Turning a 60W solid state into a 6V6 bassman

[smuc]

Hi everyone,

This is my first post and my first thread, but I hope that at least someone will find it interesting or useful.

At the end of last year I embarked on a journey of building my first tube amp from a kit, and even before that one was finished I already started thinking what would be my next project. Without knowing what I would build, I started looking for a cheap amp that I would later turn into something cool without spending a fortune on a cabinet or headshell.

Lucky as I was, I stumbled upon a dirt cheap solid state head which seemed to fit the bill. The donor was a Hartke GT60 head, which seemed a good fit for a small tube amp.

hartke-gt60-head-xl.jpg

I was always intrigued by the Tweed Bassman sound, but the 40 something watts had always put me off - especially because I rarely play my 20W tube amps past 3.

The question was - can this small headshell with only 18cm inner height house a 6V6 Bassman? To the drawing board!

Capture1.JPG

Looks like we have a winner! The new project will be a 5F6A inspired amp, using two 6V6s and Deluxe Reverb iron, with some mods from robrobinette.com:

• 3 way NFB

• Frondelli master volume

• Bias pot

• Solid State / Tube rectification

Photos of progress to follow soon!

[Tony Bones]

Looks very cool! The 5F6A is a great circuit. My recommendation is to separate the cathodes of the two input stages so that you can voice them differently. Then when you jumper the two channels you can adjust the two volume controls to mix to taste. There's magic in that method. At least leave enough extra turrets on your board so that you can try it later.

Example: https://el34world.com/charts/Schematics ... r_1988.gif

[sluckey]

I really like these conversion projects. I'll be following. Keep us posted.

[smuc]

Looks very cool! The 5F6A is a great circuit. My recommendation is to separate the cathodes of the two input stages so that you can voice them differently. Then when you jumper the two channels you can adjust the two volume controls to mix to taste. There's magic in that method. At least leave enough extra turrets on your board so that you can try it later.

Example: https://el34world.com/charts/Schematics ... r_1988.gif

Thanks for the tip! I have separate V1 cathodes on a 18W JMP that I built from a kit. The normal channel is very dark and useless on its own but when bridged, the combination is interesting.

For this project I will keep the shared cathode as a start, and maybe experiment with a split cathode if I will not like the outcome.

[smuc]

The chassis had arrived so I did a quick fit test.

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Protective foil removed, power transformer and sockets installed.

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Rectifier wired up, heater filaments are next:

image1.jpeg

[smuc]

Bias circuit done:

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PI with no grid leak resistors, which will be the MV pot:

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Just realized I have not ordered mixing resistors

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[smuc]

Question on the PSU, if someone knows your help will be appreciated!

I wanted to put a 5y3 rectifier but the specs sheet has 20uF as input capacitance. I have 2x22úF as a standard 5F6 installed, can I still use 5Y3 or do I need to go for a GZ34?


Also, making some progress...

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[Stevem]

You can make it work if you wire it up such that one / the first 20 uf cap is the resuvour cap and then you place the standby switch feeding the next 20 uf cap.

With a 6V6 type amp putting out 15 to 18 watts your gonna be pretty close to getting the 60 volt B+ drop that these 5Y3 rectos make so in that regard you might want to go with a 5AR4 as Fender did in later Deluxe reverbs!

[smuc]

Hi Stevem, thanks for the answer! I put the caps exactly as they are in the 5f6 schematic and tried it with the gz34 and I am happy with the outcome as the amp is not too stiff.

Still some cosmetics to do, but sonically I would say that the amp has passed the test. Here are my first impressions:

The good:

• Amp sounds great in most of the settings, and the NFB switch, rectification switch and the MV really make it easy to tweak in a nice palette of different sounds.

The bad:

• Amp is quite bassy past seeting 2 on the bass, but hey - it‘s a bassman

• When MV is flat out, and volumes above 7-8 the distorsion produced is a bit fizzy in the upper frequencies. Not sure if this is the result of no poweramp grid resistors, or the fact that the signal is too strong for the 6v6 tubes. My bias is at 18mA per tube, but under heavy load my multimeter shows a max of 80mA per tube. Not sure if this is something I should be worried about?

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[M Fowler]

Super nice looking build.

Mark

[Drumslinger]

looking good smuc!

[smuc]

Thanks guys! The project is slowly coming to an end...

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However, I still need to do a bit of tweaking of the circuit. The sound is way too bassy, the clean sounds good but the overdrive sound is nothing spectacular. Also, when I hit the amp with the Fuzz Face on the last rehearsal the HT fuse went out. Since I used the 5F6A schematic with 6V6 tubes and appropriate iron, I guess I need to adjust the preamp signal for the 6V6s.

Any tips on reducing the boominess? V1 cap? Coupling caps?

[xtian]

You can certainly trim the bass with coupling caps and/or cathode bypass caps. Please point to the schematic you used so we can be specific.

[smuc]

You can certainly trim the bass with coupling caps and/or cathode bypass caps. Please point to the schematic you used so we can be specific.

I was working roughly by this schematic: https://www.tube-town.net/cms/userfiles ... -schem.pdf

With some adjustments:

• PP tail 6.8K

• 6V6 grid stoppers 1.5K

[xtian]

First and most obvious target is C13 and C14, the 0.1uF post-PI coupling caps. Replace those with 0.02uF and see how it sounds. Next would be to replace the 820R shared cathode resistor R7 with individual 1K5 resistors, bypassing the bright side (C1) with 0.68uF, and the normal side with 25uF.