8 Watt SE Plexi Style Tube Amp Made from an Akai Roberts Reel Monoblock - diyAudio
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Old 27th July 2012, 08:20 PM   #1
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Default 8 Watt SE Plexi Style Tube Amp Made from an Akai Roberts Reel Monoblock

I have documented the teardown and rebuild of one of these Akai Roberts tube monoblock amps from a reel to reel machine. I have time lapse footage of the teardown, desoldering and cleanup here:

XFMR DIY Projects: Time Lapse Footage of Akai Roberts Tube Monoblock Teardown, Desodler, and Cleanup

Then after the faceplate is painted, labeled, and clearcoated here are some more time lapse videos of the full assembly:

XFMR DIY Projects: Akai Roberts Tube Monoblock to Plexi 8 Guitar Amp Conversion

Here is a picture of another one I built previously. I will have some sound samples soon.
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Old 29th July 2012, 06:49 PM   #2
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I will be posting more videos of my other projects over time. If anyone has any questions or comments feel free to let me know. I'd like to eventually put up all the info one needs to do these mods themselves.
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Old 29th July 2012, 07:24 PM   #3
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Very interesting show Jesse.

How did you get 8w out of this tube/iron combo?

I have 6 of these and am trying to decide what to do with all of them (1 pair likely to become RH84 variants)

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Old 29th July 2012, 09:03 PM   #4
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Kind of agree, vids show that you can tear down and build something, not a heck of a lot of information on the amp though. I would have been happier with a picture and a schematic.
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Old 30th July 2012, 09:41 AM   #5
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8 watts, 7 watts, 6 watts? Most stated wattages for guitar amps are kind of arbitrary to begin with anyway so I just say 8 watts cause I like the number 8. Pushed to the brink who knows? I will be putting a schematic up soon, but please keep in mind the blog is a work in progress.
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Old 30th July 2012, 06:49 PM   #6
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Typically under hifi conditions, with the B+ available a single EL84 is good for 3.5-5w in pentode (my RH84 clip at 3.9 w into an 8 ohm resistor).

dave
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Old 30th July 2012, 08:37 PM   #7
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Here is my hand drawn schematic. If anything is unclear please let me know. By the way, I know folks hate using oddball tubes, but the 6AB4 is just half of a 12AT7. If you want you can just use one section of one of those tubes. You can leave the other section unused or find another function for it. I was going to mess around with a switch that uses both sections in parallel and I can switch one section on or off.
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Last edited by tablebeast; 30th July 2012 at 09:01 PM.
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Old 30th July 2012, 08:57 PM   #8
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Quote:
Originally Posted by planet10 View Post
Typically under hifi conditions, with the B+ available a single EL84 is good for 3.5-5w in pentode (my RH84 clip at 3.9 w into an 8 ohm resistor).
Well, this amp certainly isn't hi-fi! Nothing earth shattering in the design. I sort of cobbled together a 5F6-A tone stack with a tweaked pair of differently set up preamp channels and then fed it into a tweaked Akai power section. Looking at the Marshall 5 watter that just came out I like my version a lot better!

The way I came up with the name Plexi8 is, even though it is more like a Bassman tone stack, I like the Plexi name better is all. Same idea, just tweaked. And I have tweaked it back and then made other changes. The 8 came from me working with a friend when building the first one. I told him it was something like 5-6 watts or so and when we plugged it in it was so loud he said that it HAD to be more wattage than that. A Pair of these EL84's has been claimed to have up to 20 watts with fixed bias, so I thought it wouldn't be too much of a stretch to call it an '8'.
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Old 30th July 2012, 09:37 PM   #9
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Quote:
Typically under hifi conditions, with the B+ available a single EL84 is good for 3.5-5w in pentode.......Most stated wattages for guitar amps are kind of arbitrary to begin with anyway
Not sure what your B+ is using a 250-0-250 transformer and a 6X4 rectifer, or the impedance of your OPT. I can say that 8 watts out of a single EL84 in pentode is possible but I was feeding it about 360 volts of B+ and used a 5K load. Those are RMS watts at 5% distortion at 1KHz.

JJ EL84's will survive on 420 volts of plate voltage as long as you keep the screen around 320 volts. Most others fry quickly at this voltage. I have seen over 30 watts from a pair in P-P with a 6.6K ohm load at 430 volts.
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Old 30th July 2012, 09:46 PM   #10
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Modjeski (Music Reference RM10) gets some 35 watts out of a pair of EL84 running a very high B+ (700v), regulated screens at a much lower voltage, and his own iron. (i have some notes somewhere if it is of interest)

dave
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