'63 Blonde Bassman 6G6B

[martin manning]

Hi Martin. I thought I had seen your alternative circuit, but thanks for attaching it because I actually hadn't.

I added it to the LTSpice simulation file I used for the Blackface vs 6G6-B treble sweeps I posted. Here is the result. Please check the schematic and make sure I've read your layout modification correctly. Has the same basic transition from standard TMB treble to Tweed like roll off. Also looks like it suits 10% taper which is good for the pots we can get these days. But the shape is different, where the mid scoop looks more pronounced and the treble content higher.

Yes, the circuit you modeled is correct. True, the response is somewhat different, but maybe not in a bad way. For sure, adding a couple of resistors is a lot simpler than adding a tap!

[Colossal]

Here are the two plots to show similarities and differences between the native tapped pot and Martin's mod. I am using the modded method for my build.

tapped.png

[Colossal]

I am reposting this schematic and layout as it is nice and clear.

Bassman 6G6B Schematic and Layout CLEAR.pdf

I have decided to add a second bias pot and am thinking about adding a trimmer to the PI to tune the balance. The 6G6B schematic shows 300VDC on the inverting side and 280VDC on the non-inverting side, a rather sizable difference. The gains however are fairly closely matched. Increasing the non-inverting plate resistance results in improved balance so I'm thinking a 10kB trimmer in the middle might be useful. Of course there is always the argument that there is nothing wrong with the stock amp...

6G6B PI.png

[katopan]

Yes, the circuit you modeled is correct. True, the response is somewhat different, but maybe not in a bad way. For sure, adding a couple of resistors is a lot simpler than adding a tap!

Can't argue with that! You've done well to capture a similar response.

[brewdude]

Hi Martin. I thought I had seen your alternative circuit, but thanks for attaching it because I actually hadn't.

I added it to the LTSpice simulation file I used for the Blackface vs 6G6-B treble sweeps I posted. Here is the result. Please check the schematic and make sure I've read your layout modification correctly. Has the same basic transition from standard TMB treble to Tweed like roll off. Also looks like it suits 10% taper which is good for the pots we can get these days. But the shape is different, where the mid scoop looks more pronounced and the treble content higher.

When I found out the postage cost of getting a couple sent here, and before I found out there was someone on the AGGH forum that had a stock of them locally here in Oz and would've been happy to sell me one, I made one. Don't recommend it but it gave me an authentic tapped pot with the right resistances and proper 30% taper.i
https://sites.google.com/site/chosenaud ... /tappedpot

Sorry for the sidetrack Dave. I have a soft spot for the 6G6-B so look forward to seeing your build come together.

I am absolutely amazed at the effort you took to convert a standard pot to a tapped pot. How close did you get to the original pot used by Fender?

[katopan]

Hi Brewdude!

Stupid amount of effort really. But I got a true 30% taper, right tap resistance and right overall resistance so I assume it's very close. By the time I was making the pot I had given my friend's original 6G6-B back to him so couldn't compare.

[Roe]

weber made some of the tapped 350k pots but I don't know what the tapper is. Do anyone have experience with it?

[Mark]

I bought two as replacements but I didn't like them as the taper was 10%. I still have them somewhere, I did try them in the amp but they didn't work as well as the stock pots.

[Colossal]

I had a bit of fun and replicated the '63 6G6B tube chart. I printed a handful on heavy card stock and then aged them. I pasted one into my head cab. The amp build is progressing, if a bit slowly due to year-end work and holidays. Got the input jacks wired and shielded runs done for both channels. The output section is done. Still agonizing over that Bass channel. I am leaning towards the K50 preamp but a Rocket preamp could work was well and probably would be killer with that Bassman output section.

6G6B tube chart.png

[norburybrook]

looking great Dave, I'm curious about this amp as I've a chassis sat here and this maybe a great last build.


M

[Roe]

I had a bit of fun and replicated the '63 6G6B tube chart. I printed a handful on heavy card stock and then aged them. I pasted one into my head cab. The amp build is progressing, if a bit slowly due to year-end work and holidays. Got the input jacks wired and shielded runs done for both channels. The output section is done. Still agonizing over that Bass channel. I am leaning towards the K50 preamp but a Rocket preamp could work was well and probably would be killer with that Bassman output section.


6G6B tube chart.png

I prefer the k50 preamp to the bass channel (although the k50 is on the hot side for me, esp. the 3k3 cathode resistor is a little too much)

[Colossal]

I prefer the k50 preamp to the bass channel (although the k50 is on the hot side for me, esp. the 3k3 cathode resistor is a little too much)

Hi Roe,

I am a bit concerned about this as well. I don't play high gain amps these days. I was thinking that 3k3 could be increased to 5k (maybe more?) and the mixing resistors would probably help pad down the channel a bit (?). A Rocket preamp (with either a single or paralleled triode) would fit that channel and the front panel Bass and Treble controls would be in line with the Rocket's tone stack. Plenty of gain on tap for me. The 5881 power amp should be nice. I like the Presence control on a fixed bias amp...that is an important feature.

looking great Dave, I'm curious about this amp as I've a chassis sat here and this maybe a great last build.

Thanks Marcus. I could drop one of those charts in the post, if it will persuade you to build one For me, I am chasing that inbetween world with a Les Paul; clean but not entirely. I loved the recent 102 build but wanted a low plate amp with a twist. The 6G6B Normal channel is 100k/1k5 - tone stack - 220k/2k7 - mixing resistor. I heard one of these with a P90'd 57 Les Paul and the tone was warm and clear but round and not spikey. I like a buttery smoothness with depth and authority in the low end. This amp is turning out well though and I'm adding a few bells and whistles. I just need to sort out what to put in that Bass channel

Dave

[Roe]

I prefer the k50 preamp to the bass channel (although the k50 is on the hot side for me, esp. the 3k3 cathode resistor is a little too much)

Hi Roe,

I am a bit concerned about this as well. I don't play high gain amps these days. I was thinking that 3k3 could be increased to 5k (maybe more?) and the mixing resistors would probably help pad down the channel a bit (?). A Rocket preamp (with either a single or paralleled triode) would fit that channel and the front panel Bass and Treble controls would be in line with the Rocket's tone stack. Plenty of gain on tap for me. The 5881 power amp should be nice. I like the Presence control on a fixed bias amp...that is an important feature.

Dave

6k8 or 10k works well. and decreasing the 330k resistor to ground to 150k also helps, making it more like an express

[Colossal]

6k8 or 10k works well. and decreasing the 330k resistor to ground to 150k also helps, making it more like an express

For that RC on the third stage, I would probably use a trimmer in series with a fixed resistor to be able to get the gain under control.

[jaysg]

1) I have an associate who has two of these original amps. One is brownface, the other is blackface transitional. It is a pretty great amp. That said, I don't know who uses one all the time beyond Brian Setzer.

2) I have done both the 68K and weber 4-pole pot implementations in a BF type amp. There is nothing wrong with either imo. The bass channel is surprisingly Marshall. ymmv