7C5 Idea

[dwinstonwood]

Hello all,
I've been buying some "oddball" NOS RCA tubes lately, and I have enough to string together.
This idea I came up with is probably more complicated than it needs to be, but it uses the tubes I've gathered so far.

Let me know what you all think. Maybe certain tubes would work better in different places. 

Thanks!

[passaloutre]

I just finished a 5E3 with all Loctal tubes, including 7C5. It sounds great, I say go for it!

[bmccowan]

I love doing things with different tubes. I have not used 7C5s yet, but I have a Masco amp waiting in the wings that has a pair. The other tubes I have used with success.
I do not like high gain amps - so my comments are colored by that.
It looks like a lot of gain to me. And if you end up taming the gain you still have the noise from an extra tube. I'd suggest a socket for one 7 pin preamp tube - and swapping the 6at6 and 6av6 depending on mood. Then I'd consider moving the 5879 to V2, I've used it a lot in that position. And then the 6SL7 as the PI. From what I have read, the 7C5 is a Loctal 6V6.
But mostly i want to spectate and see what you come up with.

[Latole]

Hello all,
I've been buying some "oddball" NOS RCA tubes lately, and I have enough to string together.
This idea I came up with is probably more complicated than it needs to be, but it uses the tubes I've gathered so far.

Let me know what you all think. Maybe certain tubes would work better in different places. 

Thanks!

Nice schematic you post.  It is the first time I see those tubes

[dwinstonwood]

Thanks you all! This is still at the brainstorming stage. I haven't even started planning a layout.

[dwinstonwood]

I like using PSUD2 to play around with dropping resistors and to get an idea of B+ voltages. I entered the Hammond 270CAX specs from their datasheet and the plate currents from the various tube datasheets.
The 270CAX is rated at 250-0-250 with an 81mA load and has a 170.4mA max rating. I came up with a total of about 109.7mA, so I think this PT will work.
If my calculations are close, the screen voltage on the 7C5's should be around 264VDC. The 7C5 datasheet max is 285.

https://www.hammfg.com/files/parts/pdf/270CAX.pdf?v=1697661948

[edited to correct numbers]

[passaloutre]

They are 6V6GT tubes with a different base. They can take higher than data sheet voltages. Certainly anything you would subject a 6V6 to.

[dwinstonwood]

Thanks passaloutre! I've read that in practice these older tubes are usually rugged enough to handle higher voltages. Fender definitely knew they could.

But, I already have a 270CAX, and for me it's a fun challenge to try and design a power supply that meets those datasheet recommendations. I also like the sound of power tubes at lower voltages. My last amp - based on the Valve Wizards medium-gain design - uses 6V6's with 350VDC on the plates. I think it sounds great as a home-use amp. 

David

[DummyLoad]

Use a 7F7 in place of 6SL7/6AT7/6AV6 - Same electrically as the 6SL7. Use 7C7 in place of 5879 - Now is all loctal. 

B+ node D - Decouple 5879 from that gain/CF stage with it's own R/C network, or likely will be very unstable with that much gain on tap. Sandwiching 5879 between the 6SL7 halves is going to be interesting. 470K grid stopper for 5879 is going to shave off a LOT of HF. Looks like you're building an SLO-ish OD channel amp with SLPI?

Just my 2¢ 

--Pete

[DummyLoad]

Are you SURE you want .022uF bypass caps for 6AV6/6AT7 gain stages?

--Pete

[dwinstonwood]

Thanks Pete! Those bypass caps must have been a "copy/paste" mistake. 
They should be 22uF or simply un-bypassed. Thanks for spotting that.

I originally had the 6AT6 as the Cathodyne (SLPI), and the 6SL7 as the Gain/CF stage. At some point I switched it up; not sure why. I've never used a 5879 before, so I just guessed on the grid stopper.

As has been suggested, it might make sense to just drop the first gain stage. I could then use it's B+ node ("E") for the 5879.

I appreciate the advice!

David

[dwinstonwood]

Here's version II with one less gain stage and the pentode decoupled.

Merlin states anything up to 1M can be used as a CF grid stopper, it simply increases soft clipping, and decreases headroom. I don't know what using a 500K pot as a Gain control/grid stopper will do, but it could be interesting to try!

[bmccowan]

That's looking pretty Matchless Clubmanish. A great amp.
I built an all octal 6V6 Clubman - one of my favorites (I have lots of favorites )

[passaloutre]

Use a 7F7 in place of 6SL7/6AT7/6AV6 - Same electrically as the 6SL7.

…

--Pete

That’s what I did for my 5e3

[Glenn]

That's looking pretty Matchless Clubmanish. A great amp.
I built an all octal 6V6 Clubman - one of my favorites (I have lots of favorites )

Hi bmccown. you have a pdf or like of this schematic, I
Can’t get Jschem to work on my iPad….. thanks

[bmccowan]

Looking - if I don't find one I will convert the sch file.
I think this is the same.

[dwinstonwood]

I did some searching, and the 5879 has, in fact, been successfully used as a CF driver.

And, the 6SL7 datasheets say it is well suited for PI duty.

This is version III:

[Glenn]

Thanks bmccowan I been wanting to build an all octal amp, I have all the stuff I need.
Dwinstowood, I built a Marshall super lead with 6aq5’s, these are 7 pin power tubes. I love this little amp a lot

[bmccowan]

Thanks bmccowan I been wanting to build an all octal amp, I have all the stuff I need.
Dwinstowood, I built a Marshall super lead with 6aq5’s, these are 7 pin power tubes. I love this little amp a lot

I have a small Epiphone amp with 2x6aq5. I removed the "T filter" from that amp and it sounds great beyond expectations.
Octal preamp tubes are under-appreciated nowadays. I became fond of them while rebuilding old Gibson and Valco amps before they became darlings of eBay, and also repurposing old PA amps as guitar amps. I have never found octal preamp tubes to be particularly microphonic, but I do not build high-gain amps, so  And if you are old and arthritic like me, it's a lot easier soldering those octal sockets.
It's fun to convert 9pin tube preamps to octal, but there are also tons of Valco and Gibson schematics (with a variety of head scratching mistakes) available.

[dwinstonwood]

OK, now you all have got me looking at Octal preamp triodes.

Here's one that looks similar to half of a 12AX7 with a mu of 100:
https://frank.pocnet.net/sheets/049/6/6SF5.pdf

[bmccowan]

6sf5 works fine. I have had it show up in some really early Valco amps - and I think I have an early Gibson with that tube.

[dwinstonwood]

And the 6SF5 "GT" glass versions are pretty cheap! I'll save the little 6AT6 and pick up a the Octal.
Anyway, this project is all about trying new/different things for fun. 
Thanks.

[DummyLoad]

ThreadJack - Ugh! My apologies in advance - Years ago, I experimented with an all octal amp - Deluxe-lsh if you will, the second channel being a pentode. ALL tubes were wrapped in metal, including the rectifiers. I used 6SF5 for the triodes, 6SJ7 for the second preamp channel, a pair of 5T4 full wave rectifiers, and 1614 for the final stage. Killer sounding amp, but never put to a chassis. The DC filaments were more trouble than they were worth, yes it was a bit quieter with DC to the preamps, but overall not worth the added expense and effort. The second schema is/was a much more recent revisited concept I dubbed "Rock'in Rosie Riveter", a Fenderish Super R with all octal tubes, so if Rosie rocked a roadhouse during WW2, she'd have that amp.   

--Pete 

[bmccowan]

Hey Dummy...
Those are both pretty cool and imaginative. In Metal Mania I like the idea of being able to mix the pentode and triode preamps. I had trouble with that when I tried to mix 6SJ7 and 6SL7 channels, but I admit that I got distracted by another project and set it aside before finding a sweet spot. Did you find you could mix the channels and get an interesting tone, or did you just favor one channel at a time?
Also - amp chasses never finding their way to a cabinet is an issue I am very familiar with.

[dwinstonwood]

Thanks for posting the schematics Pete. This thread can become a collection of Octal preamp designs as far as I'm concerned. 

The 6SF5GT is an interesting tube. I've been trying out a bunch of different bias settings using various plate resistors, 250V B+ and -1.5 grid volts. The smaller resistors push the quiescent plate voltage pretty far to the right. Here are two:

[DummyLoad]

Treat the 6SF5 like a 12AX7, differences are minor, e.g., slightly less transconductance and slightly higher plate resistance. Have a look at the 6F5 resistance-coupled amplifier charts, they are same electrically. RCA Edition 16 Resistance-coupled Amplifier Charts for 6F5 & 12AX7 families attached to this post for comparison and guidance.


--Pete

[DummyLoad]

I had trouble with that when I tried to mix 6SJ7 and 6SL7 - Did you find you could mix the channels and get an interesting tone, or did you just favor one channel at a time?

The MetalMania was basically an all octal Matchless DC30 of sorts, you used one channel or the other. Mixing the two as I recall, was not a pleasant experience.

Around 2012 I built a Fender Deluxe 5E3-like into a Webster Electric PA - That amp does mix triode/pentode as the 5E3 does with triodes only, amp has seriously good tone, you should revisit that. Use a 6SJ7 with Matchless DC30 EF86 values and similar preamp B+. There is a thread on the forum about it - Schematic attached to save some search time. The only things I would do differently would be to ditch the x-line Master Volume and decouple the pentode power from the triodes power rail.

--Pete

[bmccowan]

Thanks Pete,
I like that schematic. If I did not repurpose that chassis I will take a look at revising that project. I have several solid old 6V6 and 6L6 Stromberg-Carlson/Bogen/RCA PAs waiting for winter projects. That schematic will go in my potentials file.
I rebuilt a Webster organ amp into a Gibsonesque amp a while back. That Webster has seriously nice iron.
I also built an octal Deluxe many years ago 2x6SL7, 2x6V6. It'd be fun to change one of those channels to pentode.

[dwinstonwood]

Thanks for posting the Western Electric file Pete. I wasn't really aware of the 6SJ7GT pentode. Using that tube would make my preamp all-octal. So, I lifted it from your schematic! 

And, also, thanks for those RCA charts. I more or less center-biased the 6SF5GT at -1.5V.

[bmccowan]

DW - the 6SJ7 is available both as a metal tube and GT. They perform the same as far as I have experienced.
EF86 and 5879 are quite similar electrically.
Anyways - you will find that Pentode in many Gibson and Valco schematics, as well as a few early Fenders.
They were used a lot in early PAs. People complain that they are microphonic, but I have not found them to be troublesome. Some prove this by tapping them with a pencil - to which the solution is - stop tapping your tubes with pencils.

[dwinstonwood]

Thanks bmccowan. I will definitely browse through those older schematics to see how they were used. And, yes, I noticed in Pete's schematic that he has pin-1 grounded, i.e., the non-G(lass)T metal-envelope version.

Since I already bought two NOS 7C5's and some nice NOS Cinch Loctal sockets, I'm going to stick with those.

I've gotten a lot of great info from this thread, so far.

David

[dwinstonwood]

...and decouple the pentode power from the triodes power rail.

--Pete

Question: Is it OK to have the CF and its driver fed from different B+ nodes as long as the supply voltage is the same? I'd like to de-couple the 6SJ7GT in my schematic.

This Star Network is straight out of Merlin's Power Supply book.

Thanks.

[edited to fix schematic error]

[DummyLoad]

...and decouple the pentode power from the triodes power rail.

--Pete

Question: Is it OK to have the CF and its driver fed from different B+ nodes as long as the supply voltage is the same? I'd like to de-couple the 6SJ7GT in my schematic.

This Star Network is straight out of Merlin's Power Supply book.

Thanks.

Yes to decoupling pentode from CF. You're missing a cap from 6SF5 plate to volume control.

--Pete

[dwinstonwood]

...You're missing a cap from 6SF5 plate to volume control.

--Pete

Yikes! Thanks for spotting that. I updated the schematic in Reply #31.

[DummyLoad]

I see another issue with that plan I missed - The master volume control ties to a 1MΩ to ground, that's going to upset the bias of the SLPI. Consider another plan: Delete the 1MΩ, or insert a coupling cap between the treble pot wiper and the top of volume control.

Also, IMO it would sound/work? better with a 6SN7 LTPI to help recover some of the losses of the CF, the tone stack, and the SLPI as basically there are three consecutive elements of signal loss, the last bit of gain is about 10 and is the output stage itself.   

Respectfully,


--Pete

[dwinstonwood]

Thanks Pete. I've actually been thinking about ditching the CF and moving that 1/2 of the 6SL7 over to the other side of the James TS as a recovery gain stage feeding the SLPI.

If the James TS is a bad load for a pentode I can go with a six-way cap selector switch instead. That's what I have in my AC15 and it works great (thanks to sluckey for that circuit).

I've drawn something up along those lines. There are probably errors:

[DummyLoad]

Actually, I believe that the switched coupling cap ckt. was an Orange implementation in the 70's, later Matchless copied it, slucky took from both in his Dual-Lite and others?

Leave the CF, IMO it adds a lot to the overdrive character. replace the 1/2 of the 6SL7 with another 6SF6 for CF duty, use the 6SL7 or a 6SN7 as a paraphase/floating paraphase, LTPI. or Gain/Concertina pair.

--Pete

[dwinstonwood]

OK, this does make a lot more sense. Thanks.

[DummyLoad]

OK, this does make a lot more sense. Thanks.

IMO, yes, but you should really create what you think you'd like     Those are some odd values for james stack, but then again, not my taste in tone. I do prefer the Ampeg V4/VT40 james stack values for guitar, again, just my preference.


Respectfully,

--Pete

[dwinstonwood]

Thanks Pete. I think I got those James values out of Merlin's book.

There's also that TSC in the web site where I got some earlier values.

I'll look at those Ampeg V4/VT40 TS values, too.

[dwinstonwood]

It took some searching to track down the Ampeg TS values because they were originally on a small module board, and not designated on the schematics I looked at.

This image is the James TS with Ampeg values, but I lowered the 120k resistor between the pot wipers to 68k. This puts the bottom of the mid scoop at 500Hz which is the same frequency as the Fender TMB scoop.

At any rate, when I build, I'll start with these values tacked in.

[dwinstonwood]

I think I can put the tone stack on the pots. 

[dwinstonwood]

On some subconscious level I had to have been aware of this. Funny that it just struck me. 

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a_schem.gif

But, I doubt that my project will sound too much like a 15W Bassman.

[glass54]

Hi dw,

I think I can put the tone stack on the pots. 

I have done exactly that in a recent project and actually used the same RC values, Made the TC a "3 terminal network" can be changed anytime without adjustments to turret/terminal board 
Also can do a Fender TC substitution easily.
Regards
Mirek

[dwinstonwood]

Thanks Mirek, I'd like to see a photo of that!

Here's a starting point. I'm going with the Star B+ supply. I've never used it before, but it's supposed to offer better decoupling at the expense of some ripple filtering. Although, I could increase the filter cap values. I'm just keeping the reservoir (not on the board) at 16uF because of the NOS 5Y3GT.

[dwinstonwood]

I think I've finished my drawings (PDF below). The layout is intended for a blank JTM45-type chassis. But, the controls are left to right like a Fender.

I'm going with these Hammond parts:

PT 270DX: https://www.hammfg.com/files/parts/pdf/270DX.pdf?v=1697661948
OPT 1751M: https://www.hammfg.com/files/parts/pdf/1751M.pdf?v=1697661946
Choke 194A: https://www.hammfg.com/files/parts/pdf/194A.pdf?v=1697661947

[kagliostro]

Ciao Dwinstonwood

Very nice project, I like it, only regret I've no 7c5 tubes

If I can permit me to say my 2c

Why your choice for the PS node was to use the path of the old VOX AC30 ?

Also VOX in later models preferred to have the PS nodes "in series" instead of "in parallel", that way you don't use the ripple reduction due to the previous node filter

Franco


p.s.: Beautiful layout 

[mresistor]

Ciao Kag   he explained his reasoning for supply configuration in reply #44.

[kagliostro]

May be I'm short-sighted, I don't see much advantage on using that configuration 


Talking about 5Y3 tubes as far as I can know a 30uf e-cap can be used without problems, despite what is written on datasheet


Franco

[dwinstonwood]

Thanks Franco! I've never used the Star filtering arrangement before, so I want to give it a try. I want to see first hand how it works out. It is described as being better at decoupling the stages.

Having said that, I have a lot of confidence in the grounding scheme I'm using - which I've arrived at from following the advice of others. I've used it in three previous amps and they all have no audible hum.

I've haven't bought any caps yet. Yes, I did pick 16uF for the reservoir because of the 5Y3GT datasheet, but if they typically tolerate higher values I can go with 32uF:
https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Capacitors&ORDER_ID=!ORDERID!#:~:text=32uf/32uf/500v%20FT%20multi&text=Multi%20Section%20Capacitors%20and%20Clamps

If the Star arrangement doesn't work out, there's no major surgery involved in converting to series filtering; I won't have to move or add turrets. It will just look a bit weird.

Thanks,
David