Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RobBozic on June 08, 2021, 05:29:45 pm

Title: JCM800 Treble peaker or cathode bypass cap
Post by: RobBozic on June 08, 2021, 05:29:45 pm

Hi,
General question relating to say a 2204 topology.
Prior to the CF you have the 470k/470pf treble peaker. I noticed on my home brew build this cap adds a fair bit of noise.
What is the difference if I put a 330nf cap across the 820R V2A cathode and remove the 470pf?
It’s a similar boost, but in terms of what it does to the circuit, what are the differences between the two methods of adding or emphasising a particular freq.?


Thanks in advance
Rob

Title: Re: JCM800 Treble peaker or cathode bypass cap
Post by: sluckey on June 08, 2021, 05:59:18 pm

Why don't you try it and give us a report?   :icon_biggrin:

Title: Re: JCM800 Treble peaker or cathode bypass cap
Post by: RobBozic on June 08, 2021, 07:48:23 pm

Ha,
I did and to me the cathode bypass cap was less noisy, and had a similar boost (obviously). I haven't had a chance to crank it yet.


My question was about what's happening re: tube operation etc... does one method produce more harmonics, or compression etc..


Thanks
Rob

Title: Re: JCM800 Treble peaker or cathode bypass cap
Post by: jordan86 on June 08, 2021, 07:56:09 pm

You could try a different type of cap for the treble peaker if you like that tone/response more but want to compare noise: ceramic,mica,film, etc.

Title: Re: JCM800 Treble peaker or cathode bypass cap
Post by: pdf64 on June 09, 2021, 03:43:31 am

I prefer to leave any preamp clipping stage cathodes unbypassed, as the result is smoother clipping, more rounded waveform, fewer high harmonics. Obviously some degree of loss of the max potential gain, but that’s always the case.
But it would seem to be counterproductive not to try both and develop your ear and preferences in such matters.

Title: Re: JCM800 Treble peaker or cathode bypass cap
Post by: RobBozic on June 10, 2021, 02:26:34 am

Thanks for your response gents.
Curious about different cap types for this position. Marshall mostly used ceramics, but a lot of modern builders go for mica.
I’m going to put them both on a switch and compare them both.
 Interesting that a lot of the Bognor amps don’t have the V2a cap and the treble peaker cap prior to V2.