I have bought this little 5W 5k:8 ohm SE output transformer, so I wanted to make an amplifier utilizing it and as few tubes as possible. My idea is to use two PCF802 tubes, mainly because I have 10 of them, and also they allow usual two-stage preamp with tone stack, plus I can put their pentodes in parallel so I can split power dissipation (I know I can make push-pull amp with them, but I will have to buy another transformer).
This is my schematic made using calculations from Merlin's great book, and also tweaked a little in LTspice.
(https://s26.postimg.cc/a7nodr0a1/PCL802_2_W.gif)
Is this the right approach, or someone has an idea how to improve this?
LTspice code below:
Version 4
SHEET 1 2460 1172
WIRE 176 -48 -48 -48
WIRE 1168 -48 176 -48
WIRE 1248 -48 1168 -48
WIRE 1168 -32 1168 -48
WIRE -48 16 -48 -48
WIRE 1552 16 1488 16
WIRE 1584 16 1552 16
WIRE 1936 16 1680 16
WIRE 2080 16 1936 16
WIRE 176 32 176 -48
WIRE 1168 48 1168 32
WIRE 1248 48 1248 -48
WIRE 1936 48 1936 16
WIRE 2080 64 2080 16
WIRE 1552 96 1488 96
WIRE 1584 96 1552 96
WIRE 1696 96 1680 96
WIRE 1856 96 1696 96
WIRE -48 128 -48 96
WIRE 1552 128 1552 96
WIRE 176 144 176 112
WIRE 272 144 176 144
WIRE 496 144 336 144
WIRE 560 144 496 144
WIRE 704 144 624 144
WIRE 2080 176 2080 144
WIRE 1248 192 1248 128
WIRE 1312 192 1248 192
WIRE 1424 192 1376 192
WIRE 1696 192 1696 96
WIRE 1856 192 1856 96
WIRE 176 208 176 144
WIRE 496 224 496 144
WIRE 704 240 704 224
WIRE 800 240 704 240
WIRE 976 240 880 240
WIRE 1904 240 1744 240
WIRE 1936 240 1936 128
WIRE 1936 240 1904 240
WIRE -208 256 -288 256
WIRE -128 256 -144 256
WIRE -80 256 -128 256
WIRE 48 256 0 256
WIRE 128 256 48 256
WIRE 976 256 976 240
WIRE 1424 256 1424 192
WIRE 1504 256 1424 256
WIRE 1648 256 1584 256
WIRE 1808 256 1648 256
WIRE 704 272 704 240
WIRE 1424 272 1424 256
WIRE -288 288 -288 256
WIRE 1248 288 1248 192
WIRE 48 336 48 256
WIRE 1072 336 976 336
WIRE 1200 336 1152 336
WIRE 1936 352 1936 240
WIRE 976 368 976 336
WIRE -128 384 -128 256
WIRE 496 384 496 304
WIRE 576 384 496 384
WIRE 704 384 704 352
WIRE 704 384 640 384
WIRE 144 400 144 304
WIRE 288 400 144 400
WIRE 1424 400 1424 352
WIRE 704 416 704 384
WIRE 1216 416 1216 384
WIRE 1344 416 1216 416
WIRE 144 432 144 400
WIRE 1216 432 1216 416
WIRE 1344 432 1344 416
WIRE 1664 432 1664 304
WIRE 1712 432 1664 432
WIRE 1824 432 1824 304
WIRE 1824 432 1712 432
WIRE 1936 432 1936 416
WIRE 288 448 288 400
WIRE 1664 464 1664 432
WIRE 1712 464 1712 432
WIRE 976 496 976 448
WIRE 496 528 496 384
WIRE 576 528 496 528
WIRE 704 528 704 496
WIRE 704 528 640 528
WIRE 1216 544 1216 512
WIRE 1344 544 1344 496
WIRE -288 560 -288 368
WIRE -128 560 -128 464
WIRE -128 560 -288 560
WIRE 48 560 48 400
WIRE 48 560 -128 560
WIRE 144 560 144 512
WIRE 144 560 48 560
WIRE 288 560 288 512
WIRE 704 560 704 528
WIRE 1664 576 1664 528
WIRE 1712 576 1712 544
WIRE 1712 576 1664 576
WIRE -288 592 -288 560
WIRE 1664 592 1664 576
WIRE 704 656 704 640
FLAG -288 592 0
FLAG 2080 176 0
FLAG 1936 432 0
FLAG 1664 592 0
FLAG 1552 128 0
FLAG 1552 16 OUT
FLAG 704 656 0
FLAG 976 496 0
FLAG 1216 544 0
FLAG 1344 544 0
FLAG 288 560 0
FLAG 1424 400 0
FLAG 1168 48 0
FLAG -48 128 0
SYMBOL voltage -288 272 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value2 AC ac 1.
SYMATTR InstName V1
SYMATTR Value SINE(0 .1 100)
SYMBOL cap -144 240 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 43 39 VTop 2
SYMATTR InstName C1
SYMATTR Value 220n
SYMBOL res -144 368 R0
SYMATTR InstName R1
SYMATTR Value 1Meg
SYMBOL res 16 240 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL cap 64 400 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C2
SYMATTR Value 680p
SYMBOL voltage 2080 48 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 150
SYMBOL res 1696 448 R0
SYMATTR InstName Rk
SYMATTR Value 75
SYMBOL cap 1680 528 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C4
SYMATTR Value 100µ
SYMBOL Misc\\triode 176 256 R0
SYMATTR InstName U2
SYMATTR Value ecf802t
SYMBOL res 160 16 R0
SYMATTR InstName R8
SYMATTR Value 20k
SYMBOL Misc\\tetrode 1856 240 R0
SYMATTR InstName U1
SYMATTR Value ECF802p
SYMBOL cap 336 128 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 43 39 VTop 2
SYMATTR InstName C7
SYMATTR Value 47n
SYMBOL res 1920 32 R0
SYMATTR InstName R6
SYMATTR Value 150
SYMBOL cap 1952 416 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C6
SYMATTR Value 4µ7
SYMBOL Misc\\tetrode 1696 240 R0
SYMATTR InstName U3
SYMATTR Value ECF802p
SYMBOL res 1504 112 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName Speaker
SYMATTR Value 8
SYMBOL Misc\\triode 1248 336 R0
SYMATTR InstName U4
SYMATTR Value ecf802t
SYMBOL res 1232 32 R0
SYMATTR InstName R4
SYMATTR Value 20k
SYMBOL cap 1376 176 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 43 39 VTop 2
SYMATTR InstName C3
SYMATTR Value 47n
SYMBOL cap 624 128 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C5
SYMATTR Value 330p
SYMBOL res 480 208 R0
SYMATTR InstName R5
SYMATTR Value 100k
SYMBOL cap 640 368 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C8
SYMATTR Value 100n
SYMBOL cap 640 512 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C9
SYMATTR Value 47n
SYMBOL res 688 400 R0
SYMATTR InstName R7
SYMATTR Value 50k
SYMBOL res 688 544 R0
SYMATTR InstName R9
SYMATTR Value 4k
SYMBOL res 688 256 R0
SYMATTR InstName R10
SYMATTR Value 50k
SYMBOL res 720 240 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R11
SYMATTR Value 200k
SYMBOL res 992 352 R180
WINDOW 0 36 76 Left 2
WINDOW 3 29 96 Left 2
SYMATTR InstName Gain1
SYMATTR Value {Rg}
SYMBOL res 960 352 R0
SYMATTR InstName Gain2
SYMATTR Value {250k-Rg+1}
SYMBOL res 1168 320 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R13
SYMATTR Value 10k
SYMBOL res 128 416 R0
SYMATTR InstName R14
SYMATTR Value 300
SYMBOL res 1200 416 R0
SYMATTR InstName R15
SYMATTR Value 300
SYMBOL cap 1360 496 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C11
SYMATTR Value 100µ
SYMBOL cap 304 512 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C12
SYMATTR Value 100µ
SYMBOL res 1408 256 R0
SYMATTR InstName R16
SYMATTR Value 1Meg
SYMBOL cap 1152 -32 R0
SYMATTR InstName C13
SYMATTR Value 22µ
SYMBOL res 896 224 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R18
SYMATTR Value 820k
SYMBOL res 1600 240 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R19
SYMATTR Value 68k
SYMBOL AutoGenerated\\xfmr 1632 64 M0
SYMATTR SpiceModel OutXFMR_5k:8
SYMATTR InstName U5
SYMBOL voltage -48 0 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V3
SYMATTR Value 200
TEXT 816 40 Left 2 !.include TubeLib.inc
TEXT 816 80 Left 2 !;.tran .1
TEXT 848 584 Left 2 !;.step param Rg 150k 250k 10k
TEXT 848 624 Left 2 !.param Rg = 150k
TEXT 824 120 Left 2 !.ac oct 16 10 50k
TubeLib.inc can be found here: https://groups.google.com/group/utracer/attach/a605dbb10fec4/TubeLib.inc?part=0.1&authuser=0&view=1 (https://groups.google.com/group/utracer/attach/a605dbb10fec4/TubeLib.inc?part=0.1&authuser=0&view=1)
Transformer model from here: http://www.intactaudio.com/forum/viewtopic.php?t=527 (http://www.intactaudio.com/forum/viewtopic.php?t=527)
However, it seems that I didn't find those PCF802 models in TubeLib.inc, but somewhere else, so I put them into the file:
* Generic triode model: ECF802T
* Copyright 2003--2008 by Ayumi Nakabayashi, All rights reserved.
* Version 3.10, Generated on Tue Feb 24 21:58:50 2015
* Plate
* | Grid
* | | Cathode
* | | |
.SUBCKT ECF802T A G K
BGG GG 0 V=V(G,K)+0.50414128
BM1 M1 0 V=(0.0045659791*(URAMP(V(A,K))+1e-10))**-0.51818208
BM2 M2 0 V=(0.74324314*(URAMP(V(GG)+URAMP(V(A,K))/56.232595)+1e-10))**2.0181821
BP P 0 V=0.0023316308*(URAMP(V(GG)+URAMP(V(A,K))/75.658411)+1e-10)**1.5
BIK IK 0 V=U(V(GG))*V(P)+(1-U(V(GG)))*0.0013441952*V(M1)*V(M2)
BIG IG 0 V=0.0011658154*URAMP(V(G,K))**1.5*(URAMP(V(G,K))/(URAMP(V(A,K))+URAMP(V(G,K)))*1.2+0.4)
BIAK A K I=URAMP(V(IK,IG)-URAMP(V(IK,IG)-(0.0012121946*URAMP(V(A,K))**1.5)))+1e-10*V(A,K)
BIGK G K I=V(IG)
* CAPS
CGA G A 1.5p
CGK G K 2.4p
CAK A K 1p
.ENDS
* Generic pentode model: ECF802P
* Copyright 2003--2008 by Ayumi Nakabayashi, All rights reserved.
* Version 3.10, Generated on Mon Feb 23 21:42:48 2015
* Plate
* | Screen Grid
* | | Control Grid
* | | | Cathode
* | | | |
.SUBCKT ECF802P A G2 G1 K
BGG GG 0 V=V(G1,K)+0.43792508
BM1 M1 0 V=(0.0086088911*(URAMP(V(G2,K))+1e-10))**-0.63017681
BM2 M2 0 V=(0.7041669*(URAMP(V(GG)+URAMP(V(G2,K))/34.363671)))**2.1301768
BP P 0 V=0.0040790347*(URAMP(V(GG)+URAMP(V(G2,K))/48.800463))**1.5
BIK IK 0 V=U(V(GG))*V(P)+(1-U(V(GG)))*0.0023616451*V(M1)*V(M2)
BIG IG 0 V=0.0020395173*URAMP(V(G1,K))**1.5*(URAMP(V(G1,K))/(URAMP(V(A,K))+URAMP(V(G1,K)))*1.2+0.4)
BIK2 IK2 0 V=V(IK,IG)*(1-0.4*(EXP(-URAMP(V(A,K))/URAMP(V(G2,K))*15)-EXP(-15)))
BIG2T IG2T 0 V=V(IK2)*(0.80118126*(1-URAMP(V(A,K))/(URAMP(V(A,K))+10))**1.5+0.19881874)
BIK3 IK3 0 V=V(IK2)*(URAMP(V(A,K))+2600)/(URAMP(V(G2,K))+2600)
BIK4 IK4 0 V=V(IK3)-URAMP(V(IK3)-(0.0021655364*(URAMP(V(A,K))+URAMP(URAMP(V(G2,K))-URAMP(V(A,K))))**1.5))
BIP IP 0 V=URAMP(V(IK4,IG2T)-URAMP(V(IK4,IG2T)-(0.0021655364*URAMP(V(A,K))**1.5)))
BIAK A K I=V(IP)+1e-10*V(A,K)
BIG2 G2 K I=URAMP(V(IK4,IP))
BIGK G1 K I=V(IG)
* CAPS
CGA G1 A 0.06p
CGK G1 K 3.2p
C12 G1 G2 2.1p
CAK A K 2.9p
.ENDS
I used the same tube in a preamp stage.
I used the thrifty croaker schematic for it, but you are using the pentode as the output stage, so a different approach.
20k looks really low at the plates, and you're biasing it with a really low resistor. Have you tested it to see how it sounds?
This could be another nice low wattage amplifier for late night playing.
I would even go lower, with only one pentode as the output stage and use the other one as a gain stage.
I still didn't have time to build it, this is just a sketch, so I wanted to hear opinions of more experienced people, before I really build this, because I am afraid to destroy tubes. Plates are loaded with 5k total (it is impedance of a primary I have), or 10k on a single tube if we look it that way. I would go with a single pentode for output, but this is the output TF I have, and 5k is way too low for single tube.
He means 20K and 300R on the triodes.
100u seems large for cathode bypass caps...you may want to 'tune them to taste' when you have it running...just a thought.
I've chose 20k/300 on triodes because I don't need so much voltage gain there. I have tried 43k/430 (the first logical choice due to plate resistance) and it works fine in the first stage, but gives too much amplification (and output distortion) when used in second.
In both cases I am having trouble to achieve symmetrical clipping on pentode stage. Using 2k2 Rg2, 75 ohm for Rk and adding 1k8 in series with OTF primary, makes things better. Is it OK to add that series resistance to primary?
... post edited
carry on!
I also want to use usual transformers for power supply. My idea is to use 220:12 V for heaters (I will add voltage dropping resistor in series to lower heater voltage to 9V), and then 12:220 V connected to 12 V, which will produce 220 V AC for B+ rectification.
How shall I choose TF power rating? Heaters use 600 mA @ 12 V AC, and B+ will use about 70 mA @ 200 V DC, which is at least 120 mA at 220 V AC secondary, so I need 2.2 A for 12 V AC primary, if I've calculated it well. It gives total of 2.2 + 0.6 = 2.8 A for the first TF's secondary, or at least 3.5-4.0 A to be sure - may I use 220:12 V 40VA for both transformers, because I have two of a kind?
If too much gain, you can either delete triode cathode bypass caps and/or take a reduced output from a split plate load resistor, like Ampeg did in the SB12 bass amp 12AX7 first input triode.
For PT's, your logic looks sound. However, I don't believe there is anything to stop you under-running the backward transformer like using a 9-0-9 secondary as 18v driven from the 12v input transformer secondary for a reduced B+ so you don't have to waste power in large dropper resistors. Or indeed, input with a 9v and drive a 12v.
With matched transformers, are you not heading for nearer 300VDC B+ ?
Yes, it's much better to use 9V AC for heaters and then 12/220 for step-up, but I currently don't have 9V TF with enough power rating :(
I have already installed resistor voltage divider after tone stack, but you're right, I forgot that I can omit cathode bypass caps on preamp stages.
What about putting 1k8 resistor in series with OTF primary?
...
PLEASE don't put SPICE listings right in the body of the message. My scroll-finger is sore. Put them in a {CODE} (https://www.diystompboxes.com/smfforum/Themes/DIYTheme2/images/bbc/code.gif) box, which is compact and easily copied-out.
LTspice code below:
Version 4
SHEET 1 2460 1172
WIRE 176 -48 -48 -48
WIRE 1168 -48 176 -48
WIRE 1248 -48 1168 -48
WIRE 1168 -32 1168 -48
WIRE -48 16 -48 -48
WIRE 1552 16 1488 16
WIRE 1584 16 1552 16
WIRE 1936 16 1680 16
WIRE 2080 16 1936 16
WIRE 176 32 176 -48
WIRE 1168 48 1168 32
WIRE 1248 48 1248 -48
WIRE 1936 48 1936 16
WIRE 2080 64 2080 16
WIRE 1552 96 1488 96
WIRE 1584 96 1552 96
WIRE 1696 96 1680 96
WIRE 1856 96 1696 96
WIRE -48 128 -48 96
WIRE 1552 128 1552 96
----------SNIP!!!------
eft 2 !.ac oct 16 10 50k
PLEASE do not QUOTE entire listings, compounding the clutter!!
You have 10 tubes, with no apparent use, and you are "afraid to destroy" two of them? The preamp stages are hard to kill. The power stage, you can mock-up with one tube at greatly reduced output, then scale-up to two when it works OK.
However: a 5W SE transformer implies a tube of 10W-12W dissipation. This is a 1.2W pentode. You'd need all TEN of them to justify the 5W OT.
A rational operating point for 5W is 250V 50mA (12W Pdiss). For 2.5W, 175V 35mA (6W Pdiss). But two of this tube is 2.4W Pdiss. Optimized for 5K load, you want 110V 22mA (2.42W Pdiss). About 1W output, which makes your 5W lump over-kill.
Adding resistance to the load is just wasteful.
Symmetry requires the V and I to be similar to Rload. You could aim at 250V 2.4Wdiss and 10mA total current but at 5K load output is small and asymmetric.
The pentode is exceptionally small and very high gain. Good pulse-shaper (as the sales-sheet says). Poor power amp, even for small definitions of "power".
Can't you find a 6V6, EL84, 6AQ5? 200V-250V 40mA-50mA will kick household butt.
Ranko, you may wish to look at my "Lamington Junior" amp https://www.valveheaven.com/diy-amp-designs/the-lamington-junior/
It uses a pair of ECF80/ECF802 valves for 2W output. However, the output transformer is a little 100V 5W PA transformer that reflects a much higher plate impedance (20K p-p) than your 5K transformer.
(https://s8.postimg.cc/ez03fga35/Lamington_Jnr.jpg) (https://postimg.cc/image/ez03fga35/)
Quote from: PRR on August 03, 2018, 04:35:29 PM
PLEASE don't put SPICE listings right in the body of the message. My scroll-finger is sore. Put them in a {CODE}
PLEASE do not QUOTE entire listings, compounding the clutter!!
Excuse me! Had no idea that option exists! I've modified both messages.
Quote from: PRR on August 03, 2018, 04:35:29 PM
You have 10 tubes, with no apparent use, and you are "afraid to destroy" two of them? The preamp stages are hard to kill. The power stage, you can mock-up with one tube at greatly reduced output, then scale-up to two when it works OK.
However: a 5W SE transformer implies a tube of 10W-12W dissipation. This is a 1.2W pentode. You'd need all TEN of them to justify the 5W OT.
A rational operating point for 5W is 250V 50mA (12W Pdiss). For 2.5W, 175V 35mA (6W Pdiss). But two of this tube is 2.4W Pdiss. Optimized for 5K load, you want 110V 22mA (2.42W Pdiss). About 1W output, which makes your 5W lump over-kill.
Adding resistance to the load is just wasteful.
Symmetry requires the V and I to be similar to Rload. You could aim at 250V 2.4Wdiss and 10mA total current but at 5K load output is small and asymmetric.
The pentode is exceptionally small and very high gain. Good pulse-shaper (as the sales-sheet says). Poor power amp, even for small definitions of "power".
Can't you find a 6V6, EL84, 6AQ5? 200V-250V 40mA-50mA will kick household butt.
Thanks for this great explanation, I've learned a lot!
I actually though that it is better to have more powerful TF than tube's maximum power rating. I don't have any of mentioned tubes at hand, but I actually have Russian 6P3S, which is similar to 6L6. They can handle up to 72mA anode current. I also have PL84, which is rated as 12W Pa,max. Maybe them, too?
Another question: when I look at tube datasheets, there is a limiting value for anode power dissipation. Does it mean maximum dissipation at peaks, or maximum average dissipation?
Well, small power <3W guitar amps for home/studio are "a thing" these days. I like the idea compared to using a normal type with a power soak - which is also wasteful if you look at it that way.
I think you could see this plan as just a way of putting the power soak in a different place than the speaker.
If, in order to use what is to hand, and to carry through the true DIY ethic of not throwing money at everything, then what Rankot wants to do is perfectly valid to me. I do that a lot, although often keep it to myself since it's only existing because I happened to have those particular parts - so it wouldn't be worthwhile to anyone else to seek out those same parts. In the future, I may strip it all down because I've got a better use for one or more of those parts.
(https://s8.postimg.cc/ez03fga35/Lamington_Jnr.jpg)
Love that. Great to see a cheap parts design fully developed.
PL84 is definitely better choice here, it fits nice with 5k / 5W OTF. I will try to order 20k / 1W transformer for PCF802 combination.
Of course PL84 can be used as output tube, and I can combine it with any 12AX7 or similar preamp. I also have some 6N2P my brother brought me from Belarus, so I may use them for preamp. Lot's of options now. PRR really helped me understand all those ratios, I am really happy with this new knowledge!
PL84 is EL84 with different heater voltage.
It is actually more like EL86. EL84 has some 40% lower Ik,max.
Regarding power rating limit, does it count maximum at peaks Ia*Ua or mean value of dissipation?
> Regarding power rating limit, does it count maximum at peaks Ia*Ua or mean value of dissipation?
What is the tube's (any SE audio device's) dissipation on peaks?
How long does it take to heat-up a tube? (Yes, transistors are different.)
Those are anode dissipation graphs for both amplifiers:
(https://s26.postimg.cc/nnpeb80ix/anode_dissipation.jpg)
Mean value for PCF802 dual output is approx 0.7W; for PL84 it is approx 4W.
> anode dissipation graphs
Hah! Keeping your computer busy! The curves look right but I can't read the numbers.
I could throw you a fish. But let's think instead.
At idle the dissipation is V*I, no power to load.
With signal, we see three distinct extremes. Plate pulls down, low V high I. Plate kicks up, high V low I. If V and I can be made near-zero, dissipation at these extremes would be *zero*. We can't build a tube big enough to pull to zero, but dissipation will be low at these extremes. In between it must pass through the idle point, a higher dissipation.
Here's just a power amp, twice. 250V 75mA(avg). For reference the input signal is shown below, 2 cycles of Sine.
(https://s8.postimg.cc/3prmwfm8h/SE-idle-full.gif) (https://postimg.cc/image/3prmwfm8h/)
TU2 has no input signal so feels 250V*75mA= 18.7W dissipation. Blue down-triangle trace. Dead-constant because no signal.
TU1 has a big input signal and is driven near max. Red diamond line. When input goes through zero the dissipation is (near) idle dissipation. When input is *either* plus or minus the dissipation falls to near 6W. The time-scale is milliSeconds and some tube-abuse will tell you that it takes much longer than that for a tube to heat/cool. It is reasonable to find the Average over the wave, which is about 12 Watts.
The output power is 370Vpp in 3K or about 5.7 Watts. Using round numbers:
Idle --- 18W diss, 0W out === 18W total
Roar -- 12W diss, 6W out === 18W total
The amplifier runs *cooler* when pushed LOUD.
Now you have to ask yourself.... do you play loud ALL the time? Never stopping for more than a few seconds?
Radio transmitters and motor-drivers might only work maxed-out. (And they need protective relays to cut-out if signal fails.)
Speech/Music audio "never" runs flat-out ALL the time. It gets boring. And you might drop your pick. Also beer-breaks and drum solos.
So if you design an audio amplifier for the lower dissipation at FULL output, it will melt if you ever stop.
You design SE power amps for IDLE.
Great explanation, PRR!
OK, I am really safe with my PL84 design, it will dissipate 9.68W on anode, which is below it's maximum of 12W. And I"m safe with my 5W 5k:8 transformer here, right? :icon_question:
Dual PCF802 will dissipate 2.38W, which is too close to 2x1.2W, so I will have to alter this design. I recalculated everything and found that it is OK to have 150V for B+, 10k primary OTF, 2k4 for Rg2 and 100 ohm for Rk. It makes approx. 1.0-1.1W per tube, which is fine. All I have to do is to find 10k:8 SE transformer for cheap ;)
Ran , have you run this up the flagpole at Wattkins.com yet ... ( toob amps ) This is a Great Pedal sight !!!
No, I didn't. It's not easy to maintain topics on many sites, so only DIYSB is fine for now :)
I've found a very nice article about OTF calculation here: http://www.turneraudio.com.au/se-output-trans-calc-1.html (http://www.turneraudio.com.au/se-output-trans-calc-1.html)
It isn't too hard to calculate your own transformer. In fact, here's a nice calculator (http://www.py2bbs.qsl.br/calc_transf_audio.php) that does it pretty well. It's in brazilian portuguese, google translator do a decent translation to english.
Indeed, translation is really good.
This is what I get when I enter my params:
(https://s26.postimg.cc/5bmcaxs95/trafo.jpg)
If using core with 1cm middle leg width, it requires only one sheet of metal? Is it normal, as I've never seen such transformer before. Does it mind if I use bigger core - I suppose no?
Also, it proposes 0.005mm wire for primary, isn't it a little bit too thin?
That's strange, the translated page shows more parameters to insert than the original page.
Anyway, you should use something like 60Hz as the minimum frequency, 18000Hz it's the max frequency. Some transformer makers use 40Hz as the minimum frequency to get more bass signal, and some use 120Hz to offer a cheap transformer option. With a 60Hz transformer, you'll need a center leg of about 1.3 cm.
And the 40AWG wire seems fine for the primary, since the low power. I got 40AWG using the original page too. I suggest you to oversize the power a little, as a safe measure. This will also make the wires thicker.
I'll do the maths by hand later.
It seems it works better in Portuguese :)
(https://s26.postimg.cc/ffkphx9u1/trafo.gif)
Here's the math:
(https://s15.postimg.cc/h3cjzmhjr/transformer.jpg) (https://postimg.cc/image/h3cjzmhjr/)
Notice the results are somewhat different from the ones from the calculator. This is because different books use different parameters for the math. For example, the book which the calculator is based uses 7.5 for the core calculation, while the book I used for the math use 10. That results in a core a little bit bigger.
Also, you'll need to redo the math for the number of turns based on the core you'll use. Bigger cores means less turns, but also means a bigger transformer with more weight. You don't need to redo the wire gauge, since it depends on the transformer power and not on the transformer size.
For the value of B, grain oriented iron can stand bigger values before the core saturates. To get the maximum value you iron can handle, you'll need the datasheet of the iron. The guy who taught me how to do the math told me to use 10000 G as a safe value if you don't have more info on the iron.
Question: is the 2.24W power the audio power or the total dissipation (audio + heat)?
Quote from: rankot on August 08, 2018, 01:08:15 PM
It seems it works better in Portuguese :)
Yeah, I found that out too. There were extra parameters when using the calculator along with google translator. It probably messed up with the code.
I used 2.24W which is total anode dissipation (for two paralleled tubes in the circuit), and I know that real audio power is approx. 40% of that, but I wanted a little bit bigger core.
> it requires only one sheet of metal?
Parameter H, Height of stacked sheet: 1.35cm (in Portuguese).
Measure the thickness of your lams and figure how many to make 1.35cm (finger-width).
Half-inch (my measure) stack seems cheap but not unreasonable.
But foo on math. The SE audio transformer needs about 4X the iron of a straight AC transformer for the same frequency. A little bigger for low distortion. However guitar bass is higher than power frequency. So for 2.4W 82Hz audio, find a 10VA 50Hz power transformer and gut it.
The primary wire probably can be super-thin. This will not normally be stocked at a transformer rewinder (we rarely need small power at high voltage), but is common for guitar pickups.
PRR, since it is actually 0,9W of audio power (2,25W total plate dissipation), may I use 220/6V 3VA transformer in this case? It will loose some of the bottom end, but could be good enough for testing?
> it is actually 0,9W of audio power
Ah, you are correct. 6VA core is ample.
> It will loose some of the bottom end,
In principle, the bass response may be "the same" on a smaller core by winding more turns.
In practice, you are already near the smallest (and weakest!) wire you can easily buy. If the core is too small you won't be able to fit more turns.
Audio transformers usually get better the bigger they are. Not if you try to make a million for profit! But in DIY it is often affordable to be generous, and that may lead to a better, and easier-to-wind, design.
For example: most home heat/cool systems use a 24V 40VA transformer for thermostat and relays. To a heating contractor, these are like 100K pots to a pedal-builder: they accumulate under the bench. You may be able to get one just by asking. Or I can buy them at the home improvement store for $10.
Cowles says experimental SE outputs can be extra-large power transformers of appropriate ratio. An optimum design would have an air-gap, but if the iron is over-size it is not critical. Your 220V:6V 0.5A transformer looks to me like 16K:12 impedance. It may work as-is.
One question about winding interleaving - probably not important for such a small OTFs, but in case I decide to make something bigger and need to order custom made OTF - I saw few articles about that, and what I don't get is if it should be done always, or only if we need more than one secondary (4 and 8 ohm for example)?
And what happens with DC core saturation if I use simple power transformer for output?
I saw some discussion on interleaving. Some people says you should always do it, some people says it doesn't make a difference in small transformers and some people says it doesn't make a difference at all for guitar amp transformers because of the limited frequency range guitar amps have.
For DC core saturation, if you're winding an push pull transformer you should arrange the lams alternately (one E, one I, one E, one I...). The DC signal of one tube will cancel with the other, and you'll have no DC saturation. But in a single ended transformer, you'll have only one DC signal with no "other tube" signal to cancel, so the core will be under DC saturation. To avoid this, you need to put all the E lams at one side and all the I lams at the other side (check the last image of the calculator screenshot you posted). Between the E pack and the I pack, you need to put an "air gap", which is basically a thin piece of paper.
Also, when you're winding the output transformer, you should do one layer of wire, then one layer of insulation, then one layer of wire... Basically, a layer of insulation after each layer of wire.
Secondary over primary (no interleave) works fine up to 1K impedance, and higher for guitar bandwidth.
At 13K I might try half-primary, secondary, half primary, the first mode of interleaving. However it greatly increases winding troubles. And depending on gauge and turns-count, you may not have two layers of secondary.
Insulation between turns was THE standard for 400V windings. Modern enamel is tougher, and you are only 200V. And may be cramped. I wouldn't insulate between primary layers. (Do wind as neat as you can.) DO insulate between primary and secondary!!
So.... have you built it yet? ;D Math is fun, but breadboarding/prototyping is more fun ;)
Quote from: Marcos - Munky on August 08, 2018, 07:04:22 PM
For DC core saturation, if you're winding an push pull transformer you should arrange the lams alternately (one E, one I, one E, one I...). The DC signal of one tube will cancel with the other, and you'll have no DC saturation. But in a single ended transformer, you'll have only one DC signal with no "other tube" signal to cancel, so the core will be under DC saturation. To avoid this, you need to put all the E lams at one side and all the I lams at the other side (check the last image of the calculator screenshot you posted). Between the E pack and the I pack, you need to put an "air gap", which is basically a thin piece of paper.
Also, when you're winding the output transformer, you should do one layer of wire, then one layer of insulation, then one layer of wire... Basically, a layer of insulation after each layer of wire.
If I have to order new OTF, it will definitely have a gap, but I was asking in case I decide to use this power transformer mentioned above (Hahn 220:6V, 3VA). I really have no idea what is it like inside, since it is inside the plastic case and there is some kind of epoxy fill on the bottom (it is PCB mount type). So I am basically not sure if using that kind of TF could cause any damage to TF itself or to the circuit. Or core saturation is not so important in such a low power amp?
Quote from: vigilante397 on August 08, 2018, 11:52:19 PM
So.... have you built it yet? ;D Math is fun, but breadboarding/prototyping is more fun ;)
I will, in few days, just to decide which OTF to use. :icon_biggrin:
Low VA PCB transformers are supposed to be self-protecting? That is, in designed use, overload of the secondary won't burn it out and no additional fuse is necessary. I'm not sure how they achieve that - either the primary winding has such high resistance that (given that the input volts is in spec') it can survive it, or there is a self-resetting fuse inside?
Quote from: rankot on August 09, 2018, 02:15:18 AM
If I have to order new OTF, it will definitely have a gap, but I was asking in case I decide to use this power transformer mentioned above (Hahn 220:6V, 3VA). I really have no idea what is it like inside, since it is inside the plastic case and there is some kind of epoxy fill on the bottom (it is PCB mount type). So I am basically not sure if using that kind of TF could cause any damage to TF itself or to the circuit. Or core saturation is not so important in such a low power amp?
Power transformers have the lams arranged alternately. What people usually do is remove all the lams and rearrange them to add the gap. Yeah, lots of work, but you need to remove the lams anyway when you're reusing an old transformer.
Not sure of the consequences of DC saturation.
You could also try a 100v line transformer. I have one where the laminations already have a gap and are correctly stacked.
Look for one that has 10W and a lot of taps. With the 0.625W you can almost achieve 30k if you use a 8 ohm speaker and the 4 ohm tap. Over wattkins (or was it diyaudio?) they tested one of these and it produced really good results for a line transformer.
> Low VA PCB transformers are supposed to be self-protecting? .... not sure how they achieve that - either the primary winding has such high resistance that (given that the input volts is in spec') it can survive it, or there is a self-resetting fuse inside?
Many are. Either/or.
Very small parts have high surface/volume ratio. Enough surface to cool the heat that can fit inside. Also for similar geometric reasons a small transformer does have high resistance; and for economic reasons they are designed gleefully greedy. (The electric bill waste is hardly noticed, while first-cost is always noticed.)
And at larger sizes, especially when regulation matters, they have internal cut-outs.
But what happens with DC core saturation in case of my small TF (220:6V, 3VA)? I am pretty sure it doesn't have a gap (or how can I test it anyway, since it is hidden inside the plastic). Is it safe to use it as OTF?
I "think" you can make SE work in a plain PT IF the core is big enough so it can handle the DC+AC without saturating. I've seen that toroids don't handle this well so it should only be done with EI cores unless push-pull.
I don't know how dangerous the asymmetric core saturation would be, apart from sounding crap. I imagine the traffy suddenly stops being much of an inductor and generation in the secondary stops until polarity reversal cancels the DC and pulls it out of saturation. Can that generate a destructive flyback voltage? I don't know. I would expect you would get whatever dissipation happens according to the primary winding DC resistance during saturation which might only do damage if sustained.
I would attempt to manage it by sizing B+ to get plate idle current as low as possible while getting decent symmetry and volume. You may have to go down towards 150v. If you only get about 1W, provided its used with a guitar speaker, it will be loud enough for home/studio use. 1W is, after all, only 3dB below 2W.
Asked about a saturated core transformer on brazilian forum Handmades and got this answer:
(https://www.researchgate.net/profile/Jose_Oliveira27/publication/266170567/figure/fig1/AS:295817987543040@1447539912851/Concepcao-fisica-do-compensador-estatico-a-nucleo-saturado.png)
The yellow part is the saturated core response. The amp feeds a variable current to the primary, that results in a variable voltage on the secondary. In the saturated zone, we can see a big current variation translates in a small voltage variation, so the speaker won't produce too much dynamics.
Also, a saturated core temperature is higher than a non saturated core. A hot transformer probably can have it's dangers.
OK, so it seems it's best to order special SE transformer from someone. Also, Hammond 125ASE or 125BSE will fit exactly, but they're a little bit too expensive - I am trying to build this using what I have at hand. :-\
You can remove all the lamination from a power transformer and put them again forming an air gap.
Yes, I know that, but those I have are unfortunately sealed :(