Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: richmi on February 03, 2016, 04:54:03 PM

Title: Padding the Paramounts (original version). [resolved]
Post by: richmi on February 03, 2016, 04:54:03 PM

Hello all,

I need to pad my Paramounts as I have too much gain.

I searched the forum, but could not find a straight answer to my problem.

Would installing an ordinary 100k (a value I often see recommended for the Paramounts) volume potentiometer at the input be all that is required?

Thanks for any thoughts.

Richard

Title: Re: Padding the Paramounts (original version).
Post by: Paul Joppa on February 03, 2016, 06:28:43 PM

That would be the most flexible approach, and it's done in Kaiju.

Title: Re: Padding the Paramounts (original version).
Post by: richmi on February 07, 2016, 12:27:06 PM

Hello again,

I guess I do not really understand how a volume potentiometer works in this application.

I connected 100k pot (measures actually 93k), and when I did the resistance check, I realized something is not quite right.

I disconnected the red wire of the STP at T20. I connected it to the input terminal of the pot. I connected the output terminal of the pot to T20, and the ground terminal to T18.

Measurements at T20 now go from 0k to 68k (which is the result of 249k and 93k paralleled), and from 68k to 93k at the RCA jack.

Certainly, these new input impedance values must be wrong for the amp to work correctly.

I think I need some help here.

Thanks in advance.

Richard

Title: Re: Padding the Paramounts (original version).
Post by: Paul Joppa on February 07, 2016, 02:36:42 PM

Unless I am missing something, you do indeed understand, and the circuit is ready for voltage testing.

Title: Re: Padding the Paramounts (original version).
Post by: richmi on February 07, 2016, 02:39:56 PM

Hello PJ,

I was writing this message when I got a warning that a reply had been posted. Here is what I had done in the meantime.

I took a break and thought about how it works in the Crack. Then, I disconnected the 249k resistor between T20 and T18.

I now have an input impedance of 93k irrespective of how much I turn the potentiometer, and a resistance varying from zero to 93k at T20 depending on how much I turn the pot up.

Is it how it is supposed to be?

Now, since the original input impedance was 249k, can I just use a 250k potentiometer to maintain a wider range? (I still need to buy a pair of mono pots; I experimented with a stereo one.)

I have not turned on yet the amp i modified. I am waiting for confirmation that everything is okay from an expert.

Thanks again,

Richard

Title: Re: Padding the Paramounts (original version).
Post by: Paul Joppa on February 07, 2016, 03:00:47 PM

Stay with the 100K. There is some risk of treble loss with a higher-resistance pot. The value has no effect on the range of adjustment.

A linear (not "log" or "audio") pot will probably be less twitchy to adjust, but the difference is not critical.

Title: Re: Padding the Paramounts (original version).
Post by: richmi on February 07, 2016, 05:50:13 PM

I did the voltage check and everything is spot on except for B8 on the nine-pin socket where I get 4.4 and 4.6 depending on the 12AT7 I use. Manual says approx. 6 (2-28-06 revised). Not sure how important that is.

Richard

Title: Re: Padding the Paramounts (original version).
Post by: Paul Joppa on February 08, 2016, 07:07:16 AM

Good to go then - as long as the voltage at pin 8 is greater than 2.5v (and less than 25v) the hybrid regulator is working correctly.

Title: Re: Padding the Paramounts (original version).
Post by: richmi on February 08, 2016, 07:32:59 AM

Thanks PJ,

Since I worried a bit about that voltage not being within specs, I checked the other amp and I got the exact same reading. However, T9, T19 and B1 that were supposed to show 195v read 245v. A replacement of tube restored those voltages back within limit. Probably, doing a voltage check once in a while is not a bad thing.

Richard