Starting my AKAI/Roberts Mod! (Roberts 720A)

[chconnor]

Well, a year after finding a Roberts 720A at the thrift store for $10 I'm finally getting started on the 2-in-1 mods. Exciting stuff! If anyone has time for these questions, much obliged... I think these are easy, but I couldn't figure them out:

- not sure which Cinemag would be best for the input? Something from CMMI-8B3 through CMMI-10C at http://cinemag.biz/mic_input/mic_input.html? I just don't know what the trade-offs are between those options. And, if I use one of those, I understand from Rod that I want the pot to be 100k instead of 500k, right? (or 150k for the CMMI-10B)

- for switching the 6BQ5 in and out of the circuit, is it sufficient to just cut one leg of the heater voltage or is there some subtle reason I need to cut both legs? (also B+, I know).

- for the fuse on the mains, what amperage makes sense?

- is 250VDC the max DC voltage in the unit (post-mod)?

- when draining capacitors for safety's sake, besides C25/26 are the others that I should watch out for C7 and C16? The others seem too small to worry...

Thanks so much!
-c

[chconnor]

2 of those 5 questions maybe solved: just noticed the 2A fuse in the schematic from the service manual. I'll go with that unless told otherwise. And the more I contemplate the physics of the tube, the more I think I can just cut one leg of the heater.

-c

[RodC]

Most of those CineMags would do fine, I would stick with the ones that they recommend for tube pres. The CMMI-10C always looked attractive to me, more choices of ratios and impedances.

Your choice of the pot for the EF86 stage looks on track. This acts as the shunt resistor. If you can?t find a close value you can always get a higher value and put a resistor in parallel to get the value you want.

I normally solder in one of the small diode looking fuses (To save room) 1.5 to 2 amp should be fine.

If you want to take the 6BQ5 out (EL84), you can just switch one side of the heaters (If you want to switch off the heaters) You also may want to consider just a standby switch for it. (Although you will hear others say not to place a tube in standby and still feed it a signal YMMV)

When draining caps, just worry about the large electro's.

Good luck, I'm not on the TOMB much anymore, but I do get a ton of emails about these units. Sorry I didnt see this post earlier.

[chconnor]

Thanks Rod!

You also may want to consider just a standby switch for it. (Although you will hear others say not to place a tube in standby and still feed it a signal YMMV)

You suggest this just for the sake of ease/convenience over the more complicated switching, or is there another advantage?

Sorry I didnt see this post earlier.

Absolutely no problem. :-)

-c

[RodC]

Thanks Rod!

You also may want to consider just a standby switch for it. (Although you will hear others say not to place a tube in standby and still feed it a signal YMMV)

You suggest this just for the sake of ease/convenience over the more complicated switching, or is there another advantage?

-c

Mostly for the ease, and a standy is pretty common. You are going to have to switch the B+ off anyway. It was just a suggestion. You can switch the B+ and the heater for the tube and it should be fine. I pull my power tubes cause I rarely use them, and the supply voltages dont raise by much.