Dropper Resistor on a 67?

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revolver1
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Dropper Resistor on a 67?

Post by revolver1 » Mon Dec 11, 2017 3:59 am

I wanted to build a Superbass but the more i read im leaning toward trying the 45/100 circuit.

I wanted to go with EL34 rather than KT66 which is why I was leaning toward a Superbass circuit but now I realise what I've really built is a 67.

I'm using a C1998 PT and a T2562 OT, the PT has 465v or 500v taps but I'll probably go with 465v for the sake of the tubes.
Edit, sorry, C1998 O.T and T2562 P.T.

So my question is this, is it safe to use just one 10k dropper resistor? Or am I better to pair it with an 8.2k?

Is it a case of seeing what voltage I get at the plates?

If so, what voltage would you say is too much?
Last edited by revolver1 on Fri Jan 19, 2018 1:35 am, edited 1 time in total.

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Re: Dropper Resistor on a 67?

Post by neikeel » Mon Dec 11, 2017 1:36 pm

Your output tube anode voltages come straight off the first filter cap, no dropper and the screens come after the choke (whatever DCR that has), but typically 100ohms for a C1999 or 354-114, so little drop. The dropper you refer to, I presume is the one between screens and PI.
If you want a bold cleaner preamp (45/100 or early JTM) then go 8k2 on its own, be prepared to tweak.
Neil

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Re: Dropper Resistor on a 67?

Post by bill bokey » 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.

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Re: Dropper Resistor on a 67?

Post by revolver1 » Tue Dec 12, 2017 2:41 am

Thanks for the reply' s. I had understood that more resistance applied at the dropper will brown out the tone and conversely less resistance will give more head room.

Thanks for the tec explanation neikeel, I hadn't understood that before.

I have a 10k wired in there now with the next turret linked out so I can try it and if I don't like it I can always slip an 8.2k in there as well.

I'm not entirely sure what I'm shooting for with this one but I guess I'm looking for something different than I have with the others. So more head room would be interesting. I'm going to give the 270k mixers a go.

I guess I'm thinking more Cream, Hendrix, Free vibe.

I am tempted to go with the 500v tap but won't that just eat output valves?

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Re: Dropper Resistor on a 67?

Post by revolver1 » Tue Dec 12, 2017 3:00 am

Guess I'm a little paranoid, I had a reissue Plexi back in the 90's. Maybe it was bad tubes but either way it would blow an output tube and then take out the transformer. Think I blew three in total.

Not the best for an impoverished young musician...

I just want to be sure I'm not going to build an amp that is asking for trouble. That's one of the reasons I didn't choose to do an all out 45/100.

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Re: Dropper Resistor on a 67?

Post by revolver1 » Tue Dec 12, 2017 3:00 am

Guess I'm a little paranoid, I had a reissue Plexi back in the 90's. Maybe it was bad tubes but either way it would blow an output tube and then take out the transformer. Think I blew three in total.

Not the best for an impoverished young musician...

I just want to be sure I'm not going to build an amp that is asking for trouble. That's one of the reasons I didn't choose to do an all out 45/100.

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Re: Dropper Resistor on a 67?

Post by bill bokey » Tue Dec 12, 2017 9:08 am

EL34s are fine @ 500V. If you want more head room yo should really go with 500V B+.

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Re: Dropper Resistor on a 67?

Post by neikeel » Tue Dec 12, 2017 6:32 pm

I agree with Bill, go with the 500v taps on your PT and a single dropper (8k2 and 10k very similar) when you have done try posting your voltage chart.

If you are worried about tubes it is usually the screens that suffer so try 2k screen resistors or 2k5 screen resistors to drop the voltage a tad. There are other ways too (dual rail voltages like you see in the KT88 hifi and Bass amps but that is a bit of a detour for you!
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Re: Dropper Resistor on a 67?

Post by bill bokey » Wed Dec 13, 2017 2:04 am

Neil is right, I also use 2k2 screen resistors very often. It lowers the screen voltage and I find it compresses a bit more.

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Re: Dropper Resistor on a 67?

Post by revolver1 » Wed Dec 13, 2017 2:39 am

Very cool. 8) I do have some 10watt 2k2 wire wound but I was kinda saving them for a West Coast mod project and a high voltage KT88 but they are a little way off yet.

I had planned to go NOS output valves which I'm sure will stand up to it a little better. If you fellas think I'd be OK with the 500v tap I'd really like to give it a go.

I'm really into th idea of running a cleaner amp with head room and a screaming 50w in parallelle at the same time.

I will definitely post a voltage chart for this one.
Still waiting for the transformers to show up..... :whistle:

By the way Neikeel :worthy: read over your JTM 50 post yesterday and saw the links to the images.
That is some incredible work! Totally inspiring.

It's defiantly got me even more reved up for this build. Mine is certainly not even in the same league but circuit wise mine is fairly close to yours. Although mine has mostly Piher rather than Iskra and l've gone with the 270k mixers at least for now.

I also went with a 820R Carbon Comp on the cathode and a 27k CC on the negative feed back but I think I will swap these out for Iskras.

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Re: Dropper Resistor on a 67?

Post by bill bokey » Wed Dec 13, 2017 3:10 am

What NOS tubes are you planning on using ?

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Re: Dropper Resistor on a 67?

Post by revolver1 » Wed Dec 13, 2017 1:02 pm

I'd like to pick up some Millard XF2 or 3's to do it justice. They should be good for 500v. :shred:

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Re: Dropper Resistor on a 67?

Post by bill bokey » Wed Dec 13, 2017 1:57 pm

Oh yeah ! 8)
RFT, on the other hand, might not be, that's why I asked ;)

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Re: Dropper Resistor on a 67?

Post by revolver1 » Thu Dec 14, 2017 2:18 am

Got ya, RFT are not so good with high plate voltage right?

The job I'm on is finishing up in January so when I've sorted something else out and I know I'll be good for another year or so I was going to put in a bulk order for some discount.

I'm seriously thinking about only building 50w's from here on in though :lol:

This will be a new one for me, I had used all NOS in the pre amp before but never the outputs as well.
Again I'm quite excited at the prospect.

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Re: Dropper Resistor on a 67?

Post by bill bokey » Thu Dec 14, 2017 3:39 am

revolver1 wrote:
Thu Dec 14, 2017 2:18 am
Got ya, RFT are not so good with high plate voltage right?
It's more high screen voltage they don't like (supposedly above 450V). Having said that, I had RFTs in a few amps, with B+ in the 460-490V range and never had a failure.

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