Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on August 10, 2025, 09:24:57 pm
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Hi Guys,
Got some of the above valves and got a neat little circuit to use them in.
Not a lot of info for PP circuits for these guys.
Plate voltage around 315v
Cathodyne phase inverter
Circuit similar to the "Tweedle Dee"
Any input would be great
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Check out the schematic on this post Timbo
https://el34world.com/Forum/index.php?topic=23039.msg252936#msg252936
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https://frank.pocnet.net/sheets/127/6/6EM5.pdf (https://frank.pocnet.net/sheets/127/6/6EM5.pdf)
Zout = 315/(10/315) = 9,922.5R
Should be able to get away with 8k plate-to-plate load at around 300Vish for PP
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https://frank.pocnet.net/sheets/084/6/6M5.pdf
https://frank.pocnet.net/sheets/049/6/6EM5.pdf
Kinda looks like a lower output EL84.
Similar to a 6K6, compared to a 6V6.
(Edit: Tubeswell said basically the same thing as I was typing.)
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Thanks guys, 8K OT is doable
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Thanks guys
(https://i.imgur.com/Nv87Jhi.jpg?2)
I was thinking of having a valve rectifier (EZ81) and series it with a resistor to run it at 12v
Just can find the calculator that will give me the resistor value
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Easier way to get power supply sag is with a 10-20w 50R wire wound resistor in series with the FW Bridge and the reservoir cap.
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cool
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Seems much like 6AQ5 rated for vertical sweep duty; 9 pin package allows higher B+/G2 V ratings. OT 8K-10K depending on B+/bias method.
--Pete
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Hey DL,
40W 100V Line PA EI Core Audio Transformer - Altronics (https://www.altronics.com.au/p/m1130-40w-100v-line-pa-ei-core-transformer/)
These work well for a 8K primary
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Ciao TIMBO (you have a PM)
Explanation/Documentation on the .pdf file
(https://i.imgur.com/CN05UXP.jpeg)
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http://diyaudioprojects.com/Technical/Ohms-Law/ (http://diyaudioprojects.com/Technical/Ohms-Law/)
https://sengpielaudio.com/calculator-ohm.htm (https://sengpielaudio.com/calculator-ohm.htm)
Franco
p.s.: read the PM
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MAN that is mint
Thank you
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With back-to-back transformers you will find the B+ under load is much lower than 315V, maybe around 250V - they naturally sag a lot.
Also the heaters in your schem seem to be missing a connection.
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Looks like a fun project - I'm interested in what it ends up sounding like. Looking at the circuit, I was thinking that if you don't really need two channels, you could mod it to use one 12AX7 triode for a bias-wiggle tremolo like the 6G2 circuit.
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With back-to-back transformers you will find the B+ under load is much lower than 315V, maybe around 250V - they naturally sag a lot.
Also the heaters in your schem seem to be missing a connection.
Thanks mate
Just spotted it
The PT's have multiple secondary taps and can be adjusted to increase the overall voltage
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To me the secret Is to use over rated current Transformers, not to overvolt the input of the second Transformer
EDIT: Guessing a bit about the Transformers spec I'll use for T1 a 100VA unit and for T2 an 80VA unit
This will result in a 310V B+ wich will SAG only a bit (or so seems to be to me with my math)
Franco
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Hey guys, This one is gathering some interest, so I was able to get some shed time on the weekend.
(https://i.imgur.com/zp8Gqz4.jpg)
(https://i.imgur.com/K1WPoeW.jpg)
(https://i.imgur.com/6E16B4p.jpg)
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How’s the build coming along timbo? I’ve got a few 6EM5 pentodes I’d like to build a low power plexi or something similar with, keen to see how this project pans out for you!
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Hey mate, progress is slow, very busy at work atm
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Got some shed time...
(https://i.imgur.com/gci2v6Y.jpg)
Put the valves in to see where the heater voltage was sitting.
12.6v on the dot, Perfect
Circuit checked and the board is ready to graft in
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ey guys,
This one has fired up well :thumbup:
An adjustment to the power supply to bump up the HT at the rectifier was a juggle of plate voltage and plate dissipation.
Changing the connection between the Main power transformer(15V tap) and the HT transformer (now 12.5v)
has got the voltages in a good place
Maximin plate voltage 315v (256v) plate Diss 10w (8.1W)
Sounding good
NEXT
So what am I supposed to do with the 10k trimpot on the tail of the PI :scratch
I would assume it's to balance the signal on either side of the triode and done with a CRO
(https://i.imgur.com/Ra9yrJv.jpg)
(https://i.imgur.com/ioGkDKQ.jpg)
Apologies for the crap photo's, printer ran out of ink
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Cab made
(https://i.imgur.com/nmWShui.jpg)
(https://i.imgur.com/UUoMb0p.jpg)
(https://i.imgur.com/FHmqCpu.jpg)
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Ciao TIMBO
Very COOL CAB
So what am I supposed to do with the 10k trimpot on the tail of the PI :scratch
I would assume it's to balance the signal on either side of the triode and done with a CRO
Give a look to the attached .pdf, a guy at DIYItalia some years ago has build the Soma-84 (Kevin O'Connor The Ultimate Tone 5)
The purpose of the 1M pot at the cathode of the PI (called on the project Body) is to achieve a progressive transition between PP and SE output
Not that in your schematic that is the purpose of the trimmer but you can give a try with a little mod
(https://i.imgur.com/ve0BxPm.jpeg)
(https://i.imgur.com/KaOVOze.jpeg)
Franco
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Hey guys, Just when you think you are on the home stretch
(https://i.imgur.com/qcapeMk.jpg)
:sad2: :l2:
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Ciao
Franco
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Hey guys, Just when you think you are on the home stretch
(https://i.imgur.com/qcapeMk.jpg)
:sad2: :l2:
Lucky they make holes in speaker baskets nowadays…
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Hey guys, Just when you think you are on the home stretch
(https://i.imgur.com/qcapeMk.jpg)
:sad2: :l2:
Lucky they make holes in speaker baskets nowadays…
The problem is the valve hits the basket
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Hi Timbo,
V3 probably needs to do a 20mm Offset towards centre of chassis?? and a real pain to do :cussing:
Silly question (and way toooo late) How come you didn't do the 4 sockets "in line" along chassis.
You may have got away with it. :laugh:
Regards
Mirek
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... V3 probably needs to do a 20mm Offset towards centre of chassis?? and a real pain to do ...
Aluminium sheet on the top of the chassis (small if you don't care, large as the chassis top if you prefer) and new holes ?
(may be a little shield between Tube and Transformer too ?)
Franco
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Be easier to shunt the speaker 1/2” sideways methinks. Won’t line up with the baffle hole that well but who will know?
(Otherwise make a new baffle)
Might need to cut a notch in that bottom baffle rail…
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Hey guys, I liked all of those suggestions.
So I going the make a new cab.
:icon_biggrin:
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I once mounted the speaker on an angle in an early champ build.
Surprisingly the 6" speaker was too large to be mounted flush
I further justified the unusual mount thinking the speaker would be pointed more to the player if the amp was on the floor. Either way it sounds good with the "bigger" speaker.
https://stillampd.com/emery-champ-01
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Ciao scstill
https://stillampd.com/emery-champ-01 (https://stillampd.com/emery-champ-01)
VERY VERY COOL :thumbsup:
Franco
p.s.: I like your advanced 3D approach to the project :grin:
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Angle grinder.