Very Small Special

[stephenl]

Any local feedback in the preamp?

Since there's no OD, would you put all pedals in front, or better to put in an effects loop for verb, delay?

[rdavy]

Yes, major props for the creativity and execution here!
oof, bad pun alert, sorry!

[Colossal]

Very nice work, Tony! That's really creative!

[dorrisant]

Any local feedback in the preamp?

Since there's no OD, would you put all pedals in front, or better to put in an effects loop for verb, delay?

No local feedback, I think that is what gives it so much character. Has me thinking of having the LNFB on a switch and relay(s) for the next ODS I build. It takes pedals very well on the front end, very much a pedal mule. I was surprised at how much raw gain it had compared to an ODS with the OD off. Very aggressive but retained all of the articulation you'd ever want. It's not metal but it sure will rock with no outside help at all. This one didn't have even a passive loop, but I would think it would be great for modulation stuff, maybe have to build it with an internal D-lator.

Thanks for all the kind words guys... you are the best!!

As an aside... I sent Dave a board for one of these and dared him to try it... Feel free to goad him into it. I am sure he will be happy if he does.

I would post a short video clip of when the customer played for a minute before he took it home, but I can't figure out how to upload it here. He sold his Dr Zs after this...

[fred.violleau]

Wow just astonishing finish! Got time to record a small clip of that extra small special?

Cheers buddy, fantastic work!

Fred.

[norburybrook]

Any local feedback in the preamp?

Since there's no OD, would you put all pedals in front, or better to put in an effects loop for verb, delay?

No local feedback, I think that is what gives it so much character. Has me thinking of having the LNFB on a switch and relay(s) for the next ODS I build. It takes pedals very well on the front end, very much a pedal mule. I was surprised at how much raw gain it had compared to an ODS with the OD off. Very aggressive but retained all of the articulation you'd ever want. It's not metal but it sure will rock with no outside help at all. This one didn't have even a passive loop, but I would think it would be great for modulation stuff, maybe have to build it with an internal D-lator.

Thanks for all the kind words guys... you are the best!!

As an aside... I sent Dave a board for one of these and dared him to try it... Feel free to goad him into it. I am sure he will be happy if he does.

I would post a short video clip of when the customer played for a minute before he took it home, but I can't figure out how to upload it here. He sold his Dr Zs after this...

Tony,

I thought the small special was a clean amp? have you done anything different from the schematic/layout from the files section?

M

[10thTx]

DELETE

Will try to post a schematic to match the layout and repost later.

With respect, 10thtx

[mojotom]

Very nice couple of builds Dorrisant !!

I really like 6BM8/EL86 push-pull, got an old Gibson GA8T and thought it was a nice way to build a small amp rather than going for a single tube.

I was going to ask for a schem thanks 10thTX !!

[dorrisant]

Thanks again to all.

Jeff, I used this layout after reading a ton before getting the iron hot. I don't use Express PCB, so I don't want to attempt to edit your file. Sorry.

ss_layout_v2.jpg

I moved v1 cathodes to group the grounds together ((as in jelle's build (pic below)) and flipped everything 180° (top to bottom) to make it work for my layout. The parts values I used are what is on the layout... pretty simple.

Jon Small Special 4.jpg

20200603_210242.jpg

The only real deviation from that was the cathode bias resistor and bypass cap instead of the fixed bias circuit. I can't recall off hand what the resistor value is, but I know I dialed it in to as close to 100% dissipation as possible. AO-44 schematic shows 130Ω 3W II 100µF 50v. I know I wrote this value down along with the voltages, but I didn't save it in the place I would expect, so I'll probably have to get it back on the bench and report back. I know I used a 5W resistor along with the 100/100 F&T cap installed in the pic above.

[brewdude]

Is the big 5 watt resistor for global NFB?
Also, where are the dropping resistors in the PS?

Btw, your builds are very impressive—and inspiring. Thanks for sharing.

[rootz]

This has got to be in the top three coolest amps I have ever seen.

[didit]

Hello --

Joining the chorus late and hoping to keep roughly in tune, disbelieve mistakes exist of that sort. That CNC over-cut provoked those all-to-necessary brass plates, making the whole thing come together as a piece.

On thoughts regarding character. That pair of mini-power pentodes take a whole lot less voltage to send them singing. You've tied a near ideal preamp via LTP PI into cathode-biased low power output. It'll delivers overdrive without requiring anything more. Inversion version of Dumble's solution to getting tweed tone at at 1970s stage volumes. Perhaps you can sell this design to Mike Z.?

Another small deviation from the Dumble formula seems your tube rectifier. Attempt at camouflage with faux silicon bridge is cute but transparent.

Best .. Ian

[dorrisant]

Thank you all again!!

Is the big 5 watt resistor for global NFB?
Also, where are the dropping resistors in the PS?

Btw, your builds are very impressive—and inspiring. Thanks for sharing.

Yes, that 100R 5W is the NFB resistor.

Dropping resistors were not on the board yet. I am in direct contact with the owner on a local basis. I have requested the amp be brought to the shop... he needs some reel-to-reel repair, so it is a matter of time (one week or two). It will be here and I will take more pics and all measurements so I can post them here.

Ian, I have a second set of iron as well as a second board. I will find a donor chassis and start another identical build.

Also, the ss diode rectifier was there for a build I was going to do in another chassis. I did use the 6x4 rectifier and I believe I just left the diodes there. I am curious what this same preamp will do dumping into some 6V6s. Sounds like a lot of fun just finding out.

Here is a YT link for Shawn using this amp live:

https://m.youtube.com/watch?v=Ubp4ydQe7 ... e=youtu.be

If you listen, you can hear the amp rattling the snare... 10 Watts!!

Please check him out and hit "like" if you do. I'm sure some of you guys will dig it totally. If you like it, check out his album "50". I play it regularly.

This amp has no loop, so he is front-ending all effects you hear.

[10thTx]

This is the best that I can make out from the layout provided. It should be noted the layout provided does NOT match the photos or description of the actual amp.

Since I have built 6BM8 amps & that tube is still manufactured, I listed the 6BM8 instead of the ECL86. (they are both pentode/triode tubes) Note the cathode resistor being 430R/10W for the 6BM8

I added an "enhance cap" on the 51k LPTI plate resistor which helps smooth out harshness in the treble, in my experience.

I did not know what pentode grid resistors to list given the layout was fixed biased and it was noted the actual amp is cathode biased. I chose 220k to ground as that is what I use on my 6BM8 amps.

While the photos of the amp showed a tube rectifier, the layout showed a solid state rectifier, so I drew the schematic to match the layout.

I have found a post LTPI coupling cap of .1 to be a little much for my taste using 6BM8 tubes. One might consider trying a .047 or even a .03?

I added a passive effects loop which has worked for me using MXR Carbon Copy, Boss delay, Wampler Faux Echo and a Moen Pretty Dolly delay (my favorite)

I typically like around 300v to 315v on the pentode plates of the 6BM8 tubes when I can find a PT to provide that. I'd want a PT with at least 120ma.

I used the layout for the filter caps and "dropping" resistors. Don't know what choke to list?

PLEASE look this over and note any errors OR corrections preferably. Compare with layout provided. This should be considered an unproven DRAFT design regarding the 6BM8 tubes.

With respect, 10thtx

[dorrisant]

Thanks for that Jeff. I will likely know the mystery values I used sometime in the next few weeks. I'd certainly be interested in trying this with 6bm8s just to compare and dial in for your schem/layout.

The only preamp difference I see between your schematic and what I used for a layout is the 120k on the deep switch, I used 1M there.

Btw, what OT/primary impedance are you using with the 6BM8s?