ding

Another Altec 436c/SR124 p2p :p
« on: December 10, 2015, 12:02:28 AM »
There are so many threads on this but I wanted to start a new one because the other ones are very old and not too detailed when it comes to p2p. My hopes is to document my build as I go along and hopefully it will be useful to someone.

I want to do my first p2p/tube project and I decided on this one. I've been reading alot about tube circuitry lately and a bit of it is taking me some time to digest. I am certainly not very informed on vari-mu compressors so I decided to start by learning Kicad and drawing up my first ever schematic to at least try to understand where I need to go and learn something useful. As things always go I did way too much reading and research and the original simple 436c started growing with mods and other things until it became not so simple any more. So I guess a schematic was a good idea after all.

Below is the schematic I did. I'm not sure If I went overboard or if I was able to implement all the mods correctly so hopefully someone will spot some of the many errors in there before I start on this.

Things I'm not sure of...
1a. Is the threshold implemented correctly?
1b. Do I even need a threshold (is it useful for this design)?
2a. Is the balance circuitry implemented properly (neon bulb/balance pot, etc.)?
2b. Do I even need the neon bulb balancing circuit? (is it problematic without it)?
3. What input transformer to use besides the very expensive Sowter 3575 (anything from Cinemag, US builder here)?
4. Are my output transformers outputs wired correctly with the CT? Do I even need to wire up the CT?
5. Do I need a 600r termination resistor accross the output transformer? (My guess is no. I guess I should really try this one and find out for myself when I build it)

Things I think are correct...
1. The 600 ohm T-bridged attenuator.
2. The attack switch and resistor values.
3. The recovery switch wiring.

Things I need to find out...
1a. What kind of meter to get? My guess is 1mA
1b. Should I put a pot to calibrate the meter?
2. What kind of fuse to use? No guesses on that one.
3. What are the voltage values of the resistors and capacitors. My guess is 1/4w resistors in the main circuit, 1w at the psu except for that 7k 5w resistor. Caps 350v all around.

EDIT: Removed schematic
« Last Edit: December 22, 2015, 08:33:15 PM by ding »


PRR

Re: Another Altec 436c/SR124 p2p :p
« Reply #1 on: December 10, 2015, 02:10:17 AM »
"STREREO" strap and C4 appear to go to ground; a ground that goes round and round dizzlingly between the tubes and the recovery switch areas.

If no OT CT, how will the poor 6CG7 get fed?

Why does the OT have "IN" labels?

I see attack and recovery resistors but I don't see the A/R capacitor. Maybe this is what C4 really is?

6BC8 cathode current (what the GR meter reads) will be 5mA to 10mA (from tube specs shifted to lower voltage; and from the ~~33K in series with the tube). R8 diverts some of this around a lower current meter, and in the ratio of R8 to the meter's internal resistance. Since all meters today are loose-spec crap, you do NOT know R8's proper value until you have meter in hand. and the proper value will be much lower than most standard trim-pots. So don't be buying any 1% resistor here. Get a 10-pack of 100 Ohm 20% (OK, 5%) resistors, parallel the bunch (10 ohms), then with tube and meter and all running and warmed-up, snip-out resistors one by one (10r, 11r, 12.5r, 14r,...) until the meter comes on-scale and then near a top-mark. This can be very late in the build. At smoke-test and initial sound test, just throw 33r in there, wire any low-volt meter (needle is better but $3 DMM will do) across it, to see if tube current dips on large inputs (and if sound punks-out before tube current approaches 1/10th of idle current).

FWIW: I think this is a fairly advanced project for someone with no P-2-P experience. A lot of stuff going on, and your drawing suggests you can work on "neat and direct" more. A Champ-amp might be better starter project. Even cutting-up a Epi Valve Jr and re-build as P-2-P.

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #2 on: December 10, 2015, 10:31:50 AM »
"STREREO" strap and C4 appear to go to ground; a ground that goes round and round dizzlingly between the tubes and the recovery switch areas.

If no OT CT, how will the poor 6CG7 get fed?

Why does the OT have "IN" labels?

I see attack and recovery resistors but I don't see the A/R capacitor. Maybe this is what C4 really is?

6BC8 cathode current (what the GR meter reads) will be 5mA to 10mA (from tube specs shifted to lower voltage; and from the ~~33K in series with the tube). R8 diverts some of this around a lower current meter, and in the ratio of R8 to the meter's internal resistance. Since all meters today are loose-spec crap, you do NOT know R8's proper value until you have meter in hand. and the proper value will be much lower than most standard trim-pots. So don't be buying any 1% resistor here. Get a 10-pack of 100 Ohm 20% (OK, 5%) resistors, parallel the bunch (10 ohms), then with tube and meter and all running and warmed-up, snip-out resistors one by one (10r, 11r, 12.5r, 14r,...) until the meter comes on-scale and then near a top-mark. This can be very late in the build. At smoke-test and initial sound test, just throw 33r in there, wire any low-volt meter (needle is better but $3 DMM will do) across it, to see if tube current dips on large inputs (and if sound punks-out before tube current approaches 1/10th of idle current).

FWIW: I think this is a fairly advanced project for someone with no P-2-P experience. A lot of stuff going on, and your drawing suggests you can work on "neat and direct" more. A Champ-amp might be better starter project. Even cutting-up a Epi Valve Jr and re-build as P-2-P.

Yeah C4 should be attached to the wire before the attack. I guess I didn't connect that up.

As far as the OT I ran out of room on the page and I couldn't figure out how to make the page bigger so I kinda hacked something together to the left. I should really just figure out how to make the page bigger and wire it up correctly. V2 goes into TX, TX goes into the attenuation, ATT goes into XLR. As far as the CT I was more wondering about the CT on the transformers output not its input. Although I did get a pretty big chuckle out of the way you put it.

As far as the p2p goes you are right about that, and I will eventually build a little champ but this looks more of a learning experience. But maybe I will take you up on that advice. Can you give me some advice about the "neat and direct" part. I am a visual artist so neat and direct are difficult lessons for me to learn.

Thanks

DaveP

Re: Another Altec 436c/SR124 p2p :p
« Reply #3 on: December 10, 2015, 02:17:27 PM »
Ding,

A good project to start with (I've made one) but you are getting ahead of yourself.

Just get the original schematic working first before you start adding bells and whistles.  Your major problems will be:-

The 6BC8's are nowhere near being balanced halves, they were designed for series cascode circuits.  You may need 5 to get one that's usable.

Layout is critical to avoid oscillation and or motorboating.  It would help to have a 470uF cap as your final to avoid this

You need  decent sized metal film Resistors 0.75 -1W as they have less noise and withstand more abuse.

Use ohms law to work out the watts required for your resistors.

There is not enough guts in the amp to take much faster attack times, it will flunk out, but you have incorporated the 100k RS124 resistors which will give it some help.

1mA meter is fine, measure the volts across it to get FSD (use a 1.5V battery with a series pot) then you can work out the size of the variable shunt pot to take the rest of the current.

The XSM Edcor is a matching TX only rated at 50V on the primary so it's not a safe long term option, they make the XPP1 600 15k for this job!!

350V Caps are ok, 400 /450 are better, voltages rise over the steady state a lot at switch on.

Best tip  8)  Make a rough breadboard version with temporary wiring first to get the feel of it, but keep your mitts well away from the live parts.  Put a 470k bleed resistor on the power supply cap to keep you alive after switch off ::)

You may need some resistance on the heater circuit so make sure its not over voltage with your tubes attached, they won't last long at 7V!

DaveP
Soundcloud: Delayed Action.

Re: Another Altec 436c/SR124 p2p :p
« Reply #4 on: December 10, 2015, 02:58:30 PM »
You may need some resistance on the heater circuit so make sure its not over voltage with your tubes attached, they won't last long at 7V!

DaveP

I would go one further and do DC heaters.

DaveP

Re: Another Altec 436c/SR124 p2p :p
« Reply #5 on: December 10, 2015, 03:01:17 PM »
Me too, but I thought he had enough on his plate with the original

DaveP
Soundcloud: Delayed Action.

Heikki

Re: Another Altec 436c/SR124 p2p :p
« Reply #6 on: December 10, 2015, 03:27:00 PM »

As far as the OT I ran out of room on the page and I couldn't figure out how to make the page bigger so...

Click "File" on the upper left corner and there you'll find "Page Settings"

PRR

Re: Another Altec 436c/SR124 p2p :p
« Reply #7 on: December 10, 2015, 03:31:00 PM »
> won't last long at 7V!

I believe they will last more than long enough for any non-broadcast use. Receiving tube heaters "never" fail in 10,000 hours (low-cost series-string TVs an exception). He will probably be replacing tubes long before 10,000 hours for microphonics or hiss reasons. Anyway 10,000 hours (even 3,000 hours hi-volted) is a long time for an occasional-use limiter.

Agree that 6.0 to 6.6V is better, but hardly urgent.

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #8 on: December 10, 2015, 03:43:09 PM »
Layout is critical to avoid oscillation and or motorboating.  It would help to have a 470uF cap as your final to avoid this

Thanks for all the tips everyone!
@Dave, I got everything except for this part. As my final what?

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #9 on: December 10, 2015, 03:49:34 PM »
I was thinking of starting with these as input and upgrade later.

https://www.edcorusa.com/xsm10k-10k


DaveP

Re: Another Altec 436c/SR124 p2p :p
« Reply #10 on: December 10, 2015, 03:51:02 PM »
Final capacitor.

DaveP
Soundcloud: Delayed Action.

DaveP

Re: Another Altec 436c/SR124 p2p :p
« Reply #11 on: December 10, 2015, 04:08:10 PM »
Quote
I was thinking of starting with these as input and upgrade later.

I have used both the WSM and XSM types of Edcors and funnily enough, the WSM have a flatter frequency response and less ringing effects.  The WSM is rated for 10V but there is no current passing with the CV so it's just a static voltage which should be OK.  The original 436 uses a 600 ohm input but if you have enough level then I don't see why a 10k to 10k won't be OK.  The TX is unshielded so you will need to site it well away from hum fields other wise it will act like a guitar pickup!  See my tests on the U73b thread http://groupdiy.com/index.php?topic=59242.0

best
DaveP
Soundcloud: Delayed Action.

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #12 on: December 10, 2015, 10:48:07 PM »
Attempt #2. Haven't worked out the meter situation yet.

EDIT: Removed Schematic
« Last Edit: December 22, 2015, 08:33:47 PM by ding »

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #13 on: December 14, 2015, 03:04:23 AM »
This is my idea for the P2P hookup. Any suggestions? Anything I can improve on? I did this late so I'm hoping all the hookups go to the right place. :o

EDIT: Removed layout
« Last Edit: December 22, 2015, 08:34:30 PM by ding »

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #14 on: December 14, 2015, 09:19:23 AM »
I'm wondering what is the most efficient way to wire up a bypass. Anyone have any suggestions?

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #15 on: December 14, 2015, 03:53:36 PM »
Well, if no one has a better idea I'll just put a switch right before the attack switch and switch from attack, release, V3 to ground.

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #16 on: December 14, 2015, 08:01:36 PM »
I think this is the way to do the bypass.

EDIT: Removed schematic
« Last Edit: December 22, 2015, 08:34:48 PM by ding »

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #17 on: December 14, 2015, 08:23:12 PM »
Would a layout like this be more correct or should I just layout the whole thing on turret and not attach anything to components?

DaveP

Re: Another Altec 436c/SR124 p2p :p
« Reply #18 on: December 15, 2015, 07:32:28 AM »
Ding,

Best to work out the layout from first principles, I copied this from the web, can't remember where now so apologies to the writer.

CHASSIS LAYOUT AND WIRING
Review of past design practice shows us that many designers had no idea of the principles essential to optimising chassis layout.
In a wide range amplifier, the wiring and componentry act as antennae, to pick up stray and induced signals, such as ultra-sonics,
RF, hum and noise, from adjacent circuitry.
The basic rules for component layout and wiring are:
•   do not mount electrolytic capacitors close to tubes - the caps will dry out and fail
•   do not mount hot tubes near transformer windings - the windings will burn or dry out
•   do not mount driver stage tubes near transformers or mains wiring - to avoid induced interference or hum
•   do not mount exposed driver tubes near output tubes or rectifier tubes - they may pick up induced signals such as RF, hum and even audio, and trigger parasitic oscillation in the circuit
•   do not mount tubes near the edge of the chassis - they can burn the enclosure or cabinet when the chassis is installed and operating
•   do not mount input wiring near output wiring either within a stage or across stages - to avoid stray signal induction or oscillation
•   do not mount input terminals near speaker terminals or mains supply leads or plugs - to prevent instability and/or interference from induced signals
•   do not mount tubes upside down without restraining devices - they can fall out partially, with loss of one or more contact pins, or completely fall out and break
•   do not mount tubes horizontally unless their grid wires are installed in the vertical plane - to prevent grid wire sag when hot and thus uneven electron flow, or internal shorts
•   always use shielded wiring for signal pathways in the early stages - to prevent stray hum and noise pickup and induced feedback signals
•   never ground potentiometer wires to the chassis at the potentiometer - always ground directly to the relevant cathode (through a shielded co-ax lead)
•   always shield input tubes with discrete shields or a grounded metal enclosure (there are currently more than 500 million mobile telephones in the world, thousands of radio and television transmitters and significant numbers of high-powered industrial and military transmitters, and they do generate a significant amount of RF energy for your hi-fi amp to pick-up)
•    always mount tubes as close to each other as space will permit - to minimise wiring length and minimising exposure to induced signals or interference (but always leave at least a 10 mm gap between bottles for natural air cooling - more for larger tubes having more than 25 watts plate dissipation)
•   always use short leads between connection points - to minimise wiring length and minimising exposure to induced signals or interference
•   never connect pins on tube sockets directly together with straight or solid or heavy wire unless there is a loop to enable each socket pin to locate properly about its tube pin - tube pins do not bend well to match socket misalignment, so some movement in the socket terminals is essential
•   always mount tube sockets such that the heater pins are aligned with the direction of wiring - to minimise heater lead length
•   always twist heater wires together - to neutralise AC radiation to nearby wiring and components (cancel-out hum signals)
•   always ground grid and cathode resistors (and bypass caps where applicable) to the cathode pin, if grounded, or to the grounded end of the cathode resistor. One very effective technique is to install the resistors vertically, straight up from the socket pins, then simply join the ends to be grounded together (neatly). Connect them via a wire to chassis ground. This bundle of components does not need further support as they are a strong structure mounted directly from the tube socket. Some miniature sockets have a centre tube spigot for shielding the pins from each other - this should be also grounded. The tube shield provides a useful structural support
•   where practicable, install interstage capacitors with the outer layer connected to the previous stage plate - to minimise hum pickup - vintage caps often had a band printed on one end to indicate the outer layer
•   try to layout the chassis following the circuit diagram (schematic) - this method actually works!!
•   ensure the amplifier is well ventilated - to ensure adequate cooling and to facilitate long-term component life. If necessary, install a cooling fan
•   always insulate and isolate high voltage and mains power circuitry - death is permanent!!
•   always use an isolating transformer between the mains power source and the rectifier and heater/filament system (to prevent electrocution)
•   always insulate and isolate high voltage capacitors - particularly large capacitance units - they can store electrical energy for several days. An amplifier switched off may still be hazardous or even lethal
•   always use an enclosed METAL chassis system - if necessary fit a removable lid/cover for component access. 20 gauge galvanised steel sheet (1 mm) thick is ideal but aluminium is more suitable for large chassis to reduce weight. Note that it is not possible to solder components to aluminium using tin/lead solder so all chassis joints must be tightly screwed together.
•   never use 50/50 tin/lead solder - that is for plumbers. Use only a high quality 60 tin/40 lead solder having a cored flux. When soldering components to prevent dry joints (imperfect metallurgical joint bond) always allow time for the joint to thoroughly heat before removing the soldering iron. Printed circuit board conducting strip tends to separate from the Bakelite/fibreglass base board if overheated - best to avoid them.
•   always ground or earth the chassis (to prevent electrocution)
•   the earth side (outer shielded cover) of the input shielded lead must be grounded to the amplifier chassis to prevent hum pickup, however modern television receivers do not have an isolating power transformer and use a "hot" chassis system. This means that if you connect yourself - or someone else - between the TV chassis and ground there is a high likelihood of death. If connecting audio signal leads to a TV receiver or similar device ensure there is an isolating capacitor in series with each lead - ie both live and earthed input leads, to prevent direct connection between the amplifier and the hot (ungrounded) source. If you are not qualified to do this then take your system to a qualified technician - you only have one life!!

Best
DaveP
Soundcloud: Delayed Action.

ding

Re: Another Altec 436c/SR124 p2p :p
« Reply #19 on: December 22, 2015, 08:52:12 PM »
Here is the final schematic before I go for this thing. The only thing I'm not sure of is the meter resistor so I left it without for now see what happens with the 1ma meter. I'm thinking I'm going to turret most of it for better troubleshooting.