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Reply With QuoteIt's a bleeder resistor. It takes the voltages in the amp down to a safe level within a minute or so after powering off. Does not affect the sound. (This should be mandatory for all DIY amps...)Originally Posted by daz
i'm about ready to order parts for a 4x6v6 build with a cascaded pre like i did with my 18 watt. The schematic is a 36 watt with el84's from 18watt.com that i edited my pre onto and changed the pin #'s on the power tubes to reflect 6V6's. I modded my 18 to use 6v6 and really liked it, so i wanted to do the same by building the 36 watter but changing a few things to work with 6v6 outputs. So the schematic has PA values that are for el84's because i haven;t changed those not knowing what to use. i hilighted components who's values i either definatly need to be changed like the cathode resistors and screen resistors, and some that probably should but i'm not sure. If i only had 2 questions that could be answered it would be what screen and cathode resistors should i use, and should i use one of each per pair like on the schematic. Also, most 6v6 amps seem to use 1.5 grid blockers. should i use the same or stick with the 8.2k's on the schematic?
To give you an idea of what i'm shooting for, i want a fairly high gain (yes, i know it's REAL high, but the pre drawing is just an unfinished general idea) pre with a lot of headroom in the PA. The tone i want should have a squishy feel, sound very complex, and sound closest to a marshall tone. JCM's have always been my fav. I definatly want to stay away from that real hard/stiff feeling attack.
So if you could tell me what values i need or probably should use it would help me get the parts on order finally. Theres one R at the beginning of the B+ referencing ground with a question mark, and i'd like to know whether it's necassary and what it even does. Don't bother with the pre, as thats just a basic skeleton that will likely be change and possibly a lot. (possible one less stage and V1 paralleled) the PT will be a hammond 270HX and the OT is a 4k 40 watt heyboer. aside for the 1st most important question as to the cathode and screen values, anything you notice that might seem funky please critique. I'm flying 1/2 blind here. I know this is a lot to ask, but i have to try and figure out what to order so please give me any advice you can. (aside from "give it up" that is) Heres the schematic, thanks....
http://img518.imageshack.us/my.php?image=6v6aj1.jpg
It's a bleeder resistor. It takes the voltages in the amp down to a safe level within a minute or so after powering off. Does not affect the sound. (This should be mandatory for all DIY amps...)
Thats awesome. I too can't understand why all amps don't have that. All mine will from now on tho!
Big change in plans.....after posting at several forums about 6V6 amps i got some replies that caused me to change my mind. I decided to either go back to EL84's, or more preferably if i can, EL34's which are my fav tube to begin with. However, i already have my OT, a heyboer with a 4k primary at 30 something watts. (I suspect higher because it's huge compared to another 25 watt heyboer i have)
So i have 2 questions..
1)-i was thinking of using one of weber's EL34 schematics to build the PA section, and i didn't realize that according to his specs i can probably use mine for a pair of 34's. I thought 4k would be way off, but in his brit kits that are 40 watts it show the WO18343 model OT which shows a 4k primary like mine has and about the same wattage. can anyone tell me why i could not use my 4k heyboer for this? Heres the OT... https://taweber.powweb.com/store/018343sch.jpg
2)- i was going to use the hammond 270HX PT (270HX 176VA, sec. 275-0-275, DC ma 200, Fil.#1(rct) 5.0v @ 3a ct, Fil.#2(htr) 6.3v @ 6.0a ct. EACH $60.99. ) but i need to know whether i still can. I would imagine the voltage at 275-0-275 is going to be too low for EL34's? If so, what hammond wouuld you suggest, or at least what ratings should i look at?
Thanks
Interested to hear what was said that put you off using 6V6's, as i've a 4x 6V6 amp that i like a lot.
Using EL34's at low HT voltages should allow it to be biased class A, but remember that they take 1.5A heater current per valve.
Sorry. Just reread and saw it was 2 EL34's not 4.
If you are going to order a Chinese ouput tranny from Weber, just get a Chinese power tranny too.
You can use the:
https://taweber.powweb.com/store/022798sch.jpg
It is a cheaper, "clone" of my custom wound Heyboer PT, used in my 35 to 50 watt amps.
I spec'd them with a second high voltage tap at a lower range for my cathode biased 6L6 amps using a 5U4GB rectifier.
Bruce,
I didn't say i was going to buy a OT, just a PT. That is unless i did a typo somewhere earlier. (which is entirly possible of course) I have a heyboer thats 4k and the wattage as i was just told via email response from heyboer is 50 watts. So it looks like i'm good to go for any of the possibilities i was considering....EL84, EL34, or 6V6. And i'm looking into EL34's at the moment since it's my favorite tube. I just need to either find a schematic who's PA and PSU i can follow pretty much to a T so that i don't have to ask a hundred questions before i can start buying parts and building. The weber 40 watt brit amps might be a possibility in which case i might then buy that PT. But i'm not sure yet. I'm burning to start ordering parts tho, and the wait is killin me !
Well I've done quite a bit of consulting work with some other builders and one of them has a small production, KILLER 4xEL84 amp with a 40-50 watt Super Reverb sized OT and that Hammond 270HX PT.
Last edited by Bruce / Mission Amps; 05-02-2008 at 07:30 AM. Reason: another frickin' typo
I wanted to buy a hammond because i heard good things and they're easy to find and inexpensive. Think i'm going with the 272jx tho because i decided i'm building a EL34 amp. In fact i'm getting the order together now. Can't wait.
I've used that one and it is a pretty good PT.
Here's a tip... most all of the generic Hammond PTs I've used over the years will actually make about 6% to 12% higher secondary voltage then what they are advertised at.
So your 300v PT might actually have 318vac to 336vac on the secondary before rectification and filtering.
Which will mean nothing to your EL34s but in cathode biasing, it is usually easier to nail them down with less B+ then too high of B+.
So plan accordingly.
What rectifier do you plan on using?
If a GZ34/5AR4, you will probably see about 410vdc to 440vdc.
I know it is a big spread... but that is why some of us spend thousands of dollars over a number of different orders with custom tranny companies to get the actual voltages we want.... and Hammond years ago, Hammond used to
build custom trannys for me so I have a good history with using them.
GZ34/5AR4, yes. But on that voltage..wow, someone else told me a VERY different scenario. He said that i should get low 300's (i think he said around 335) on the plates ! I hope he's closer on that one because i was shooting for a lot less than 440V !!!
Well, whatever happens happens because i already placed the orders.
EDIT: come to think of it, i believe he said 350, and also calculated in a 25v loss for cathode bias. maybe you didn't calculate that? Then again that would still be more than he estimated. I'm also using a choke if that matters.
The voltage specification for the transformer secondary is at maximum
current draw, which in your case is 250ma. If you draw less your voltage
will be higher. I was surprised when my 320-0-320 PT showed 335vac on
each leg and my B+ on standby was 450v. Things settled down to reasonable
levels with everything running properly.
Paul P
OK if this person thinks the B+ will be in the low 300v range then you need to start questioning his/her opinion or experience.
Depending on how high of idle current you run your two power tubes, the B+ will be around 1.25 to 1.30 times the measured hi-v AC going into the rectifier.
Although it is possible Hammond has corrected some of these PT issues, I betting they have not and you'll see over 400vdc with the amp idling in normal ranges... assuming you are not going to run the power amp in class A idle conditions.
Here is why.
If the PT is rated 300v-0-300v under a 250ma load, and it follows what my experience has been over the years with generic Hammond PTs, then you'll have at least this:
300v x 1.06 = 318, times 1.25 (accounting for medium current voltage drop through a rectifier tube) = 397vdc.
I would not be surprised to hear it was actually higher then that, (maybe as much as 420vdc to 440vdc) though because (most Hammond PTs are wound with 115vac to 117vac primary voltages in mind, and it seems like modern line voltages are running higher then 120v plus, your amp (at idle) will not be stressing the PT very much.
Your actual line voltages from your wall will be a big factor here.
But even at that there is nothing wrong with those voltages for a pair of cathode biased El34s.
Yes, you can subtract the cathode voltage from the B+ on the plates.
Ok, so lets say it's 400 and i subtract what, 30v for CB? So 370. That would be ok, but i was hoping for lower just because i want the tone to be spongier than it is with typical marshall voltages. But then again i really don't know WTF i'm doing anyways, so whatever i end up with will just be pure luck anyways if it's good !!!
http://img379.imageshack.us/my.php?image=el34ns2.jpg
Bruce, what about if he uses a choke filtered supply like you showed me a year or so back? Will that take things too low?
It is going to be a choke. a 20h 100ma to be exact.
If you're going with a choke filter (choke comes before any caps) it'd better
be rated for the 250 ma of your PT, or whatever maximum current you're
planning on.
If it's a capacitor filter with a choke after (which I believe is the case) you
should be ok.
Paul P
I was told to get a 100, and that even a 75 would be ok. Heres the schematic below, is that ok the way it is?
http://img3.freeimagehosting.net/ima...c69dba0f75.jpg
What I was asking about was a choke input where the choke is in series before any caps, which is what Paul was talking about in his first line. That method brings down B+ quite a bit.
Like this http://music-electronics-forum.com/a...4&d=1180230529. Notice that the choke is in series before any filter caps.
I agree he'd need a higher rating than I used for my single ended build.
Ok, then i'm fine as is. By the way, does anyone know what value the choke in the matchless chieftain is? The schematic is illegible.
A decent rule of thumb for a choke input filter is to use .9 x the measured AC secondary voltage while under load... that's the hard part... under load.
I have a test jig with 100 watt resistors to come up with this crap but a home builder is kinda at the mercy of one at a time experimentation.
A choke input is probably not needed nor wanted in this amp though.
That will bring the B+ down pretty low.
Actually, I wouldn't be over concerned with too high of B+ even if it ended up being 420-440v with cathode biased EL34s.
If you want more sag and lower B+ use a softer rectifier tube, like a 5U4GB.
Daz if you were using 250 ohms to 300 ohms as your cathode biasing resistor, the idle current, per EL34, with 30vdc on the cathodes would be 50ma to 60ma each.
If your plate to cathode voltage was then also say, 420v -30v = 390v then the power tubes would be idling at around <20 to 23 watts.
That is a bit hot for modern EL34s but because of the lower plate to cathode voltage, wouldn't really hurt them.
A decent way to add a little sag and still use the PT with some fat idle current would be to limit the B+ a little.
What I would do is put some series resistance on the high voltage AC to the rectifier tube.
Maybe a 120 to 150ohms at 5 to 10 watts on each leg.
With the tubes drawing about 80ma, there would be about a 10 drop at idle... before the rectifier and filter cap. But with the amp really cranked up the current through those resistors will be much higher and so will the voltage drop.
If the power stage pulled 150ma to 200ma at high volume, then the drop would be 18vac to 24vac... and that would be some real sag.
Just an idea.
Last edited by Bruce / Mission Amps; 05-06-2008 at 08:31 PM.
Thanks Bruce. Of course as usual some of this is over my head enough to generate more questions.
So i shouldn't have bought it? I was going on the advice of others, so i'm curious why they told me this. But if you really believe this is the case, should i use a R in it's place, and if so what value?A choke input is probably not need in this amp though
Is this a plug and plat tweek? In other words, can i just plug it into the socket with no changing of pin wiring?If you want more sag and lower B+ use a softer rectifier tube, like a 5U4GB
What value should i use in order to bring the bias to a happier medium?Daz if you were using 250 ohms to 300 ohms as your cathode biasing resistor, the idle current, per EL34, with 30vdc on the cathodes would be 50ma to 60ma each.
If your plate to cathode voltage was then also say, 420v -30v = 390v then the power tubes would be idling at around <20 to 23 watts.
That is a bit hot for modern EL34s but because of the lower plate to cathode voltage, wouldn't really hurt them.
As to the sag idea, i may try that if i feel it needs more, but maybe start with lesser values and work up to where it feels right.
Thanks
There are two forms of power supply filters commonly used with tube amps.
The "capacitor input to filter" which has a capacitor directly after the
rectifier, and the "choke input to filter" which has the choke directly after
the rectifier. If you look in the RCA Receiving Tube Manual under 5AR4/GZ34
you'll see that for an example power transformer the "capacitor input" filter
will provide 475vdc whereas the "choke input" filter will provide 275v. The
two filters have different advantages and disadvantages.
Your schematic shows a capacitor directly after the PT so you have a
"capacitor input" first filter, and will get the higher voltage.
You can just swap the tubes but you should count on having to play with
the power supply dropping resistors to get the voltages you want for the
different stages. Having a range of resistor values on hand is nice when
you get to this point so you don't have to wait a week or more between
each trial.
Paul P
Thanks Paul. thats some stuff i really needed to hear and understand. I had no idea the choke would drop the v so much when it's placed first. Only thing that puzzles me is, when you use it as i am isn't it then going to drop the PI and preamp stages an awful lot?
No, but I'd have to pull out my books to remember why. On my amp the
voltage is hardly dropped at all. I measured 391v before the choke and 389v
after the choke, and this is with a larger than usual current draw from my
preamp.
Paul P
The choke after a first filter cap will only drop the B+ rail by the DC resistance of the coil in the choke and that's pretty much all.
The first filter cap will charge up to the peak DC voltage from your rectifier.
Remember that 300vac from the PT? Well it won't be 300v but let's ignore that for a minute.
That is RMS voltage not peak voltage.
The peak voltage form your PT is actually 1.414 x the rms or .707 of the 300v on each side of the PT with respect to the center tap, which is 600v.
As a matter of fact, that PT is actually a 600vct tranny. That means 600v with a center tap!
600v x .707 = 424v.
1.414 x 300 = 424v.
The only thing in between those numbers is the rectifier and what the impedance of that device is.
However all these Hammond PTs make about 6% to 12% more voltage then they are rated at... especially when lightly loaded and then run at normal line voltages, which around here are 122v-126vac.
Most of those Hammond trannys were spec'd out with primary windings seeing 115vac to 117vac. That makes a difference.
So the actual peak voltage could be as high as:
(1.06 x 300v) x 1.414 = 450vdc to, (1.12 x 300) x 1.414 = 475vdc when rectified under no load and then filtered with a high voltage cap.
That 6% to 12% still applies when you use a choke input filter instead of a capacitor input filter because it is before the choke or cap.
But, the choke input will limit the peak AC voltage from the PT because it impedes the peak voltage to something less then the RMS voltage.
In inductors, AC voltage is bucked while current passes rather unimpeded.
That is why I said to roughly use .9 times the 300v rms number.
.9 x 300v = 270v.
Now take the 300v and use the rule of 6% more, or x 1.06.
1.06 x 300v = 318v
I think you will see at least that from the PT and probably more.
318 x .9 = 286vdc... that too low for this amp.
All the current needed to run all the high voltages in the amp has to pass through this choke and needs to be BIG, probably at least as high of current rating as the PT.
That is why I said don't use a choke input filter with this PT unless you want really low DC voltages for EL34s... and that means through out the entire amp.
It is fine with a pair or quad of EL84s though.
The normally seen screen choke after the plate supply, (and it is MUCH smaller, supplying only screen current to the EL34s or what ever and the tiny load from the preamp tubes), has nothing to do with dropping the B+ rail in these numbers. That choke is exposed to B+ that has already been filtered and smoothed by a big capacitor that has been charged up to the peak DC so there is very little AC component left.
I doubt you'll see more then a few volts drop with a choke used after the first B+ node to the screens and the other preamp tubes.
OK
This is why you need to read up on all this stuff before wearing me out! ha ha
Last edited by Bruce / Mission Amps; 05-07-2008 at 06:22 AM. Reason: more typos
Ok well as long as thats the case you don't have to explain it to me .....i'd never understand it anyways
I guess I don't know what choke you bought.. what are the specs?Thanks Bruce. Of course as usual some of this is over my head enough to generate more questions.
So i shouldn't have bought it? I was going on the advice of others, so i'm curious why they told me this. But if you really believe this is the case, should i use a R in it's place, and if so what value?
Is this a plug and plat tweek? In other words, can i just plug it into the socket with no changing of pin wiring?
What value should i use in order to bring the bias to a happier medium?
As to the sag idea, i may try that if i feel it needs more, but maybe start with lesser values and work up to where it feels right.
Thanks
And I am not clear if this is just a small screen node choke or a big input filter choke.
You have to experiment with the cathode biasing resistor based on the B+... you can't just pull it out of your ah, hat... it doesn't work that way.
Very low voltages will use a smaller one.. 220ohms maybe,,, high B+ will need a larger one.. 330ohms maybe. etc.
Yes with that PT you can use just about any octal socket rectifier I can think of.... well the ones used in any guitar amps. You know, GZ34, GZ30, 5U4GB, 5U4, 5V4GA, 5Y3GT etc etc.
Last edited by Bruce / Mission Amps; 05-06-2008 at 08:37 PM. Reason: added
Hammond P-T193C
Specifications
Rating: 20 Henries, 100 mA
Max. VDC: 600
Resistance: 181 ohms
Mounting centers: 2" x 2.1875 vertical mount
Weight: 3.3 lbs.
Heres the schematic showing where i was going to put the choke.....
http://img3.freeimagehosting.net/ima...c69dba0f75.jpg
As for choosing a cathode R, how then do i go about it? I suppose i should use a safe value like a 330 ohm so i can start the amp then calculate what i need by the voltages? if so, is there a layman's tutorial on how to go about this that you or anyone knows of ?
I really tried hard to get every tiny bit of info i could before i ordered the parts. I asked every question i could think of at many forums, and once i had everything i thought i needed to order the parts and build the amp, the answers i got elsewhere are then shown to be apparently flawed. I was told by several people that 250 ohms would be fine even tho they knew exactly what i was doing and how the voltages were not yet known. Now i hear this and unless i can find out how to do this i'm kinda screwed. It never ends.
Last edited by daz; 05-06-2008 at 10:05 PM.
WOW!
I got all of that Bruce!!! Thanks, that's why I asked the question, to get a lesson.
Ok, i have another question. Why do i need the choke? Can i do without it? Why am i asking? Because i just recieved 1/2 my parts order which included the choke. God god man ! I've had amps with chokes, but not like this. This Effing thing is huge ! It's bigger and much heavier than my 18 watter's OT. The PT is of course heavy as is the OT, and between the 3 of them i'm gonna need a forklift to carry this thing. Tell me again why i need this $32.00, 50 pound chunk of metal?
In my amp it reduces AC ripple on B+ from 3v rms to .07v rms. Seeing asTell me again why i need this $32.00, 50 pound chunk of metal?
1mv is audible I think they're pretty useful.
Hammond says your choke weighs 3 lbs
Paul P
Thats about right, 3 Lbs. The chokes i've seen seem like they probably weight 8 Oz. I can't understand why this one has to be so friggin big. Chassis is gonna be stressed with all this weight. Probably should have looked for a punched steel one.
Yours does seem to be pretty heavy. My choke, 3H, 100 ohms,
90 ma (I think), is not much smaller than yours but weighs about
one pound. I see weight as a good thing when it comes to amps.
Paul P
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