I am gathering parts for my first tube amp and am ready to order some OPTs, but I'm wavering on which ones are best for the job. The amp will be ~4.5W output, SET, with a good quality build. The load should be approximately 7k ohm. The speakers are ~93dB, single driver, with a useful freq response of 40-20k Hz.
I like the Edcor line because they are affordable, good looking, and USA made. So lets stick with them unless there is a compelling reason not to.
The relevant choices from their catalog are:
Model, Power, Primary, Secondary, FR
GXSE10-8-8K, 10W, 8K Ohms, 8 Ohms, 40-18k
GXSE15-8-8K, 15W, 8K Ohms, 8 Ohms, 40-18k
CXSE25-8-6.5K, 25W, 6.5K Ohms, 8 Ohms, 20-20k
I've eliminated the XSE series because the freq response is narrower than I would like.
The questions coming to mind that I am unable to answer are:
The two GXSE models are identical in specs other than power handling and weight. I've read that generally, the heavier the OPT the better, but it sounds in this case like the increased weight is just due to the increased power handling. For a 4.5W amp, should there be any difference in sound quality?
The CXSE has better FR and a massive 25W power handling. Would these provide any benefit for a 4.5W amp feeding speakers that are only good to 40Hz?
Which would you choose for a project like this? Feel free to suggest other brands if you feel they would better meet my needs.
Thanks for the help!
I like the Edcor line because they are affordable, good looking, and USA made. So lets stick with them unless there is a compelling reason not to.
The relevant choices from their catalog are:
Model, Power, Primary, Secondary, FR
GXSE10-8-8K, 10W, 8K Ohms, 8 Ohms, 40-18k
GXSE15-8-8K, 15W, 8K Ohms, 8 Ohms, 40-18k
CXSE25-8-6.5K, 25W, 6.5K Ohms, 8 Ohms, 20-20k
I've eliminated the XSE series because the freq response is narrower than I would like.
The questions coming to mind that I am unable to answer are:
The two GXSE models are identical in specs other than power handling and weight. I've read that generally, the heavier the OPT the better, but it sounds in this case like the increased weight is just due to the increased power handling. For a 4.5W amp, should there be any difference in sound quality?
The CXSE has better FR and a massive 25W power handling. Would these provide any benefit for a 4.5W amp feeding speakers that are only good to 40Hz?
Which would you choose for a project like this? Feel free to suggest other brands if you feel they would better meet my needs.
Thanks for the help!
Hi I am not sure did You mention the output tube type You want to use in SE?
...because of the tube internal resistance that is of merit when we talk about the OT primary inductance and that is from importance for the proper bw
not the transfer ratio...
.
probablu some pentode in pentode mode operation?
that is could be read from 7K and 4.5W power?
.
...because of the tube internal resistance that is of merit when we talk about the OT primary inductance and that is from importance for the proper bw
not the transfer ratio...
.
probablu some pentode in pentode mode operation?
that is could be read from 7K and 4.5W power?
.
When designing my first SE amp, I bought the XSE15-8-5k and CXSE25-8-5k transformers and tried both in the same configuration. The XSE15 is really quite good -- especially for the money ($20). The CXSE25 offers a bit tighter bass, and much more transparent highs. At least that was my experience.
If you are just looking to get your feet wet with the tube amp design and aren't overly confident in your design skills, don't want to spend too much money all at once, get the XSE or GXSE. But if you're fairly confident that you'll like the tube sound and can handle the design, want a solid amp that offers really good performance, I'd opt for the CXSE-series of transformers.
Hope this helps.
~Tom
If you are just looking to get your feet wet with the tube amp design and aren't overly confident in your design skills, don't want to spend too much money all at once, get the XSE or GXSE. But if you're fairly confident that you'll like the tube sound and can handle the design, want a solid amp that offers really good performance, I'd opt for the CXSE-series of transformers.
Hope this helps.
~Tom
The output tube is the 6N6G. A somewhat uncommon dual dissimilar triode, but makes for a very simple, low parts count amp. I am building the recommended configuration from the data sheet here:
http://www.mif.pg.gda.pl/homepages/frank/sheets/084/6/6N6G.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/084/6/6N6G.pdf
John,
Could you explain that a little further? I do not understand the relationship between plate resistance and load, and what happens when they are not correctly matched.
Surely the 6N6G is able to operate in SE mode since the data sheet indicates that is what it's intended for? Is it then a matter of sourcing an OPT with the appropriate characteristics?
I have a box of these tubes, and would like to make something useful of them if possible, but if it's really a bad idea I suppose I could scrap the design.
Could you explain that a little further? I do not understand the relationship between plate resistance and load, and what happens when they are not correctly matched.
Surely the 6N6G is able to operate in SE mode since the data sheet indicates that is what it's intended for? Is it then a matter of sourcing an OPT with the appropriate characteristics?
I have a box of these tubes, and would like to make something useful of them if possible, but if it's really a bad idea I suppose I could scrap the design.
I am not familiar with that tube but According to the data sheet it is looking for a 7k load, As you already have them, I say go for it. If it doesn’t work out, there are plenty of tubes that will work well into a 8k transformer and you will have lost almost nothing. I have used the GSXE10-8-8k in a 6V6 amp and they sound very good for the money…John
Dude,
In addition to John's warning about RP, think about 5% distortion and inadequate power O/P. Paul Joppa's 102 dB. rule tells us the minimum power O/P for 93 dB. sensitive speakers is 8 WPC. If you want to use 6N6Gs, go push/pull. A 12AT7 based LTP will give you the 15 V. of drive needed from slightly less than 1 V. of I/P. That's excellent and a line stage will not be needed, if a CDP is the signal source. Edcor's GXPP15-6-10K should be fine with 8 Ω single driver speakers. You might think about a small amount of NFB, for damping factor and distortion reasons. Roll extreme LF info. and infrasonic noise off with a cap. at the amp's I/Ps. A 30 Hz. F3 appears reasonable.
Thanks for the suggestion Eli. I'm a bit scared of the P-P scheme due to several circuit elements I don't understand well enough to design myself, like how to implement NFB and what an LTP is.
I'm not too worried about the power output. I have a small room and have been perfectly happy with the 5wpc t-amp I've been using on these speakers.
I'd love it if someone could drop some knowledge on me and explain why a high Rp is a bad thing.
I'm not too worried about the power output. I have a small room and have been perfectly happy with the 5wpc t-amp I've been using on these speakers.
I'd love it if someone could drop some knowledge on me and explain why a high Rp is a bad thing.
It's routine to use a load of 3X RP in combination with a "conventional" triode. The much loved 2A3 has an 800 Ω RP and it's frequently combined with 2.5 KOhm O/P trafos. Sylvania's 6N6G data sheet shows a RP of 24 KOhms, which suggests (sic) a 75 KOhm primary. However, that's anything but a "conventional" triode. Frankly, I'm still trying to make sense of the wiring diagram. FWIW, I see a cathode follower directly coupled to a common cathode power triode. Perhaps the data sheet has incorrect numbers and the RP is 2.4 KOhms and the μ is 5.8, which does "jive" with a 7 KOhm O/P trafo primary. Also, μ = 58 is very large for a power O/P type.
LTP = long tailed pair, AKA cathode coupled, AKA Schmitt, is a phase splitter based on a differential gain block.
The small signal circuitry in the attached "El Cheapo" schematic should serve you well, with few adjustments. With a combined draw slightly in excess of 200 mA. from 4X 6N6s, the PSU needs to be rethought.
Attachments
Thanks again Eli. I think you might be onto something with the incorrect Rp supposition. Of the data sheets for the 6B5 (6-pin version of the 6N6), some indicate a 24k Rp and some show it as 2.4k. I believe, however, that the μ of 58 is correct. This tube is described as having the equivalent of a 76 driver triode directly coupled to a 6AC6 power triode in a single envelope, so that gain rating seems believable.
I think I'll buy the cheap Edcors and try it out. So the relevant part of my original question remains unanswered - for an amp of this output power, is there likely any sonic advantage to the GXSE15-8-8K over the GXSE10-8-8K?
As an aside, the 6N6G is pin-out swappable with a 6V6 pentode (and others). Only a cathode resistor and bypass cap need be added, and the Edcors have UL taps. So this could be an easy conversion if the 6N6G's turn out to be horrible.
I think I'll buy the cheap Edcors and try it out. So the relevant part of my original question remains unanswered - for an amp of this output power, is there likely any sonic advantage to the GXSE15-8-8K over the GXSE10-8-8K?
As an aside, the 6N6G is pin-out swappable with a 6V6 pentode (and others). Only a cathode resistor and bypass cap need be added, and the Edcors have UL taps. So this could be an easy conversion if the 6N6G's turn out to be horrible.
High Rp typically indicates the need for a large amount of inductance in the OPT. This is usually not the case with triode connected Pentodes and typical triodes, but you must be careful when using pentodes in their native state. Some DHT's do require quite a bit of inductance, like the 45 and that is only a problem in SE use due to current draw. A 70 watt sized OPT is needed to properly load one of these and for only a watt and a half out. True, the 45 has probably the lowest distortion of any triode, BUT....
To correctly size an OPT to a tube you need the load line or impedance ratio and the transconductance. Add them in parallel and then add the direct DCR of the OPT primary and the reflected DCR of the secondary in series, to obtain the working impedance you must meet with inductance at the lowest frequency of interest. Divide the impedance by 2 X Pi and the lowest frequency of interest. This inductance number will provide -3 db at this frequency. To obtain - 0.5 db multiply the first inductance by 2.76, for - 1 db multiply by 1.9.
Bud
Ok, so the take-home message is that if the OPT primary inductance is not large enough for the given Rp, low frequencies will be rolled off.
I would like to attempt to calculate F(-3dB) for my own education. I'm confused by your instructions for calculating the working impedance, specifically paralleling the impedance ratio and transconductance. I have requested inductance and DCR specs for the above transformers from Edcor, and the transconductance for the 6N6G is published as 2400 μmohs. The impedance ratio for this 8k transformer would be 1000:1. So would the formula then be:
working impedance = ???
F(-3dB) = (working impedance / pri inductance) - 2*pi
I would like to attempt to calculate F(-3dB) for my own education. I'm confused by your instructions for calculating the working impedance, specifically paralleling the impedance ratio and transconductance. I have requested inductance and DCR specs for the above transformers from Edcor, and the transconductance for the 6N6G is published as 2400 μmohs. The impedance ratio for this 8k transformer would be 1000:1. So would the formula then be:
working impedance = ???
F(-3dB) = (working impedance / pri inductance) - 2*pi
(1/8000+1/2400) + pri DCR + sec reflected DCR [(pri n / sec n) squared X sec DCR] = effective nominal load impedance that must be met with inductance.
Divide this by 2 Pi and the frequency of interest to obtain an inductance that will provide -3db at that frequency. Multiply that number by 1.9 for -1 db and 2.76 for - 0.5 db.
I strongly urge you to buy Edcore transformers for development work. They, along with Hammond products, are well made, reliable and quite cost effective. Then, after the amp is stable and tweaked to your liking, decide whether your taste runs to the clear, clean, tonally correct and slightly edgy transients, without a great deal of internal gradient structure to notes and transients, style of OPT, or a very dense gradient structure to notes and transients, with equivalent tonal clarity but a slightly softer overall presentation, due to slightly higher distortion. This will determine whether you move to amorphous core or stick with E/I construction. At the top echelon of performance, both are very good and cost about the same.
Bud
Divide this by 2 Pi and the frequency of interest to obtain an inductance that will provide -3db at that frequency. Multiply that number by 1.9 for -1 db and 2.76 for - 0.5 db.
I strongly urge you to buy Edcore transformers for development work. They, along with Hammond products, are well made, reliable and quite cost effective. Then, after the amp is stable and tweaked to your liking, decide whether your taste runs to the clear, clean, tonally correct and slightly edgy transients, without a great deal of internal gradient structure to notes and transients, style of OPT, or a very dense gradient structure to notes and transients, with equivalent tonal clarity but a slightly softer overall presentation, due to slightly higher distortion. This will determine whether you move to amorphous core or stick with E/I construction. At the top echelon of performance, both are very good and cost about the same.
Bud
Ok, that makes sense. As an exercise, I did the calculation using the measured specs of the Edcor GXSE10-8-5k which I found in the following thread: diytube.com :: View topic - EDCOR XSE and GXSE basic specs and measurements
GXSE10-8-5K: Primary DCR 244 ohms, sec DCR 0.5 ohms, inductance 17H, turns ratio 25/1
The math:
working impedance = (1/pri imp+1/transcond) + pri DCR + sec reflected DCR [(turns ratio) squared X sec DCR]
working impedance = (1/5000+1/2400) + 244 + (25)^2 *.5
working impedance = 556.5 ohm
F(-3dB) = (working impedance / pri inductance) - 2*pi
F(-3dB) = (556.5 / 17) - 2*pi
F(-3dB) = 26.45 Hz
Look good to you? I'd be just fine this low frequency response since my speakers don't go that low anyway.
What I'm not seeing is where the Rp factors into all this. These formulae seem to ignore it.
Thanks for walking me through this. I feel like I'm learning some actual design skills.
GXSE10-8-5K: Primary DCR 244 ohms, sec DCR 0.5 ohms, inductance 17H, turns ratio 25/1
The math:
working impedance = (1/pri imp+1/transcond) + pri DCR + sec reflected DCR [(turns ratio) squared X sec DCR]
working impedance = (1/5000+1/2400) + 244 + (25)^2 *.5
working impedance = 556.5 ohm
F(-3dB) = (working impedance / pri inductance) - 2*pi
F(-3dB) = (556.5 / 17) - 2*pi
F(-3dB) = 26.45 Hz
Look good to you? I'd be just fine this low frequency response since my speakers don't go that low anyway.
What I'm not seeing is where the Rp factors into all this. These formulae seem to ignore it.
Thanks for walking me through this. I feel like I'm learning some actual design skills.
Math looks good, 17.4 Henry needed for -3 db @ 20 Hz.
You need to be very careful about what various mfg's called various items, in their tube data sheets. There was no consensus I can find for the Rp meaning, nor Ra (anode) Rk or Plate Resistance, so I just go with transconductance, if I can find their number, or I derive it from max usage values.
If I am in control of the amplifier and how the tube is used I use the DC plate voltage and DC current as the transconductance portion. The other half is the load line actually chosen for the AC performance. This is just a nominal value but does describe the usage fairly well. So, Rp, Plate Resistance, Ra, Rk are not always the same usage, from tube spec paper to tube spec paper. Occasionally you will get Rp, Rk and transconductance in the same document. Take the transconductance number and the load resistance that describes the load line you intend to use, based upon maximum linearity from the tube curves.
Bud
You need to be very careful about what various mfg's called various items, in their tube data sheets. There was no consensus I can find for the Rp meaning, nor Ra (anode) Rk or Plate Resistance, so I just go with transconductance, if I can find their number, or I derive it from max usage values.
If I am in control of the amplifier and how the tube is used I use the DC plate voltage and DC current as the transconductance portion. The other half is the load line actually chosen for the AC performance. This is just a nominal value but does describe the usage fairly well. So, Rp, Plate Resistance, Ra, Rk are not always the same usage, from tube spec paper to tube spec paper. Occasionally you will get Rp, Rk and transconductance in the same document. Take the transconductance number and the load resistance that describes the load line you intend to use, based upon maximum linearity from the tube curves.
Bud
6N6 for SE
I don't know about the 6N6, but I have been running a 6B5 se for just over 3 years now in my shop and it sounds great. To my ears I cannot hear much of a difference in sound VS a SE 6V6. They do require 7k load resistance on the plate, and if I am not mistaken the old stock opt's that I used with mine are 8k. I built it just for the fun of it with scrap matching parts from the junk box, and have been happy ever since.
This amp will not win awards for looks or boutique parts, but is satisfiying none the less.
Here is a circuit http://www.makearadio.com/amplifiers/12ax7-6n6.php that may get you started in the right or (wrong?) direction. I have not built this but his other projects work exceptionally well.
Have a good day,
Michael (mltube)
I don't know about the 6N6, but I have been running a 6B5 se for just over 3 years now in my shop and it sounds great. To my ears I cannot hear much of a difference in sound VS a SE 6V6. They do require 7k load resistance on the plate, and if I am not mistaken the old stock opt's that I used with mine are 8k. I built it just for the fun of it with scrap matching parts from the junk box, and have been happy ever since.
This amp will not win awards for looks or boutique parts, but is satisfiying none the less.
Here is a circuit http://www.makearadio.com/amplifiers/12ax7-6n6.php that may get you started in the right or (wrong?) direction. I have not built this but his other projects work exceptionally well.
Have a good day,
Michael (mltube)
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