CF driver Va - Vk voltages in SSS consensus?

[ElectronAvalanche]

Dear all,

I revisited my SSS 002 clone again (actually my take on 002 with a different PCB layout, sans Reverb but with integrated Dumbleator look here: https://ampgarage.com/forum/viewtopic.php?f=4&t=24607 ) and checked the voltages.

The japanese schematic shows voltages that are all 15% off, since the AC wall voltage was 102V as oppsoed to 120V. As already found by some other knowledgeable forum members, V1 must be a 5751 to get to the approx. 213V on V1a and V1b anodes (correpsonding to the 185V volts which are 15% lower due to the 102V mains). The 5751 also really sounds nice IMHO. But this has all been noted before. Just wanted to confirm.

Now to my question:

What is the consensus regarding he extremely high anode voltage of around 430V on the CF post PI driver tube? I also measure 430V there in my amp and read that some here on the frorum suggest to introduce a series resistor between the plates of V6 a(post PI CF) and (E) = Screen node to drop voltages below 400V.

Any longterm experiences in tube life with the high voltages?

Any impact on sound with lower (<400V) voltages on the plates of V6?

Preferred voltage for plates?

Is the high voltage on V6 plates due to a mistake in the japanese schemo (missing a dropping resistor + bogus measurement? I find this unlikely.

Did Dumble not care, or where tubes back in the dayy more resistant to high plate voltages?

I might just put in a dropping resistor and see what happens, but wondered what the consensus here is. What do the commercial sellers of SSS clones do ?

best regards and thanks,

Electron

[martin manning]

There is this alternative bipolar power supply using a standard 60V bias tap: https://ampgarage.com/forum/viewtopic.p ... 11#p368411

[ElectronAvalanche]

928E15C9-FD66-4602-B1CB-C69922A8066F.jpeg

Hi Martin,

thank you, I saw your suggestion, but am hesitant to do such a deep modification to the amp. Given that the original 002 is rocking along nicely after all those years on stage.

Was Dumble wrong? Is there a piece of info missing in the japanese schemo?

I looked at the SVT schematic (version V9] and Ampeg used 350V on the plates of a 12BH7. And Ampeg SVTs seem also to hold up nicely with extreme abuse.

https://drtube.com/schematics/ampeg/v9pwr.gif

B+ for the 12BH7 is taken from point E. See schematic on top of this post.

So maybe lowering the B+ node for the CF to 350V might help? I need to figure out what that would mean for that stage.

Best,

Electron

[JD0x0]

Dont forget actual plate voltage is Plate to cathode. So if your CF has 180VDC on the cathode and 430 on the plates, that's actually like the tube is seeing a 250V plate voltage AFAIK. That's also why some Phase Inverters get away with over 300VDC on the PI plates, while the limit for the tubes is ~300VDC, the cathode usually runs around 70-80V.

Also worth noting is the Reverb driver on Fender amps often runs around 400V on the plates, and that's only got about 2VDC on the cathode. Those generally seem to hold up, likely because they're usually not dissipating too close to max spec.

[martin manning]

The Japanese schematic shows -40V on the cathodes and 379V on the plates, so there is 419V Va-k at idle. There is a 220k resistor between the cathode and its supply node, which is at -328V, so with no current flowing the cathode is at that voltage (scary for the h-k insulation), and that would be the case when the amp is on standby. Ampeg went with a bipolar supply for the SVT. In that case the positive voltage is 350V, but the negative is -150. Much more reasonable for Vh-k.

[rootz]

But this is different. The cathodes are more likely at -50V, so there will be 480V plate to cathode in an ecc83. Quite obviously not what they were designed for. And this is when on, not standby.

Any reports on failing cf’s in sss amps due to this. If not, it might seem going beyond the limits. But if it works, it works.

Edit, this is in response to JD0x0’s post.

[ElectronAvalanche]

@rootz:

indeed, I measure 483V at idle from anode to cathode.


Scary!!!!


I like Martins suggestion, but that would mean to totally rebuild the bias supply. And still makes me wonder what Dumble thought and what some long time gigging users of 002 clones experienced.

Electron

[mojotom]

Quite interesting !

I noticed on the block diagram of a 150W ODSR he used on the same taps as the rectifier another rectifier on each side (a resistor and a diode) to form a negative bias supply then use a 4k7 alter the choke (with its filtering cap) for the CF (DC coupled of not) stage and a 470R for the PI stage.

That power supply could be what I will try for that kind of amp (I have to try Martin bipolar power supply too).
That’s what I’ve seen from AN pictures of 005 also and on paper it felt « better » than the Japanese schem.

The funny thing is I had a look at that same block diagram ages ago when I bought that Pitman book (20 years ?) and just pass on it, this is just a block diagram, felt dumb now noticing that power supply topology was there all that time

A good 12AT7 NOS in new condition is still something available at a good price around $40 for a solid military Mullard tested and will keep up quite a while on the reverb position.
I would be sad to throw a NOS 12AX7 on that CF spot with that much voltage considering how expensive they are now.

Got to ask Tony why he uses an AT there.

[talbany]

There are several reasons why I use an AT. Foremost is I was burning through too many 12AX's I used to play every weekend 2 and sometimes 3 gigs and would go through one every several months. It eventually became an issue so I converted it an AT. I liked the AX (when it was working) as they seemed to help give the amp a bit more life and not quite as stiff feeling as the lower gain AT's and BH7's but to be honest is not that big a deal for me.
I suppose it depends on how reliable you expect the amp to be and how often and how you use it.At the time I was gigging quite a bit so I needed the extra reliability that the AT seemed to provide. I generally replace the Driver with another balanced one on every power tube change and that seemed to take care of the issue (2 tubes a year)
BTW.. My fave PI Driver tube was a Telefunkin smooth LP 12AX gave the amp a great feel but it's just not practical burning through a NOS hard to find tube for gigging. Now I generally stick with Sovtek AT is a nice cheap current production tube that has seemed to be fairly reliable.

My #002 was built right from the hand drawn schematic from Japan back in 2012 IIRC which was right when I got hold of the thing and have used the amp quite a bit since I built it. At first I was planning on doing some upgrades to help reliability and performance however as long as I keep an eye on the tube life and address any issues quickly I'ts been a reliable work hoarse for me over the years and don't really have a bad thing to say about it other than the filters and reverb (if you really crank it can get a bit noisy). better cables and the right tubes in the mixer helped that out quite a bit. So in the end I have no problem recommending to builders (If they want) to stick to the schematic will be a fine and fairly reliable amp just be aware of the slightly higher maintenance requirements.

Tony

[ElectronAvalanche]

Hi Tony,

many thanks for your most valuable insights!

Can you elaborate what changes you made for AT or BH7 use?

Alsodid you ever consider to lower the B+ for the CF?

How did the AX type tubes fail? Did they simply stop working or did you experience weak performance or did it take some other components wih it?

Best Regards and thanks,

Electron

[talbany]

Hi Tony,

many thanks for your most valuable insights!

Can you elaborate what changes you made for AT or BH7 use?

Alsodid you ever consider to lower the B+ for the CF?

How did the AX type tubes fail? Did they simply stop working or did you experience weak performance or did it take some other components wih it?

Best Regards and thanks,

Electron

Oh Man! I knew you were going to ask that I wish I could remember what I did as this has been so long ago. I may have tweaked the voltages a bit but if I did it was not a major mod or I would have remembered that, old age.. As far as the AX's failing? One time I remember having a power loss or a volume drop. Some other times it was just noise crackling or possible arching going on inside the tube but nothing catastrophic like a blown fuses or burnt resistors.
If I get inside the thing in the next few days I'll take some measurements and see if anything was done to modify it. I will also say that both Bill Dunham (Sebago Amps) and myself built one at the same time with the boards we designed and he built his just like mine opted for the 12AT as well and don't remember him having any problems with the AT as well although I think he never really stuck with the AX due to the problems I was having.
BTW.If I would have done it over again or if I were to build one for the general public I think I would add a separate winding on the PT (ala SVT) to power the PPI Driver section, but since I was only building the 1 just used a Twin style PT..
Sorry I couldn't be of much help!
Take Care!
Tony

[ElectronAvalanche]

Dear Tony,

Thank you for your reply! Yes, I also seem to forget more and more...... aging is the hardest part.

I saw a schemo of the Sebago Texas Flood on the Sloclone forum and it showed a 12AT7 for the position, but no changes to the cathode resistors (220k) or the B+ (438V).

Joey from Sloclone said that one simply would lower the B+. One does not need more than what one hits the power tubes grids with.

I will take a look at this and report back.

Electron

[ElectronAvalanche]

Update:

I installed a 22k (2W) resistor in series between B+ and the CF driver anodes and dropped the voltage to 380V (B+: 435V), this calculates to 2.5mA for the CF driver. I measured the bias and the output tubes lost 1mA.

I do not hear much of a difference between the high CF driver voltage and the lowered one and assume this will not make much of a difference or only at extremely high volumes.

BTW: the plates measure -47V when on standby, and this voltages slowly drops to -27V over time. Of course putting the amp in operate mode (standby off) results in the measured 380V on the plates.

So now I think I can play the amp with less anxiety that the CF driver tube may malfunction. Installing a 12AT7 would probably be even more cautious, albeit I am not really sure 12AT7 have a higher max Va-Vk rating.

Electron

[martin manning]

Interesting. No question you don't need the higher anode voltage. Did you happen to measure the cathode voltage when on standby?

[ElectronAvalanche]

Interesting. No question you don't need the higher anode voltage. Did you happen to measure the cathode voltage when on standby?

Hi Martin,

I forgot to measure the cathodes while on standby. I will do that the next couple of days, I put the chassis back in the cabinet already.

Electron