Dropper Resistor on a 67?

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neikeel
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Re: Dropper Resistor on a 67?

Post by neikeel » Thu Dec 14, 2017 5:42 pm

I have found RFTs pretty good.
Save my Mullards for 500v plus
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Re: Dropper Resistor on a 67?

Post by Roe » Wed Dec 20, 2017 4:51 am

bill bokey wrote:
Mon Dec 11, 2017 3:29 pm
For bass preamp I prefer 500V to 465V. The amp will be punchier and cleaner. I find 465V works best with a lead circuit.
Agreed! I like the one 8k2 with the bass/jtm preamp
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Re: Dropper Resistor on a 67?

Post by Roe » Wed Dec 20, 2017 4:54 am

neikeel wrote:
Thu Dec 14, 2017 5:42 pm
I have found RFTs pretty good.
Save my Mullards for 500v plus
RFTs are excellent. I got an amp with 495v, never had a problem. The sag is important though. One option is to add a small flying screen resistor - 130R or 220R for instance
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Re: Dropper Resistor on a 67?

Post by revolver1 » Sun Dec 24, 2017 4:44 am

Any recomendations on which RFT to look out for?

Any years better than others?

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Re: Dropper Resistor on a 67?

Post by Roe » Wed Dec 27, 2017 12:26 pm

not the last ones from the late 80s
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Re: Dropper Resistor on a 67?

Post by revolver1 » Thu Jan 04, 2018 3:21 am

8) Do you guys have any recommendations for filtering?

I was going to go with dual 16uf pre, 2x dual 32uf screens, dual 32uf PI and 2x dual 32uf mains.

I'm kinda wondering would I be better with 2x dual 50uf for mains....

Any thoughts?

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Re: Dropper Resistor on a 67?

Post by neikeel » Thu Jan 04, 2018 5:16 am

I would go 32/32 (with 10k dropper between) in preamp
32+32 in parallel for PI
x2 dual 32 for mains (so 64+64 in series for 32uF)
the screens I would experiment as you may find 32+32 in series for 16uF feels nice and greasy to play you may ghost lots
Dont forget the choke. If you go that low then 8-10H rated at 250mA and DCR of around 100ohms will be needed (IMO).
I am sure Roe will have opinions on this :)
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Re: Dropper Resistor on a 67?

Post by Roe » Thu Jan 04, 2018 6:54 am

I like 3h chokes, even with 16uf on the screens. But it does ghost if you crank the amp with 33uf on the mains. 50uf on the mains reduces ghosting greatly, even with 16uf on the screens. But this presupposes excellent grounding, a balanced driver and good filter caps. Removing some deep bass may also help.
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Re: Dropper Resistor on a 67?

Post by revolver1 » Thu Jan 04, 2018 1:45 pm

Ok verry 8). Looks like a bit of experimenting will be in order.

I have a 3h choke coming but the ghosting issue is why I thought of maybe going 50/50 on the mains. I haven't tried 32/32 so I'm keen to try if anything to make it different from the others.

I have 10k droppers on the pre, so far like you I think I prefer the dual 32uf but dual 16uf has something about it I like and again to make it different from the Super lead and 1987.

Guess I'll have to experiment with the pre, screens and mains. Bring it on.... can't wait.

Now if I could just get my hands on these trannies......

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Re: Dropper Resistor on a 67?

Post by revolver1 » Sun Jan 07, 2018 6:29 am

I've been thinking long and hard on the filtering advise. I sure do like greasy!

I did want to try 2x dual 32's on the mains but with the 3h choke I think it's going to be ghost city.

I've ordered another dual 16uf, so I'm going to pull the dual 50uf I have on the pre of my 2203 and replace it with a dual 32. So now I will go with 2x dual 50uf for mains, 2x dual 16uf screens, dual 32 PI and dual 32uf pre. Should do the trick.

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Re: Dropper Resistor on a 67?

Post by revolver1 » Wed Jan 10, 2018 1:47 am

Another question. I keep reading that most are not so keen on the 220k mixers. I think Shakti wrote they have more snap but I'm not too sure what that means?

Is it worth trying? For an idea of tone who would have had an amp with these? Or should I just go with 470k?

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Re: Dropper Resistor on a 67?

Post by neikeel » Wed Jan 10, 2018 6:52 am

If you use 270k mixers ideally use 560pF Lemco type cap, they are slightly cleaner, obviously depending on where the volume pots are. If you want grind and crunch or using 470pF or 500pF caps then use the red tubular ceramics and 470k mixers. I tried both in my JTM50 block end clone and the original with 270k sounded and felt different (new trannies and filter caps) even with the same tubes. Switching the clone to 470k got it closer to the original (why as the 270k were Iskras and had not drifted up).
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Re: Dropper Resistor on a 67?

Post by revolver1 » Thu Jan 11, 2018 2:11 am

Yes, sorry 270k not 220k. Excellent info, thanks!
Looking like there are a few bits that will require experimenting with.

Definitely want to go with the high voltage tap, so every thing I know is out the window. There's definitely some experimentation required here with this one.

Really want to try not to build a bunch of amps that all sound and feel the same. But I suppose this can be tricky with a pre conceived sound in your head.

Think I'm going to try and shoot for head room and punch, more classic Cream or free than loads of grind.

I picture that if I do the high voltage West Coast mod this amp will be somewhere in between that and the 1987 and Superlead.

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Re: Dropper Resistor on a 67?

Post by Roe » Thu Jan 11, 2018 9:20 am

these amps ghost less with cold bias, something that works with higher voltages.
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Re: Dropper Resistor on a 67?

Post by revolver1 » Thu Jan 11, 2018 12:58 pm

Cheers Roe that's good news. I only have experience with the 69 Superlead but with that I definitely prefer a colder bias. I guess it's easier on the tubes too :wink:

I was just reading over that evolution of the plexi article and was pleasantly 're assured I'm headed in the right direction. I do think I need to swap out those 270k mixers for 470k though....

Hopefully next weekend I will get the chassis together as far as I can and I'll start another thread and get some pics up.

The material for the board looks really nice I think.
Very excited about this build.

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