Ryan & Erwin's SSS #002 Madness

[erwin_ve]

If voltage is a concern on the driver tube (also 12bh7) , a good alternative is the JJ ecc99.
A amp builder in the Netherlands I spoke to use this tube as a replacement for the ampeg svt 12bh7 driver tube after many failing 12bh7.

[Richard1001]

Richard, the McIntosh schematic you linked is where the Odyssey amps came from, it is not a CF (cathode follower, ie the signal is taken from the cathode of the second tube) as most SSS amps (which come from SVT), but it is a “plate follower”(ie the signal is taken from the plate of the second tube), basically AC vs DC. Also tube compliment on these amps is different, the second PI tube is actually a 12BH7 and will see a bit over 300V on plates, and the tube will get extremely hot

I can't follow you. In the schematic i posted i can see a 12ax7 cathodefolower with 430 on the anode and the negative bias for the powertubes on the cathode direct tied to the powertube grids (DC ). The 12ax7 seems to get the signal from the 12bh7 and this is AC coupled by .22 caps. The 12bh7 is indeed at a lower voltage.

[Richard1001]

I made a screenshot of the cathodefolower in the schematic

[Bombacaototal]

Richard, I was thinking in terms of the LTPI but I understand what you mean although in this case technically the ax7 is the second follower tube, but still valid point about the voltages

[martin manning]

It is a solid way to lower the B+ on V7 Martin, but In my opinion a change in the applied anode voltage will result in a different perfomance of the tube and therefore will have some influence on the sound of the amplifier. (either good or bad) I think the transconductance and internal resistance of the tube changes. (according to the application notes)

True, but the difference in gain and frequency response is negligible, and the only difference I see in an FFT is is more second harmonic using the lower voltage +/- 180V bipolar supply. It's at -64 vs. -74 dB, though, so also insignificant IMO. I can't see how there would be any noticeable difference in sound.

[martin manning]

Erwin and a few have been helping with the schematic fixes. I'm pretty confident about it and happy to adjust if anyone finds a problem. https://github.com/colganr/LTSpiceCircu ... Export.pdf

Thanks Ryan, I believe that's the same as the one I have. It's missing the reservoir cap, the bias circuit is incomplete (not connected to the HT with a rectifier), the FET node is not connected to the power rail, and there is no FET circuit.

I'll go through it, see what I can find, and mark up your .pdf. I know the Japanese schematic has some errors, and I'll flag those differences as I see them. First installment (preamp) is below. Are people building directly from this schematic or are there others?

[rccolgan]

Thank you for the corrections, Martin. I'll add the changes and update GitHub.

Folks are building per the layout and relying less on the schematic unless its for secondary troubleshooting. Primary troubleshooting/verification is done by tracing on the layout. My understanding is that layouts are less intimidating for new builders vs schematics since it represents the physical product in front of them. It took me a while to understand this (I'm a "traditionally educated" electrical engineer who is comfortable reading a schematic) however it seems like Ceriatone, Mojotone, etc has created an amp-gateway-drug with their layouts. Coming full circle, a proper schematic is overdue so I'll carve out time to do so. Thank you, everyone, for the contributions thus far!

[martin manning]

I firmly believe that everyone should learn to read schematics. Having both is ideal.

I looked at the layout for the first time today, very nice! I’m afraid I can’t fully understand the notes, though, as they are not clearly referenced to the particular places on the layout to which they apply. Adding corresponding numbers for each is highly recommended.

Instead of more mark-ups, I’ll just summarize here:

The filter section:
Looks fine, but I like the response of my improved filter circuit better :^)

The mixer section:
Looks fine, except that the cathode bypass cap on the reverb input is 5u on the hand-drawn schematic, 1u on your layout and schematic. Note the cathode voltage on the CF stages is high, ~200V, so a tube with robust h-k insulation is needed.

The reverb section:
The reverb enable switch grounds/ungrounds both the send and recover wipers on the hand-drawn schematic and on your layout, but it’s only shown on the send in the schematic, Also, the XLR footswitch jack is not shown.

On the hand-drawn schematic, the FB from the mixer is linked to the reverb transformer driver tube’s anode, which seems odd, and an odd mistake, as is the comment written next to it “From pi.” The recovery stage does have the 100-ohm tail and the same relationship to the mixer as the filter recovery stage, so it seems likely that you have it in the right place. Any thoughts on that?

The power Section:
Looks fine, but I’ll suggest an alternative to the bias adjustment shown in the layout that does not require a 2W pot: Replace the 130k resistor going to ground from the junction of the 1M grid resistors with a standard 150k pot or trimmer (the element taking the place of the 130k), then connect its wiper to ground with a 220k resistor. This will provide a fail-safe adjustment from 89k to 150k. The 2W pot can then be replaced with a fixed resistor to get a suitable voltage range.

The power supply:
Needs a total re-draw matching the layout, and I’d include Rlchsrd’s suggested diode across the screen cap. One odd thing is the 60uF cap on the PI node shown in the hand-drawn schematic. Hard to say whether that was intentional or not.

[Richard1001]

It is great to have a readable schematic. I looked at yours and have some thoughts i would like to share.

I think on the handdrawn schematic the feedback to the reverb mixer from the reverb transformer the comment is: 'to yel' not ' to PI' (bad handwriting), i think it is a reference to the yellow wire used. Either way, i also think this is a mistake and the feedback is not on the transformer but the 100 ohm resistor.

The low filter looks in reverse on your schematic. I think low and high should both increase clockwise.
On the high filter the hand drawn schematic shows the 3nF cap as seperate position on the switch to ground. In your schemattic its wired to the other caps and ground. I think the handdrawn schematic is right on the filters. The purpose of the high filter i believe is not to have a linear increase of the same frequency's, but to increase the mid and low mid responce as well (shifting the frequency's ) turning the knob higher. This corresponds with a youtube demonstration of the filters from a Dumbleland special.

In the powersupply I used the bigger filtercap on the screens, first after the choke. (in my case 47uF, not 60uF, because Mundorf dit not have a 60uF Hv cap) To me this would make sense for the response of the poweramp. The easy wat to wire it on the board is between the 20uF caps. So it would look as if the 20uF cap is first after the choke. I think the guy drawing the japanese schematic might have made a mistake here.

With the reverse diode the voltage on the CF cathodes will not go below -70 volts after startup. I found no need for a bias pot, but if you would want one, Martins suggestion would be the way to go i think.

Also, I wondered if anyone has tryed to wire the reverbtransfomer in reverse like drawn on the japanese schematic? (black to signal input, green to ground).

[martin manning]

Thanks for the comments, Richard.

Yes, the note does indeed look like "from yel."

On the 3n cap connection on the high filter: In the plots below the two options are shown. In the first, isolating the 3n produces a huge treble boost in all the other positions, which suggests that it is an error. Connecting the 3n to the common terminal of the other caps and then to ground produces a much more reasonable result. The third plot is the revised filter I developed, which evens out the increments and reduces interaction.

I agree on the direction of rotation. I would prefer the clockwise direction to be increasing bass/treble.

I'm not too impressed with the TMB EQ, either, and I wonder if a Skyliner tone stack wouldn't be a better option.

[Richard1001]

I can clearly see what you mean on the plots, but it doesn't sound like that. Are you isolating the filters in the plots or taking the whole amp (with feedbackloops etc) into account? Also the TMB controlls work realy well i think. Nice tracking on the whole range of all the pots and good sensitivity. Only when de basspot is turned fully counter clockwise (zero) , the middle pot has little effect. This is to be expected. I used 250k lin for treble, 100k lin for mid and 250k audio for bass.
My amp and filters sounds a lot like this:
https://youtu.be/zthE05CGFvY
You need headphones or a good speaker to hear what happens.

[martin manning]

The plots above are results for the whole preamp and mixer up to the power amp input. The NFB around the filter section has very little effect at all, which is not surprising considering the 470k/100Ω divider (that's the 100Ω shown as 1k in the hand-drawn schematic). I don't see anything wrong with the model, but I'll keep playing with it.

Perhaps I was too hard on the tone stack. It's the same basic arrangement as the ODS, and on second look it appears to be ok. Skyline would be a little more effective on the high end, I think.

Interesting observation: the Guitar/Mic switch is identical to the Rock/Jazz switch, which means you can also have PAB with a middle off position.

[nwilderman]

What are the mounting nut plates required for this chassis layout? I’ve tried the recommended 10-32 K1000-3 and MK1000-3 from Aircraft Spruce but neither fit the chassis mounting.

[Richard1001]

I could not find the right 'two lug anchor nuts' either. After measuring, looking at datasheets and ordering some, i concluded that it is (at least for me) impossible to find the right ones. Nothing I found at different stores would fit the chassis. I decided to cut square holes in the chassis and use cagenuts. They are available everywhere.

[martin manning]

Thank you for the corrections, Martin. I'll add the changes and update GitHub.

Sure, just trying to understand the circuit a little better. I've made some good progress in modeling and checking it out beyond the pieces I already had.

I'm still wondering about the two different values of cathode bypass caps in the mixer section, There's a 5u on the reverb side and 1u on the dry side in the hand-drawn schematic, both are 1u on the layout. I don't know if adding more bass to the wet signal makes sense or not. Any thoughts on that from you or anyone else?