Interstage cap

[trioid]

Hi All,

I have some good quality oil caps (630V or higher), but they are 0.22 uF. Now, the grid cap on the 300B is sized at 0.1 uF on the stock amp. I suspect doubling the capacitance would be OK, but have I missed something? Related question is -- is there much to be 'gained' in clarity by doing this?

A search of the forum has not answered this specifically, but maybe I missed it?

Thanks!
Jim

[Grainger49]

It should not make a difference.  If it were a lower value you might get a deep bass roll off. 

Doubling might allow noise in the 10 Hz to 20 Hz range through  But if your speakers are not FLAT in that range and your room isn't over 56 feet long, you will not notice that difference.

[trioid]

Hi Grainger,

Thank you for your response! Should not be a problem with the low f issue in my case. I think I will give this try as time permits, and will post my impression.

Later,
Jim

 

[Paul Joppa]

It's probably not a problem in this application; values of up to 0.47uF are not uncommon.

There is a slight tradeoff with the startup transient - the cap is charged through the grid resistor, leaving a positive grid voltage that decays rapidly, as the 300B filament is heating up. Very large coupling capacitors are know to produce problems, some tubes are more sensitive that others - I speculate that the sensitive ones start to emit electrons especially quickly after the application of filament power. I don't have a definitive way to determine the tradeoffs (the cathode bypass capacitor also plays a role). The EML is the only tube I would be nervous about - the probability of a problem is small but the consequence could be costly.

If you have current (rev 1.1) Paramount or the the soft-start board in an older one, this problem has been addressed.

[trioid]

Hello Paul,

As always, thank you for the additional education, and the warning. I do run EML 300B tubes (standard version). Also FYI, the electrolytic cathode bypass cap has been replaced with a Solen 47 uF, 630V cap. I am not sure about the version of the amp, other than to say that the schematic I have is dated Feb 25, 2006, Rev 3.2.  (I do not have the soft start upgrade).

I really appreciate the information, and understand that there is some (non-zero) risk if this is not a Rev 1.1 version.

Cheers,
Jim

[trioid]

Hi all,

Can one of you let me know if the Rev 3.2 of the 300B amp is the Rev 1.1 version?

Thanks!
jim

[Paul Joppa]

The Paramount v1.1 manual (copyright 2011) includes the following schematics:

Driver Stage for Paramount, Rev. 3.52 Dec 28, 2010

Paramount 300B Amp and PSU, Rev. 3.2 Feb 25, 2006

Paramount 2A3 Amp and PSU, Rev. 3.2 Feb 25, 2006

The Paramount Soft Start Retrofit kit (copyright 2010) also had the Driver Stage for Paramount, Rev. 3.52 Dec 28, 2010 diagram.

[trioid]

Thank you Paul. That is quite clear. Now, I apologize for being so 'pedantic' (uneducated, really), but I am still not clear on this:

I have the updated version of the 800B amp and PSU. However, I have an older version of the Driver Stage. I am not clever enough to know if that will make this larger grid cap safe.... So that is my last question for now, and I appreciate your patience.

[Paul Joppa]

I cannot say that it will make it safe, or that it will not. I know that the original 0.1uF is OK, I believe Jac (at EML) has approved it and it's typical of many other 300B amps as well. And I also know that Jac has cautioned against using very large values of capacitor in this kind of circuit; I understand and agree with his reasoning on that. I expect that he has seen examples of damage from such designs. What I don't know is how big is too big.

To determine that for certain, one would have to do an extensive experimental study, almost certainly damaging at least a few tubes in the process - I'm not volunteering for that job! The critical question is what combination of positive grid voltage and filament warmup sequencing leads to damage, and there just isn't enough data available to say that for sure.

[trioid]

Hi Paul,

That concludes my questions! 

I understand the situation. At this point, I think I will just leave those Cu foil/oil, 0.22 uF caps for another, less risk-prone, experiement.

Much appreciated,
Jim