Quad II Mods/Tweeks

[Johan Potgieter]

Paul,

If I am permitted a few ideas and some questions?

Croft's modifications were in the power supply; there are silicon diodes and a 6080 pass valve controlled by a pair of ecc83s which are crammed underneath.

WHAT!!

How on earth did he manage that without roasting everything inside that rather submarine-like cramped quarters? And to what purpose??

I presume it was a misguided desire to have a regulated power supply - something which would have made no audible difference to a class-A amplifier. Sorry, but a lot more could have been achieved with a quite high power supply filter output capacitance at a fraction of the cost.

The amplifiers arrived fitted with EL34s which I do not think are a good idea - partly because the need 1.5A fpr their heaters. I refuse to pay stupid money for GEC ones. Once upon a time there was in Crystal Palace a TV Repair shop; he had lots of brand-new EL 37s at 15/- each (75p)

Anyway as he's gone I chose Russian 5881s and also 6L6GCs; they are excellent and best of all all the voltages are spot on and they draw only 0.9A for their heaters. I'm delighted with them.

Exactly so. The EL34s could have made an academic difference to the distortion since their Gms are almost double that of the KT66, but their optimum operating conditions are different to those of KT66. And the heater current, as you say ... (Also note that KT66s are within 10% of 6L6s. I currently use 6L6GCs in all my amplifiers. As the dissipation here is only about 20%, the 5881s will also do.)

Next I took Chris Beeching's advice an replaced the EF86s with Brimar's 6BR7s a little rewiring of the bases and changing a resistor or two gave good audible improvement ...

I have great respect for Chris, but am not sure what he intended with this. The operation of the EF86s and 6BR7s are quite similar here (you may find greater spread in the characterisitics of either than the listed differences). Still.

When I get the Quads back I should like to replace the input circuitry with a triode diff pair.

Here I would like to pause and take a step back. What do you hope to achieve by such a (drastic) departure from the original design? What audible improvement are you after?

Keep in mind that because of the local 6L6 cathode feedback, the maximum anode outputs of the phase inverter will need to be >100Vpp at full output. You will still have some 100V available to drop over the common cathode resistor (more if you intend to use a semiconductor CCS), but your net stage gain will be about 15% that of the EF86, with either an equivalent drop in NFB and increase in distortion - or converted to the same NFB as before, the required signal input will now be 9Vrms.

Fine - it is your amplifier and your decision, but why deviate that far from the original design? To repeat: For what purpose?

I'll be removing the 6080 and the two ECC83s of course and I'll build some 317 regulators and there'll be a standby switch too. My chosen valve will give gain of about 20 and as my line stage gives 20 I should be well away.

So, having adapted the NFB to the same as before, your input requirement will then be about 450mV. That would appear quite adequate for feeding from a CD player of modern FM tuner. (Again, I see no advantage to a regulator here, and there are safer choices at these voltages than a LM317 - can't recall at present. Unless you mean regulating only the pre-amp's h.t. The advantage there is more one of eliminating h.t. ripple than the necessity of a constant voltage.)

Paul, hope to be forgiven for all that; mostly alternative thinking; please, no fault-finding. As said, it is YOUR amplifier. Good fortune and let us know how things turn out!

 

[Johan Potgieter]

Follow:

Sorry, Paul; my stupid, I think. Of course the 6080 could then go into the socket left by the rectifier. For an awful momnet there I thought that was also underneath!!

(Hope to be pardoned for a blond moment - time of night here is 02:12 am)

 

[7N7]

Paul,

If I am permitted a few ideas and some questions?



WHAT!!

How on earth did he manage that without roasting everything inside that rather submarine-like cramped quarters? And to what purpose??

I presume it was a misguided desire to have a regulated power supply - something which would have made no audible difference to a class-A amplifier. Sorry, but a lot more could have been achieved with a quite high power supply filter output capacitance at a fraction of the cost.



Exactly so. The EL34s could have made an academic difference to the distortion since their Gms are almost double that of the KT66, but their optimum operating conditions are different to those of KT66. And the heater current, as you say ... (Also note that KT66s are within 10% of 6L6s. I currently use 6L6GCs in all my amplifiers. As the dissipation here is only about 20%, the 5881s will also do.)



I have great respect for Chris, but am not sure what he intended with this. The operation of the EF86s and 6BR7s are quite similar here (you may find greater spread in the characterisitics of either than the listed differences). Still.



Here I would like to pause and take a step back. What do you hope to achieve by such a (drastic) departure from the original design? What audible improvement are you after?

Keep in mind that because of the local 6L6 cathode feedback, the maximum anode outputs of the phase inverter will need to be >100Vpp at full output. You will still have some 100V available to drop over the common cathode resistor (more if you intend to use a semiconductor CCS), but your net stage gain will be about 15% that of the EF86, with either an equivalent drop in NFB and increase in distortion - or converted to the same NFB as before, the required signal input will now be 9Vrms.

Fine - it is your amplifier and your decision, but why deviate that far from the original design? To repeat: For what purpose?



So, having adapted the NFB to the same as before, your input requirement will then be about 450mV. That would appear quite adequate for feeding from a CD player of modern FM tuner. (Again, I see no advantage to a regulator here, and there are safer choices at these voltages than a LM317 - can't recall at present. Unless you mean regulating only the pre-amp's h.t. The advantage there is more one of eliminating h.t. ripple than the necessity of a constant voltage.)

Paul, hope to be forgiven for all that; mostly alternative thinking; please, no fault-finding. As said, it is YOUR amplifier. Good fortune and let us know how things turn out!

Hullo.

A lot to respond to there.

Guy Croft has quite good reputation apparently, but anyway that's how the amplifiers are. There is indeed a lot crammed inside.

I noticed an improvement when I fitted the 6BR7s and would recommend them. As for the output valves there is also the idea of converting from the 6L6/5881s to STC 5B/255M which are essentially Loctal-based 807s - which of course is really a 6L6 isn't it.

Why convert from pentodes? Well my line stage is looking very promising; it uses 955 diff pairs, with triode cathode followers with individual pentode loads (triode-pentode 6AW8A). Prototype tested -3dB at 180kHz and 955s in diff pairs THD driving a 30k pad at 19v rms was -57dB with 3rd at -70dB. The cathode followers will of course drive anything, so as I like diff pairs, triodes and fiddling with amplifiers I thought that another diff pair in the Quad II might be a nice idea. And in addition, the line stage has balanced outputs. I fancy triode-strapped E282F (mu = 25 gm 30) but I'll need 30mA for them if I can steal it from Quad's rather feeble power transformer Of course I won't be running the 6080 and the 2 ECC83s so that should save a bit. As for feedback I have no idea; I do not understand it, never have. It's supposed to be easy with diff pairs, but I tried it once and everything went haywire. That was my last experiment with feedback.

If necessary perhaps I'll run the output valves as triodes; after all 10 watts is half as loud as 100w! And I'll have the wonderful output transformer with its cathode winding to help me. And I could go fixed bias; that would win me some volts and the 6l6/5881/5b/255M will be cruising.

I would use regulators to make the amps quiet and ripple-free - a separate regulated g2 supply if I remain beam tetrode. No problems with high-voltage 317 regulators. I built the line stage ten years ago and fired it up for the first time today; it has two 317 regulators. First reg's design voltage is 400V, result 401V; second reg (anodes of cathode followers) is 335V and gave this value, I built a few HV 317 regs over the years and found that if one takes care they are fine.

Finally you ask quite reasonably what audible improvement I am after. The answer to be honest is none really. It would be great if it sounds better but it's quite good now. The aim is aesthetic rather than audible and I love valves; that's it!

Thanks so much for your message and I hope you feel like writing again.

Best wishes

Paul