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Old 25th February 2022, 10:13 PM
bikerhifinut bikerhifinut is offline
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Default Info on the Pre3/4 output transformers.

Can anyone tell me what the turns ratio is on the pre3 output transformer?
Wired normal, I do realise they have 4 separate secondaries after a chat with Matthew.
And can anyone confirm the gain in standard form of the pre3/4 without any interstage attenuation?
I'm in the process of a rebuild of my "Frankenstein" Pre4, giving it a treat of an official WD remote control board instead of the occasionally flaky ebay special I threw in when I first knocked it up and also the latest PCB designed for the pre4, which looks very nice i must say, a lot roomier and the grounding at first sight looks better done.
But my plan is to use fixed attenuation rather than the adjustment pots as i do now. I am aiming for around 6dB of gain which suits my main power amp nicely.
Andy.
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Old 2nd March 2022, 12:18 PM
John Caswell John Caswell is offline
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Default Re: Info on the Pre3/4 output transformers.

Hi Andy,
Right just measured mine with the following results,
Turns ratio - 16.5:1 therefore
Impedance ratio - 271:1
I measured the output impedance when working to be close to 10R which gives an anode load of 2K7 for the ECC82.
I may be wrong but I am pretty sure I measured the gain to be 6X (half sec'y) and 12X (full sec'y) which equates to 15dB and 21dB.
Hope this helps all and sundry.
I haven't had a chance to try out the unit using 1,2,3 & 4 secondary sections but will do so in the near future.

John

Last edited by John Caswell; 2nd March 2022 at 12:19 PM. Reason: Add info
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Old 2nd March 2022, 01:23 PM
bikerhifinut bikerhifinut is offline
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Default Re: Info on the Pre3/4 output transformers.

Thanks very much indeed John.
Thats a big help.
I think initially I will just use a fixed 6dB attenuator ie halve the signal using equal values of fixed resistor in a PD which will give 9dB or just less than 3x Gain which I feel will fit nicely in to my power amp sensitivity of 1V.
As an aside is it possible for me to have a copy of the revised circuit diagram and board layout as I did buy a genuine board from Matthew but no associated Circuit diagram, as I see a few changes to the power supply in and more obviously the attenuation arrangement looks a bit more complicated than a straight Potentiometer PD.
I do like your layout John, it makes sense to me to have the dropper resistor and HT decoupling capacitor on board. And the earthing runs seem much better to my untrained eye.
By the way I'm back on a refurbished PSU3 board again after a glitch took out the regulated supply again and this time it wasnt the LD1086 but seems to be the unobtanium (at least from official sources) pass device. I ordered up some off a Chinese eBay shop but I think its a gamble on them being genuine.
Anyhow I took the original no name 100uF HT capacitors and they measured quite low at 80uF on the component analyser, the ESR seemed OK but with only 3.5V across fro the analyser i'd take that with a pinch of salt. I replaced with some known Good F&T 100uF and also some 100uF in the positions after the dropper resistors to make a sort of CRCLC filter.
Voltages all came in fine, slightly high but in agreement with my very high Mains which is usually around 251V and sometimes hits the legal maximum of 253V, and not so high to cause me any concern for any of the preamplifiers I use.
The Phono3 is still showing a bit of the LF instability that caused me to go regulated in the first place, but definitely attenuated from original and interestingly absent if the Pre3/4 is in circuit. This I think will be the output transformers not having an LF response that low so kind of naturally filtering out the low level Speaker cone flap etc.
More on that in another thread I think as I figure its an incentive to do a complete rebuild of Phono3 rather than dig around looking for the culprit R and C.
Cheers

Andy.

Last edited by bikerhifinut; 2nd March 2022 at 01:31 PM. Reason: typos
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Old 10th March 2022, 10:07 PM
bikerhifinut bikerhifinut is offline
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Default Re: Info on the Pre3/4 output transformers.

Not sure if this warrants its own thread but here goes:

I bought the up to date latest PCB for the pre4 to upgrade the previously fitted pre3 board in my "frankenstein" Pre4 Homebrew lash up in a modded HD3 case, which if memory serves me right was used in the original PC era prototype pre4.

I've never been completely happy with it and after a bit of head scratching I figured that for one reason or another the signal grounding using the original pcb wass less than ideal.
So on inspection when it arrived I was impressed with the spacious layout and also that there were a lot fewer earth points to join up etc as I am sure that was the reason why I was chasing hum loops down for a long time after the initial build.
I took a look at how the attenuation potentiometer was effected and it took a bit of reverse engineering but I realised quickly enough that R2A and R2B were connected by a trace that meant no extra link wires were needed if I wanted to leave the pot out of circuit, which I did.
So I figured a more elegant way of dealing with the extra gain that I dont need would be to adjust the gain in the 2 stage circuit. That was why I was asking about the transformer turns ratio.
Anyway I figured that leaving the second stage unsullied was the best approach rather than faff around with feedback etc. And from experience with another homebrew preamp build I figured that an unbypassed cathode resistor on the first stage could be the answer, the degenerative cathode feedback would reduce the overall gain and quite possibly tidy up the already low distortion a bit. A fag packet calculation and experience from another preamp gave me the value of 7.2k for around 6dB gain and 3.6k for 12dB gain, it's the way Velleman dealt with their ECC82 preamp kit and I'm not ashamed to do a bit of plagiarism, and anyway its discontinued.
So in the absence of a 7.2 k I settled for 7.5, although 6.8 would probably have worked just as well, its not so critical here. Keeping the original 33k anode load too. I also reduced the HT by the use of a 47k resistor at R1 instead of the 1k on the pre3 diagram, I've got a 300V to 310V HT off PSU3 here with our very high mains. Anyway that works a treat bringing the HT at the anode to the intended 170V give or take. The HT to the second Valve via the output transformers was left as per Pre3, i.e. directly wired off the decoupling capacitor and 1k resistor as per original. I used a 100uF F&T on the PCB for the first stage after the 47k dropper.
So switch it on and all measured well. Initial listening test indicated this board could be a lot quieter than the pre3, certainly completely hum free and no discernible hiss. So far so good. I did use good quality standard metal films on the important resistors in the signal Path, matched per side on the DMM. a pair of nice Samwah 470uf 105degC on the second stage cathode.
However when I checked the gain, I found with a 6dB gain on the first stage, the total amplification was basically 0dB i.e Unity gain although there was promise in the sound quality, the level from the Squeeze box Touch wasnt really high enough. Bearing in mind my power amp needs 1V to drive to full power.
So I took out the 7.5k and subbed in a pair of 3.6k for a hoped for 12dB gain on that stage, on measurement afterwards the whole preamp was now showing 6dB gain on a 500Hz and 1Khz input from the signal generator.
Boxed it up and with a pair of US made raytheon 5814 (ECC82 equivalent) in and also tried a pair of NOS ex BBC Brimar 12AU7 for comparison. Both came in very similar.
I like what its doing now, it's got loads of low end thrutch which I felt was missing before and I can now see what all the fuss about transformer output preamps is about.
With this layout you would be ok with 0.1uF between stages, assuming a 1meg gridleak into the second stage, gives a cutoff around 1.5hz i think.
I used a pair of Soniqs 470nF polyprops that I had in the toybox looking for a good home and I'm well happy so far.
Ok it may not be for everyone in here, but it works for me and my needs and its an easy mod that you can reverse easily. I would recommend veropins on the component locations that are different from standard to save damaging the PCB.
Its certainly been interesting for me, and also has brought out the basic qualities of pre3/4 .
You could do a similar job on the pre3 board too.
Cheers.
Andy.
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