Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeJunkie on January 18, 2020, 08:03:33 am

Title: Need Expert Help with Schematic Discrepancies
Post by: TubeJunkie on January 18, 2020, 08:03:33 am

I'm researching an amp build, and have come across two sets of schematics for the same amp, but they have some differences.  None are dated, so I don't know which is the most current.  I could really use some help figuring out which is correct, and why, and what the tonal/functional differences are between the differnt values, and different locations.  Or, is one correct and one wrong?  The differences are shown below in the Schematic Differences JPEG.  In the JPEG, the image on the left is from Set 1, and the one on the right is from Set 2:

Set 1 Pre-Amp
http://2.bp.blogspot.com/-PvHVV3SDaTU/UWjhYsYe__I/AAAAAAAACS4/IXqCi65NHVQ/s1600/Excelsior+preamp.JPG (http://2.bp.blogspot.com/-PvHVV3SDaTU/UWjhYsYe__I/AAAAAAAACS4/IXqCi65NHVQ/s1600/Excelsior+preamp.JPG)

Set 1 Power Amp
http://1.bp.blogspot.com/-Ap00oYhq574/UWjhY-jjEGI/AAAAAAAACTA/bNS_3lIZ5S4/s1600/ExcelsiorSchematic.JPG (http://1.bp.blogspot.com/-Ap00oYhq574/UWjhY-jjEGI/AAAAAAAACTA/bNS_3lIZ5S4/s1600/ExcelsiorSchematic.JPG)


Set 2 Pre-Amp
https://images.reverb.com/image/upload/s--JioSPy24--/f_auto,t_supersize/v1533842432/ilygvkcbfg2vuws7wxzt.jpg (https://images.reverb.com/image/upload/s--JioSPy24--/f_auto,t_supersize/v1533842432/ilygvkcbfg2vuws7wxzt.jpg)

Set 2 Power Amp
https://images.reverb.com/image/upload/s--X-fTPyIq--/f_auto,t_supersize/v1533842437/erotiwpmubvxjylj1le8.jpg (https://images.reverb.com/image/upload/s--X-fTPyIq--/f_auto,t_supersize/v1533842437/erotiwpmubvxjylj1le8.jpg)

Title: Re: Need Expert Help with Schematic Discrepancies
Post by: shooter on January 18, 2020, 08:41:23 am

.01uf is sorta a defacto value in trem circuits

the bottom values are all within "tweak to your like", flip a coin to start, or start small n parallel for easy tweaking

Gonna stab at.EDIT .05uF below the line

Title: Re: Need Expert Help with Schematic Discrepancies
Post by: sluckey on January 18, 2020, 09:50:52 am

PRE-AMP... These two caps and one resistor form a high pass filter designed to pass the instrument signal and reject the tremolo signal. Either may be correct but I'd bet on the .01µF caps.

PRE-AMP - TREMOLO... Both circuits are wrong. See attachment.

PRE-AMP - GUITAR INPUT... This is a typical James tonestack. It's more common to see a small value cap on top of the Treble pot and a larger value cap on the bottom of the Treble pot.

REVERB... Either cap is acceptable. Doubt you could tell the difference. Who knows which is actually in the amp.

Title: Re: Need Expert Help with Schematic Discrepancies
Post by: TubeJunkie on January 18, 2020, 11:33:30 am

Thanks shooter and sluckey.  I believe I'll go with the .01uf over the .0047uf for the PRE-AMP.  For the GUITAR INPUT, both schematics show a .001uf cap before the tone pot (attached).  Since the most common arrangement for a Jones tonestack is a smaller cap on the top and a larger cap on the bottom, then the correct value should be .0022uf and not 200pf?  And, any value from 20uf up to 50uf should work for the REVERB?