Critique my schematic, please

[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

[d95err]

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.

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...)

[daz]

Thats awesome. I too can't understand why all amps don't have that. All mine will from now on tho!

[daz]

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

[Wakculloch]

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.

[Wakculloch]

Sorry. Just reread and saw it was 2 EL34's not 4.

[Bruce / Mission Amps]

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 .... 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

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.

[daz]

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 !

[Bruce / Mission Amps]

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.

[daz]

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.

[Bruce / Mission Amps]

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.

[daz]

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.

[Paul P]

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

[Bruce / Mission Amps]

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.

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.

[daz]

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 !!! So when it come down to it, whatever the voltage i'm just hoping for tone and really don't know what voltage is going to make my ears happiest. of course i can always lower the B+ too. By the way, below is the schematic i edited to reflect what i'm building, tho the blued out preamp section is not. That will be very different. In fact i'm using the same cascaded pre i did in the 18watt at first, then i'll tweak it as necassary to mate well with the el34's . Feel free to critique this one as well, as the 6v6 is definatly out having already ordered all the parts for this el34 build.

http://img379.imageshack.us/my.php?image=el34ns2.jpg