SSS Cathode Follower and Bias Voltage in a 50W Fender Bassman Converted to Vibroverb

[Krinkle]

I just installed the 100k resistor that was missing from the High Filter section of the MCE schematic. The High filter works fine now. The 100k needs to be in the circuit, otherwise there will be no change in tone when switching positions, except for bypass of course.

Now on to the Deep and Bright switches! I'll play around with the tone controls, and the volumes, but so far I can't hear any change when activating the switches.

[CHIP]

Thanks for all of the input everybody, very appreciated.

I have converted the rest of the circuit so it has gone from a 1968 silver faced 50W Bassman (it was screwed up, somebody modded it before me) to a Vibroverb/Bassman to a full 002 build attempt now, using the "May Contain Errors" schematic (using modded power supply resistor values and minus the reverb as I didn't have enough tubes, but I have a Catalinbread Topanga and I'll fight anybody to the death who says they can tell the difference between it and real spring reverb ). This amp tone is far more to my liking than the previous incarnations.

I used Dumble's bias circuit (off one side of the HT with 3k3 resistor and cap) and added a 25k pot in series with a 100k resistor in place of the 130k and it was easy to bias. I don't really want to tear it apart anymore as I'm enjoying the tone so much so I'll keep it and see what happens. I don't have the 470k's installed. If they are crucial somebody please let me know. How did they get in the schematic anyways, I don't see them in the original hand drawn version?

My CF plate voltage is in the 300's, well under 400VDC, I can't remember the exact value right now and I don't have my notes with me. The power supply schematic that I used had an extra 27k resistor off of the PT screen node to provide a lower CF voltage.

I notice that the CF 12AX7 flashes a little when I power on the amp. I read somewhere about the voltage possibly stressing the tube at power on, can't remember where. If the tube blows I'll look to see if there's a soft start circuit that's not too complex that can keep that from stressing the tube.

The extra 27k resistor that is providing the lower CF voltage, could you please post that schematic. I see a 27k resistor on Mhartmans layout, but that is for the reverb driver supply.

[markusw]

I just installed the 100k resistor that was missing from the High Filter section of the MCE schematic. The High filter works fine now. The 100k needs to be in the circuit, otherwise there will be no change in tone when switching positions, except for bypass of course.

Now on to the Deep and Bright switches! I'll play around with the tone controls, and the volumes, but so far I can't hear any change when activating the switches.

Could you post a link to the layout you used?

[Krinkle]

I'll try to find that schematic with the 27k. If not I'll scan it and post it.

[Krinkle]

The layout I'm using is one I did in AutoCAD. It's not complete but I could post a pdf of what I have. I can share the ACAD file as well, as all of my boards are in there and are to scale, in case anyone wants to mod an old 50W bassman head like I did.

I also have a bill of materials in the project section at Mouser that I can share.

I'm questioning my deep and bright circuits now as they don't seem to change the tone, so it may have errors, but I'll share it anyways and put the typical disclaimer.

[Krinkle]

Here's a pic;

http://s793.photobucket.com/user/Darrin ... i.jpg.html

[Krinkle]

Before anybody asks, I realize that my OT primaries are backwards from a color code perspective. That bassman has one of the stupid OT's that has the colors backwards and when I first modded it I ended up swapping primaries and just left it.

The first time I modded it I got that big squeal some of us know, not a pleasant experience.

[Krinkle]

I was searching this site earlier today and read that the V1b 100k volume pot should be 1M. I changed it out and the bright switch causes a change in tone now!!

It also fixes another problem that I didn't mention. Previously I couldn't get the master volume to 2 without a massive volume increase. Now the volume and master work together as I would expect.

I still can't really detect much change with the Deep switch, so it didn't fix that. Anybody else know any secret fixes to more errors?

I also read that the 270k resistor in the filter section should be 2k7. I tried that and I found that it caused the High Filter effect to lessen greatly. It was very hard to detect a change in tone when switching so even though it gave me a little more signal, I left the resistor at 270k.

Next up I read that the 1k in the feedback circuit of V1b should be 100R like the first stage, instead of 1k. I'm just going to clip a 100R over the 1k with some alligator cables. If it sounds better I'll desolder and change it out.

[Krinkle]

I changed the V1b 1k to 100R and it made a big difference. The amp certainly got "livelier" if not a hair dirty (on Rock, with the FET boost jacked a bit). I have to play with it for a little while. It was clean to the point of being a bit sterile before the change. I'm not sure if it's not clean enough now, I heard that this amp is supposed to be dead clean at all volumes, but the Welagen amp from the Muzz clip on Youtube gets a nice little OD that I'm getting now. I have a 1.5 k pot in the same style as the old bias pot. I'm going to put that in series with the 100R resistor and see if there's a sweet spot that I like. That way I don't have to drill a hole in the amp and I can adjust the pot over a few days to see what I like.

[Krinkle]

I guess I can't attach an AutoCAD 2013 file of my layout? I says I have an invalid extension.

[Krinkle]

Just played for 2 hours straight, I'm keeping the 100R on V1b, not even bothering with the pot I mentioned. That 1k has to be an error, the amp sounds amazing now.

The only issue I have now is it seems to be a little noisy. Just enough to bother me. It hums a little with no guitar plugged in and the noise increases as I turn up the volume. I was not particularly careful with the grounding, going with the typical Fender random grounding style. I'm not going to tear the amp apart but I'll try a few things.

The first thing I did was put the chassis back in the head case. It has mesh on the underside of the top inside panel. I was hoping that would fix it but no change.

I didn't use any shielded wire either.

Anybody got any experience with noisy Dumbles?

[Krinkle]

Here's what I have for my layout. I've already changed some things so this is not perfect but it will give you an idea of how an SSS circuit (minus reverb) can fit in a 4x12AX7 50W Bassman head.

The amp sounds amazing with the so called error corrections (I haven't seen the inside of a SSS so I'm just going by my ear) except for the noise, which I will start tackling tonight.

[larsvictor]

Marcusw,

thanks for the information concerning the high and low filter section. I compared your schematic to the sss_002_revision_7-174-781 ("may contain errors-schematic") and to the original drawing of SSS #002 from Japan there are indeed minor differences. Your understanding of that network makes sense to me and if you have good experience with it, that may lead us to little errors in the known schematics.

The high filter in the original drawing and in your design seem to be the same, whereas the circuit in the "mce schematic" has the 3nF capacitor to ground in all filter positions. I will check out if that alters the sound.

Lars

[Krinkle]

Where did that hand drawn pic come from? I see some Japanese writing on it. Is there more than one Japanese hand drawing? The only one I have seen has a dashed box where the filter circuits are.

[larsvictor]

It was posted 5 years ago by TAG member Lefty.

http://ampgarage.com/forum/viewtopic.php?f=5&t=17827