HAmmond Conversion Question
Working on a Hammond conversion, got a question.
I gutted the preamplifier of the first AO-35 chassis, taking out or rewiring mostly everything before the 2.2 meg resistor in front of the phase inverter. Now the preamp has 68k on the single input and and a 1meg grid resistor on the first triode, 1.5k at each preamp cathode. The volume and tone are between the two gain stages. I used this volume/tone
It's switchable between unbypassed preamp cathodes (good clean until 4/5 up the knob) and 1uf bypass caps, giving a good crunch as the volume increases (25uf didn't seem to give me that much more gain or more mids). It's quiet as a mouse at idle without the bypass caps and there's just a little hiss with them. I plan on adding two .001 caps in parallel with the plate resistors to roll off some treble, as the amp is on the bright end. Otherwise, it sounds pretty good.
SO -- question: Given the tiny chassis and fixed number of available triode gain stages, is there a better/less crude way of having switchable "gain" than switching those cathode caps in and out?
Re: HAmmond Conversion Question
Hey buddy - just got wind of the switch of the change of venue for this thread - which is pretty :cool:
Any road - have you got a schematic for this amp?... Switching cathode bypass caps in and out is a noisy affair and likely the reason you are looking for an alternative?...:wah: What are you wanting to use the amp for?... is it to go back into a Hammond or for some other purpose?...
BTW - the Leslie that has been offered to me is a HL-822 and it's going to be a long haul project ie completely stripped down and re-built... active crossovers, multiple tube amps...
Looking forward :D
YR
Re: Hammond Conversion Question
Quote:
Originally Posted by
yankeerob
have you got a schematic for this amp?... Switching cathode bypass caps in and out is a noisy affair and likely the reason you are looking for an alternative?...:wah: What are you wanting to use the amp for?... is it to go back into a Hammond or for some other purpose?...
YR
Thanks for your response Rob. It's a guitar amp now, no plans to re-deploy for organ use. I'm looking for circuit design advice.
Here's a link to the preamp section of the Hammond AO-35 amp schematic as I gutted the preamp and made changes.
Right now it's got 100K resistors on each preamp triode plate, 1.5k on each cathode and 1uf cathode bypass caps switchable in/out on each triode.
Basically, I'm looking to have the amp switchable between a good clean and good grind. (and I don't really have the know-how - learning as I go)
I read and re-read a bunch of articles (Aiken, Valve wizard, etc) and sought advice on the forums, never got any good answers, didn't want to employ a
master volume as the chassis is really cramped and I didn't know which type to use.
So -- I made the cathode bypass caps switchable. It's not that noisy actually, just more gain as you get higher up the volume knob. There's gotta be a better way though. Switchable preamp cathode resistance..?
Re: Hammond Conversion Question
Quote:
Originally Posted by
jeru
Thanks for your response Rob. It's a guitar amp now, no plans to re-deploy for organ use. I'm looking for circuit design advice.
Here's a link to the preamp section of the Hammond AO-35 amp schematic as I gutted the preamp and made changes.
Right now it's got 100K resistors on each preamp triode plate, 1.5k on each cathode and 1uf cathode bypass caps switchable in/out on each triode.
Basically, I'm looking to have the amp switchable between a good clean and good grind. (and I don't really have the know-how - learning as I go)
I read and re-read a bunch of articles (Aiken, Valve wizard, etc) and sought advice on the forums, never got any good answers, didn't want to employ a
master volume as the chassis is really cramped and I didn't know which type to use.
So -- I made the cathode bypass caps switchable. It's not that noisy actually, just more gain as you get higher up the volume knob. There's gotta be a better way though. Switchable preamp cathode resistance..?
You can mess around with that and the bypass cap value... both have an effect. Start with small increments to get an idea of how it will affect the gain... Strangely enough - I just picked up a brand new AO-68 (literally unused and still in the original box) complete with original tubes for just under 250USD... I will have a look at the AO-35 over the weekend and have a think :D
Re: HAmmond Conversion Question
Great deal -- that old Hammond stuff is great.
I'd love to hear any suggestions that you have, Rob.
This issue is coming up again for me now, in the context of a Webcor amp that I'm
converting. I have only one 12a_7 tube feeding two 12AB5s in push-pull -- one
triode has to serve as the cathodyne PI, leaving only one triode gain stage. instead
of just picking Plate/Cathode resistor and bypass cap values and soldering everything
together to see what I end up with (as I pretty much did with the Hammond), I'm going
to experiment a good bit with jumper wires and different values to try to get the most
out of that single 12ax7 preamp triode.
Like I say, any wise words appreciated.:salud:
JR
