SSS CF Driver Compression Questions

[bcmatt]

Probably my biggest complaint about my SSS is just the lack of natural compression compared to most of my other amps. I feel really confused because it seems like SRV wanted an amp that didn't compress at all and that was the purpose of the SSS. But then most of the SSS type amps with youtube demos seem WAY more compressed than my amp.

After reading this description of the purpose of the CF Driver:
Cathode Follower Driver section.12AU/12BH7
The main advantage to the cathode follower directly into power tube grids is losing the effects of grid-blocking from the coupling caps recharging and causing momentary distortion when power tube grid current occurs. The amp is overall tighter (electrically) without the time constant of the coupling caps. There also seems to be an added transparency, clarity and overall sheen to the tone with that style of driver for sure. The amp just sounds bigger and has more authority. Since the driver tube no longer has to drive a bias network or the miller effect of the multiple power tubes, I think overall bandwidth is improved along with a tighter more controlled and extended bass response. The driver also adds a unique compression to the amp that is instantly noticeable ( like the amp has a built-in compressor). The driver circuit also enhances the amps ability to sustain for long periods of time!

I feel like the attack on my notes are like super hurty-loud. No compression at all. The notes ring out just fine but it's hard to tame that initial attack. The amp just wants to let it all though. I'm wondering if I need to tune my CF follower somehow. I also wonder if my amp overall has less gain than most SSS amps... I have to put the amp volume(gain) up to 10 just to get any hint of breakup. So, I'm always setting it to at least 7 just to keep any overdrive pedal from not boosting the volume like crazy.

The Level/Master volume of the amp is before the effects loop and PI, so do people usually need to crank the master just to get any hint of compression from their CF driver, or does it do it's thing regardless of how much signal it receives somehow?

I can't actually play any other SSS to compare mine to, but is seems like youtube is telling me mine should be compressing way more....

[Guy77]

I have found that adjusting the voltages on the Phase Inverter and the CF plates can effect the level of compression.
Try lowering them and see if it gets you what you want.

Cheers
G

[bcmatt]

I have found that adjusting the voltages on the Phase Inverter and the CF plates can effect the level of compression.
Try lowering them and see if it gets you what you want.

Cheers
G

OK, thanks! I was wondering about something like that. What sorts of voltages did you end up settling on?
Did you also record your cathode voltages?

[bcmatt]

Is your level of compression affected by the level of your master volume?

[Guy77]

I have found that adjusting the voltages on the Phase Inverter and the CF plates can effect the level of compression.
Try lowering them and see if it gets you what you want.

Cheers
G

OK, thanks! I was wondering about something like that. What sorts of voltages did you end up settling on?
Did you also record your cathode voltages?

I like how it sounds when the Phase Inverter and CF plates are around 240v. This was with SSS #005. That's a good point about the Master Volume. I never experimented with that. I will test that out when I have another one finished later in the year.

Happy Easter and Happy Holidays!

Guy

[bcmatt]

I like how it sounds when the Phase Inverter and CF plates are around 240v. This was with SSS #005. That's a good point about the Master Volume. I never experimented with that. I will test that out when I have another one finished later in the year.

Happy Easter and Happy Holidays!

Guy

Happy Easter to you too!

Since my CF Driver has it's own power supply (It was my amp build where Martin Manning first drew up his design for CF Driver power and Bias Supply)... It's plates see 190V.
However, all the rest of my plate voltages are much higher. The PI plates are currently reading at 298V and 312V.

The PT is from an early 70s Traynor, so with my modern high mains voltage (often around 125VAC) , even my heaters are seeing 6.7VAC. I think I'll start by adding a couple thermisters before the PT to bring the voltage down on the whole amp. Then I guess I'll see where I'm at. I had bought some CL-80s and CL-90s for bringing down voltages on my 74 Deluxe Reverb... and I knew I'd probably want to do it on some other amps...

[Guy77]

I like how it sounds when the Phase Inverter and CF plates are around 240v. This was with SSS #005. That's a good point about the Master Volume. I never experimented with that. I will test that out when I have another one finished later in the year.

Happy Easter and Happy Holidays!

Guy

Happy Easter to you too!

Since my CF Driver has it's own power supply (It was my amp build where Martin Manning first drew up his design for CF Driver power and Bias Supply)... It's plates see 190V.
However, all the rest of my plate voltages are much higher. The PI plates are currently reading at 298V and 312V.

The PT is from an early 70s Traynor, so with my modern high mains voltage (often around 125VAC) , even my heaters are seeing 6.7VAC. I think I'll start by adding a couple thermisters before the PT to bring the voltage down on the whole amp. Then I guess I'll see where I'm at. I had bought some CL-80s and CL-90s for bringing down voltages on my 74 Deluxe Reverb... and I knew I'd probably want to do it on some other amps...

I was in an Engnater amp a while back and they used those CL-80s and 90s similar to how you described.
Curious to hear how much voltage they drop and how it turns out for you.
Cheers
Guy

[bcmatt]

So, I finally got around to installing a couple CL-90A Thermisters in series on a terminal strip after the power switch. They are each supposedly rated for 2A, so I was hoping that having them in series next to each other they are good for about 4 amps. They seem to be working fine. They brought the line voltage from about 123.5VAC to 119VAC before the Power Transformer. I fed the little transformer that provides the power tube bias voltage as well as the B+ for the Cathode Follower tube from the terminal strip before the Thermisters bring it down since the CF Driver plates are already lower than the rest of the amp (around 190VDC).
Interestingly, my filament supply is still up at about 6.6VAC. I am thinking this may be due to only running 2 of the potential 4 6L6s... not enough current load on the PT to pull the voltage down?

I was looking at the voltages on ViperDoc's recent #002 Schematic, and decided to try get my preamp plates down to similar on that document (208V) while also trying to get around 235V PI plates like you mentioned. I hit those by changing my 2.2K dropping Resister up to about 12K.

With my Power tube Plate voltage at about 444VDC, I re biased to about 80mA on the singular 1 ohm cathode resister...

2 6L6s = 60 Watts
60W/444V = 136mA
So biasing at about 60% plate dissipation gives me about 80mA. I think I had biased significantly colder before. (Although I didn't check before installing the thermisters).

I do feel like a combination of bringing plate voltages down and biasing a little warmer is bringing more of that clean sustain out of this amp.
I also tried rebalancing the PI trim pot. The two plates want to be rather separate with the current 12ax7 I popped in there. But, by putting the trim pot all the way to one side, I can get them within about 8V of each other. with the input side being higher like suggested. I will probably mess more with this and with some different 12ax7s.
I think I am getting more of the typical SSS clean compression out of this amp. I don't have any other SSS to actually play and compare it to, so who knows...
It's all subtle changes, but I can play clean with my Strat for quite a while without getting bored, so maybe it is doing something special. I also might just be getting better at dialing out that aggressive pick attack using the filters and other countless tone-stack controls. I imagine an SSS should be able to dial in some aggressive attack when wanted too, so it makes sense to me that it shouldn't always feel compressed... you should probably have to work for it a bit?

[erwin_ve]

A part of the compression is coming from the LNFB loops. Especially when you drive the first volume pot a little more.
Not all SSS are the same as are the LNFB loops.
Which one do you have?

Erwin

[martin manning]

So, I finally got around to installing a couple CL-90A Thermisters in series on a terminal strip after the power switch. They are each supposedly rated for 2A, so I was hoping that having them in series next to each other they are good for about 4 amps.

The same current flows through both, so you are still limited to 2A.

Interestingly, my filament supply is still up at about 6.6VAC. I am thinking this may be due to only running 2 of the potential 4 6L6s... not enough current load on the PT to pull the voltage down?

That, and the fact that a change in the primary voltage will result in a smaller absolute change on a secondary that is a fraction of the primary as opposed to a multiple of it.

I was looking at the voltages on ViperDoc's recent #002 Schematic, and decided to try get my preamp plates down to similar on that document (208V) while also trying to get around 235V PI plates like you mentioned. I hit those by changing my 2.2K dropping Resister up to about 12K.

IMO the sweet spot for 12AX7 plate voltage is ~180 to 200V.

I also tried rebalancing the PI trim pot. The two plates want to be rather separate with the current 12ax7 I popped in there. But, by putting the trim pot all the way to one side, I can get them within about 8V of each other. with the input side being higher like suggested. I will probably mess more with this and with some different 12ax7s.

Find one that is better balanced and use the method described in the PI balance thread, here: https://ampgarage.com/forum/viewtopic.p ... 89#p416389 The plate voltage differential is not a good indicator of balance.

[bcmatt]

The same current flows through both, so you are still limited to 2A.

OK, thanks! This is the first time I could seem to find a clear answer on this. That actually makes sense I guess I could have measured across one of them to test. When measuring across the pair of them, I do see that the current spikes to about 2.2A for a millisecond when switching off standby before staying around 1.1A no matter how loud I play... maybe up to 1.2A when playing really loud. Does this seem like something they should be fine with, or ought I switch to the CL-80s that are rated for 3A?

IMO the sweet spot for 12AX7 plate voltage is ~180 to 200V.

Good to know, I'll see if I can drop a little more. Thanks! I do think I ought to be getting more "bloom" out of this design, so I want to keep trying things till I can get it. I won't be tempted to really play this amp unless it is doing something a little magical. I've also learned recently that I need to start more with rolling a few tubes before blaming a circuit for potentially worn out preamp tubes.

Find one that is better balanced and use the method described in the PI balance thread, here: https://ampgarage.com/forum/viewtopic.p ... 89#p416389 The plate voltage differential is not a good indicator of balance.

Thanks Martin, I was trying to find that thread yesterday, but stopped on the first page. I'll try your recommended method.

[bcmatt]

A part of the compression is coming from the LNFB loops. Especially when you drive the first volume pot a little more.
Not all SSS are the same as are the LNFB loops.
Which one do you have?

Erwin

Thanks for that idea. I think I changed it a little a couple years ago. I'll take another look. I used to have a LNFB switch on the front panel that I removed... but not sure which values I stayed with... Let me check that.

[martin manning]

The 2A rating on the NTC limiter is probably for continuous operation so exceeding that by a bit on startup is probably ok.

[bcmatt]

The 2A rating on the NTC limiter is probably for continuous operation so exceeding that by a bit on startup is probably ok.

OK Thanks. I'll leave them for now and work on the other stuff.
I put another tube (Tung-sol 12ax7) in the PI and it balances a lot better plate voltage-wise. They're currently both at about 240V. I'll see about getting it lower. V1, v2, and V4 are just a bit above 200V. I see that V3 (the 12at7 Reverb driver) is up at about 344V. Is there an ideal range that you often try to get those in too?

[bcmatt]

A part of the compression is coming from the LNFB loops. Especially when you drive the first volume pot a little more.
Not all SSS are the same as are the LNFB loops.
Which one do you have?

Erwin

My build is adapted slightly from MHarman's Layout which I believe is based on #004.

So I see that I did rip out the LNFB completely from V1 because I did have the switch on the front panel (instead of an FET input) and I felt after a while that it wasn't doing enough to warrant a weird looking switch above the input. I may put that part of the circuit back in since I'm now chasing subtleties that I may not have noticed before...