Self adjusting cathode bias

powdog

Junior Member
Joined
Oct 13, 2008
Messages
48
Reaction score
75
Hi all. I'm thinking of building a single ended amp along the lines of the Emery Sound Superbaby. Something that I can experiment with using different output tubes. The Superbaby uses a "self-adjusting cathode bias" that allows you to swap out a 6V6, 6L6, EL34, KT66, KT88, and other similar tubes without any bias adjustment. Assuming a power transformer with a 325-0-325 secondary, what would be an "ideal" R/C cathode bias network that would allow swapping all of the above mentioned tubes? Thanks alot for the help.

-Powdog
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Hi PD: subject to some discussion below, the most commonly seen R/C bypass is 250 ohms/5 watts and 25uf/25V. Having looked at group of schematics for SE and PP amps, the lowest R was 200 ohms (Gibson GA8 Gibsonette, parallel 6V6s) and designs above 250 ohms included the 5E1 and AA764 Champs at 470 ohms and the highest in a brief survey was the SE 6BM8 Gibson GA1RT at 680 ohms. As you know, the lower the value of R, the lower/cooler the idle current.

The bypass caps were either 20uf - 25uf (no meaningful difference) or 50uf (Ampeg Reverberocket R12R, 6V6s). In those schematics that had cathode voltages, the design objective is consistently between 16V and 19V. Finally, nearly all of these designs rely on the older RCA Tube Manual data for maximum plate dissipation guidelines; guidelines routinely ignored by Mr. Leo Fender who certainly knew his beans.

One of our BBers tele4tone started a thread recently about his Class of 57 Amp, a design similar in concept to the Emery. I went and looked at the Class of 57 and Emery sites - killer power transformer on the Superbaby btw and neither provided an accessible schematic. Giving them both the benefit of the doubt, they had to have spent a lot of time trying to optimize a power supply for group of tubes that present such a wide range of voltage handling characteristics.

That's all 'code' for how does a fixed plate voltage, fixed bias cathode resistor, and fixed heater supply maximize tone in a group of tubes whose outputs in SE config range from 5-6 watts for a 6V6 to as many as 16 watts for a KT88? ... and what about the screen supply; sometimes at the same voltage (EL84/Marshall), sometimes not (6V6/Gibson) and sometimes regulated (6L6-6550/Guild) admittedly all PP designs?

I guess what I'm saying is that your finished product will give you a chance to listen to a variety of power tubes and that would be pretty neat but without some close figuring at least on screen supply, there's a chance that the distinction might not be a great as you expected ... not trying to discourage ... it's more about expectations. Also, tele4tone tone just posted in his own thread this morning about his disappointment with his Class of 57 .. take a look over there. Good luck! John
 

JonL

Junior Member
Joined
May 6, 2009
Messages
12
Reaction score
0
John, I think in your first paragraph you mean to say that a HIGHER cathode R value gives lower/cooler idle current?
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
JonL said:
John, I think in your first paragraph you mean to say that a HIGHER cathode R value gives lower/cooler idle current?
Thank you Jon; yes, the higher the resistance between anode and ground, the less current would flow. John
 

powdog

Junior Member
Joined
Oct 13, 2008
Messages
48
Reaction score
75
Hey CJ. I appreciate the input. Probably better to make bias resistors and OT primary impedence switchable to best accomodate different tubes.

-PD
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
powdog said:
Hey CJ. I appreciate the input. Probably better to make bias resistors and OT primary impedence switchable to best accomodate different tubes.
Hi PD; you may have confused the OT primary and secondary; the secondaries can be switched to accomodate varying speaker impedances but the primary impedance, on the side facing the output tube(s), is fixed in the design of the transformer. One of its functions is to match the output impedance of the output tubes to the impedance of the speakers. It turns out that the output tube's impedance is a function of its operating voltage. For example, according to its data sheet, a 6V6 operating at 315V has an output impedance of 8.5k which drops to 5K when operating at 250V. For an EL84 at 250V Zout is 5.2K and probably higher if the voltage were higher.

I guess my point is that if you're going to design a power supply to support several different output tubes and considering that there are far fewer choices in primary impedances in single-ended OTs, you might want to consider choosing an output impedance first and manipulating B+ and screen voltages to get the closest match you can between tube and transformer impedance. Otherwise, you could work up a power supply for which there is no good match in OTs. Sorry I can't be more helpful. J
 

powdog

Junior Member
Joined
Oct 13, 2008
Messages
48
Reaction score
75
You're right, as usual. I have a Mercury Magnetics OT that has multiple 8 ohm taps (secondaries), each optimized for either 6L6's (5K), or 6V6's (8K). I'm not an Electrical Engineer, and I think I realize my limitations, so I'd better stick to what I know and stop trying to reverse-engineer that which has been designed by a far brighter mind than my own. As always, thanks for your input.

-PD
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Hi PD: thanks for the compliment but I've had more than my share of screw-ups ... didn't work, sounded crappy, or blew up in my face :oops: :( ... and I know what it feels like to run into my own technical limitations. But if you think you have a high-grade OT, my suggestion would be to stick with the better-worn path; discussed Dave Hunter's book and here a couple years back by our BBer K'apn; switchable preamp cathode bypass caps and some form of switchable negative feedback applied to any of the Gibson GA5 / Fender Champ / Silvertone 1471/1481 circuits - all 6V6 designs or adapt them to EL84.

If you want something with more meat on the bone, you might considering building a Gibson 6BM8-based GA1RVT. Probably a Seth Lover design and the only SE amp I know of to have both reverb and trem; and provides the features using only 3 tubes not counting the 5Y3. It can do this because both the 7199 and 6BM8 are two-fers; two different tubes in the same bottle. Both are triode/pentodes. The 7199 uses its pentode half as the first gain stage.

It's worth nothing that the EF86, a pentode and popularized in Vox amps is making a comeback with both scratch and commerical amp builders; our BBer Fungus Young has a Winfield built around the EF86 and divided by 13 markets an EF86 model. The 7199, while dated and a little pricey, provides both initial gain using it's pentode half and reverb drive using it's triode half. The 6BM8 - also a split tube - uses its front/triode half for the trem oscillator and its pentode half for output. The 6EU7 can/should be replaced with a 12AX7 for reverb recovery and PI/driver duty.

If you went this route, you could still mess with switchable bypass caps and negative feedback. Anyway, find a design that interests you and maybe add some bells and whistles that, for the sake of manufacturing economies, were never considered or just left out. Good luck! John
 

powdog

Junior Member
Joined
Oct 13, 2008
Messages
48
Reaction score
75
Hey CJ.

I feel your love of the triode/pentode tube. A few years ago I picked up half a dozen Hammond AO-44 reverb amps off of eBay. They run a pair of 6GW8/ECL86's. It wasn't till I acquired a Guild T-1, which also runs 6GW8's, that I pulled out the AO-44's and started messing with them. I must have built, torn down and rebuilt that amp 6 or 7 times before I was thrilled with the result. Replaced the Germanium transistor preamp with a 6AT7 (ala Fender Harvard), standard Volume/Tone controls, SS rectifier, and a switchable NFB loop. Probably 10 watts and the size of a small loaf of bread, but will drive a 4 x 12 cab beautifully (and everybody out of the room). I'll try and attach some pics.

I'll check out the GA1-RVT. I'm always on the lookout for my next favorite amp.

-PD

http://i452.photobucket.com/albums/qq24 ... ics152.jpg
http://i452.photobucket.com/albums/qq24 ... ics150.jpg
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Congratulations PD! what a fine-looking piece of work 8) Love that silky maple. The Hammond reverb amp must be a little like a 'Hoffman' amp retro-fitted to a GA18 in this LTG thread. I did some looking but didn't find any documentation on the Hoffman product and the folks on Doug Hoffman's BB and Doug himself didn't know anything about it. I'm sure there are other examples but I think the best-known application of the 7199 was/is in the Dynaco Stereo 70 probably the best-selling tube amp of any kind; was sold retail and in kit form. It showed up in Gibson's GA1RVT and for reverb drive/recovery in the tweed version of the GA19RVT later dropped in favor of a 6C4.

Anyway, congratulations again on your recovery/rebuild of the Hammond reverb amp; did the other ones get the same treatment w/ the fancy cabinet? John
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Hi PD; at the risk of over-cooking the subject, link to a closed eBay auction for a Gibson GA1RT These were sold mostly with tweed coverings; this one with the rarer/later 'tree bark' tolex and in VG condition w/ the mahogany footswitch. It's pretty rare; caught up in the period when Gibson was rushing stuff in and out of its product lineup.

Chassis view:
ga1rvt.jpg
 

powdog

Junior Member
Joined
Oct 13, 2008
Messages
48
Reaction score
75
Hey CJ. Sorry I didn't get back to you sooner, but I live in the sticks and only have access to high speed internet during the week.

I've attached links to pics of the other finished AO-44 amps. The small head cabinet is Mahogany, and the combo cab is Walnut and Spalted Birdseye Maple. I made the combo for a guy that has a small home studio where he writes and records his own music. He was looking for a small studio amp to jam and record with, but could also drive a large 2 x 12 or 4 x 12 cabinet. The 1/4 inch jack on the back of the amp is wired to bypass the internal 8" speaker when an external speaker cab is plugged in. Best of both worlds. He must have listened to 8 or 10 of my amps before he chose that one. Another guy in L.A. has one of the AO-44 head cabs, and he plays it through a 4 x 12 JBL cabinet. He swears that tonally it blows away his 100 watt Carvin tube head, and says it really pushes alot of air. Can't believe its only 10 watts.

You mentioned the comeback of the EF86. Have you ever used these in a combo? What a pain. I've built a couple AC-15's and 18 Watters with EF86 preamps, and they are noisy as hell. I've got a small arsenal of NOS EF86's that are okay in a head cabinet, but any vibration sets off the alarm. The RFT's seem to be the quietest of the bunch. But if you can get them isolated, they do sound fabulous.

Anyway, I'm still looking at the GA1. What kind of power/output tranny values should I be using with a single ended 6BM8? Thanks for the help. -Ethan

http://i452.photobucket.com/albums/qq24 ... iniamp.jpg
http://i452.photobucket.com/albums/qq24 ... ofront.jpg
http://i452.photobucket.com/albums/qq24 ... mbotop.jpg
http://i452.photobucket.com/albums/qq24 ... boback.jpg
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Hi PD; stunning work, absolutely stunning! The soul of simplicity, tone, and beauty. No, I haven't worked with the EF86; I'm not much of a scratch builder ... more of a tinkerer really. The boys over on Hoffman Amps BB are very strong technically and several of them are working on EF86-based projects. I don't follow the EF86 threads that closely but there's been mention of the noise/vibration problem including using O-rings as vibration dampers on the socket ears ... like the old dried out ones in tweed Gibson amps.

There are three Gibson / 6BM8 designs; GA1RT (remote reverb effect) w/ 285V on the plate, the GA1RVT (on-board reverb version pictured above) running at 265V, and the GA8T, a twin P/P 6BM8 design w/ tremolo Schematic here that has the voltage down to 255V. Weber's 022772 has both 300 and 330VAC secondaries. The 6BM8 data sheet indicates 5.6K load resistance. Weber has a W022905M with 7K primary impedance and Mojotone has this SE OT rated for use with a Gibson GA5, a SE 6V6 design but the data sheet doesn't specify primary impedance. and I don't really know how closely matched these two values have to be.

Anyway, I don't think the plate B+ is all that critical anyway; 280-300VDC should do just fine; hope this helps. John
 

powdog

Junior Member
Joined
Oct 13, 2008
Messages
48
Reaction score
75
On a personal level, I gotta agree that an 8" speaker isn't big enough, certainly if your jamming with a drummer and bass player. But from what I've been told, they're perfect for studio work. Easy to mike, you can drive the amp to meltdown without driving everyone out of the studio, and you can record that 8" speaker to sound like a much bigger amp on tape.

-PD
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Well ... the transformers in the vid make the suggested Weber model look like a doorbell step-down transformer. :shock: There might be one out there but I've never seen a 6BM8 design, SE or P/P, operating at over 300V. Fender ran early 6V6 amps at voltages way over those recommended in the older RCA manual / tube data sheets. It's also true that many/most/all 6L6 and EL34 designs operate way over 400V. 8" not so good for stand-alone work but many a Champ has been used for demo'ing and recording with 8" speakers. The tone in the vid is pretty cool but I wonder if it isn't more a function of the amp's iron than the speakers?
 

Default

Super Moderator
Platinum Supporting
Joined
Jul 30, 2007
Messages
13,596
Reaction score
3,008
Location
Philly, or thereabouts
Guild Total
11
I was banging through Taco when I was waiting for the exterminator to show up a couple of weeks back and I was surprised at how early the alnico 8 was breaking up. It would probably be a good idea to install a speaker jack so I could try different speakers. It's not like I'm going to kill the collector value of an unknown Mexican practice amp. :roll:
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Default said:
.... It's not like I'm going to kill the collector value of an unknown Mexican practice amp...
Yes .... but how can you place a value on an undiscovered oil well ... ?
 
Top