Dumble Modded Bassman Layout/Schematic

[Charlie Wilson]

Yes, Alzuz1961. That did help. I think I can see brown black orange on the second cathode resistor, 10k just like a JCM800. Cold clipper.
CW

[pompeiisneaks]

all excellent information, I'll get some time to update the layout correctly, then I'll start trying to create a useful schematic as well.

Thanks!

~Phil

[Charlie Wilson]

Looking at the power supply in Aaron's schematic. I notice several of the Bassman models do have the extra filter cap. I am curious where he came up with the dropping resistors.
CW

[pompeiisneaks]

Yeah no idea, guess he either went off common bassman droppers or off a typical ODS? I've not peeked yet at either though.

~Phil

[pompeiisneaks]

yeah that doesn't seem to fit any known bassman I've seen yet. THe classic 5F6A has 4.7k instead of 2.2k then they both have 10k, and there's no third dropper on the bassman.

the AB165 is 1k, 4.7k and 27k so... thoughts on a 'good' one?

~Phil

[Mr. dB]

Ugh, my head hurts after this. I couldn't figure out how the 47pf ceramic fit in and then I realized it might be a snubber.
CW

Doesn't the 47pf go from the first lug on the pot to the lower right lug on the switch? Looks like the heat shrink is where an extension was soldered to the cap lead, then it goes down behind the green cap. No?

[Mr. dB]

Yes, Alzuz1961. That did help. I think I can see brown black orange on the second cathode resistor, 10k just like a JCM800. Cold clipper.
CW

Is it still a cold clipper if it's cap bypassed?

[Charlie Wilson]

The droppers I think are taken from a JCM 800. Yes dB the 47pf cap does appear to have an extension and I believe it is a wire to that ground area below the switch. It just doesn't make any sense as part of the jazz/rock switch network. What I mean by cold clipper is that the tube is biased pretty cool. A bit fatter sounding with the bypass cap but still biased cool. I keep hoping that someone that has built one of these amps will chime in.
CW

[Charlie Wilson]

yeah that doesn't seem to fit any known bassman I've seen yet. THe classic 5F6A has 4.7k instead of 2.2k then they both have 10k, and there's no third dropper on the bassman.

the AB165 is 1k, 4.7k and 27k so... thoughts on a 'good' one?

~Phil

Phil, I think maybe this should be thought of as a modified JCM 800 design rather than a 5F6A. The later Bassman amps do have the extra filter cap and I would have to assume there is an additional power supply node since he could have simply ran a wire from the top of the clean 100k plate to the CF follower supply if there was not. I think Aaron's dropping resistors seem like a good place to start. The JCM 800 has about 470 volts on the plates of the power tubes and the Bassman has about 430 volts, so reducing the JCM 800 first 10k dropping resistor to 2.2k seems like a good idea and then keeping the addition two 10k dropping resistors.
CW

[pompeiisneaks]

yeah that doesn't seem to fit any known bassman I've seen yet. THe classic 5F6A has 4.7k instead of 2.2k then they both have 10k, and there's no third dropper on the bassman.

the AB165 is 1k, 4.7k and 27k so... thoughts on a 'good' one?

~Phil

Phil, I think maybe this should be thought of as a modified JCM 800 design rather than a 5F6A. The later Bassman amps do have the extra filter cap and I would have to assume there is an additional power supply node since he could have simply ran a wire from the top of the clean 100k plate to the CF follower supply if there was not. I think Aaron's dropping resistors seem like a good place to start. The JCM 800 has about 470 volts on the plates of the power tubes and the Bassman has about 430 volts, so reducing the JCM 800 first 10k dropping resistor to 2.2k seems like a good idea and then keeping the addition two 10k dropping resistors.
CW

Sounds good, I'm almost done with the layout updates and I've gotten maybe 3/4 of the way through the schematic, I'll have updates soon, just been a busy weekend.

Thanks for all the help!

~Phil

[Aaron]

Looking at the power supply in Aaron's schematic. I notice several of the Bassman models do have the extra filter cap. I am curious where he came up with the dropping resistors.
CW

Hi Charlie,
Sorry to all for not reposting a revised schematic with the 475k resistors. As Dustin said at the start, if you substitute 470k where 45k is shown in the schematic amp works and sounds fine.
This amp is a modded AB165? modded circuit so already has the extra dropping resistor. The values are what I used in the amp. Might have started with JCM800, and then adjusted accordingly to get voltages I was after.
Regarding the Jazz/Rock Switch, I don’t think it one, but maybe a modded bass cut/boost? Usually with J/R switch the 100k/.005 is located at the terminals at the same end, where on this switch the 100k/.005 are at opposite terminal ends?
Thanks,
Aaron

[Charlie Wilson]

Hey Aaron, I think the photo is a bit deceptive. If you look just under the blue .005 you can see the cap lead twist back up towards the upper terminals. The wire at the bottom terminals that looks like the cap lead, I believe, is the jumper wire going across the two bottom terminals.
CW

[pompeiisneaks]

So, I've attached an updated schematic from what we think we've found on the layout photos, I've also updated the layout.

Another set of eyes would be greatly appreciated

~Phil

[Charlie Wilson]

Anybody have any ideas on what the weird jumper and eyelet right next to the first cathode cap and resistor eyelet is?
CW

[Charlie Wilson]

Phil, power supply. Main filters and B+1 on the other side of the choke. Screen supply B+2 where B+1 is now. PI supply after 2.2k and remove one of the 10k droppers.
CW