Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => PC Express and JSchem - Schematics and Layout diagrams => Topic started by: tubenit on October 26, 2011, 06:53:40 am
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DummyLoad has been kind enough to help me with this thread. Many thanks to his efforts and schematics.
Note below there is a "cheat sheet" listing alot of information on these tubes in one place.
I view the Hoffman Amp forum as one of the most innovative forums on amp design & certainly my 1st "go to" forum for ideas.
In light of that, I was interested in trying to create a good reference for low watt push/pull (not single ended) amps.
The guidelines is that it has be "around" 9 watts or less and not draw more than 90ma current.
I also am not interested in exploring push/pull triodes in this thread at all (no 12AU7 designs for example). There are other forums that cover that idea.
PLEASE make any comments or suggestions that relate to these specific tubes. IF you have built a push/pull amp design around one of these tubes please share it with us. I have tried to list one GIF version schematic for each tube amp design for those who do not have ExpressSCH.
I will leave this thread up for about a week in Tweaks and then move it to the ARCHIVES to preserve it.
With respect, Tubenit
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EL91/6AM5 amp (experimental design not built)
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6AU6 amp
This is a design built by Zambo. I also included some experiment (not built ideas)
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6BM8 amps
Little Wing DaGeezer
56T Tubenit
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I have NOT built this exact design but I did use a 5Y3GT in my Tweed Overdrive with 6K6's and it sounded GREAT!
This design allows for a lower voltage PT.
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I am currently in the process of building this amp, so this schematic and layout will probably change.
The other design is by Darryl who has built many of these.
EDIT: This tube seems to be remarkably mid and bass oriented for a small tube. I found myself continuing to add more treble
to the design and then adding "smoothing caps" to keep the tone & OD smooth sounding.
EDIT 12-24-11 I have worked on tweaking this amp for a month. It sounds fantastic! NO oscillation issues at all. I can get a clean or an OD tone by changing pot settings. The new "boost" works better than the original PAB I had.
This is a really sweet amp with a nice rich tone. The bass on this small amp sounds great. It does NOT sound small!
EDIT 3-2001 I changed the 5879 V1 in the original SoLow Watt to a 12AV7. It has a MUCH smoother, clearer, more transparent OD with sweeter harmonics. Sustain is still fantastic. The 12AV7 is a better choice for playing rhythm and lead both. In light of that, I recommend building this with the 12A_7 wiring for V1. You can use a 5751, 12AY7 or 12V7 (which I like).
I also have added another idea for a Tweed BluezMeister ECL84. I probably would use a 250-0-250 PT with a 5Y3GT rectifier. I would probably use a 12A_7 for the LTPI instead of the ECL84 triodes. That way I can control the gain to the ECL84 pentodes. The VVR would be optional using a 250-0-250 PT. I'd probably leave it out.
With respect, Tubenit
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Another experimental design that should use about 50ma in push pull.
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Design by DummyLoad Thanks!
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DL drew up this 7 pin HoSo56 type amp. Attached is the schematic AND a list of interchangeable tubes. However, note the cathode resistor value and other values may need to be difference even though the pin out is the same.
with respect, Tubenit
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yet another - based on 6F6 metal or GT. A1 P-P should make ~~8-9W @ ~270V B+.
--DL
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Another suggestion was this one.
would try to design an EF80/6BX6 output stage (that would be pin compatible with EF183 EF184 EF85) but i really don't know how. i can design a basic triode preamp stage by now but no poweramp yet.
why? you'll be much happier with a low power output pentode. if you must, with the screens at 200V and with 25K Ra-a; use 180ohm Rk for the pair. bias should be around 3V.
Tubenit
EDIT posted on TAG and reportedly a proven design that has been built and played.
Jumpered out the first 4.7k resistor in the power supply and I have around 230V on the plates of the output tubes. Also the cathode resistor for them is somewhere around 360 ohms I think. Used a 70V line transformer for the OT. A 120V - 120V to 6V - 6V transformer with one of the 120V transformer windings for the HV supply.
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Somebody gave me a couple ECL85s. Would these be usable for a low watt push pull amp like the above
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They look pretty similar to the ECL84 ........ BUT the max voltage rating is only 250v instead of 300v. So, you'd need
to factor that into the build. I really liking the tone of the ECL84 tube.
With respect, Tubenit
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OK, here is another ECL84 idea ........ BUT you could substitute almost any of the small pentode tubes for this.
The "CPI" stands for cathodyne phase invertor.
With respect, Tubenit
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Hello all,
I know this is an old thread, but this seems a good place for this.
I've been looking at projects I have parts lying about for and I got interested in Tubenit's TBM 6K6. Has anyone built this version?
I'm seeing 3 footswitch relays, but a slightly different configuration on this. I'm thinking a footswitch for the OD and toggles for the boosts, but if someone's built this, maybe they can chime in.
I used the original Geezer board layout and re-drew it for the 6K6 values.
Another pair or two of eyes would be appreciated - please let me know if you see any mistakes.
Tubenit or others admins, if this is not the place for this, please let me know.
Schem and layout:
Thanks in advance,
mike
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I think I'd do one relay for clean/OD and then do spdt toggles as you suggested.
IF you don't have the headroom that you desire, try V1 as 12AY7, V2 as 5751 and V3 as a 12AT7.
You could try a 5V4 rectifier for 300v on the plates using a PT with 250-0-250.
Please keep us posted how the project is going. I played a TBM with 6K6's and liked it.
I liked the HoSo56 with 6BM8's somewhat better.
With respect, Tubenit
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Hey Tubenit,
Thanks for the reply. I've got the PT for this and the 6k6s so..
I liked the HoSo56 with 6BM8's somewhat better.
Which one is that? The Little Wing or the 56T? Is it on this thread? And what is HoSo? I'm sure it means something...
Thanks for the suggestions on the tubes, I think I've got everything but that 5V4.
mike
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Hey Tubenit and all,
I still didn't find that design, but I'm doing Sluckey's lil' maggie too, so 6GW8's not the same but similar-ish. I have most of the parts I need for this too. (at least the expensive bits).
I'm going to give the TBM 6k6 a go, start collecting up what I don't have, etc.
Any recommendations for the OT?
Thanks in advance for your help,
mike
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Found/fixed error in layout near OD Level/cut circuit.
Updated layout attached.
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Added PPMIV to my earlier schematic/layout.
Found/fixed resistor value on row 21.
Think this is good. If anyone is bored/ambitious/ocd - can you give it a look?
I stole the PPMIV values/idea from Tubenit's Mini-TOS 6K6 version. (Thanks Tubenit!)
mike
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Hey everyone (Tubenit?),
Can anyone recommend a OT for this? I have an SE 5.1:8ohm laying about, is that usable? (I'm leaning no here)
I've read a Marshall 18 watt style for the full TBM, should I go with that or a standard p/p Fender style 8/8.5:8 ohm (like a PR). Or, being that this is a lower watt version, something else perhaps?
thanks in advance everyone.
mike
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You can't use a SE transformer in a P/P circuit.
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Hey Sluckey,
Yeah, I didn't think so. I'm making assumptions too. I was given a chassis with Amethyst PT and OT (never heard of 'em). Looked like a SE design, but the part #s are an OEM thing so specs were not to be found. I sent a couple emails to them in the UK only to find they were a custom run for someone. I've had two different people drop me chassis half full of stuff over the last few months. The second one was a DR build, PT/choke, bunch of stuff. Been fun seeing what I can do with 'em.
While I got ya, what do you think for the OT?
m.
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Here's my tried and tested all small signal pentode low watt push pull amp.
I had a bunch of old Mullard EF80 and EF184 valves from the '50s and 60s in the junk box (the EF184 was the successor to the EF184) so I decided to try them as triode strapped for the preamp/PI and as pentodes for the pp output pair.
I rolled my own small transformers, used the guts from a 20VA 6.3v filament supply transformer for the power and a 100v audio line transformer for the 50:1 OT.
The 6.3v filament transformer had 2 x 10VA secondaries, I unwound one and added a HV secondary.
The transformers are available for about £10 for the PT and £5 for the 100v line transformer, both from UK suppliers.
https://soundcloud.com/dougie-tamson/albatros-ef80-push-pull-amp
https://soundcloud.com/dougie-tamson/wcm-ef80-pp
https://soundcloud.com/dougie-tamson/ef80-pp-5-n-dime-ot
Doug.
Edit, just checked the coupling caps in the 'as built' amp, I used 22nF instead of the the 0.1uF as per the schematic.
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Hmm, forgot the bypass caps in the preamp, also want to try V2 in pentode mode, should give a big overdrive/boost and try to make it switchable...