OT [TUBE] can someone please check this for me

[jimmy]

hi everyone

i PROMISE this is the last time i will bother you with amp questions. i was recently considering my next amp project, (champ, 18 watt etc) and have come across the Z20 from www.diyguitaramp.com and it looked GREAT. unfortunately i cant get the correct transformer, so i had to make some modifications. i am a liiiiiittle confused about the output stage setup and the power supply. please check the schem for me and tell me if its ok?

*EDIT: see schem below*

Cheers
Jim

P.S. on duncans PSU calculator, how do you determine what the 'load' will be for a particular project?

[blandman74]

Well, I'm not sure about what voltages you're seeking for B+1 or B+2.  I can say, though, that you might want to consider doing a little more filtering of the power supply voltages for B+1, since I'm not sure that a capacitor alone will do it.  Try throwing a choke in right after the combination of the filter cap and 470k resistor (but before the dot marked B+ obviously).  You *could* use a resistor instead (it's cheaper) but considering the current you'll be drawing, I would use a choke.

A choke with the appropriate current rating and at least 2 or 3 Henries would do it, and make it whisper quiet.

Since you're using solid state rectification, you should add a standby switch on the center tap of secondary of the PT.

As for the question you're asking, I don't use the Duncan Amps software, so I don't know the answer to that.  But mostly you'll just need to know what current you'll be drawing. Figure at least 1 ma for each preamp tube, 3-5 ma for EL34 screens, and whatever the data sheets say for the EL34 plate current draw.  Throw in a cushion of 20% and you'll have the rough current draw.  Then use Mr. Ohm's law to calc the voltage drops.

I know it wasn't what you're asking, but I thought I'd contribute a little bit.

[jimmy]

thanks for the input! as for the B+1 or the B+2 voltages, i also have no idea what voltages i hope to achieve. i was just guessing.

standby switch - good idea, i forgot about that.

filter choke - i wasnt sure how or what to do to these so i left it out. ill stick one in.

as for the ohms law bit, thanks anyway, its more than i know... ill give it a go.

anyone else?

Cheers
Jim

[jimmy]

righto got my photobucket account. heres the schem.

EDIT: see below for schem again, didnt update, think it had something to do with temporary internet files...

anyone?

thanks
Jim

[Jim Jones]

Hey Jim,

Have a look at an old Marshall 50-watter schem for more ideas.  The B+ 1 (centre tap of the OT), should connect before the choke, while the screen supply (B+ 2) should connect after the choke.  Also, the node at the top of the schem that you have marked B+ 1 should actually be B+ 2...everything that's "downstream" of the power tube screens should connect after the screen supply.  Again, check out an old 50-watter Marshall schem to see what I mean. www.schematicheaven.com or www.drtube.com are good places to look.

Hope this helps some!

Jim

P.S.  Also, I don't think you'll need that 22K resistor that's in series with the choke...

P.P.S.  I think I'd do a "B+ break" standby switch rather than the PT centre tap lift you have drawn in.  I'd use that methode with cathode biased amps but if you're amp is fixed bias, you'll be smacking the power tubes with plate voltage (and without bias voltage) for an instant.  It may not be the end of the world, but something to think about.

[ED]

Looks pretty good to me my friend.

I see youve got a 1650K output transformer. 50 watt output. Same one ive got in my room right now, cept i still havent quite got all the stuff i need to start(perspex & wire).

On alot of marshalls they have 50uF filter caps, while on your schem you have 22s. I just thought maybe thats too low for a 50 watt, and seein as you based your amp arond tha Z20,  they might not do there job properly. I dunno though. I just noticed most of the bigger amps have higher values. Also the 1k 1W resistors you have on yours are 10K 1W i think. Im no expert   as you know though.

Looks good man
ED

[jimmy]

thanks Jim, your input has been noted and applyed. much appreciated.

ED!!! hows it doin? good point about the caps and resistors, its not so much the wattage but the voltage i think that might affect it, as there is many more volts in this one than in the Z20. noted and applyed.

tell you what though, i am STILL listening to Tuesday's Gone  

anyone else?

thanks
Jim

[Peter Snowberg]

That's looking really good.

Three things..... (all having to do with the phase inverter)

1. The 470pF across the PI plates is often a bandage applied to stop oscillations. You may want some value here or you may not, but if you do I would doubt that you would need as much as 470pF. That's the same size as the treble cap in the tone stack.  :shock: Try some variation in that value.

2. You have a serious error in the PI. It should work as is IF you move the 0.022uF cap on the grounded grid side down so that it sits between the junction of the 22K and 2K2 resistors and the junction of the grid on the right side of that tube with the 470K grid load resistor.

3. It looks like the schematic you pulled the PI from has a negative feedback loop. You don't have such a beast on yours so two ways you could change things..... the first would be to just eliminate that 2K2 resistor with short. The PI will work better when the grounded grid side has the lowest impedance to ground you can give it. The second way to go would be to add a presence control with a pot, a cap, and tap intp the ouput side of the transformer. I generally like a little presence, but only a little. No NFB can be too bright while the stock values Fender used for years make things dull and lifeless. There is a happy medium to be had there.

4. If you want to wire up sockets for both EL34s and 6L6/5881s, remember that the 6L6 is a beam tetrode while the EL34 is a true pentode. Be sure to wire up pin 1(!). If you want to swap back and forth between power tubes, consider adding a good quality DPDT switch to select two different bias pots. That way you set them once with a meter and then you can compare without having to re-dial a new value. Just swap tubes and flick the switch.

Take care and best of luck on your build!,
-Peter

[Peter Snowberg]

one more thing.....

With the volume controls like that you're looking at a 500K input impedance. That's a bit low and it will muddy things up. I would eliminate the second volume control and just connect the jacks together. In the fender approach the switch is used on the second jack to put two 68K resistors in parallel when only using the 1st jack. That 34K less of input resistance should help out and the value is still plenty big for RFI supression. Some amps don't include series resistance at all. Try a bunch of values and see what you like there. Maybe a 500K pot in series before the 1M volume pot will be what you settle on.

I bring this up because you only have two stages and a passive tone stack so you'll probably need all the gain you can get.

Take care,
-Peter

[puretube]

agreed, Peter;
but the 470p cap is located at an order lower impedance part of the circuit than in a tone stack.
AC30 got 4700p (IIRC) in series with a pot ("High Cut") at that location.

[jimmy]

wow thats some GREAT info there. thanks alot. ill update the schem.

thanks again! any other suggestions?

cheers
Jim

[jimmy]

ok heres the NEW updated schem. i had to change the name around because the last one would change - i think it had something to do with my temporary internet files  :?

anyways here it is:


i think its almost there! anyone else?

thanks alot
Jim

[chumpito]

Regarding the standby switch, I've read that it's completely unecessary.  The standby switch was only needed for high voltage applications (kV) and since your amp would definately fall way below high voltage you shouldn't need it.

[jimmy]

aha, now thats one thing i DO know...

the standby switch isnt really necessary, but it lengthens the life of your tubes, as they are being immediatly slammed with the full voltage coming from the power supply without having heated up. while this is acceptable, the added stress on the tuubes will shorten their life, and as they are expensive, that completely sucks.

cheers
Jim

[petemoore]

What voltages are the tubes to see at the plates?
 I have some  12AX7 gain stages in boxes at 30V...I'm wondering what they'd be like running them at higher voltages.
 Also, I have some EL84's here, and in some evetuality, plan on wiring them into an output amp section, Ppower supplies being the present 'hurdle'.

[jimmy]

well, i dont know. im really just guessing. according to duncans PSU designer, the tubes will be getting 445 volts, and thats with a load of 5k, but i dont know what the loads meant to be, so i couldnt tell you  :lol:

anyone who does know what the voltage will be?

thanks
Jim

[puretube]

those 1/2W resistors parallell to the PS caps:
make that 2x 270k 1/2W in series each;
normal half-watters are specd. for 250V, only  (usually)

1W wouldn`t hurt, though...

[jimmy]

i was not aware of that 250V thing, thanks!

any other suggestions?

thanks
Jim

[Peter Snowberg]

I see only one more change before you warm up your soldering iron.

The standby switch needs to move to the right so it sits between the junction of the two diodes and the 80uF filter cap. The other benefit of that position is that the bias supply is always on even with the high voltage off.

I'm a big believer in having a standby switch in any circuit where the tubes are seeing over 50 volts on the plates. Why have cathode stripping if you don't need it?

Whoops... one more thing in the input jacks... If you just switch the lower jack switch connection directly to ground, you'll have it. In that situation use only the bottom jack for maximum signal, only the top jack for hotter signals you want to attenuate, or both at the same time for dueling guitars.

Now the only changes are "additions" . Consider adding some high voltage diodes from ground to the 6L6 plates (cathode pointing to the pates) to eat up spikes that normally decrease tube life and then add a 230V MOV across the plate connections to complete the spike eater.

Take care,
-Peter

[jimmy]

aaahhhhh thanks again peter! added and updated. one thing - whats the schematic symbol for a MOV (metal oxide varisistor? - i hope so...)?

few last questions:
1) how many watts will this thing put out
2) what can i expect the B+ voltages to be
3) how do i work out the 'load' on duncans PSU!??!?!

thanks ALOT guys, ill post the complete schematic in just a tick.

cheers
Jim