power amp bass response

[hebaton]

Hello, I have a 67 Traynor YBA-3 that I had changed onto a guitar amp buy rebuilding the entire front end. Although I replicated the Fender AB763 ( twinn reverb ) preamp section ( no reverb or tremolo ) I still find the amp extremely bass heavy.
Is there something in the power section or the phase inverter circuit that could be modified to tame that ? I see 0.1uf caps before the phase inverter, would changing those to .022uf for example help with that ? and is there any danger to damage anything down the line in experimenting with different values in that position... or elsewhere ?


Thank you !

[martin manning]

You can try reducing the 0.1u at the PI input and/or the two 0.1u after the PI to 0.047 or 0.022. No danger of anything bad happening.

[roberto]

I'd say that it is not linked to the equalisation per se, nor on reducing lows, but to the way the amp reacts with the impedance of the speaker.
Try to add a presence in the classical way on the PA NFB loop, so that the amp is more speaker dependant (lower DF on higher frequencies).

[pdf64]

The TR AB763 had a 1nF cap at the power amp input, for some models intended for closed back cabs it was even lower, eg 500pF.

[martin manning]

]Try to add a presence in the classical way on the PA NFB loop...

Looks like it already has that.

[pdf64]

Note that if a typical 10 to 15% audio taper pot has been used for the treble control, it may be beneficial to try setting it higher than you’d expect if used to a real Fender, as they typically used a treble pot with a taper of about 30%.

[roberto]

]Try to add a presence in the classical way on the PA NFB loop...

Looks like it already has that.

The schematics I've found show a presence out of the nfb loop.
Can you link the one you are referring to?

[martin manning]

It's YBA-3A that has the non-traditional NFB with the presence control outside the loop. https://www.thetubestore.com/traynor-schematics

[roberto]

The link you provided shows that the presence is outside the NFB loop in the YBA3.
https://www.thetubestore.com/lib/thetub ... ematic.pdf

[martin manning]

The link you provided shows that the presence is outside the NFB loop in the YBA3.
https://www.thetubestore.com/lib/thetub ... ematic.pdf

Look in the title block of the drawing. That's a YBA-3A schematic.

[roberto]

So in the link you provided there's not the schematic I've asked you to please provide.
Can you please link a reliable schematic of the amp?
Thanks

[martin manning]

Same link. Click on YBA3 Manual. The schematic is there.

[pdf64]

There’s 3 yba3 schematics in that manual. The 1st has the regular 5F6A type presence control, 2nd and 3rd don’t.
1st is hand noted as being the 1st model, 2nd looks to be introduced Jan 13 1967, 3rd is the same basic drawing as the 2nd, but starts from revision 10, Nov 1970.
The amp of this thread is said to be from 1967, so

[martin manning]

I guess the OP could just have a look?

[roberto]

Ok, so there are 353 possibilities over 365 that the amp has the presence outside the nfb loop as I was saying.
If so, adding it would most probably help with highs.

That thing apart, I agree that a direct intervention of the OP would be the best option.