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Newbie: Hi there and a ECL86 question
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Old 12th February 2019, 11:37 PM   #21
Galu is offline Galu  Scotland
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Here's an interesting thread which includes comparisons between 82s and 86s:

PCL86 - Worth a try?
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Old 13th February 2019, 07:59 AM   #22
snaarman is offline snaarman  United Kingdom
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Thanks for that.

I haven't checked but I guess the ECL82 has the same pinout as the ECL86 - so I could throw this idea together with a cheap 82 and still trade up to the 86 if it shows signs of promise.

I do have a second idea based on the same circuit. Instead of a small amp triode/pentode/OP transformer/speaker - I was considering triode/pentode/OP transformer/load as an overdrive pedal. And possibly still using the risky positive feedback idea... Watch this space :-)

Pete
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Old 13th February 2019, 08:46 AM   #23
tristanc is offline tristanc  United Kingdom
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Newbie: Hi there and a ECL86 question
Quote:
Originally Posted by snaarman View Post
I haven't checked but I guess the ECL82 has the same pinout as the ECL86 - so I could throw this idea together with a cheap 82 and still trade up to the 86 if it shows signs of promise.
They don't have the same pinouts... I've just built a SE PCL82 "hifi" amp - partly due to the transformer I had to hand. There seem to be plenty available and so far I like the sound. I had some PCBs made up which I guess could be used for guitar easily enough. Merlin also had some on his web store, but I think they're sold out...
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Old 13th February 2019, 09:15 AM   #24
snaarman is offline snaarman  United Kingdom
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Hmm. Not the same pinout. That's annoying. Well, I ordered an ECL82, some caps and tagstrip, so the game is afoot.

Mental note, buy card and chocolates - because Valentines day tomorrow. I might need to grovel a bit before all these parts for "yet another amplifier" arrrive :-)
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Old 13th February 2019, 11:24 AM   #25
klingo is offline klingo
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Hi,

a single ECL for guitar is a bit quiet the 82 more than the 86. For positive feedback you can try a link tying both (unbypassed) cathode with resistor/cap play with them to stay in stable area. I once tried PF from the cathode follower to the preceeding cathode stage turning the pot until it SQWEEEEEEE.....Finally i delete it because the thing was not switchable without loud pops.

Have you some germanium transistors left? you could built a treble booster like the range-master or better (to me) this one: npnboostv2 which can be adapted (bias & polarity) for pnp germanium as well.
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Old 13th February 2019, 11:32 PM   #26
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by snaarman View Post
Well, I ordered an ECL82, some caps and tagstrip, so the game is afoot.
I have a hunch you may not have enough voltage gain, unless you have high-output humbucker pickups and plan to play only rhythm guitar with 100% clean tone.

One possibility to consider: add a MOSFET input stage ahead of the ECL82. Something like the little LND150 ( LND150 - Depletion Mode MOSFETs ) might do the trick, wired as a common source gain stage, maybe with an unbypassed source resistor to set the voltage gain to where you need it.

I have seen a variant of this idea that uses a JFET / MOSFET totem-pole (cascode). The JFET is on the "bottom" and provides low input noise and a little quadratic nonlinearity for "valvey" warmth. The MOSFET is the upper device, converts the JFET output current to an output voltage, and isolates the JFET from the several-hundred-volt power supply.

I encountered the "positive feedback inside negative feedback" thing many years ago, in an op-amp circuit that synthesizes a negative output resistance. I built and measured it, and it was well behaved and quite stable. For years I thought I had invented both the circuit and the concept, but no, it turns out somebody got there long before me and spoiled my fun.
Quote:
Originally Posted by snaarman View Post
Mental note, buy card and chocolates - because Valentines day tomorrow.
Thanks for the reminder! We were snowed in for about four days, so I didn't get my Valentine's Day shopping done. I may have to stop at a few shops on my way back home tonight!


-Gnobuddy
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Old 14th February 2019, 07:06 AM   #27
snaarman is offline snaarman  United Kingdom
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Thanks for those ideas.. I am kinda expecting this one valve amp won't have enough gain. Otherwise the world would be full of them. So maybe positive feedback will just add enough to save the day? I'm planning that the volume control adds the PFB as you turn it to the loud end. I'm also thinking of back to back diodes on the PFB loop to create some distortion if needed.

Cascade FETs are a neat idea, particularly if you operate them from conventional valve HT. I admit I have looked at cathode bias DC voltage in the past and wondered if I could use that as a form of LT for a transistor stage, such as effect loop or reverb driver. I'm much more at home with silicon - and I once designed a 200w bass amp. But for these amps I have taken a "no transistors" vow
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Old 14th February 2019, 09:49 AM   #28
klingo is offline klingo
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You may find some ideas for positive feedback in this book /page65 http://www.tubebooks.org/Books/Atwoo...t%20Design.pdf
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Old 14th February 2019, 09:59 AM   #29
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by snaarman View Post
Mental note, buy card and chocolates - because Valentines day tomorrow. I might need to grovel a bit before all these parts for "yet another amplifier" arrrive :-)
I prefer watching Peter Weir's Picknick at Hanging Rock today !
Best regards!
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Old 14th February 2019, 10:54 AM   #30
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Sorry for the OT, buy I had sent some stuff to your email, and a PM, but it appears that I can't reach you...
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