AKAI/Roberts 6BQ5 Power Mod Question

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AKAI/Roberts 6BQ5 Power Mod Question

Post by fmhurst » Mon May 10, 2010 9:08 am

First, since I've only been a member of the board for a couple weeks, let me tell you a little about my interests. I've done mod work on old Fender '70s tube amps (sliver face to black face sorts of mods). I also repair stereo and mono tube amps. My son is a left-handed guitar player, so I've modded guitars for him, both redoing the nitrocellulose finishes and the electronic side (hard to find lefty guitars exactly how you want them). I'm active on the Audio Karma and Guitar Reranch forums from time to time.

So I tried to tag this question on the "Tablebeast" thread and should have known better - both for trying to hijack the thread and expecting a response buried on another page.

So here's the question: If you want to use just the the preamp section of the AKAI M7-M8/Roberts 720/770X can you put in a switch to cut the power to the output tube?

I have a Roberts/Akai RTR that I want to mod to use primarily for the tube pre-amps for recording. But I want to use the amp sections occasionally. Conventional practice (Rod Childers) is to pull the output tubes if you are using them for pre-amps only. This reduces the heat buildup and puts less draw on the PTs. But pulling and reinstalling the tubes requires pulling the units out of the rack or case. Not to mention the wear and tear on the tube and socket.

I asked the question on the AudioKarma site and got this as one of the responses from GordanW:

"Could probably be done... but, I'd use a triple-pole switch. Here's what the three sections would do:

1) Cut B+ to the output transformer

2) Un-bypass a resistor (which would also need to be calculated as for value and installed) in the power supply, which would drop the voltage of the power supply, so that everything would receive the same voltage as when the output tubes were in there. In other words... install a resistor in-line with the power supply to the preamp, which would reduce the B+ to the right level when the output tubes are NOT turned on, and use this one section of the switch to bypass this resistor when the output tubes ARE turned on.

3) Cut the heater supply for the output tubes (to avoid cathode stripping, from running them without B+). You'd have to re-wire the heaters to separate the output tubes from the preamp tubes.

If you did that, I don't think there's any reason it won't work.

One caveat- you would want to put the new dropping resistor early in the power supply... just before the stage where the output tubes tap of for B+ to the center tap of the output transformers. This would ensure that no caps are run on too much voltage.

One more caveat- you'd need to use a switch, rated for at least the B+ voltage. Many switches are rated to 600v... that would do it, in most cases..."

So, has anyone modded an Akai/Roberts and put a power switch in to eliminate the power to the power tubes ? Is the resistor needed to reduce the voltage out of the power supply as GordonW suggests? Seems to me that pulling the output tube as many have done would have the same impact to voltage as cutting power to the tube?

Seems to make sense but if anyone has a mod in place to cut power to the output tube, I'd like to know how you did it.


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Post by RodC » Wed May 12, 2010 4:39 am

I have done it. I just cut the B+ and the heater voltage. I didnt worry about the PS going up in voltage because of the reduced load. I dont go up by much and mostly just gives you a bit more gain.
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Post by fmhurst » Wed May 12, 2010 1:54 pm

Thanks, Rod. Confirmation on the approach from the king of AKAI/Roberts mods. Your mod info on your site is great! I'm still putting together the documentation and ordering parts. In June I should have time to mod and will post pictures of the completed unit.

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