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SRPP ANZAÏ tube preamplifier

tubelectron said:
As you can see, like you I relied on a JJ 40+20+20+20 500V multi-cap. I thought that the replacement Pwr Xfo I installed was also a Mercury, but it is a Classic Tone.

T
Click to expand...

What do you bias those Bendix 5992 at? I bias them a little lower than US made 6V6GT. The voltages that Fender puts onto their 6V6GT is a bit high for the 5992. So, I compensate by running lower idle current. I think that last amp I biased to like 8-9-watt range. Seemed to make very good tone and last long time.

Lately, I haven't seen many true NOS Bendix tubes. Their 6106 is an excellent slow-start 5Y3GT substitute. Raytheon and GE also made an excellent slow-start 5Y3 type, the 6087/5Y3WGTB.

6087_GE_JAN1.jpg
 
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Mr Boochie said:
What do you bias those Bendix 5992 at? I bias them a little lower than US made 6V6GT. The voltages that Fender puts onto their 6V6GT is a bit high for the 5992. So, I compensate by running lower idle current. I think that last amp I biased to like 8-9-watt range. Seemed to make very good tone and last long time.

Lately, I haven't seen many true NOS Bendix tubes. Their 6106 is an excellent slow-start 5Y3GT substitute. Raytheon and GE also made an excellent slow-start 5Y3 type, the 6087/5Y3WGTB.

View attachment 2961168
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Here are below the values on my SF PR, wher you have 0.030A x 357V=10.7W total dissipation per tube, for a grid bias at -27.5V :

1692371769337.png

Yes, I have a stock of Bendix 5992 that I bought at least 30 years ago in a local military surplus that had closed since.

T
 
tubelectron said:
Here are below the values on my SF PR, wher you have 0.030A x 357V=10.7W total dissipation per tube, for a grid bias at -27.5V :

View attachment 2961582

Yes, I have a stock of Bendix 5992 that I bought at least 30 years ago in a local military surplus that had closed since.

T
Click to expand...
So, you have about 357VDC on the plates? Is that with a 5Y3GT or 5U4GB rectifier?

Thanks!
 
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tubelectron said:
Here is the complete schematic of my modified SRPP ANZAÏ :



T
Click to expand...

Doesn't the 100-ohm resistors (to simulate ground reference) work only on the AC side of the heater line?

Just curious. Thanks!
 
Last modification today... And great improvements !

1 - Level compensation between TONE OFF and TONE ON modes :

I noticed a significant drop in loudness level when switching from the TONE ON to the TONE OFF mode. I solved this issue by :

- only switching the TC RLC network instead of the whole TC stage.
- leaving all the time the 12AU7 recovery/output stage engaged as a permanent output buffer
- replacing the TC network by a compensated attenuator matching the attenuation level of the TC all controls at noon.

1692573910627.png

Output level between the two modes are exactly matched, plus the response to square waves at 20kHz is neatly improved.

2 - Correction of the square waves signal at low frequencies for any source and any mode :

Square waves were very steep at low frequencies, it even started to tilt significantly at 1khz...

1692574034605.png

Square waves at low frequency are now flat top, no more steep.

My modified SRPP ANZAÏ can play now in the Big Leagues...

1692574557853.png

Last step to come : back faceplate engraving - in next September.

T
 
As it was, the tone bypass basically made it a passive "preamp", no active bits at all. Not surprising the gain dropped
 
gadget73 said:
As it was, the tone bypass basically made it a passive "preamp", no active bits at all. Not surprising the gain dropped
Click to expand...

Yes, it's surprising that a gain matching was not considered in the original design between the Tone ON and OFF modes : it would have been an easy improvement. But the French Audio School of the late 80s - early 90s was a bit too much in a "the less in the signal, the best it is" direction.

Mr Boochie said:
I'm liking the 12AU7 buffer/impedance match stage. :thumbsup:

How's the sound? :idea:
Click to expand...

Well, the timbre difference between Tone IN and OUT has vanished, hence to gain compensation and also the permanent insertion of the 12AU7 output buffer in all modes and all sources selections. That's beneficial, of course. The output impedance of the 12AU7 stage alone should be circa 2400 ohms. Nonetheless, the volume pots A50K stays placed at the end of the circuit, as in the original design : it allow a better S/N ratio at low volumes.

The SRPP ANZAÏ is reputedly a high dynamic and high clipping headroom margin preamplifier. The mentioned specs of the original circuit were alrerady amazing (I don't think I need to translate...) :

1692600537949.png

I will indeed make some measurements to check and report it there, you guess it... ;)

About the sound, a serious listening comparison with some other preamps owned by my friends and me should be made to give a reliable advice : ARC SP3A1 and SP7, NAD 114, McIntosh C712... I do not have the complete list !

For the moment, what I can say is that the modified circuit operates flawlessly : no adverse noise or mbalance of any sort, good punchy sound, and excellent signal to noise ratio. In other words : a good challenger for listening comparisons...

T
 
Some measurements made on my Modded SRPP ANZAÏ...

Via the PHONO section :

1692625125718.png

The output capability of the preamp set in Phono section is at least 52VRMS before clipping.
The input capabilty of the preamp set in Phono section is 137mV @1kHz sinus.

Via the CD/AUX section :

The voltmeter monitors the output in V TRMS.

1692623630157.png

1692623680662.png

1692623706524.png

Max input capability of the CD/AUX section before clipping : over 10VRMS... which is the limit of my Signal Generator output - to know the real max capability, I'll have to shift to my old FERISOL C-901 tube Audio Generator from 1953, which can provide up to 40VRMS output :

1692625383938.png

Level comparison Tone ON / Tone OFF, all tone controls at noon :

1692624034114.png

1692624061616.png

The difference in output is negligible now.

Final schematic :

1692625472287.png

1692625502665.png

T
 
tubelectron said:
Some measurements made on my Modded SRPP ANZAÏ...

Via the PHONO section :

View attachment 2964501

The output capability of the preamp set in Phono section is at least 52VRMS before clipping.
The input capabilty of the preamp set in Phono section is 137mV @1kHz sinus.

Via the CD/AUX section :

The voltmeter monitors the output in V TRMS.

View attachment 2964475

View attachment 2964476

View attachment 2964478

Max input capability of the CD/AUX section before clipping : over 10VRMS... which is the limit of my Signal Generator output - to know the real max capability, I'll have to shift to my old FERISOL C-901 tube Audio Generator from 1953, which can provide up to 40VRMS output :

View attachment 2964502

Level comparison Tone ON / Tone OFF, all tone controls at noon :

View attachment 2964480

View attachment 2964481

The difference in output is negligible now.

Final schematic :

View attachment 2964503

View attachment 2964505

T
Click to expand...
You have some awesome test equipment!

The 12AU7 is like the perfect impedance buffer tube. I'm sure that you've already read Valve Wizard section on this circuitry --- but here's the link.

https://www.valvewizard.co.uk/accf.html
 
Mr Boochie said:
You have some awesome test equipment!

The 12AU7 is like the perfect impedance buffer tube. I'm sure that you've already read Valve Wizard section on this circuitry --- but here's the link.

https://www.valvewizard.co.uk/accf.html
Click to expand...
Yes, valvewizard is an excellent and well-written website about tubes :cool: - I do not always think to have a look to it, though... :confused:

The difference observed at low frequencies with the RIAA equalization is certainly due to the increase from 0.22µF to 2.2µF for the first interstage capacitor. Providing a slight increase level at those low frequencies is not a bad point, as it will give deeper bass but it will also made the circuit more sensitive to warped records... ;)

T
 
Update : I could make some refinments and measurements on my SRPP ANZAI :

- RIAA equalization in the PHONO section is met at +/-1dB between 20Hz and 20kHz.
- PHONO input saturation @1kHz sinus is 130mVRMS for 50VRMS output.
- CD/AUX input saturation is 25VRMS for 50VRMS output.
- Bandwidth in CD/AUX section is 5Hz-45kHz@-1dB and 2.5Hz-85kHz@-3dB.

Unfortunately I no more have distortion measurement facilities - and I plan to have one, when is the real question - but by my experience, looking at the triangle and sine waves, the max THD% should be around 1% at worse.

Just in regard of the max output level capability (and hence the dynamic margin) of the SRPP-ANZAÏ measured at 50VRMS, my ARC SP3A1 offers 25VRMS and my McIntosh C712 offers 8VRMS according to their respective specs - certainly with a lower THD%, I guess...

T
 
so gain of about 3db / 2x in the line stage if I read that right ?

honestly no reason you should need anywhere near 50v so it sounds like its got more than enough headroom assuming the gain is in fact about 2.
 
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