tube amp

[jag142]

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

[SHiFTY]

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!)

[jag142]

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.

[Eli Duttman]

I have 6 ohm speakers, sensitive at 91db.

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

[Boris_The_Blade]

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/A...0Amplifier.pdf

[Eli Duttman]

Originally posted by Boris_The_Blade
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/A...0Amplifier.pdf

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.

[sorenj07]

Originally posted by Eli Duttman
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.

Quick question: How many watts would it take to bring 98dB speakers to 102dB listening levels?

[Boris_The_Blade]

since a 98dB speaker is specified at 1W/1m, and a 3db increase is double the power (and the fun) ... it will take a little over 2 watts...to be exact: 10^(+4/10) = 2.51... then it is about 2.5 watts to push it to 102dB SPL

[ray_moth]

The Trimax power transformer is 425-0-425 @ 300mA secondary. The A & R model PT1934 with a HT secondary of 350-0-350 @ 175mA

The Trimax should be able to provide enough current for a stereo amp using low-medium power tubes in PP (6V6, EL84 etc.) but the A & R cannot. Neither transformer has enough capacity to drive a stereo amp using bigger tubes (6L6, EL34 etc.)

[Tubes4e4]

Hi Eli,

Originally posted by Eli Duttman
Hammond O/P trafos will not do for Williamson topology.

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