Best to check the bias voltage. Korg warns that if the filament is red then
it's too high. This is regulated by the 9,1V zener, so check that as well.
I have been using Toshiba Jfets Idss 3 to 5 mA, work just fine. Fairchild
J113 parts about 8 mA Idss.
Source resistor for the DIY board current source is for setting high Idss
Jfets to lower value.
Plans are that the kit will have the Fairchilds with the appropriate
Source resistor values, typically 100 to 221 ohms.
People with matched parts and also Idss at 8 mA or less can short the
Source to Ground on the current source Jfet.
I noticed with the NuTube mosfet buffer circuit I have (not yours) that gain was mis-matched between the two sections by about 0.4 dB. I raised the gain in the lower output tube channel (ch 2) by lowering the plate resistor value a bit until channel gains matched. Do you advise this type of matching for your B1 Korg circuit?
Myself, I don't worry about .4 dB much, but you can trim it out that way
or by attenuation elsewhere.
If you have a distortion analyzer, you would also want to adjust for the
distortion figure when you change the load resistor.
Well, my contact at RS is 'interested in the project' and 'may be able to help with some discounted Nu-Tubes'. So how many would we want? Is it worth me getting a 60unit carton and bulk posting some to whoever is doing the European distribution? I really want rough numbers before asking for a price.
Thank you. I just looked at the specs for the Alps "Blue Velvet" RK27-series 2-gang pot and it is specified that the two channels will only track within 3 dB. More possible difference than I had assumed.
I'm not the most experienced DIY amp guy hence the next question. I'm trying to use some of the stuff I have collected over the years and have a couple of encapsulated 24V 50VA toroidal transformers that I would like to relieve from door stop duties.
I was wondering if I could use one of them to make the power supply for the Korg Preamp, and if someone could point me at an appropriate PSU design that I could use?
Sorry I don’t have the confidence to launch out on my own design and possibly stuff up the good sound I’m anxiously anticipating.
I think I read somewhere that the Korg board could cope with more the 24V coming in?
Should I put the PSU in a separate case?
I was wondering if I could use one of them to make the power supply for the Korg Preamp, and if someone could point me at an appropriate PSU design that I could use?
Sorry I don’t have the confidence to launch out on my own design and possibly stuff up the good sound I’m anxiously anticipating.
I think I read somewhere that the Korg board could cope with more the 24V coming in?
Should I put the PSU in a separate case?
DQ, yes, you can use one of them as long as you are prepared to lose some voltage in a CRCRC filter and also a Cx multiplier with a FET.
24V x 1.414 = 33.9 V
this is after a diode bridge at the first cap.
then put some series resistance between caps to bring it down to approx. 28VDC.
Ohms law is your friend here, the total current draw will help you calculate the value of resistors.
e.g. if I = 40mA then (33.9 - 28V) / 0.04 = 147 ohm, this is the total resistance that you'll need, divide by the number of RC stages to get the individual resistors.
don't forget resistors power ratings.
in the example above (0.04 x 0.04 mA) x 147 = 0.24 W so 1 - 2W resistors will be fine.
then a cap multiplier with IRF610, 620, 640 or any other suitable power FET (on a small heatsinK) and voila - 24V.
then you'll have to think of the filament supply for the Korg.
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