTM you're using. I'm considering the options - I probably won't be building this for awhile yet, but I like to start thinking about a project that excites me even if it's not on the bench. I'm looking at the Sowter in the event that I end up mounting the transformer externally on the rear of the chassis and want some shielding (again, just considering various options and layouts).
I'm enjoying following your build progress!
chrispbass
Well-known member
Understand and thanks!
I'm really roughing this out at the mo, and trying not to get too caught up in detail (too many years of that), however, I'm trying to grab hold of items in the 'swirling cloud' around my head, and tame them until I own them.
I/we will share the lows and highs and together we will crack it!
chrispbass
Well-known member
Continuing the journey, I built up a psu today and began testing voltages etc!
Heres the schematic with Z's measured voltages which show some differences from the original schematic.
When measuring mine, there are a few minor discrepencies (to be expected), but pretty close, however there are a few things to sleep on with regards to the 6ss7 remote cut off valves. I'll offer more info on my 'protoype' over the coming days.
So Using the schematic with Z's voltages, I began to calculate plate currents (which tbh, I've never done before) and if I'm peeing in the wind please correct me!
Lets start with V3 (6SN7).
So in steady state, we have a drop of 245V - 72.5V across the plate resistor (100k) = 172.5 volts. Divide by 100K = 1.725mA plate current.
This gives a cathode resistor voltage of 1.725mA x 1.5K = 2.5875 Volts which is pretty close to the measured value on the schematic.
Simiarly for V4 (6SN7) we have a voltage drop of 245V - 243.4V across one half of the O/P Tx primary (plate resistor)
Not knowing exactly which Tx is used (Edcor WSM likely), but I just looked up the data for the UTM I'm planning on using which has a primary resistance (DCR) of 370 ohms so used that. Dividing by 2 = 185 ohms
so we have 1.6V divided by 185 ohms = 8.65 mA.
This gives a cathode resistor voltage of 8.65mA x 1K = 8.65 Volts
Checking the plate characteristics on the data sheet.
These seem to tally with the graph although, V3 sits at the lower end of the curve (less linear). Im not sure if this good/correct etc. There is negative feed back to V3, and although I understand the basic principles...I'm not going to go there at the mo!
So on with V1 and V2 - the remote cut-off valves used for gain reduction (6SS7).
We have a voltage drop of 89.5 - 50 = 39.5 volts across the plate resistor (10K), giving a plate (anode) current of 3.95 mA.
Looking at the schematic, there is a shared resistor in the cathode path of 220 ohms. I may be wrong but if there is 3.95mA going through this, then the current must be doubled when calculating the cathode voltage.
This will give 2(3.95mA x 220 ohm) + 1(3.95mA x 68 ohm) = (1.738V + 0.2686) = about 2 Volts.
These are the plate characteristics for the 6SS7. As you can see, its not following the graph, but in this application its a pentode connected as a triode, and there is no graph for that. I dont know how radically the specs will change.
One thing that did occur to me is the meter. At FSD (zero dB reduction) it will be drawing 1mA through the divider of 220/68. Does that have any effect on the cathode biasing?
Heres the schematic with Z's measured voltages which show some differences from the original schematic.
When measuring mine, there are a few minor discrepencies (to be expected), but pretty close, however there are a few things to sleep on with regards to the 6ss7 remote cut off valves. I'll offer more info on my 'protoype' over the coming days.
So Using the schematic with Z's voltages, I began to calculate plate currents (which tbh, I've never done before) and if I'm peeing in the wind please correct me!
Lets start with V3 (6SN7).
So in steady state, we have a drop of 245V - 72.5V across the plate resistor (100k) = 172.5 volts. Divide by 100K = 1.725mA plate current.
This gives a cathode resistor voltage of 1.725mA x 1.5K = 2.5875 Volts which is pretty close to the measured value on the schematic.
Simiarly for V4 (6SN7) we have a voltage drop of 245V - 243.4V across one half of the O/P Tx primary (plate resistor)
Not knowing exactly which Tx is used (Edcor WSM likely), but I just looked up the data for the UTM I'm planning on using which has a primary resistance (DCR) of 370 ohms so used that. Dividing by 2 = 185 ohms
so we have 1.6V divided by 185 ohms = 8.65 mA.
This gives a cathode resistor voltage of 8.65mA x 1K = 8.65 Volts
Checking the plate characteristics on the data sheet.
These seem to tally with the graph although, V3 sits at the lower end of the curve (less linear). Im not sure if this good/correct etc. There is negative feed back to V3, and although I understand the basic principles...I'm not going to go there at the mo!
So on with V1 and V2 - the remote cut-off valves used for gain reduction (6SS7).
We have a voltage drop of 89.5 - 50 = 39.5 volts across the plate resistor (10K), giving a plate (anode) current of 3.95 mA.
Looking at the schematic, there is a shared resistor in the cathode path of 220 ohms. I may be wrong but if there is 3.95mA going through this, then the current must be doubled when calculating the cathode voltage.
This will give 2(3.95mA x 220 ohm) + 1(3.95mA x 68 ohm) = (1.738V + 0.2686) = about 2 Volts.
These are the plate characteristics for the 6SS7. As you can see, its not following the graph, but in this application its a pentode connected as a triode, and there is no graph for that. I dont know how radically the specs will change.
One thing that did occur to me is the meter. At FSD (zero dB reduction) it will be drawing 1mA through the divider of 220/68. Does that have any effect on the cathode biasing?
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KrIVIUM2323
Well-known member
Does it have an effect on cathode biasing?
Yes it does but it's minimal, main purpose is to have a bit margin for zeroing the meter and keep it zeroed when tube age.
Your math seems spot on to me.
Yes it does but it's minimal, main purpose is to have a bit margin for zeroing the meter and keep it zeroed when tube age.
Your math seems spot on to me.
chrispbass
Well-known member
Thanks K.
Yes I suppose it’s only adding a very small voltage across the 220 ohm resistor and thanks for validating that I’m calculating using the correct methods…that was the main thing.
Small steps on a long journey etc!
Hopefully will help someone else on that journey too
