Roland Bolt-60 Modification

[proffett]

I just picked up an old Bolt-60 combo, (solid state preamp section, tube power amp section with 2 6l6 and a 12AT7 phase inverter) and after a bunch of internet research, have found a few mods that I would like to do to it.

http://bolt60.no.sapo.pt/documents/b...0Schematic.jpg


First off, there's a mod to add a switch to disconnect the feedback to the power tubes, which is supposed to give it more 'punch' and 'loudness.'
I'm using it for bass, (has a Peavey Black Widow) and it does sound good, but more 'punch' is always good. Is this actually what I would get by doing this mod? Or would it be something else entirely... more distortion, gain, etc. which I don't want...

Another mod I found reference to is an 'adjustable slope resistor control' to adjust the tone and 'pitch the amp where you want it'
There's no diagram or any other description of this mod... dos this kind of mod ring a bell to anyone? Maybe not with this particular amp but with any amp?

Also, the specs I could find on this thing call for a 12AT7 for the phase inverter, but the tech who worked on it put in a 12AX7... I understand the 12AT7 is a lower gain tube... what is the function of the phase inverter, and how would the wrong tube affect tone here?

Thanks in advance for any helpful info I might be able to glean from this forum... if there's anyone here with experience with this amp who might be able to answer some questions or at least shed some light...

Dan

[Gokarna]

Hi Dan,
I just was searching for the Roland Bolt-100 and found your questions

First off, there's a mod to add a switch to disconnect the feedback to the power tubes, which is supposed to give it more 'punch' and 'loudness.'
I'm using it for bass, (has a Peavey Black Widow) and it does sound good, but more 'punch' is always good. Is this actually what I would get by doing this mod? Or would it be something else entirely... more distortion, gain, etc. which I don't want...

Another mod I found reference to is an 'adjustable slope resistor control' to adjust the tone and 'pitch the amp where you want it'
There's no diagram or any other description of this mod... dos this kind of mod ring a bell to anyone? Maybe not with this particular amp but with any amp?

I did, along a lot's of others, also this two mods to my Roland Bolt-30,
which has nearly the same circuit as the Bolt-60 ( I also have that one).

1.The "feedback-Switch" is a nice feature, it adds a lot of punch and loudness to the amp. But also, the amp turns faster into feedback and overdrive-sound, If you switch to "disconnected".
Still, I like it, but I can't tell you, how it will be with a bass-guitar. I have OEM Electro-Voice 12L speaker installed.

2. Also the "adjustable slope resistor" is very nice. You can add a lot of "bottom" with this or even make very clean, less bass fender sound.
Only thing here is, that I didn't manage to make the potentiometer completly noiseless, if you turn it - you can hear a kind of grumbling.

Also, the specs I could find on this thing call for a 12AT7 for the phase inverter, but the tech who worked on it put in a 12AX7... I understand the 12AT7 is a lower gain tube... what is the function of the phase inverter, and how would the wrong tube affect tone here?

From my experience, the difference in sound isn't very big. You can try yourself with 12AT7, 12AU7 or 12AX7 (ECC81, ECC82 & ECC83) . Just try youself what will sound best to yourself.
To explain the function of the phase inverter will be too much for this answer, google for it or find some forums, where you can learn the basics of tube-amp building.

If you like, contact me and I'll send you some photos or can even re-draw a schematic for your Bolt-60. It'll take a little time, but i'll do it.

I hope, my english is not to bad,

Regards

Gokarna