Need help with fresh build....

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Can anyone tell me what the purpose of the yellow highlighted wire is in this schematic...when I attach it, it burns up the 510 ohm 5 watt resistor highlighted in red...I know that it's the center tap for the 6.3vac windings, but why send it there??( noise reduction?? i assume). I'm sure everything else is wired OK , triple checked all voltages, and amp seems to be fine without it connected...

[John]

If I'm reading it right, you're connecting the CT to the B+. Always a bad idea. That should be going either to ground, or to the junction of those 2 220k resistors (that would elevate the heaters).

*edit* I don't think it should go to those resistors either... I think that 56k resistor paralleled with the 25/25 cap is supposed to give a bit of an "elevation" to the CT ground. Fear not, sluckey or someone will soon be along that *knows* what it's all about!

[sluckey]

The purpose is to elevate the filament circuit to reduce heater to cathode leakage, ie, reduce filament hum. However, there should be a big resistor in that line just to the right of C21. This would make a voltage divider to pick off a small amount of dc from the B+ line to elevate the heater center tap. Without that big resistor you have applied full B+ to C21 and it probably pooped and shorted. Now you have a dead short to the B+ line and that will burn up that sag resistor.

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Well, that was quick...THANKS JOHN
I was going nuts because it was on both versions of the schematic that i was working off of.
Here they are:

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Thanks Sluckey....can I safely connect it to ground if I'm ok with the resulting filament hum (seems minimal)

[kagliostro]

R31 is labeled 510 (so meaning 510R) - R42 is labeled 50K (56k in the other schematic)

and the CT of the heater winding is connected at the junction point of the two resistors

may be R42 is 50R instead of 50K or 56K

assuming B+ = 400v / R31 = 510R / R42 = 50K

the tension on the junction is 395.96v

assuming B+ = 400v / R31 = 510R / R42 = 56K

the tension on the junction is 396.39v

assuming B+ = 400v / R31 = 510R / R42 = 50R

the tension on the junction is 35.71v

assuming B+ = 400v / R31 = 510R / R42 = 56R

the tension on the junction is 39.58v

K

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Thanks K,
A little over my head, but I get the jist of it and I appreciate your help.
I'll try that and report on my results....
All I do is follow the schematics, but this is a great way to learn.

What is your opinion of using a separate dc regulated preamp filament supply off of a toroid? (for future projects)
In a hi gain preamp to try to minimize hum.....

I assume that's what Mesa was doing by using dc in my Rectoverb combo V1 (only)

[kagliostro]

The schematic you posted seems a sort of Vox AC30 Bright (Top boost) and normal channel

in some version of the AC30 also Vox used DC heater but I think that is something to be used principally on PCB amps where you can't twist the wires

here an example



though I am surprised that the positive of the bridge is connected to ground

There are so many amps that use AC heater (with elevation or not) that using DC is something to reserve to very high gain amps, not that I think that is detrimental to have DC heaters, but is a complication of the circuit to use probably only if strictly necessary

EDIT: Looking better to your circuit seems you require one other 510R resistor connected between R31 and R42 or you'll give less of 40v to the grids of the final tubes and the remaining circuit, so your Heater CT is to be connected between the junction of this new 510R resistor and R42 to have a correct voltage and an adequate elevation voltage for the heaters

K

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Thanks for taking the time to respond again, I truly appreciate your responses...
I'm in a heavy learning stage, and all of this is good stuff...

This particular amp was givin to me to modify, and I decided to get rid of the PCB...so I replaced it with a PTP Bogner Ecstacy overdrive preamp circuit that I hand wired directly off of a print I found on this site...It sounds great but that center tap wire was just killing me.
Here's the schematic for the Bogner. All I did was insert a Master Volume right before the phase inverter in the Sovtek and kept the whole power section in tact. So now it's a single channel high gain amp with a true master volume...what a difference

That white center tap wire was just staring at me

I'll insert that resistor tomorrow and report back

Thanks again...priceless

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Just to be sure... I should add this resitor here?

[sluckey]

I'll insert that resistor tomorrow and report back

NO!

If you put another 510Ω resistor between R31 and R42 you'll just pop both resistors. The schematic is WRONG.

Just totally remove R42 and C21 and the yellow highlighted wire. Then connect the filament CT directly to ground.

[sluckey]

This schematic clearly shows the BIGGG resistor I mentioned in my first post...

http://www.el34world.com/charts/Schematics/files/sovtek/Sovtek_MIG50-60.pdf

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You guys are the best...thanks so much for your time
I'm really tempted to just put it to ground and live with any hum, BUT, I always prefer to learn the truth.
Now you've got me thinking...I'm going to try to trace the path on the old board and see if i can figure out the value from there.
I see that 1M in the other schem.....I just don't know how to do the math for the correct value for my application

I'm sure you guys will let me know...I really can't thank you enough!!

[sluckey]

I'm going to try to trace the path on the old board and see if i can figure out the value from there.

Should be easy to locate if the R# is on the board.

Personally, I'd connect the CT to ground and be done with it. But if you really want to elevate the filaments, go for it. But replace that 25µF/25V cap. It's been stressed to the max with over 400 volts on it. I bet if you check it you'll find that it's shorted.

I just don't know how to do the math

Just use 500v B+ as an example. Your actual B+ is probably lower...

P = E²/R = 500²/1,000,000 = 0.25 watts. Double that wattage for a safety margin. So, 1/2 watt resistor is fine. The small error due to ignoring the other resistor simply increases the safety margin for the wattage size.

[John]

Personally, I'd connect the CT to ground and be done with it.

I'm a new guy at this too. I always listen to what Sluckey says.

(and most everyone else too, they all know lots more than me! ) 

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Thanks again, again Sluckey....i'll probably just go to ground,,, and look for other ways to reduce noise.
For the amount of gain this preamp has now, It's really not that noisy.
That wire was the LAST wire I connected in this build, and i couldn't believe my eyes when that 510 ohm 5w resistor started smoking and opened up. I HAD TO KNOW WHY.

I didn't even know what elevation was until I asked this question today, and your answers taught me a lot. Then I went further and read into it a little more and it made me think about AC vs. DC for filament supplies for the future....which led to more reading...If you get a minute give me your opinion on the subject...including the use of a separate toroid xfmr for a regulated dc supply for the preamp filaments (i have no ties to tradition)
The reason it's so important to me is that I did this mod for a friend and may now build one from scratch for myself because i like the sound so much, and it's a really simple circuit......anyone looking for a high gain preamp REALLY needs to check out that Bogner schematic

[kagliostro]

in front to Sluckey I always bowed, his experience is really great

here is the Merlin's explanation about heater elevation

http://www.freewebs.com/valvewizard/heater.html

DC elevation: DC elevation is often used when a valve in the circuit has a high cathode voltage. The heater voltage is elevated to a higher level to avoid exceeding the maximum heater-cathode voltage rating of the valve. This is done simply by 'adding' a DC voltage to the heater supply. The heaters still operate at 6.3V (or whatever you're using), but the AC component 'floats' on top of a DC voltage.
 This method is also used to reduce audible heater hum, because the leakage current between heater and cathode becomes more-or-less constant when the voltage between heater and cathode is greater than a certain threshold (which will depend on the type of valve)*. Therefore, although there is still leakage current, its is constant and therefore the noise voltage appearing at the cathode becomes DC, which is inaudible!
This effect works in both 'directions' too, so we can either elevate the heater supply (which is usual) or make it lower than the cathode voltage. Reference voltages used are typically between 10V and a few tens of volts.

A typical way to apply the DC reference is by connecting the centre tap (real or artificial) to the cathode of a power valve, providing the power valve is cathode biased of course. The bias voltage of most power valves is usually more than 5V, and this will be 'added' to the heater voltage.
 The other common method is to take the DC reference from a potential divider from the HT (useful if the amp has fixed biased power valves). Typical voltage references are around 20V to 90V, placing the heater supply well above the potential of most cathodes in the amp.
 The potential divider should have a fairly high resistance so there is no significant current drawn from the HT (it can also serve as the bleeder path for the HT smoothing capacitors).
 The lower resistor in the divider (R2) should not be excessively high or the maximum heater-to-cathode resistance may be exceeded. Many data sheets do not quote this so it is advisable not to make it greater than 100k. A fairly large value capacitor (C1) can also be added to ensure a smooth DC reference and to prevent the 50Hz heater hum reaching the HT supply. It's actual value is not critical, anything over 10uF should be fine

I wrongly told to use 510R & 56R resistor (I apologize)

but with 510K & 56K resistors (with B+ = 400v / added resistor = 510K / R42 = 56K) I think it will be usable

what do you think about ?

K

[DummyLoad]

looks like 1M/68K - hard to read, but believable at about 14.7:1 ratio of B+; about 27V with 400V B+

--DL

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You guys are the best.....keep it coming

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So do we all agree on 510K in the spot we discussed earlier?(inline between R31 and R42) 1 watt? ( I've got a couple flameproof 510k 1w lying around)
Actual no load B+ at this connection point is: 517 vdc
With the center tap connected to ground, my preamp filaments are @ 5.95 vac

I have no time limit on this amp, so for educational reference, I'm going to give it a shot...
With no experience in elevation, I will just need more help with taking measuremnets...
I guess I'll be taking DC volt readings off of my ac filament connections....what are the ideal readings for optimal elevation?

Sluckey, if you tell me to just go to ground and get on with my life, I'll do it...

What other guitar amps commonly use elevation?...why not just regulated DC?

I thought the bad schematic was just a ploy by those Soviet bastards to blow my amp up!

[Willabe]

what are the ideal readings for optimal elevation?

I've used a dc stand-off on 3 of my builds with no problems so far. Their quite, was easy to do on a new build.

Kevin O'Connor uses this on his amps and in his TUT series books he lists 70 to 80 dcv for the stand-off voltage. Some say this is too high and seem to go for 30 to 40 dcv. IIRC Ampeg used it on some of their amps and took the stand-off from the cathode of the output tubes. I think in stereo tube amps this was a more common thing than MI amps?

I like to take the stand-off tap from latter down stream on the B+, less ripple/quieter.

why not just regulated DC?

It's more work, more $$ in parts and it takes up more space. I've read KOC say a dcv stand-off works better than regulated dc anyway. A number of guys on his forum who have tried both agree with him and found the dcv stand-off just as quite or better.

I've never tried regulated dc heaters.


                           Brad        

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Thanks Brad, I'm really impressed with the amount and quality of help that I've received from this site in 1 day since joining..

That's great info, and will inspire even more reading on my part....I really like the idea of tapping it futher downstream (there's just a lot going on in my layout at that original physical point)

[Willabe]

Here's how I did mine. Ones with a CT on the PT heater's wind, the other's heater wind does not have a CT.

KOC recommends a 1w flame proof for the B+ droping R because of heater surge currents at start up.

                        Brad        

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Nice,,, I'm just a big sponge at this point...

What do the filament dcv readings look like at the tube sockets? should I expect to see the 40v there? half? I hate being stupid/asking potentially stupid questions

[Willabe]

If you look closer at the drawings it says 70 to 80 dcv on the heaters. And like I said some think that's too high and/or not necessary to go that high to get good results. Me, I'm gonna go with KOC on this.    

It's just a voltage divider taking a dcv off of the B+, with a filter/smoothing cap to kill any ripple. Depending on where you tap the B+ will make you have to adjust the stand-off voltage divider. KOC and Merlin are a little different on what value R to use for the R that goes to ground. Again I'm going with what KOC says on this. IIRC in his TUT books he is using a flame proof 100K@1w. The smaller you make this R the more you bleed off B+ current to ground. So I like the higher value KOC recommends. Someone posted not long ago that in _some_ tube characteristics books there is a limit of,          , what was it again? DCV between the filament and cathode? I don't recall it correctly, but it turned out that 100K was a little too high and Merlins value here was in line with the book?

So far I've had no problems with using a 100K.    


                              Brad      

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Thanks for the simplified explanation. Sometimes you just need to hear it broken down that way to wrap your head around it...especially since this topic is brand new to me. I totally get it. Just not much for the math.

I repair welding machines for a living and we just spend an aweful lot of time trying to keep the dc away from the ac

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OK, so if anyone's still with me on this...
just for the record...
I went back to the original pc board and traced the resistor that IS missing on the schematic
It's positively 1M and it's inline, right where you guys said it should be...
I'll assess hum without the elevation, and then hook it up and re-assess, so I learn something, and you guys will get my report on the results of my findings, including a dcv measurement
Thanks again,
Dave