At the time this manual was written the Simple SE has only been tested as a stand alone single ended amplifier. I have not used it in other applications yet. Since it was designed for stand alone amplifier use, this manual only covers this application.

I have built and tested a few different amplifier configurations that all share the same basic design. It is recommended that you duplicate one of these designs if this is your first amplifier, or you want to copy a known good design. Even if you use one of the standard configurations you should expect some variation from my measured values because of tube and line voltage variation.

About 95% of the Simple SE's ever built were made this way. It is a well tested and proven combination. If this is your first amp build, this is the best place to start. The amp runs with a B+ of around 450 volts using 5K ohm output transformers and can be run in triode or ultralinear mode. All of the popular output tubes can be used.

The primary winding should match the line (mains) voltage available at your location. A universal (multi winding or multi tapped winding) primary can be used if it supports your line voltage.

The high voltage secondary should be 700 to 750 VCT (350-0-350) to (375-0-375) it should be rated for at least 175 mA DC output. An 800 VCT (400-0-400) transformer will result in too much B+ voltage and should NOT be used. This winding MUST have a center tap.

The rectifier filament winding should be 5 VAC at at least 2 amps. If a 5U4 or 5R4 rectifier is used this winding must be at least 3 amps. A center tap on this winding is OK, but it will not be used.

The main filament winding should be 6.3 VAC at at least 5 amps. A center tap on this winding is OK, but it will not be used.

The transformer may have additional windings or a bias tap. These will not be used.

For builders in the USA, Canada, or other 120 volt 60HZ countries the following transformers have been used:

Many builders outside the USA have used the Hammond 374BX. It is a universal primary design and can be used in the US or just about anywhere in the world. Many builders feel that Hammond's 300 series transformers exhibit higher build quality than the 200 series. There are likely other transformers available to builders in other countries that I don't know about. If it matches the requirements listed above, it should work. Edcor lists a clone of the Hammond 374BX called an XPWR059. 120/240 primary, 375-0-375 @ 175 ma, 50V @ 50 mA, 6.3V CT @ 6A, and 5V CT @ 3 A. I have not tried it though.

The OPT is the most critical component in any tube amplifier, and one that is not likely to be upgraded later. The ability of an OPT to reproduce low frequencies is directly related to its size. There are plenty of different types of output transformers available for tube amplifiers. The prices go from $15 USD each to well over $500 USD each. How do you choose? First let me state the requirements:

The Simple SE is a single ended amplifier. The output transformer MUST be designed to work in a single ended amplifier. A push pull transformer will NOT work. Severe distortion will result. The ideal primary impedance for this design is 5K ohms. The secondary impedance should match your speaker impedance. The transformer should be rated to handle the power produced by this amplifier. This could be as low as 5 watts in triode mode, or as high as 15 watts in UL (Ultralinear) mode. The output transformer must have a UL mode tap if you intend to use UL.

The performance of an output transformer is generally related to its size and therefore cost. It will have the biggest effect on the sound quality in the amplifier. I usually recommend that you start with the best transformers that the budget allows even if it means using cheaper tubes. You will upgrade the tubes later, probably not the transformers. See this page for more info.

The small OPT's in the 1 to 2 pound (weight) range will generally exhibit distortion on heavy bass notes. They may be OK if your speakers do not reproduce these frequencies, OR you use an active subwoofer. The small Edcors sound fairly good on my Yamaha NS-10's since their 7 inch woofers can't do much below 70 Hz. Popular transformers in this range are the Edcor XSE15-8-5K and the Hammond 125CSE. I have used both of these, and they work well within their limitations. I prefer the Edcor and it is cheaper. The Edcor has a UL tap, but is available in a single output impedance only (either 4 or 8 ohms). The Hammond has multiple output taps, but NO UL tap (triode mode only).

Both Edcor and Hammond have larger versions of these transformers. The Hammond 125ESE and the Edcor GXSE series have been used by Simple SE builders who report good results, but I have not tried either of them.

There are several transformers in the 5 to 7 pound range. These are probably the best choice for most builders. A transformer of this size can deliver full solid bass through a big speaker, without compromising the high frequencies, IF it is properly designed AND built. I have used these with good results. The One Electron UBT2, a custom "10 watt" Electra - Print, and an older "Transcendar" I got from Ebay. The Transcendar transformers that I used are no longer made. They were surplus "left over from a production run" I bought a pair and liked them so well that I bought 10 more pairs. There are still Transcendars available on Ebay. Several Simple SE builders have used them with good results. Electra-Print still makes very good transformers, but they are considerably more expensive than when I got them. The One Electrons are available from Antique Electronics Supply. I have used a pair of "Eastern Audio " transformers that are also common on Ebay. The pair that I have are 600 ohm for another project. They are rated at "50 watts", this is a joke, they are 8 to 10 watt transformers. The build quality is slightly worse than any of the others that I mention, but might be OK if the price is cheap enough. There are dozens of transformers in this size range that I have not tried.

There are also larger transformers available. I have tried the Hammond 1628SEA and the Edcor CXSE25-8-5K. The Hammond is huge at 11 pounds. It will produce full powered bass down to 20 Hz. It will also roll off some of the high frequencies and cost about a watt of output power. It makes bass that can be heard INSIDE the house across the street when connected to my 15 inch OB speakers. This is unreal for a SE amp. The big Hammond does not do well when connected to super high efficiency speakers. The Edcor is smaller than the Hammond at 9 pounds. It gives up very little in the big bass department, but offers serious improvements in the efficiency and high frequency department. I liked the big Edcor so much that I stole it from my Simple SE for use in a 300B powered Tubelab SE. These are very good transformers and may be cheaper than some of the smaller units. Keep in mind that both of these are BIG and HEAVY, verify chassis fitment and strength!

Several users have had problems with the FRED diodes from IXYS. I have noticed that the old ones (3 years ago or older) will live forever, but the newer production diodes are prone to failure. This has to do with their "avalanche performance" which was not specified on the IXYS parts. I no longer recommend IXYS diodes. I have been using Fairchild Semiconductor ISL9R8120P2 without issue, but Fairchild may be eliminating this part. Mouser has 5000 of them in stock. Understand that these diodes are only required if you want the solid state diode option and may be omitted. Simply leave them out if you only want to use a tube rectifier.

I advise adding an "INRUSH CURRENT LIMITER" in series with the line (mains) power input to the amplifier. This small component acts like a 120 ohm resistor when it is cold (initial turn on) and gradually reduces to about 2 ohms as it warms up. This greatly reduces the surge that occurs when the amp is turned on. This makes life far easier on the tubes and capacitors. Some users also add another in series with the center tap on the high voltage winding on the power transformer (red - yellow wire). This reduces the surge experienced by the rectifier tube as it warms up. The Simple SE runs the 5AR4 close to the maximum surge rating, and many new production tubes (especially JJ) are not up to the task. I use the CL-90 from GE sensing. it is DigiKey number KC009L-ND

If you already have transformers from another project I have included these amplifier simulations done with TubeCad SEamp Cad to give you an idea of where to start. Choose an output tube, and find the appropriate table. Then find the B+ voltage and load impedance that most closely matches the transformers that you have. You can then choose a cathode resistor value for a given amount of tube dissipation. The power output, distortion and damping factor values are also given. Keep in mind that these are just computer simulations based on a typical vintage vacuum tube.

Power Output Simulations for EL34


Tube Type	B+ voltage	Load Z	Vp	Vk	Rk	Ik	Diss (idle)	Pwr Out	Dist - 2nd	Dist 3rd	DF


EL34	500	5000	485	43	560	77	34	7.58	7.8	0.5	4.44
EL34	500	5000	486	44	620	71	32	7.67	8.3	0.5	4.3
EL34	500	5000	487	45	680	66	29	7.76	8.8	0.4	4.17
EL34	500	5000	488	45	750	61	27	7.81	9.5	0.3	4.04
EL34	500	5000	489	46	820	56	25	7.87	10.1	0.3	3.92
EL34	500	5000	490	47	910	52	23	7.49	10.7	0.1	3.79

EL34	500	3000	493	44	620	72	33	10.6	12.3	0	3.13
EL34	500	3000	494	45	680	67	31	10.6	13.3	0.1	3.04
EL34	500	3000	494	46	750	61	29	9.98	14.1	0.3	2.94
EL34	500	3000	495	47	820	57	27	9.89	15.4	0.5	2.86
EL34	500	3000	495	47	910	52	24	9.66	16.9	0.8	2.76

EL34	450	5000	435	37	470	79	32	6.29	7.1	0.5	4.59
EL34	450	5000	435	38	510	74	30	6.1	7.2	0.5	4.47
EL34	450	5000	436	38	560	69	28	6.23	7.7	0.4	4.35
EL34	450	5000	437	39	620	63	26	6.33	8.2	0.4	4.21
EL34	450	5000	438	40	680	59	24	6.44	8.8	0.4	4.09
EL34	450	5000	439	41	750	54	22	6.17	9.1	0.2	3.97

EL34	450	3000	442	38	470	81	33	8.57	9.7	0.3	3.35
EL34	450	3000	443	39	510	76	31	8.71	10.4	0.2	3.26
EL34	450	3000	443	39	560	70	29	8.78	11.3	0.1	3.17
EL34	450	3000	444	40	620	64	27	8.32	11.9	0.1	3.07
EL34	450	3000	444	41	680	60	25	8.32	13	0.2	2.98
EL34	450	3000	445	41	750	55	23	8.26	14.3	0.4	2.89

EL34	400	5000	382	30	330	92	33	4.62	5.7	0.5	4.96
EL34	400	5000	383	31	360	86	31	4.49	5.8	0.4	4.85
EL34	400	5000	384	32	390	81	29	4.68	6.1	0.5	4.74
EL34	400	5000	385	32	430	75	27	4.84	6.5	0.5	4.61
EL34	400	5000	386	33	470	70	25	4.69	6.6	0.4	4.49
EL34	400	5000	387	34	510	66	24	4.86	7	0.4	4.39

EL34	400	3000	391	31	330	94	35	6.96	7.8	0.4	3.63
EL34	400	3000	392	32	360	88	32	6.7	8	0.4	3.55
EL34	400	3000	392	32	390	83	30	6.93	8.6	0.4	3.47
EL34	400	3000	393	33	430	77	28	6.63	8.9	0.2	3.37
EL34	400	3000	393	34	470	71	26	6.79	9.6	0.2	3.28
EL34	400	3000	394	34	510	67	25	6.94	10.4	0.1	3.2

EL34	350	5000	329	24	220	107	33	3.03	4.5	0.4	5.37
EL34	350	5000	330	24	240	101	31	3.23	4.8	0.5	5.26
EL34	350	5000	332	25	270	93	29	3.4	5	0.5	5.11
EL34	350	5000	333	26	300	86	26.5	3.28	5.1	0.4	4.98
EL34	350	5000	334	26	330	80	25	3.46	5.4	0.4	4.86
EL34	350	5000	335	27	360	75	23	3.36	5.5	0.4	4.75
EL34	350	5000	336	28	390	71	22	3.54	5.8	0.4	4.65
EL34	350	5000	337	28	430	65	20	3.7	6.2	0.4	4.52
EL34	350	5000	338	29	470	61	19	3.58	6.3	0.3	4.41

EL34	350	3000	340	24	220	111	35	4.78	5.9	0.4	3.95
EL34	350	3000	340	25	240	104	33	5.04	6.3	0.5	3.86
EL34	350	3000	341	26	270	95	30	4.82	6.5	0.4	3.75
EL34	350	3000	342	27	300	88	28	5.02	7	0.4	3.65
EL34	350	3000	342	27	330	82	26	5.23	7.5	0.4	3.55
EL34	350	3000	343	27	360	77	25	5.04	7.7	0.3	3.47
EL34	350	3000	343	28	390	72	23	5.26	8.4	0.3	3.39
EL34	350	3000	344	29	430	68	21	5.03	8.7	0.2	3.3
EL34	350	3000	344	29	470	62	20	5.19	9.5	0.1	3.22

Power Output Simulations for 6L6GC


Tube Type	B+ voltage	Load Z	Vp	Vk	Rk	Ik	Diss (idle)	Pwr Out	Dist - 2nd	Dist 3rd	DF


6L6GC	500	5000	486	41	560	73	33	5.23	7.5	0.5	3.24
6L6GC	500	5000	487	42	620	68	31	5.24	7.9	0.5	3.16
6L6GC	500	5000	488	43	680	64	29	5.28	8.4	0.5	3.08
6L6GC	500	5000	488	44	750	59	27	5.28	8.9	0.4	3
6L6GC	500	5000	489	45	820	55	25	5.29	9.4	0.4	2.94

6L6GC	500	3000	493	42	560	74	35	6.52	10.2	0.4	2.39
6L6GC	500	3000	494	43	620	69	32	6.47	10.9	0.4	2.34
6L6GC	500	3000	494	44	680	64	30	6.45	11.6	0.4	2.29
6L6GC	500	3000	494	45	750	60	28	6.39	12.3	0.3	2.23
6L6GC	500	3000	495	46	820	56	26	6.35	13.1	0.3	2.19

6L6GC	450	5000	434	34	430	79	32	3.69	6	0.5	3.42
6L6GC	450	5000	435	35	470	74	30	3.78	6.4	0.5	3.35
6L6GC	450	5000	436	36	510	70	29	3.88	6.7	0.5	3.28
6L6GC	450	5000	437	37	560	66	27	3.95	7.1	0.4	3.21
6L6GC	450	5000	438	38	620	61	25	3.99	7.5	0.4	3.13
6L6GC	450	5000	439	39	680	57	23	4.04	8	0.4	3.06

6L6GC	450	3000	442	35	430	80	34	5	8.4	0.5	2.52
6L6GC	450	3000	443	35	470	76	32	5.41	9.2	0.5	2.47
6L6GC	450	3000	443	36	510	71	30	5.16	9.4	0.4	2.42
6L6GC	450	3000	444	37	560	67	28	5.19	10	0.4	2.37
6L6GC	450	3000	444	38	620	62	26	5.2	10.7	0.4	2.32
6L6GC	450	3000	445	39	680	58	24	5.21	11.4	0.4	2.27

6L6GC	400	5000	381	25	270	94	34	2.46	4.6	0.5	3.73
6L6GC	400	5000	383	27	300	88	32	2.57	4.8	0.5	3.65
6L6GC	400	5000	384	27	330	83	30	2.68	5.1	0.5	3.58
6L6GC	400	5000	384	28	360	79	28	2.79	5.4	0.5	3.51
6L6GC	400	5000	385	29	390	75	27	2.91	5.7	0.5	3.45
6L6GC	400	5000	386	30	430	70	25	3	6	0.5	3.37

6L6GC	400	3000	391	26	270	97	36	3.51	6.1	0.5	2.74
6L6GC	400	3000	391	27	300	91	34	3.62	6.6	0.5	2.68
6L6GC	400	3000	392	28	330	85	32	3.74	7	0.5	2.63
6L6GC	400	3000	392	29	360	80	30	3.58	7.1	0.4	2.58
6L6GC	400	3000	393	30	390	76	28	3.71	7.6	0.4	2.54
6L6GC	400	3000	393	31	430	71	26	3.79	8.1	0.4	2.49

6L6GC	350	5000	330	19	180	103	32	1.41	3.5	0.4	3.95
6L6GC	350	5000	331	20	200	97	31	1.52	3.7	0.4	3.88
6L6GC	350	5000	332	20	220	92	29	1.64	4	0.4	3.81
6L6GC	350	5000	333	21	240	88	28	1.76	4.2	0.5	3.75
6L6GC	350	5000	334	22	270	82	26	1.86	4.5	0.5	3.67
6L6GC	350	5000	335	23	300	77	24	1.78	4.5	0.4	3.59
6L6GC	350	5000	336	24	330	72	23	1.88	4.8	0.4	3.52
6L6GC	350	5000	336	25	360	68	22	1.99	5.1	0.4	3.46

6L6GC	350	3000	340	19	180	107	35	2.12	4.6	0.4	2.9
6L6GC	350	3000	341	20	200	101	33	2.26	4.9	0.5	2.85
6L6GC	350	3000	341	21	220	95	31	2.17	4.9	0.4	2.8
6L6GC	350	3000	342	22	240	90	29	2.31	5.3	0.4	2.75
6L6GC	350	3000	342	23	270	84	27	2.42	5.7	0.4	2.7
6L6GC	350	3000	343	24	300	79	26	2.53	6.2	0.4	2.64
6L6GC	350	3000	343	24	330	74	24	2.65	6.6	0.4	2.59
6L6GC	350	3000	343	25	360	70	23	2.77	7.1	0.5	2.55
6L6GC	350	3000	344	26	390	66	22	2.66	7.2	0.4	2.51

Power Output Simulations for KT88


Tube Type	B+ voltage	Load Z	Vp	Vk	Rk	Ik	Diss (idle)	Pwr Out	Dist - 2nd	Dist 3rd	DF

KT88	500	5000	480	43	430	100	44	6.29	4.8	0.3	4.44
KT88	500	5000	482	44	470	93	41	6.44	5.1	0.3	4.34
KT88	500	5000	483	45	510	88	39	6.58	5.3	0.3	4.25
KT88	500	5000	484	46	560	82	36	6.69	5.6	0.3	4.14
KT88	500	5000	485	47	620	76	33	6.77	6	0.2	4.03
KT88	500	5000	486	48	680	70	31	6.85	6.4	0.2	3.93
KT88	500	5000	487	49	750	65	29	6.9	6.8	0.1	3.82

KT88	500	3000	490	44	430	102	46	9.42	6.9	0.2	3.25
KT88	500	3000	491	45	470	95	43	9.08	7.3	0.1	3.17
KT88	500	3000	492	46	510	90	41	9.19	7.8	0.1	3.1
KT88	500	3000	492	46	560	83	38	9.22	8.4	0.1	3.03
KT88	500	3000	493	48	620	77	35	9.18	9.2	0.2	2.95
KT88	500	3000	493	48	680	71	33	9.15	10	0.4	2.87
KT88	500	3000	494	49	750	66	30	8.69	10.7	0.6	2.79

KT88	450	5000	429	36	330	109	43	4.66	4.1	0.3	4.63
KT88	450	5000	430	37	360	102	40	4.82	4.3	0.3	4.53
KT88	450	5000	431	38	390	96	38	4.99	4.5	0.3	4.45
KT88	450	5000	432	39	430	90	36	5.12	4.7