6AS7 Push Pull

[pre65]

Good choice Steve, but...

I wanna know who's gonna be 'twinned' with a Svetlana Winged C - Forward and aggressive in the midrange. ????

DTB

How about Ken Wood from Catalunya ?

[Cressy Snr]

Tweaked the current down a bit through the sinks to correctly accommodate the 12E14s

mmmwwaah!

[Cressy Snr]

I did a bit more tweaking of the driver stage today nothing more radical than bumping up the current a tad on the first stage to 1mA, just to lift the loadline up a bit
The supply to the 12E14 constant current sinks was finally given snubbing caps around the silicon bridge rectifier which dropped the hiss at the speakers to near inaudibility. An additional benefit was a further smoothing out of the top-end

The best improvement was however connecting the beam plates up on the 12E14s. These are on pin 6 and unlike most other beam tetrodes, need to be connected to the cathode via an external link. Pin 6 is normally left unconnected on the EL34 pentode.

Not a lot of people realise that a beam tetrode works even better when the beam plates are connected to something sheesh. This small oversight must surely qualify for the wally of the week award

Never mind. I'm taking the amp to Simon's place at the end of this week to give it a chance on some big speakers. The result of this will decide if it is worth building some bigger Metronomes in the new year.

On Simon's Fostexes I'll probably realise what I've been missing by using small speakers.

Steve

[simon]

On Simon's Fostexes I'll probably realise what I've been missing by using small speakers.

Or not, as the case may be...

[Cressy Snr]

More tweaking of the driver stage this evening.

The 400-0-400 CLC power supply giving 530V to the drivers has been converted to a simple LC setup which has dropped the voltage to 320V.

Why do this?
Well I began to question the need for a voltage this high, given the reliability problems I have had with this section of the amp. First the fire in the solid state rectified supply that took out the whole of the driver stage, then the second HT decoupling cap failure that tried its best to destroy a perfectly good NOS GZ37. Lastly peace of mind. In "Zen and the Art" Robert Pirsig talks at great length about this "peace of mind" factor when applied to machines.

Pirsig argues that if the machine does not give the operator peace of mind then the machine is wrong. I was forever wondering what was going to blow up next despite the fact that I was getting great sound. Therefore the amp was wrong... period.

Having another look at the drive requirements of the 6AS7 I decided I could get down to around 300V of HT in the driver stages and still drive the finals to full power.

Simulating the driver stage supply on PSUD minus the first cap and with just the two paralleled 150H 8mA Hammond chokes, followed by a 150uF capacitor, gave a fast rise time, no overshoot or ringing, 10mV p-p ripple (perfectly adequate for push- pull) and an output of 320V with bugger all ripple current worth worrying about.

One pair of cutters and a snipped out first cap later and we have exactly the 320V predicted by PSUD.

Measuring the voltages at the 6SN7s and 6SL7s you'd think the sections had been designed straight out of the RCA Resistance Coupled Amplifier data sheets so near are the figures for both valves.

Peace of mind restored. Instant Karma.

Steve

[Nick]

Pirsig argues that if the machine does not give the operator peace of mind then the machine is wrong.

Yep, I can sing up to that, however in this case as the machine was designed by the operator, maybe its more of a question of reflecting the mind of the builder

I think it would have been interesting to hear a conversation between Persig and Sakuma San.

[Cressy Snr]

Yep, I can sign up to that, however in this case as the machine was designed by the operator, maybe its more of a question of reflecting the mind of the builder

Well I suppose when the design is your baby, then like Pirsig's narrator's friend John, who couldn't bring himself to fix the dripping faucet, even though it must have been driving him crazy, you will persevere with something even after it has twice blown up, before eventually coming to terms with the fact that in the driver stage area, your design sucks and needs a bit of a rethink.

I think it would have been interesting to hear a conversation between Persig and Sakuma San.

Well after having read a few papers on the metaphysics of Quality it seems that the Japanese would find nothing revolutionary in Pirsig's philosophy. They already know about it and have done for several hundred years.

Mr Sakuma would probably find it quite odd that it would take a Westerner such as Pirsig 30 years of mind bending work to come up with a structure of thought that his own countrymen use every day of their lives without even thinking about it.

Steve

[Cressy Snr]

I've been simplifying the amp a bit today by taking out the CCS on the first stage 6SL7 voltage amps. Forced into it really after shorting out one of the LM317 regulators to the HT with my meter probe whilst testing voltages in order to do an accurate schematic. Let's just say it made me jump.... an OOYAHBASTAD moment

I've been reduced to 500R shared cathode resistors in the former 1st CCS position now. There was no difference to the sound without the CCSs at the front of the amp, so I shan't be putting them back. The second stage CCSs remain, as of course do the beam tetrode 12E14 CCSs on the output valve pairs

I suppose with a good quality input transformer/phase splitter you have near perfect balance from the outset. Any remaining imbalances ought to be taken care of first by the CC sunk 6SN7 drivers then simply maintained in the output stage by the 12E14 sinks.

Looks like CCSs on all three stages was maybe a tiny bit of overkill on my part?

Here's the (hopefully) final schematic revision. 300K on 6SL7s has been replaced by 220K after the HT was dropped down.

Did anyone say Ultrapath caps?........Oh no here we go again! Stop it!!

Seriously though, I want now to draw up a PSU schematic for the amp with a view to picking your brains Nick/Andrew on possible mods in that department. The AC heaters are fine. I'm thinking more along the lines of regulating the HT to the outputs and the 12E14 current sinks. Don't know where to start on that so any advice would be welcome. I'll put up a PSU schematic as soon as I have collected all the back-of-a-fag-packet working drawings into something presentable.

Steve

[Cressy Snr]

Here is the PSU

Driver stage fed by choke input supply, rectified with GZ37

Beam tetrode CCS and output valves fed by 3 section cap input filters

Output valves on separate 625VA toroidal transformer

Big value caps could mean the PSUs feeding the big valves might be "slow" and treble softness could be an issue?
The SS rectified PSUs are a hangover from the OTL.

Any comments or suggestions for improvements would be much appreciated.

Steve

[Nick]

I would be tempted to try a smaller first cap on the main supply. That 2200uf cap could cause big current spikes, if it wasnt for the resistor, maybe try a smaller cap and a smaller resister. And maybe some snubbing aroud the diodes is worth trying.

[Andrew]

Hi Steve,

How much current do the various bits driver/main etc draw?

cheers,

-- Andrew

[Cressy Snr]

Hi Andrew

The whole amp is a fairly low current device

The voltage amp driver stage is drawing around 15mA

The output stage draws approximately 200mA in total (4 X 50mA)

The constant current sinks are sinking 270mA altogether (100mA per output pair plus 35mA each of their own draw via their screen grids.

Nick
I'll try a lower first cap on both the CCS and the power stage PSUs.

Steve.

[Andrew]

Steve,

Some food for thought.

I have managed to get 250v @ close to 100mA through a LM317 as a HT regulator. For a power or driver stage these regs are quiet enough on their own. The voltage limit for the design are the high voltage transistor on the input; if you use a MJE340 it will be about 350v on input, a transistor cascode may get you higher still. The reg will need to drop voltage, so 350v input gives a fair bit less on the output.

I would suggest that 100mA for a mono channel is do-able, with careful parts selection, at highish current thermal limits start to get to you on these regs because of good old "I squared R". Large heatsinks are needed etc. They should be capable of much more but the original design by Maida was limited to 50mA and this is where I have set my stall as a sensible working limit (a bit Zen-ish I know). He did this to limt the short current. However, 100mA should be OK.... the first pass at the LCR phono drew 100mA out of a LM317 in this config - but the various bits got blummin' hot which is where I found out why Maida had set his at 50mA. Bear in mind LM317 are not expensive, however, so one for each power valve might be a better way forward. You could easy build 4 off LM317 regs for the price of the valves on a single valve series reg.

For the driver, a cascode would be needed for 330v-ish, well I reckon that would be close enough to 350v on the input to be marginal on a single MJE340. At 15mA current is not a problem.

You could wire one up such a reg in a day, easy, takes me a couple of evenings, on a bit of strip board.

As you know, however, if you short one of the little buggers they don't half go bang - and that's with the protection circuitry.

I have also got a working design for a shunt reg, using valves, that is able to buck some PSU noise, tho' I wouldn't use it for that purpose alone, and given the PP nature of your amp then the PSU shouldn't need shunting in the same way an SE amp does.

Otherwise you're in valve series reg territory and, for that, Nick's your man.

One last thought, as Nick once said to me, and it was very sound advice, he couldn't have debugged his reg without a 'scope, having been there I would agree.

cheers,

-- Andrew

[Cressy Snr]

Thanks Andrew, I'll look into this after Christmas. The idea of one reg per valve on the power stage sounds good.

I took the amp to Simon's place today to see how it would fare driving bigger speakers. It was quite good!

I'd love to hear it on Nick's and Steve's baffles too at some point in the future.

New year project will surely be a bigger pair of Metronomes.


Steve.

[simon]

Thanks for coming over and bringing the amp Steve, I had a really good day.

The amp sounds very good, this is the least PP-sounding PP amp I've heard. My horns were able to show the control it has in the bass, which we didn't really hear that at Steve's, for obvious reasons. Bigger metronomes should work really well for you Steve.

Comparing my 45SE amp confirmed to me that I find 45s coloured, at least in my amp. Lovely top, but lower down, whilst there's lots of weight it kinda thumps and doesn't reveal much fine detail. I need to go back to the 2A3s to cross reference.

As I'm home alone this evening and the neighbours are away I thought I'd dig the WE91s out again and dust the KRs off. I'm running them directly from the Squeezebox with Hammond 1627 opts, and at first it was quite a shock. Lots of brightly lit midrange but a bit bass light. I think some of this is down to the Hammonds - the AE c cores have more bass weight.

But as they've warmed up and my ears have become reaccustomed I'm really enjoying them, they sound much more neutral now. Probably me. The PSUs are cap input and I think I might get a worthwhile improvement from choke input. Just need a couple of largish mains tx to try unfortunately.

I changed the 300B filaments to AC and this may have helped. Checking the voltage though I'm getting 5.4V AC which I would have thought was a little high. Should I be concerned and put some resistance in there to bring it down?