Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on May 12, 2021, 10:53:10 pm

Title: Why does the JCM800 use a coupling cap before the power section?
Post by: markmalin on May 12, 2021, 10:53:10 pm

Using Robinettes JCM800 schematic, there's a 0.022uf coupling cap between the input of the phase inverter and the master volume.  I'm wondering why this is needed if the caps in the tone stack block all the DC already.  Is this additional cap redundant, or is it used for tonal reasons?

Thanks guys!
Mark

Title: Re: Why does the JCM800 use a coupling cap before the power section?
Post by: glass54 on May 12, 2021, 11:04:57 pm

Hi Mark
DC isolation for Pot. Bias on Grid (V3A) 24V will be severely compromised.
This would vary with pot position, esp when pot wiper is closer to Ground.
The 22nF Cap also sets the Lower Freq break point for that section of circuit.
Pots also don't like DC on them esp Carbon (adds a little unwanted Fx  :laugh:)
Kind regards
Mirek

Title: Re: Why does the JCM800 use a coupling cap before the power section?
Post by: 2deaf on May 13, 2021, 12:32:38 am

Quote from: markmalin on May 12, 2021, 10:53:10 pm

Using Robinettes JCM800 schematic, there's a 0.022uf coupling cap between the input of the phase inverter and the master volume.  I'm wondering why this is needed if the caps in the tone stack block all the DC already.  Is this additional cap redundant, or is it used for tonal reasons?

All long-tailed pairs like this have a capacitor in that position.  They won't work without it.

The grid doesn't really have 24V on it.  Well, it does when you put a meter on it, but it has 36.5V on it as soon as you remove the meter.

The input impedance of that LTP PI is higher than it looks so that the .022uF cap passes all audio frequencies.  No tonal reason there.

Title: Re: Why does the JCM800 use a coupling cap before the power section?
Post by: glass54 on May 13, 2021, 12:40:44 am

Additional useful info:
https://www.aikenamps.com/index.php/the-long-tail-pair (https://www.aikenamps.com/index.php/the-long-tail-pair)
It's a good read and I'm sure there are others.
Kind regards
Mirek

Title: Re: Why does the JCM800 use a coupling cap before the power section?
Post by: markmalin on May 13, 2021, 08:42:57 am

Thanks, guys.  That helps.  And I'll read that article from Aiken.  Thanks


So if I have a single ended output stage (no phase inverter), is a cap still needed?  Here's a schematic I'm working with.  I guess at the end of the day I'm wondering if I should put a coupling cap in my circuit between the grid of the EL34 and the master like the Marshall schematic.

Title: Re: Why does the JCM800 use a coupling cap before the power section?
Post by: sluckey on May 13, 2021, 08:54:17 am

Quote from: markmalin on May 13, 2021, 08:42:57 am

So if I have a single ended output stage (no phase inverter), is a cap still needed?  Here's a schematic I'm working with.  I guess at the end of the day I'm wondering if I should put a coupling cap in my circuit between the grid of the EL34 and the master like the Marshall schematic.

It's not a question of phase inverter or no phase inverter. The question is "Does the grid operate with a voltage other than zero?" If yes, then a cap is required.

No cap is required between your EL34 grid and the MV. In fact, if you put one there, the EL34 will not work properly.

Title: Re: Why does the JCM800 use a coupling cap before the power section?
Post by: markmalin on May 13, 2021, 09:45:20 am

Quote from: sluckey on May 13, 2021, 08:54:17 am

It's not a question of phase inverter or no phase inverter. The question is "Does the grid operate with a voltage other than zero?" If yes, then a cap is required.

No cap is required between your EL34 grid and the MV. In fact, if you put one there, the EL34 will not work properly.

I see.  Thanks for the explanation, slukey. When I fired up the amp for the first time I measured the voltage at the grid of the EL34 because I was trying to understand.