It lives! 6G6B Bassman micro SE 6AK6

mountainhick

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Alrighty!

Uh TLDR warning.

Concept turned into reality. This has been a long haul and great learning experience for me. I've asked so many questions here over many months regarding many aspects of this build, and I am forever grateful for all the generous help!

This has been the starting point for me to explore mixing and matching design features, using retasked parts to build from a combo of already established design and designing some things from scratch.

The preamp as it is now is a 6G6B bass channel. The power amp a parallel single ended pair of 6AK6.

This grew out of a PT from a Regency MR10 radio, which also provided a 6ak6 and 12 ax7.

I started by gutting the radio chassis, and designed the board, layout etc for that chassis, welding on a new chassis top, And configured layout to fit. Well, not quite. The power supply design and layout wasn't finalized, and when the time came, I had a helluva time time trying to make it fit. Lesson #1: if you don't have to, don't cram!

So I transplanted the whole works into a different, larger old Sparton radio chassis. Same deal, welded on a new top for a blank slate for the layout.

I subsequently planned another amp design (Project#2) to fit the MR10 tiny chassis, that also filled 4 tube places, it'll fit, but the Antek PT I have for it takes about 1/3 of the real estate! Maybe this tiny chassis is better suited to a micro with a pair of 6f4p or something. I'll build a new chassis from scratch for project #2!

So, successful startup on variac and lightbulb limiter.

Under load, it is obvious the PT is designed for 110-112vac supply. Filament demand for the radio 2.4A, and my filament load is 0.9A, so I added dropping resistors. I am now at 6.03V on 122.5 wall voltage with parallel pairs of 1 ohm resistors on each side. Should I leave it there or try tripling in parallel to tweak closer to 6.3v?

Edit: updated with pics, PS schematic shows actual operational voltages. Tiny bit high to 6ak6 plates, otherwise spot on design targets!

And in the pic, you will notice a couple resistors reaching up to space. This was the result of trying a voltage divider for FX loop impedance. Sucked way too much signal so detached for now.

Oh, and MV is 100k, not 1M. Seems like a pretty usable range since the 6ak6s run with lower signal voltage. I might experiment with this value

IMG_20220905_130922.jpg
 

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mountainhick

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Congratulations on a successful build! Now we need some sound clips.
Thanks!

I've just spent a little time with it. It can be a little harsh with some settings, very strong treble on treble, have to dial back on guitars tone. I might dial bit back with cap adjustment in the amp, but will play some hours before any tweaks. A tiny bit of reverb in the effects loop and Wow, it sings!! It is already exceeding expectations. I can set it so rolling back volume knob on guitar is sweet and clear, and into pretty good drive turning it up.

I'll eventually post sound, but it may be some time. I am still unable to use my left hand well after the tablesaw/ thumb accident, and currently our internet is out. I've been posting this from my phone, and don't have unlimited data.
 

mountainhick

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I guess I'll re-ask the question buried in the tldr:

Should I further tweak dropping resistors' load on filament supply to raise 6.09 to closer to 6.3 vdc?
 
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mountainhick

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I would probably do it (I am like that) but being 0.2V low is not a big crime.
I would. Dropping the filament voltage makes heat and I don't like heat. What is the voltage without the resistors? Iow, can you get by without them?

Added a third resistor in parallel, now 6.41VDC with 122.5V wall voltage. Call it good, or order other resistor values next electronics order to get it right on? I checked wall voltage today too, my home variance seems to be 121.7-123.5
 

Lowerleftcoast

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Call it good.

Before you came back with your 6.41V (VDC?) post, I was going to write, "If it was a vintage amp I would leave it at 7V."
10% high isn't a deal breaker for me. OMMV. 6.41V will keep the tubes comfy.

Carry on.
 

mountainhick

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I've spent some more time with this amp, played both Tele and SG through it. Takes a bit of tweaking settings for each guitar and once there, Holy cow it is excellent! I am actually kind of blown away. I want to share some sound but it's going to take cobbling together some way to record. Need a good recording to do it justice.

If anyone is interested enough to copy the design, I think it will be worth your while, but there are a couple details in the schematic that are not exact. I'll update this sometime soon. I'll also post my layout/wiring diagrams.
 

mountainhick

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Please do! I'm always in interested in new build ideas for the upcoming winter months.

Got some time, cleaned up the schematics. voltages on PS schematic are actual. PT was from a Regency MR-10 radio, obviously configured for 117V which is why the 6 dropping resistors on the heater supply. I had the 1 ohm a plenty so used a few.

Layout drawings still to come:

6G6B-6AK6-Final-preamp-schematic.png

6G6B-6AK6-Final-power-amp-schematic.png

6G6B-6AK6-Final-PS-schematic.png
 

dan40

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Thank you for the schematic. Now I'm torn between building Rob's micro Bassman with EF80's or your design with the 6ak6 power tubes and the 6G6B preamp.
 

mountainhick

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Thank you for the schematic. Now I'm torn between building Rob's micro Bassman with EF80's or your design with the 6ak6 power tubes and the 6G6B preamp.

Here's a layout.

Couple notes:

-I am not that great with DIY layout creator yet. Things could be a bit better optimized.

-And likewise, double check against the schematic.

-My PT was a one off model from an old radio. You will need to confirm or redraw PS layout for your own needs depending on the PT you select.

-The PS layout is not what I actually did, but the circuit is the same. I had odd sized radial caps and had to customize. This layout is compact and will work for radial caps. If you use axials, you'd need to rearrange.

-Heater wires left out to avoid clutter in the layout diagram.

-With the PT a one off, I left out the resistors for dropping the heather voltage.

-Your choice for indicator lamp wiring

LMK if you want the actual DIY creator file to work with

(LOL! I just found half a dozen things to optimize, layout edited/updated. Please LMK any thoughts or errors.)


6G6-B-6AK6.png
 
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mountainhick

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Getting around to a cabinet. I am using a 1940's Sparton radio cabinet, the one that this chassis was scrounged from. It started as a double decker with amp space, and top section for a 6x9" speaker.

Spartoncab-1.jpg



I cut off the bottom section, and extracted the bottom panel, cut out a couple corners to fit the bracing and glued the bottom panel back in. (Not shown: I also added corner blocks after this gluing/clamping on the underside that bring the bottom corners flush with the bottom edge, and reinstalled the feet so it stands proud of the deck.

Spartoncab-2.jpg


As is now. I am not delighted with the old school dark appearance, I think I am going to refinish, sand off the black paint to reveal the wood and hopefully brighten its appearance a bit. remaining tasks:

-refinish
-face plate
-grill. I will probably use cane.
-add mounting brackets to chassis and attach to the cabinet. I think i will just rivet 90' angled metal tabs with tapped holes that can be secured with machine screws from the underside.
-pilot light

Spartoncab-3.jpg
 
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