E/PCF802 2W amplifier

[rankot]

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.



Is this the right approach, or someone has an idea how to improve this?

LTspice code below:

Code Select Expand

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

[rankot]

TubeLib.inc can be found here: 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

However, it seems that I didn't find those PCF802 models in TubeLib.inc, but somewhere else, so I put them into the file:

Code Select Expand

* 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

[thomasha]

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.

[rankot]

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.

[thermionix]

He means 20K and 300R on the triodes.

[GibsonGM]

100u seems large for cathode bypass caps...you may want to 'tune them to taste' when you have it running...just a thought.

[rankot]

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?

[stringsthings]

... post edited

carry on!

[rankot]

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?

[anotherjim]

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+ ?

[rankot]

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.

[rankot]

What about putting 1k8 resistor in series with OTF primary?

[stringsthings]

...

[PRR]

PLEASE don't put SPICE listings right in the body of the message. My scroll-finger is sore. Put them in a {CODE} box, which is compact and easily copied-out.

Code Select Expand

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.

[gcwills]

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.

[rankot]

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.

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?

[anotherjim]

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.


Love that. Great to see a cheap parts design fully developed.

[rankot]

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!

[PRR]

PL84 is EL84 with different heater voltage.

[rankot]

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?