Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dinkotom on December 06, 2012, 03:19:04 am

Title: Differences between Magnatone 260 and 260A
Post by: dinkotom on December 06, 2012, 03:19:04 am

Here I've got an older scheme of this amplifier (model 260) where the PI is connected directly (without a .01 and 10M resistor), as opposed to my 260A (which has the cap and resistor). I rewired it and got zero sound from the amp! I'm sure I'm connected it correctly, there's only two wires to it, so I wonder if it can work this way at all (without a .01 cap and 10M resistor), ie, is this schematic for 260 correct. Thank you in advance.

model 260:
http://happygolucky2.blogspot.com/2012/12/magnatone-260-schematic.html (http://happygolucky2.blogspot.com/2012/12/magnatone-260-schematic.html)
 
model 260A:
http://3.bp.blogspot.com/-1kGryza5IDg/TZlhqG1vKGI/AAAAAAAAAI8/zu06pK88TeA/s1600/magnatone%2B260A%2Bschematic.jpg (http://3.bp.blogspot.com/-1kGryza5IDg/TZlhqG1vKGI/AAAAAAAAAI8/zu06pK88TeA/s1600/magnatone%2B260A%2Bschematic.jpg)

BTW, the tonestacks are also different, I rewired it as on 260 schem and got much fuller and fatter cleans and of course farty bass when cranked. The difference in sound between 260 and 260A models is like between tweed and BF Fenders.

Title: Re: Differences between Magnatone 260 and 260A
Post by: sluckey on December 06, 2012, 07:39:20 am

Quote

I wonder if it can work this way at all (without a .01 cap and 10M resistor)

It should work just fine.

Title: Re: Differences between Magnatone 260 and 260A
Post by: PRR on December 06, 2012, 09:23:50 pm

Notice that the volt-amp stage bias-bleed is 120K in one and 220K in the other. This will set different DC plate voltage on the volt-amp. When cathodyne is direct-coupled, if its grid (volt-amp plate voltage) is too high, the cathodyne will bottom-out. The R-C network coupling lets the cathodyne find its own bias.

Title: Re: Differences between Magnatone 260 and 260A
Post by: dinkotom on December 07, 2012, 02:32:08 am

Ah, now I remember, I completely forgot that I did incorrectly connect one version of the tonestack, (I changed a lot of different tonestacks and PIs in this amp in past few days so I must've lost the plot at some point), and the amp wasn't working at all for some time, so I wrongly attributed that problem to the PI from 260 amp. Sorry, disregard the original question, I was again trying to improve on a fine working thing.  :icon_biggrin: