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General Category => VALVE archive => Topic started by: najo49 on September 04, 2012, 05:18:48 AM

Title: C/4s original SEX ,voltage questions.
Post by: najo49 on September 04, 2012, 05:18:48 AM
Hi I just put a set of the c4s boards in (I built them up myself). i have many of the Brainiac changes to the circut as well. I am getting just 200 v going in to the board and 116 going out. I plays fine,the voltage in the old valve mag says a considerable larger voltage. the 1k resistor is removed in the PS. Any thoughts,this may take paul or doc to comment. Thanks
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 04, 2012, 05:39:22 AM
Unfortunately, I can't locate any information on the C4S installation and my memory does not include any details either. Do you have a manual or circuit?

The voltages ae quite acceptable, however. I would speculate that there is a series resistor between the power supply and the C4S input, which drops a substantial voltage. But changing that, and thus changing the input voltage, won't change the plate output voltage, so it won't affect the performance.
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 06, 2012, 07:33:02 AM
Thanks Paul, I did all the work according to the Valve schematic,and docs schematic on the c4s board. In that he says there was 300 V going in .  then  a resistor 60 ohm,then the 2907 ,350 b and and a 60k resistor to gnd. after the leds. but I only have 190 going in not 300v. But as you say ,and the fact it works fine may be I should just consider it a success,and for get about it. The rest of the mods were done according to a valve piece you wrote on SEX changes in valve. In fact the original schematic contains no voltages ,if I had them, perhaps I could have figured this out. Thanks again, let me know if you have any further thoughts. I love these amps,but i thinks i am pretty much alone in this, I guess all the bottleheads did this 16 years ago and have moved on. Jan
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 06, 2012, 05:11:07 PM
If you tell me what issue of VALVE has the C4S schematic, I'll look it up and estimate the voltages. If you can find Doc's post of a schemo, I'll look at that too. There is probably some reason fro the 190 volts!
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 06, 2012, 06:41:18 PM
Paul, It was in vol. 6 no. 1. Jan
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 07, 2012, 06:08:38 AM
OK, thanks. If you look at the schematic again, you will see a 6K resistor between the 410v and the 300v. You probably don't have 410v at the hot end of that resistor, or else that resistor is not actually 6K. Once we know which of those possibilities is correct, we can look for ways to fix it.
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 07, 2012, 10:22:09 AM
Paul, I  have the original 1k wirewound removed ,I have the higher47uf cap in and no 6k resistor. I have 124v going into the c4s and  116 going out. Let me know what you think. jan
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 07, 2012, 03:19:49 PM
So, are you saying that the high voltage is now 124v instead of the normal 410 volts? And that it was 200volts a couple days ago? If so, obviously something is seriously wrong, and it has nothing to do with the C4S installation!

There were several versions of the original S.E.X. amp, I have only one version of the manual, and yours is modified anyhow. So instead of the manual voltage checks, how about these voltages:

AC filament voltage on the 6DN7s (pin 7 to pin 8) - should be close to 6.3 volts

AC high voltage transformer - from ground to the leads that go directly to the rectifier diodes - should be around 350v

DC power supply voltage, relative to ground and measured at the positive terminals of the PSU capacitors - should be over 410 volts

Tube voltages, relative to ground and at pins 1-2-3-4-5-6

These should give us a good understanding of what is going on. Believe me, if your earlier voltages are correct, then when we get this puppy working right you will be amazed at how much better it sounds!
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 07, 2012, 04:24:02 PM
Paul, That last post was not accurate and was miss leading. My high voltage is 453 and filliments are  6.3. I have no voltage dropping resistor where there is shown a 6k in docs c4s valve article . My voltage is however 124 going into the boards. If I disconnect the input lead to c4s the voltage at that terminal where that lead goes to is 190. after the second ps filter the 47uf one there are not any resistors. So it seems to me I should put 6k 3w right after the 47uf cap. I hope this makes more sense.jan
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 08, 2012, 06:49:02 AM
Paul here are the voltages at each pin first the valve with the input from RCA.or the left one with power cord away from you. 9.3v,425,15.5,0,109,3.14,125,125 the other tube is  9.7,424,15,84,425,132,125,125   Jan.
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 08, 2012, 10:32:40 AM
Thanks, that helps a lot.

You do not have the circuit wired as in the VALVE circuit. The driver triode sections (pins 4, 5, and 6) are still connected as a mu follower (the original SEX circuit), but in the VALVE circuit those two sections are in parallel. You also do not have the C4S connected to a third (new) RC filter, consisting of a 6K/3W resistor and a new 47uF/450v capacitor. I suspect the C4S is connected to midpoint of the mu follower circuit, rather than the new filter off the main high voltage power supply. Finally, you report around 9v at teh power section grids (pin 1) but I suspect that is 9mV, i.e. 0.009 volts.

A couple things to watch out for here:

1) the 6DN7 sections are connected backwards from what everyone thinks - section 1 is pins 4-5-6, section 2 is pins 1-2-3.   

2) The old C4S boards are very confusing and counter-intuitive as to which resistors are in series and which are in parallel.

Those of us who were around back then have, I believe, all made the above mistakes. I know I have! Hence the reminder.
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 09, 2012, 07:30:51 AM
Paul, Thanks soooo much. A little more info. I have the 47uf cap installed,but as I said no 6k resistor.after the 47u the high voltage supply splits one to red transformer lead, the other then goes thru a 270k resistor to  pins 8,7,6 each lead has a resistor before connection at the above mentioned pins. On the other tube I have 2 LEDs connected to pin 4. c4s to pin 5. The connection to pin 5 was a 12k. resistor which I removed and put the c4s in its place. I will now try to get things connected as you say, Is adding the 9v battery  supply worth doing? i am also buying a set of tfa 204s from mike. considering all the transformer available today is that still the best choice? Thanks again, for sharing your infinite wisdom.
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 09, 2012, 02:43:54 PM
Wow - this is getting more interesting, and more complicated, by the minute! An interesting historical puzzle for sure! Yes, the TFA-204 is still one of the best available. I assume you are getting the TFA-204 series feed transformer, not the TFA-2004 parallel feed version.

I now have the following references:

1) My own original SEX manual, version 1.1, 1996.

2) My article "Braniac's S.E.X. Changes, VALVE v.4 no.10 October 1997 pp. 15-16

3) The circuit "ActiveS.E.X. from VALVE v.6 no. 1 1999 p. 13 which depicts an amp converted to parallel feed and with a C4S plate load on the driver, and also incorporating some of the ideas from 2)

The circuit 3) is the most recent, however it does not include a power supply circuit, and it depicts the parallel feed mod (called ParaS.E.X.) which is different from the TFA-204 mod. Here are some notes to make sense of all this:

Power supply: The original in 1) was a CLC (10uF/10henries/10uF) followed by a series 1K power resistor. The modification described in 3) is to replace the last 10uF with 47uF450v and remove the 1K resistor (replace it with wire). This will give about 410v at the output plates.

The same circuit 3) shows a 6Kohm, 3Watt resistor and a second 47uF 450v capacitor; this is a decoupling network for the driver stage, which drops the voltage to a safe level (300v) for the C4S transistor. Notice this - there are TWO 47uF/450v capacitors per channel.

The power supply in 1) also shows a voltage divider of 270K and 100K, with a 0.1uF capacitor across the 100K. The hot end of the 270K goes to the power supply 410v, the common point where  the two resistors meet goes to pin 8 of the tubes, and the other end of the 100K goes to ground. I would suggest reducing teh 100K to around 50K; the higher value was only needed with the original driver circuit. You can put a second 100K in parallel with the existing one to get 50K.

Finally, the 6.3v winding goes to pin B7 and B8 with a twisted pair. Another twisted pair connects B7 to A7, and B8 to A8.

Output stage: This is pins 1, 2, and 3 of the tubes. They are in parallel - A1 is connected to B1, A2 to B2, and A3 to B3. In the circuit 3) the cathodes (A3/B3) are connected to the old output transformer but for the TFA-204 they should connect to a 390 ohm resistor whose other end is grounded. The circuit 3) shows that to be a 390 ohm 2 watt resistor but for the TFA-204 I recommend a 5 watt resistor. This resistor is bypassed with a 220uF 35v electrolytic, as shown in 3).

The grids (A1/B1) goes to ground through a 270K resistor, and to the driver through a 0.47uF capacitor. I recommend a small grid stopper, 200 to 500 ohms metal film, connected right at each socket pin 1, with the other wires to pin 1 going to the other end of the stopper resistor.

The plates (A2/B2) are shown in 3) going to the old output transformer and a parafeed capacitor. For the TFA-204, the plates should go to the blue output transformer wire. The red output transformer wire goes to the power supply 410v point.

Driver stage: The driver triodes are pins 4, 5, and 6. They are in parallel, same as the power section A4 connected to B4, A5 to B5, and A6 to B6. The grid (A4/B4) goes to the input RCA jack as usual.

The cathode (A6/B6) goes to ground through a 500 ohm resistor. Circuit 3) shows 1000 ohms, which is incorrect. The 500 ohms resistor must be rated at least 1/2 watt; I would use a 1 watt myself. It can be any value from 470 to 500 ohms. It is bypassed with a 220uF or greater capacitor rated 16 to 35 volts.

The C4S input comes from the second 47uF capacitor, about 300volts. The C4S output goes to the plate (A5/B5) and from there to the 0.47uF coupling capacitor. The C4S has a ground lead to signal ground as well. Note that the C4S has three 50K 1W resistors in series from the LED string to ground; this is an unusual arrangement so be sure you have it correct.
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 11, 2012, 06:49:16 AM
paul, I can not thank you enough for taking the time to post all this for me. everything is clear to me . I think the most misleading for me was the difference between the original schematic posted in valve and the ,version in your 3. last version which I did not have available to me. My amp pretty much was spot on for that version, except for input which is not paralel wired.,The the key was your explaining that there should be 2 47uf caps, a 47uf and a 6k  line to the c4s from the 410v.  Thanks again for your willingness to share with others ,but before we end this two more questions, they may be stupid, but I do not understand the second chock,if it is one, which is mounted on top. My understanding is the the small tranny inside connected to output is the opt. or is this a plate chock?, or the parasex opt and my old tranny  wired as a plate chock? but  the tranny on top has 4 leads to it and this is not like a simple chock but looks more like an a opt to me. I am wondering if I have a tfa 204 already or a parafeed ,or a different opt transformer  Further it does not show up on the schematic at all,except in the vol 6 no 1 c4s schematic, which makes me ask this question. ???. and lastly would the 9v battery mod be worth doing?
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 11, 2012, 01:07:20 PM
In reverse order ... the 9-v battery is a revision of the mu follower - the original input circuit It was to bias the top triode, which acts as a current source. When you install the C4S, it replaces that part of the circuit, so you take that extra triode and put it in parallel with the original first triode to get more drive current. Thus, teh 9-v thing has no place in teh C4S driver circuit.

Now, then reference 3) circuit shows the parallel feed version of the original S.E.X. The original output transformer (on top, with four wires) is re-purposed as a plate choke, with the secondary wired in the cathode as a cathode feedback. It's not a lot of feedback, only 10% of the primary winding, but it does make a difference. The small transformer with many wires is the output transformer - it can be small because it handles only the audio signal, while the plate choke handles all the DC current.

The TFA-204 is large, black, and has bell ends. The core is about 2.7" tall, 2.2" wide, and the core is 1.75" thick. It also has six wires - red and blue are the primary, black is the output ground, and yellow-orange-brown are the output taps for speakers. I don't remember which is 4, 8, or 16 ohms - when the transformers arrive they will have some notes. To install it you would remove both the 4-wire transformer/choke on top and the small output transformer underneath. The chassis plate already has the requisite four holes to mount the TFA-204 on top.
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 13, 2012, 04:33:14 PM
Paul, I got everything wired as per your instructions. Two things the 3w 6k gets hot! And there is no sound,at all. I am using the original output trans. . The out put from pin 3 goes to the orange wire on top mounted trans. and also thru a 3.3uf solen cap and on to the orange lead of the outpt trans.Does that snd. right to you? All the LEDss light up on c4s and the output voltage into c4s is 298v. I have checked and double checked but now have reached a point I thought I would seek your help. J.
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 13, 2012, 06:07:45 PM
Yes, the 6K will get pretty hot. I would have chosen a 5 watt resistor, today, but that was the design back then and I didn't want to invent yet another design! The hotter that resistor, the less hot the C4S transistor, and hence the longer it will life. :^)

So far as I can tell, there is no reason it would not work if it is wired as per the circuit in ref 3) with the small changes I suggested. I don't think I actually specified that circuit myself; I thought you were asking for wiring applicable to the TFA-204. So there's no way for me to tell exactly how it is wired now. Perhaps if you ran the tube voltage checks again I could uess whether the tube part is working as expected. Better include the voltages on the four wires of the original output transformer (now the plate choke and cathode feedback coil). The current parafeed output transformer probably has a label which will identify the wires by color; they are not standard and of course I don't remember the color code used 13 years ago so tell me which wire goes to where and I'll see if that makes sense to me. Best I can do at this distance.
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 15, 2012, 09:59:10 AM
Paul, I have wired the amp in parallel feed is that the right term? I have two questions 1. the 6k 3w is running very hot and seems ready to go up in smoke at any time. It is putting out 298v to the input to the c4s. and it is putting out the proper 225V at its output. My voltage after the input chock are high 453 compared to the 410 or so I would rather see. Thats running on a varic at 117V. I am running the original output transformers as my  204s have not arrived. I am not getting sound out now. Is this because I can not get sound until I get the new transformer in? I will just give you the discription of my transformers as they are wired now. The orange lead to top trans comes from pin 3,the lead to the output trans goes from same pin 3 thru a 3.3up solen to orange lead of this smaller under mounted small trans. the output come from this small original output trans. to binding posts. j
Title: Re: C/4s original SEX ,voltage questions.
Post by: Grainger49 on September 15, 2012, 10:32:01 AM
Some basic electrical calculations.  

You can get the power dissipated in the resistor by first getting the voltage across it.  Measure the voltage across the 6k resistor.  Power is V2/R, that is the voltage squared divided by the resistance, 6000 ohms.  

You want the resistor to have some headroom since it is under the hood.  So if it is a 3W resistor and it is dissipating 2+ watts then you can get a 5W resistor to replace it.  A 5W resistor will run cooler and the temperature under the hood won't derate the power rating to below what it can sustain steady state.
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 15, 2012, 11:56:25 AM
That's not enough information for me to work from. See my previous post.
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 16, 2012, 06:25:55 AM
Paul, I will not take anymore of your valuable time. I will end by giving you the current transformer wiring. upper xformer ,    Red  is from high voltage of power supply. blue lead comes from pin 2 orange connects to pin 3, . green from upper, goes thru a 220 uf, electrolytic bypassed with a 270K resistor to gnd. the small output has a connection from pin 3 thru a 3.3 solen to the orange lead. bl.,brn,red,yel are not connected. Out put to binding posts is via blk. and 16 ohm yel. to speakers. I am going to have to wait several weeks for the x formers the tfa 204s to arrive. Thanks you so much for your help,I have really learned alot,and as you have noticed I have alot to learn. You are so kind to share your knowlege with me. J.
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 17, 2012, 07:11:04 AM
grainger, Thanks for your input. I see the ohms law, and I appreciate the heads up. I have now got that sorted out.The amp does not produce sound ,and I assume thats because I have converted to parallel feed,from the original mu follower. and the original output cannot work. I am waiting on a pair of tfa 204s,but I wanted to listen to it until the new xformer arrives.
Title: Re: C/4s original SEX ,voltage questions.
Post by: Grainger49 on September 17, 2012, 07:19:56 AM
Hey!  I put the 10 year old C4S boards into my FP 2 a few months back.  I got the incoming and outgoing voltage leads swapped.  It drove me nuts!  (short trip!)

I spent a couple of hours reversing the LEDs.  To no avail.  Then PJ got my head straightened out and it sounds awesome!

I had to turn down my sub by a couple of clicks.  Maybe I should turn it down another.  The bass is much firmer and extended.
Title: Re: C/4s original SEX ,voltage questions.
Post by: Paul Joppa on September 17, 2012, 10:29:53 AM
Paul, I will not take anymore of your valuable time. I will end by giving you the current transformer wiring. upper xformer ,    Red  is from high voltage of power supply. blue lead comes from pin 2 orange connects to pin 3, . green from upper, goes thru a 220 uf, electrolytic bypassed with a 270K resistor to gnd. the small output has a connection from pin 3 thru a 3.3 solen to the orange lead. bl.,brn,red,yel are not connected. Out put to binding posts is via blk. and 16 ohm yel. to speakers. I am going to have to wait several weeks for the x formers the tfa 204s to arrive. Thanks you so much for your help,I have really learned alot,and as you have noticed I have alot to learn. You are so kind to share your knowlege with me. J.
It's no problem, I am genuinely interested in seeing you get this thing working. Sometimes I make short answers when I'm in a hurry, so apologies if I seemed curt.

The reason I asked for tube voltages is to be sure all four tube sections (two per tube) are operating correctly. The high voltage (453 vs. 410 expected) might be caused by one section failing to draw enough current. Check the voltages at the actual tube socket pins, don't rely on wires that connect one socket to the other - there could be a bad connection or a broken wire.

The old output transformer (now the plate choke with feedback coil) appears to be connected correctly. I assume the "270K" resistor is actually 270 ohms? I would encourage you to raise it to 390 ohms, 5 watts. The higher wattage will give a longer life, and the higher resistance will reduce the total current through the plate choke, reducing its distortion.

From your description, there is only one lead of the small output transformer connected, so it's no wonder there is no sound. The 3.3uF Solen should go from pin 2 of the tube socket (NOT pin 3) to the orange lead of the transformer (I assume that it the lead marked "1.25 watts" on the transformer). The black lead of that bundle is shown in ref 3 going to ground.

Your numbers show 155 volts across the 6K resistor (453-298), which by Ohm's Law would be about 26mA current. This would be 4 watts dissipation, exceeding the rating - no wonder it is hot! Best to check the resistance in case the heat has damaged the resistor. I would encourage you to replace it with a 6K 5watt resistor - but not until we get the high voltage down to where we expect it to be. But the circuit says you should have only 18mA (total of 16mA to tube pin 5, and 2mA through the three 50K resistors on the PC board). The plate voltage is close enough (225v vs. 250v expected) so I expect the 16mA is correct - leading me to suspect the 50K resistors may not be wired in series. The old boards were very confusing in that area, so it's worth inspecting them. There should be three resistors in series, with 100v across each one (adding up to 300 volts across the string).

Incidentally, I do recognize how much of a pain it is to be entirely clear and complete in posts like these, where we can't see what the other is doing. But we are usually successful eventually!
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 19, 2012, 07:27:39 AM
Paul, Thanks,I just thought you must become tired of dealing with a beginner. I am now retired,a musician ( string bass) playing with various Orchestras . I am tryi ng to learn enough to partially at least design a few things. And I have quite a collection of tube amps. I have a Heathkit wm 6, 70w push pull, ever seen one? They are very rare only 2 k were built.But I have many others all rebuilt by me.

I am waiting on the 204 x formers. But I did something fun with the sex amp that is working and still wired in the first version. I have a transformer,it is 117v primary and a pair of 19v secondaries  and a pair of 7.5 v secondaries. I hooked it up in the place of the original transformer. primary to the 3.3 up cap output and the other lead grounded to sex gnd. I used the 19v secondaries on gnd. and the other as the positive out put. And to my surprise GREAT things came out. It had more power, I do not know how much but it was a big improvement on the original trans. And no feed back except what the chock adds. Why does this work?It has a oval magnet structure and the windings on each side of the oval. And I tried the 7.5 outputs and they sounded pretty much the same. I have no idea what the output resistance is but I thinkk the 19v outputs sound best. What do you think. J.
Title: Re: C/4s original SEX ,voltage questions.
Post by: Hank Murrow on September 19, 2012, 10:14:42 AM
I am now retired,a musician ( string bass) playing with various Orchestras . I am trying to learn enough to partially at least design a few things. And I have quite a collection of tube amps. I have a Heathkit wm 6, 70w push pull, ever seen one? They are very rare only 2 k were built.But I have many others all rebuilt by me.

Dear J; Indeed, I remember that Heathkit 70watter, because I built one back in the days of mono when I was 15 or so and drove a Stephens 15" coax loudspeaker in a Voice of the Theater system I built myself. Hey, it could be heard on the ridge a half mile distant from my parents' house! Think "Night on Bald Mountain" in the balmy California evening air..... Went on to Dynakits until they went solid state, but now Bottlehead Customs has built my Final Music System, and with Orcas & BUF Subs I am a very happy camper. Just hoping I get a couple more decades to enjoy it.

Cheers, Hank
Title: Re: C/4s original SEX ,voltage questions.
Post by: najo49 on September 21, 2012, 03:58:44 AM
Hank, I use a pair of altec speakers for my low power system, the 15 ohm vintage 15 inch woofer, horn and driver. I can not imagine what a 70 watt speaker would do with them, loud !. I love my wm 6s and I have completely restored them,paint,teflon wiring ,all the resistors and caps . They look like new and and hands down the best push pull I have ever heard,they are paired with the proAc 2.5s or the pro ac tab 50 sigs in my high power system. Thanks for you reply, j.