Southern summer SE fun build project idea

[tubeswell]

I have a PT with 240-0-240 100mA and 6.3V 2A and a fairly chunky 25Wish SE gapped OT 8k Pr with 4, 8 and 16R secondaries, plus the rest of the bits I need to get this rolling.

I've fancied a parallel SE amp for some time and also like the idea of an EF86 with Merlin's triode/pentode morph control. I also dig trem and have a spare triode and a pile of IRF820s so...

Here is the 1st cut at the schematic. Any suggestions/comments/criticism feedback is welcomed please. (The voltages are just guesses)

I am particularly interested in whether anyone here has any reckoning about adding NFB (none at present on the schematic), and also the relative sizes of the trem depth pot and the output stage grid load resistor (highlighted in Blue) - I was wondering whether a 100k pot and a 180k grid resistor would be more appropriate, or maybe a 100k pot and a 220k resistor?

TIA

[loudthud]

The B+ seems to be kind of low for that OT. How much power do you think this amp will produce? The morf control looks interesting, what's it supposed to do?

[tubeswell]

The PT is fairly large for the ratings given and is wound by a local guy who's PTs are pretty conservatively rated. I have used a similar one in a stand-alone reverb and got 360 on the 6V6 plate. But I have a few other PTs in my stash that I could chuck at it if I don't like it. Similarly with the OT.

The morph control is out of Chapter 3 of Merlin's 1st book in the section on 'Switchable and variable pentode designs'. I guess its like a 'texture' control but I aim to find out. According to Merlin's theory, with the pot turned CW the screen bypass cap is connected directly to ground like a normal pentode, and with it turned ACW the screen bypass cap and the other cap are in series for triode connection of the screen to the plate (to AC). In between you are supposed to get anything from screen compression to ultra linear operation at intermediate settings. This intrigues me.

[Wombaticus]

The PT is fairly large for the ratings given and is wound by a local guy who's PTs are pretty conservatively rated. I have used a similar one in a stand-alone reverb and got 360 on the 6V6 plate. But I have a few other PTs in my stash that I could chuck at it if I don't like it. Similarly with the OT.

The morph control is out of Chapter 3 of Merlin's 1st book in the section on 'Switchable and variable pentode designs'. I guess its like a 'texture' control but I aim to find out. According to Merlin's theory, with the pot turned CW the screen bypass cap is connected directly to ground like a normal pentode, and with it turned ACW the screen bypass cap and the other cap are in series for triode connection of the screen to the plate (to AC). In between you are supposed to get anything from screen compression to ultra linear operation at intermediate settings. This intrigues me.

Looks fun! I did a much more complixified version of Merlin's morph control for ECL86 pentode output tubes in a PP stage (required a couple of MOSFETS and an expensive 2W dual pot). It was a neat and worthwhile experiment, but in practice the knob ended up staying parked near the full pentode end. But I would expect very different results with the morph on the preamp, and very much look forward to hearing how it turns out for you. You'll surely find that it acts somewhat as a volume control as well.

As for NFB, I doubt it will be needed or wanted. But in parallel SE amps I've done, I've injected it into the cathode of the driving triode. It's fairly straightforward to implement a presence control as well (I do it with a pot with an SPST switch so the NFB can be removed entirely for a "rude" mode). The inevitable cost would be the loss of the bypass cap, but it may not be missed anyway..Just eyeballing it without crunching any numbers at all, I'm guessing you may find it a bit bassy as drawn, and with the pentode you should have adequate gain on tap even without bypassing that stage.

[tubeswell]

FWIW I finished the chassis yesterday - reported here: http://music-electronics-forum.com/t23443/

[mooreamps]

The B+ seems to be kind of low for that OT. How much power do you think this amp will produce? The morf control looks interesting, what's it supposed to do?

Something about sliding the EF-86 between pentode mode and triode mode... I'm not so sure what effects on the sound that would really give you, but I'm not a guitar player, so I can not comment on that.. Electrically, it should vary the output impedance of the tube, so maybe some sort of limited range preamp gain control..


-g

[tubeswell]

Well FWIW it is up and running now (except for the trem which needs a rethink and is disconnected). Story here: http://music-electronics-forum.com/t23443/ . There is a substantial increase in gain with the morph contol, and the way it is set up in the amp, it acts to transfer the amp from what is a 5F2A-like amp in triode mode (I built it incorporating a 5F2A tone stack in the end), into a howling beast in pentode mode. I haven't fully explored the morph control at all settings with vol and tone pots. I will get to that later.

Overall initial impression, is I think I would definitely do this again in a PP amp like a tweed deluxe (although the SE amp has its own primitive mojo)

[tubeswell]

New Trem schematic concept idea

Okay, what I am thinking now is to aim to drive the output tube grid into cut-off by making the trem swing into really -ve voltage. (The PT has a 45VAC bias tap). I don't know if this will work, but please bare with me, and please please do chime in if you know how it could be done a better way etc.

Anyway the theory is that at present the grid sits at 0V (being a cathode-biased output tube'n'all), and I think that if I design the trem to swing only in the negative-voltage range, with a concomittent -ve bias adjustment on a dual-ganged pot (that is the same pot as the trem depth control), that I could sort of match the amount of -ve voltage required at the trem voltage swing mid-point to be set relative to the trem depth, so that the trem's most-positive swing was always at (or near enough to) 0V, and the output tube was driven into really cold bias, so that the trem doesn't adversely affect the normal biasing of the output stage (as it otherwise appears to be doing after I bench-tested a prototype of the previously dreamed-up incarnation). Please refer to the concept schematic* attached. TIA

* in the concept schematic the light-grey shaded line is meant to represent the dual-ganged pot, which in theory when turned CW both increases the trem depth and increases the -ve voltage of the trem swing mid-point)