'63 Blonde Bassman 6G6B

[dorrisant]

It's so beautiful.jpg

[Colossal]

It's so beautiful.jpg

Thanks again for the boards!

[deiseldave]

Wow! Absolutely fantastic build. Such attention to detail. Great job!

[Colossal]

Wow! Absolutely fantastic build. Such attention to detail. Great job!

Thanks Dave!

[angelodp]

Dave, its a beautiful build and i am sure sounds great. I am wondering if the use of shielded runs necessitated any compensation for the loss of hi-frequencies?

Ange

[Colossal]

Dave, its a beautiful build and i am sure sounds great. I am wondering if the use of shielded runs necessitated any compensation for the loss of hi-frequencies?

Ange

Thanks Ange. No, plenty of sparkling highs (without being obnoxious) and clarity on tap with both channels.

[Colossal]

Just as a postscript on this thread...I was able to play and turn this amp up in a band situation over the holidays. It sounded really good on both channels. I played through it as did another guy and we both used Les Pauls. It was very smooth and the high end had great clarity and detail with no harshness. The other fellow normally plays through a Peavey Classic 50 and he said the Bassman sounded like "it had a pedal in front it", referring to the almost reverb like presence to it.

[didit]

Hello Dave -

Excellent postscriptive ending to a beginning. Curious and trusting of your opinions on the "bass" channel of the amp as compared to the "normal", and any other points of reference you might also have to share.

Best .. Ian

[Guy77]

Looks great Dave! I noticed you used those green resistors , KOA/Kiwane.
I use those too. They have a nice rich sound.

Cheers

Guy

[katopan]

Just as a postscript on this thread...I was able to play and turn this amp up in a band situation over the holidays. It sounded really good on both channels. I played through it as did another guy and we both used Les Pauls. It was very smooth and the high end had great clarity and detail with no harshness. The other fellow normally plays through a Peavey Classic 50 and he said the Bassman sounded like "it had a pedal in front it", referring to the almost reverb like presence to it.

Awesome!

[Colossal]

So, just an update on on the Blonde Bassman. I have spent some time with this amp in a band situation. I believe it is well broken in and performing well. It is currently mated to an Avatar THD Compact 2x12 loaded with a pair of 8Ω 20W Celestion Heritage Greenbacks wired to 4Ω. For those interested, the [stock] Bass Channel sounds thicker than the Normal channel and has a notable percussive, poppy quality to it, probably owing to the cathode follower right off V1a and driving the tone stack. It can sound plump and bluesy. The Normal channel is clear and warm sounding with a little less of that dark thickness, but plenty of shimmering harmonic complexity. I used Martin's 75k/1M tapped treble pot mod. I opted to use the Normal channel for the band as it cut a little more, but I like both channels. I have to say, it's a nice amp to play. It will overdrive but it's a smooth and clearish singing tone with a slight edge, not the kerrang of a British pentode amp. We played Mary Jane's Last Dance and the Bassman worked well for that, getting that clear, warm but sparkly, cutting tone. I used the 5kB/0.1uF Presence circuit, not the 4k7/25kB/0.1uF version as I prefer the former. I like that Avatar compact cabinet. It's just a hair smaller than an AC30 cabinet, quite light, and sounds great.

[skidbrake]

Your bias test points solution is genius! I'm looking at adding a PI balance trim pot and noticed that the trim pots I have are rated for 100V max. I've researched Piher, CTS and other trim pot brands and none of them seem rated for this service. Since this junction is seeing 400+VDC from the rail, have you had any issues with the trim pot you used? Perhaps there is a brand I have not found ... thoughts?

Thanks for sharing your build notes and pictures with us. Fantastic work! Happy holidays.

Well, here it is.


Bassman 6G6B Circuit.jpg

Bassman 6G6B Chassis.jpg

Bassman 6G6B Cab.png

Bassman Tube Chart.jpg


This was a "parts on hand" build; a bit of a mashup. First, a big thank you to Mark Fowler for the chassis and faceplates. Thank you, my friend. Thanks and appreciation also to Tony Dorris (dorrisant) for CNC milling my boards. I gave him the dxf files and he graciously milled them. I took some inspiration from Tonemerc's 6G6B build (https://ampgarage.com/forum/viewtopic.p ... 47#p226747) using (Kiwame) carbon films for plates and signal path and (Takman REY) metal films for cathodes and grid leaks, as I had these on hand from past builds. The amp is very quiet at idle, aside from the usual gain hiss. The power transformer is a Blonde Bassman clone with the primary wound for 117VAC, so I variaced down the mains to set the idle current. A while back, I had some little step down transformers with multiple primary and secondary taps wound and will dedicate one to the amp when I put it in a box. This is a stock 6G6B build with a few mods:

1) Dual bias pots, one for each 5881. This worked out quite well and current can be dialed in precisely to the desired value for each tube. I used an Apex matched set of Tung-Sol 5881s. The Fender 6G6B schematic shows -54VDC on the grids but at about 436VDC on the plates, this amounted to 17mA per tube. I warmed it up to about 50% dissipation at 26mA per tube or -50VDC on the grids.

2) Heaters elevated to DC. There is a divider and cap tapped off the bias tap giving +60VDC. I like to do this with amps that have cathode followers and pretty much any amp.

3) LED lamp in place of the #47 bayonet lamp. Again, another divider off the bias tap to supply a 3.2V warm white LED. It is biased to 15mA with a 330R. For fun I paralleled the divider with a 10nF ceramic and it makes the lamp fade perfectly when the amp is powered down. A bit of nostalgia for the old incandescent #47 lamps.

4) The SS rectifier diodes are also bypassed with 10nF ceramics, Dumble style. Again, had a bunch of these caps on hand, and, why not?

5) Bias test points on the back. I repurposed the Death Cap ground switch to switch between the 1R current sense resistors on the power tubes. Click it left and it's the left 5881, click it right and it's the right one. Works great and quite handy.

6) The OT has a 4R and 8R tap. NFB is taken off the 4R tap and wired to the extension speaker jack and 8R is wired to the main jack.

7) PI balance trim pot. I used a 10k trimmer between the 82k and 100k plate loads on the PI. It is set for 300V on the inverting side and 294V on the non-inverting side. You can definitely hear the effect. It sounds very fat and sustains like a bell. I think this is a very useful feature.

8 ) Martin Manning's Tapped Treble pot mod (1M1 & 75k across 500kA) on the Normal Channel. Sounds great. Nice sweep. See here: https://ampgarage.com/forum/viewtopic.p ... 43#p174243

9) Presence pot is wired Marshall style, 5kB pot and 100nF to ground off the wiper. Yes it scratches, but I don't care and it's much better than the 25kB -100nF parallel to 4k7 version, IMO.

I used wire nuts on the load and neutral connections for Phil S. Phil, I know how you love wire nuts Don't worry, the leads are soldered and held by both the wire clamp and strain relief. Tonally, so far, I am very happy with the sound. The Bass channel is unique and sounds great. Nice and full. I decided to just build the amp stock at Katopan's behest. Good call, Craig. The Normal channel sounds killer. I haven't had a chance to really open it up yet.

[Colossal]

Your bias test points solution is genius! I'm looking at adding a PI balance trim pot and noticed that the trim pots I have are rated for 100V max. I've researched Piher, CTS and other trim pot brands and none of them seem rated for this service. Since this junction is seeing 400+VDC from the rail, have you had any issues with the trim pot you used? Perhaps there is a brand I have not found ... thoughts?

Thanks for sharing your build notes and pictures with us. Fantastic work! Happy holidays.

Hello Skid,

Welcome to the forum and thanks for the nice comments. I enjoyed playing the amp with a cover band this summer and it sounded great, I must say. As for the voltage balancing trim pot in the phase inverter, this method is used in Dumble amps and this thread might be of interest to you: https://ampgarage.com/forum/viewtopic.php?f=4&t=18048. The trim pots themselves only "see" a small difference in voltage across them.