ARC SP-6A Tube Preamp

[chiily]

Don't forget that grid stoppers need to be pure resistive, thus any film types trimmed in spirals is contraproductive and might even provoke instability!
Old type carbon composit is what is needed !
Test with these resistors you will see what happend.

Interesting, that's what I've seen before as grid stoppers before...I had some Dales handy.

The schematic from the Mk 2 SP-8 has the grid stoppers as 1% tolerance. I guess that as the only carbon comps or films I can find in the UK are 5%, then 5% is good enough as the value is not critical.

Ordered some 1k2 carbon composites 0.5W and 0.25W 5%.

 

[chiily]

And on another upbeat note, the preamp doesn't induce hum in the amp anymore when playing. Though the hum does come back if I switch to mute... weird...

 

[Salectric]

Interesting, that's what I've seen before as grid stoppers before...I had some Dales handy.

The schematic from the Mk 2 SP-8 has the grid stoppers as 1% tolerance. I guess that as the only carbon comps or films I can find in the UK are 5%, then 5% is good enough as the value is not critical.

Ordered some 1k2 carbon composites 0.5W and 0.25W 5%.

The grid stoppers don’t need to be closely matched. But the type of resistor used is important.

 

[chiily]

The grid stoppers don’t need to be closely matched. But the type of resistor used is important.

Interestingly, looking at the BoM for Mk 2 of the SP-8 shows that grid stoppers for V2/3/5/6 are metal film, 1/4 and 1/2 watt.

 

[DasguteOhr]

Interesting, that's what I've seen before as grid stoppers before...I had some Dales handy.

The schematic from the Mk 2 SP-8 has the grid stoppers as 1% tolerance. I guess that as the only carbon comps or films I can find in the UK are 5%, then 5% is good enough as the value is not critical.

Ordered some 1k2 carbon composites 0.5W and 0.25W 5%.

If you install a grid stopper, this will affect the phase shift. Since the tube V6 has a feedback loop to V4, it can happen that the voltage becomes positive earlier, thus becoming unstable and oscillating. Any small inductance added will make this worse.
Before you change the resistors reduce gain in the feedback loop, i.e. reduce the gain (6db) as described in the circuit diagram. Trying is better than studying. Nothing can break with this. Maybe the oszillation is gone.
The basic problem is still interference from the power supply. Did you insert C17( 150uf-220uf after the ECC83 or is it already there?

 

[chiily]

Another day, and another learning experienceView attachment 145937

...and what looks like a step backwards.

I've installed grid stoppers (1K) on both sides of V5 and V6. At that appears to have triggered some oscillation, well I think it is that, though certainly a change to the output spectra.

Hmm, I will try bypassing the ones on V6 with a bit of wire...

I'm such an idiot. Because I've had the front of the amp off I didn't have the input selected to the right input, doh!!

This should be better



Anyway, having the front off the preamp has let me have another go at getting the dress nuts on the switches straight rather than skewed, it looks much better now.

 

[chiily]

If you install a grid stopper, this will affect the phase shift. Since the tube V6 has a feedback loop to V4, it can happen that the voltage becomes positive earlier, thus becoming unstable and oscillating. Any small inductance added will make this worse.
Before you change the resistors reduce gain in the feedback loop, i.e. reduce the gain (6db) as described in the circuit diagram. Trying is better than studying. Nothing can break with this. Maybe the oszillation is gone.View attachment 145944
The basic problem is still interference from the power supply. Did you insert C17( 150uf-220uf after the ECC83 or is it already there?

I've already inserted the gain reducing resistors, my little Leak ST20 is very sensitive.

Which C17 are you referring to please? There's one in the PSU that is deleted in my version of the SP-8, 1983, the last of the Mk Is.

 

[chiily]

Really dumb question, which I think I know the answer to having reading about grid stoppers and how they are essential in audio circuits.

I noticed that the SP-8 Mk 2 has grid stoppers on the grids of the PSU valves, V7/V8



Should I bite the bullet and added grid stopper resistors here too?

 

[Atmasphere]

then 5% is good enough as the value is not critical.

The value isn't critical. You just need something to stop the oscillation so there is a minimum value; once above that all that happens is you curtail high frequencies due to Miller Effect.

I think the next thing is to get to the source of the hum. How does the output of the filament supply look on the 'scope? For that matter, what do the outputs of the two high Voltage supplies look on the 'scope?

One thing that is a common problem with single-ended components is ground loops. To reduce ground loop problems, the chassis (which should be grounded via the power cord into the building's electrical ground) should not be the same thing as the audio ground. This is one of the reasons RCA connectors are supplied with insulators so when they are mounted to a chassis they are insulated from it.

You can find out if this is a problem simply by measuring the resistance from the ground of an RCA connector to the actual chassis. If the Ohmmeter reads less than 1 Ohm you have a problem. A simple way to correct this of course is to lift the ground of the AC power and see if the hum goes away. If that proves to be the case then you have to sort out a plan of how to go about fixing it.

 

[DasguteOhr]

I've already inserted the gain reducing resistors, my little Leak ST20 is very sensitive.

Which C17 are you referring to please? There's one in the PSU that is deleted in my version of the SP-8, 1983, the last of the Mk Is.

View attachment 145956

ok. I would still increase the 2uf cap (C 18) a bit, 10uf maybe. you can use the coupling caps if you change them to 3.3uf. of course they have to withstand 450 volts. I don't know what models are in there.

 

[DonH50]

Interestingly, looking at the BoM for Mk 2 of the SP-8 shows that grid stoppers for V2/3/5/6 are metal film, 1/4 and 1/2 watt.

They just need to be carbon comp(osition) or film (carbon or metal), preferably non-inductive, but not wire-wound power resistors.

 

[Atmasphere]

Really dumb question, which I think I know the answer to having reading about grid stoppers and how they are essential in audio circuits.

I noticed that the SP-8 Mk 2 has grid stoppers on the grids of the PSU valves, V7/V8



Should I bite the bullet and added CC resistors here too?

Yes. You really want to shut down any possibility of oscillation.

And as others have said, don't use an inductive resistor! I've had good luck with metal film. Carbons are good too but they tend to be noisier.

 

[DonH50]

Also either surface-mount or with very short leads.

@Atmasphere do you have current data on resistor noise for audio? When I last checked a few years ago, carbon comp where the worst, then metal (depending upon materials and construction), then carbon film. Metal was better than carbon film at higher frequencies, but at audio the Caddock and Vishay metal films I measured were actually higher in noise than carbon film at "DC". Curious - Don

 

[DasguteOhr]

One of best resistors ever for me

Amtrans AMRG Carbon Film Resistors | HIFICollective

Amtrans AMRG Carbon Film Resistors

www.hificollective.co.uk

 

[Atmasphere]

Also either surface-mount or with very short leads.

@Atmasphere do you have current data on resistor noise for audio? When I last checked a few years ago, carbon comp where the worst, then metal (depending upon materials and construction), then carbon film. Metal was better than carbon film at higher frequencies, but at audio the Caddock and Vishay metal films I measured were actually higher in noise than carbon film at "DC". Curious - Don

We use Caddock resistors as an option, but they are not the typical kind you usually see compared to the Vishays. We have to get them custom built for us. They are the lowest noise metal films we could find. I don't see them in their catalog at all.

We use a variety of metal film parts. Some are Draloric, a Vishay brand. We've been seeing recently a resistor that looks like a 1/4 Watt size but is actually rated 1 Watt. These have been low noise too. I don't like the carbon films so much - the ones I've seen have been noisier, though not as noisy as the regular carbon composition parts. Generally speaking the noise of the resistors is not significant WRT the noise that vacuum tubes make. But I am careful to avoid larger resistive values in series with the signal if I can.

 

[Salectric]

I have tried many different types of resistors as grid stoppers. While the values are not critical, the type of resistor is. The best sounding grid stoppers IMO are Riken carbon film. They stopped production a number of years ago but a few outlets may still stock them.

 

[microstrip]

In fact, as far as I remember, Caddok is one the few existing independent companies supplying ultra low noise precision resistors for instrumentation. Vishay slowly acquired most of the competition along the years - last time I checked the best I could find were Vishay Z-Foil Resistors and VHP Series Vishay Foil (not searching for audio purposes, BTW). I heard about low noise precision wirewound resistors but I have never seen them.

 

[DasguteOhr]

I have tried many different types of resistors as grid stoppers. While the values are not critical, the type of resistor is. The best sounding grid stoppers IMO are Riken carbon film. They stopped production a number of years ago but a few outlets may still stock them.

successor amrg resistors post # 114

 

[DonH50]

I have tried many different types of resistors as grid stoppers. While the values are not critical, the type of resistor is. The best sounding grid stoppers IMO are Riken carbon film. They stopped production a number of years ago but a few outlets may still stock them.

See post #114: https://www.whatsbestforum.com/threads/arc-sp-6a-tube-preamp.38354/page-6#post-1033194 -- @DasguteOhr posted a link to the company that took over (allegedly).

Edit: NM, he beat me to it!

 

[DonH50]

In fact, as far as I remember, Caddok is one the few existing independent companies supplying ultra low noise precision resistors for instrumentation. Vishay slowly acquired most of the competition along the years - last time I checked the best I could find were Vishay Z-Foil Resistors and VHP Series Vishay Foil (not searching for audio purposes, BTW). I heard about low noise precision wirewound resistors but I have never seen them.

I haven't looked for a while since getting out of the design side of things and retiring. Wirewound resistors actually have the lowest noise, but have so many other issues (inductance and high tempco among them) that they are rarely used IME. Even for power applications thick-film metal resistors have pretty much taken over, likely driven by the proliferation of SMPS that require good high-frequency performance (high self-resonance frequency).