HI.
I am looking at building a (stereo) tube amplifier. my story is this, I have obtained a large power supply with a couple of large transforms and chokes. described as follows
.....a large power supply chassis. The chassis has two very large power transformers, one Trimax and one A&R, and two very large Trimax chokes as well as 4 valves and high voltage caps etc. The Trimax power transformer is 425-0-425 @ 300mA secondary. The A & R model PT1934 with a HT secondary of 350-0-350 @ 175mA and Heater windings appear to include 1 x 5VCT @ 3A, 2 x 6.3V CT @ 3A. The chassis measures 400mm W x 200mm H x 260mm D.
I sound a bit like a man who finds a set of keys, and decides to build a car to suit. I was thinking of using the williamson design, using el34's. I know I can buy something without the stress, but a bit like aman and a mountain (or a chicken and the road) there are somethings that just need to be done.
Can anyone suggest if this is the best use of the transformers I have, or if not, any other or better suggestions for me.
I am looking at about 20 - 30 watts per channel.
I have 6 ohm speakers, sensitive at 91db.
cheers
I am looking at building a (stereo) tube amplifier. my story is this, I have obtained a large power supply with a couple of large transforms and chokes. described as follows
.....a large power supply chassis. The chassis has two very large power transformers, one Trimax and one A&R, and two very large Trimax chokes as well as 4 valves and high voltage caps etc. The Trimax power transformer is 425-0-425 @ 300mA secondary. The A & R model PT1934 with a HT secondary of 350-0-350 @ 175mA and Heater windings appear to include 1 x 5VCT @ 3A, 2 x 6.3V CT @ 3A. The chassis measures 400mm W x 200mm H x 260mm D.
I sound a bit like a man who finds a set of keys, and decides to build a car to suit. I was thinking of using the williamson design, using el34's. I know I can buy something without the stress, but a bit like aman and a mountain (or a chicken and the road) there are somethings that just need to be done.
Can anyone suggest if this is the best use of the transformers I have, or if not, any other or better suggestions for me.
I am looking at about 20 - 30 watts per channel.
I have 6 ohm speakers, sensitive at 91db.
cheers
Does the Trimax 850V unit have any heater windings? You could just use this one and ditch the other power transformer to clear some space on the chassis.
Rule of thumb is you get 1.2 times AC voltage. So you might get 500VDC out of that, useful for EL34 PP or more power with KT88s PP, or if you want to use vintage tubes, you could go for aussie made 807 pentodes. Put a voltage regulator tube in there to drop a couple of hundred volts for your small signal tubes and regulated screen grids (g2).
You will need: 2 matched output transformers. The specs on these will determine what you can build... Get ones with ultralinear tappings if you can.
However you can start simple- use the smaller transformer and build a 6V6 or EL84 PP amp, or even a SE amp ( simplest design ever!)
Rule of thumb is you get 1.2 times AC voltage. So you might get 500VDC out of that, useful for EL34 PP or more power with KT88s PP, or if you want to use vintage tubes, you could go for aussie made 807 pentodes. Put a voltage regulator tube in there to drop a couple of hundred volts for your small signal tubes and regulated screen grids (g2).
You will need: 2 matched output transformers. The specs on these will determine what you can build... Get ones with ultralinear tappings if you can.
However you can start simple- use the smaller transformer and build a 6V6 or EL84 PP amp, or even a SE amp ( simplest design ever!)
Thanx for the reply.
1. no, the 850 v trimax does not appear to have heater windings. It does have outputs of 200, 230 & 250. So I assume that I will need to use both transformers If I go with the original plan
2. start simple and work up from there: great idea. can you suggest a design? I would be happy to sacrifice some power, as I have reasonably sensitive speakers. Afterall, the wright brothers didnt design a jumbo jet as their first project.
1. no, the 850 v trimax does not appear to have heater windings. It does have outputs of 200, 230 & 250. So I assume that I will need to use both transformers If I go with the original plan
2. start simple and work up from there: great idea. can you suggest a design? I would be happy to sacrifice some power, as I have reasonably sensitive speakers. Afterall, the wright brothers didnt design a jumbo jet as their first project.
That's not especially sensitive. Think 89 dB. efficient connected to the 4 Ohm taps of O/P trafos. Using the 4 Ohm taps improves damping factor, which I suspect you will need. Paul Joppa's 102 dB. rule, which suggests that an amp/speaker combo should be capable of 102 dB. SPL peaks (1 M. distance), tells us that you need approx. 32 WPC. PP EL34s wired UL will do the job for you.
SHiFTY is correct. The Trimax 425-0-425 power trafo will do NICELY in powering a PP EL34 stereoblock. Having to use a separate filament trafo is a blessing in disguise, as you can SS rectify the B+ without worrying about SS diode noise sneaking into the heater supply. A CL150 inrush current limiter between the diodes and the 1st filter cap. will slow B+ rise down enough.
I'm linking the Mullard 5-20 schematic, which could serve as a point of departure for the design ultimately used. Switch to "fixed" bias for the finals. For several reasons, a 12AT7/ECC81 is a BETTER choice than the 12AX7/ECC83 in the phase splitter "slot". Also, with the peak 2 VRMS drive from a CDP, a tube with less gain than the EF86 is (IMO) appropriate in the voltage gain position.
edit: linked schematic
I'd recommend the Williamson like you mentioned. B-e-a-utiful sounding amp when done right. Do it ultralinear for lots of power. I haven't heard an EL34 Williamson amp, but i suspect with all the feedback it will not sound too different than the 5881 and KT66 Williamsons I've listened to (Heathkit W4 ultralinear, craftsmen 500a triode). I'd recommend 6SN7's for the front end as opposed to 12AU7s or the like noval tubes. You will need decent output transformers for this though. Hammonds might not do.
http://www.clarisonus.com/Archives/Amp_Design/Williamson 1952 The Williamson Amplifier.pdf
http://www.clarisonus.com/Archives/Amp_Design/Williamson 1952 The Williamson Amplifier.pdf
Boris_The_Blade said:
Hammond O/P trafos will not do for Williamson topology. Absolutely top notch O/P "iron" is necessary or the phase shifts will add up and cause instability. A well done Williamson is NICE, but not so easy to achieve at "modest" cost. The Mullard topology has fewer phase shifts, which reduces exposure to instability.
Also, getting a 12AT7 in the signal path of PP amps is a GOOD thing. The 'T7 mates with PP "finals" to yield a "waterfall" for the NET harmonic distortion spectrum. That "waterfall" is PLEASING.
Hi Eli,
Ah, come on, this only holds true if one is not able and willing to get the needed stepped attenuators and Zobel networks right for the OPTs being used.
Mind you, I don´t dispute that better iron will make that task easier. But it is necessary in any case, if you want an unconditionally stable Williamson style amp. Which the original Williamson is not - regardless of its excellent Partridge iron.
Regards,
Tom Schlangen
Eli Duttman said:
Ah, come on, this only holds true if one is not able and willing to get the needed stepped attenuators and Zobel networks right for the OPTs being used.
Mind you, I don´t dispute that better iron will make that task easier. But it is necessary in any case, if you want an unconditionally stable Williamson style amp. Which the original Williamson is not - regardless of its excellent Partridge iron.
Regards,
Tom Schlangen
It depends on what output transformers and valves/tubes you can get locally. Single ended transformers cannot be used for push-pull, and vice versa. See if you can score some old stereo amp or console and raid it for parts. The bigger the OPTS the better. Ask any old radio HAM guys, they usually have piles of transformers around.
Push pull has more power, but SE has (arguably) more finesse and is very simple. I'm currently using a 5W single ended on my 91db speaks and they sound just fine.
www.tubelab.com/simplese.htm
looks like a good first project, and the bloke also posts on diyaudio.
Or you can build the El-Cheapo (like I will be!)
http://www.decware.com/cgi-bin/yabb2/YaBB.pl?num=1126228937/255
Push pull has more power, but SE has (arguably) more finesse and is very simple. I'm currently using a 5W single ended on my 91db speaks and they sound just fine.
www.tubelab.com/simplese.htm
looks like a good first project, and the bloke also posts on diyaudio.
Or you can build the El-Cheapo (like I will be!)
http://www.decware.com/cgi-bin/yabb2/YaBB.pl?num=1126228937/255
Tubes4e4 said:
One of the problems in the Williamson design are that the RCs are tight clustered. According to the original diagram the first time constant is 0.05u x 0.47M = 0.024s and the other time constant is 0.25u x 0.1M = 0.025s. That’s not a good start for a feedback amplifier, IMHO.
Jan E Veiset
Thanx for all the replies.
If I assume correctly, the jury is not out on the 425 transformer running el34's, also, the 350 A & R transformer will not do either.
Output tranfsformers for the willinason design will be difficult, if not costly to obtain.
As this is my first real project of this magnitude, and having limited funds, I really need to use resourses wisely. So, as eli suggested I may be able to use both transformers to power smaller tubes, such as 6V6, as confirmed by ray.
RE output transformers, Hammond make a variety that can be used with a variety of tubes, including 6V6 (and el34 for future projects/ mods) which are reasonably inexpensive and easy to get.
Next question, is, can anyone suggest a amp design, using 6V6 tubes PP that I might beable to construct with the transformers I have, and what kind of power output could I anticipate.
If I assume correctly, the jury is not out on the 425 transformer running el34's, also, the 350 A & R transformer will not do either.
Output tranfsformers for the willinason design will be difficult, if not costly to obtain.
As this is my first real project of this magnitude, and having limited funds, I really need to use resourses wisely. So, as eli suggested I may be able to use both transformers to power smaller tubes, such as 6V6, as confirmed by ray.
RE output transformers, Hammond make a variety that can be used with a variety of tubes, including 6V6 (and el34 for future projects/ mods) which are reasonably inexpensive and easy to get.
Next question, is, can anyone suggest a amp design, using 6V6 tubes PP that I might beable to construct with the transformers I have, and what kind of power output could I anticipate.
ray_moth said:
Hi Ray,
The old Dynaco ST-70 produces 2 x 35W rms both channels driven (I've measured it on at least 30 samples) with four EL34 using a power transformer that most assuredly cannot deliver anywhere close to 300mA on a continuous basis and that with a rectifier tube having a maximum continuous current rating of just 250mA.
The 120W monoblocks I used to manufacture used 1.1KVct @ 300mA rated power transformers and 2 x 5AR4 and have proven to be quite reliable over the last 8yrs or so.
Certainly a 300mA rated power transformer ought to be sufficient for a class AB PP stereo power amplifier comparable to an ST-70 based on my experience.
I would seriously suggest spending as much money on output trannies as you can afford, they make or break the amp in my opinion. Vintage ones are often the best. If your output transformers have ultralinear taps, you have more options.
I think if you are going to the trouble of scratch building a PP amp, build one with more power than 6V6. Here's my suggestion: 807 fixed bias PP. The 807 is a 6L6 type with a topcap and they are cheap and rugged.
Have each 807 running 500V@40mA, with regulated screen grids at 300V, total standing current would be less than 200mA, 50% safety margin. This can easily be powered off the Trimax.
You could have a separate power supply from the other transformer for the lower voltage valves. This would get you about 50W per channel, any 6L6PP schematic can be used.
I think if you are going to the trouble of scratch building a PP amp, build one with more power than 6V6. Here's my suggestion: 807 fixed bias PP. The 807 is a 6L6 type with a topcap and they are cheap and rugged.
Have each 807 running 500V@40mA, with regulated screen grids at 300V, total standing current would be less than 200mA, 50% safety margin. This can easily be powered off the Trimax.
You could have a separate power supply from the other transformer for the lower voltage valves. This would get you about 50W per channel, any 6L6PP schematic can be used.
I agree with Kevin. The 425-0-425 300 mA. trafo should be good for somewhat more than 250 mA. of B+, if it's SS rectified and pseudo choke I/P (cLC) filtered. Regulation should still be quite decent if the B+ rail is tweaked to 425 V. Surely, the EL34 "veterans" around here can suggest a good Class "AB" idle current for that rail voltage. Only filament and "fixed" bias power remain to be supplied. KISS and stand each EL34 cathode on a 10 Ohm resistor.
This thread's originator, jag142, has indicated that budget is a concern. The Edcor model CXPP60-4-4.2K should work well in a Mullard style circuit. In the US, Edcor "iron" costs less than Hammond. Whether or not that holds true in "Oz", I can't say.
This thread's originator, jag142, has indicated that budget is a concern. The Edcor model CXPP60-4-4.2K should work well in a Mullard style circuit. In the US, Edcor "iron" costs less than Hammond. Whether or not that holds true in "Oz", I can't say.
DEar all.
thanx for all the valuable input.
given my resources, and experience, I am looking at shiftys suggestion of a 6L6 PP, or an 807 fixed bias PP design.
I have done some searching and found a Schematic for 6L6 Push-Pull by David Hafler & Herbert I. Keroes, rated at 20W.
Alternitavely, I found an old thread in the forum from a few years ago posted by shifty re building an 807 fixed bias amp. Shifty, would you think this would be an appropriate first project of this type?
appreciate your input
Regards
jag142 (Greg)
thanx for all the valuable input.
given my resources, and experience, I am looking at shiftys suggestion of a 6L6 PP, or an 807 fixed bias PP design.
I have done some searching and found a Schematic for 6L6 Push-Pull by David Hafler & Herbert I. Keroes, rated at 20W.
Alternitavely, I found an old thread in the forum from a few years ago posted by shifty re building an 807 fixed bias amp. Shifty, would you think this would be an appropriate first project of this type?
appreciate your input
Regards
jag142 (Greg)
Hi
I'm looking for a diy project and I saw the attached yesterday.
About the schematic (old) on page 14 of the books.... are there some updated or tweaking?
thanks
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