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Reworking a current build from an all triode PP to SE
Reworking a current build from an all triode PP to SE
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Old 9th September 2019, 06:12 PM   #1
Lingwendil is offline Lingwendil  United States
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Default Reworking a current build from an all triode PP to SE

So, I'm almost done with a PCB based build of my flea amplifier, and will be stripping the original p2p prototype and building my first personal SE amplifier in years. On the original chassis I have a 50VA power transformer, a mosfet ripple filter that gives about 290 volts under load, and four octal sockets, input RCAs, and speaker binding posts that will stay in the chassis. I will be using 6V6GT( Russian 6P6, actually) and a pair of surplus fender champ style output transformer for my outputs, which will leave me with two octal sockets for input circuitry. Planing on going for tetrode, and maybe triode connected, if the drivers can handle it. I would like to use global negative feedback.

So far I've toyed with two ideas for it. Either a CCDA front end with 6SL7 feeding a 6SN7 cathode follower, or a 6SL7 totem pole. No desire to go SRPP, unless there is a compelling reason to.

I suppose I could make some adapter discs to convert to nine pin input tubes if needed, and then could also use 6N1P, or 6N2P, or similar tubes, limited to what I have on hand.

What would you do?
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Old 9th September 2019, 06:29 PM   #2
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Which is the input sensitivity you need? Phono, radio, guitar???
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Old 9th September 2019, 07:24 PM   #3
Lingwendil is offline Lingwendil  United States
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I usually leave it hooked to the TV, so something from 1V peak for full output would be good and versatile enough for most of what I'm likely to do with it, as that's what the current flea ampl is at and it does well. It'll either be there or used with a tablet or phone that will do 300mV RMS in the garage, for background workshop tunes.

I'm leaning towards more gain so that I can have enough headroom to play with for feedback, and a 6SL7 totem pole should give me around 30~35x which should be usable for tetrode, but maybe not triode. If I go triode (might as well with the 6V6, it really is a fine triode, if not the most powerful) I'd rather have a bit more gain.

I'm thinking that the 6SL7 into 6SN7 would be best, that way I can go for the bypassed cathode up front and a cathode follower for oomph driving the triode connected grid capacitance. Direct coupled to the preceding anode should still be plenty of headroom for swing.
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Old 11th September 2019, 03:52 AM   #4
kodabmx is offline kodabmx  Canada
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Reworking a current build from an all triode PP to SE
You could go "old school" and use 6SJ7 input

http://www.mif.pg.gda.pl/homepages/f...049/6/6SJ7.pdf

Found this on a Korean forum...

Click the image to open in full size.

Last edited by kodabmx; 11th September 2019 at 03:57 AM.
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Old 11th September 2019, 02:53 PM   #5
Lingwendil is offline Lingwendil  United States
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Would if I had a pair this is a "sweep the floor" rebuild that will be limited to parts that I've already got on hand.

I'm thinking a totem pole might be a good way to go. Maybe bite the bullet and try an SRPP just for the hell of it? At modest swing like I'm going to need, and with some global negative feedback I think it would be plenty good. If I'm not mistaken kevinkr did a 6SL7 SRPP into 45 SE a few years back and had good things to say about it. Triode connected 6V6 is (allegedly) very close to 45, so might be worth a shot?

Anybody done totem pole with bypassed cathode resistor on the bottom tube? Gain should be close to mu that way unless there's something I'm missing.
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Old 11th September 2019, 03:24 PM   #6
kodabmx is offline kodabmx  Canada
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Reworking a current build from an all triode PP to SE
If you aren't using gNFB, I'd just do a 6SN7 CCDA into the 6V6. If you need the extra gain for gNFB, Then I'd go 6SN7 into 6SN7 into 6V6, all common cathode.

You could also do this: 6V6 SE3

Just sub 6SL7 for 12AX7
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Old 11th September 2019, 04:24 PM   #7
pblix is offline pblix  United States
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I would do this...except swap the 6L6 and go with your plan of using 6V6. I already made a board I haven't had time to test yet. Too many projects...

Click the image to open in full size.

Last edited by pblix; 11th September 2019 at 04:26 PM.
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Old 12th September 2019, 03:57 PM   #8
Lingwendil is offline Lingwendil  United States
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^ If I had a pair of 6SH7 or 6SJ7 I would try something like that Financially I'm not in a position to sink any additional money into my hobbies for a while, but I happen to have quite a bit of parts on hand, other than higher quality iron.

Maybe a 6SN7 beta follower, like Morgan Jones featured in his Designing Valve Amplifiers books? Should be an interesting solution that would be something different. A 6SN7 in this configuration shouldn't have an issue driving a triode connected 6V6 in this setup, and overall distortion should be low without negative feedback. A pair of cheap Russian 6H8C/6SN7 should be fine here, and I have a dozen or so on hand.


Just found the cheap transformers that I had on hand, and they're 7.5k to 8/4/16 ohm. They are for the Epiphone valve junior, and are rated at 5W with a 6V6. I doubt they have bandwidth past 10khz, but they'll do for a quick garage amp rebuild like this. Might be able to extend the bandwidth with some feedback, but without a way to test it there's not much point.

This thing is for background tunes while sawing wood, beating on seized bolts, etc. When we end up in a place with room for bigger, better speakers out in the shop I'll build a bigger rig with nice parts.
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Old 12th September 2019, 04:38 PM   #9
pblix is offline pblix  United States
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Two triode stages in cascode act like a pentode in a lot of ways. Merlin's page is pretty helpful:

The Valve Wizard

The schematic I posted was discussed in another thread briefly. The accompanying article is an interesting read, and the whole purpose of the circuit was to utilize budget OTs, sort of like what you are doing. Here is a post with the article:

Local feedback between grid-cathode

The images may be hard to read, so here is a link to a pdf:

https://www.americanradiohistory.com...o-1955-Mar.pdf

Last edited by pblix; 12th September 2019 at 04:44 PM.
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Old 12th September 2019, 06:57 PM   #10
kodabmx is offline kodabmx  Canada
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Reworking a current build from an all triode PP to SE
Quote:
Originally Posted by Lingwendil View Post
This thing is for background tunes while sawing wood, beating on seized bolts, etc. When we end up in a place with room for bigger, better speakers out in the shop I'll build a bigger rig with nice parts.
Exhibit A: MKll Tripath TA2024 Amplifier Board Module 2x15watt tested Audio Power HIFI NEW | eBay
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