Please critique my tube amp schematics

[tubegeek]

The circuit in the "Super SE AC4" schematic you linked to is exactly what is shown on p. 95 of Merlin's book. P. 94 has a couple of suggestions for variable bypass on the pentode's screen. The TubeCad article does take this one step further with the triode driving and buffering the screen - it's a step up in complexity from the other design. I'll wait until Merlin OK's it before I make a link with his two schematics on p. 94, I don't want to cut and paste pages from his book unless it's Ok with him - and it very well might not be.

[Roger Martin]

Would anyone be so kind to suggest me a verified small tube amp schematic like 10-30 watt ?

Thank you

[amptramp]

Even our best gurus may want to peruse some of the designs and derivations.

I have no illusions that you were calling ME out with that one, amptramp but I do just want to chime in with an enthusiastic agreement. John Broskie (Mr. TubeCAD)  afforded me the privilege of meeting him on a trip to NYC some years back and it was a very enjoyable afternoon. Just about anyone interested in tubes - or hybrid circuits for that matter - should read everything on that site from A to Z and then some. John has an enormous talent for investigating standard circuits with fresh approaches and he has redesigned a huge number of basic blocks that most people take for granted. The only problem with his writing is that there are too many ideas flowing from him to know which ones to try!

Actually, I wasn't calling you out specifically.  Even our top people like P.R.R. and R.G. would find it informative and entertaining and I have never heard of anyone suffering from excess knowledge.  I do like Broskie's logical explanations that use the "suppose we increase the grid voltage by one volt..." derivations that are about as easy to follow as anything I could imagine.  His stuff on the µ-amp/SRPP is excellent and that subject comes up here on a number of occasions.

[JRB]

Would anyone be so kind to suggest me a verified small tube amp schematic like 10-30 watt ?

Thank you

Excuse me for going a bit off topic but here are some links:

Apparently this dude sells decent kits I haven't bought one but you see his name pop up al over the tube amp forums,
https://taweber.powweb.com/store/kits.htm

The AX84 community made some kind of starter project.
http://www.ax84.com/

VOX and Fender have put up the schematics of their amps on their website,
http://www.fender.com/en-NL/support/articles/?section=downloads&category=amplifier-schematics
http://www.voxamps.com/support/circuits/

And then there is dr.tube
http://www.drtube.com/en/library/schematics

[Roger Martin]

Would anyone be so kind to suggest me a verified small tube amp schematic like 10-30 watt ?

Thank you

Excuse me for going a bit off topic but here are some links:

Apparently this dude sells decent kits I haven't bought one but you see his name pop up al over the tube amp forums,
https://taweber.powweb.com/store/kits.htm

The AX84 community made some kind of starter project.
http://www.ax84.com/

VOX and Fender have put up the schematics of their amps on their website,
http://www.fender.com/en-NL/support/articles/?section=downloads&category=amplifier-schematics
http://www.voxamps.com/support/circuits/

And then there is dr.tube
http://www.drtube.com/en/library/schematics

Awesome, JRB. Many thank yous. Sorry to jump out of topic, guys 
Let me read them first lol

[patricks]

Thanks tubegeek - don't worry about posting the pages from Merlin's book. It took me quite a while to find a schematic where I knew the "morph" control was implemented, so my take guess is that he won't want them posted. I'm on the lookout for a second-hand copy of his book, if anyone's got one they don't need/want. I may end up just buying a new one, though; they seem to be the sort of thing that people get and don't let go of. Plus, the second-hand first editions are selling for twice the price of the second editions!
Thanks for the calculations, too. I'm picking things up as I go, but every now and then I hit a roadblock so this helps.

Roger, if this is a first build, I've heard lots of good things about the ax84 kits. They seem like the easiest and cheapest way to start; I'd go with one, except that I'm looking for some pentode flavour in my preamp
Ceriatone sells a few different kits; Trinity amps sells kit versions of their amps, too.
As far as schematics go, el34 world has schematics from just about every well known brand you can think of.
As an aside, if this is a first build and you want 10-30 watts to get something that's useful at stage volume, maybe consider building a smaller amp and putting more efficient speakers in it. In all my searches, I came across this thread on ax84 saying that one 100db speaker driven at 10 watts is just as loud as four 95db speakers driven with 10 watts. Much easier using sensitive speakers than building a more complicated amp - at least that's what I'm planning for my first build

[Roger Martin]

Thanks tubegeek - don't worry about posting the pages from Merlin's book. It took me quite a while to find a schematic where I knew the "morph" control was implemented, so my take guess is that he won't want them posted. I'm on the lookout for a second-hand copy of his book, if anyone's got one they don't need/want. I may end up just buying a new one, though; they seem to be the sort of thing that people get and don't let go of. Plus, the second-hand first editions are selling for twice the price of the second editions!
Thanks for the calculations, too. I'm picking things up as I go, but every now and then I hit a roadblock so this helps.

Roger, if this is a first build, I've heard lots of good things about the ax84 kits. They seem like the easiest and cheapest way to start; I'd go with one, except that I'm looking for some pentode flavour in my preamp
Ceriatone sells a few different kits; Trinity amps sells kit versions of their amps, too.
As far as schematics go, el34 world has schematics from just about every well known brand you can think of.
As an aside, if this is a first build and you want 10-30 watts to get something that's useful at stage volume, maybe consider building a smaller amp and putting more efficient speakers in it. In all my searches, I came across this thread on ax84 saying that one 100db speaker driven at 10 watts is just as loud as four 95db speakers driven with 10 watts. Much easier using sensitive speakers than building a more complicated amp - at least that's what I'm planning for my first build

Yes, Patrick thank you for the advice.
I need it 10 watts or so.....use a reliably good quality sensitive speakers.....Jensens ?
Any idea for simple Marshall or Bugera maybe ? Which one does it sound with lots of harmonics and colorful mojos. For blues to metal purposes.
And what kind of speakers, brands or sensitivity value to choose from ?

[JRB]

18Watt Marshall maybe?

Pushpull EL84's in the poweramp and one 12AX7 as a preamp with a 12AX7 as phase inverter in between. Doesn't get a lot less complicated then that.

[patricks]

use a reliably good quality sensitive speakers.....Jensen ?s

I'm not an expert on speakers, but the way I've approached it for this build is to think of the type of amps I want to emulate, then find out what speakers are in 'em. Then, do some googling to see who makes clones (gives you more choice - sometimes the clones have higher sensitivity than the originals) and pick the one that suits. Weber, Eminence, Celestion and Warehouse guitar speakers are the ones I know of.

I'm thinking I'll go for a WGS Reaper (Celestion G12H30 clone rated for 30 watts and 98db sensitivity) or similar for mine. That's another thing to think of, the speaker power rating. There wouldn't be much point putting a G12H65 in mine, cos it's rated for 65 watts and a 5 watt amp wouldn't get it moving properly.

Speakers make such a big difference to the sound of the amp that it's hard to say what'll be best until you've heard your amp played through them. The easiest way to get going is probably to hunt on Craigslist for a loaded cab, or put up a "wanted" ad, then swap speakers later on after you're up and running.

[tubegeek]

Actually, I wasn't calling you out specifically.  Even our top people like P.R.R. and R.G. would find it informative and entertaining...

No, I know that! I do NOT consider myself anything like a "top guru" especially in the company we get around here. I'm not that impressed with myself, trust me. My feeling is, I know what I know, and what I don't know, I shut the hell up about.

Better to be silent and be thought a fool than to speak up and remove all doubt!

[patricks]

Better to be silent and be thought a fool than to speak up and remove all doubt! Wink

Yup. Case in point, in all my gabbing, I completely overlooked some good advice that came out of my own mouth - the ax84 amps. The P1 kit will be my easiest, cheapest way of getting this project off the ground. It's only $250 including valves and transformers - I couldn't put the components together myself for that. So the plan will be:
1. build P1
2. Rawk!
3. Get a 6GH8 and mod the P1 with some pentode flavour
4. Repeat steps 2 and 3 to my heart's content.

The bonus is that after I mod it, I'll have a spare 12ax7 to use as a cathode follower and paraphase inverter, or a buffered effects loop, or whatever else takes my fancy if I decide to expand the amp. The chassis has plenty of room to fit more tubes and the power transformer that comes with the kit has plenty of current available to run extra tubes, too.
Plus, the doberman amps site sells Merlin's preamp book for $25 - almost half of what it goes for on Amazon!

The only thing I'm not quite clear on is why the P1 schematic says that the B+ out of the rectifier is 266 volts, when the Hammond website says that the 269AX transformer puts out 380V rms. I could understand if it was 380V peak-peak, cos that'd work out to 270V rms. Unless the PT for the P1 is meant to be the 269AX and not the EX. I can always ask 'em...

[tubegeek]

For full wave, cap input: V(DC) = .707 V(ACRMS)

Good summary document here from Hammond: http://www.hammondmfg.com/pdf/5c007.pdf

[patricks]

Excellent, thanks very much! 

[patricks]

For full wave, cap input: V(DC) = .707 V(ACRMS)

Good summary document here from Hammond: http://www.hammondmfg.com/pdf/5c007.pdf

Resurrecting the thread cos I'm confused (again) - on Merlin's site, on the page about two phase, full wave recitifers it says that the voltage after rectification increases, whereas the Hammond site says that it decreases...

[psychedelicfish]

The rectified voltage will be higher than the AC voltage, because the transformer's output voltage will be an RMS voltage, or the average voltage. When you rectify the AC, the resulting DC voltage will be the peak voltage (roughly 1.4 times the AC voltage), not the average.
When you start pulling something like the max current your transformer can supply, the voltage the transformer supplies will drop, or "sag". This "sag" also happens when you pull current through a tube rectifier, because they, like all electronic components, aren't perfect, and have an internal resistance which will cause a voltage drop when you pull lots of current.
In Hi-Fi designs you would try to avoid the power supply "sag" as much as possible, by using Si rectifiers, which have a far lower internal resistance than a tube rectifier, and by using a transformer rated for at least twice the max power you will draw. However, guitar amps aren't Hi-Fi amps, and guitarists tend to like the "sag", which is partly the cause of the "natural compression" tube amps have.
So in short, running the amp at low volumes the rectified voltage will be higher, and running it at high volumes will cause the supply voltage to drop, perhaps below the voltage the transformer is supposed to supply.

[patricks]

Thanks - that makes sense and it's what I initially thought. Problem is, though, that the Hammond sheet says that the DC voltage will be less than the AC RMS voltage for that type of rectifier (full wave - using two diodes - with capacitor input).
The DC(peak) = 1.4*AC(rms) matches up with their formula for a full wave bridge rectifier with a capacitor input load, but not the one mentioned above.
There must be something basic I'm missing.

[tubegeek]

There must be something basic I'm missing.

Indeed there is.

If you look at the center-tapped secondary/two diode circuit, and think about which diodes are switched "on" and which "off" during the positive and negative sections of the AC cycle, you'll see that basically a center-tapped secondary is hitting the cap with half its voltage during each half-cycle: current is not flowing through one half of the secondary. The loop is from one end of the secondary, through the cap., to the center tap. The other end is connected to a diode that is switched off.

The four diode bridge is a much more familiar circuit today, but a two-diode circuit was much more typical in the days of vacuum rectifiers. It can be arranged so that only one heater circuit and one cathode is needed to rectify with two anodes. I think this would not just be doubled for four anodes: you'd need three cathodes and heater circuits to make it work, a "top," a "bottom," and a "middle."

If you cut the (end-to-end) secondary voltage in half and then convert that value from RMS to peak, it works out. There is very little drop across the vacuum rectifier because the circuit isn't drawing a heck of a lot of current, even so, I bet that AX84's value is a little higher than you'll actually measure. On the other hand, the transformer will be running high due to the light load - so maybe it comes out in the wash.

I'll tell you what confused the crap out of me at first: why the positive voltage appeared on the vacuum rectifier's cathode instead of the anode.


From the page you linked:

For example, with a transformer rated at 300-0-300Vrms, the DC voltage after rectification will be close to:
1.4 * 300 = 420Vdc

Note he is multiplying a half-secondary by 1.414, same as multiplying the end-to-end value by .707.

[patricks]

Of course, thanks so much!
It took a little time to rattle around in my brain and sort out half-secondaries and which side switches when, but I've got it now. I didn't realise until now that Hammond quotes their power transformers' end-to-end voltages and that I could calculate either 1.4 multiplied by half the end-to-end voltage, or 0.7 multiplied by the full end-to-end voltage.

I'll tell you what confused the crap out of me at first: why the positive voltage appeared on the vacuum rectifier's cathode instead of the anode.

Aw, man! Now my brain hurts again. Does the current flow backward (anode to cathode) in a rectifier valve?

[amptramp]

I'll tell you what confused the crap out of me at first: why the positive voltage appeared on the vacuum rectifier's cathode instead of the anode.

Aw, man! Now my brain hurts again. Does the current flow backward (anode to cathode) in a rectifier valve?

Electrons inside the tube flow from cathode to anode.  The capacitor is connected from the transformer centre tap which is the negative output and the positive side of the capacitor is connected to the cathode of the rectifier.  When the plate goes positive, electrons flow from cathode to plate, removing electrons from the side of the capacitor that is connected to the cathode and sending them through the transformer winding to the centre tap where they are added to the electrons on the negative side of the capacitor.  When the plate goes negative, no current flows and the capacitor retains its charge - positive on the cathode side and negative on the centre tap side.

What adds to the difficulty is that the diode only conducts when the cathode voltage is pulled higher than the capacitor voltage.  Thus, the current waveform is a series of spikes where the average current is equal to the spike current times the duty cycle of the time the diode is conducting.  Inductor input filters are used where the load requires more current than the diodes can carry in this type of pulsed operation.  But choke input filters have difficulties of their own.

[patricks]

Ah, that makes sense, thanks heaps 
Much easier to think in terms of the movement of electrons.